The Coral Reef of South Molokaÿi, Hawaiÿi

Scientific Investigations Report 2007-5101

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2007-5101

U.S. Department of the Interior

U.S. Geological Survey

The Coral Reef of South Molokaÿi, Hawaiÿi

Portrait of a Sediment-Threatened Fringing Reef
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The Coral Reef of South Molokaÿi, Hawaiÿi—

Portrait of a Sediment-Threatened Fringing Reef

Edited by Michael E. Field, Susan A. Cochran, Joshua B. Logan, and Curt D. Storlazzi

Scientific Investigations Report 2007–5101

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Field, M.E., Cochran, S.A., Logan, J.B., and Storlazzi C.D., eds., 2008, The coral reef of south Molokaÿi, 

Hawaiÿi—portrait of a sediment-threatened fringing reef: U.S. Geological Survey Scientific Investigations Report 

2007-5101, 180 p.

Cataloging-in-publication data are on file with the Library of Congress (http://www.loc.gov/).

Produced in the Western Region, Menlo Park, California

Manuscript approved for publication, April 29, 2008

Text edited by Peter H. Stauffer 

Design and layout by David R. Jones
iii 

CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vi

INTRODUCTION

The South Molokaÿi Reef: Origin, History, and Status. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

	 	 	 Michael E. Field, Susan A. Cochran, Joshua B. Logan, and Curt D. Storlazzi

EVOLUTION OF THE SOUTH MOLOKAÿI FRINGING REEF

1 Antecedent Substrate Underlying the South Molokaÿi Fringing Reef and Implications

for Reef Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Walter A. Barnhardt, Eric E. Grossman, and Bruce M. Richmond

2 Views of the South Molokaÿi Watershed-to-Reef System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17

Michael E. Field, Joshua B. Logan, Pat S. Chavez Jr., Curt D. Storlazzi, and Susan A. Cochran

3 Shape of the South Molokaÿi Fringing Reef: Trends and Variation . . . . . . . . . . . . . . . . . . . . . . . . . .33

Curt D. Storlazzi, Joshua B. Logan, and Michael E. Field

4 Molokaÿi—Two Different Types of Reef Growth in the Past 8,000 Years . . . . . . . . . . . . . . . . . . . . .37

Mary S. Engels and Charles H. Fletcher

CHARACTER OF THE SOUTH MOLOKAÿI FRINGING REEF

5 Reef Corals and the Coral Reefs of South Molokaÿi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

	 	 	 Paul L. Jokiel, Eric K. Brown, Ku’ulei S. Rodgers, and William R. Smith

6 The Status of the Reefs Along South Molokaÿi: Five Years of Monitoring . . . . . . . . . . . . . . . . . . . 51

	 	 	 Eric K. Brown, Paul L. Jokiel, Kuÿulei S. Rodgers, William R. Smith, and Lucile M. Roberts

7 Coral Reef Fishes and Fisheries of South Molokaÿi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

	 	 	 Alan M. Friedlander and Kuÿulei S. Rodgers

8 The Seaweed and Seagrass Communities of Molokaÿi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67

	 	 	 Jennifer E. Smith, Heather L. Spalding, Ryan Okano, and Celia M. Smith

9 Benthic Habitat Maps of the South Molokaÿi Coral Reef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77

Susan A. Cochran

NATURAL CHANGE TO THE MOLOKAÿI REEF

10 Sea Level and its Effects on Reefs in Hawaiÿi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .101

	 	 	 Eric E. Grossman

11 Waves and Their Impact on Reef Growth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .105

	 	 	 Curt D. Storlazzi, Michael E. Field, Eric K. Brown, and Paul L. Jokiel

12 The Freshwater Cycle on Molokaÿi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109

	 	 	 Gordon W. Tribble and Delwyn S. Oki

13 Ground Water and its Influence on Reef Evolution. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

	 	 	 Eric E. Grossman, Joshua B. Logan, Joseph H. Street, Adina Paytan, and Pat S. Chavez, Jr.

14 Light, Calcification, and Carbonate Sediment Production on the Molokaÿi Reef Flat. . . . . . . . . . .117

	 	 	 Robert B. Halley and Kimberly K. Yates

RIDGE-TO-REEF: SEDIMENT SHED FROM THE ISLAND

15 People, Land, and the Reefs of South Molokaÿi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

	 	 	 Lucile M. Roberts and Michael E. Field

16 Invasive Mangroves and Coastal Change on Molokaÿi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .129

	 	 	 Mimi D’Iorio

IMPACT OF SEDIMENTATION ON THE MOLOKAÿI REEF

17 Sediment on the Molokaÿi Reef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .137

	 	 	 Michael E. Field, R. Scott Calhoun, Curt D. Storlazzi, Joshua B. Logan, and Susan A. Cochran

18 Measurement of Toxicity in Reef Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .145

	 	 	 R. Scott Carr and Marion Nipper

19 Sediment Mobility Along Molokaÿi's Fringing Coral Reef: Evidence From Sediment Traps . . . .147

	 	 	 Michael H. Bothner, Richard L. Reynolds, Michael A. Casso, Curt D. Storlazzi, and Michael E. Field

20 Causes of Turbidity on the Molokaÿi Reef Flat and Resulting Sediment-Transport Patterns . . . . .153

	 	 	 Andrea S. Ogston, M. Katherine Presto, Curt D. Storlazzi, and Michael E. Field

21 The Effects of a Kona Storm on the Molokaÿi Reef: November and December 2001 . . . . . . . . . .159

	 	 	 Michael E. Field, Michael E. Bothner, Pat S. Chavez, Jr., Susan A. Cochran, Paul L. Jokiel,

	 	 	 Andrea S. Ogston, M. Katherine Presto, and Curt D. Storlazzi

SUMMARY

The Molokaÿi Coral Reef Today, and Alternatives for the Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . .167

	 	 	 Michael E. Field, Susan A. Cochran, Joshua B. Logan, and Curt D. Storlazzi

Glossaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .171

Fieldwork Activities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
iv

FOREWORD

Molokaÿi, with the most extensive coral reef in the main Hawaiian 

Islands, is especially sacred to Hina, the Goddess of the Moon. As 

Hinaalo, she is the Mother of the Hawaiian people; as Hinapukuÿa, 

she is the Goddess of Fishermen; and in the form Hinaÿopuhalakoÿa, 

she is the Goddess who gave birth to coral, coral reefs, and all 

spiny marine organisms. Interdependence between the reef’s 

living resources, the people, and their cosmology was the basis for 

management of Molokaÿi’s coastal waters for over a thousand years.

The ancient residents of Molokaÿi built the greatest concentration of 

fishponds known anywhere, but their mastery of mariculture, something 

needed now more than ever, was lost after near genocide from exotic Western diseases. Subsequent destruction of the native vegetation for exotic 

cattle, goats, pigs, sugar cane, and pineapple caused soil erosion and sedimentation on the reef flat. This masterful volume clearly documents that 

soil washing into the sea is the major threat to the reef today. Abandoned 

fishponds, choked with sediment, now act as barriers and mud traps, making damage to corals less than it would otherwise would have been.

The role of mud and freshwater from land in preventing coral reef 

growth, clearly articulated in Charles Darwin’s first book, The Structure and Distribution of Coral Reefs, is the major theme of this book. 

All around the tropics, coral reefs have died from huge increases in 

terrestrial sedimentation that resulted from destruction of hillside forests for cash-crop agriculture and pastures in the colonial era, especially in Latin America, Asia, and the islands of the Caribbean and 

Indo-Pacific. It is obvious that one cannot manage the coastal zone as a 

unit separate from the watersheds that drain into it. Yet there has been 

surprisingly little comprehensive scientific study of these impacts.

In this landmark volume, U.S. Geological Survey researchers and their 

colleagues have developed and applied a remarkably integrated approach 

to the reefs of Molokaÿi, combining geology, oceanography, and biology 

to provide an in-depth understanding of the processes that have made these 

reefs grow and that now limit them. They have joined old fashioned natural 

history of marine animals and plants with study of the geological evolution 

of the island, hydrology, meteorology, and land-use history, to an arsenal of 

new methods of remote sensing, including aerial photography, laser ranging, infrared thermal mapping, seismic reflection, in-situ instrumentation 

to measure chemical parameters of water quality, and direct measurements 

of the physical driving forces affecting them—such as wave energy, currents, sedimentation, and sediment transport. They provide a level of documentation and insight that has never been available for any reef before. 

A remarkable feature of this book is that it is aimed at the people of Molokaÿi to inform them of what is happening to their reef 

and what they might do to preserve their vital resources. The scientific data and interpretations are expressed in unusually clear and 

comprehensible language, free of the professional jargon that makes 

most technical publications impenetrable to the public that most 

needs to know about them, yet without loss of scientific rigor. 

Here readers will see clearly explained the whole path of soil loss, 

from the impacts of wild pigs and goats at higher elevations, deforestation of the hills for cattle pasture at lower levels, and denudation of low 

lands for cash crops. The resulting biological impoverishment has bared 

the soils, which wash away in flash storms, smothering the inshore reefs, 

whose growth was already limited because they had grown right up to sea 

level. The data in this book show that the mud doesn’t get far if it is washed 

into the sea during a big storm with heavy waves. Afterwards this mud 

keeps getting stirred up by every succeeding storm, spreading and affecting corals over wider areas until it is finally washed out of the system—and 

that only happens if there is no more new mud washing onto the reef.

I saw this myself a few years ago in Pilaÿa Bay on Kauaÿi, where 

a bulldozed hillside of abandoned sugar cane fields had slumped right 

on top of a coral reef following exceptional rains. Years later, the 

algae species were zoned in a way that clearly mapped the distribution of nutrients washed into the bay, most likely from fertilizers bound 

to the eroded soils. That pattern closely mimics, on a small scale, that 

shown in Molokaÿi in this volume, where the inner reef is covered with 

algae, zoned by species in a way that points to land-based sources of 

nutrients, while the outermost reef slope is still coral dominated, and 

the deep algae seem to indicate deep-water nutrient upwelling. 

What of the future? The Hawaiian Islands have been exceptionally fortunate to be spared the worst coral heatstroke death from high 

temperatures, at least to date. So far, the worst global warming impacts 

have luckily been small in this region, and the small number of people on 

Molokaÿi has kept population densities, and sewage pollution, low compared to the more developed islands. Nutrients from years of sugar and 

pineapple fertilization, and the washing of this soil onto the reefs, show 

clear influences on the pattern of algae on the reef. Even at very low levels of nutrients, well below that which drives algae to smother and kill 

coral reefs, more algae is present. Soil erosion control is therefore the 

key to better management of both nutrients and turbidity on Molokaÿi 

reefs. To that end land management actions mentioned in this book, such 

as suppressing wild fires and eliminating wild goats and pigs, could 

be made even more effective if supplemented by active erosion control 

using plants whose roots bind the soil effectively in place. Through all of 

these efforts, Hina and the people of Molokaÿi could be happy again!

Thomas J. Goreau, Ph.D.

President, Global Coral Reef Alliance
v 

PREFACE

Two themes pervade this publication. One is the beauty, grandeur, and 

majestic resource that the fringing coral reef off south Molokaÿi represents to the people of Molokaÿi and Hawaiÿi—and indeed to people 

everywhere. The other theme is the threats that face that same reef. 

Hawaiian reefs, like other reefs worldwide, harbor an incredible 

diversity of plant and animal life, and that richness is reflected in the 

cultural traditions of native peoples and in prosperity from tourism. However, coral reefs are in decline worldwide (Hughes and others, 2003a), and 

Hawaiÿi and Molokaÿi are no exception. An abundance of land-derived 

silt has washed from Molokaÿi’s slopes and threatens the health of the 

island’s reef by blocking light and choking the reef inhabitants. Conditions on Hawaiÿi’s reefs vary from island to island and location to location (some areas show signs of improving), and the causes of degradation are both natural and human induced. Natural disturbance by large 

waves is being exacerbated by increased sediment runoff. Islands erode 

by natural processes, but the activities of humans—deforestation, agriculture, cattle and sheep farming, introduction of feral grazers, increased 

wildfires, construction —often cause the release of sediment at higher 

rates than occur naturally. Nowhere is this more evident than on the 

island of Molokaÿi. In 1979—a full two decades before U.S. Geological Survey (USGS) scientists visited Molokaÿi to study sediment on 

the reef—the famous ocean explorer Jacques-Yves Cousteau wrote: 

One of the most serious dangers to coral reefs is sedimentation. 

This threat is dramatically illustrated in Hawaii. With the arrival of 

Westerners, upland soil was plowed for sugar cane and pineapple. 

The impact of the resulting erosion has been tragic. Since 1897 the 

shoreline of Molokai has advanced as much as a mile and a quarter across the reef flat. Elsewhere off Molokai, the reef is overlaid 

with four to 27 inches of red-brown silt. (Cousteau, 1985, p. 191)

It is noteworthy that of all the troubled reefs in the world, Cousteau singled 

out Molokaÿi as the example for sedimentation on reefs. 

Starting in 1999, the USGS, along with university and agency partners, 

initiated a study to map the reef off south Molokaÿi and describe the coral, 

analyze the sediment, and measure the processes that suspend and transport 

sediment on the reef. This publication reports the results of those endeavors.

The importance of identifying and monitoring the effects of landbased pollution has become crucial within the United States, as evidenced 

by priorities set by the U.S. Task Force on Coral Reefs. One of the key 

topics in the April 2004 Report of the U.S. Commission on Ocean Policy 

(U.S. Commission on Ocean Policy, 2004) is the need for “an ecosystem 

and watershed-based management” approach to ocean pollution, and that 

report highlights “the astounding decline of coral reef ecosystems” and “an 

urgent need to address the identified, major factors causing coral declines.” 

In Hawaiÿi, local representatives of the U.S. Coral Reef Task Force specifically developed and established a Local Action Strategy (LAS) and steering committee to address land-based sources of pollution and their impact 

on reefs. The LAS is essentially a locally-driven roadmap for collaborative and cooperative action among Federal, State, and nongovernmental 

partners to identify and implement priority actions needed to reduce key 

threats to valuable coral-reef resources. In the state of Hawaiÿi, Molokaÿi 

was designated one of the key LAS sites. To protect and sustain coral 

reefs, broad understanding of the processes involved is necessary—how 

sediment is derived in upland regions, how it is transported through the 

watershed, and its fate on the reef. Many positive signs indicate that science and management are embracing that need for understanding. 

This volume supports and augments those goals through presentation 

of new information on the behavior and fate of sediment on the Molokaÿi 

coral reef. The following chapters discuss the evolution of the reef, the 

natural processes that affect the reef, and the role of sediment in inducing 

change on the reef. This information is intended to help make decisions—

on Molokaÿi and elsewhere—that can lead to sustaining healthy reefs.

We hope this volume will be of benefit first and foremost to the people 

of Molokaÿi. The fringing coral reef bordering the south shore of their island 

is perhaps the most magnificent coral reef in the Hawaiian Island chain. 

In addition to its intrinsic value as an important ecosystem, the reef holds 

a rich heritage of cultural and historical associations and meanings for the 

island residents. Today the Molokaÿi reef is at a crossroad—the evidence 

of damage from heavy land use is clear, and yet procedures for its protection through improved land-use practices are already underway. It is the 

people of Molokaÿi who will ultimately decide the fate of their coral reef. 

Measures to decrease sediment runoff, control fish collection, and establish protected zones are all actions they can take to help preserve the reef. 

We hope this publication will also be of value to all coastal 

people living harmoniously with neighboring coral reefs. Some of 

the causes of reef degradation are global, but many remain local. 

It is largely through local decisions and local actions that protection of coral reefs can be assured for future generations.

Michael E. Field, Susan A. Cochran,

Joshua B. Logan, and Curt D. Storlazzi
ACKNOWLEDGMENTS

The information presented in this volume derives largely from research 

undertaken by the U.S. Geological Survey (USGS) to meet one of its principal goals—to establish the geologic framework of ecosystem function and 

structure. Some of the research was supported by our partners at the University of Hawaiÿi and the National Oceanographic and Atmospheric Administration (NOAA). All of the authors and editors fully recognize that the success of their studies resulted in very large measure both through the technical 

expertise provided by our colleagues in Hawaiÿi and on the mainland and 

from the assistance and friendship of the people of Molokaÿi.

Our studies benefited greatly from the advice, assistance, and knowledge 

of the land and reef shared by Molokaÿi residents. There are many friends on 

Molokaÿi to acknowledge, but most certainly at the top of the list is Captain 

Joe Reich, owner of the Alyce C, whose keen eye and understanding of the 

waves, wind, and the reef itself made for safe and productive experiments time 

after time after time. We could not have done it without you, Joe.

Among the many people who assisted us, we wish to thank in particular the following. Ed Misaki of the Nature Conservancy, Debbie Kelley 

and Sheila Cox of the USDA, and the members of the Molokaÿi Watershed 

Advisory Group collaborated in our studies. Lyndon and Janis Dela Cruz 

assisted with data collection, instrument maintenance, and other lastminute duties beyond the call of friendship. Harbormaster Bernie Straehler, Young Brothers manager Al Ranis, and the rest of the employees at 

Young Brothers stored our mountains of gear and allowed us temporary 

workspace at times over the years. Aka and Porter Hodgins assisted with 

additional transportation and storage of fieldgear. The late Bill Kapuni 

filled numerous scuba tanks for us over the years, and we’re glad he was 

able to share his manaÿo with us. Roland (Rolly) Moran provided boats, 

motors, and the uncanny ability to repair anything or have any obscure part 

one might need on a small island. Sam Rawlins taught us “crazy professors” about fishponds. Mervin Dudoit assisted with small-boat work. Jerry 

Johnson took our team under wing and introduced us to many folks on 

the island. Molokaÿi General Hospital provided us with x-rays of cores. 

The staff members of Hotel Molokaÿi, Molokaÿi Shores, Molokaÿi Vacation Rentals, and Molokaÿi Fish and Dive were always helpful, as were 

Molokaÿi residents Bill Feeter, Donna Haytko-Paoa, Yolanda Reyes of 

Molokaÿi Ranch, and finally, the FedEx drivers who carted our endless 

boxes of instruments and gear.

Many colleagues at the USGS and in universities provided invaluable 

technical and laboratory assistance on Molokaÿi and in our laboratories in 

California, Arizona, and Massachusetts. From the USGS Coastal and Marine 

Geology team in Santa Cruz and Menlo Park, California, we thank Mike Torresan, Hank Chezar, Tom Reiss, Charlene Tetlak, Olivia Buchan, Simon Barber, Ann Gibbs, Kurt Rosenberger, Carolynn Box, Becky Stamski, and Eric 

Thompson. From the USGS Coastal and Marine Geology team in Woods 

Hole, Massachusetts, we thank Rick Rendigs, Ray Davis, Paul Lamothe, and 

Sandy Baldwin. From the USGS in Flagstaff, Arizona, we thank Pat Chavez, 

Stuart Sides, Miguel Velasco, JoAnn Isbrecht, and Rian Bogle. From the 

USGS in Denver, Colorado, we thank Rich Reynolds. From the USGS in St. 

Petersburg, Florida, we thank Nate Smiley. From the USGS and Texas A&M 

University–Corpus Christi, we thank James Biedenbach, Russell Hooten, and 

Linda Price-May. From the USGS in Honolulu, Hawaiÿi, we thank Delwyn 

Oki. From Colorado State University, we thank Paul Ayers and Sara Legoza. 

From the University of Washington, we thank John Crockett.

This volume contains contributions from many authors on diverse and 

complex topics, and this combination makes smoothness and consistency 

between chapters inherently difficult. Peter Stauffer and David Jones were 

responsible for editing and melding the contributions into a single volume. 

They were extremely helpful and, in our view, successful in doing so. The 

individual chapters have also benefited measurably from superb, thoughtful suggestions by the following colleagues, and we thank them for their 

patience and advice:

Alan Allwardt, USGS, Santa Cruz CA

Carl Berg, Hanalei Watershed Hui, Hanalei HI

Eric Brown, National Park Service, Kalaupapa HI

Brad Butman, USGS, Woods Hole MA

Dave Cacchione, USGS, Menlo Park CA

Scott Calhoun, Associated Earth Sciences, Kirkland WA

Kathy Chaston, University of Hawaiÿi, Mänoa HI

Pete Dartnell, USGS, Menlo Park CA

Brian Edwards, USGS, Menlo Park CA

Chip Fletcher, University of Hawaiÿi, Mänoa HI

Alan Friedlander, NOAA/The Oceanic Institute, Waimänalo HI

Ginger Garrison, USGS, St. Petersburg FL

Ann Gibbs, USGS, Santa Cruz CA

Eric Grossman, USGS, Santa Cruz CA

Bob Halley, USGS, St. Petersburg FL

Stacy Jupiter, James Cook University, Townsville, Queensland

Ilsa Kuffner, USGS, St. Petersburg FL

Ian Macintyre, Smithsonian Institution, Washington DC

Mark Monaco, NOAA, Silver Spring MA

Marlene Noble, USGS, Menlo Park CA

Andrea Ogston, University of Washington, Seattle WA

Delwyn Oki, USGS, Honolulu HI

Adina Paytan, University of California, Santa Cruz CA

Greg Piniak, NOAA, Beaufort NC

Bruce Richmond, USGS, Santa Cruz CA

John Rooney, NOAA, Honolulu HI

Dave Rubin, USGS, Santa Cruz CA

Chris Sherwood, USGS, Woods Hole MA

Gene Shinn, USGS/University of South Florida, St. Petersburg FL

Chris Sommerfield, USGS, Woods Hole MA

Jon Stock, USGS, Menlo Park CA

Gordon Tribble, USGS, Honolulu HI

Jingping Xu, USGS, Santa Cruz CA

We thank our on-the-ground “ridge-to-reef” colleagues, Gordon 

Tribble, Jon Stock, Pat Chavez, and Jim Jacobi, for their insight and willingness to share their knowledge of tropical watersheds. Lastly, but importantly, this study received solid support from leaders in the U.S. Geological 

Survey Coastal and Marine Geology Program and the U.S. Geological 

Survey Western Region.

vi

Captain Joe Reich and his boat, Alyce C.
View of south Molokaÿi, looking eastward from hills above Hale O Lono.

Introduction
View south from Molokaÿi Shores condominium complex. The island of Lanaÿi is in the background.
Introduction

160° W 158° 156°

22°

N 

20°

NiÔihau

KauaÔi

OÔahu MolokaÔi

LnaÔi

KahoÔolawe

Maui

HawaiÔi

0 50 100 Miles

0 100 200 Kilometers

Penguin Bank

P a c i f i c

O c e a n

KauaØi

Sea level

OØahu Molokai Maui

HawaiØi

Hot

spot

Rebound

Depression

Pacific lithospheric plate

Asthenosphere

INTRODUCTION

The South Molokaÿi Reef: Origin, History, and Status

Michael E. Field1, Susan A. Cochran1, Joshua B. Logan1, and Curt D. Storlazzi1

Molokaÿi is the fifth youngest island in the long chain of volcanoes 

and volcanic remnants that compose the Hawaiian archipelago 

(fig. 1). The archipelago extends from the Island of Hawaiÿi (the 

“Big Island”) in the southeast past Midway Island, to Kure Atoll in the 

northwest, for a total distance of about 2,400 km (1,500 mi). Beyond Kure 

Atoll, the chain continues as a series of submerged former islands known 

as the Emperor Seamounts, which extend to the Aleutian Trench off the 

coast of Alaska. Evolution of the entire Hawaiÿi-Emperor volcanic chain 

represents a time span of nearly 80 million years (Clague and Dalrymple, 

1989). The volcanic chain is a result of gradual and persistent movement 

of the Pacific lithospheric plate (the sea-floor crust and rigid uppermost 

part of Earth's mantle) over a deep fracture (or hot spot) that extends down 

to the astenosphere, a less rigid part of the mantle (fig. 2). Plumes of 

molten lava flowed onto the sea floor, repeatedly creating massive shield 

volcanoes that exceed 10,000 m (33,000 ft) in relief above the surrounding 

sea floor. The growth of each volcano is a process that takes half a million 

years or more to construct most of its mass through sequential volcanic 

phases—submarine, explosive, and subaerial—of shield growth.

Once formed, each massive island volcano is carried northwestward on the Pacific tectonic plate at rates of 8.6 to 9.2 cm/yr (Clague 

and Dalrymple, 1989). The postshield processes of alkalic volcanism, 

subsidence, landslides, rejuvenated volcanism, weathering, sediment 

deposition, and reef growth have all markedly influenced each volcano’s 

present-day shape. Subsidence of each island is rapid at first (rates of 

2 mm/yr or more; Moore and Campbell, 1987; Moore and Fornari, 1984; 

Campbell, 1986) in response to the extraordinary weight of large volumes of lava loaded onto the crust. As each island cools and slides northwestward with the sea-floor crust, it continues to subside at decreasing 

rates, down to the order of 0.02 mm/yr (Detrick and Crough, 1978). The 

sheer volume of rock that accumulates at each volcano ultimately leads 

to failure and partial collapse—each island has had spectacular landslides 

that are amongst the largest on earth (Moore and others, 1989). The large 

areas of irregular topography on the sea floor around the islands (for 

example, north of Molokaÿi and northwest of Oÿahu) attest to the magnitude of these events (fig.1).

The normal processes of surface erosion and stream runoff modify 

volcano slopes early in an island’s history. Those processes, along with 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

development of soils, which occurs relatively quickly in humid volcanic 

terrain, lead to transport and deposition of sediment in alluvial fans, flood 

plains, and narrow coastal plains. The final process in island shaping is 

the establishment of coral reefs in shallow waters that are protected from 

large waves. Corals colonize exposed rock surfaces very quickly (Grigg 

and Maragos, 1974; Grigg, 1983), and it is likely that they become established early in the evolutionary history of each island. The development 

of coral reefs—the massive limestone structures capped by a living ecosystem that border many Hawaiian Islands—takes much longer (Grigg, 

1987). Each reef is a thick (meters to tens of meters) packet of reefal 

limestone that likely accumulated over multiple stages of sea-level shifts 

(Grossman and others, 2006; Grossman and Fletcher, 2004; Sherman and 

others, 1999). In most locations in Hawaiÿi, modern coral cover is only 

a thin living veneer on top of older reef structures that formed during an 

earlier time under different conditions (Grigg, 1983, 1998)

 Molokaÿi—Making of a Unique Island

Despite the similarity in origin and physical processes that it shares with 

the other Hawaiian Islands, Molokaÿi is unique in many ways. Its marked 

east-west orientation and its relatively storm- and wave-protected south shore 

are two important factors that have contributed to development of a spectacular coral reef that is perhaps the most extensive and continuous fringing reef 

in the Hawaiian chain.

In its original configuration, Molokaÿi was part of a very large island 

(Maui Nui) that included Länaÿi, Maui, Kahoÿolawe, and Penguin Bank 

(fig. 1). As the massive, contiguous island subsided, Molokaÿi, Länaÿi, and 

Penguin Bank became an island separate from the others about 300,000 

to 400,000 years ago (Juvik and Juvik, 1998). Penguin Bank eventually 

submerged, and Länaÿi and Molokaÿi became separated by a small chan- Figure 1. Physiographic map of the eight main Hawaiian Islands, which are the 

most recent additions to the approximately 2,400-km (1,500-mi) -long Hawaiian 

chain of islands, atolls, and pinnacles that extends from the Island of Hawaiÿi (the 

“Big Island”) northwest to Kure Atoll. Molokaÿi formed between 2.0 and 1.7 million 

years ago and is the fifth youngest in the line of more than 107 volcanic edifices 

in the Hawaiian chain (Clague and Dalrymple, 1989). Only the volcanoes forming 

Länaÿi, Maui, Kahoÿolawe, and Hawaiÿi are younger. 

Figure 2. Diagram illustrating the formation of the Hawaiian volcanoes by the 

eruption of lava rising from a “hot spot”—a thermal plume emanating from deep 

in the Earth’s mantle. As the Pacific Plate of the lithosphere moves northwestward at rates of about 9.0 cm/yr (3.5 in/yr), a new volcano forms over the hot 

spot every 100,000 to 500,000 years. Vertical arrows show the depression of the 

lithosphere under the island of Hawaiÿi from the weight of newly formed volcanic 

edifices and the subsequent rebound of the lithosphere (as shown under Oÿahu) 

surrounding the depression.
21°

10'

 N

157°20' W 157°10' 157° 156°50'

0

0 5 Miles

5 10 Kilometers

SEDIMENTARY ROCKS VOLCANIC ROCKS 

Beach sand Dunes

Alluvium

Lithified

dunes

Marine

limestone

Kalaupapa

basalt

(with vent)

Upper differentiatied

member (with vent)

Caldera

complex

Basalt

(with vents)

Lower basalt

member (with vent)

Holocene

Holocene and

Pleistocene

Late

Pleistocene

West Molokai volcanic series

erosional unconformity

East Molokai

volcanic series

Late Pleistocene

Great

Erosional

Unconformity

Pleistocene

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

4

volcano (Wailau) formed about 1.76 Ma, and it too was capped by postshield volcanic flows (1.5 to 1.35 Ma) and a rejuvenated phase of eruptions at about 0.57 to 0.35 Ma (fig. 3; Juvik and Juvik, 1998). The geology of Molokaÿi, as originally mapped by Stearns and Macdonald (1947; 

fig. 4), illustrates the island’s evolution by a series of distinct, sequential 

events: (1) formation of the west Molokaÿi volcano, (2) formation of the 

east Molokaÿi volcano through two sequences of volcanism, (3) formation 

of the high sea cliffs (pali) of the north coast by a gigantic landslide, (4) 

formation of a small shield volcano and caldera forming the Kalaupapa 

Peninsula, and (5) erosion of the volcano slopes and deposition of alluvium 

in channels, fan deltas, and coastal plain deposits (Stearns, 1966; Stearns 

and Macdonald, 1947; Walker, 1990).

The narrow shape of the island and the very high (nearly 1,200 m or 

3,900 ft) pali on the northern side of Molokaÿi—among the highest sea 

cliffs in the world—resulted from a catastrophic landslide following the 

construction of east Molokaÿi volcano. The landslides that characterize the 

volcanoes of the Hawaiian chain are some of Earth’s largest, and among 

the Hawaiian slides, the Molokaÿi slide (named the Wailau Slide) is one of 

the largest (Moore and others, 1994). A large part of the island collapsed, 

scattering huge volcanic blocks (as much as 6 km, or 4 mi, in diameter) 

and debris more than 150 km (90 mi) across the sea floor (Moore and 

others, 1989). Following the landslide, rejuvenated volcanism created the 

low-relief Kalaupapa Peninsula that abuts the cliffs of north Molokaÿi. 

The island of Molokaÿi today has a shape reflecting these formative processes (fig. 5).

Figure 3. Original drawings from Stearns and Macdonald (1947; 

reproduced in Stearns, 1966) showing the development of the 

island of Molokaÿi. Stages 1 through 3 show the two volcanoes 

forming between 1.9 (west) and 1.75 (east) million years ago 

and converging to form a single, overlapping volcanic ediface. 

Stearns and Macdonald thought that the two volcanoes were 

isolated later by rising sea levels associated with an intergalcial 

period (stage 4 in the diagram). Long after Molokaÿi was formed, 

posteruptive volcanism produced the Kalaupapa Peninsula 

(stage 5). Stage 6 shows the island as it exists today. Stearns and 

Macdonald’s work was published before anyone had inferred that 

giant landslides formed the pali on the north side of the island 

(see, for example, Moore and others, 1989), and so that episode 

is missing from their scenario.

Figure 4. Geologic map of Molokaÿi, from 

Stearns and Macdonald (1947). Distinguishing 

features of the geology include the distinction between the west Molokaÿi (Mauna Loa) 

volcano (pink) and the layered lava flows that 

constitute the east Molokaÿi (Wailau) volcano 

(orange and rust brown), the posteruptive volcanism (dark grey) that formed the Kalaupapa 

Peninsula against the base of the north pali 

(following the giant landslides that formed those 

sea cliffs), and the alluvium (light green) that 

forms a thin band of coastal and deltaic deposits along the south coast. 

nel about 100,000 to 200,000 years ago (Juvik and Juvik, 1998). The two 

islands were attached to each other by an isthmus on multiple occasions and 

as recently as 8,000 years ago, during periods of low sea level associated 

with global continental glaciations (Grigg and others, 2002).

Molokaÿi was built by two distinct volcanoes, west and east Molokaÿi, 

that formed about 1.9 and 1.76 million years ago, respectively (Clague 

and Dalrymple, 1989). The formation of the west Molokaÿi shield volcano 

(Mauna Loa) at 1.9 Ma (million years ago) was followed by a postshield 

phase of eruption from 1.8 to 1.75 Ma that capped the caldera (large crater 

at the summit of the volcano, fig. 3; Stearns, 1966). The east Molokaÿi 
Introduction

Kamakou (4,970 ft, 1,515 m)

(highest point on Wailau volcano) 

Mauna Loa (1,424 ft, 434 m)

(highest point on Mauna Loa volcano) 

PuØuØuao (404 ft, 123 m)

(highest point on Kalaupapa volcano)

1000

1000

4000

0

0 5 Miles

5 10 Kilometers Slope, in degrees

0 to 2.5 

2.5 to 5

5 to 10

10 to 20

20 to 40

>40 

NiØihau

KauaØi

OØahu

MolokaØi

LnaØi Maui

KahoØolawe

HawaiØi

0 50 100 Miles

0 100 200 Kilometers

<4.5

4.5Ð5.0

5.0Ð5.5

5.5Ð6.0

6.0Ð6.5

6.5Ð7.0

7.0Ð7.5

7.5Ð8.0

>8.0

<10.1

10.1Ð11.2

11.2Ð12.3

12.3Ð13.4

13.4Ð14.5

14.5Ð15.7

15.7Ð16.8

16.8Ð17.9

>17.9

Meters per

second

Miles per

hour

Mean wind speed

at 30-meter elevation

The South Molokaÿi Reef, Hawaiÿi: Origin, History, and Status

5 

Figure 5. Maps showing the topography and steepness 

of slopes on the island of Molokaÿi. The island is dominated by two volcanoes: Mauna Loa on the western 

side of the island and Wailau on the eastern side. The 

topographic map (top) shows the dominance—in both 

area and relief—of the Wailau volcano (east) over the 

Mauna Loa volcano (west). Mauna Loa has the gently 

sloping shield shape characteristic of Hawaiian volcanoes. Wailau, in contrast, has areas of steep-sided 

valleys and some of the highest seacliffs in the world on 

its northern edge. These seacliffs resulted from a giant 

landslide, among the largest on Earth, which bisected 

the Wailau volcano, leaving debris on the sea floor 

more than 150 km (90 mi) to the north. The Kalaupapa 

Peninsula is the most recent volcanic addition to the 

island, having been formed by eruptions subsequent 

to the giant landslide. The caldera of Kalaupapa (large 

crater at its summit) is now the site of a brackish lake 

nearly 240 m (800 ft) deep. On the slope map (bottom), 

steeply sloping land is shown in red and gently sloping 

land is shown in blue. The map highlights the relatively 

low slopes that characterize the west volcano and the 

Hoÿolehua saddle between the volcanoes. The steepest 

slopes occur on the pali on the north shore and in the 

deeply incised gulches on the large east volcano. The 

maps were derived from a U.S. Geological Survey 10-m 

digital elevation model (DEM).

Figure 6. Map showing mean wind speed 30 m (100 ft) 

above the surface for the region of the eight main 

Hawaiian Islands. Note the large variability in wind 

speed around the islands, in particular the greater 

speeds in the channels between the islands, caused by 

topographic steering. In this region, mean wind speeds 

over the ocean are highest during the summer tradewind period. During May through September, winds 

over the ocean exceed 20 km/h (12 mi/h) 50 percent of 

the time and are dominantly out of the northeast. Along 

the south Moloka’i coast, trade winds approach the 

shoreline from the southeast because they are steered 

around the east Molokai volcanic cone (1,500 m or 

4,970 ft high). Data from AWS Truewind (http://www.

awstruewind.com/wind-view.cfm, last accessed April 29, 

2008).

sphere. In the tropical Pacific north of the equator, the trade winds blow 

from the northeast from a region of high pressure known as the Pacific 

Anticyclone (or Pacific High). The center of this high-pressure system is 

typically located well north and east of the Hawaiian Islands. The Pacific 

Anticyclone moves with the seasons, reaching its northernmost position in 

the summer. This brings the main part of the trade winds across Hawaiÿi 

from May through September, during which time they blow 80 –95 percent 

of the time (Western Region Climate Center, 2006; fig. 6). Trade winds 

have relatively high moisture content, which yields frequent showers on 

the windward coasts and at higher elevations but leaves the leeward coasts 

(south and west) dry. This pattern is clearly depicted by a map of the 

distribution of annual rainfall on Molokaÿi (fig. 7). Only a relatively small 

amount of rainfall occurs during the summer months on the leeward coasts, 

as shown by a 12-year rainfall record from Kaunakakai (fig. 8). 

There is often a strong daily variation in wind direction and speed 

around the islands because of the increased solar heating of the islands 

relative to the ocean. By late morning or early afternoon, the sun’s rays 

typically heat the island and the overlying air mass to the point that the 

air rises, drawing in more cool air off the sea. This daily intensification of 

onshore winds during the daytime is often referred to as a “sea breeze.” 

Conversely, at night, as the island cools relative to the ocean, the overlying 

air mass cools and sinks, causing offshore flow. Thus, whereas out on the 

open ocean the winds may blow steadily at 10 km/h (6 mi/h), they might 

exceed 20 km/h (12 mi/h) off Molokaÿi’s south coast during the day and be 

less than 3 km/h (2 mi/h) at night.

Tropical storms and hurricanes are infrequent events that generally 

occur during the summer and early fall. Hurricanes are rare in Hawaiÿi 

(only two have made landfall in the past 24 years), but even if they do not 

Molokaÿi Today: Climate, Land, and the Reef make landfall in Hawaiÿi, they may pass close enough to the island chain 

Seasons, Trade Winds, and Rainfall 

Early Hawaiians recognized only two six-month seasons, “kau,” when 

the weather was warmer and the trade winds blew steadily, and “hoÿoilo,” 

when the weather was cooler and the trade winds were less consistent 

(Western Region Climate Center, 2006). Weather analysts today recognize 

these two seasons with a slight modification: the winter season, hoÿoilo, is 

seven months (October through April) and the summer season, kau, is only 

five months (May through September). Rainfall, wind, and storm patterns are 

quite different for the two seasons. The summer season is warmer, drier, and 

dominated by northeast trade winds, and widespread rainstorms are rare. In 

the winter season, temperatures are cooler, trade winds diminish, and lowpressure storm systems bring rain. Each of these two seasons is influential in 

shaping the land and transporting sediment to the ocean. 

Summer Patterns: Persistent Trade Winds and Low Rainfall

The persistent trade winds that blow in a general east-to-west direction 

are one of the most prominent circulation patterns in the Earth’s atmo-
2004 2005 2006

Rainfall, in inches

Rainfall, in centimeters

Summer

Winter

0

4

8

12

16

0

10

20

30

40

1995 1997 1999

Year

2001 2003 2005

Rainfall, in inches

Rainfall, in centimeters

0

2

4

6

8

10

12

14

16

18

0

5

10

15

20

25

30

35

40

45

Figure 7. Map showing the distribution of annual rainfall on Molokai. The highest elevations on the eastern half of 

the island receive more than 400 cm (160 in), and the amounts decrease to the south and west, reaching annual 

values of less than 40 cm (15 in) along coasts on the western part of the island. Molokai and the other main Hawaiian Islands lie within the trade-wind zone, an area of prevailing easterly surface winds caused by global atmospheric 

circulation. When these warm, moisture-laden winds meet the island’s volcanic mountains, they are forced to rise 

abruptly and cool. The moisture then condenses to form clouds and precipitates out of the atmosphere in the form of 

rain. This phenomenon is evident in this map showing the average annual rainfall: most precipitation occurs on the 

windward, northeastern portion of the island, where high elevations and steep watersheds are found. Much of the 

southern and western portions of the island receive little rainfall by comparison, because they are blocked from the 

moisture-bearing wind by the volcanic cone to the east. This accounts for the drier climate found in Kaunakakai and 

other areas on the south coast. This map was derived from the Hawaii Statewide Planning and Geographic Information System Program (Giambelluca and others, 1986).

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

6

to generate heavy rains, high winds, and great waves on the coasts (fig. 9; 

Fletcher and others, 2002).

Winter Patterns: Decreased Trade Winds and Occasional Storms

From October through April, Hawaiÿi is located to the north of the 

main trade-wind belt. During the winter the trades still blow across the 

islands, though only 50-80 percent of the time and with wind speeds that 

exceed 20 km/h (12 mi/h) only about 40 percent of the time. It is during 

the winter season that light, variable winds are most common, as are occasional very strong winds. The high Hawaiian volcanic mountains have a 

pronounced topographic effect on the winds, both in winter and in summer, 

funneling the winds through the channels between the islands and causing 

the local wind speeds to be much higher than those observed over the open 

ocean (fig. 6).

Major storms occur most frequently between October and March, 

bringing heavy rains often accompanied by strong winds. The rainfall 

record from Kaunakakai (fig. 8) shows the increase in rain in the winter 

months. Three main classes of disturbances produce major storms. Cold 

fronts sweep across the islands, bringing locally heavy showers and gusty 

winds. Low-pressure systems, known locally as Kona storms, bring widespread heavy rains, often accompanied by strong winds from the south 

or west. Hurricanes strike the islands infrequently but can result in major 

impacts to coasts and reefs when they arrive.

Drainage Patterns and the Making of an Ahupuaÿa 

The southern slopes of Molokaÿi are dominated by a series of elongate, 

shallow-to-deep gulches that are important for the transport of soil from island 

slopes to the coast; they are also are important to the cultural traditions of 

native Hawaiians. More than 80 individual gulches reaching to the south coast 

can be identified on topographic maps of Molokaÿi (fig. 10). The gulches range 

from tens to hundreds of meters in width and meters to tens of meters in depth; 

a few are more than 100 m (330 ft) deep. The longest gulches (for example, 

Kaunakakai, Kawela, Kamalö) emanate from the higher east Molokaÿi volcano. In general, gulches on the south slope of the west Molokaÿi volcano are 

shorter, narrower, and shallower. The prominence of the gulches on the southern slope of the east Molokaÿi volcano results from three factors that influence 

stream incision: the greater height of the volcano, its steeper slopes, and the 

higher rainfall amounts than on the west Molokaÿi volcano. 

The early Hawaiians used a complex system of land division and management based on the natural form and boundaries of island watersheds. 

Islands were subdivided into wedge-shaped parcels, or “moku,” extending from the mountain crest to the shoreline. Each moku was subdivided 

Figure 8. Graph showing monthly rainfall between July 1994 and January 2007 

as recorded by a rain gauge at Kaunakakai on Molokaÿi’s south coast. Note the 

distinctly different rainfall patterns between the winters (October through April) 

and the summers (May through September). Rainfall in the summer is derived 

from the trade winds and typically is less than 5 cm (2 in) per month. Winter periods, however, derive rain from north Pacific low-pressure systems and commonly 

yield 15 cm (6 in) or more per month. The inset graph for 2004 through 2006 

shows clearly this pattern of most rain occurring in winter periods (brown bars) 

and little rain occurring in most summer periods.

into smaller wedge-shaped parcels, termed “ahupuaÿa,” which followed the 

shapes of the watersheds; each had resources that were managed in a way to 

be self-sustaining (Roberts, 2001). These resources included upland trees, 

fertile lands for taro production, and coastal areas for salt and fish. Early 

Hawaiians recognized that effective sustainability of the ahupuaÿa required 

management of the resources and restrictions on the plants and fish that 

could be harvested, as well as restrictions on the seasons during which they 

could be harvested. This recognition of the connectivity and interdependence 

of the different parts of the ahupuaÿa—upland ridge, stream valley, coastal 

area, and reef—is as valid today as it was centuries ago. 

A key component of the ahupuaÿa was the construction of coastal fishponds to raise fish (Faber, 1997). Ancient fishponds are a part of the coastal 

landscape in Hawaiÿi, and particularly so on Molokaÿi’s south shore, where 

they number more than 60, the greatest number for any comparable area in 

the State. The fishponds, constructed between the 13th and 15th centuries 

60

60

80

100

200

300

400

40

Trade winds 

Contour lines of average annual precipitation, in centimeters 

0

0 5 Miles

5 10 Kilometers
Introduction

MAP AREA

Kanukuawa

Fishpond

Fishpondhiomu

Fishpond

Permanent or intermittent stream 

KamhuØehuØe

Fishpond

KinØohe

Fishpond

Keawa Nui

Fishpond

Phloa

Fishpond

ØUalapuØe

Fishpond

Kalokoiki

Fishpond

Paialoa

Fishpond

Mikiawa

Fishpond

Kamal

0 3,000 Feet

0 500 1,000 Meters

Kolo

Wai a Hewahewa

Pphaku

Kamal

Kamiloloa

Kawela

Wailua

Mapulehu

ØhiØia

Kaunal Kaunakakai

Hakina

Waihnau

Manawainui

ManeØopapa

KluapeØelua

KalamaØula

MoØomomi

KaØa

WaiØaleØia

Intermittent stream 

Permanent stream 

Major watershed boundary 

Hlawa Wailau Waikolu Pelekunu

0

0 5 Miles

5 10 Kilometers

Raymond 1983

Fernanda 1972

Nina 1957

Hurricane (wind speeds 4 mi/h)

Tropical Storm (wind speeds 39Ð73 mi/h)

Tropical Depression (wind speeds <38 mi/h)

MolokaØi

HawaiØi

KauaØi Gil 1983

Iniki 1992

Iwa 1982

Dot 1959

Gilma 1988

Irah 1963

Unnamed 1958

Eugene 1993

Kanoa 1957

Diana 1972

Dela 1957

Daniel 2000

Hiki 1950

Daniel 1982

Jova 1981

The South Molokaÿi Reef, Hawaiÿi: Origin, History, and Status

7 

(Summers, 1964; fig. 11), played an important role in early Hawaiian life. 

Today they remain important cultural landmarks, and because of their large 

number and large size (many are tens and even hundreds of acres) they exert 

an influence on coastal processes.

Fishpond walls were built with basalt and limestone boulders on the 

shallow submerged limestone surface of the reef flat in water less than 1 

m (3 ft) deep (fig. 12). The walls measured 1 to 6 m (3–20 ft) wide and 0.6 

to 2 m (2–6 ft) high, and could be hundreds of meters long. Some of the 

fishponds (referred to as “loko kuapä”) were entirely enclosed except for a 

slotted gate (“makaha”) that allowed small fish to enter but restricted them 

from leaving when they increased in size. Other fishponds (“loko ÿume iki”) 

had low walls that were submerged at high tide; walled lanes led into and 

out of the ponds. Hawaiian pond builders, recognizing the dominant east-towest wind and current patterns, particularly on the eastern part of Molokaÿi, 

placed the lanes leading into the ponds on the eastern side and the lanes leading out on the western side.

Today, these ancient fishponds influence the coastal environment in 

several ways. Fishponds are natural accumulation sites for sediment, both 

that which is discharged directly from stream mouths and silt settling from 

turbid waters on the Molokaÿi reef flat. Once deposited, the sediment tends 

to remain in the fishponds (early Hawaiians cleaned the ponds with bamboo 

rakes) and plants gain a foothold. Mangroves have migrated along the south 

coast from their originally introduced location at Päläÿau and taken hold in 

many fishponds. Once established, mangroves increase the sediment-trapping 

efficiency of the pond, which then tends to fill more quickly, creating more 

area for plant growth. A second effect of ponds is the steering of coastal currents by the massive stone walls that extend out onto the reef flat. Two recent 

studies (Presto and others, 2006; Calhoun and Field, 2008) have shown that 

Figure 9. Tracks of major tropical storms that have affected the eight main Hawaiian Islands since 1945. Tropical storms are categorized by their measured 

wind speed, based on a 1-minute average. A tropical depression has wind 

speeds that are less than 38 mi/h. A tropical storm has wind speeds from 39 

to 73 mi/h. A storm is categorized as a hurricane once the wind speeds reach 

74 mi/h. The last major hurricane to strike the islands was Hurricane Iniki in 

September 1992. Data from the Pacific Disaster Center (http://www.pdc.org/

geodata/world/stormtracks.zip, last accessed April 29, 2008).

Figure 10. Stream drainages and major watersheds on Molokaÿi. With the exception of the streams that drain the wet 

northeastern valleys, the majority of streams on Molokaÿi are fed only by surface runoff and therefore flow intermittently—

during periods of high precipitation. When heavy rainfall events occur, many of the streams are subject to rapidly increasing flows and flooding. These high flows are a hazard to people and property, and they also deliver high concentrations 

of sediment to the coral reef. This map was derived from the Hawaiÿi Statewide Planning and Geographic Information 

System Program (Giambelluca and others, 1986).

coastal flow of water and transport of sediment follow the lobate shape of the 

ponds; silt is more easily trapped in the spaces between protuberant ponds 

(see fig. 11). The residents of Molokaÿi have made several valiant efforts to 

restore selected fishponds to their original condition. Through State funding, 

the Waialua Fishpond project demonstrated, with very strenuous volunteer 

labor, the feasibility of restoring two fishponds at Waialua (State of Hawaiÿi 

Department of Lands and Natural Resources, undated).

The Remarkable Coral Reef of South Molokaÿi

The configuration of Molokaÿi and neighboring islands has played a 

large role in the development of the extensive coral reef bordering the south 

coast. The island’s elongate east-west orientation perfectly blocks the large 

north Pacific swell. To the south, Länaÿi, Maui, and Kahoÿolawe provide an 

effective shield against destructive storm waves and south Pacific swell. This 

protection, along with the low gradients of the submerged shelf and the presence of narrow coastal plains and alluvial fans that capture sediment shed 

from the hillsides, provided an optimum setting for coral growth.

The south Molokaÿi reef is not the result of coral growth in the past century or even the past millennium; nor is it the result of continuous growth. 

Accumulating evidence on its age and structure indicate that much of the 

Figure 11. Aerial photograph and interpreted map of part of the south Molokaÿi 

coast, showing the abundance of fishponds. Most of the fishponds are partly to 

completely infilled, and others have only submerged remnants present.
Wave height Mean sea level

Wave length

Peak

Trough

Water

depth

Direction of wave propogation

Wave

orbital

motions

Wave period—the time between the 

passage of two successive wave peaks or 

wave troughs past a fixed point

North Pacific Swell

Northeast

Tradewind

Waves

Southern Swell Kona Storm Waves

282¡

45¡

MolokaÕi

320¡

123¡

0¡

78¡

147¡

236¡

187¡

258¡

194¡

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

8

Forces Acting on the South Molokaÿi Reef

Waves, Tides, and Currents

More than any other single physical process, waves control not only 

the reef shape and the types of coral that grow on the reef, but even whether 

or not a reef can form. Waves on the ocean’s surface are mechanical energy 

traveling through water. The water particles in a wave move in a circular 

motion; these wave orbital motions decrease in size with depth and become 

more elliptical (“flatter”) where the wave orbits interact with the sea floor in 

shallow water. Every waveform has certain properties used to describe it, as 

shown schematically in figure 13.

The wave regime off Molokaÿi can be characterized by four end-members, as shown in figure 14—North Pacific swell, Northeast trade-wind 

waves, Southern swell, and Kona storm waves (Moberly and Chamberlain, 

1964). The North Pacific swell is generated by strong winter (October–

May) storms as they track from west to east across the North Pacific and 

has wave heights on the order of 3 – 8 m (10 –26 ft) and wave periods of 

10 –20 seconds. The Northeast trade-wind waves occur throughout the 

year but are largest from April through November, when the trade winds 

blow the strongest. These waves have wave heights on the order of 1–4 m 

(3–13 ft), but they have very short periods of 5 – 8 seconds. Southern swell, 

generated by storms in the Southern and Indian Oceans from April through 

October, typically has small waves (heights ~1–2 m; ~3 – 6 ft), but they have 

very long periods (~14 –25 seconds). Kona storm waves occur infrequently 

when local fronts or extratropical lows pass through the region. Kona storm 

waves typically have wave heights on the order of 3 – 4 m (10 –13 ft) and 

wave periods of 8 –12 seconds.

Tides are the rhythmic rise and fall of sea level driven by the interaction between gravitational and inertial forces of the Earth, Moon, and Sun. 

Although the Moon’s mass is 27 million times less than that of the Sun, 

because of its much greater proximity to the Earth (400 times closer), the 

Moon imparts twice the gravitational force on the Earth and the oceans. 

The Moon’s gravity pulls at the ocean’s water, dragging it towards the 

Moon in a bulge on the side of the Earth closest to the Moon. On the 

opposite side of the Earth (away from the Moon, where the Moon’s gravitational influence is less), the ocean’s inertial forces exceed the gravitational forces, causing the water to move away from the Earth and also form 

a bulge. Over the rest of the Earth’s surface, the two forces are relatively 

balanced. The two bulges in the ocean stay essentially aligned with the 

Moon as the Earth rotates, and as a coastal area passes under each bulge, 

it experiences a tidal high when it passes through the bulge and a tidal low 

when the shore is outside of the bulge.

The Earth completes a full revolution relative to the Moon every 

24 hours and 50 minutes (the diurnal tidal cycle), and thus the two daily 

high tides occur 12 hours and 25 minutes (the semidiurnal tidal cycle) apart. 

Different points on the Earth’s surface experience different tidal patterns 

because the Earth’s axis of rotation is inclined relative to the position of the 

Moon. The shape of the ocean basins also plays a role. Most areas have two 

Figure 14. Wave regime for the island of Molokaÿi, modified from Moberly and Chamberlain (1964). The arrows and arcs denote mean directions and range of direction, 

respectively, for a given wave type. Numbers on arrows are azimuths of the direction 

waves come from. North Pacific swell occurs throughout the year and generates some 

of the largest waves in the Hawaiian region, especially during the months from October through May. Northeast trade-wind waves occur throughout the year (50 percent 

of the time in the winter months, 90 percent of the time in the summer months), but 

are largest from April through November when the trade winds are strong and consistent. Southern swell, which also occurs throughout the year, generates waves that are 

typically largest and most frequent from April through October, when they occur up to 

50 percent of the time. Kona storm waves, which are neither frequent nor consistent, 

occur less than 10 percent of the time during the winter months. Note that the region 

off south-central Molokaÿi, where the coral reef is largest, is shadowed from most of 

these waves by the surrounding islands and by the island of Molokaÿi itself.

Figure 13. Diagram showing the parameters used to describe ocean surface waves

reef originated during earlier periods of reef growth, and it is quite likely that 

the reef was built during a number of intervals when sea level was at or near 

its present position (Engels and others, 2004, Barnhardt and others, 2005). 

In some ways the reef is robust, but in many ways it is not. The material 

presented in ensuing chapters points out two distinguishing characteristics 

of the Molokaÿi coral reef—it is remarkable and it is fragile. It is remarkable 

in its size, continuity, and high degree of coral cover. It is fragile in that it is 

a thin living coral surface built upon an old eroded coral surface. The entire 

reef is susceptible to changes induced by coastal development, runoff, fishing, and many other activities.

Figure 12. Oblique aerial photographs (taken in 1998) of fishponds along the 

south shore of Moloka’i. A, The large Känoa (foreground) and Aliÿi fishponds jut 

out onto the reef flat along the central coast just west of Kawela. Note the accretion of the coast on the updrift side (east, or closest to viewer) of the ponds and 

erosion on the downdrift side (west, or far side). B, Submerged remnants of a 

fishpond on the reef flat off Wave Crest Condominiums, near Kalaeloa. The size 

and extent of fishponds on the reef flat, such as shown here, makes them a factor 

in sediment transport on the south shore and reef flat.

A

B

Bruce Richmond, USGS
Introduction

170° W 165° 160°

Hawaiian Islands

155° 150°

15°

20°

25° N

Vector arrows denote

wind speed and direction

50 cm/s

25 cm/s

Aug 8 Aug 13 Aug 18 Aug 23 Aug 28 Sept 2 Sept 7 Sept 12

Water level, in meters

2001

-0.5

0

0.5

Neap Spring Neap Spring Neap

7 hours

5 hours

10 hours

HawaiØi

3 hours

Figure 17. Map displaying approximate travel times required for tsunami waves 

generated anywhere on the rim of the Pacific basin to reach Hawaiÿi (U.S. Geological Survey, 1997). The travel times are estimated assuming a tsunami speed 

of 1,000 km/h (600 mi/h). Therefore, the contours on the map are spaced at 

1,000 km, or one hour of travel time.

The South Molokaÿi Reef, Hawaiÿi: Origin, History, and Status

9 

high tides and two low tides each day, a pattern that is termed a semidiurnal 

tide. In Hawaiÿi, as in many places, the amplitudes of the two high tides and 

two low tides experienced each day are unequal, creating what is termed a 

mixed semidiurnal tide (fig. 15).

When the Sun and the Moon are aligned relative to the Earth, the 

combined gravitational forces of the Moon and Sun cause a very strong tide 

called a “spring” tide; spring tides happen approximately every 14 days at 

the times of new and full Moons. When the Moon and Sun are at a right 

angle with the Earth (first and last quarter lunar phases), their gravitational 

forces act separately from one another, causing a relatively weak “neap” tide 

approximately every 14 days. Both spring and neap tides are highlighted in 

figure 15. The tidal range, which is defined as the maximum water height 

at high tide minus the minimum height at low tide, is greatest during spring 

tides and least during neap tides. Although the full tidal range in Hawaiÿi is 

relatively small compared to other coastal areas on Earth, tides are important 

to the reef in terms of transporting nutrients and larvae and flushing sediment 

and other pollutants.

The currents in the northwest Pacific Ocean form a large basin-scale 

clockwise circulation, called a gyre, centered at about 28° N latitude. At the 

latitude of Hawaiÿi, the flow is roughly from east to west and increases in 

intensity southward (fig. 16). At the ocean’s surface, however, these largescale ocean currents are strongly influenced by surface winds. This causes 

complicated circulation patterns, especially in the vicinity of the Hawaiian 

Islands, where topographic steering of the surface winds occurs. Circulation 

in the sheltered lees of islands is characterized by vigorous and changing 

eddies or swirls, causing significant temporal variability in surface current 

speed and direction (Flament, 1996).

Infrequent Events: Tsunamis

A tsunami is a very long period wave generated by a large, rapid displacement of water in the ocean. The name derives from the Japanese term 

Figure 16. Diagrammatic map of average surface currents in the central Pacific 

Ocean, based on 40,000 observations of ship drift, 85,000 observations of satellite-tracked drifting buoys, and 8,000 modern current measurements (modified 

from Flament, 1996). The major current direction is westward through the main 

Hawaiian Islands, and a large gyre forms in the lee of the Big Island centered 

near 19° N, 163° W. This large-scale feature is caused by the islands standing 

in the path of the broad and steady trade winds, causing an ocean-atmosphere 

interaction that results in an island wake, similar to a river’s flow around a bridge 

piling (Xie and others, 2001). Circulation in the lee of the islands and close to 

shore is characterized by vigorous smaller scale eddies or swirls (not visible at 

the scale of the figure), which cause significant local variability in surface current 

speed and direction.

Figure 15. Forty days of tide data collected off 

Kamiloloa in 2001, displaying south Molokaÿi’s mixed 

semidiurnal tidal regime. The data set encompasses 

more than two complete spring-neap tidal cycles, with 

the peak spring tides on August 18 and September 2 

and the minimum neap tides on August 11 and August 

25. Molokaÿi’s mean tidal range is 0.43 m (1.4 ft), while 

the spring and neap tidal ranges are 0.91 m (3.0 ft) and 

0.32 m (1.1 ft), respectively. Because the rough topography of south Molokaÿi’s broad fringing reef restricts flow, 

especially over the shallow reef crest, there is generally 

a time lag between the tidal elevations on the fore reef 

and those on the reef flat, with the maximum heights on 

the reef flat occurring after those on the fore reef (Storlazzi and others, 2004).

for “great wave in harbor.” Landslides, earthquake faulting, submarine volcanic eruptions, and even meteorites have caused tsunamis throughout Earth’s 

history. Over the course of human history, earthquakes have been the most 

common tsunami-generating source (estimated 95 percent), followed by violent volcanic eruptions, such as those of the Indonesian volcanoes Tambora 

in 1815 and Krakatau in 1883 (Soloviev, 1982).

Tsunamis are often hundreds of kilometers in length (the wavelength), 

but in deep water they may be only a few centimeters to a meter or so in 

height. Tsunamis travel at several hundred kilometers per hour in the deep 

ocean basins and thus can propagate across the entire Pacific Ocean in less 

than a day’s time (fig. 17), potentially devastating coastal areas very far from 

where they originated. Hawaiÿi, located in the center of the seismically active 

Pacific Basin, is susceptible to tsunamis generated from around the basin’s 

rim. In the 20th century alone, deadly earthquake-generated tsunamis struck 

the Hawaiian Islands in 1946, 1952, 1957 (fig. 18), 1960, and 1975. Overall, 

26 tsunamis have been recorded in the Hawaiian Islands since 1812, or one 

every 7 years on average, although many of them have been inconsequential 

(Lander and Lockridge, 1989).

Because of their long wavelengths, tsunamis act like shallow-water 

waves even in the deep ocean basins. Although their heights in deep water 

are very small, they become much taller when they enter into shallow 

water because increasing drag on the sea floor slows them down. However, 

when tsunamis strike islands with narrow, steep shelves, they tend to have 
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

10

Figure 18. This series of three historical 

photographs (A-C, in chronological sequence) 

shows the arrival of a tsunami at Laie Pt. 

on the island of Oÿahu in March 1957. This 

tsunami was generated by a magnitude 8.6 

earthquake located about 3,700 km (2,300 mi) 

away in the Aleutian Islands of Alaska.

larger run-up heights than when they strike coastal areas with broad, gently 

sloping shelves where more of the tsunami’s energy can be dissipated. 

Like shallow-water waves, tsunamis are also susceptible to energy loss by 

refraction around and dispersion between islands. Although the Hawaiian Islands are, in general, susceptible to high tsunami run-up because of 

their lack of broad shelves, the south shore of Molokaÿi is protected by the 

surrounding islands of Oÿahu, Länaÿi, Kahoÿolawe, and Maui and has been 

affected less by large tsunamis than other Hawaiian locations. During the 

1946 tsunami, for example, wave run-up heights on the north, east, and 

west sides of Molokaÿi averaged 10 m (35 ft), while those along the south 

shore averaged only 2 m (7 ft). Fletcher and others (2002) rank the tsunami 

hazard along the south Molokaÿi fringing reef moderately high at both the 

eastern (Kamalö to Kümimi) and western (Hale O Lono to Päläÿau) ends 

of the island and moderately low along the central portion (Päläÿau to 

Kamalö) of the reef, where it is sheltered by Länaÿi and Kahoÿolawe.

No studies have actively investigated the impact of any of these tsunamis on coral reefs in Hawaiÿi. Observations from the December 26, 2004, 

Indonesian tsunami suggest that most of the damage to reefs throughout 

the Indian Ocean occurred in embayments, where the force of the tsunami waves was concentrated. Much of that damage was caused by debris 

dragged over the reefs by receding waters, rather than by the waves themselves (Fernando and others, 2005). 

A

B

C

Henry Helbush

Results from Recent Research on the Molokaÿi Reef

The foregoing summary of the processes that shape the Hawaiian 

Islands is a prelude to the main focus of this volume—the coral reef off south 

Molokaÿi. The chapters of this publication present our current scientific understanding of the reef itself, including its overall character and the processes that 

influence its long-term health. Chapters 1 through 9 summarize the origin of 

the reef and the character and distribution of its inhabitants, particularly corals, 

fish, and algae. Following those discussions, chapters 10 through 14 document 

key natural processes—waves, currents, runoff, and ground water—and how 

they affect the reef. Finally, chapters 15 through 21 address the topics of sediment runoff and toxic pollutants—why and how they are delivered to the reef, 

and the behavior and ultimate fate of island-derived material on the reef.

Molokaÿi is but one volcanic island in the tropics among very many 

worldwide. Its fringing coral reef is likewise only one among very many. It 

is therefore likely that many other reefs are influenced by a similar suite of 

physical processes and that many of those reefs also face threats from island 

runoff, similar to those reported here for Molokaÿi. With an eye towards those 

other reefs, as well as to Molokaÿi, the closing discussion following the chapters reviews the issues facing the Molokaÿi reef and the pathways that might 

be undertaken to ensure the long-term survival of one of the most spectacular 

reefs in the Hawaiian chain.
Evolution of the South Molokaÿi Fringing Reef

The fringing reef off south Molokaÿi, like the 

island itself, is the product of many cycles of 

growth and destruction. The reef we see today 

is a unique combination of modern growth over 

a volcanic surface shaped by ancestral patterns 

of sea-level change and reef accretion.

Aerial view of Molokaÿi reef.
Coral on the Molokaÿi fore reef.
tion that does not preserve or amplify preexisting topographic irregularities. 

The high-relief pinnacles, mounds, and spur-and-groove features of the modern reef system have developed over a broad, flat-lying platform that exhibits 

only limited local relief. In this chapter, a conceptual model is presented to 

explain the possible origin of the antecedent substrate that is buried beneath 

the Molokaÿi reef.

Seismic-Reflection Profiling and Reef Evolution

The U.S. Geological Survey used an acoustic technique known as 

“seismic-reflection profiling” to investigate the long-term development 

of the Molokaÿi reef. Seismic-reflection profiling relies on sound waves 

to image the geologic framework of the reef and adjacent areas of the sea 

floor. The seismic-reflection system consists of (1) a sound source that 

generates an outgoing pulse of sound and (2) a receiver that records the 

time required for each pulse to complete a two-way trip through the water 

column, the materials the reef, and back to the boat (fig. 2). A computer 

analyzes and plots the returning sound waves on paper as a series of dark 

and light bands that depict the thickness and geometry of materials making 

up the ocean floor (figs 3 and 4). These new data have important implications for understanding regional sea-level history and patterns of coastal 

change (Barnhardt and others, 2005). To ensure that marine mammals were 

not affected by the sound, we tested the equipment with a calibrated hydrophone and employed independent observers as required under a permit 

from the U.S. National Marine Fisheries Service. 

Antecedent Substrate

The most prominent feature observed in the seismic-reflection profiles is 

a flat-lying, highly reflective surface that lies beneath the fore reef and marginal shelf. The strong reflection, indicated by the red line in figures 3 and 4, 

was generated by a sharp contrast in physical properties between the material above and below this surface. It might represent, for example, a layer of 

reef rock overlying volcanic rock or two stacked layers of reef rock that have 

different porosity or degree of cementation, perhaps due to differences in 

age and degree of weathering. The low-relief surface dips gently seaward at 

Walter A. Barnhardt1, Eric E. Grossman2, and Bruce M. Richmond2

The development and physical characteristics of coral reefs are closely 

related to the geological evolution of the islands where they form. 

According to the theory proposed by Charles Darwin (1842), the 

shape and structure of reefs evolve over geologic time, reflecting the gradual 

subsidence of tropical oceanic islands balanced by the upward growth of corals. Over shorter time scales, reef evolution is controlled by cycles of falling 

and rising sea level (Daly, 1915) that are, in turn, driven by the expansion 

and decay of large polar ice sheets. Erosion during lowstands of sea level, 

when the reef is subaerially exposed, and renewed coral growth during highstands of sea level modulate reef development. 

The fringing reef along the southern shore of Molokaÿi (fig. 1) is a 

massive geological structure that has developed over thousands of years 

by the accumulation of corals and other organisms with calcium carbonate skeletons. Corals are colonial animals that inhabit shallow waters 

along tropical shorelines and require a stable substrate for attachment. 

Reef-building corals commonly become established on preexisting, hard 

surfaces, such as old volcanic lava flows or sedimentary rock, which can 

include ancient reefs. 

Geologists consider the antecedent substrate to be an important factor 

that determines the shape of modern reefs (Purdy, 1974; Grigg and others, 

2002). In their view, patterns of coral growth are largely controlled by the 

configuration of the foundation that lies beneath the reef. Corals need sunlight and so preferentially grow on bathymetrically higher parts of the sea 

floor. In addition, these bathymetric highs allow corals to grow undisturbed 

by sediment resuspended from the bottom, which can suffocate, abrade, 

and kill the reef-building organisms. New reef growth serves to amplify 

irregularities of elevation and slope that already exist on shallow parts of 

the sea floor. These irregularities commonly represent features that formed 

when sea level was lower, such as old stream valleys with raised banks that 

serve as preferred attachment points for corals. Off the south-central coast 

of Molokaÿi, for example, corals have encrusted the margins of a former 

stream channel (Päläÿau), which meandered across the coastal plain before 

sea level rose and submerged it (see Storlazzi and others, this vol., chap. 3). 

At a broad regional scale, however, subsurface mapping of the 

Molokaÿi fringing reef shows evidence for a different style of reef evolu1

 U.S. Geological Survey Coastal and Marine Geology, 345 Middlefield Rd., Menlo Park, CA 94025; 

current address: U.S. Geological Survey Woods Hole Science Center, 384 Woods Hole Rd., Woods Hole, 

MA 02543

2

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

Figure 1. Map of Molokaÿi showing the locations of seismic-reflection profiles 

discussed in subsequent figures.

Figure 2. Block diagram showing the basic operation of a seismic-reflection 

profiling system. Pulses of sound are generated by the system and travel different paths (dashed lines) through water, sediment, and reef rocks. Some of the 

energy is reflected by the ocean floor back to the hydrophone, which records the 

timing and strength of each pulse. Some energy also penetrates the ocean floor 

and is reflected from interfaces between materials of different properties, such 

as a layer of loose carbonate sand overlying well-cemented reef rocks. Longer 

ray paths require longer travel times, so reflections from the buried surface that 

underlies the reef arrive later than reflections that are generated closer to the 

ocean floor.

Evolution of the South Molokaÿi Fringing Reef

0 2 4 Miles

0 4 8 Kilometers

157°

21°

10' 

N 

156°50'

21°

05' 

157°10' W

MolokaØi

Section line

(see fig. 3) Section line

(see fig. 4)

Ocean surface

Ocean floor

Acoustic receiver

(hydrophone)

Acoustic source

(sparker)

Buried platform

Carbonate

sand

Coral reef

Antecedent substrate

USGS

CHAPTER 1

Antecedent Substrate Underlying the South Molokaÿi Fringing Reef and Implications for Reef Development
14

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

Figure 4. Cross section oriented perpendicular to the coast about midway between 

Kaunakakai and Kamalö. A, Seismic reflection profile. B, Line-drawing interpretation of 

profile. Compared to the profile in figure 3, 

the buried platform in this area is flatter and 

extends farther seaward (approximately 2 

km or 1.2 mi) of the reef crest.

Figure 3. Cross section across 

the narrow, steeply dipping 

shelf south of Molokaÿi. A, Seismic reflection profile, with inset 

showing modern reef front. The 

dashed line at lower left indicates 

the lowstand position of sea level 

at about –130 m (427 ft) during 

the last ice age. B, Line-drawing 

interpretation of profile. The red 

line represents a buried platform, 

which is etched into the antecedent substrate and extends approximately 1 km (0.6 mi) seaward 

of the reef crest. The dark lines 

labeled “multiples” are created 

by sound pulses that have been 

reflected more than once; they do 

not represent geologic structures.

slopes of 1°– 2° and locally crops out on the sea floor in locations offshore of 

the fore reef. It was traced in the subsurface for about 60 km (37 mi), nearly 

the entire length of the island.

Judging from its morphology and setting, the buried platform probably 

represents an ancient reef flat that formed at a previous highstand of sea level 

or, more likely, a wave-cut terrace that was eroded into the flank of Molokaÿi 

during the last ice age (fig. 5). We believe that erosion by coastal and terrestrial 

processes produced the wide, flat platform during the long period of subaerial 

exposure when sea level was lower. During that time, streams flowed across 

the exposed areas of former sea floor and locally incised deep channels. The 

platform was later recolonized by corals as glaciers melted and the ocean again 

rose to its present level. In the course of the sea-level rise, the stream channels 

were filled with sediment and preserved beneath the modern reef. Note that the 

uniformly low relief of the buried platform contrasts with the irregular shape of 

the overlying modern reef, which exhibits rugged pinnacles and mounds. 

Deep-Water Sediment

If erosion created the buried platform when sea level was lower than 

today, we need to account for the large volumes of material that have been 

removed. Where did all the eroded sediment go? Erosion by waves, currents, and streams probably carried sediment off the reef and deposited it 

offshore in deep water. Indeed, large deposits of stratified or layered sediment, locally more than 40 m (131 ft) thick, occur in deep water seaward 

of the modern reef front (fig. 3). The wedge-shaped deposits exhibit a 

relatively flat top, a sharp break in slope at a depth of about 130 m (427 ft), 

and a smooth, steep slope that dips farther offshore. Internal reflections 

dip seaward and are slightly S-shaped. The morphology and depth of the 

sediment wedge suggests that the deposit was graded to the lowstand of 

sea level that occurred during the last ice age approximately 21,000 years 

before present.

Summary

The geologic development of coral reefs is closely linked to sea-level 

change. Fluctuations in sea level are well documented in the Hawaiian Islands 

and, without doubt, have considerably influenced the development of coral 

reefs (see Grossman and others, this vol., chap. 10). The internal structure of 

the Molokaÿi reef records a complex history of reef building that was modulated by changes in global sea level and erosion by oceanic and terrestrial processes. As submarine volcanoes grew out of the sea to form the island, corals 

and other carbonate-secreting organisms colonized the shallow nearshore environment. High-resolution seismic-reflection data have enabled us to accurately 

map the morphology of the antecedent substrate on which the reef complex 

formed. The buried platform underlying the modern reef probably represents 

a wave-cut terrace that developed at some past time; a full understanding of its 

origin and age will require direct sampling with cores.

A

B

South North

0

40

80

120

160

200

0

40

80

120

160

200

Depth of

sea-level

lowstand

Traveltime, in milliseconds

Stratified, 

deep-water 

sediment Antecedent substrate

multiples Multiples

Ocean

floor

Sea level

Buried platform

Coral reef

0

40

80

120

160

0

40

80

120

160

Depth, in meters

0 500 1,000 1,500 2,000

Distance, in meters

R1

Reflective surface

(red line below)

0

40

80

120

Traveltime, in milliseconds

0

40

80

Depth, in meters

0

40

80

120

0

40

80

0 500 1,000 1,500 2,000 2,500

Distance, in meters

A

B

South North Sea level

Ocean floor

Buried platform

Antecedent substrate

Multiples

Coral reef

Section line

Section line
Antecedent Substrate Underlying the South Molokaÿi Fringing Reef and Implications for Reef Development

15 

The rugged surface of the modern fore reef bears no resemblance to 

the planar morphology of the buried platform beneath. The pinnacles and 

spur-and-groove features (see Storlazzi and others, this vol., chap. 3) have 

high local relief, but the underlying platform is smooth and gently sloping. In more seaward locations, the shelf steepens and dips down at a high 

angle, restricting the potential area for reef growth during sea-level highstands to a narrow band that is, on average, about 1.5 km (0.9 mi) wide.

Figure 5. Diagrams showing a conceptual model of four steps in the development of the Molokaÿi reef in response to sea-level changes. Thin dashed lines 

represent island profiles in the preceding panel.

Deeper parts of the sea floor off Molokaÿi have received carbonate sand 

and rubble from reworking of coral reefs and terrigenous sediment from erosion of the island’s volcanoes. Although the relative contributions are poorly 

understood, a significant volume of sediment has accumulated in a thick, 

wedge-shaped deposit. This stratified sediment was probably transported across 

the steep shelf and sequestered offshore, mostly during the last ice age as sea 

level fell and remained lower than the present shoreline for thousands of years.

Evolution of the South Molokaÿi Fringing Reef

Incised 

stream

channel

Stratified sediment

Erosion and

transport to

lowstand

shoreline

Reef

Fore reef flat

Coral reef

Buried platform

Erosional

surface

Channel-fill

deposits

Coral reefs

Antecedent

substrate

~10 m

~1000 m

Sea level

SEA-LEVEL HIGHSTAND

(prior to ice age ~125,000 years ago)

SEA-LEVEL LOWSTAND

(during ice age ~21,000 years ago)

RISING SEA LEVEL

(after ice age ~5,000 years ago)

SEA-LEVEL HIGHSTAND

(today)

STEP 1

Previous interglacial period approximately 

125,000 years ago, before the last ice age began. 

Sea level is at or near the present highstand 

elevation. Coral reefs grow in shallow waters 

along the margins of the island, similar to the 

modern setting. These reefs and/or underlying 

volcanic rocks form the antecedent substrate 

that was mapped with seismic-reflection 

profiles.

STEP 2

Last glacial maximum approximately 21,000 

years ago (late Pleistocene). Global sea level 

drops to depths of about 130 m (427 ft), 

exposing older reefs and marginal shelf areas. 

The shoreline migrates in a seaward direction. 

Coastal and terrestrial processes erode and 

rework the antecedent substrate. PlØau and 

other streams incise their channels deeply into 

the emergent landscape. Stratified sediment is 

deposited at the lowstand shoreline and in 

deeper water offshore.

STEP 3

Postglacial period approximately 5,000 years 

ago. Ice sheets are melting and sea level is 

rising. Shoreline migrates landward across 

formerly exposed areas of the island. Waves 

and currents further erode the antecedent 

substrate, creating a relatively broad, flat 

platform (red line). Incised stream channels fill 

with sediment.

STEP 4

Present conditions. The rate of sea-level rise 

has decreased, allowing coral reefs to develop 

in shallow water near the coast. Reef deposits 

bury the wave-cut platform (red line), which is 

observed in seismic-reflection profiles beneath 

most of the MolokaØi reef complex.
Molokaÿi reef west of Kaunakakai Harbor.
CHAPTER 2

Views of the South Molokaÿi Watershed-to-Reef System

Evolution of the South Molokaÿi Fringing Reef

Michael E. Field1, Joshua B. Logan1, Pat S. Chavez Jr.2, Curt D. Storlazzi1, and Susan A. Cochran1

The fringing coral reef along the south coast of the island of Molokaÿi 

is one of the most extensive fringing coral reefs in the Hawaiian 

chain. For a distance of about 40 km, from Hale O Lono on the west 

to Kamalö on the east, live coral coverage is estimated to be 60 to 90 percent (see Jokiel and others, this vol., chap. 5), higher than on all but the most 

protected reefs in Hawaiÿi. The reef is actively accreting in depths between 

about 6 and 27 m (20 and 90 ft), creating, as these views presented here 

show, spurs, grooves, knobs, pits, and pinnacles that are home to thousands 

of marine organisms. Winter waves from the north restrict active reef accretion beyond the sheltered portion of the coast, but the old reef surface in 

those areas is still populated by numerous individual coral colonies, invertebrates, and fish. For more information about the distribution of coral see 

Jokiel and others (this vol., chap. 5), and for more information about the distribution of waves along the reef see Storlazzi and others (this vol., chap. 11).

The images of the south Molokaÿi coast in this chapter were created 

using a combination of aerial photography and high-resolution bathymetric 

data. The aerial photography was collected in 1993 by the National Oceanic 

and Atmospheric Administration (NOAA). The aerial photographs were 

scanned at 800 dots per inch (dpi), yielding an approximate pixel resolution of 1 m, then georectified, color-balanced, and mosaicked by the U.S. 

Geological Survey to produce a seamless image map covering most of the 

coastal area on the south coast of Molokaÿi. The aerial image maps were 

then digitally combined with high-resolution bathymetric data to produce 

the image maps shown in this chapter. 

The high-resolution bathymetry was collected in 1999 and 2000 using 

an airborne laser-imaging system called SHOALS (Scanning Hydrographic 

Operational Airborne Lidar Survey) operated by a private company for the 

U.S. Army Corps of Engineers. The SHOALS system employs lidar (light 

detection and ranging) technology, with laser pulses in two wavelengths, to 

very accurately determine water depth at points roughly every 4 m along the 

bottom. For the Molokaÿi surveys, lidar data were collected at a 2-m spacing, 

and more than 12 million depth measurements were collected. The system is 

mounted in an aircraft, which allows it to quickly cover large areas. The vertical accuracy of the measurements is within 15 cm of the actual water depth. 

Because it is an optical technology, the maximum depth lidar can measure is 

dependent on water clarity; a general rule of thumb for the SHOALS system is 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

2

 U.S. Geological Survey Flagstaff Science Center, 2255 North Gemini Dr., Flagstaff, AZ 86001; 

current address: Northern Arizona University, Department of Geography, PO Box 15016, Flagstaff, AZ 86001

that it penetrates approximately 2.5 times the depth that a person can discriminate objects visually. During the Molokaÿi surveys the approximate maximum 

depth range recorded by the lidar system was 35 to 45 m (115 to 150 ft). 

Each of these two different data sets is extremely useful on its own. 

The aerial photography provides valuable insight into the types of reef 

habitat that are found in the shallow areas off the south side of Molokaÿi. To 

the trained eye, different reef environments can be distinguished from one 

another in the photography, and this information can be used to create maps 

depicting bottom type and habitat type (see Cochran, this vol., chap. 9). In 

addition to shallow water environments, the aerial photography also shows 

areas of land adjacent to the reef. This, in combination with elevation models, can provide important information about the natural and human-caused 

processes that may affect the reef, such as wave erosion or land clearing. 

The usefulness of aerial photography, however, is limited to the shallow water environments that can be seen with the human eye. Even in the 

relatively clear water found in some areas of the Molokaÿi reef, the aerial 

photography begins to show low contrast in water deeper than about 10 m. 

At these depths the information provided by the lidar data help to fill in the 

picture of the reef. The dense network of lidar depth measurements can be 

used to create a three-dimensional computer image of the reef. This image 

allows us to gain information about the shape of the reef in areas that would 

otherwise be too deep to “see” with conventional aerial photography. By 

overlaying an artificially colored and shaded image on the aerial photography, we can develop a more complete picture of the ridge-to-reef environment, from the land all the way to depths greater than 40 m. This combination of high-resolution bathymetric data and high-quality photographs is a 

powerful and unprecedented tool for interpreting the geologic processes that 

contribute to the reef’s development. 

In the following pages, views of the south Molokaÿi reef and the adjacent land are shown using combined aerial photography and lidar. Recent 

advances in computer-aided mapping technology allow us to display these 

data sets in both two-dimensional map view and in vertically exaggerated 

three-dimensional perspective views. Using these two types of images and 

our observations from field studies, we point out features and patterns on 

each map sheet that are significant for understanding the processes that have 

affected, and continue to affect, the evolution of the reef environment. Linkages between the shape of the reef and the processes that shape it are further 

explored in the following chapter (Storlazzi and others, this vol., chap. 3).

"Blue holes" and channels off Kamalö.
1

2

3

4

5

Kamanakai Gulch

WaihiØi Gulch

Manawainui Gulch

PlØau

Punakou

Gulc

h

Direction of view

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

18

Central Molokaÿi Reef

This image is an expanded perspective view of the central portion of the coral 

reef off south Molokaÿi. Distinctive features include its pervasive rhythmic 

spur-and-groove formations (1, 2), like those that characterize many other 

reefs around the world, and the reef channel (3) at Päläÿau, remnant from a 

stream valley that was eroded when sea level was lower than present. Note that 

the channel is surrounded by solid reef at its terminus (4). Also note the large 

solution pits (5), partially filled with sediment, east of the channel. These pits, 

also commonly referred to as “blue holes,” result from dissolution of the reef 

by flowing water, either as surface streams during times of lower sea level or as 

flow of submarine ground water under present conditions, followed by collapse 

of the overlying surface. Approximate distance across the bottom of the image 

is 7.5 km (4.7 mi).
Hakina Gulch

LØAU

POINT

Kahalephaku

Kanalukaha

KamkaØip

0 0.5 1 KILOMETER 0 0.25 0.5 MILE 

1

3

2

4

1

1

157°18' W 157°17' 157° 16'

21°

07'

N 

21°

06' 

21°

05' 

Views of the South Moloka

ÿi Watershed-to-Reef System

19 

Evolution of the South Molokaÿi

Läÿau Point

 Fringing Reef

The western end of Molokaÿi is an arid, eroded surface characterized by 

large exposures of reddish-brown soil densely to sparsely covered by 

scrub vegetation and grasses (1). The exposures result from low annual 

rainfall coupled with heavy livestock grazing throughout much of the 

20th century. The exposed soil is a source of sediment that is carried to 

coastal waters during infrequent flood events. 

The western beaches (2) of south Molokaÿi are generally wider, more 

continuous, and composed of coarser sand than the central and east-

ern beaches. Two factors contribute to this: wave energy and sediment 

sources. First, the western end of the south Molokaÿi coast receives 

higher wave energy than farther east, where waves are blocked by the 

island of Länaÿi. The southwest coast also receives refracted wave 

energy from north Pacific swell, and the high wave energy sorts the 

beach deposits and removes the smaller, lighter particles of fine sand 

and silt. The second factor has to do with the local availability of beach 

material. Much of the western shoreline of south Molokaÿi is underlain 

by older beach sands and beach rock that provide a ready source of 

material for the modern beach. The adjacent reef also provides a source 

of coarse material for the beaches.

The modern reef (3) on the westernmost part of south Molokaÿi is not 

well developed. The inner shelf consists of a dipping surface composed 

of shore-parallel bands of sand and old reef limestone; individual coral 

colonies and algae mantle the limestone. This end of the island does 

not have a well-defined reef crest, allowing waves to break closer to the 

shore (4) than they do farther east. The barely discernible dark underwa-

ter bands in the photograph are exposures of older reef limestone with 

scattered colonies of coral. Studies of the limestone deposits in this area 

show that active reef growth ceased about 5,000 years ago (see Engels 

and Fletcher, this vol., chap. 4). No lidar data was collected in this area, 

so only aerial photography is shown. 
40

5

25

Hale O Lono

Harbor

Kolo Wharf

Hlena

Hlena

Gulch

K Hinawai Gulch

One-o-Hilo

Gulch

Ono

p

ala

ni Gulch

2

3

4

2

5

Refracted 

wave 

energy

1

10

20

30

10

20

30

40

5

157°15' W 157°14' 157°13' 157°12'

21°

06' 

N 

 21°

 05' 

21°

04' 

0 0.5 1 Kilometer

0 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

20

Hale O Lono 

The southern slopes of Molokaÿi are heavily dissected by deep gullies 

(1), many of which discharge onto the coastal plain, whereas others (2) 

extend almost completely to the shoreline. Those gullies that meet the 

shoreline directly are capable of delivering sediment onto the reef during 

heavy rains and flooding. East of Hale O Lono Harbor, the reef shows 

the first indication of rhythmic spur-and-groove patterns (3), a common 

shape of coral reefs that is derived by coral growth along ridges oriented 

perpendicular to the dominant swell direction. Note that off Hale O Lono, 

the spur-and-groove morphology curves to the southwest (4), probably 

because large North Pacific swell wraps around the island and approaches 

the reef from the southwest, inducing growth along that trend. The road to 

the town of Mauna Loa can be seen in the upper left of the image (5).
Wai a Kne

Kolo

Gulch

Hikauhi 

Gu cl h

Wai a

Kne

Gulch

Naninanikukui Gulch

K

kk

Gulch

P

u

nakou Gulch

2

4 3

1

1

2

2

3

5

10

20

30

40

10

30

40

5

5

25

157°11' W 157°10' 157°09' 157°08'

21°

06'

N 

21°

05'

0 0.5 1 Kilometer

0 0.25 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Views of the South Molokaÿi Watershed-to-Reef System

21 

Evolution of the South Molokaÿi

Wai a Käne Gulch

 Fringing Reef

Red-brown soils (1) are exposed on the hill slopes along the western part 

of south Molokaÿi as a result of low rainfall and heavy cattle grazing 

(Roberts, 2001). This 1993 photo captured images of plumes of turbid 

water on the inner reef flat (2). The fore reef here is composed of wellorganized, evenly spaced spur-and-groove morphology (3) with some 

large shallow solution pits (4) and a few spurs in deeper water (5). The 

deeper spurs, although colonized on their surface, are probably remnants from earlier periods of more active reef growth, as documented by 

Engels and others (2004). 
Päläÿau

Päkanaka Fishpond

Kamanakai Gulch

Manawainui Gulch

10 20

30

40

5

10

10

30

40

5

5

30

21° 157°07' W 157°06' 157°05'

07' 

N 

21°

06' 

21°

05' 7

3

4

1

5

2

6

8

7

0 0.5 1 Kilometer

0 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

22

Päläÿau

The coastal area at Päläÿau is characterized by an extensive mud and salt 

flat (1) that formed from heavy flooding and runoff in the early 1900s 

and an extensive mangrove forest (2) that was established in 1903 to 

curb the heavy sediment runoff (D’Iorio, 2003). The elongated structure 

trending northeast-southwest (3) east of the mangroves is a shrimp farm. 

The reef at Päläÿau is dissected by a meandering channel (4) that resulted 

from stream erosion during a period of lower sea level (>12,000 years 

ago). Note that the reef is not breached at the end of the channel (5), 

possibly from coral growth since that time, or because the water flowed 

through the porous reef rather than over it. East of the channel the reef 

flat is a broad, barren surface (6) covered by thin deposits of stained 

carbonate sand, giving way to an algae-covered rock surface to the east. 

Near Päläÿau, the reef undergoes a subtle change in orientation, reflecting the overall change in island morphology. The trends of the fore reef 

on each side of the channel (7) mimic the change in orientation of the 

shoreline: to the west, the shoreline and reef trend west-southwest; to the 

east they trend east-southeast. East of the channel, the middle part of the 

reef is characterized by large pits (8), which likely result from extensive, 

long-term dissolution by fresh water flowing through the reef, followed 

by roof collapse.
N

N

K a u n a k a k a i G u l c h

PuØumaninikolo 

(162 ft)

(49 m)

KalokoØeli

Fishpond

PuØuluahine

(1,210 ft)

(369 m)

KualapuØu

(1,017 ft)

(310 m)

Kaunakakai Kamiloloa

1

2

3

3

PlØau

1

2

3

4

5

6

Direction of view Direction of view

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Views of the South Molokaÿi Watershed-to-Reef System

23 

Evolution of the South Molokaÿi Fringing Reef

Blue Holes near ÿUmipaÿa

This perspective view of the central Molokaÿi reef near 

ÿUmipaÿa shows that the inner reef is characterized by a wide 

reef flat (1) and large, sediment-infilled pits (blue holes) (2). 

The east-west alignment of the inner walls on the pits (see also 

image of Päläÿau area) possibly indicates a regional control 

related to shoreline history or reef structure. The outer reef has 

a pronounced spur-and-groove structure (3, 4), which gives 

way to isolated reef pinnacles (5) and a sediment-covered terrace (6) seaward of the reef. Approximate distance across the 

bottom of the image is 850 m (0.5 miles). 

Kaunakakai to Kamiloloa

This perspective view of central Molokaÿi shows the alignment of Kaunakakai Gulch with the reef channel (1) west and 

offshore of the wharf (2). The channel is interpreted to have 

formed by stream water flowing out of the gulch when the 

ocean was at a lower level and the shoreline was farther south 

(offshore of the present shoreline). The base of the actively 

growing reef (3) lies at a water depth of about 28 meters 

(~90 feet). Approximate distance across the bottom of the 

image is 4.5 km (2.8 miles).
Kaunakakai

Kapuiwa

Coconut Grove

ØUmipaØa

Kaluaapuhi

Fishpond

Kaunakakai

Harbor

3

4

1 5

2

6

3

10

10

30

40

10

30

40

5

5

157°04' W 157°03' 157°02' 157°01'

21°

07' 

N 

21°

05' 

21°

04' 

0 0.5 1 Kilometer

0 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Kaunakakai

The town of Kaunakakai is the largest town on Molokaÿi and sits adjacent to the largest and most important harbor on the island. In 1934 the 

Kaunakakai Wharf (1) was rebuilt to its present size and configuration. 

It is nearly 1 km long and solid, and thus it is influential in blocking the 

westward flow of water and sediment along the reef flat. For more information about sediment movement along the reef, see Ogston and others 

(this vol., chap. 20). To the west of the wharf (2), the shoreline is eroding at Kapuäiwa Coconut Grove (D’Iorio, 2003), perhaps in response 

to sediment trapping by the wharf. The reef flat on both sides of the 

wharf (3) is very broad and mantled by thin layers of muddy sediment 

and macroalgae. The reef is dissected by the Kaunakakai channel (4), a 

former stream channel remnant from a period of lower sea level (similar 

to Päläÿau channel). Note that west of the Kaunakakai channel the fore 

reef exhibits very well defined rhythmic spur-and-groove morphology 

(5), but east of the channel the morphology is subdued and flattened 

(6). This pattern reflects the active, healthy growth of coral west of the 

harbor and the limited, stunted growth that extends to the east for several 

kilometers.

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

24
One AliØi Park

Moku

Kamiloloa

KalokoØeli

Fishpond

AliØi

Fishpond

Knoa

Fishpond

Ønini Gulch

4

1

5

2

6

3

1

1

10

20

30

40

10

20

30

15

20

30

30

5

25

35

5

30

157° W 156°59' 156°58' 156°57'

21°

05' 

N 

21°

04' 

21°

03' 

Isobath interval: 5 m

0 0.5 1 Kilometer

0 0.5 Mile 

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Views of the South Molokaÿi Watershed-to-Reef System

25 

Evolution of the South Molokaÿi

Kamiloloa

 Fringing Reef

Fish ponds (1) are historical manmade features along the entire south 

coast of Molokaÿi, and they are particularly prominent along this section of the coast. Note the infilling of the fishponds with sediment (solid 

lines) and mangrove trees (dashed lines). The reef flat here is broad and 

nearly flat (2), with a thin covering of muddy sediment and macroalgae. The fore reef shows signs of erosion or non-growth to the west (3), 

gradually broadens toward the east (4), and becomes segmented by major 

re-entrants (5). Rainfall and slope gradients are higher on this part of the 

island than on the western part of south Molokaÿi, contributing to more 

extensive dissection and erosion on island slopes here (6) compared to 

farther west. 
Kawela

Kpapa

East Fork Kawela

Gulch

Waiakuilani Gulch

Pnhh

Fishpond

KawiØu

Fishpond

Kanukuawa

Fishpond Pahiomu

Fishpond

KamhuØehuØe

Fishpond

4

5

2

6

3

1

5

6

6

10

20

5

15

10

20

20

10

20

30

156°57' W 156°56' 156°55' 156°54'

21°

04' 

N 

21°

03' 

21°

02' 

Isobath interval: 5 m

0 0.5 1 Kilometer

0 0.5 Mile 

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

26

Kawela Gulch 

Kawela Gulch (1) is a major pathway of water and sediment to the south 

coast of Molokaÿi. Deposition of sediment has created a pronounced 

alluvial fan onshore (2) and a pronounced lobate delta at the coast (3). 

The inner reef flat here, as farther to the west, is a flat, sediment-covered 

surface (4) with little coral growth. In contrast, the outer reef flat has 

appreciable amounts of live coral (Rodgers and others, 2005) growing 

on low-relief ridges (5) oriented perpendicular to the coast. For more 

information on the low-relief coral ridges, see Storlazzi and others (this 

vol., chap. 3). Although several coral species appear within 400 m of the 

shoreline, coral coverage across the entire reef flat out to 700 m from 

shore is generally low (<10 percent) with few exceptions (Rodgers and 

others, 2005). The reef crest and fore reef are characterized by large 

reentrants, or embayments, and pits (blue holes) (6). 
N

N

1

3

Knoa

Fishpond

KamhuØehuØe

Fishond

Pahiomu

Fishpond

AliØi

Fishpond

Ønini Gulch

Kawela Kamal

One AliØi Park

2

2

1

1

Direction of view Direction of view

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Views of the South Molokaÿi Watershed-to-Reef System

27 

Evolution of the South Molokaÿi Fringing Reef

ÿOnini Gulch to Kawela Gulch

This perspective view of the reef off One Aliÿi Park in east-central Molokaÿi shows the changing shape of the reef. The spurand-groove structure, dominant farther west (see, for example, 

the image of Kaunakakai) is not well defined in this area; 

individual spurs (1) are discontinuous and of various orientations, indicating that coral growth has not been uniform in this 

area. The large reentrant (2) in the reef may have been the site of 

stream outflow during a period of lower sea level. Approximate 

distance across the bottom of the image is 2 km (1.25 miles).

East toward Kamalö

This perspective view shows the change in coastal orientation that occurs at Kamalö. It is this change that leads to higher 

wave energy east of Kamalö, where the coast is more exposed 

to winter swell. The near part of the view (1) shows the reef flat 

that is sheltered from the high wave energy that reaches the reef 

east of Kamalö. Wave energy plays a major role in the development of the reef. East of Kamalö (2), reef growth is limited. In 

contrast, the fore reef west of Kamalö (3) is well developed and 

densely covered with live coral to depths of 28 meters (~90 feet), 

and small isolated coral colonies may be found deeper. Note the 

prominent blue holes, which are deeper and more abundant on 

the eastern end of the reef, perhaps because of the higher levels 

of precipitation and water flow. Approximate distance across the 

bottom of the image is 1.3 km (0.8 miles).
Kamal

Kamal

Gulch

Kamal

Harbor

KamhuØehuØe

Fishpond

KinØohe

Fishpond

Keawa Nui

Fishpond

Kalaeloa

Harbor

Phloa

Fishpond

ØUalapuØe

Fishpond

Keawa

Nui

Gulch

KaØpah

u

Rid

ge

Kamakou 

(4,970 ft, 1,515 m)

(3.3 km north)

Kamakou 

(4,970 ft, 1,515 m)

(3.3 km north)

5

2

6

3

4

1

7

8

10

20

30

25

10

20

30

5

5

5

30

20

no data

no data

5

156°53' W 156°52' 156°51' 156°50'

21°

04' 

N 

21°

03' 

21°

02' 

Isobath interval: 5 m

0 0.5 1 Kilometer

0 0.5 Mile 

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

28

Kamalö 

Kamalö is the point where the south Molokaÿi coast changes orientation 

and consequently becomes more exposed to wave energy eastward; the 

area also essentially marks the east end of the accreting reef of south 

Molokaÿi. Whereas the reef to the west of Kamalö appears to be actively 

accreting, the reef surface to the east consists largely of ancestral reef 

deposits intermittently colonized by live coral. Kamalö Gulch (1) and 

its adjacent gulches are the most prominent gulches along the south 

shore. Running steeply down from the island’s highest peak (Kamakou, 

1,515 m or 4,970 ft), the gulches are wide and deeply entrenched. The 

large amounts of sediment that have been delivered through this system 

over time have built a large alluvial fan and coastal plain complex (2). 

The present mouth of Kamalö Gulch (3) discharges runoff directly into 

deep water through one of two paleochannels (4) incised by stream flow 

across the reef when sea level was lower than it is today. It is likely that 

the fine-grained sediment plumes formed during flood discharge from 

the gulches are driven west along the shoreline by prevailing winds. 

To the east of the paleochannels lie three pronounced lineations (5, 6, 

and 7) of deep holes in the reef flat; all three likely mark the location of 

freshwater flow (surface as well as ground-water discharge) across and 

through the reef, dissolving older reef limestone in the process. At the 

landward end of the string of elongated blue holes (6) is an artificially 

created pit (8) formed by dredging in the 1970s to create a harbor. 
Waialua

Mapulehu Gulch

Kamakou (4,970 ft)

(1,515 m)

OlokuØi (4,602 ft)

(1,403 m)

2

1 N

2

1

3 3

Kamal Kalaeloa

Harbor

Kpeke

Fishpond

Keawa Nui

KinØohe Fishpond

Fishpond

N

2

1

2

Direction of view Direction of view

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Views of the South Molokaÿi Watershed-to-Reef System

29 

Evolution of the South Molokaÿi Fringing Reef

Kalaeloa

This perspective view of the Kamalö region and farther east 

shows that the reef is relatively smooth with numerous large 

blue holes (1). The blue holes are typically 6 to 24 meters (20 to 

80 feet) deep and, as already pointed out, likely formed through 

dissolution of the preexisting reef limestone. The irregular reef 

surface (2) marks the transition between the actively accreting 

reef to the west and the sparsely covered limestone surface to 

the east. Approximate distance across the bottom of the image is 

2.7 km (1.7 miles).

Honomuni

This perspective view shows that the eastern reef of south 

Molokaÿi is characterized by a smooth surface (1) with numerous channels and blue holes. Many of the large gullies dissecting 

the steep land surface align with channels incised into the reef 

(2). There is a complete absence of spur-and-groove formation 

by coral accretion in this area because of the high wave stresses. 

Live coral colonies are relatively sparse on the limestone surface 

(3). Approximate distance across the bottom of the image is 

2.8 km (1.7 miles).
PkoØo

L

ahia

m

anu Gulch

KaluaØaha

Kuakea

Gulch

PunaØula

Gulch Mapulehu

Stream

ØAhaØino Gulch

KaØopea Hina

Fishpond

NØaupala

Fishpond

Kpeke

Fishpond

PkoØo

Harbor

ØUalapuØe

Fishpond

No high resolution aerial imagery available

10

20

30

10

20

30

5

5

5

6

2

4

1

3

156°49' W 156°48' 156°47'

21°

05' 

N 

21°

04' 

21°

03' 

0 0.5 1 Kilometer

0 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

30

Pükoÿo Harbor 

The western shoreline of the Pükoÿo Harbor image displays good examples of remnant fishponds on the inner reef flat; the stone walls have been 

knocked down, but the submerged boulders outlining the ponds are still 

evident in the aerial images (1). Pükoÿo Harbor itself (2) is the remains 

of a fishpond that has been altered by dredging and emplacement of 

fill material. The original rectangular shape of the fishpond was altered 

in the early 1970s in an attempt to develop a marina. However, after 

completion of the dredging and accompanying reshaping of the coast in 

this area, the project was abandoned. The existing fishpond (3) on the 

reef flat and its upslope gulch are aligned with a channel through the reef 

(4), another example of freshwater dissolution and erosion of older reef 

limestone, as is the channel at (5). The fore reef along this part of the 

island is smooth (6) and lacks the distinctive spur-and-groove pattern that 

characterizes the reef west of Kamalö (as seen, for example, on the fore 

reef on the Kaunakakai image).
Kmimi

Waialua

Waialua

Gulch

Honouli Wai Stream

Phakuloa

Cape Hlawa

(3 km)

End of high resolution bathymetry data

No high resolution aerial imagery available

2

3

1

1

5

15

25

35

5

21° 156°46' W 156°45' 156°44' 156°43'

08' 

N 

21°

07' 

21°

06' 

0 0.5 1 Kilometer

0 0.5 Mile 

Isobath interval: 5 m

No lidar data

10

30

50

70

90

110

130

0

10

20

30

40

Depth below mean

lower low water

Feet Meters

Kahinaphaku

Fishpond

Kümimi Point

This map includes the east end of the south Molokaÿi fringing reef. 

The island slope is carved by numerous gullies (1), but there is little 

net accretion of sediment in the form of deltas or coastal-plain deposits 

along the shoreline. This reflects both the soil stability provided by lush 

vegetation as well as high-energy conditions along the shoreline that 

quickly move any sediment transported to the coast by stream runoff. 

The reef flat, nearly 2 km wide in the center of the island (see the image 

of Päläÿau), is barely 0.5 km wide on this image (2) and disappears 

altogether at Kümimi (also known as 20-Mile Beach). This section of 

the coast is subject to repeated high wave energy (Storlazzi and others, 2005), which inhibits growth of the common reef-building coral 

species. The surface of the fore reef here (3) is colonized by scattered 

coral colonies, dominantly Pocillopora meandrina (rose coral), which is 

commonly found in high wave settings in Hawaiÿi. Active reef accretion 

is not occurring here; the fore reef in this area is simply the remnant of a 

former reef-building period.

Views of the South Molokaÿi Watershed-to-Reef System

31 

Evolution of the South Molokaÿi Fringing Reef
View from Kawela ridgetop across the Kalohi Channel.
Evolution of the South Molokaÿi Fringing Reef

21°

05' 

 N

156°50' W 156°45' 

0 2 4 Kilometers

0 1 2 Miles

PkoØo

Kmimi

Cloud cover

Map area

Mud

Ridge-and-runnel

Breaking waves

Sand

Mud and algae

Ridge-and-runnel

Reef crest

Fore reef

A

B

CHAPTER 3

Shape of the South Molokaÿi Fringing Reef: Trends and Variation

Curt D. Storlazzi1, Joshua B. Logan1, and Michael E. Field1

Ever since sailors started mapping the approaches to natural harbors 

at low latitudes, the complex morphology of coral reefs has been 

evident. Whereas the shallowest portions of the reef was the most 

important feature for sailors to map because of its obvious hazard to navigation, it was clear on fringing reefs that the majority of the reef lay seaward, extending into water depths of more than 40 m (132 ft). Although 

most sandy coasts are relatively straight and gently sloping, carbonate reefs 

display a wide variety of shapes, including extremely sharp boundaries and 

nearly vertical faces. The goal of this chapter is to document the morphology 

of the reef off south Molokaÿi, Hawaii, and from this infer what processes 

may have led to its present form.

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

General Reef Morphology

The fringing coral reef lying off the south shore of Molokaÿi extends 

more than 40 km (24 mi) alongshore. The reef pinches out westward roughly 

7 km (4 mi) from the west end of the island and eastward about 20 km 

(12mi) from the east end of the island (fig. 1). From the shoreline out to 

deeper water, the reef can be generalized into three main zones: the reef flat, 

the reef crest, and the fore reef.

The reef flat is a low-relief, roughly horizontal surface with relatively 

sparse coral cover (see Cochran, this vol., chap. 9, for all benthic habitat 

types and coverage percentages) that extends from the shoreline to the reef 

crest (fig. 2). The reef flat is on average approximately 1 km (0.6 mi) wide 

and reaches a maximum width of more than 1.5 km (0.9 mi) offshore from 

the saddle between the two basaltic shield volcanoes that make up the island. 

The relatively flat inner half of the reef flat transitions into alternating shoreperpendicular low ridges of coral and sand-filled troughs on the outer half of 

the reef flat. The ridges are on the order of 0.1–1.0 m (0.3–3.3 ft) high and 

are covered by a variable percentage of live coral, with greater percentages 

of live coral farther offshore. The intervening depressed channels are mostly 

filled with sand and some coral gravel; these features have been termed 

“ridge-and-runnel” structures by Blanchon and Jones (1997).

The water over the reef flat is generally shallow, typically less than 

2m (7 ft) deep. In certain locations, however, there are “blue holes” on the 

reef flat that have nearly vertical walls and extend to depths of 25 m (83 ft) 

or greater (fig. 3). Many elongated, shore-normal blue holes correlate to 

onshore drainages (fig. 4) and are probably related to discharge from the 

adjacent streams or to submarine ground-water discharge through the reef 

(see Grossman and others, this vol., chap. 13). Blue holes may have been 

formed by stream incision or by freshwater-induced dissolution during 

periods of lower sea level and have subsequently partially filled in by new 

growth during the recent sea-level highstand. It is not clear, however, which 

process or processes are responsible for the shapes we see today. 

The reef crest, where most deep-water waves break, is well defined 

along most of the reef off southern Molokaÿi. Lying at a water depth of 1–2m 

(3 –7 ft), it is characterized by irregular morphology, scattered corals, rubble 

deposits, and numerous patches of sand. The reef crest is dominated by 

encrusting coralline algae and robust lobate and encrusting corals.

Offshore of the reef crest, from depths of 5 to 30 m (17–99 ft), lies the 

fore reef, the zone of highest coral cover. It is generally characterized by 

Figure 1. LANDSAT visual satellite image of the east end of the south Molokaÿi 

reef, showing the tapering of the reef to a narrow band at Kümimi.

Figure 2. Aerial photographs of the inner reef flat (characterized 

by mud) and the outer reef flat (characterized by ridge-and-runnel 

topography) off south-central Molokaÿi. Note the houses for scale in 

both images. A, Oblique photograph of the reef flat just east of One 

Aliÿi Park. B, Vertical aerial photograph of the reef flat off Kamiloloa. 
Waialua

Mapulehu Gulch

Kamakou (4,970 ft; 1,515 m)

OlokuØi (4,602 ft; 1,403 m)

Kpeke Fishpond

N

No lidar

data

Feet Meters

0

Depth,

 below sea level

40

80

120

10

0

20

30

40

1

15

Depth,

in meters

0 200 Meters

0 1500 Feet

0 500 Meters

0 1500 Feet

A

B

C

a

a

b

b

c c

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

34

shore-perpendicular ridges of coral (coral “spurs”) that are separated by 

shore-perpendicular patches of sand (sand “grooves”). These features are 

common along wave-exposed reefs and have been termed “spur-and-groove” 

(SAG) structures (fig.5). Coral cover on the fore reef is typically between 

70 and 90 percent (see Jokiel and others, this vol., chap. 5, for descriptions 

of all corals). The reef typically ends at a depth of about 27 m (89 ft), with 

hard corals giving way to sand flats dominated by patches of calcareous 

Halimeda algae, similar to what was observed by Dollar (1982) on other reefs 

in Hawaiÿi.

Large-Scale Morphology (1–10 km)

In order to understand the large-scale shape of the reef, 36 shore-perpendicular depth transects were made extending from the shoreline out to the 

base of the present reef. The transects were spaced at roughly 1.5-km intervals and were constructed from SHOALS bathymetric data (see Logan and 

others, this vol., chap. 2, for a further discussion of SHOALS). The locations 

of the profiles and selected depth profiles of the reef along these transects 

are shown in figure 6. For reference, the projected slope of the land surface 

beneath the reef is extended through the reef profiles to provide some insight 

to the likely cross-sectional area of the reef complex. No shallow reef flat is 

present at the ends of the island, as shown in profiles 2 and 36; field observations show a very thin veneer of live coral overlying volcanic substrate. 

Along the central portion of the fringing reef between Päläÿau and Kamalö 

(profiles 13 through 27), a well-defined reef flat extends more than 1,200m 

(3,960 ft) offshore. The reef along this stretch of coast has a steeper slope 

from just above 20 m (66 ft) to the 30 m (99 ft) isobath, which corresponds 

to the toe of the present-day living reef observed during scuba transects, 

similar to that observed by Dollar (1982). Intermediate between these areas 

of no clearly defined reef crest and well-defined reef flat are transitional 

regions, as shown by profiles 6 and 33. These transitional sections display a 

reef flat on average 500 m (1,650 ft) wide and a less steeply sloping fore reef 

that lacks the abrupt increase in slope between the 20 m and 30 m isobaths 

seen in profiles 13 through 27.

Figure 3. Examples of 

“blue holes” in the reef flat 

of south Molokaÿi. A, Aerial 

photograph showing the 

dark blue color of the water 

in a blue hole off Kakahaiÿa. 

B, SHOALS bathymetry of 

the same area. C, Aerial 

photograph of a series of 

blue holes in the reef flat 

just east of Kamalö.

Figure 4. Perspective image of the land (grey scale) and reef (color-coded bathymetry) near Waialua, showing the correlation between terrestrial stream drainages and paleostream channels incised in the fringing reef off south Molokaÿi. 

Depths from SHOALS data. Approximate distance across the bottom of the 

image is 2.8 km (1.75 mi).

Figure 5. Underwater photograph of a spurand-groove (SAG) structure off ÿUmipaÿa. The 

height between the sand-floored groove and 

the top of the coral spurs in the photograph 

is about 1.5 m (4.9 ft); the width of the groove 

is about 2 m (6.6 ft). Three of the main reefbuilding corals can be identified in the photograph: (a) Porites compressa (vertical fingers); 

(b) Montipora capitata (horizontal plates); and 

(c) Pocillopora meandrina (cauliflower). Wavegenerated symmetrical ripples cover the sand 

bed. View is seaward.

Tom Reiss, USGS
Evolution of the South Molokaÿi Fringing Reef

0 4 8 KILOMETERS

0 2 4 MILES

0

Depth, in meters

20

Projected slope of volcanic cone

0

20

0

20

2

Profile

2 6 13 17

23 27

33

36

6

13

17

23

27

33

36

0

20

0

20

0

20

0

20

0

20

0 500 1,000 1,500

Distance offshore, in meters

2,000

Reef profile

Spur width, in meters

Longitude, in degrees

60

80

100

120

140

0

1

2

3

Spur height, in meters

5

10

15

20

Depth, in meters

0 4 8 Kilometers

0 2 4 Miles

157°

21°

10' 

N 

156°50'

21°

 05' 

157°10'

157°10' 157° 156°50'

157°20' W

0

40

Depth,

in meters

Shape of the the South Molokaÿi Fringing Reef: Trends and Variation

35 

Figure 6. Depth profiles of the reef off south Molokaÿi, showing the variation 

in development of the reef complex alongshore. The dashed lines are projections of the slope of the volcanic cone through the reef profiles to provide 

some insight to the likely cross-sectional area of the reef complex. Note that 

the reef is almost nonexistent at the ends of the island (profiles 2 and 36) 

and extends more than 1,500 m (4,950 ft) offshore of the central portion of 

the island (profiles 13 though 27).

Figure 7. Plots showing the variation in spur-and-groove (SAG) morphology 

along the reef off south Molokaÿi. Each of the data points represents the mean 

spur height or width calculated over a 500-m (1,650 ft) segment of the line. 

Note that along the 5-m (16 ft) isobath mean spur height is higher near the 

ends of the island, while at greater depths (15 – 20 m, 50 –66 ft) the heights are 

greater near the middle of the island. Along the 5-m isobath, and to a lesser extent along the 10-m (33 ft) isobath, mean spur width increases to the east end 

of the island. At greater water depths (15 –20 m), spur width tends to be greatest near the middle of the island. The curves are running averages to display 

how spur height and width tend to vary alongshore. 

Small-Scale Morphology (10–100 m)

As part of an experiment to better understand small-scale reef morphology, almost 4,800 spur-and-groove (SAG) structures were mapped from the 

SHOALS bathymetric data along the 5-m (17 ft), 10-m (33 ft), 15-m (50ft) 

and 20-m (66 ft) isobaths, with roughly 1,000 SAG structures measured per 

isobath. See Storlazzi and others (2003) for more information on how the 

calculations were made. Along the 5-m isobath (fig. 7), mean SAG heights 

were typically less than 1 m, with the lowest heights occurring off the 

middle of the island. Spurs at the 5-m isobath typically have much lower 

total coral cover than those along deeper portions of the reef. At this depth, 

the coral species that are most commonly observed are Pocillopora meandrina and the more robust Porites lobata. The alongshore trend at the 10-m 

isobath shows the lowest mean SAG heights at the east end of the island 

and a general increase in height from east to west. The trends in mean SAG 

height along the 15-m and 20-m isobaths displayed the greatest heights 

(>3m) at the middle of the island. The spurs observed between the 10-m 

and 20-m isobaths typically have much higher percentages of live coral than 

those at 5 m. The deeper spurs tend to have slightly lower coral coverage at 

both the east and west ends of the island, where exposure to large waves is 

greater and they are covered by more robust species of coral (P. lobata and 

P. meandrina), than along the sheltered central portion of the reef where 

Porites compressa is dominant at similar depths.

Two trends were observed in the calculated mean values of SAG 

width. Mean width along both the 5-m and 10-m isobaths increases 

towards the east end of the island, with the widths being slightly greater on 

average along the 10-m isobath than along the 5-m isobath (fig. 8). Mean 

SAG widths along the 15-m and 20-m isobaths exhibit a different trend 

from that observed along the two shallower isobaths, with the largest width 

values generally at the center of the island.

On average, the shallowest (0.7 m) and least variable mean SAG 

heights are along the 5-m isobath, whereas the largest (1.6 m) and most 

variable mean heights are along the 15-m isobath (fig. 8A). Similarly, the 

widest (104 m) and most variable mean SAG widths are observed along the 

15-m isobath, whereas the 5-m isobath displays on average both the narrowest (71 m) and least variable widths (fig. 8B). The variation in the ratio 

of spur height to width with depth displays a pattern similar to spur height 

and width, increasing from the 5-m isobath to the 15-m isobath, then 

decreasing slightly from there to the 20-m isobath (fig. 8C). This trend 

suggests that spurs tend to become more peaked (greater height to width 

ratio) with increasing water depth. Overall, field observations showed that 

the grooves tend to be wider relative to the spur width in shallow water 

depths (especially in more exposed areas) than in deeper water, where the 

grooves tend to be narrower.

Numerous short, narrow SAG structures occur at shallow water 

depths (fig. 9), whereas in mid water depths fewer, wider, and taller spurs 

dominate, followed by a reversal back to more numerous, short and narrow spurs lower on the fore reef along the 20-m isobath. The numerous 

short, narrow SAG structures either truncate or merge together to form the 

fewer, taller SAG structures along the 15-m isobath. The tall, broad SAG 

structures deeper on the fore reef typically terminate and are replaced by 

numerous, low mounds or new spurs, often separated by areas of fine sand 

from the shallower, continuous SAG structures. This variation in SAG 

morphology causes a change in slope, with a relatively low slope between 

the 5-m and 15-m isobaths giving way to a steeper slope between the 15-m 

and 20-m isobaths, forming a morphology similar to the shelf-edge “buttresses” observed by Blanchon and Jones (1997) on the Caribbean Island 

of Grand Cayman.
Meters Feet

Depth below sea level

100

60

20 10

20

30

0

0

600 Feet

200 Meters

200

100

300

Isobath interval 1 meter

30

30

20

20

20

30

20

10

10

10

30

Map area

0 0 1 50 70 90 110 130 0.005 0.015 0.025

Spur height, in meters

Depth below sea level, in meters

Spur width, in meters Height/width ratio

2 3

5

10

15

20

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

36

Figure 9. Color-coded image of the SHOALS bathymetry offshore of Kaunakakai 

overlaid with 1-m (3 ft) contours to highlight spur-and-groove (SAG) morphology. 

The white lines denote interpreted spur traces. Note that most of the short, narrow spurs at shallow water depths merge to form taller, broader spurs around the 

15-m (50 ft) isobath. Most of the spurs at the 15-m isobath bifurcate with depth or 

simply truncate and are replaced by smaller, more closely spaced SAG structures. Some of the structures around the 30-m (99 ft) isobath are coral-covered 

mounds isolated by sand from the SAG structures at shallower depths. Also note 

the transition from the broad reef flat at the top of the image at a depth of 2–3 m 

(7–10 ft) to the steeper sloping fore reef between 10 m (33 ft) and 30 m (99 ft), 

below which is the more gently sloping insular shelf.

Implications Of Observations To Understanding 

Recent Reef Development

The shape of the reef is a direct result of coral growth rates and patterns, 

and thus the shape of the reef conveys important information about the external factors that affect reef development. The primary physical constraints on 

rocky corals along a single exposed reef are food, light availability (Stoddart, 1969; Dustan, 1979), and the forces imposed by ocean waves. Light 

has been shown to decrease rapidly (logarithmically) with increasing water 

depth. Light is critical to coral growth because the symbiotic algae (zooxanthellae) housed in the corals require light for photosynthesis. Water motion 

induced by surface waves dominate the near-bed flow field on the inner 

portion (<35m depth) of the south Molokaÿi shelf, where significant reef 

has developed. Wave-induced motions, and therefore wave-induced forces 

on corals and the sea floor, decrease rapidly (exponentially) with increasing 

depth. These wave-induced motions can, when very high, physically break 

corals, and at slightly lower levels they may inhibit settlement by juvenile 

coral polyps.

The width of the reef and the relative heights and widths of SAG structures provide an indication of where reef development may be greatest through 

the combination of high light availability for photosynthesis but relatively 

low wave-induced forces for the species that compose the reef community off 

south Molokaÿi. Maximum calculated mean spur height and width, and thus of 

SAG development, were measured along the 15-m isobath. In shallower depths 

(5–10 m), the high light availability appears to be offset by the high waveinduced forces, keeping SAG structures small, whereas along the 20-m isobath 

and deeper, decreased light availability inhibits high relative growth despite the 

low wave-induced forces (Dollar, 1982).

The reef flat is widest along the central portion of the south Molokaÿi 

shore and narrowest at the east and west ends of the island, even though 

light penetration at these shallow depths is relatively uniform. This further 

suggests that wave-induced forces, which vary spatially (see Storlazzi and 

others, this vol., chap. 11), might be a primary control on large-scale reef 

morphology along south Molokaÿi.

Figure 8. Variation in mean spur-and-groove (SAG) morphology with water 

depth. A, Mean ±1 standard deviation of spur height. B, Mean ±1 standard deviation of spur width. C, Mean ±1 standard deviation of the ratio of mean spur height 

to mean spur width. Note how all three parameters’ means and their variability 

are smallest at the 5-m isobath, largest at the 15-m isobath, and then slightly 

less at the 20-m isobath than at the 15-m isobath. These variations are likely a 

tradeoff between light available for photosynthesis and wave energy.

A B C
Evolution of the South Molokaÿi Fringing Reef

10

20

30

10

CORE 2

CORE 1 

CORE 3

CORE 4

CORE 8

CORE 9

CORE 10

CORE 5

CORE 6

CORE 7

Hale o Lono

Harbor

Kanalukaha

Hakina 

Gulch

21°

06'

 N

21°

05'

157°16' W 157°15' 

60

80

10

30

70

157°10' W

Wai a Kne

Hikauhi Gulch

Wai a

Kne

Gulch

Nani Nanikukui Gulch

CORE E

CORE F

CORE G

CORE H

CORE A

CORE D

CORE B

CORE C

CORE K

CORE L

CORE I

CORE J 

CORE M

CORE N

Hale O Lono Wai a Kne Gulch

0 0.25

0 0.5

0.5 Mile

1 Kilometer

Hale O Lono

Wai a Kne Gulch

CHAPTER 4

Molokaÿi—Two Different Types of Reef Growth in the Past 8,000 Years

Mary S. Engels1 and Charles H. Fletcher2

Reef growth in the main Hawaiian Islands during the current warm 

period (known as the “Holocene Epoch,” the past 10,000 years of 

Earth history) has been described in detail by only a few scientists, 

mostly conducting studies on the island of Oÿahu. These studies provide 

information that has been used to formulate a model of reef growth around 

the island of Oÿahu (Grossman and Fletcher, 2004, Fletcher and others, 

2001). The model proposes three major components of the Oÿahu reef system, including (1) fossil reefs dating from the Pleistocene, a geologic epoch 

covering the period 1.8 million to 10,000 years ago, (2) reefs formed during 

the Holocene, and (3) a thin veneer of modern coral and coralline algae capping older reef structures or growing directly on volcanic rock. 

According to the model developed for Oÿahu, Holocene reef growth 

was restricted to the narrow zone between the intersection of the sea floor 

with the wave base (the deep limit of water motion caused by waves) and the 

outer edge of the Pleistocene reef shelf ringing the island. Heavy wave action 

tends to limit reef development because of high shear stress on the sea floor, 

direct concussion, and sediment abrasion. At places around Oÿahu where reef 

growth has not been limited by waves (for instance, deeper portions of the 

reef in Kailua Bay and in protected Hanauma Bay), a sea-level rise of 1 to 2m 

between about 4,000 and 2,000 years ago provided new environments for reef 

growth and sediment production (Fletcher and Jones, 1996; Grossman and 

Fletcher, 1998). Fletcher and others (2001) suggested that when sea level fell 

to its present position over the past few thousand years, reef development in 

shallow waters diminished and the centers of reef growth shifted from reef flat, 

reef crest, and upper fore reef to the lower fore reef and deeper waters seaward 

of the shallow island shelf. Their model assumes that the majority of Holocene 

growth on Oÿahu is limited to protected areas of low wave energy or to depths 

below wave base but still within the zone of sufficient sunlight to permit coral 

growth. One example of such a location is where ancestral stream channels cut 

through a fringing reef during periods of lower sea level. 

The locations and histories of Holocene-age reefs around the other 

Hawaiian Islands are largely unknown, although the reports of Rooney and 

others (2004) and Engels and others (2004) shed light on reef growth on 

Molokaÿi and Kauaÿi. Rooney and others (2004) used cores of Holocene reef 

from Kauaÿi, Oÿahu, and Molokaÿi in support of a hypothesis that reef accre1 University of Hawaiÿi at Mänoa, Department of Geology and Geophysics, Honolulu, HI 96822; 

current address: Sea Education Association, P.O. Box 6, Woods Hole, MA 02543.

2 University of Hawaiÿi at Mänoa, Department of Geology and Geophysics, Honolulu, HI 96822.

tion during middle and late Holocene time was suppressed, and in places 

ended, by the onset of heavy North Pacific swell resulting from amplification 

of El Niño events about 5,000 years ago. Engels and others (2004) provided 

a model of reef accretion on the south shore of Molokaÿi that compared reef 

in the area of Hale O Lono wharf to reef at Wai a Käne Gulch. That study is 

important for comparison to the Oÿahu system and thereby improves the overall understanding of how reefs developed in Hawaiÿi during the Holocene.

Site Overview 

The two reef sites chosen for this study are located at Hale O Lono wharf 

and Wai a Käne Gulch, both on the southwest shore of Molokaÿi (fig. 1). 

Figure 1. Satellite images of Hale O 

Lono and Wai a Käne areas showing 

location of drill cores. Hale O Lono, 

located 5 km east of Läÿau Point, is affected by large North Pacific swell waves 

during the winter months. Wai a Käne is 

located approximately 15 km east from 

Läÿau Point and is mostly protected from 

North Pacific swell. 

These sites were selected for their close proximity but contrasting physical 

characteristics. At Hale O Lono, 5 km east of Läÿau Point, sparse fields of live 

coral (mostly Pocillopora meandrina, Porites lobata, and species of the genus 

Montipora) grow on a much older (early Holocene) fossil limestone reef. The 

site does not have a typical reef structure (see Cochran, this vol., chap. 9). 

Rather, the sea floor is a fossil reef slope, interspersed with sand fields, that 

dips gradually away from shore. The amount of fine-grained terrestrial sediment in the water at Hale O Lono is typically small because of the arid nature 

of the adjacent uplands. But large amounts of runoff associated with episodic 

(seasonal) rainfall events may cause temporary turbidity lasting several days. 

Additionally, resuspension of carbonate sand can be significant during large 

swell events (see Bothner and others, this vol., chap. 19).
Living coral cover, in percent 

0 100

Bed shear stress, in Newtons per square meter

P. lobata, massive

P. lobata, encrusting

Montipora sp., encrusting

P. meandrina, stout branching

P. compressa, finger branching

< 5

5Ð10

> 10

Water depth, in meters

Low-energy community

Mid-energy community

High-energy community

Coralline algae cover, in percent High

High

Low

Low

Coralline algae cover, in percent

MolokaØi coral/algal assemblage model

0.6

0.5

0.4

0.3

0.2

Figure 4. Model of the modern coral/algal assemblages on the south Molokaÿi 

reef. This model (from Engels and others, 2004) relates wave-induced near-bed 

shear stress with percent living coral cover, dominant coral species, and coral 

morphologies. How these elements interact allow us to predict where different 

types of bottom cover (such as coral, coralline algae, sand, and rock) might be 

found on the reef. 

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

38

By contrast, Wai a Käne, 10 km to the east of Hale O Lono, has a 

typical fringing reef structure with a reef flat, reef crest, and fore reef. The 

reef flat is buried under 2 cm to more than 1 m of fine terrestrial mud that 

comes from upland runoff (see Field and others, this vol., chap. 17). Live 

coral cover is low near the reef crest, perhaps suppressed by the high sediment content of runoff. Coral cover increases with depth across the fore 

reef, which is dominated by spur and groove features (see Field and others, 

this vol., chap. 2, and Storlazzi and others, this vol., chap. 3). The dominant sediment on the fore reef is carbonate sand. 

Because of their close proximity (approximately 10 km), these sites 

experience a similar wave climate, with one notable exception—the reef 

at Wai a Käne is protected from damaging North Pacific swell. During 

the summer, both sites are affected by Southern Ocean swell, Kona storm 

waves, and trade wind waves (see Storlazzi and others, this vol., chap. 11). 

During the winter months, trade wind waves diminish and North Pacific 

swell refracts around Läÿau Point, affecting Hale O Lono more than Wai 

a Käne. The Hale O Lono site also commonly experiences strong tidal 

currents, being adjacent to the Kaiwi Channel that separates Oÿahu and 

Molokaÿi.

Methodology 

For this study we surveyed the modern reef environment of western 

Molokaÿi to develop an understanding of the community of corals and 

coralline algae under varying water depths and wave stresses. We use this 

understanding to interpret fossilized communities in drill-core samples at the 

two sites and reconstruct past wave stresses and water depths earlier in the 

Holocene. Details of the study are available in Engels and others (2004). The 

results of our survey are specific to the reef system on the western portion 

of the Molokaÿi south shore. Our survey summary relates dominant members of the coral community, their growth forms and percentage cover, and 

percent coralline algae cover to water depth and wave stress (the force on 

the reef system due to wave action; see Storlazzi and others, this vol., chap. 

11). By comparing fossil coral samples in drill cores to the present-day coral 

and algae distribution, we were able to determine coral communities, growth 

forms, and percentage coral cover of the fringing reef during past periods of 

the Holocene Epoch. These relations provide a method for inferring probable past water-depth and wave-stress environments that governed past reef 

development. From this we determined a likely geologic history for the reef 

at the two sites.

We surveyed the reef surface at each site using a technique called the 

line intercept technique. This technique consisted of placing a 10-m line on 

the reef surface and recording every change in the composition and character of the sea floor under the tape (fig. 2). Twenty-seven transects at Hale O 

Lono and 19 transects from Wai a Käne were collected in a range of depths. 

More than 50 different kinds of information were collected, including depth, 

bottom type, coral types, algal types, growth morphologies, invertebrates, 

and others.

chap. 11) to understand the relations to the modern, depth-related coral/algal 

assemblage (fig. 4). In general, coral cover increases with depth and with 

decreasing near-bed shear stress, while coralline algae cover increases with 

increasing near-bed shear stress. By comparing core samples (figs. 5, 6) to 

the modern coral/algal community, we were able to assess differences in reef 

accretion at Hale O Lono and Wai a Käne.

The reef at Hale O Lono is early to middle Holocene in age, ranging 

from at least ~8,100 years old to ~4,800 years old, whereas the reef at Wai a 

Käne is less than 1,000 years old. During early Holocene time, reef growth 

at Hale O Lono appears to have occurred in discrete events, with the centers 

of growth shifting landward under rising sea level. The rising sea level was 

responsible for opening new habitat for colonization, but it appears to have 

risen at variable rates throughout early to middle Holocene time. Indeed, 

there is a major change in the type of coral seen in some of the cores at Hale 

O Lono at an age between ~8,100 and ~7,900 years. One hypothesis explaining this change is that a rapid acceleration of sea-level rise led to sudden 

deepening of the sea floor, forcing the coral/algal community to shift from a 

shallow-water type to a deep-water type.

By about 4,800 years ago the reef stopped growing altogether at Hale 

O Lono, leaving the present fossil sea floor. Although there are live coral 

Figure 2. Information on the modern 

coral community structure is collected 

using a line intercept technique. In 

this technique a 10-m line is placed 

along the bottom, as shown in this 

image, and every change in bottom 

type is recorded along the length of 

the transect. The length of line viewed 

in this image is approximately 2.5 m.

Figure 3. Divers working to reinsert 

the hydraulic drilling apparatus into 

a bore hole on the reef surface. For 

every section of core drilled, the 

full bit must be removed and then 

replaced with a new extension rod 

attached to an empty core barrel.

In order to collect reef cores, we employed a diver-operated hydraulic 

drill system (fig. 3). We collected 10 cores from Hale O Lono and 14 cores 

from Wai a Käne. The cores averaged 2–5 m in length. All cores were split 

for archiving, then subsampled for 14C radiometric dating (a method of 

determining age in years by measuring the concentration of the radioactive 

isotope carbon-14), X-ray diffraction analysis (a method of determining mineralogy and factors that might influence the measurement of sample age), 

and thin-section analysis (a rock preparation in which a sample is ground 

to ~0.03-mm thickness and mounted on a microscope slide to determine its 

structure and cementation history).

Results and Discussion 

Results from the reef surface surveys were combined with modeled 

wave-induced near-bed shear stresses (see Storlazzi and others, this vol., 
Evolution of the South Molokaÿi Fringing Reef

1A

1B

3A

4A

5A

5B

8A

8B

2A

2B

0 0

5,000 cal yr BP

6,000 cal yr BP

8,000 cal yr BP

1

2

3 4

5 6 7

8 9 10

5.5

8.5

14.0 14.0

17.7 17.7 17.7

21.0 21.0 21.0

Modern mean sea level

Hale O Lono core logs

Shore

Core

length

(m)

Core label 1

0

1

Site depth (m) 5.5 Encrusting cor-algal bindstone

Mixed skeletal rubble

Unconsolidated floatstone

Branching coral framestone/bafflestone

Massive coral framestone

No recovery

SITE INFORMATION FACIES

1A 4,812

1B 5,427

2A 5,648

2B 6,492

3A 6,719

4A 6,667

5A 7,914

5B 7,961

8A 8,100

8B 8,147

DATES

(in cal yr BP)

0

1

3

0

1

0

1

0

1

3

0

1

0 0 0

A3

A2

A1

C2

C1

D1

D6

D2

D3

D4

D5

I2

I1

I3

K3

K1

K2

N2

N1

A1

C1

C2

D1

D2

D3

D4

D5

D6

A2

A3

K1

K2

K3

N1

N2

I1

I2

I3

900 cal yr BP

600 cal yr BP

300 cal yr BP

Modern

A

4.0 Encrusting cor-algal bindstone

Mixed skeletal rubble

Unconsolidated floatstone

Branching coral framestone/bafflestone

Massive coral framestone

No recovery

FACIES

A1 Modern

A2 303

A3 500

C1 Modern

C2 282

D1 Modern

D2 Modern

D3 60

D4 277

D5 725

D6 910

 I1 Modern

 I2 140

 I3 398

K1 Modern

K2 Modern

K3 300

N1 83

N2 502

DATES

(in cal yr BP)

4.0

5.5 5.5

6.1

9.1 9.1 9.1 9.1

9.8

10.7

12.8

14.3

18.3

19.8

A

B C

D

E F G H

I

J

K

L

M

N

Wai a Kne Core Logs

0

0 0 0 0 0

0

0

0

0

0

0

1

2

3

0

1

0

1

2

3

4

5

1

2

1

2

3

1

2

3

1

2

1

2

3

3

1

1

2

Shore Modern mean sea level

Core

length

(m)

Core label

0

1

Site depth (m)

SITE INFORMATION

Molokaÿi—Two Different Types of Reef Growth in the Past 8,000 Years

39 

Figure 6. Drill core logs and radiometric ages from Wai a Käne. Limestones were classified according to the Embry and Klovan (1971) 

modification of the Dunham (1962) scheme. This site is a young (< 1,000 yr) reef structure, extending seaward and upward from the deeper 

environment of the fore reef. Contours indicate regions of similar age and highlight the seaward stepping or prograding nature of this reef. The 

dominant rock type in these cores is unconsolidated floatstone (a type of rock in which chunks of coral rubble are surrounded by, or “float,” in a 

sediment matrix). The unconsolidated floatstones represent accumulated reef debris that has not yet had time to solidify into a reefal limestone. 

Figure 5. Drill core logs and radiometric dates from Hale O Lono. Limestones were classified according to the Embry and Klovan (1971) modification of the Dunham (1962) scheme. The age of 

the reef at the sea floor increases with distance offshore. No modern ages are associated with this 

site, suggesting little or no reef growth over the past ~4,800 yr. Contours indicate regions of similar age and highlight the shoreward stepping nature of reef growth in this area. The dominant rock 

type associated with cores 5 –7 is branching coral framestone, a type of rock composed of a coral 

framework filled with carbonate sediments and then solidified. According to the modern coral/algal 

assemblage model, branching corals grow in low-energy, deep-water (>10 m) settings. Thus, their 

fossilized remains indicate a similar environment at the time of coral deposition ~7,900 yr ago. All 

the remaining cores at this site have coral types that are more closely associated with high-energy, 

shallow-water depositional environments. This distinct change in depositional environments suggests a possible rapid increase in the rate of sea-level rise ~7,900 yr ago, followed by a slowing of 

sea-level rise and a return to high-energy, shallow-water depositional environments. 
colonies growing on this old surface today, the reef as a whole does not 

appear to be experiencing net accretion. The fact that there has been little 

net reef growth since ~4,800 years ago is likely a result of a combination 

of two things, (1) lack of new habitat, related to falling sea level following 

the “Kapapa” sea-level highstand in the late mid-Holocene (Grossman and 

Fletcher, 1998), further compounded by island uplift due to lithospheric 

flexure (Watts and ten Brink, 1989) and (2) an increase in El Niño Southern 

Oscillation (ENSO) intensity and accompanying increase in extreme levels 

of North Pacific winter swell, as suggested by Rodbell and others (1999), 

Moy and others (2002) and Rooney and others (2004). Corals may survive 

the typical north swell occurring during Hawaiian winter, but immense 

waves, often arriving during particularly intense El Niño years, may clear the 

sea floor of all accumulated growth since the last huge event and reset the 

sea-floor clock back to its 4,800-year age.

The young reef at Wai a Käne, by contrast, has experienced significant 

accretion over the past 1,000 yr. The reef-flat area is limited by shallow water, 

sediment accumulation, and poor water quality related to the adjacent watershed. The reef crest is characterized by crusts of coralline algae and some 

encrusting coral growth. Seaward of the reef crest in less than 5-m water depth, 

reef development is limited by low accommodation space in the water column 

because of falling sea levels over the late Holocene and slow present rise of 

sea level. In deeper waters, greater than 5-m depth, thick stands of Porites

compressa and other types of corals grow vertically. The reef is extending 

vertically and laterally (seaward) in this region because it is not limited by high 

wave energy and related shear stresses. Without rapidly rising sea level, the 

focus of reef growth has shifted to the deeper environments along the seaward 

margins of the reef, where there is room to grow outward and upward. At 

depths exceeding 15 to 20 m, reef growth is limited by substrate (sandy vs. 

hard bottom) suitable for colonization.

Age discrepancies between Hale O Lono and Wai a Käne make direct 

comparison of growth histories impossible, but analysis of the sites together 

yields a picture of shrinking habitat suitable for reef accretion because of 

increased wave energy since the middle Holocene and lack of accommodation space because of falling or stable sea levels in the late Holocene. These 

combined conditions have caused the shift of net reef accretion from the 

area of the reef flat to the deeper waters of the lower fore-reef slope at Wai 

a Käne, whereas at Hale O Lono high wave stress continues to suppress all 

reef growth. The Pleistocene foundation of older reef so prevalent on Oÿahu 

is not found on Molokaÿi, although additional drilling is necessary before 

this possibility can be confidently eliminated.

Consistent with the Oÿahu model, the major control on reef growth, at 

least on the western south shore of Molokaÿi, is wave-induced shear stresses 

from large interannual North Pacific winter swell, probably originating with 

amplification of the El Niño process in the middle Holocene. This factor 

would be accentuated and brought to greater influence by the falling sea 

level of the late Holocene and the relatively stable (slowly rising) sea level 

of the modern era. The setting of our studied reefs along a uniquely straight 

coastline provides the ability to examine the influence of high wave stress 

along a clearly definable gradient, where wave stress increases with proximity to Läÿau Point. The fact that such different accretion histories were 

observed within a space of only 6 km proximity suggests how dynamic and 

amazing reef growth strategies can be within a relatively small area of generally similar environmental setting but differing wave influences.

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

40
Fish and coral of the Molokaÿi reef.

Character of the South Molokaÿi Fringing Reef

Arguably one of the most extensive fringing coral reefs in the Hawaiian chain, 

the reef off south Molokaÿi is host to a 

rich and diverse population of coral, 

fish, and algae. The distribution of these 

organisms, and changes in their abundance, are of foremost importance for 

monitoring the health of the reef.
Sunset from Molokaÿi Shores condominium complex.
Character of the South Molokaÿi

CHAPTER 5

 Fringing Reef

Reef Corals and the Coral Reefs of South Molokaÿi 

Paul L. Jokiel1, Eric K. Brown2, Kuÿulei S. Rodgers1, and William R. Smith1

Rich and diverse marine benthic habitats exist along much of the 

south Molokaÿi coral reef, as demonstrated by maps of the area 

(see Cochran, this vol., chap. 9 for a complete discussion of benthic 

habitats). Habitats that support corals generally contain a characteristic 

assemblage of coral species that can be referred to as a “coral community.” 

Composition of each coral community is controlled by numerous environmental factors that influence recruitment, growth, and mortality of each 

component species. Different species show different adaptations to various 

environmental conditions, which enable some species to thrive in certain 

habitats but not others. 

Corals that develop framework reefs in tropical waters are long-lived 

sedentary organisms that have a very narrow range of tolerance to temperature, salinity, water clarity, sedimentation, and nutrients. Therefore, 

corals can serve as excellent indicator species. Data on coral abundance 

and species composition allow evaluation of prevailing environmental 

1

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744

2

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744;

current address: Kalaupapa National Historical Park, P.O. Box 2222, Kalaupapa, HI 96742

conditions and changes over time. Corals are the keystone species that 

form the reef framework through secretion of limestone skeletons. The 

abundance and diversity of reef fish are directly related to coral community development (see Friedlander and Rodgers, this vol., chap. 7 for a 

complete discussion of reef fish on Molokaÿi). The ecological importance, 

economic value, and environmental sensitivity of corals make them ideal 

subjects for monitoring and assessment efforts. Brown and others (this vol., 

chap. 6) discuss results from several years of monitoring of coral growth 

off south Molokaÿi.

The Corals of South Molokaÿi

Distinguishing among the hundreds of species of reef corals in the 

world is based largely on microscopic features of the coral skeleton as 

well as on skeletal growth form. However, the number of coral species in 

Hawaiÿi is small, and they are relatively easy to distinguish from each other 

in the field (Maragos, 1977; Fenner, 2005). There are five dominant species 

Figure 1. Encrusting and platelike 

growth forms of Montipora capitata

dominate vertical faces and overhangs on the fore reef. Photograph 

taken at depth of 5 m (15 ft) at 

Päläÿau, Molokaÿi.

Figure 2. Platelike and encrusting 

growth forms of Montipora capitata

(on the right) and encrusting Montipora patula (darker colony on the left) at 

a depth of 3 m (10 ft) on the shallow 

fore reef at Päläÿau, Molokaÿi.

Figure 3. Purple-polyp form of Montipora patula (top of photograph) compared to brown variety at a depth of 3 

m (10 ft) on the fore reef at Päläÿau, 

Molokaÿi. The purple-polyp form of M. 

patula should not be confused with 

Montipora flabellata, which has a 

deep blue coloration throughout the 

colony.

Figure 4. The three dominant species 

of Montipora compete for space on 

shallow reef off Kamalö, Molokaÿi. The 

blue species Montipora flabellata (upper center) struggles to overgrow the 

brown encrusting species Montipora 

patula (lower center). The branching 

growth form of Montipora capitata surrounds the two encrusting species in 

this photograph.

Figure 5. Montipora flabellata assumes a very unusual branched form 

on the reef flat of south Molokaÿi 

though symbiosis with a small shrimp 

known as Gammaropsis sp. (see 

text).

Figure 6. Montipora studeri (= Montipora incrassata) at depth of 3 m (10 

ft) at Kamalö, Molokaÿi, with a small 

plate of Montipora capitata showing in 

the upper right.

of reef-forming corals on the reefs of south Molokaÿi. They are the “rice 

corals” Montipora capitata (previously known as Montipora verrucosa), 

and Montipora patula; the “finger coral” Porites compressa; the massive 

“lobe coral” Porites lobata; and the “cauliflower coral” Pocillopora meandrina. South Molokaÿi reefs are also inhabited by a number of other species 

of reef-forming and soft corals that range in abundance from rare to common. These species increase biodiversity, have great aesthetic value, and are 

of basic scientific interest, but they account for only a small percentage of 

the total living coral cover.

Abundant Species of Hard Corals

Montipora capitata (figs. 1, 2, 4) is clearly the most widespread species off south Molokaÿi. It is highly variable in growth form and occurs in 

all coral habitats, including isolated rocks on the muddy inner reef flat. Its 

range extends to a depth of more than 30 m (100 ft) on the deepest portions of the offshore fore reef, where it takes on a frail and highly branched 

growth form reminiscent of a finely branched Anacropora. Vaughan (1907) 
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

44

reported that M. capitata (which he called Montipora verrucosa) had the 

most variable growth form of any coral species. In shaded environments 

M. capitata forms large platelike structures; in calm-water environments it 

assumes a branched morphology; and in areas of high wave action it takes 

on an encrusting growth form. 

Montipora patula (figs. 2, 3, 4) is an encrusting and plate-forming 

species that does not have a branching growth form. M. patula is somewhat 

more restricted in range than M. capitata, being more common in environments of higher wave energy and less abundant in environments subject to 

high sedimentation rates.

Montipora flabellata (figs. 4, 5) is a blue encrusting species commonly found in shallow, high-wave-energy environments (Jokiel and others, 2004). This species does not normally have a branching growth form 

and, by itself, cannot compete with branched species. However, unique 

branched colonies of M. flabellata occur on the outer reef flat at south 

Molokaÿi that have not been reported from other locations in Hawaiÿi. 

The unusual branched morphology is the result of a mutualistic symbiosis between the coral and a small shrimp. The shrimp is an unidentified 

species of gammarid amphipod in the genus Gammaropsis. The shrimp 

forms a tube, and as the tube grows outward, the coral skeleton grows and 

encrusts the tube. Each branch of the coral thus has a hollow tube running 

up the center and an opening at the tip that provides a home for this small 

crustacean. In this way the coral is able to form a branched colony while 

the amphipod is given the protection of the coral skeleton (fig. 5). To the 

casual observer the opening at the end of the branch resembles a terminal 

polyp, and this growth form could easily be mistaken for a member of the 

genus Acropora.

Montipora studeri (fig. 6) is uncommon and generally found in wavedisturbed habitats, but isolated colonies have been observed as deep as 

10 m (30 ft) along the south Molokaÿi coastline. Fenner (2005) recently 

revised this species name to Montipora incrassata Dana, 1846.

Porites compressa is a conspicuous coral and occurs in abundance at 

depths of 10–30 m (30–90 ft) on the fore reef (fig. 7) and can be found in 

most environments at lower abundance. The species is also successful in 

many shallow calm-water environments, such as along the rim of “blue 

holes” and in the shallows off Kamalö Pier. P. compressa is endemic to the 

Hawaiian Archipelago (Veron, 2000) and is an important framework coral. 

Its highly branched growth form provides cryptic habitats and cover for 

many species. 

Porites compressa is most abundant in deeper areas on platforms and 

on the tops of ridges where wave energy is low (Dollar, 1982). This species can outgrow most other corals for space through rapid vertical growth 

of branches. Several encrusting species of coral are associated with this 

community, but they are generally forced to live in the spaces between the 

P. compressa branches because of the ability of this branched species to 

outgrow the encrusting forms. Occasionally some encrusting species can 

gain the upper hand. For example, Montipora patula can be seen overgrowing a branch of P. compressa (fig. 8) in some environments. Other encrusting species, such as Pavona varians (fig. 9), can be found deeper within 

the branches of P. compressa as well as in cryptic habitats, such as under 

ledges or in crevices. Pavona maldivensis (fig. 10) also follows this pattern, but is rare off south Molokaÿi.

The species in the genus Porites with more massive growth habits are 

ultimately classified by microscopic structures of the skeleton, but they can 

be distinguished from each other in the field off south Molokaÿi by certain 

diagnostic features. Porites lobata is the most common species of Porites. 

This species is generally yellow in color and forms massive to encrusting 

colonies. P. lobata is a long-lived species able to withstand periodic storm 

waves that remove the fast-growing and more delicate branching species 

that would otherwise overgrow the colonies (Grigg, 1983). Some shallow environments are dominated by colonies of P. lobata measuring 5 to 

10 m in diameter (fig. 11). The polyps of this species are located in deep 

Figure 7. The deep fore reef is dominated by the “finger coral” Porites compressa along much of the south Molokaÿi 

coastline that is not affected by large 

waves. Photograph at 10-m (30-ft) depth 

off Päläÿau, Molokaÿi.

Figure 8. The “finger coral” Porites 

compressa (top) must grow vertically at 

a rate faster than the encrusting competitor Montipora capitata in order to survive. 

Photograph taken at depth of 10 m (30ft) 

at Kamalö, Molokaÿi.

Figure 9. Pavona varians is an encrusting coral often found in shaded environments.

Figure 10. The encrusting coral 

Pavona maldivensis grows in crevices 

and under ledges.

Figure 11. The shallow fore reef is 

commonly dominated by large yellow 

colonies of Porites lobata. Photograph 

taken at depth of 2 m (6 ft) at Kamalö, 

Molokaÿi.

Figure 12. Porites evermanni at 

a depth of 2 m (6 ft) at Kamalö, 

Molokaÿi.

Figure 13. Porites brighami on the 

shallow reef flat near Päläÿau, Molokaÿi. 

Note the deep calices in comparison to 

Porites evermanni (fig. 12).

Figure 14. The shallow fore reef and 

reef crest are commonly dominated by 

the rose or cauliflower coral Pocillopora 

meandrina. Photograph taken at depth 

of 2 m (6 ft) off Kamiloloa, Molokaÿi.

Figure 15. Pocillopora eydouxi

growing at a depth of 10 m (30 ft) off 

Päläÿau, Molokaÿi.
Reef Corals and the Coral Reefs of South Molokaÿi

45 

Character of the South Molokaÿi Fringing Reef

calices (Veron, 2000). By comparison, the closely related species Porites

evermanni has very shallow calices (fig. 12). At Molokaÿi, P. evermanni

is uncommon and is light blue-grey in color. It can form large massive 

colonies but is generally restricted to water depths of less than 1–2 m 

(3–6ft) off south Molokaÿi. This coral has been classified as Porites lutea

by Fenner (2005) because the two cannot be distinguished from each other, 

and P. lutea is the older name. A third species, Porites brighami, has very 

deep calices (fig. 13) and forms very small colonies that seldom exceed 

5cm (2 in) in diameter. P. brighami is relatively rare off south Molokaÿi 

and only occurs in shallow, high-wave environments, such as the reef crest 

or outer reef flat. 

Pocillopora meandrina has a dense, hard skeleton that resists breakage (Rodgers and others, 2003) and thus is able to dominate shallow areas 

characterized by high wave energy where other species cannot persist 

(fig. 14). Pocillopora ligulata is a species closely related to P. meandrina, 

which occurs in environments of high wave energy. It has the general 

appearance of P. meandrina but with very thin and ragged branches. Pocillopora eydouxi is similar to P. meandrina but with thick large branches 

(fig. 15). This species occurs from the reef flat through the shallow fore 

reef at depths of 1–10 m (3–30 ft), generally as isolated large colonies. 

P. eydouxi is very vulnerable to extreme storm surge because of its large size.

Less Common Species of Hard Corals

Pavona duerdeni (fig. 16) has a very dense skeleton that can form very 

massive colonies with ridgelike structures. In the Hawaiian Islands this 

species is more common on south-facing shores. This species is typically 

found on the upper fore reef, reef crest, and seaward edge of the reef flat.

The encrusting coral Lepastrea purpurea (fig. 17), which has a hard, 

dense skeleton that resists abrasion, is commonly found on the reef crest 

and outer reef flat.

Porites rus is uncommon on most Hawaiian reefs. Scattered colonies occur on the reef crest on south Molokaÿi in the Kaunakakai region 

(fig. 18). An unusual situation occurs on the outer shallow reef flat and 

shallow fore reef on the west side of the Kaunakakai channel. This area is 

dominated by an extremely large population of P. rus colonies that covers 

an acre or more. 

The solitary, free-living coral Fungia scutaria (fig. 19) is an easily 

recognized hard coral. This species is typically found tucked into crevices 

in the reef. Although not common in Hawaiÿi, F. scutaria can be locally 

abundant.

Cyphastrea ocellina (fig. 20) can be found on reef flats, the reef crest, 

and the shallow fore reef. Porites lichen is another species with a similar 

distribution (fig. 21) with small greenish cryptic colonies less than 1 in 

(2.5cm) in diameter. The calices of this species are often aligned to form 

rows. P. lichen reported from Hawaiÿi is not the same species as P. lichen

found in other areas of the Pacific, where they commonly grow to a much 

larger size. Therefore, this Hawaiian species has recently been reclassified 

as Porites cf. bernardi Vaughan, 1907 by Fenner (2005).

Pocillopora damicornis is a finely branched species typically found in 

shallow, calm-water environments. Off south Molokaÿi, P. damicornis is 

found on the reef flat, despite muddy sediment and competition from macroalgae (fig. 22). Psammocora stellata (fig. 23) occupies the same reef-flat 

habitats as Pocillopora damicornis, but it is less abundant. This species 

occurs primarily on the shallow reef flat and is often found growing on 

rocks and rubble very close to shore. 

Soft Corals and Octocorals

Several “soft corals” and an octocoral are found on the reefs of south 

Molokaÿi. These do not contribute to the formation of the reef, because 

they do not form a calcium carbonate (limestone) skeleton. Although these 

species are uncommon off south Molokaÿi, they are interesting organisms that contribute much to the biodiversity and beauty of the reefs. The 

soft coral Sinularia molokensis is extremely rare (fig. 24). Only a few 

colonies are known to exist in the world, and all of these are recorded 

Figure 16. Pavona duerdeni colony 

growing in shallow fore reef area at 

Päläÿau, Molokaÿi.

Figure 17. The encrusting reef coral 

Leptastrea purpurea at Päläÿau, 

Molokaÿi.

Figure 18. Porites rus on the reef 

crest at Kamiloloa, Molokaÿi.

Figure 19. “Mushroom coral” Fungia scutaria on outer reef flat at 

Kamiloloa, Molokaÿi. 

Figure 20. Cyphastrea ocellina on 

outer reef flat at Kamiloloa, Molokaÿi.

Figure 21. Porites lichen (= Porites 

cf. bernardi) on outer reef flat off 

Kamiloloa, Molokaÿi. 

Figure 22. Pocillopora damicornis on 

inner reef flat off Kamiloloa, Molokaÿi.

Figure 23. Psammocora stellata on shallow inner reef flat at Kamiloloa, Molokaÿi.

Figure 24. Sinularia molokensis is a 

very rare soft coral and known only from 

a few colonies that occur on Molokaÿi.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

46

Major Environmental Factors Affecting Coral 

Community Structure off South Molokaÿi

Physical Factors

Currents and Waves

Wave energy is an important factor controlling development of coral 

reef communities (Storlazzi and others, this vol., chap. 11; Grigg, 1983). 

Anomalous and extreme wave events such as hurricanes can pulverize 

corals, but such catastrophic events are very infrequent, so the prevailing 

wave regime is of greater importance over the long run. Many fast-growing 

species do not recruit, grow, or survive in high-wave-energy regimes. Corals 

adapted to strong water motion cannot compete effectively in environments 

of weak water motion. Each major reef-building species modifies its morphology to fit environmental conditions; thus the prevailing wave regime 

shapes the structure of the coral community. For example, the growth form 

of the coral Montipora capitata is delicate and branching in calm water, but 

becomes encrusting in areas with strong water motion. 

In extreme situations, wave impact retards or prevents establishment of 

reef coral communities. The large North Pacific swell that strikes the north 

coast of Molokaÿi during the winter can move boulders the size of trucks 

(fig. 28). Such waves can fragment and abrade all except the most waveresistant coral species, which persist on isolated outcrops and in protected, 

recessed areas where wave force and abrasion are minimized. The elongate 

east-to-west orientation of Molokaÿi shields much of the reef along the 

south coast from the North Pacific swell. North Pacific swell wraps around 

the extreme east and west ends of the island, influencing reef development 

in these areas (see Storlazzi and others, this vol., chap. 11 for more information on waves). The southeast coast from Kamalö to Hälawa is severely 

affected by the large northeast trade wind swell passing through the Pailolo 

Channel. The south coast of Molokaÿi falls into the “wave shadow” of surrounding islands, but it is potentially vulnerable to the infrequent storm 

waves associated with hurricanes or “Kona” storms from the south. Hurricane Iwa caused extensive damage to the Kaunakakai wharf during late 

November 1982 and presumably damaged the surrounding fragile reefs 

(Capt. Joe Reich, oral commun., 2001). A paradox is that such extreme 

events can also remove years of accumulated sediment and thereby rejuvenate coral growth on a reef. 

Light 

Light is absolutely necessary for reef growth. Corals are plant-animal 

symbioses that require sunlight as a primary source of energy. Therefore, 

their lower depth distribution is set by light penetration. Waters above the 

ocean reefs off south Molokaÿi are of sufficient clarity to permit development of rich Porites compressa communities to depths of 30 m (100 ft). At 

greater depths, the substrate is unsuitable for corals, and the reef gives way 

to a sand terrace dominated by the alga Halimeda (see Smith and others, this 

vol., chap. 8 for more discussion of alga distribution). High turbidity inshore 

because of fine sediment from land runoff dramatically reduces light penetration on the shallow reef flats. The impact and processes that influence 

sediment runoff and turbidity are discussed in more detail in chapters 17 

through 21. 

Temperature 

Temperature is a primary physical factor governing reef coral distribution (Wells, 1957). The optimum growth temperature for Hawaiian reef 

corals is 27°C (81°F), although they can tolerate prolonged temperature of 

from 20°C to 29°C (68 –84°F) and short exposure to temperatures of 18°C 

to 31°C (64 –88°F). Under calm sunny periods during the summer months, 

temperature on the inner reef flat can approach the upper lethal temperature 

for these reef corals.

Salinity

Reef corals have been described as having very narrow salinity tolerance (Wells, 1957), but corals and coral reefs are known to occur under 

natural conditions at salinities ranging from 25 parts per thousand (ppt) to 

42 ppt (Coles and Jokiel, 1992). Reef kills caused by low salinity associated with flood events have been reported throughout the world, as well 

as from Hawaiÿi (Jokiel and others, 1993). Salinity on the reefs of south 

Molokaÿi is generally close to that of the open ocean, which is on the order 

of 34 –35 ppt (Ogston and others, this vol., chap. 20). Hawaiian corals can 

tolerate salinity of 15 ppt for several days without dying. This represents 

a 50-percent dilution of seawater with freshwater. During extreme floods, 

such low salinity can occur near stream mouths on the reef flat, but the 

freshwater is rapidly mixed with seawater that is pushed over the reef crest 

by wave action. Freshwater is much less dense than seawater, so it often 

Figure 25. Palythoa tuberculosa

with polyps expanded (far left) and 

contracted (near right) on the reef 

crest at Kamiloloa, Molokaÿi.

Figure 26. Zoanthus pacificus is 

uncommon off south Molokaÿi but 

is found on shallow areas near 

the east end of Molokaÿi and more 

rarely in deeper water along the 

rest of the coastline.

Figure 27. This small colony of 

Anthelia edmondsoni was photographed at a depth of 10 m (30 ft) 

off Päläÿau, Molokaÿi.

Figure 28. The force of large storm 

waves from the North Pacific prevents 

development of a reef framework 

off the north coast of Molokaÿi. Note 

diver in the lower portion of the figure. 

Photograph taken at Kalaupapa on the 

north coast of Molokaÿi.

from Molokaÿi. S. molokensis occurs on the fore reef at depths of 1–6 m 

(3–20 ft). Palythoa tuberculosa is found in shallow, high-wave-energy 

environments on the reef crests (fig. 25), but this species rarely occurs on 

the deeper reefs. Zoanthus pacificus is uncommon on the reefs of south 

Molokaÿi, but it has been observed in shallow water at the east end of the 

island and in water as deep as 10 m (30 ft) off Pälä’au (fig. 26). Anthelia

edmondsoni is a small endemic octocoral that is common in shallow, highwave environments off many of the Hawaiian Islands but rare off south 

Molokaÿi. Note the presence of eight tentacles (fig. 27). Colonies of this 

species are generally velvetlike and of purple color in shallow water and 

more brownish in deep water. 
Reef Corals and the Coral Reefs of South Molokaÿi

47 

Character of the South Molokaÿi Fringing Reef

forms a surface layer that does not reach corals in deeper water. Rising 

and falling tides further flush the reef flat. Thus, live coral can be found 

scattered across the shallow reef flat even close to shore. Floods definitely 

represent a major but episodic stress for corals on the Molokaÿi reef flats, 

but corals continue to persist in this environment. 

Substrate Type

Reef corals generally require a hard substratum. A substratum of sand 

and mud is unsuitable for coral settlement and growth. Corals can, however, develop in these areas through initial settlement on isolated outcrops 

of hard rock. Accretion of skeletal material over many generations of coral 

will produce larger coral mounds or ridges surrounded by mud or sand. 

The solitary coral Fungia scutaria (see fig. 19) can persist on unstable 

rubble substrate because of unique hydrodynamic properties (Jokiel and 

Cowdin, 1976). 

Sand and mud are transported onto and off the reef in response to currents and waves (see Bothner and others, this vol., chap. 19 and Ogston and 

others, this vol., chap. 20). Passage of sediment through coral communities 

can smother new coral settlements and encrusting corals and can even bury 

large coral colonies (fig. 29), which are killed and sometimes uncovered later 

by subsequent erosion. 

Depth 

Depth of water is an important factor for corals, mainly because light 

and water motion diminish with depth. Deeper corals are subjected to less 

risk from wave energy under normal conditions. However, these corals are 

limited by lower light intensity, which causes a reduction in growth. 

Land Impact

Terrigenous runoff carries freshwater, fine sediment, and nutrients onto 

the reef (Field and others, this vol., chap. 21; and Roberts and Field, this 

vol., chap. 15). The shallow inshore reefs of Molokaÿi have been profoundly 

affected by increased runoff and sediment from two centuries of improper 

land-management practices. A major impact to the inshore is the extreme 

sedimentation of fine mud.

Biological Factors

Competition 

The limited amount of space available on a coral reef results in competition. Corals compete with each other as well as with other organisms for this 

resource. In optimal environments (for example, high light and moderate 

water motion), the fast-growing encrusting or branching species have the 

advantage. They have lightly constructed skeletons that allow them to rapidly 

overgrow other species (fig. 30). This eventually leads to total dominance 

by the single most successful competitor (see example in fig. 7). Such weak 

skeletons, however, cannot persist in high-wave environments, where the 

slower growing corals with dense skeletons have the advantage. 

Predators and Parasites

Numerous organisms feed directly on coral tissue. Various butterfly 

fish feed exclusively on coral polyps, and large parrotfish will occasionally scrape tissue and underlying skeletal material from live corals (Brock, 

1979). The nudibranch mollusk Phestella sibogae feeds on Porites coral. 

The snail Drupella, and the “cushion starfish” Culcita feed directly on a 

variety of corals. The flatworm Prosthiostomium montiporae feeds on the 

coral Montipora. Generally, coral predators and parasites do not inflict 

severe damage on coral colonies. A notable exception is the “crown of 

thorns starfish” (Acanthaster planci), which will eat the tissue off of entire 

coral colonies (fig. 31).

During 1969 –70, a large aggregation estimated to consist of 20,000 

Acanthaster planci was observed off south Molokaÿi (Branham and others, 

1971). They were feeding selectively on Montipora capitata. The infestation occurred on the rich coral reefs between Kawela and Kamalö. At that 

time the coral cover in the area was estimated visually and was reported to 

consist of approximately 90 percent Porites compressa and about 5 percent 

Montipora capitata. The area of uniform coral cover was reported to be 

approximately 1 km wide and extended to depths of 30 m (100 ft), where 

the bottom becomes a sandy slope. This description generally fits conditions that exist today, suggesting that major changes have not occurred in 

the reefs over the 1971–2004 time period. The State of Hawaiÿi Department of Fish and Game undertook extensive surveys and eradication efforts 

after discovery of the infestation (Onizuka, 1979). Divers killed a total of 

approximately 26,000 starfish between 1970 and 1975 by injecting them 

with ammonium hydroxide. Additional surveys were conducted throughout 

the State of Hawaiÿi, but no other infestations were detected at that time. 

Disease

All organisms are subject to disease, and corals are no exception. However, little is known about the diseases of corals. Recently, interest in this 

area has grown tremendously because of serious outbreaks of coral disease 

in many parts of the world (Richardson, 1998). Off south Molokaÿi, very 

few diseased corals have been observed on the fore reef, where conditions 

Figure 29. Movement of 

sediment across the bottom can smother and later 

uncover large coral colonies, 

as well as new coral settlements. Photographs of live 

Pocillopora meandrina (top) a 

buried colony (middle) and an 

uncovered dead colony (bottom) were taken at a depth 

of 3 m (10 ft) off Kamiloloa, 

Molokaÿi.

Figure 30. The coral Pocillopora 

meandrina (center of picture) is being overgrown by the fast-growing 

purple Montipora flabellata (bottom 

of picture) and by the branching 

growth form of Montipora capitata

(top of picture). Photograph taken 

at depth of 3 m (10 ft).

Figure 31. Adult Acanthaster 

planci or “crown of thorns starfish” 

off Kamiloloa, Molokaÿi, feeding on 

the coral Montipora capitata. The 

starfish is approximately 25 cm (10 

in) in diameter.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

48

are very favorable to coral growth (fig. 32). Dead and dying corals are 

more common on the highly stressful reef flat and the damaged fore reef 

off Kamiloloa. Corals that are weakened by adverse environmental conditions, such as high sediment loading, high organic input, and reduced light 

availability, are under stress. Increased environmental stress makes corals 

more prone to disease, as is the case with other organisms.

Bioerosion

Along with the physical forces of wave action, organisms play a significant role in abrading the reef framework. The rasping action of grazing 

fish (fig. 33) and urchins wears away carbonate rock (Brock, 1979). Boring 

sponges and boring mollusks degrade and weaken coral skeletons (Hutchings and Peyrot-Clausade, 2002).

Herbivorous Fish 

Herbivorous fish crop competitive algae, to the advantage of reef corals 

(Karlson, 1999). Overfishing can reduce the number of herbivores on a reef 

and thereby tip the balance in favor of the algae (Hughes, 1994). A healthy 

population of these fish off south Molokaÿi (Friedlander and Rodgers, this 

vol., chap. 7) will keep competitive algae under control.

Distribution of Corals on South Molokaÿi Reefs

Coral Zonation from Deep Ocean to Shore

Deep Fore Reef

The deep fore reef (fig. 34) extends from a depth of approximately 

10m (30 ft) out to the sand shelf at 30 – 40 m (90 –120 ft). This zone consists of coral platforms bisected by sand channels. In areas of high coral 

coverage, the tops of the platforms are dominated by a Porites compressa

community, whereas the vertical sides are dominated by a Montipora spp.

community.

Shallow Fore Reef 

The shallow fore reef extends from a depth of approximately 2 m (6 ft) 

to a depth of approximately 10 m (30 ft). This zone is characterized by spurand-groove morphology consisting of sand channels, ridges, and buttresses 

(Cochran, this vol., chap. 9). In many places there is a shallow shelf extending seaward that receives the brunt of the wave action. 

Reef Crest

Diverse wave-resistant coral species and encrusting coralline algae 

characterize the reef crest. This area receives extremely heavy wave impact, 

resulting in low coral cover. Diversity of coral species, however, can be 

relatively high. Dominant species, such as Porites compressa, Montipora flabellata, small Pocillopora meandrina, Pocillopora ligulata, and Montipora

studeri are often found in this zone, as well as in the shallow fore reef and 

the seaward edge of the reef flat. Leptastrea purpurea is an encrusting species with a hard skeleton that is found on the reef crest, shallow fore reef and 

outer reef flat, but with a somewhat scattered distribution. Pavona duerdeni

occurs on the reef crest and extends deeper into the shallow fore reef. This 

species has a hard dense skeleton and is resistant to waves and sediment 

abrasion. 

Reef Flat 

Shoreward of the reef crest there is a strong environmental gradient on 

the shallow reef flats of south Molokaÿi. The seaward edge of the reef flat is 

subjected to waves breaking over the reef crest and to strong currents. Small 

carbonate rock outcrops surrounded by coral rubble characterize the seaward 

reef flat. Depth diminishes shoreward of the reef crest. Wave energy is dissipated as the water decreases in depth moving toward the shoreline (Denny, 

1988). Toward shore the substrate becomes sandy with scattered rocks. Near 

the shoreline the sand grades into mud. 

The coral community on the outer reef flat is characterized by high coral 

diversity and low coral cover. Wave disturbance prevents the dominance 

of substratum by the more rapidly growing species, so many other species 

Figure 32. Coral Montipora patula

at Kamalö, Molokaÿi, showing rapid 

death of diseased tissue exposing 

a white coral skeleton. 

Figure 33. Parrotfish rasp away 

encrusting organisms along with 

carbonate substratum on the 

reef crest at Päläÿau. Grazing by 

fish and urchins is a major erosive force on the reefs of south 

Molokaÿi, producing carbonate 

sand and fine carbonate mud in 

the process.

Figure 34. This generalized profile shows the distribution of the five most 

abundant species of corals along a typical section of reef between Kamalö and 

Kawela, Molokaÿi. Relative abundance of each species at each point along the 

cross section is represented by the thickness of the color bar.

Figure 35. This aerial image shows the portion of the south Molokaÿi coast from 

Kawela (left) to Kamalö (center) and Uÿalapu’e (right). The areas of darker blue 

on the image on the reef flat are channels and “blue holes.” The straight-edged 

hole in the reef flat (right center, near shore) is from a dredging operation in the 

late 1960s. Distance across the image is approximately 7 km.

Relative distribution of the five most abundant corals on the south MolokaØi reef

Montipora patula

Montipora capitata

Pocillopora meandrina

Porites lobata

Porites compressa

Mud Sand Rubble Hard substratum

Carbonate reef platform

Volcanic bedrock

Inner reef flat Outer reef flat Shallow

fore reef Deepfore reef

Deep

sand

terrace

Reef

crest
Reef Corals and the Coral Reefs of South Molokaÿi

49 

Character of the South Molokaÿi Fringing Reef

rarely found in deeper water occur on the outer reef flat. All of the common 

species are found here, mixed with several species not common in deeper 

water. The mushroom coral Fungia scutaria is a solitary free-living coral 

that is not attached to the reef. This coral can live on the unstable rubble 

and fore reef. Porites cf. bernardi (previously known in Hawaiÿi as Porites

lichen) is a small encrusting species that is characteristically apple green in 

color with calices arranged in rows. Colonies are generally less than 2 cm (1 

in) in diameter and are found hidden in crevices and under overhangs on the 

carbonate outcrops. Cyphastrea ocellina is another common encrusting species that occurs here in similar habitats.

One of the unique features of the south Molokaÿi reef is the system of 

deep “blue holes” that lie to the east of Kamalö (fig. 35). This portion of the 

reef is cut by areas of deeper water. The holes are partly in alignment and 

may be remnants of submerged stream valleys that have been cut off and 

partly filled in by reef growth. Perhaps the remaining holes are areas where 

reef development was retarded by freshwater and sediment discharged from 

Kamalö Gulch. Another hypothesis is that the blue holes are karst features 

caused by ground-water dissolution with consequent undermining of the 

carbonate reef structure (Grossman and others, this vol., chap. 13). The coral 

community growing along the edges of the blue holes is similar in composition to the coral communities found in Käneÿohe Bay, Oÿahu. The dominant 

coral is Porites compressa, with small amounts of Pocillopora damicornis, 

Cyphastrea ocellina, and Montipora spp.

Distribution of Corals Along the South Molokaÿi Fore Reef

Estimated coral cover, by dominant species, along the 10-m (30-ft) 

depth contour is shown in figure 36. These data were obtained by visual 

estimates from the surface rather than by detailed transects. Data recorded 

by the type of visual survey used in this chapter can show a slightly higher 

total coverage, biased in favor of the branched species, when compared to 

the digital methodology used for the monitoring stations (Brown and others, 

this vol., chap. 6). Estimation of coral over a wide area by the visual method 

has a resolution of from 20 to 40 cm, whereas the digital methodology has a 

resolution of a few millimeters. Furthermore, there is a parallax effect when 

viewing a large area by the visual method from the surface, which results in 

seeing the sides of the branching finger corals and not the intervening spaces 

between branches. The visual method does not account for the small dead 

areas between the branches, nor will it record coverage by the small cryptic 

encrusting species. Nevertheless, a recent comparison study by Jokiel and 

others (2005) has demonstrated that the various coral survey methods commonly used in Hawaiÿi yield very similar estimates for coral cover.

 It is apparent that the coral distribution along the 10-m (30-ft) depth 

contour is bimodal, with two areas having high coral cover and three areas 

having very low coral cover. On the western end of the south Molokaÿi reef, 

the extremely high wave energy from the North Pacific swell wraps around 

Läÿau Point and suppresses coral development (Storlazzi and others, 2002, 

2003). Coral coverage increases rapidly to the east of Hale O Lono and 

remains high along the coastline to Kaunakakai, where a sharp decrease is 

observed. Coral cover from Kaunakakai to Kawela is quite low. Coverage 

increases sharply between Kawela and Kamalö, where the coastline turns 

northeast and another sharp decrease is noted. Low coral coverage along 

the southeast coast from Kamalö to Hälawa on the east can be attributed 

to the destructive wave impact of the northeast trade wind swell (Storlazzi 

and others, this vol., chap. 11). The existence of low coral cover between 

Kaunakakai and Kawela is not easily explained.

Damage Along the Kaunakakai-Kawela Sector

A detailed visual survey was conducted by teams of scuba divers from 

Kaunakakai to Kawela along a series of transects run perpendicular to shore 

from depths of 30 m (100 ft) to 10 m (30 ft) (fig. 37). West of Kaunakakai 

and east of Kawela the coral cover is very high at all depths. Between 

Kaunakakai and Kawela, however, is an area of damaged reef with low to 

nonexistent coral cover. Two important features of the coral distribution 

were noted. First, damage increases westward with depth from Kawela to 

Kaunakakai. Second, the distribution is essentially bimodal, with either low 

coverage (0 –20 percent) or high coral coverage (80 –100 percent), separated 

by a very sharp intervening boundary. 

Damage Along the Coast from Kamalö to the East End of Molokaÿi

The drop in coral cover between Kamalö and the east end of the island 

can easily be explained by high wave stress due to northeast trade wind swell 

(Storlazzi and others 2003; Storlazzi and others, this vol., chap. 11). The 

break in both coral cover and wave stress is remarkably sharp.

Anthropogenic Change: Man and Corals of the 

South Molokaÿi Reef

Over the past two centuries, improper land-use practices have led to 

accelerated erosion of the watersheds, with increased sediment and nutrient 

input onto the coral reefs (Roberts and Field, this vol., chap. 15). Initially the 

problem was overgrazing by cattle and sheep, but overgrazing continues to 

Figure 36. Distribution of reef corals along the fore reef of south Molokaÿi at a depth of 10 m (30 ft). 

Map shows sampling sites (black circles); depth contours in meters.

Coral cover, in percent

Longitude, in degrees

0

40

80

157¡10' 157¡ 156¡50'

Total cover

Porites lobata

Porites compressa

Montipora spp.

Pocillopora meandrina 

21°

 10' 

N 

 21°

 05' 

100

100

100

100

300

300

500

500

100

300

Kaiwi Channel

Kalohi Channel

Kaunakakai

Wharf

Kamiloloa

Kamal

Kalaupapa

Hale O Lono

ØìLIO POINT

LØAU

POINT

HLAWA

POINT

Mauna Loa

PkoØo

Kmimi

One AliØi

Wai a Kne

PlØau

Kawela

P A C I F I C O C E A N

0 2 4 Miles

0 4 Kilometers

157°10' W 157° 156°50'

Pailolo Channel
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

50

this day as a result of uncontrolled feral ungulates (deer, pigs, and goats) that 

are rapidly increasing in number on the watersheds. Initial cattle- and sheepranching operations were followed by extensive development of plantation 

agriculture (sugar cane and pineapple), which further contributed to the 

sediment and runoff problem (Roberts and Field, this vol., chap. 15). More 

recently, there are reports of introduced algae, which could negatively affect 

the shallow inshore reef communities (Smith and Field, this vol., chap. 8).

Fishing is an important activity on the reefs of south Molokaÿi, but the 

low human population has less impact on the reefs than what is observed 

on the more populated Hawaiian Islands (Friedlander and Rodgers, this 

vol., chap. 7). Effects of future climate change on the coral reefs of south 

Molokaÿi could be disastrous if current warming trends continue (Hughes 

and others, 2003). Hawaiian reefs have already begun to experience the 

impact of mass bleaching events (Jokiel and Brown, 2004).

Some of the impacts of coastal construction were discussed previously 

in this chapter. Damage due to dredging has also occurred on the reefs of 

south Molokaÿi. Kamalö was a prime fishing area with extensive coral cover 

before a series of aborted dredging operations in the area that began in the 

late 1960s. The dredging occurred on the inner reef flat east of Kamalö 

near Kalaeloa Harbor. The prevailing westward currents carried silt from the 

dredge operation down the coast and well past Kamalö (fig. 38). The fine silt 

covered reefs downcurrent, killing the coral. The area took on the appearance 

of a wasteland—everything was covered with fine silt, and the fish left the 

area (Capt. Joe Reich, oral commun., 2001). Even after the company went 

bankrupt and abandoned the dredging operation, the fine sediment continued 

to remobilize whenever the wind speed and wave heights increased. Chronic 

turbidity and sedimentation prevented any recovery of the reefs for many 

years. As fine sediment was winnowed out and transported offshore, the area 

slowly began to improve. These reefs showed signs of recovery by mid 1970s. 

Recovery was well underway by the early 1980s, with full recovery by 1990. 

The reefs off Kamalö presently appear to be “pristine,” but much of the area 

actually represents a regenerated reef that was heavily damaged by siltation.

Conclusion

The coral reefs off south Molokaÿi represent a great living treasure that 

has been damaged by increased sedimentation resulting from improper landuse practices and from coastal construction. Nevertheless, a large portion of 

the fore reef to the east of Kaunakakai and to the west of One Aliÿi remains 

relatively pristine. Diverse habitats in this area support rich coral reef communities. The reefs of south Molokaÿi represent a significant source of 

sustenance, recreation, and inspiration to all and must be managed carefully 

to prevent further decline.

Figure 37. This diagram shows coral cover off south Molokaÿi from 

Kaunakakai (left) eastward to Kawela (right) at depths from about 

10 m (30 ft) to about 25 m (80 ft). The figure represents coral cover 

on the reef face as viewed from seaward. 

30

Depth, in feet

Depth, in meters

40

50

60

70

80

157.04 157.02 157 156.98

Longitude, in decimal degrees

80Ð100

60Ð80

40Ð60

20Ð40

0Ð20

Coral cover,

in percent

15

20

10

Kaunakakai Kamiloloa Kawela

Figure 38. This map shows the area of damage near Kamalö resulting from a 

dredging project in the late 1960s.

0 .5 1 Mile

0 1 2 Kilometers

Dredge

site

Area of severe damage

Kamal
Character of the South Molokaÿi Fringing Reef

0 2 4 Miles

0 4 8 Kilometers

157°

21°

10' 

N 

156°50'

21°

05' 

157°10' W

PlØu

Kamiloloa

Kamal

KakahaiØa 10-m water depth

3-m water depth

1-m water depth

Coral Study Sites

CHAPTER 6

The Status of the Reefs Along South Molokaÿi: Five Years of Monitoring

Eric K. Brown1, Paul L. Jokiel2, Kuÿulei S. Rodgers2, William R. Smith2, and Lucile M. Roberts2

I

n Hawaiÿi, coral reefs are subjected to a variety of natural and anthropogenic stresses at several spatial and temporal scales (Grigg and Dollar, 

1990). The intensity and duration of these factors can both directly and 

indirectly alter the physical and biological structure of the reef (Connell 

and others, 1997). Natural factors, such as acute wave disturbances (Dollar 

and Tribble, 1993), freshwater inputs (Jokiel and others, 1993), and predator outbreaks (Done, 1992) can affect the shallow-water reef communities 

at small spatial (10–100 m) and temporal (0 –1 yr) scales. Chronic disturbances, such as non-point source pollution, are more difficult to detect 

because changes to the community landscape are subtle and occur over 

longer time periods (Pastorok and Bilyard, 1985). How different types of 

disturbance influence coral community structure is only beginning to be 

understood and quantified (Edmunds, 2000).

Disturbances can lead to a deteriorating or “unhealthy” coral population if recovery of the community does not occur or if the community is 

replaced by another perceived to be less desirable. DeVantier and others 

(1998) defined a “high quality” or “healthy” reef as one with high diversity 

of corals and associated biota and a strong reef-building capacity. The reefbuilding capacity is usually represented by high species richness and high 

absolute percent cover of hard corals (DeVantier and others, 1998). Szmant 

(1996) has incorporated temporal changes in her definition of reef health 

by stating that shifts from reefs dominated by corals to areas dominated by 

macroalgae signal the decline of a reef from a healthy state to an unhealthy 

one. These definitions, however, tend to depict reefs as static, steady-state 

systems without incorporating cyclical variation in substrate cover, coral 

growth, or recruitment. Categorizing reefs as “healthy” based primarily 

on high coral cover does not take into account geographic areas such as 

Hawaiÿi that naturally have low to moderate coral cover. Many of these 

areas are in good condition compared to other regions of the world (Wilkinson, 2000). Long-term monitoring programs can clarify natural cycles present in the system over time and provide an overview of population trends 

in the reef community. In addition, long-term monitoring is necessary to 

understand the role of natural and anthropogenic processes in changes in 

reef ecosystems (Hughes and Connell, 1999).

Long-term monitoring involves repeated surveys of organisms and/or 

environmental parameters at selected sites over time (Rogers and others, 

1

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744;

current address: Kalaupapa National Historical Park, P.O. Box 2222, Kalaupapa, HI 96742

2

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346 Käneÿohe, HI 96744

1994). Documenting changes in the community structure at various spatial 

and temporal scales using a well-designed monitoring program can assess 

the condition (such as deteriorating, improving, or undergoing a phase shift) 

of various reefs and focus research on causal links among the various factors (Hughes, 1993; Done and Reichelt, 1998). In Hawaiÿi, long-term coral 

reef monitoring projects have been conducted to detect change in coral 

cover at a small spatial scale at Kahe Point (Coles, 1998), Hanauma Bay 

(C. L. Hunter, written commun., 2000), Honolua Bay, Kahekili Park, Puamana, and Olowalu on Maui (Brown, 1999), and Molokini (B. N. Tissot, 

written commun., 2000).

The fringing reef along the south shore of Molokaÿi is considered to be 

one of the best-developed reef tracts in the Hawaiian archipelago (Storlazzi 

and others, 2003). This reef tract, however, has been subjected to a variety 

of natural (Branham and others, 1971) and anthropogenic (Roberts, 2001; J. 

Reich, oral commun., 2001) stresses during the course of human habitation. 

Consequently, recent research efforts have focused on the response of the 

reef system to these stresses and the possible causal mechanisms.

The purpose of this chapter is to provide baseline spatial and temporal 

data for reefs along the south shore of Molokaÿi. Percent coral cover and 

population dynamics (for example, recruitment) of the abundant coral species at three sites are described using two approaches. First, visual quadrats 

and digital video transects were used to analyze historical development of 

each reef over the past 5 years. Second, a more detailed but smaller scale 

approach used fixed photoquadrats to track individual colonies over the same 

time period to document life-history patterns (for example, growth, recruitment, and mortality) of various species at each reef. These patterns helped 

characterize the present condition of these reefs and identified areas undergoing changes that could be explored with future research studies.

Methods

Study Sites

Three long-term monitoring sites were established along the south 

coastline of Molokaÿi to examine changes in coral cover over time (Päläÿau, 

Kamiloloa, and Kamalö) (fig. 1). At each site two stations were sampled, at 

3-m (10-ft) and 10-m (33-ft) depths. Sites were haphazardly selected along a 

perceived gradient of sediment stress and wave exposure. Historical erosion 

at Päläÿau suggested that this site would show evidence of severe sediment 

deposition (Roberts, 2001). Eastward towards Kamiloloa, higher coral cover 

was predicted because of lower sedimentation levels and a more wave-sheltered environment. Finally, at the easternmost site of Kamalö, coral cover 

was expected to be highest and protected from all but the most severe storm 

events. These study sites were incorporated into the Hawaiÿi Coral Reef 

Assessment and Monitoring Program (CRAMP) monitoring sites in 2000 as 

Figure 1. Study sites at Päläÿau, 

Kamiloloa, and Kamalö along the south 

shore of Molokaÿi, Hawaiÿi. At each site 

there are 3 stations, at depths of 1 m, 3 

m, and 10 m (3.3 ft, 10 ft, and 33 ft). The 

1-m station at Kakahaiÿa served as the 

inshore station for Kamalö because of 

inaccessibility to suitable habitat near the 

Kamalö 3-m and 10-m stations.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

52

percent coral cover, mean percent coral cover by species, species richness 

(number of species per transect) and diversity were calculated for each site.

Life-History Patterns

A set of five fixed photoquadrats at each 3-m and 10-m station was used 

to examine growth, recruitment, and mortality of individual corals (for example, Hughes, 1985; Porter and Meier, 1992) (fig. 2B). At the shallow 1-m stations, sample size was increased to 10 photoquadrats to compensate for the 

more dynamic environment. Scanned slides from the photoquadrat sampling 

were analyzed using image-processing software to determine horizontal areal 

coverage of each coral colony by species. The fate (dead, exposed, fusion, 

areal growth, re-emerged, recruit, and shrinkage) was tracked over the time 

period of the study. More detailed methods for photoquadrats are described 

in Brown (2004) and Brown and others (2004); examples of image analysis 

(fig. 3) and life-history processes (fig. 4) are included here. Only the recruitment data are presented because of the complexities of life-history dynamics that are beyond the scope of this chapter. Recruits were defined as new 

colonies with a minimum detectable size of 0.5 cm in diameter (area 0.2 cm2

) 

and no previous record of settlement in a particular location within the plot. 

Typically this size represents colonies of approximately 1 to 2 years of age 

and is defined as “visible recruitment” in contrast to “invisible recruitment” 

(<0.5 cm in diameter) (Fitzhardinge, 1993).

Figure 2. Digital video-transect (A) and photoquadrat (B) methods used to 

survey the benthic community. Ten permanent (fixed) 10-m (33 ft) transects were 

initially selected at random within a grid 2 m × 100 m (6.6 ft × 330 ft) along either 

the 3-m (10 ft) or 10-m isobath and videotaped annually from a height of 0.5 m 

(1.6 ft) above the substrate. Five photoquadrats at each station were randomly 

established in 2000 within the same grid and sampled at least once a year. At the 

shallow (1-m; 3.3-ft) station, sample size was increased to 10 photoquadrats to 

accommodate the more dynamic environment.

Figure 3. Examples of how coral colonies were tracked within a photoquadrat 

using computer image analysis. The top photographs depict the raw images 

in 2000 (left) and 2001 at the Kamiloloa 1-m (3.3 ft) station. The bottom row 

shows the labeled colonies after analysis. Each unique colony was labeled and 

measured for total two-dimensional area by digitally tracing the colony boundary 

using SigmaScan®.

Data Analysis

The long-term coral cover data were analyzed using a General Linear 

Model (GLM) repeated measures ANOVA design. Planned comparisons 

or contrasts were used to examine changes in total coral cover within each 

station from the initial survey to the last survey. The recruitment data were 

also analyzed using a General Linear Model (GLM) repeated measures 

ANOVA design. Planned comparisons or contrasts were then used to examine temporal trends in recruitment levels within each station and between 

stations across depths. For the purposes of this chapter, statistical patterns 

were reported without the accompanying probability values. A more detailed 

description of data analysis techniques can be found in Brown (2004).

Results

Trends in Coral Cover

A total of 13 coral species were documented across all of the six 

stations in the initial baseline survey in 2000 (table 1). Overall, the top 5 

species with the highest coverage were Montipora capitata (21 percent), 

Porites compressa (9 percent), Montipora patula (8 percent), Porites lobata

(3 percent), and Pocillopora meandrina (1 percent) (fig. 5). Coral cover 

was highest initially at the Kamalö 10-m and 3-m stations (75 percent at 

both stations) and lowest at the Kamiloloa 10-m station (<1 percent). At 

the Kamalö stations, Montipora capitata (25 percent at 10 m; 59 percent at 

3 m) and M. patula (24 percent at 10 m; 9 percent at 3 m) had the highest 

percent cover, followed by Porites compressa (25 percent at 10 m; 1 percent 

at 3 m), P. lobata (<1 percent at 10 m; 3 percent at 3 m), and Pocillopora

meandrina (<1 percent at 10 m; <1 percent at 3 m). The community assemblage at the Päläÿau 10-m station was also dominated by Montipora species (M. capitata 33 percent; M. patula 13 percent), but there was a large 

component of Porites species (P. compressa 25 percent; P. lobata 1 percent). The Päläÿau 3-m station actually had a higher percentage of Porites

species (P. compressa 5 percent; P. lobata 13 percent) than Montipora

species (M. capitata 7 percent; P. lobata 3 percent) and a small percentage 

of Pocillopora meandrina (2 percent). As noted previously, the Kamiloloa 

10-m station had the lowest coral cover (<1 percent), which was distributed 

over 7 species. The Kamiloloa 3-m station had slightly higher coral cover 

(4 percent) but only 6 species were observed. P. meandrina was the most 

abundant coral at this shallow station (table 1).

The long-term trends in coral cover showed stations declining, recovering (increasing), and remaining stable (fig. 6). Since 2000, coral cover at the 

two stations at Päläÿau has remained relatively stable (fig. 6). The one exception was the 10-m station, which experienced a significant drop in absolute 

cover of 12 percent (17 percent relative) in 2004. The decline in cover was 

primarily attributed to Porites compressa (10 percent absolute, 40 percent 

relative). In contrast, the 3-m station at Päläÿau experienced a decrease of 

<2 percent (5 percent relative decrease) in total coral cover from 2000 to 

part of the larger statewide effort to document changes in coral cover (Jokiel 

and others, 2004).

Three additional stations were set up at 1-m depth (3.3-ft, on the reef 

flat) to document life-history patterns in highly turbid water (Päläÿau, 

Kamiloloa, and Kakahaiÿa). The shallow stations on the reef flat at Päläÿau 

and Kamiloloa were inshore of the deeper 3-m and 10-m stations (fore reef). 

Kakahaiÿa served as the inshore station for Kamalö because of inaccessibility to suitable habitat near the Kamalö 3-m and 10-m stations. These shallow stations were established at transitional locations on the reef flat where 

coral cover first became prevalent (approximately 5 percent) as one moved 

from shore out to the reef crest. Theoretically, these areas represented the 

landward edge of live coral and would most likely be influenced by anthropogenic processes nearshore.

Trends in Coral Cover

Sites were monitored annually from 2000 to 2004 using the CRAMP

protocol (Brown and others, 2004; Jokiel and others, 2004) with digital 

video transects (fig. 2A). Percent cover was tabulated for coral (by species), 

macroinvertebrates (for example, urchins, sea stars), and other benthic substrate types (coralline algae, turf algae, macroalgae, and sand). Total mean 

A B
Figure 5. Chart showing mean percent substrate cover among all six of the 3-m 

(10-ft) and 10-m (33-ft) sites off south Molokaÿi in 2000. See table 1 for data and 

list of common names. Substrate types and coral species with 0.1 mean percent 

cover or less are not included on chart. 

2000

Growth

Shrinkage

Death

2001 2000

Fission

Fusion

Recruitment

2001

Sand

0.8%

Montipora flabellata

0.2%

Montipora capitata

20.8%

Turf algae

50.9%

Macroalgae

4.6%

Coralline algae

0.7%

Porites lobata

3.0%

Porites compressa

9.4%

Pocillopora meandrina

1.1%

Montipora patula

8.3%

The Status of the Reefs Along South Molokaÿi: Five Years of Monitoring

53 

Character of the South Molokaÿi Fringing Reef

Figure 4. Examples of different life-history processes documented 

from 2000 to 2001 in photoquadrats. In the first year of this project, all 

colonies were given a unique number, even though it is highly probable that many of the colonies were the result of prior fission events. 

After the initial labeling, colonies (including fragments from documented fission events) were followed each year to calculate growth, 

shrinkage, and record total mortality. Colonies in the fission category 

represented any daughter colony separated from the parent colony. 

Fission also included fragmented colonies that fell into the plot and 

were still loose. Colonies that had undergone fusion were consolidated 

into one larger initial colony without any evidence of prior boundaries. 

Recruits were defined as new colonies with a minimum detectable size 

of 0.5 cm (0.2 in) in diameter (area 0.2 cm2; 0.03 in2) and no previous 

record of settlement in a particular location within the plot. Typically 

this size represents colonies around 1 to 2 years of age and is defined 

as “visible recruitment” in comparison to “invisible recruitment” (diameter <0.5 cm; 0.2 in).

2004. This small decrease was not statistically significant and could easily 

be attributed to measurement error. Consequently, the reef at the 3-m station 

was considered to be stable.

Coral cover at the Kamiloloa 10-m station showed a statistically 

significant increase of 5 percent in absolute coral cover (646 percent relative increase) since 2000 (fig. 6). The two principal species accounting for 

the recovery were Montipora capitata (4 percent absolute increase, 1,550 

percent relative) and M. patula (1 percent absolute increase, 1,300 percent 

relative). Pocillopora meandrina and Porites lobata were minor components of the assemblage. In contrast, coral cover at the Kamiloloa 3-m station has remained relatively stable over the past 5 years (fig. 6) even though 

there has been a 1 percent decrease (30 percent relative) in coral cover. This 

change was statistically significant but probably not ecologically important, because coral cover was initially very low. P. meandrina, which was 

the most abundant coral in terms of coverage, has declined by 2 percent 

(49 percent relative) during this time period. Other species (for example, 

M.capitata and M. patula) that were not recorded in the initial surveys have 

begun to appear in the benthos, suggesting that a community shift is occurring in the assemblage.

The Kamalö 10-m station experienced a statistically significant decline 

in coral cover of 16 percent (21 percent relative decline) from 2000 to 

2002, but it has since stabilized around 62 percent. This is still significantly 

lower than in the initial survey (fig. 6). The decrease in cover was primarily 

because of loss of Porites compressa, which declined steadily from 25 percent to 6 percent (76 percent relative decline) over the past 5 years. In comparison, percent cover of Montipora capitata has remained constant around 
Table 1. Baseline percent cover by substrate type at the six south Molokaÿi CRAMP stations in 2000. Dashes (–) indicate none observed.

Substrate Type Taxon Common Name1 Päläÿau

3 m

Päläÿau

10 m

Kamiloloa

3 m

Kamiloloa

10 m

Kamalö

3 m

Kamalö

10 m Mean

Coralline algae <0.1 0.6 <0.1 0.3 0.1 3.1 0.7

Macroalgae 0.2 2.6 4.3 19.6 0.4 0.1 4.5

Turf algae 69.3 22.9 87.4 78.7 24.4 21.1 50.6

Sand 0.1 – 4.5 0.2 – – 0.8

Coral Leptastrea purpurea Crust coral <0.1 – <0.1 – – – <0.1

Montipora capitata Rice coral 6.5 32.7 – 0.2 59.5 25.2 20.7

Montipora flabellata Blue rice coral – <0.1 – – 0.8 0.1 0.2

Montipora patula Sandpaper rice coral 3.0 12.9 – 0.1 9.2 24.3 8.2

Pavona maldivensis Maldive coral – <0.1 – – – – <0.1

Pavona varians Corrugated coral – – – – <0.1 <0.1 <0.1

Pocillopora damicornis Lace coral <0.1 – 0.1 – 0.1 – <0.1

Pocillopora eydouxi Antler coral 0.2 0.1 – 0.1 0.2 – 0.1

Pocillopora meandrina Cauliflower coral 1.5 0.6 3.5 0.1 0.6 0.2 1.1

Porites brighami Brigham’s coral – – <0.1 0.1 0.1 <0.1 <0.1

Porites compressa Finger coral 5.3 24.7 – – 1.0 25.2 9.4

Porites lobata Lobe coral 13.0 1.5 0.1 0.3 3.0 0.2 3.0

Unknown coral 0.1 0.1 <0.1 <0.1 0.2 – 0.1

Total percent coral cover 29.6 72.4 3.7 0.9 74.6 75.2 42.7

Species richness2 9 9 6 7 11 8 8

1

Common names are from Fenner (2005).

 2Species richness is the total number of coral species documented at a site.

PlØau

Kamiloloa Kamal

 Coral study sites

 3-m depth 

 10-m depth

Montipora

Porites

Pocillopora

Other

Coral Genera

Kamal

Year

Kamiloloa

0

40

80

0

40

80

0

40

80

0

40

80

Coral cover, in percent

Depth: 3 m 3 m 3 m

Depth: 10 m 10 m 10 m

2000 2001 2002 2003 2004 2000 2001 2002 2003 2004 2000 2001 2002 2003 2004

PlØau

0

10

20

0

10

20

*

***

***

***

**

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

54

25 percent while M. patula has actually increased 5 percent (22 percent relative). Coral cover at the 3-m station at Kamalö also suffered an initial decline 

of 22 percent (30 percent relative) from 2000 to 2001 (fig. 6). The coral community, however, seems to be recovering, as indicated by increases in coral 

cover for both M. capitata (6 percent absolute, 16 percent relative) and 

M. patula (6 percent absolute, 61 percent relative). The recent increase in 

cover, however, has not compensated for the initial loss, and consequently, 

cover at this station remains significantly depressed (fig. 6).

In summary, temporal trends across sites indicate that four of the six stations (Päläÿau 10 m, Kamiloloa 3 m, and Kamalö 3 m and 10 m) experienced 

a statistically significant drop in coral cover after the initial survey (fig. 6). 

In contrast, the Päläÿau 3-m station appeared to be holding steady, while 

coral cover at the centrally located Kamiloloa 10-m station increased significantly. Some of the changes in coral cover may not be ecologically important 

because of the confounding effects of possible measurement error with the 

low absolute values. In the case of the Kamalö 3-m station, recovery appears 

to be taking place after an initial decline in 2001. The trends will become 

clearer with continued monitoring over the next decade.

Life-History Patterns—Recruitment 

A total of 221 new coral recruits were documented in the 60 photoquadrats from 2000 to 2004. The five most abundant species of recruits 

were Montipora capitata (89), M. patula (32), Pocillopora damicornis (26), 

Porites compressa (23), and Porites lobata (22) (fig. 7). Another five species 

(Cyphastrea ocellina, Montipora flabellata, Pavona maldivensis, Pocillopora

damicornis, and Porites lichen) formed 13 percent of the new recruits in 

the photoquadrats. The majority of Montipora recruits were observed at the 

Kamiloloa 3-m and 10-m stations (fig. 8). Spatially, Porites recruits formed 

a larger component of the recruiting classes at the Päläÿau 3-m, Kakahaiÿa 

1-m, and Kamalö 3-m stations than at the Kamiloloa stations. In comparison, 

pulses of Pocillopora recruitment were sporadic and appeared during various 

times at the Päläÿau 1-m, Päläÿau 10-m, Kamiloloa 1-m, Kamiloloa 3-m, and 

Kakahaiÿa 1-m stations.

Recruitment rates varied by depth within a site. The number of new 

recruits at the Päläÿau 1-m and 3-m stations declined after 2001, but this was 

not a statistically significant change (fig. 8). The recruitment rate in 2001 

was 2.7 recruits/m2

/yr at the 1-m station and 3.0 recruits/m2/yr at the 3-m 

station. These levels decreased to 0.3 and 0.6 recruits/m2

/yr, respectively, by 

2004. In contrast, recruitment levels increased at the 10-m station from 

0 recruits/m2

/yr in 2001 to 2.4 recruits/m2/yr by 2004. This increase was not 

statistically significant because of the high variation in recruitment numbers.

At Kamiloloa, recruitment rates at the 1-m station were relatively stable 

across the 4 years and ranged from 0.9 to 1.8 recruits/m2

/yr (fig. 8). The 

recruitment rate at the 3-m station was 6.1 recruits/m2

/yr in 2001 and then 

jumped up to 8.5 recruits/m2

/yr in 2002. In 2003 and 2004, recruitment 

dropped off to 2.4 and 3.0 recruits/m2

/yr, respectively. The 10-m station at 

Kamiloloa followed a similar pattern, with high recruitment in 2001 (17.0 

recruits/m2

/yr) followed by an increase in 2002 (23.0 recruits/m2/yr) and then 

Figure 6. Trends in coral cover for the 

abundant genera at the deeper Molokaÿi 

stations (3-m and 10-m; 10-ft and 33-ft) 

from 2000 to 2004. Mean ± 1 standard 

error with a sample size of 10 transects 

at each station. Significant changes 

in coral cover from the initial survey 

in 2000 to the last survey in 2004 are 

denoted on the figures (* = p <0.05, 

** = p <0.01, *** = p <0.001). Note the 

different Y-axes for the Kamiloloa 3-m 

and 10-m stations.
PlØau

Kamiloloa

Kamal

*

*

Year

KakahaiØa

0

2

4

6

0

2

4

6

0

2

4

6 1 m 1 m

PlØau

Coral recruits, in numbers per square meter per year

0

2

4

6

0

8

16

24

0

2

4

6 3 m 3 m

0

2

4

6

8

0

8

16

24

32

0

2001 2002 2003 2004 2001 2002 2003 2004 2001 2002 2003 2004

2

4

6

8

10 m 10 m

Montipora

Porites

Pocillopora

Other

Coral Genera

Kamiloloa

Kamal

1 m

3 m

10 m

PlØau

Kamiloloa

Kamal KakahaiØa

 Coral study sites

 1-m depth

 3-m depth 

 10-m depth

Montipora flabellata

3.2%

Montipora capitata

40.3%

Cypastrea ocellina

0.5%

Porites

lobata

10.0%

Porites lichen

4.1%

Porites

compressa

10.4%

Pocillopora

meandrina

5.0%

Pocillopora

damicornis

11.8%

Montipora

patula

14.5%

Pavona maldivensis

0.5%

Figure 7. Chart showing percentage of new coral recruits by species among all 

nine of the stations off south Molokaÿi from 2000 to 2004. See table 1 for data 

and list of common names.

The Status of the Reefs Along South Molokaÿi: Five Years of Monitoring

55 

Character of the South Molokaÿi Fringing Reef

a subsequent decline in 2003 and 2004 to 3.0 recruits/m2/yr. The declines in 

2003 and 2004 from the preceding years were statistically significant.

Recruitment rates at the Kakahaiÿa 1-m station showed a general 

decline from 5.2 recruits/m2/yr in 2001 to 0.9 recruits/m2/yr in 2004 (fig. 8). 

This decrease was not statistically significant even though it represented a 

fivefold drop in recruitment. The most likely explanation for the apparent 

decrease is that recruitment fluctuated dramatically among the photoquadrats, especially in 2001.

The 3-m and 10-m stations at Kamalö displayed an even more pronounced and steady decline in recruitment than at Kakahaiÿa (fig. 8). At the 

3-m station, recruitment rates dropped from 2.4 recruits/m2

/yr in 2001 to 

0 recruits/m2/yr in 2004. The 10-m station experienced a tenfold decrease in 

recruitment, from 6.1 recruits/m2

/yr in 2001 to 0.6 recruits/m2/yr in 2004. 

Despite the dramatic reduction in recruitment rates, these declines were not 

statistically significant because of the high variability in recruitment rates 

among photoquadrats. 

In summary, recruitment rates at Päläÿau were generally lower in comparison to the other sites at each of the depths sampled (fig. 8). In comparison, 

Kamiloloa had the highest recruitment rates, and this was attributed to the high 

initial recruitment of Montipora spp. Over time, most of the stations experienced a drop in recruitment rates, except for the Päläÿau 10-m station, which 

increased, and the Kamiloloa 1-m station, which remained at a consistently 

low level. Deeper stations generally had higher recruitment rates than the corresponding shallow stations at a given site. The Päläÿau 10-m station, however, 

had low initial recruitment rates compared to the shallow stations, but in 2004 

recruitment was more than threefold higher than the shallow stations.

Figure 8. Trends in juvenile (minimum detectable size of 

0.5 cm/0.2 in diameter) recruitment (number of recruits/m2/

yr) of the abundant coral genera at the Molokaÿi stations 

(1-m, 3-m, and 10-m; 3.3-ft, 10-ft, and 33-ft) from 2000 to 

2004. Mean ± 1 standard error. Significant changes in coral 

recruitment from the initial survey in 2000 to the last survey 

in 2004 are denoted on the figures (* = p <0.05). Note the 

different Y-axes for the Kamiloloa 3-m and 10-m stations.

Discussion

Trends in Coral Cover

Analysis of long-term monitoring data at stations along the south shore 

of Molokaÿi over the past 5 years has documented patterns of reef decline, 

recovery, and stability. Stations spanning a distance >20 km, such as the 

Päläÿau 10-m, Kamiloloa 3-m, and Kamalö 3-m and 10-m stations, all experienced declines in coral cover since 2000, although the Kamalö 3-m station 

appeared to be recovering (fig. 6). In contrast, coral cover at the centrally 

located Kamiloloa 10-m station was increasing, while less than 200 m away 

at the 3-m station it decreased. In addition, the Päläÿau 3-m station, which is 

only 250 m from the 10-m station, appears to be holding steady compared 

to the declining 10-m station. What explains these differences in community 

trends over varying distances?

Coral community structure at each of the stations reflects the influence of natural and anthropogenic factors along the fringing reef tract. The 

principal natural factors on the south shore of Molokaÿi, discussed separately in other chapters in this volume and elsewhere, include wave exposure 

(Storlazzi and others, this vol., chap. 11), tidal cycle (Storlazzi and others, 

2004), wind patterns (Storlazzi and others, this vol., chap 11), freshwater 

intrusion (Grossman and others, this vol., chap. 13), and natural erosion 

from land (Roberts and Field, this vol., chap. 15). Wave disturbance has been 

proposed as the primary factor shaping coral reef communities in Hawaiÿi 

(Grigg, 1983; Jokiel and others, 2004). The long-term trends in coral cover 

on Molokaÿi suggest that stations within a site at a scale of several hundred 

meters are influenced differently by wave events. Indeed, Connell and others 

(1997) observed that reefs in close proximity (30-300 m) at Heron Island in 

the Great Barrier Reef varied in their response to natural disturbances such 

as cyclones over a 30-year period. Edmunds (2002) also found contrasting 

patterns in long-term (12 yr) cover data from two sites in the U.S. Virgin 

Islands separated by <1 km. He attributed this pattern to the different depths 

and the coastal features that protected one reef from the impact of Hurricane 

Hugo in 1989.

In the present study at Molokaÿi, it appears that depth may also play 

a role in producing differential trends in coral cover, but the trends were 

not the same at a given depth. Disturbances (for example, waves, flooding, 

coastal development) over the past 5 years may have been of sufficient magnitude to alter coral community structure between stations within a site. For 

example, the cooler average water temperatures at the shallow Kamalö and 
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

56

Kakahaiÿa stations suggests that freshwater intrusion has been higher than 

at deeper stations and stations further west (E. K. Brown, written commun., 

2004). Spatial patterns in historical precipitation levels on Molokaÿi support this observation (Giambelluca and Schroeder, 1998). Heavy rainfall in 

Hawaiÿi has been known to result in reef “kills” caused by freshwater intrusion at the surface on shallow reef areas (Jokiel and others, 1993). This may 

explain the sudden decline in coral cover from 2000 to 2001 at the Kamalö 

stations. Precipitation levels during this time period, however, were not 

abnormally high (National Climatic Data Center, v://www.ncdc.noaa.gov/

oa/ncdc.html, accessed August 31, 2007), so this scenario does not appear 

likely. Biological factors such as predation, parasitism, disease, competition, and bioerosion can also play a role and add to the dynamic changes that 

occur on reefs.

The primary anthropogenic factor on Molokaÿi appears to be sedimentation from erosion (Roberts, 2001) and the subsequent resuspension 

of the sediment along the reef flat (Ogston and others, 2004; Storlazzi and 

others, 2004; Presto and others, 2006). Initial site selection for this study 

was based on a perceived sediment stress gradient from Päläÿau, which 

was believed to be the most heavily impacted site, to the more easterly 

Kamalö, with relatively little sediment influx. Roberts (2001) reported that 

cattle ranching in western and central Molokaÿi in the mid-to-late 1800s 

and early 1900s resulted in a severe loss of native vegetation, producing 

upland erosion and subsequent deposition at Päläÿau. Parts of the shoreline 

at Päläÿau have prograded approximately 200 m since the late 

1800s (see D’Iorio, this 

vol., chap. 16, for a 

further discussion of historical shoreline changes). Consequently, it was 

hypothesized that the sediment influx would have detrimental impacts on 

the adjacent reef. Clearly this was not the case at Päläÿau, where moderate to high coral cover has been coupled with the relatively steady state in 

community structure. It appears that the introduction of the mangroves in 

1902 (Roberts, 2001), coupled with the reduction in cattle grazing, served 

to retain upland sediment close to shore and reduce sediment influx onto 

the reef. Kamiloloa farther east was the depauperate site, with some of the 

lowest coral cover in the state (Jokiel and others, 2004). Surveys of coral 

cover at 1-km intervals along the 10-m isobath indicated that coral reefs at 

Kamiloloa are situated in a pocket of low (<5 percent) coral cover surrounded by regions of extremely high (>60 percent) coral cover (Storlazzi 

and others, 2005). Thus, it appears that most of the anthropogenic stress is 

now focused on the region between Kawela and Kamiloloa because of the 

sediment influx and resuspension (Storlazzi and others, 2004).

An important point to consider in the development of a coral reef is 

the coral community structure at the initial observation. The Päläÿau 10-m 

and Kamalö 3-m and 10-m stations all had high coral cover >50 percent in 

2000. Consequently, it is much more likely that coral cover would decline 

rather than increase (Hughes, 1993). In comparison, the Kamiloloa stations 

had <5 percent coral cover, so the probability of coral cover increasing is 

much greater. Therefore, long-term trends must be interpreted with caution.

Recruitment Patterns

Biological processes such as recruitment could explain coral-cover 

temporal patterns at stations in close proximity. Recent evidence suggests 

that many marine populations of corals, fish, molluscs, and crustaceans once 

considered open to larval input at a large spatial scale are in fact relying 

more on self-recruitment than on outside sources (Kingsford and others, 

2002; Sponaugle and others, 2002; Swearer and others, 2002; Warner and 

Cowen, 2002). Perhaps the coral populations in this study are self-seeding 

and therefore heavily influenced by the local adult community. Harriott 

(1992) hypothesized this relationship at an isolated subtropical reef (Lord 

Howe Island), but in that case the coral recruits came from brooding corals 

rather than the broadcast spawners documented in this study. Brooding corals have exhibited a shorter time to competency (Harriott, 1992) and a larger 

dispersal potential than broadcast spawners (Richmond, 1988), which would 

facilitate self-seeding. Consequently, it is not directly apparent in this study 

that self-seeding is occurring, but genetic analysis of the coral recruits and 

surrounding adult population may help resolve this question. Supply-side 

considerations indicate that a decline in living tissue would subsequently 

reduce reproduction and ultimately recruitment (see, for example, Hughes 

and others, 2000). The lag time, however, between the decline in coral cover 

and the subsequent reduction in local recruitment may not be evident in the 

short time frame of this study.

Spatial patterns in recruitment rates at the Molokaÿi stations generally 

corresponded to trends in coral cover. For example, the Kamiloloa 10-m 

station had the highest annual recruitment rate among the 9 stations (fig.8), 

and this was the only station that experienced an increase in coral cover. 

This increase was attributed to the high initial recruitment of Montipora

spp. in 2001 and 2002, which also most likely resulted in the abundance of 

this genus in the community structure (fig. 6). Other examples included the 

low recruitment rate at the Päläÿau 10-m station coupled with the declining 

coral cover at this same station. Some recruitment patterns, however, did not 

match the trend in coral cover. An example is the high annual recruitment 

at the Kamiloloa 3-m station (fig. 8) and the low coral cover that was actually decreasing (fig. 6). This pattern suggests that high water motion and the 

resulting sediment stress (Storlazzi and others, 2004) are limiting recruitment 

success at this station. Other possibilities exist to explain the poor relationship between recruitment and coral cover trends at certain stations. Perhaps 

the source of recruitment is from nearby reefs outside the sampling area, or 

maybe differential mortality is occurring at the stations for all age classes.

Temporal patterns in recruitment are more difficult to interpret because 

of the highly variable nature of recruitment (Hughes and others, 1999a; 

Brown, 2004) and the short time frame of this study. For example, the 

higher initial recruitment at each of the stations may simply be an episodic 

recruitment pulse rather than a high, sustained level of recruitment along the 

Molokaÿi coastline followed by a subsequent decline. Therefore, understanding temporal patterns in recruitment probably requires monitoring over time 

scales commensurate with coral reproductive patterns (that is, decades).

In general, recruitment rates for similar species and genera were lower in 

this study compared to other studies around the globe (table 2). Higher rates 

were documented by Smith (1992) for Porites astreoides (12.1 recruits/m2

/yr) 

in Bermuda and by McClanahan (2000) for Porites spp. (3.3 recruits/m2

/yr) in 

the Maldives (table 2). Connell (1973) and Connell and others (1997) reported 

recruitment rates averaging 5 recruits/m2

/yr for all coral taxa on recovering 

reefs subjected to periodic cyclones in the Great Barrier Reef. In their study, 

the highest recruitment rates (mean 8 to 13 recruits/m2

/yr) occurred at reefs 

that experienced the greatest absolute declines in percent coral cover (Connell and others, 1997). Smith (1992), however, observed similar recruitment 

rates at both damaged (12 recruits/m2

/yr) and control (13 recruits/m2/yr) 

reefs in Bermuda. In addition, Loch and others (2002) documented recruitment rates of 12 recruits/m2

/yr at reefs in the Maldives that had experienced 

severe bleaching during the 1998 El Niño event. Consequently, high recruitment rates associated with various disturbance events imply either that larval 

sources are not necessarily local or that adult fecundity was unaffected by 

the storms, at least within the spatial scale (100 m) of the sampled adult 

community.

Perhaps the coral communities on Molokaÿi, with low coral recruitment in comparison to other areas, have not experienced disturbances of 

sufficient magnitude to open up suitable substrate for new recruits. Indeed, 

the highest recruitment rates were observed at stations (Kamiloloa 3-m and 

10-m) with the lowest coral cover. If these Kamiloloa stations had experienced a recent disturbance, then substrate could have become available for 

settlement from adjacent coral communities with high cover. Larger scale 

qualitative surveys have documented coral-rich communities east and west 

of Kamiloloa (Storlazzi and others, 2005) that could easily have accounted 
Site Region Genus/Species Common Name1

Recruitment 

(number of 

recruits/m2/yr)

Reference

Oÿahu, Hawaiÿi Pacific Montipora capitata Rice coral 0 Polacheck (1978)

Maui, Hawaiÿi Pacific Montipora capitata Rice coral 4.5 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Montipora capitata Rice coral 1.4 Brown and others, this study

Maui, Hawaiÿi Pacific Montipora flabellata Blue rice coral 0.3 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Montipora flabellata Blue rice coral 0.1 Brown and others, this study

Great Barrier Reef Pacific Montipora foliosa * 0.3 Connell (1973)

Great Barrier Reef Pacific Montipora hispida * 0.3 Connell (1973)

Maui, Hawaiÿi Pacific Montipora patula Sandpaper rice coral 1.0 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Montipora patula Sandpaper rice coral 0.5 Brown and others, this study

Maldives Indian Montipora spp. 0.4 McClanahan (2000)

Great Barrier Reef Pacific Pocillopora damicornis Lace coral 0.7 Connell (1973)

Molokaÿi, Hawaiÿi Pacific Pocillopora damicornis Lace coral 0.2 Brown and others, this study

Oÿahu, Hawaiÿi Pacific Pocillopora meandrina Cauliflower coral 0.2 Polacheck (1978)

Maui, Hawaiÿi Pacific Pocillopora meandrina Cauliflower coral 1.0 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Pocillopora meandrina Cauliflower coral 0.2 Brown and others, this study

Maldives Indian Pocillopora spp. 0.1 McClanahan (2000)

Great Barrier Reef Pacific Porites annae Nodule coral 0.7 Connell (1973)

Bermuda Caribbean Porites astreoides Mustard hill coral 12.1 Smith (1992)

Florida Caribbean Porites astreoides Mustard hill coral 1.3 Miller and others (2000)

Oÿahu, Hawaiÿi Pacific Porites compressa Finger coral 0 Polacheck (1978)

Maui, Hawaiÿi Pacific Porites compressa Finger coral 1.7 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Porites compressa Finger coral 0.3 Brown and others, this study

Maui, Hawaiÿi Pacific Porites lobata Lobe coral 1.1 Brown (2004)

Molokaÿi, Hawaiÿi Pacific Porites lobata Lobe coral 0.3 Brown and others, this study

Great Barrier Reef Pacific Porites lutea Mound coral 0.9 Connell (1973)

Florida Caribbean Porites porites Finger coral 0.4 Miller and others (2000)

Maldives Indian Porites spp. 3.3 McClanahan (2000)

1

Common names are from Fenner (2005) for Pacific species and Humann (1993) for Caribbean species. Species denoted with an * have no documented 

common name.

 8

 4

0

4

8

Change in coral cover, in percent

KauaØi OØahu MolokaØi KahoØolawe Maui HawaiØi

The Status of the Reefs Along South Molokaÿi: Five Years of Monitoring

57 

Character of the South Molokaÿi Fringing Reef

for the high recruitment seen at these stations. It is important to note, 

however, that disturbance events are not necessarily associated with wave 

phenomena and do not necessarily promote recruitment. For example, 

sediment stress, either acute or chronic, might actually reduce the amount 

of available substrate for recruitment (Rogers, 1990; Fabricius, 2005). 

Given the high recruitment levels at the two Kamiloloa fore reef stations 

coupled with high sediment accumulation (Storlazzi and others, 2004) and 

resuspension (Ogston and others, 2004), it appears that sediment may not 

influence recruitment rates as much as other factors such as adult fecundity 

(Hughes and others, 2000). Lack of sediment data at the other stations, 

however, limits any spatial and temporal conclusions regarding disturbance 

type and recruitment.

In Hawaiÿi, other studies of juvenile recruits (>0.5 cm in diameter) on natural substrates have included Polacheck (1978) on Oÿahu and Brown (2004) on 

Maui. Polacheck (1978) found no Montipora spp. or Porites spp. recruits and 

lower Pocillopora meandrina recruitment rates (0.18 recruits/m2

/yr) in comparison to this study (mean 0.26 recruits/m2

/yr) (table 2). His study sites off 

Waikïkï may not have been conducive for good recruitment because of large 

areas of unconsolidated sediment (Coyne and others, 2003), which can inhibit 

survival of new recruits (Fabricius, 2005). Polacheck’s study was also 1 

year in duration and may not have detected episodic recruitment events. In 

contrast, Brown (2004) documented high recruitment rates for all species 

at his west Maui sites (table 2). The difference in recruitment rates between 

these studies can possibly also be explained by the lower coral cover, and 

thus more available substrate, at the west Maui stations (Brown, 2004). It 

should be noted, however, that areas existed on both islands with plenty 

of available space but low recruitment. Thus, other factors (for example, 

sedimentation stress, larval availability) must also be contributing to the 

low recruitment observed in these photoquadrats.

Statewide and Global Comparisons of Reef Condition

Spatial patterns in coral cover indicate that reefs along the south shore 

of Molokaÿi have both some of the highest levels and some of the lowest 

levels of coral cover found in Hawaiÿi (Jokiel and others, 2004). Sections of 

the reef around Kamalö contain some of the most densely packed coral communities in the State. Temporal patterns, however, reveal that the Molokaÿi 

sites may not be faring as well as other sites in Hawaiÿi. From 2000 to 2002, 

the six CRAMP stations on Molokaÿi discussed here experienced the largest decline (by island) compared to the other 54 CRAMP stations on Kauaÿi, 

Oÿahu, Kahoÿolawe, Maui, and Hawaiÿi (Jokiel and others, 2004) (fig. 9). 

The decline was not statistically different from those stations on Oÿahu and 

Maui, but declines in coral cover on Oÿahu and Maui corresponded to high 

human populations. In contrast, Molokaÿi has a small population relative to 

land area (Juvik and Juvik, 1998), so causal mechanisms have focused on 

poor land-use management, attempted coastal development (Roberts, 2001), 

and the subsequent sediment influx and resuspension on the reef (Ogston and 

others, 2004). Efforts are currently underway on Molokaÿi to control feral 

ungulates and revegetate upland areas.

Table 2. Average coral recruitment on natural substrates for juveniles (minimum detectable size of 0.5 cm/0.2 in diameter 

and no previous settlement record) of coral species and genera in the Caribbean, Pacific, and Indian Oceans. 

[Recruitment rates for Molokaÿi are averaged across all nine stations for the four years 2001–2004.]

Figure 9. Mean percent change (±1 standard error) in coral cover by island at 

sites across the state of Hawaiÿi from 1999 to 2002 (from Jokiel and others, 2004).
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

58

Reefs in the Hawaiian archipelago appear to be doing better than reefs 

in other parts of the world. Of the 20 regions examined in the latest biennial 

review, “Status of Coral Reefs of the World: 2004,” the Hawaiÿi region had the 

highest percentage of reefs (93 percent) at low or no threat level (Wilkinson, 

2004). Only 1 percent of the reef area in the region is considered destroyed 

(Waikïkï) compared to a high of 65 percent in the Persian Gulf (Wilkinson, 

2004). The south shore of Molokaÿi, however, was included in the 5 percent 

of Hawaiÿi’s reefs at the threatened stage. Local residents are taking steps to 

reverse this pattern by changing upslope land-use patterns and limiting coastal 

development. These proactive measures may not be enough to protect nearshore reefs, because changes in global climate are elevating water temperatures in Hawaiÿi above critical thresholds for corals (Jokiel and Brown, 2004). 

If annual water temperatures continue to increase with extended durations 

of seasonal high temperature anomalies, then reefs along the south shore of 

Molokaÿi may not recover. In addition, long residence time and resuspension 

of sediment may compound the problem (Ogston and others, 2004; Presto and 

others, 2006). The reefs in decline may already be responding to these chronic 

disturbances, which could lead to the demise of the Molokaÿi reefs. Coral reef 

ecosystems, however, operate in a cyclical fashion over time (Karlson, 1999), 

and therefore the decline we are currently witnessing may simply represent the 

downward portion of the community cycle before the upswing. Indeed, one 

station (Kamalö 3-m) is already showing signs of recovery.

Conclusions

Temporal trends in coral cover indicate that four of the six CRAMP stations on Molokaÿi (Päläÿau 10-m, Kamiloloa 3-m, and Kamalö 3-m and 10-m) 

experienced a statistically significant drop in coral cover from 2000 to 2004. 

In contrast, the Päläÿau 3-m station appeared to be holding steady, while coral 

cover at the centrally located Kamiloloa 10-m station increased significantly. 

Temporal patterns reveal that Molokaÿi may not be faring as well as other sites 

in Hawaiÿi, but from a global perspective the reefs along this coastline are 

better off than elsewhere. Spatial patterns in recruitment rates at the Molokaÿi 

stations generally correspond to trends in coral cover but there are exceptions 

(for example, the Kamiloloa 3-m station). Recruitment rates of comparable 

coral species and genera are generally lower along the south shore of Molokaÿi 

than at other sites around the State and globally.

Molokaÿi has the longest contiguous reef tract in the main Hawaiian 

Islands, but there are signs that the reef community along the south shore may 

be experiencing a difficult period in its evolution. Coral recruitment patterns 

and anthropogenic stressors will need to be closely monitored to see if the 

reef ecosystem can continue to sustain itself. Projected changes in climate and 

sea level will further exacerbate an already tenuous situation along this coastline and could lead to the demise of an outstanding fringing reef area.
Character of the South Molokaÿi

CHAPTER 7

 Fringing Reef

Coral Reef Fishes and Fisheries of South Molokaÿi

Alan M. Friedlander1,2 and Kuÿulei S. Rodgers3

Fish provide food, cultural identity, and commerce to a broad majority 

of the local population of Molokaÿi and are an integral component 

of the marine environment. Fishes transfer energy from one location 

to another by their movement and thereby enhance the overall productivity 

of the ecosystem. Herbivorous fishes help control the spread of alien and 

nuisance limu (seaweed), while predators help to maintain high turnover 

rates and increase productivity within the ecosystem. The high proportion 

of species that are endemic (found nowhere else on earth) makes Hawaiÿi an 

important location for global marine biodiversity.

Importance of Coral Reef Fishes to Hawaiian Culture

Fishing has been a way of life for Hawaiian people for centuries, with 

fish and shellfish providing the major protein source (Kamakau, 1839; 

Titcomb, 1977). Native Hawaiians depended on fishing for survival, which 

motivated them to acquire a sophisticated understanding of the factors that 

caused limitations and fluctuations in their marine resources (fig. 1). Using 

their familiarity with specific places and through much trial and error, 

Hawaiian communities were able to develop ingenious social and cultural 

controls on fishing that fostered, in modern terminology, “sustainable use” 

of marine resources (Hui Malama o Moÿomomi, 1995). The traditional 

system in Hawaiÿi emphasized a code of conduct that was strictly enforced 

(Friedlander and others, 2002a; Poepoe and others, 2007). Harvest management was not based on a specific amount of fish, but on identifying the 

specific times and places that fishing could occur so it would not disrupt 

the basic processes and habitats of important food resources. Understanding when, where, and how to conduct fishing so that it was compatible with 

local resource dynamics required an intimate knowledge of natural rhythms 

and processes associated with these resources (Hui Malama o Moÿomomi, 

1995; Poepoe and others, 2007).

1

 NOAA, National Ocean Service, National Centers for Coastal Ocean Science, Center for Coastal 

Monitoring and Assessment–Biogeography Branch

2

 The Oceanic Institute, Makapuÿu Point/41-202 Kalanianaÿole Hwy, Waimänalo, HI 96795

3

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744

Decline in Coral Reef Fishes in Hawaiÿi 

A shift from purely subsistence to commercial fishing occurred with 

the arrival of the British and American whaling fleets during the early 

1800s (Schug, 2001). After contact with Westerners, a breakdown of the 

traditional system of laws and regulations (kapu) and the demise of the 

watershed (ahupuaÿa) as a management unit led to the virtual elimination of 

traditional Hawaiian fisheries management practices (Smith and Pai, 1992; 

Lowe, 2004).

Over the past 100 years the coastal fisheries in Hawaiÿi have undergone enormous changes (Shomura, 1987, 2004). The early 1900s saw a 

rapid change from subsistence to a cash economy and large increases in the 

commercial landing of fish and other marine resources ensued (Cobb, 1905; 

Schug, 2001). Following statehood, Hawaiÿi saw a rapid growth in tourism, 

an increasingly urban resident population, and the continued development 

of shoreline areas for tourism and recreation. These changes resulted in 

another change in the character of the coastal fisheries—they became dominated by recreational anglers and a greater number of part-time commercial fishers, who curtailed their fishing to take advantage of more lucrative 

economic activities (Friedlander, 2004). 

Commercial landings for a number of important species have shown 

dramatic declines since the 1900s (fig. 2). These declines in fish abundance 

and size, particularly around the more populated areas of the state, are 

likely the cumulative result of years of chronic overfishing (Shomura, 1987; 

Friedlander and DeMartini, 2002). Other factors contributing to the decline 

of inshore fisheries include a growing human population, destruction or 

disturbance to habitat, introduction of new fishing techniques (for examples, 

inexpensive monofilament gill nets, scuba, spear guns, power boats, sonar 

fish finders, GPS), and loss of traditional conservation practices (Brock and 

others, 1985; Lowe, 1996; Friedlander and others, 2003).

Importance to Biodiversity Conservation 

In addition to food and recreation, the wide variety of fishes observed 

in Hawaiian waters are also important to the overall health of the coral reefs 

and ultimately of the islands. Herbivorous parrotfishes and surgeonfishes 

feed on limu (seaweed) and help to prevent the overgrowth of limu on corals. Other fishes, such as weke (goatfishes), feed on shrimp and crabs in 

the sand and transfer that energy back onto the coral reef, thus enhancing 

Figure 1. Hawaiians have traditionally harvested the sea for sustenance and 

cultural purposes. Photos courtesy of Hawaiÿi State Archives.
Km (whitesaddle goatfish, 

Parupeneus porphyreus)

Uhu uliuli (spectacled parrotfish, 

Chlorurus perspicillatus)

Kole (goldring 

surgeonfish, 

Ctenochaetus 

strigosus)

Hnlea lauwili (saddle wrasse, 

Thalassoma duperrey)

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

60

the productivity of the entire ecosystem. Because fishes move energy from 

one part of the ecosystem to another, they are good indicators of large-scale 

ecosystem health.

Conserving coral reef fishes is also important to global marine biodiversity. The faunas of isolated oceanic archipelagos like the Hawaiian 

Islands represent species conservation hotspots that have become increasingly important because of continual losses of biodiversity on coral reefs 

worldwide (DeMartini and Friedlander, 2004). Owing to its geographic 

isolation, the proportion of endemic fishes (found nowhere else on earth) 

in Hawaiÿi is the highest of any known tropical marine ecosystem (Randall 

1996, DeMartini; and Friedlander, 2004). Many of the endemic fish species 

in Hawaiÿi (fig. 3) are some of the most common, like hïnälea lauwili (saddle 

wrasse, Thalassoma duperrey) and kole (goldring surgeonfish, Ctenochaetus

strigosus). Other endemics are valued food fishes like uhu uliuli (spectacled 

parrotfish, Chlorurus perspicillatus) and kümü (whitesaddle goatfish, Parupeneus porphyreus). The loss of endemic species not only affects Hawaiÿi 

but also represents a loss of global genetic diversity.

The purpose of this chapter is to describe the fish and fishery resources 

of the south Molokaÿi reef tract on the basis of information obtained from 

scientific surveys, as well as fisheries catch data and local knowledge.

Methods

Description of Survey Methods

Sampling Stations

Three permanent monitoring sites (Päläÿau, Kamiloloa, Kamalö) have 

been established on the south Molokaÿi reef tract to measure coral growth 

and benthic cover (see Brown, this vol., chap. 6). At each site, one fish sampling station was located at 3-m depth and another at 10-m depth. Surveys 

along four transects, separated by 5-m gaps, were conducted at each station. Additional samples were collected using a stratified random sampling 

approach, where samples were stratified by major habitat type using NOAA’s 

benthic habitat maps (Coyne and others, 2003) (fig. 4). These additional stations were classified as either shallow (≤ 5 m) or deep (10–20 m).

Figure 2. Statewide commercial catch data from Hawaii Department of Land 

and Natural Resources/Division of Aquatic Resources (DLNR/DAR) from 1948 

to 2005. Data for 1990 from market surveys conducted by the United States Fish 

Commission. Adapted from Friedlander and others, 2005. Data sources: Cobb, 

1905; DLNR/DAR, unpublished data. 

Figure 3. Endemic species (found nowhere else on earth) represent some of the 

most common and economically important 

fishes found in Hawaiÿi.

Figure 4. Satellite image showing sampling locations for fishes along the south 

shore of Molokaÿi.

J.E. Randall J.E. Randall J.Watt J.Watt

Fish Surveys

At each station, data were collected on fish, habitat complexity (rugosity), and benthic cover. Fish assemblages were assessed using standard 

underwater visual belt-transect survey methods (fig. 5; V.E. Brock, 1954; 

R.E. Brock, 1982). A scuba diver swam each 25-m by 5-m transect at a 

constant speed and identified, to the lowest possible taxon, all fishes visible 

within 2.5 m to either side of the centerline (125 m2

 transect area). 

Total length (TL) of fish was estimated to the nearest centimeter and 

converted to weight using a standard length-weight conversion. Total length 

was converted to standard length (SL) using the methodology presented by 

FishBase (http://www.fishbase.org, last accessed April 29, 2008). An index 

of relative dominance (percent frequency of occurrence times percent total 

numbers or percent total biomass) was calculated for comparison among taxa. 

Fish taxa were categorized into four trophic categories according to various 

published sources and FishBase. Species diversity was calculated from the 

Shannon-Weaver Diversity Index (Ludwig and Reynolds, 1988): H’=S (pi

 ln 

pi

), where pi is the proportion of all individuals counted that were of species i. 

21°

05'

 N

157° 156° 157 55' °05' W

PlØau

Kaunakakai

Kamiloloa

Kawela

Kamal

Cloud

25

25

25

125

100

150

200

200

Isobath interval 25m 

0 1 2 Miles

0 2 4 Kilometers

Map area

11 Sites

3 Sites 1 Site

8 Sites

2 Sites

Sampling location for fishes

Moi Ð Threadfish

(Polydactylus sexfilis)

1900 1940 1960 1980 2000

0

10

20

30

ÔÔio Ð Bonefish

(Albula spp.)

1900 1940 1960 1980 2000

Year

0

100

50

150

300

260

340

Km ÐWhitesaddle Goatfish

(Parupeneus porphyreus)

1900 1940 1960 1980 2000

0

10

20

200

400

600

Ulua Ð Jacks

(Carangidae)

1900 1940 1960 1980 2000

0

10

30

40

50

20

60

Landings, in thousands of pounds
Coral Reef Fishes and Fisheries of South Molokaÿi

61 

Character of the South Molokaÿi Fringing Reef

Trophic Categories (primary food items and representative taxa)

• Herbivores: plant eaters; primarily parrotfishes and surgeonfishes 

• Invertebrate feeders: feed on crustaceans, mollusks, and other 

 invertebrates; representatives of this group includes goatfishes and 

 trigger fishes 

• Piscivores: primarily fish eaters; jacks including kähala (amberjack, 

 Seriola dumerili) and uku (green jobfish, Aprion virscens)

• Planktivores: plankton feeders; damselfishes and some unicornfishes

Benthic Habitat Assessments

To assess the characteristics of benthic populations (percent cover, species richness and diversity of corals, algal functional groups, and substrate 

cover), high-resolution digital images were taken along transects using a 

digital camera in an underwater housing (fig. 6A). To measure reef-surface 

complexity, a small-link chain (1.3 cm per link) was draped along the full 

length of the centerline of each transect (fig. 6B). Care was taken to ensure 

that the chain followed the contour of all natural fixed surfaces directly below 

the transect centerline. The ratio of distance along the reef-surface profile to 

linear horizontal distance gave an index of spatial relief or rugosity. 

Results

Description of Fish Assemblage

Dominant Species

A total of 99 taxa of fishes were recorded on quantitative transects 

between January 2001 and February 2002. Dominance of fishes on south 

Molokaÿi reefs was calculated on the basis of contribution of each species to 

the total biomass or numerical abundance multiplied by the proportion of samples in which that species occurred (fig. 7). The introduced taÿape (bluestripe 

snapper, Lutjanus kasmira) accounted for 22 percent of the total fish biomass 

observed on transects, but this represented a single school of approximately 

500 individuals observed on only one transect at Kakahaiÿa. The dominant 

species by weight was the bullethead uhu (Bullethead Parrotfish, Chlorurus

sordidus) accounting for 10 percent of total fish biomass and occurring on 

70 percent of all transects. Mäÿiÿiÿi (brown surgeonfish, Acanthurus nigrofuscus) was the most important species by number and second most important by 

weight. It occurred in 74 percent of the transects and accounted for 13 percent 

of the numerical abundance and 5 percent of the total biomass observed. The 

endemic kole (goldring surgeonfish, Ctenochaetus strugosus) was also abundant and occurred on 62 percent of the transects and made up 11 percent of the 

numerical abundance and 6 percent of the total weight. Hïnälea lauwili (saddle 

wrasse, Thalassoma duperrey), also an endemic species, was the most ubiquitous species, appearing on 94 percent of all transects.

Dominant Families

The top five families listed below, plus jacks, accounted for nearly 90 

percent of the total biomass observed on transects along the south Molokaÿi 

reef tract (fig. 8). Surgeonfishes, primarily mäÿiÿiÿi and kole, were the dominant family by both numbers and weight. Several schools of manini (convict 

tang, Acanthurus triostegus) were encountered, ranging in size from 43 to 88 

individuals per school. The bullethead uhu was the most important parrotfish 

species by weight and number, followed by the palenose uhu (Scarus psittacus). Triggerfishes were the third most important family by weight and sixth 

by number. Humuhumuÿeleÿele (black durgon, Melichthys niger), humuhumu 

hiÿu kole (pinktail durgon, Melichthys vidua), humuhumu nukunuku äpuaÿa 

(reef triggerfish, Rhinecanthus rectangulus), and humuhumu lei (lei triggerfish, Sufflamen bursa) were the dominant species in the family. Hïnälea lauwili 

made up most of the wrasses by weight and number, while taÿape accounted 

for most of the snappers, followed by uku (green jobfish, Aprion virscens).

Figure 6. Underwater habitat assessment method on the south Molokaÿi reef. 

A, A diver using an underwater video camera to record benthic cover. B, A diver 

is measuring habitat complexity using a rugosity chain marked with orange flags.

Figure 7. Index of relative dominance (percent frequency of occurrence times 

percent total number or percent total biomass) for the five most dominant species of fish appearing on transects along the south Molokaÿi coast.

Figure 8. Index of relative dominance (percent frequency of occurrence times 

percent total number or percent total biomass) for the five most dominant fish 

families appearing on transects along the south Molokaÿi coast.

Figure 5. Underwater fish survey method. The left image shows a diver recording 

information on fishes along a belt-transect. In the right image, a diver is calibrating length estimates using model fishes.

A B

0 200 400 600 800 1,000

Index of Relative Dominance

Bullethead uhu

Bullethead Parrotfish

Chlorurus sordidus

MØiØiØi

Brown Surgeonfish

Acanthurus nigrofuscus

Kole

Goldring Surgeonfish

Ctenochaetus strigosis

Hnlea lauwili

Saddle Wrasse

Thalassoma duperrey

HumuhumuØeleØele

Black Durgon

Melichthys niger

Number

Biomass

Index of Relative Dominance

Number

Biomass

Snappers

Wrasses

Triggerfishes

Parrotfishes

Surgeonfishes

Lutjanidae

Labridae

Balistidae

Scaridae

Acanthuridae

0 1,000 2,000 3,000
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

62

Dominant Trophic-Category Guilds

Herbivores were by far the most important feeding guild observed, with 

parrotfishes and surgeonfishes dominating this group (fig. 9). Bullethead 

uhu accounted for 23 percent of the total herbivore biomass, followed by 

kole (13 percent) and mäÿiÿiÿi (12 percent). Invertebrate feeders were the 

next most abundant feeding guild by both numbers and weight, with nearly 

70 percent of the biomass consisting of taÿape. Large piscivores were not 

commonly observed in the survey area, but the few large individuals contributed disproportionately to the overall biomass relative to their numbers. 

Kähala (amberjack, Seriola dumerili) made up 69 percent of the total piscivore biomass, followed by uku (19 percent), and the introduced roi (peacock grouper, Chephalopholis argus, 8 percent). In contrast to piscivores, 

small planktivores were numerically common (9 percent of total numerical 

abundance), but accounted for only a small proportion (3 percent) of the 

total fish biomass. 

Native, Endemic, and Introduced Species

A total of 612 native reef and inshore fishes (down to 200 m or ~600 ft) 

have been identified from the Hawaiian Islands (Randall, 2007). Endemic 

species (found nowhere else on earth) accounted for 37 percent of numerical abundance and 29 percent of biomass on the south Molokaÿi reef tract 

(fig. 10). Kole was the most abundant endemic species by both weight and 

number. Manini (convict tang, Acanthurus triostegus sandvicensis) is an 

endemic subspecies that was the second most important endemic species 

by weight and third by number. Hïnälea lauwili was numerically the second most common endemic and, despite its small size, accounted for 

12 percent of total endemic biomass. 

At least 13 species of introduced marine fishes have become established in Hawaiÿi (Eldredge, 1994). Of these species, taÿape, toÿau (blacktail snapper, Lutjanus fulvus), and roi have established viable breeding 

populations in the state. Taÿape and roi have been particularly controversial 

because they have adapted well to Hawaiian waters and are often blamed 

for depletion of desirable species by competition or predation (Friedlander 

and others 2002a; Oda and Parrish, 1981; Randall, 1987). The introduced 

taÿape accounted for 22 percent of the total fish biomass observed on 

transects, but, as noted earlier, represented a single school of 500 individuals. Roi accounted for less than 10 percent of the weight of introduced 

species and was observed on about half of all transects (mean = 0.48 

individuals/125m2

).

Spatial Distribution of Fish Assemblage Characteristics 

The number of species, biomass (weight of fishes), number of individuals, and diversity (an index that combines number of species and number 

of individuals) are important characteristics of the fish assemblage and 

are useful in describing and comparing assemblages among stations. The 

deep transects at Päläÿau and Kakahaiÿa had the highest values for most 

fairly consistent among most stations (fig. 13). The exceptions included the 

deep Kakahaiÿa site, which was dominated by a large school of 500 taÿape, and 

the shallow Päläÿau site, where high numbers of surgeonfishes were recorded. 

Diversity was high among stations, with no apparent trends observed (fig. 14).

Table 1. Ranking of fish assemblage characteristics by survey location. 

[Individual variables were given a ranking score from 1 (highest value) to 9 (lowest value). See figures 

11–14 for actual values. Overall rank was determined by averaging the ranking scores of the four 

individual parameters (1=highest, 9=lowest).]

Location Depth Species Biomass Number Diversity Overall Rank

Päläÿau Deep 1 6 3 1 1

Kakahaiÿa Deep 2 1 1 8 2

Päläÿau Shallow 41 2 2 6 3

Kaunakakai Shallow 41 3 6 2 4

Kamalö Shallow 6 5 4 4 5

Kakahaiÿa Shallow 3 8 7 3 6

Kamiloloa Deep 7 4 5 7 7

Kamalö Deep 9 7 8 5 8

Kamiloloa Shallow 8 9 9 9 9

1

Tied 

Spatial Distribution of Fish Trophic Structure 

Herbivores dominated at all shallow sites, with the exception of 

Kamiloloa (fig. 15). The high percentage of invertebrate feeders there 

(76 percent) was a function of overall low biomass, primarily herbivores. 

Low overall biomass at the shallow location at Kakahaiÿa also resulted in a 

higher proportion of invertebrate feeders (28 percent), primarily a mixture 

of goatfishes, wrasses, and butterflyfishes. Planktivores were not common 

at any of the shallow sites.

Although herbivores dominated at the deep stations at Päläÿau and 

Kamalö, they were not as abundant overall as at shallow stations. At 

Kakahaiÿa, invertebrate feeders (almost exclusively taÿape) account for 

nearly half of the total fish biomass. Kähala and uku were responsible for 

the high predator biomass (42 percent) at this location. The relatively high 

percentage of invertebrate feeders (19 percent), piscivores (19.5 percent), 

and planktivores (16 percent) at Kamiloloa was a function of overall low 

biomass, primarily of herbivores.

Comparison with Locations Statewide

Overall species number, biomass (kg), and species diversity along 

the south Molokaÿi reef tract range near the state median when compared 

with other locations statewide (Rodgers, 2005). Päläÿau, Kaunakakai, and 

Kakahaiÿa possessed values for most fish assemblage characteristics higher 

than the state average, but most fish assemblage characteristics at Kamiloloa 

fish assemblage characteristics (table 1). Diversity was low at Kakahaiÿa, 

owing to the dominance of a school of 500 taÿape at one deep survey site. 

Kamiloloa and Kamalö had the lowest values for most assemblage characteristics at both deep and shallow sites. 

The number of species was comparable across depth strata, with the 

lowest values for both depths occurring at Kamiloloa and Kamalö (fig. 11). 

Biomass was highly variable among, as well as within, stations. The biomass 

at the deep Kakahaiÿa sites (3.60 t/ha) was more than 12 times greater than at 

the adjacent shallow sites (fig. 12). The number of individuals observed was 

Figure 9. Percentages of total biomass and numerical abundance among major 

trophic fish guilds along the south Molokaÿi reef tract. 

Figure 10. Mean percentages of endemic, indigenous, and introduced species 

along the south Molokaÿi reef tract.

Percent

Number

Biomass

0 20 40 60

Planktivores

Piscivores

Invertebrate

feeders 

Herbivores

Number

Biomass

Percent

0 20 40 60 80

Introduced

Endemic

Indigenous
Coral Reef Fishes and Fisheries of South Molokaÿi

63 

Character of the South Molokaÿi Fringing Reef

Figure 11. Mean species richness of fishes (number of 

species) observed on transects along south Molokaÿi reef 

tract. Orange = deep sites (10 to 20 m), blue = shallow 

sites (≤ 5 m).

Figure 12. Mean fish biomass (in tons per hectare) 

observed on transects along south Molokaÿi reef tract. 

Orange = deep sites (10 to 20 m), blue = shallow sites 

(≤ 5 m).

Figure 13. Mean number of individual fish (in thousands 

per hectare) observed on transects along south Molokaÿi 

reef tract. Orange = deep sites (10 to 20 m), blue = shallow sites (≤ 5 m).

Figure 14. Mean species diversity (calculated using the 

Shannon Weaver Diversity Index) observed on transects 

along south Molokaÿi reef tract. Orange = deep sites (10 to 

20 m), blue = shallow sites (≤ 5 m).

and Kamalö were well below the state average (fig. 16). Fish biomass at 

Kakahaiÿa was among the highest observed biomass values in the state, 

while only 5 km away, Kamiloloa had some of the lowest observed biomass 

values statewide.

Fisheries of South Molokaÿi 

The broad reef fringing the southern shore of Molokaÿi is probably the 

most productive reef flat for the harvest of reef fishes and invertebrates in the 

main Hawaiian Islands (Bartram 1992). This extensive area supports a wide 

variety of commercial, subsistence, and recreational fisheries (table 2). The 

only consistent long-term source of data on the coastal fisheries of Hawaiÿi is 

the commercial landings database maintained by the State Division of Aquatic 

Resources (DAR). Commercial landing data between 1998 and 2002 were 

used to examine characteristics of the commercial fishery along the south coast 

of Molokaÿi (reporting areas 310 and 314). The average annual commercial 

harvest during this time period was just less than 10,000 kg (22,000 lbs) This is 

in sharp contrast to the early 1980’s, when approximately 68,000 kg (150,000 

lbs) per year was harvested in the same location (Bartram, 1992). 

Fisheries catch statistics alone are unreliable because of underreporting by commercial fishers and a large, resident recreational and subsistence 

fishing catch that goes unreported. The subsistence reef-fish fishery harvest 

may be as high as 45,000 kg (100,000 lbs) a year, or more than four times 

that of the commercial harvest (Bartram 1992). Information on subsistence 

and recreational components of the fishery was obtained from local knowledge, interviews, and written sources. 

Commercial Landings by Gear Type and Species

Deep-sea handlining, primarily for ÿulaÿula koaÿe (long-tailed red 

snapper, Etelis coruscans, known more commonly by its Japanese name, 

Table 2. Major fishing methods employed along south Molokaÿi reef tract.

[Adapted from Bartram (1992).]

 Gear type Character of fishery General areas fished

Spearing Mostly recreation/subsistence Middle to outer reef flat in areas with 

coral heads and other vertical relief

Throw netting Subsistence Inner reef flat, fishponds, mangrove 

thickets; water depths to waist-deep

Surround netting Commercial/subsistence Harbors, man-made basins, reef channels; water depths >20 ft; targeting 

akule and other schooling species

Drag netting Subsistence Inner reef flat, fishponds

Bullpen netting Commercial Outer reef flat (5 –10 ft depth) in sandy 

and other areas of low relief

Gill netting Commercial/subsistence Reef flat (usually inner and middle); 

low-tide depths of 5 –6 ft; areas of 

low bottom relief

2.13 2.22 1.90 2.35

2.36 1.96 1.45 2.42 2.11

2.13 2.22 1.90 2.35

16.32 7.08 16.32 7.08

2.36 1.96 1.45 2.42 2.11

8.34 13.65 7.97 4.61 7.38

6.46 8.06

8.34 13.65 7.97 4.61 7.38

6.46 8.06

0.30 1.07 0.72 0.48 0.09

0.28 0.38 3.60 0.27

0.30 1.07 0.72 0.48 0.09

0.28 0.38 3.60 0.27

20.00 19.00 13.50 15.50

15.18 13.66 20.25

18.50

18.50

20.00 19.00 13.50 15.50

15.18 13.66 20.25

18.50

18.50

Deep sites (10 to 20 m)

Shallow sites (< 5 m)

Species richness, in number of species

Mean number of individual fish, in thousands per hectare

Mean species diversity, calculated using the Shannon Weaver Diversity Index

Fish biomass, in tons per hectare

0 1 2 Miles

0 2 4 Kilometers
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

64

cent of the total catch (fig. 17B). Parrotfishes accounted for 30 percent of the 

catch by diving, followed by kole (23 percent), heÿe (octopuses, 20 percent), 

ula (spiny lobster, Panulirus spp., 1 percent), and ulua (8 percent) (fig. 18A). 

The trolling catch consisted of ono (wahoo, Acanthocybium solandri, 48 

percent), mahimahi (dolphinfish, Coryphaena hippurus, 31 percent), ulua (9 

percent), aku (skipjack tuna, Katsuwonus pelamis, 7 percent), and kawakawa 

(wavy-back tuna, Euthynnus affinis, 5 percent) (fig. 18B). 

Table 3. Commercial fisheries landings data from Hawaii Department of Land 

and Natural Resources, Division of Aquatic Resources for reporting areas 310 

and 314 for 1998-2002 by gear type. 

[Values are annual means for five-year period.]

Fishing method No. fishers No. trips Kg caught Lbs caught Value ($)

Deep-sea handline 6.8 43.0 1,601 3,523 16,555

Gleaning 5.6 185.2 2,400 5,280 7,923

Inshore handline 10.6 64.8 1,697 3,734 7,339

Net 8.6 41.0 1,910 4,203 5,531

Dive 8.0 48.0 1,113 2,449 5,187

Troll 24.4 90.4 1,170 2,573 3,068

Grand Total 64.0 472.4 9,891 21,763 45,603

Noncommercial Fishing Activities 

The noncommercial harvest of fish and invertebrates is an important 

food source for many Molokaÿi residents. The Governor’s Molokaÿi Subsistence Task Force (1994) found that among Hawaiian families surveyed, 38 

percent of all food was acquired through subsistence, and Molokaÿi residents relied on subsistence to a much greater extent than those of neighboring islands. Interviews conducted with co-op members of the Molokaÿi Ice 

House, Inc., in 1991 indicated average catches of reef fishes and akule to be 

about 90 kg (200 lbs) per person per year (Bartram, 1992). If this average is 

applicable to all Molokaÿi households with subsistence fishermen, then the 

subsistence reef fishery harvest may approach 45,000 kg (100,000 lbs) annually (Bartram, 1992). 

There are a number of subsistence and recreational fishing activities 

that Molokaÿi residents practice along the south shore. Shore-based fishing 

includes throw-netting and pole and line fishing. Locals pole fish from shore 

for päpio (juvenile jacks, Caranx spp.), weke and ÿoama (juvenile goatfishes, 

Mulloidicthys spp.), and halalü (smaller sized akule). Crabbing with lantern 

is still common, but less so than in the past. Throw netters mainly target moi 

(Pacific threadfin, Polydactylus sexfilis), äholehole (Hawaiian flagtail, Kuhlia

spp.), enenue (chubs, Kyphosus spp.), and ÿamaÿama (mullet, Mugil cephalus). In addition to fishes, other marine resources are locally collected for 

food. Crabbing with spotlight or flashlight on rocky shores for ÿaÿama crab 

(Grapsus tenuicrustatus) is still common. Limu (seaweed) species such as 

limu kohu (Asparogopsis taxiformis) and manauea (Gracilaria coronopofolia) are harvested along the beach and upon inshore flats. Collecting of opihi 

(limpets, Cellana spp.) also occurs in rocky shoreline habitats toward the 

east end of the south shore. 

onaga), ÿulaÿula (red snapper, Etelis carbunculus, known commonly as ehu), 

and ÿöpakapaka (pink snapper, Pristipomoides filamentosus), constituted 16 

percent of the total weight landed and 36 percent of the value of catch from 

1999 to 2002 (table 3). This was followed in total value (1998 to 2002) by 

gleaning for limu manauea, also known as ogo (seaweed, Gracilaria spp.) 

(3,837 kg/8,441 lbs., $24,102) and ÿopihi (limpets, Cellana spp.) (558 

kg/1229 lbs., $3,543), which are the major species taken using this method. 

The inshore handline catch was dominated by akule (bigeye scad, Selar

crumenophthalmus) (80 percent by weight and 69 percent by value) with 

the remaining portion consisting of various reef fishes and invertebrates 

(fig.17A). Netting caught a diverse assemblage of fishes, with weke (goatfishes, Mulloidicthys spp.) and ulua (jacks, Carganidae) making up 73 per- Figure 17. Catch of 

major species off south 

Molokaÿi between 1998 

and 2002 by (A) shallow handlining and (B) 

netting. Numbers are 

weight, value, and percentage of total weight.

Shallow sites

(< 5 m)

Deep sites

(10 to 20 m)

0 1 2 Miles

0 2 4 Kilometers

Fish biomass by 

trophic categories

Shallow sites

(< 5 m)

Deep sites

(10 to 20 m)

Herbivores

Invertebrates

Piscivores

Planktivores

Pl`au

Kaunakakai

Kamiloloa

0

0.2

0.6

Rank among statewide locations0.8

0.4

1.0

PlØau Kaunakakai Kamiloloa KakahaiØa Kamal

Species

Biomass

Diversity

State median

ØØ

Soldierfish

635 lbs., $2,199

4%

Uku

Green jobfish

486 lbs., $1,169

3%

Ulua

Misc. jacks

312 lbs., $374

2%

HeØe

Octopus

789 lbs., $2,368

5%

Laenihi

Razorfish

841 lbs., $3,605

5%

Weke nono

PflugerÕs goatfish

1%

Akule

Bigeye scad

13,369 lbs., $23,817

90%

A

B

Weke

Goatfish

7,598 lbs., $5,742

40%

Moi

Pacific threadfin

710 lbs., $1,255

4%

ØAmaØama

Rock crab

572 lbs., $1,627

3%

ØØio

Bonefish

1,180 lbs., $48

6%

Akule

Bigeye scad

1,181 lbs., $1,762

6%

Manini

Convict tang

1,441 lbs., $1836

8%

Ulua

Misc. jacks

6,239 lbs., $9,880

33%

Figure 15. Pie charts showing proportions of fish biomass by trophic categories observed at five shallow (depth ≤ 5m, 

top row) and four deep (depth >5m, 

bottom row) locations along the south 

Molokaÿi reef tract.

Figure 16. Rank for fish-assemblage characteristics along the south Molokaÿi 

reef tract (see figs. 11, 12, 14 for details) relative to locations statewide (data 

from Rodgers, 2005). A rank of 0.5 equals the State median.
Coral Reef Fishes and Fisheries of South Molokaÿi

65 

Character of the South Molokaÿi Fringing Reef

Offshore, the extensive reef flat off the south shore of Molokaÿi supports 

a variety of fishing activities. Diving for kole, heÿe, uhu, manini, kümü, ÿüÿü 

(soldierfishes and squirrelfishes, Holocentridae), uma-uma lei (orangespine 

unicornfish, Naso literatus), kala (bluespine unicornfish, Naso unicornis), 

palani (eyestripe surgeonfish, Acanthurus dussumieri), pualu (yellowfin 

surgeonfish, A. xanthopterus), moana ukali (goatfish, Parupeneus spp.), 

ula and ula papa (slipper lobster, Parribacus antarcticus) occurs often. Gill 

netting and surround netting are common methods as well. Gill netting is 

most frequent along the inshore edge of the fringing reef. The most common species caught are weke, ÿamaÿama, päpio, öÿio (bonefish, Albula spp.), 

kala, and enenue. Surround netting is frequently used for particular species 

like akule that are usually found in deeper waters such as channels, bays, and 

outside the fringing reef. Trolling and boat-based spinning for päpio along 

the reef flat and outer reef crest is also a popular activity among residents. 

Other activities in this area include line fishing in sand channels for oÿio and 

outside the fringing reef for weke ÿula (Mulloidicthys vanicolensis), päpio, 

uku and laenihi or nabeta (razorfish, Iniistius spp.). Netting for kona crab 

(Ranina ranina) is also conducted in these sandy areas. Farthest off the south 

shore, trolling for pelagic species such as ono and mahi mahi and bottomfishing for onaga, ehu, and ÿöpakapaka, are performed by both commercial 

and recreational fishers.

The importance of the fishery resources to the local community is very 

great. The harvest of marine resources still serves as a considerable supplement to locals’ incomes and is a significant part of local culture. Lüÿau for 

graduations and birthdays are still events that ‘necessitate’ many customary foods, such as ÿopihi. In the absence of large supermarkets, marine 

resources are still very much a large part of the local Molokaÿi diet, especially in comparison to other islands such as Oÿahu. 

South Molokaÿi Fishponds 

Ancient Hawaiian fishponds (loko iÿa), dating back to the 13th century, are unique in the world and have been recognized as the most extensive and productive systems of early aquaculture. It has been estimated 

that close to 500 fishponds producing more than 900,000 kg (2,000,000 

lbs) of fish each year were in use before Western contact in 1778 (Summers, 1971). This phenomenal feat of engineering is evidenced by its 

longevity, with many of the original ponds still in existence. As a highly 

efficient form of resource management, fishponds provided a sustainable 

protein source that could be harvested during rough oceanic conditions 

and by those no longer able to fish. Representing not only survival, the 

fishponds had an inseparable relationship to religious beliefs and political 

practices.

The loko kuapä shoreline pond enclosed by a rock wall, is the most 

familiar of the four basic types of fishponds. The multipurpose sluice 

gates (mäkahä) evolved into a system of gates and channels that regulated 

the flow of water, allowed entrance to fingerlings, and facilitated harvest. 

Within these fish farms, constructed of basalt and coral, numerous fish 

species and limu (seaweed) were raised and harvested. The most common 

included awa (milkfish, Chanos chanos), ÿamaÿama, awaÿaua (ladyfish, 

Elops hawaiensis), äholehole, and ulua and päpio, while some species, 

such as the moi, kümü and öÿio, were reserved for the aliÿi (royality).

This extensive modification of the coastline altered both downslope 

and alongshore sediment transport while shifting nearshore current patterns (Roberts, 2001; Ogston and others, this vol., chap. 20). Fishpond 

cultivation rapidly declined with increasing urbanization and coastal 

development, changes in land tenure, loss of knowledge of pond management practices, and decline in the native population (Hlawati, 2002). 

Because these events affected Molokaÿi less than the other Hawaiian 

Islands, more intact fishponds remained there. By 1930, however, grazing and agricultural activities on Molokaÿi contributed to their abandonment as the majority filled with sediment. Today, remnants of more than 

70 ponds remain on the south shore of the island of Molokaÿi (fig. 19). 

Keawanui and ÿUalapuÿe fishponds have been federally recognized as 

national historical landmarks. A wetland bird sanctuary has been created 

with lands surrounding and including the freshwater Kakahaiÿa fishpond. 

Some ponds have been restored and are currently being used by local 

residents (Wyban, 1992).

Strong advocates of preservation and restoration of the remaining fishponds have renewed community interest in complementing a subsistence 

lifestyle. These historic structures provide a significant archeological, 

spiritual, and cultural link with the past. They hold important significance 

as an educational resource, in preserving cultural values, and in stimulating economic opportunities within the community. 

Traditional Fisheries Management

There has been a renaissance of traditional community-based fisheries management throughout the Pacific, and rediscovery of these traditional 

techniques offers great promise for improving the management of marine 

fisheries. In coastal communities, fishermen (and women) combine empirical information on fish behavior, the physical environment, and fish habitats 

to determine when, where, and how to fish (Ruddle, 1994). This information 

in some communities has been passed down over many generations, and 

traditional knowledge can play an important role in designing effective fishery management systems (Johannes, 1997). Many of the management tools 

used today, such as closed areas, closed seasons, size limits, and restricted 

access, were used by Pacific Islanders centuries ago to manage their fisheries 

resources (Johannes, 1978) 

A number of communities throughout Hawaiÿi are currently strengthening local influence and accountability for the health and long-term sustainability of their marine resources through revitalization of local traditions and 

resource knowledge (Friedlander and Brown, 2004). The community in the 

Hoÿolehua Hawaiian Homesteads on the island of Molokaÿi exemplifies this 

trend (Poepoe and others, 2007). Community-sanctioned norms for fishing conduct are being reinforced through continual feedback based on local 

resource monitoring, education, and peer pressure (Friedlander and others, 

2002b; Poepoe and others, 2007). 

The most effective means to elicit proper conduct of fishing is by 

educating young people in the community to understand that they have 

responsibilities, as well as rights, for marine resource use. The continuation 

of traditional Hawaiian practices in and around Moÿomomi Bay (fig. 20) 

helps to maintain social and cultural identity and provides reinforcement of 

values shared by the Hoÿolehua community (Poepoe and others, 2007). The 

repetition of subsistence fishing activities is one of the ways that knowledge, 

Figure 19. Aerial views of south Molokaÿi fishponds. Houses give indication of scale. 

Erica Muse

Figure 18. Catch of 

major species off south 

Molokaÿi between 1998 

and 2002 by (A) diving 

and (B) trolling. Numbers are weight, value, 

and percentage of total 

weight.

M

Bigeye emperor

352 lbs., $521

4%

Enenue

Rudderfish

431 lbs., $584

Ulua 4%

Misc. jacks

758 lbs., $842

8%

Ula

Spiny lobster

1,094 lbs. $10,011

11%

HeØe

Octopus

2,000 lbs., $3,889

20%

Kole

Goldring surgeonfish

2,251 lbs., $3,079

23%

Uhu

Misc. parrotfish

3,023 lbs., $4,665

30%

A

Aku

Skipjack tuna

812 lbs., $106

7%

Ulua

Misc. jacks

1,011 lbs., $252

5%

Mahimahi

Dolphinfish

3,526 lbs., $3,942

31%

Kawakawa

Bonito

611 lbs., $623

5%

Ono

Wahoo

5,590 lbs., $9,.671

48%

B
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

66

values, and identity are transferred to succeeding generations. Cultural survival is thus entwined with resource conservation. Each community will have 

to develop management strategies that are compatible with their own unique 

situation. Environment, history, and resources will all dictate what type of 

management regime is best suited for each individual community.

Summary

The fish assemblage of the south Molokaÿi reef tract provides food, 

recreation, commerce, and cultural identity to the local human population. In 

addition, it is an integral component of the south Molokaÿi marine environment, helping to maintain a healthy ecosystem. The large number of endemic 

species on these reefs attests to the unique composition of the ecosystem and 

its importance in global marine biodiversity. 

The broad reef fringing the south shore of Molokaÿi is probably the 

most productive reef flat in the main Hawaiian Islands for the harvest of reef 

fishes and invertebrates. The fish assemblage represents a diverse fauna that 

Figure 20. Local traditions and 

resource knowledge help to guide 

fishing practices in the Hoÿolehua 

community on the north shore of 

Molokaÿi

is healthier than those in more heavily exploited urban areas around the 

State, but it has shown signs of decline in overall health and quality over 

time. The average annual commercial catch has declined from a peak of 

68,000kg (150,000 lbs) per year in the early 1980s to roughly 10,000 kg 

(22,000 lbs) today. In addition, the men and women of the fishing community have expressed concern about the declining size and abundance 

of several important resource species such as kümü and moana kale (blue 

goatfish, Parupeneus cyclostomus). There is a great need for more localized control of these resources in order to rehabilitate existing stocks and 

ensure sustainable harvests in the future. The revitalization of some of the 

existing fish ponds could help to provide additional food resources for 

the community as well as revive local culture. To be effective, fisheries 

management must function within a specific local context. Communities 

and their individual members should exercise control over local inshore 

marine resource use and be accountable for the health and productivity of 

local resources (Poepoe and others, 2007). Wise use and conservation are 

essential in order to provide improved utilization and appreciation of these 

resources for future generations. 
Character of the South Molokaÿi

CHAPTER 8

 Fringing Reef

The Seaweed and Seagrass Communities of Molokaÿi

Jennifer E. Smith1, 2, Heather L. Spalding2, Ryan Okano2, and Celia M. Smith2

The marine environment around the island of Molokaÿi has many 

different types of habitats and unique marine communities. 

The focus of this chapter is to describe the marine plant communities around the island, with a general focus on those off the south 

shore. Both seagrasses (true marine plants) and the seaweeds (which 

are not considered to be “true” plants, see below) will be discussed in 

detail. Seaweeds or marine algae are ocean-dwelling photosynthetic 

organisms that belong to several diverse evolutionary lines, the most 

prominent being the green algae (chlorophytes), the brown algae (phaeophytes), and the red algae (rhodophytes). There are many other groups 

of photosynthetic organisms in the ocean, including photosynthetic 

bacteria (cyanobacteria), but they are not the focus of this chapter. 

The Hawaiian Islands support more than 500 species of marine 

algae (Abbott, 1999; Abbott and Huisman, 2004), and species new 

to science are still being discovered on a regular basis. To Hawaiians the word “limu” is used to describe seaweeds, many of which are 

important for food and/or cultural activities. The term seaweed specifically refers to photosynthetic marine organisms that are members of 

a disparate kingdom of organisms known as kingdom Protista. These 

organisms are not considered to be true plants, even though the evolutionary ancestor of land plants was a green alga. They are generally 

simpler in organization, lacking typical plant organs such as leaves, 

roots and stems, and they do not reproduce by making fruits and seeds. 

Because seaweeds are immersed in water, they generally do not need 

specialized organs to take up water and nutrients (although some seaweeds do have specialized structures). Instead, the entire algal thallus (body) can photosynthesize and absorb nutrients from the water.

There are true marine plants, known as seagrasses, which also live 

completely immersed in seawater and are often quite abundant in shallow back-reef environments on coral reefs. Seagrasses can form dense 

meadows, which can be important nursery habitats for a number of 

commercially important fish and invertebrate species (Mumby and 

others, 2004; Dorenbosch and others, 2005; Gratwicke and Speight, 

2005; Valentine and Heck 2005). The following sections will focus 

1

 National Center for Ecological Analysis and Synthesis, University of California Santa Barbara, 

735 State St., Suite 331, Santa Barbara, CA 93101; current address: Scripps Institution of Oceanography, 

University of California San Diego, 9500 Gilman Dr., Mail Code 0202, La Jolla, CA 9208

2

 University of Hawaiÿi, Department of Botany, 3190 Maile Way, Honolulu, HI 96822.

on the benthic (bottom dwelling) communities around Molokaÿi, with 

an emphasis on seaweeds, seagrasses, and some cyanobacteria. 

A number of physical and biological factors influence the distribution and abundance of seaweeds and seagrasses on tropical reefs (Lobban 

and Harrison, 1994). Physical factors include substrate type, temperature, 

salinity, light levels, hydrodynamics or wave exposure, nutrient availability (nitrogen and phosphorous ), sedimentation, and others. Some of 

the most important biological factors affecting the distribution and abundance of seaweed communities on coral reefs are predation or grazing 

by herbivores (both fish and invertebrates that eat seaweed) and competition with other benthic organisms (other seaweeds, seagrasses, and corals) 

(Carpenter, 1986; Hughes and others, 1999b; McCook, 1999; Miller and 

others, 1999; Smith and others, 2001; Stimson and others, 2001; Thacker 

and others, 2001; Jompa and McCook, 2002; Diaz-Pulido and McCook, 

2003; Jompa and McCook, 2003; McClanahan and others, 2003). 

One of the most intriguing areas of research in coral reef ecology has 

been to understand how and why corals dominate the landscape in tropical marine environments while plants dominate most other ecosystems 

on the planet (Odum and Odum, 1955). Reef ecologists generally believe 

that a combination of physical and biological factors favor the dominance 

of corals over algae in healthy or pristine reef-slope areas. Humans, however, can have dramatic effects on coral reef communities by altering any 

of the above factors, often shifting the competitive edge away from the 

corals and in favor of faster growing algae (Hughes and others, 1999b; 

Hughes and Connell, 1999; Jackson and others, 2001; Aronson and others, 2003; Hughes and others, 2003; Pandolfi and others, 2003a, b; Bellwood and others, 2004; Pandolfi and others, 2005). There are, of course, 

always exceptions, and it is worth mentioning that in the remote Northwestern Hawaiian Islands seaweeds are often more abundant than corals 

(Vroom and others, 2005) in the absence of any chronic human disturbance 

(this is generally believed to be the result of low water temperature).

From a biological perspective, dense populations of grazers (for 

example, surgeonfish, parrotfish, and sea urchins) on healthy reefs help 

keep algae cropped to low levels (Carpenter, 1986; McCook, 1999; Miller 

and others, 1999; Smith and others, 2001; Thacker and others, 2001; Belliveau and Paul, 2002; McClanahan and others, 2003). Generally, on 

healthy or pristine reefs where fish and invertebrate are abundant, the 

algae that are present are either very small turfs that have fast growth 

rates or are either chemically and/or physically defended against grazers 

(Carpenter, 1986). Experimental evidence suggests that the loss of gazers 

from reefs through overfishing or from disease outbreaks can allow algae 

to begin overgrowing corals and eventually cause a phase shift in which 

long-lived, slow-growing corals are replaced by fleshy, fast-growing seaweeds (Hughes and others, 1987; Hughes, 1994; Hughes and others, 1999b; 

McCook, 1999; Jackson and others, 2001; Bellwood and others, 2004). 

From a physical perspective, the availability of nutrients, primarily 

nitrogen and phosphorus, can also influence the growth rates and abundance of algae on reefs (Smith and others, 1981; Bell, 1991; Bell, 1992; 

Lapointe, 1997; Smith and others, 2001). Most reefs have developed in 

the clear, warm, and nutrient-deficient waters of the tropics. The coral 

Figure 1. Map of Molokaÿi showing the algae 

sampling sites surveyed between 1999 and 

2003. See text for further details.

Kalaupapa

Hlawa Point

PlØau

Øìlio Point

LØau Point 

0

0

2 4 Miles

2 4 Kilometers Algae sampling location

Pphaku

Kaunakakai Harbor

Kalaekapu

Hlawa

Kaunakakai

Kamiloloa

Kamal

PkoØo

Hale o Lono
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

68

Figure 2. The brown alga Endarchne binghamiae is commonly found growing on intertidal boulders in the Hawaiian 

Islands. The length of this species ranges from 5 to 20 cm 

(2 to 7.8 in).

Figure 3. The filamentous brown algae Hinksia mitchelleae, which is usually found growing in small clumps either 

on boulders or shallow basalt benches in the intertidal 

zone. Hinksia is unsually no bigger than 3 cm (1.2 in) in 

diameter.

Figure 4. A typical Hawaiian intertidal boulder field showing the purple shingle urchin grazing on algae. The pink 

crusts are crustose coralline algae (CCA) and the small 

brown turfs are the filamentous brown alga Hinksia. 

Figure 5. The brown alga Turbinaria ornata is commonly 

found growing in tidepools and in other environments of 

high wave energy. Individuals of Turbinaria can range from 

1 to 30 cm (0.4 to 12 in) in length.

organisms and their symbiotic zooxanthellae (microscopic single-celled 

algae that live inside the coral tissue) are extremely efficient at using 

nutrients and are able to survive with very little external nutrient input. 

Reef algae and seagrasses, on the other hand, need external sources of 

nitrogen and phosphorus to survive. These organisms are able to absorb 

nutrients from the water column, sand, or sediment. On healthy reefs 

the low input of nutrients generally limits the growth of algae. In some 

cases where nutrient inputs are high (for example, from sewage outfalls, terrestrial runoff containing agricultural fertilizers, landscaping, 

river runoff, sedimentation), algae may be able to overgrow corals and 

become dominant, even occasionally forming large blooms, as can be 

seen on some of the shallow reefs of Maui (Smith and others, 2005). 

The introduction of exotic or nonnative species has caused numerous problems in the Hawaiian Islands, both on land and in the sea. These 

invasive species are seen as one of the largest threats to global biodiversity, 

because they are often able to outcompete native species (Vitousek and 

others, 1997) and even cause extinction events. In Hawaiÿi, this is particularly a problem because there are so many endemic species (species 

which are only found in Hawaiÿi) that are highly specialized to specific 

habitats. Several species of marine algae have been introduced to Hawaiian reefs, primarily for experimental aquaculture (Smith and others, 2002). 

Unfortunately, many of these species have spread from their initial points 

of introduction (mainly on Oÿahu) and have become quite abundant on 

the shallow reef-flat environments around the Hawaiian Islands, including Molokaÿi (Smith and others, 2002; Smith and others, 2004; Conklin 

and Smith, 2005; Smith and others, 2005). Some of these exotic species 

are a particular concern because they have been shown to cause reductions 

in species diversity and coral cover on Oÿahu (Smith and others, 2004). 

Different Algal Communities and Functional Groups 

Marine algae can be classified in several different ways. The most general classification for reef algae includes three functional groups: macroalgae, 

turf algae, and crustose coralline algae (Littler, 1980; Littler and others, 1991). 

Macroalgae are generally taller than a few centimeters and include hundreds of 

species in the Hawaiian Islands. These species can be either fleshy or calcified 

(containing calcium carbonate). Calcified species usually have a whitish hue 

and may have a chalky texture. Turf algae represent a community of very small 

(less than a few centimeters tall), usually filamentous algae including hundreds 

of species and even juvenile stages of larger algae. Algal turfs are an important 

food source for many reef fish species. Finally, crustose coralline algae (CCA), 

or heavily calcified pink crusts, look like a nonliving pink pavement. The 

CCA are incredibly important for reef building, as they are known to cement 

corals and other reef components together. They are also extremely strong 

and can withstand very large wave stresses. CCA commonly form an “algal 

ridge” along the reef crest, which is the portion of the reef where waves break 

(Macintyre and others, 2001). It is the CCA and the algal ridge, along with the 

corals themselves, that build reef structure. CCA also play an important role in 

the development of reefs because they are the primary substratum upon which 

many coral larvae settle (Harrington and others, 2004). Essentially, the CCA 

not only help to build reefs but they facilitate the persistence of corals as well. 

Algal Communities of Molokaÿi 

The remaining sections of this chapter will discuss the benthic marine 

algal communities around Molokaÿi, ranging from Hälawa Bay in the east, 

around the south shore, and ending at Päpöhaku Beach Park in the west 

(fig.1). The north shore of the island is generally very difficult to access and 

has not been surveyed or assessed by the authors, so it will not be discussed 

here. Brief descriptions of the marine communities from each of the other 

coastal regions (west, south, and eastern coasts) will be accompanied by 

photographs of some of the dominant algal species or groups found in these 

respective areas. The majority of the data presented here come from two years 

of shallow-water surveys that were conducted by snorkel, and only a limited 

amount of data is available to describe the offshore reef-slope communities 

from Kamalö, Kamiloloa, and Päläÿau (fig. 1). Lastly, the deep-water algal 

communities off the south shore will be discussed briefly, using the results of 

some recent (2004) submersible and ROV (remotely operated vehicle) surveys. 

The Eastern Shore 

The eastern shore of Molokaÿi is characterized by basalt points, bays 

with sandy beaches, and narrow, shallow reef flats. The shoreline is part 

of the windward shore and is regularly exposed to waves and wind. At 

Hälawa Bay, large boulders make up much of the intertidal zone, and these 

boulders are colonized by the brown algae Endarachne binghamiae (fig. 

2) and Hincksia mitchelliae (fig. 3). The boulders also provide substrate 

for CCA and many invertebrates such as the shingle urchin, Colobocentrotus atratus or häÿukeÿuke (fig. 4) and several species of limpets (ÿopihi) 

are common. The basalt benches and tidepools common along most of the 

eastern shore are generally dominated by several species of large brown 

algae, including Turbinaria ornata (fig. 5) and Sargassum (limu kala) 

(fig. 6), as well as CCA. In the shallow subtidal environments many spe-
The Seaweed and Seagrass Communities of Molokaÿi

69 

Character of the South Molokaÿi Fringing Reef

Figure 8. The red alga Melanamansia glomerata is a very 

common component of shallow and deeper reef communities in Hawaiÿi. Many other species of algae commonly 

grow on top of Melanamansia, making it sometimes difficult to identify in the field. Individuals of Melanamansia

are usually around 10 cm (4 in) tall.

Figure 9. Portieria hornemannii is a very common species 

of red alga found on shallow reef flats and deeper reefslope environments in Hawaiÿi. Portieria is a delicate alga 

usually less than 10 cm (4 in) tall. 

Figure 10. The calcified red alga Dichotomaria marginata is 

quite common in Hawaiÿi, where it is usually found growing 

on steep to vertical surfaces such as basalt walls or slopes. 

Individuals are usually less than 15 cm (5.9 in) tall.

Figure 11. Galaxaura species are commonly found on the 

exposed windward coast of Molokaÿi.

Figure 12. Tricleocarpa is a branching calcified red alga 

that is common in the shallow subtidal environments 

around Hawaiÿi. This alga commonly grows on hard substrates in subtidal areas on both the east and west shores 

of Molokaÿi. Trichleocarpa is usually more heavily calcified 

than Galaxaura and is usually no taller than 15 cm (5.9 in).

Figure 13. The calcified green alga Halimeda discoidea is 

common in the shallow reef-flat environments on the windward shore of Molokaÿi. This species can also be found on 

the outer portions of the extensive shallow reef flats off the 

south shore. H. dicoidea grows in small rosettes reaching 

heights of 5–10 cm (2–4 in).

Figure 6. One of the three species of Sargassum (limu 

kala) commonly found in Hawaiian intertidal environments, 

especially in tidepools or on shallow reef flats. Sargassum

species are some of the largest seaweeds in Hawaii (on 

average 20 cm/8 in tall) and can reach heights of nearly 

1m (3 ft).

Figure 7. The red alga Asparagopsis taxiformis (limu kohu) is 

common in shallow reef-flat environments, exposed benches, 

and subtidal reef environments. This alga is also known as 

limu kohu and is regularly eaten after being fermented to 

prevent poisoning from the high concentrations of ammonia 

contained in the alga. Asparagopsis is a delicate alga that is 

usually less than 10 cm (4 in) tall.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

70

cies of red algae are common, including Asparagopsis taxiformis (limu 

kohu) (fig. 7), Melanamansia glomerata (fig. 8), and Portieria hornemannii (fig. 9), as well as some calcified red algae, including Dichotomaria

spp. (fig. 10), Goalaxaura spp. (fig. 11), and Tricleocarpa (fig. 12). Numerous other species of algae are present, either out in the open such as Halimeda discoidea (fig. 13) or in the cracks and crevices, hidden away from 

the mouths of herbivorous fish and sea urchins. Algae present include, 

among many others, species of Dictyosphaeria (fig. 14), Codium (fig. 15), 

Dictyota (fig. 16), and of course turf algae (fig. 17) and CCA (fig. 18). 

The South Shore 

The Reef Flat

The south shore is characterized by very wide (100s of meters) shallow (0 –2 m, 0 – 6 ft) reef flats (Storlazzi and others, this vol., chap. 3), 

which provide an extensive amount of habitat for marine algae. Most 

of these shallow reef flats are composed of a soft sandy or muddy bottom (Cochran, this vol., chap. 9) and will only support growth of algae 

or seagrasses that can anchor themselves into the soft substrata. Most 

notably, the endemic seagrass Halophila hawaiiana is quite abundant 

(fig. 19). Interestingly, H. hawaiiana exhibits two different growth forms 

on Molokaÿi. The common morphology is shown in figure 20, where the 

leaves of the seagrass are oval; the other growth form is shown in figure 

21, where the leaves are more linear in shape. It is unclear if these two 

growth forms are different genetic entities (different species) or if they 

are just responding to differences in environmental conditions. Nonetheless, it is interesting that they are both found growing side by side on 

Molokaÿi. Halophila decipiens looks very similar to the oval morphotype 

of H. hawaiiana and may also be present on Molokaÿi. These seagrasses 

are particularly important on shallow reef-flat and/or lagoonal areas for a 

number of reasons. Primarily, they help to stabilize sediments by anchoring their roots into the soft bottom, inhibiting erosion and preventing 

sediment transport onto the fore reef, where it could smother the coral 

community and associated fauna. Seagrasses are also an important food 

source for many organisms, including sea turtles. Furthermore, seagrasses are known to provide habitat and shelter for numerous species 

of fishes and invertebrates during different stages of their life cycles. 

In addition to the seagrasses, several species of macroalgae are found 

on these shallow reef flats growing on small pieces of reef rubble or other 

occasional areas of hard bottom, including some weedy nonindigenous 

invasive species. The invasive red alga Acanthophora spicifera (fig. 22) is 

quite common along the entire south coast and can be found growing from 

shore all the way out to the reef crest. This alga can be seen overgrowing or smothering many species of coral (fig. 22). Another invasive red 

alga, Hypnea musciformis (fig. 23), can also be found on the south shore 

but is most common near the town of Kaunakakai. Hypnea grows abundantly on the shallow reef flats and sometimes washes ashore (fig.

24). 

This alga forms massive blooms on the island of Maui, likely as the 

result of increased nutrient input from shore (Smith and others, 2002). 

Several species of Gracilaria can be found on Molokaÿi’s south shore. 

Gracilaria coronopifolia (limu manauea) (fig. 25) grows in large clumps 

on the shallow reef flats on Molokaÿi’s south shore. This species is regularly harvested for food and can be found in the common raw fish salad 

known as “poke.” Gracilaria parvispora (long ogo) (fig. 26) can occasionally be found growing attached to small rocks close to shore on the reef 

flats, but this species is also harvested regularly and is uncommon in the 

wild. Both ogo and manauea are grown on Molokaÿi in fishponds or other 

areas for human consumption. The invasive species Gracilaria salicornia (gorilla ogo) (fig. 27) is also present on the shallow reef flats of the 

south shore. This species was introduced to Molokaÿi in the 1980s from 

Oÿahu (the Oÿahu population originated from a population found in Hilo, 

which was most likely accidentally introduced by ships from the Philippines before the 1900s, during the whaling era). G. salicornia was only 

found at a few locations on Molokaÿi—specifically Pükoÿo fishpond and 

in front of the canoe club at Kaunakakai Harbor—but it may be spreading. This species is invasive on Oÿahu, where it is one of the most dominant species in Waikïkï and in Käneÿohe Bay (Smith and others, 2004).

Several other species of red algae can be found on the shallow reef 

flats of Molokaÿi, but Spyridia filamentosa (fig. 28) is probably among 

the most common. This species is easily recognized because the numerous small hairs that cover the thallus usually trap sediment, giving it a 

dirty appearance. Other common species include members of the brown 

Figure 14. The green alga Dictyosphaeria cavernosa, 

otherwise known as “bubble algae,” is common on Hawaiian reefs in both the shallow subtidal and deeper reef-slope 

environments. It is often seen growing between corals as 

shown in this picture, where it is growing under a colony of 

the coral Montipora capitata. Dictyosphaeria has very large 

cells, which are often visible to the naked eye. 

Figure 15. The green alga Codium edule commonly grows 

on the bottom in shallow subtidal environments. There are 

numerous other species of Codium (limu waiwai’ole) in 

Hawaiÿi, some of which are commonly consumed in salads. 

Figure 16. Several species of Dictyota are common in 

Hawaiÿi, including the two pictured here. Dictyota sandwichensis is often an iridescent blue-green to yellow color, 

whereas Dictyota acutiloba is more of a caramel-brown 

color. The various species of Dictyota can vary in size from 

less than a centimeter to several centimeters in length. 

Figure 17. The mixed species communities of small 

filamentous algae (less than 2 cm/0.8 in tall), otherwise 

known as turf algae, are a very common and important 

component of reef ecosystems. These communities serve 

as the primary food source for numerous species of grazing fish and sea urchins. The turfs have extremely fast 

growth rates and so are considered to be an extremely 

efficient energy source for the reef community.
The Seaweed and Seagrass Communities of Molokaÿi

71 

Character of the South Molokaÿi Fringing Reef

Figure 18. Crustose coralline algae (CCA) are pictured 

here in various shades of pink. CCA are heavily calcified 

red algae that help to cement and bind reefs together. 

These organisms are quite common in all reef zones in 

Hawaiÿi and include around 25 species, although very little 

is known about their taxonomy. CCA look more similar to 

pink cement or rock than they do to other algae. 

Figure 19. A meadow of the endemic seagrass Halophila 

hawaiiana growing along the south shore of Molokaÿi. 

Leaves of Halophila are usually less than 5 cm (2 in) in 

height, but meadows can cover several hundred square 

meters. 

Figure 20. The common growth form of the endemic 

Hawaiian seagrass Halophila hawaiiana (note the oval or 

paddle-shaped leaves) growing on the shallow reef flats of 

south Molokaÿi. 

Figure 21. The more elongated and linear-shaped leaves 

of a different growth form of Halophila hawaiiana can also 

be found growing on the shallow reef flats of Molokaÿi.

Figure 22. The invasive red alga Acanthophora spicifera

can be seen in the foreground of this picture, where it is 

growing over the coral Porites lobata. This alga is one of 

the most common species of algae on the south shore of 

Molokaÿi and can be found from Hale O Lono Harbor in the 

west all the way to Kalaekapu in the east. A. spicifera was 

first introduced to Oÿahu in the 1950s by a heavily fouled 

barge that originated from Guam. A. spicifera is usually 

less than 15 cm (5.9 in) tall, with small spike-like side 

branches. 

Figure 23. The invasive red alga Hypnea musciformis was 

first introduced to Oÿahu in the 1970s for experimental 

aquaculture and has now spread to all of the main Hawaiian Islands except the Big Island of Hawaiÿi. It is quite 

common on the south shore of Molokaÿi, and it forms large 

blooms on the island of Maui.

Figure 24. The red alga Hypnea musciformis washed up on beaches. A, A small band of H. musciformis lines the beach 

of Kamiloloa at low tide. B, Thousands of pounds of this alga wash onto the beaches every day on Maui (photo taken at 

north Kïhei). 

Ron Philips

A B
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

72

Figure 25. Gracilaria coronopifolia, or limu manauea, 

growing on the shallow reef flat along the south shore of 

Molokaÿi. This alga has thin branches (1 mm), but an individual can grow to the size of a bowling ball.

Figure 26. Gracilaria parvispora, or long ogo, grows on 

the shallow reef flat along the south shore of Molokaÿi. 

This species is also commercially cultivated on Molokaÿi 

in fishponds and other areas for food (limu poke) and is 

also used as garnish for meals in restaurants. This species 

is usually very long and wispy, with branches growing to 

50cm (19.6 in) in length. 

Figure 27. The invasive red alga Gracilaria salicornia,

or gorilla ogo, growing near Kaunankakai Harbor. This 

alga was introduced to Molokaÿi from Oÿahu in the 1980s 

for aquaculture projects that were later abandoned. This 

species is highly invasive on Oÿahu and has the potential 

to become a problem on Molokaÿi. G. salicornia is usually 

yellow, orange, or brown in color and can grow in large 

clumps 30 cm (12 in) or more in size.

Figure 28. The red alga Spyridia filamentosa is quite 

common on the shallow reef flat of Molokaÿi. This species 

is often covered with a dusting of sediment and seems 

to be able to tolerate growing in extremely muddy areas. 

Spyridia has soft delicate branches and can grow up to 

20cm (7.8 in) in height.

Figure 29. The lightly calcified brown alga Padina growing 

on the reef flats near Kamiloloa. This species is common in muddy areas and can grow quite large, with blade 

diameters up to 25 cm (10 in).

Figure 30. The green alga Caulerpa sertulariodes growing in the sand on the shallow reef flat of the south shore 

of Molokaÿi. Most species of Caulerpa have small rootlike rhizoides, a horizontal runner, and upright blades. In 

this species the blades resemble a fern frond but are only 

2–3cm (0.8–1.2 in) tall.

Figure 31. The filamentous green alga Cladophora 

vagabunda growing intermixed with the red alga Spyridia 

filamentosa (lower right). Most species of Cladophora

are very fine and delicate, but they can grow quite large 

(~20cm/7.8 in tall), often resembling green cotton candy. 

Figure 32. The green alga Ulva fasciata, or limu pälahalaha, 

is common in shallow nearshore environments, especially 

near nutrient or fresh sources such as springs or streams. 

Ulva is also known as sea-lettuce because its bright green 

blades resemble leaves of lettuce grown on land.
The Seaweed and Seagrass Communities of Molokaÿi

73 

Character of the South Molokaÿi Fringing Reef

algal genus Padina (fig. 29), which are easily recognized by the rings 

of calcification that lightly cover the flat leaf-like thallus. Several species of Caulerpa can be found in Hawaiÿi, and Caulerpa sertularioides

(fig. 30) is quite common on Molokaÿi. The calcified green alga, Halimeda discoidea (fig. 13) is locally abundant, growing in small clumps. 

Other green algae such as the filamentous Cladophora vagabunda (fig. 

31), the blade Ulva fasciata (limu pälahalaha) (fig. 32), and the commonly eaten Enteromorpha spp. (limu ÿeleÿele) (fig. 33) are also common, especially in areas close to shore near fresh water sources such 

as springs or stream mouths. The cyanobacteria Lyngbya majuscula

(otherwise known as swimmer’s itch because it produces a toxin which 

can irritate the skin; fig. 34) grows on top of other algae on the reef 

flats. Lyngbya seems to bloom seasonally in the summer, when it can 

become quite abundant; by the fall it nearly completely disappears. 

In summary, the shallow reef flat and back reef habitats on Molokaÿi 

are dominated by macroalgae, seagrass, soft sediment, and reef rubble. 

These areas seem to be quite productive, and because of numerous factors including extensive habitat, they support a diversity of algal species. 

The high amount of terrigenous sediment deposited in these areas may 

also contain both organic and inorganic nutrients, thus potentially fertilizing the algal communities. The three invasive nonindigenous algae 

found on the south shore reef flats seem to be able to outcompete native 

species in other areas around the Hawaiian Islands and therefore are of 

concern for marine resource management. The reef flats are important 

resources, because they support numerous fish, invertebrate, and seaweed 

species, of which many are harvested for subsistence consumption. 

The Reef Slope

There is a limited amount of information known about the algal communities in the deeper (2 –20 m, 6 –60 ft) reef-slope environments around 

Molokaÿi. However, in a single survey in 2002, two divers collected more 

species of algae (as many as 85) from Päläÿau, Kamiloloa, and Kamalö than 

from any other sites surveyed around the eight main Hawaiian islands that 

same year. These sites represent some of the highest diversity sites for algae 

that the authors have seen. Although most of the species were members of 

the turf community and were small filamentous forms, several species of 

red macroalgae were also quite common. Species such as the fleshy redbladed Kallymenia sessilis (fig. 35) and Halymenia sp. (fig. 36) were present. The brilliantly iridescent red algal species, including members of the 

genera Halicrysis (fig. 37), Dasya (fig. 38), Martensia (fig. 39), and Neomartensia (fig. 40) and Acanthophora pacifica (fig. 41) were all present. 

In the deeper areas off the reef slope, the green alga Halimeda

kanaloana becomes the dominant benthic organism in soft sandy areas. 

This endemic species forms extensive meadows (fig. 42) in the soft bottom areas off the south shores of Molokaÿi, Läna’i, Kahoÿolawe, and Maui. 

Figure 33. Enteromorpha (this alga is now considered 

to be in the genus Ulva, but it often grows in tubes rather 

than sheets), or limu ÿeleÿele, can be seen growing at the 

land-sea interface and is often common in brackish water 

where springs or streams enter the marine environment. 

Individual tubes of Enteromorpha can reach more than 

30cm (12 in) in length.

Figure 34. The cyanobacterium Lyngbya majuscula is often 

found growing on top of other algae such as Sargassum. 

This species often forms large ephemeral blooms in the 

summer. It also produces a toxin that has been known to 

cause skin rashes in swimmers.

Figure 35. The fleshy red-bladed Kallymenia sessilis

growing in the interstices of the coral Monitpora capitata. 

Kallymenia is usually 5–10 cm (2–4 in) in height.

Figure 36. The large, dissected, red-bladed Halymenia is 

common in wave-swept subtidal habitats in Hawaiÿi. Blades 

of Halymenia can reach up to 40 cm (15.7 in) in height, but 

most individuals are 20 cm (7.8 in) or less.

It generally occurs in depths ranging from less than 1 m to more than 90 

m (300 ft) (Verbruggen and others, 2006). All species of Halimeda are 

heavily calcified and are important sand producers. When these organisms 

reproduce, they relocate all of their intracellular material into their gametes, which are then released by the thousands into the water column—only 

an empty white skeleton of the adult plant is left. Usually within a day, the 

adult plant crumbles into a pile of small calcium carbonate discs that eventually break down into sand (fig. 43; Field and others, this vol., chap. 17). 

This process is easy to recognize in areas where Halimeda is abundant, 

because the plantlike segments are still identifiable. Although there are 

several other sand producers in the tropics, Halimeda is likely to be among 

the most important because of the fast turnover rates of these plants. 

The reef slope communities along the southeast and southwest shores 

of Molokaÿi are largely dominated by hard corals, but the algal communities are also incredibly diverse and are composed of some rare species. 

The reef slope communities near Kaunakakai have very low coral cover 

(Jokiel and others, this vol., chap. 5), and are predominantly covered in 

fleshy algae such as Spyridia and thick stands of turf algae. Along most 

of the south shore, the green alga Halimeda forms extensive meadows 

(fig.42) in the waters beyond the reef slope, extending out into deep water.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

74

Figure 37. The golden, iridescent red alga Halycrisis is 

common at Päläÿau. This alga grows tightly adhered to hard 

substrates and is no taller than 5 cm (2 in).

Figure 38. The delicate iridescent red alga Dasya irridescens is shown here growing next to the coral Porites 

lobata (lobe coral). Dasya grows up to about 10 cm (4 in) in 

height.

Figure 39. The fragile and iridescent red alga Martensia 

fragilis. This alga can vary in color from blue to purple to 

red and orange and is usually 5-10 cm (2-4 in) in height.

Figure 40. The delicate mesh-like iridescent red alga Neomartensia flabelliformis resembles gauze. This species is 

very fragile and is usually 5-15 cm (2-5.9 in) in height.

Figure 43. Halimeda kanaloana. A, A close-up of live H. kanaloana. B, Sand that is composed largely of recently dead 

Halimeda (note the oval-shaped disks).

Figure 42. A meadow of the calcified green alga Halimeda 

kanaloana off Kamiloloa in 11 m (35 ft) of water. This species forms extensive meadows on the deep reef slopes off 

Molokaÿi and Maui. Most species of Halimeda are important 

producers of sand in the tropics. This species is among the 

largest Halimeda in Hawaii and can be more than 30 cm 

(12 in) tall.

Figure 41. The blue to purple iridescent red alga Acanthophora pacifica. This species is far less common than the 

invasive counterpart, Acanthophora spicifera. A. pacifica is 

usually about 5 cm (2 in) in height.

A B
The Seaweed and Seagrass Communities of Molokaÿi

75 

Character of the South Molokaÿi Fringing Reef

Figure 44. Large (~20 cm/7.8 in) green blades of the 

genus Ulva at 77-m (254 ft) depth. This chlorophyte covers 

large expanses of this deep water rubble zone.

Figure 45. Two different Codium species at 67-m (221 ft) 

depth; one is long (~15 cm/5.9 in) and lightly branched, 

while the other is short and more densely branched.

Deep-Water Communities

In the deeper waters off south Molokaÿi, the bottom is a mix of soft 

sand, hard carbonate, rubble, and large areas covered by small balls 

of calcified red algae called rhodololiths. Areas that were surveyed 

include depths from about 60 to 120 m (~200 to 400 ft) on three different occasions in 2004 with a submersible and a remotely operated 

vehicle. Despite the low light levels at these depths, there was a high 

abundance and diversity of macroalgae. The most common large macroalgal group encountered was the chlorophyte Ulva (fig. 44), with 

Ulva sp. blades up to 2 m (6 ft) in length. This genus usually occurs in 

the intertidal zone, yet we found that it was common (as much as 25 

percent cover) from 60 to 100 m (~200 to 330 ft) depths. Other common green algal genera were Codium (fig. 45) and Halimeda (fig. 46). 

At least three different species of Codium and three different species of 

Halimeda were observed, although species identifications are still pending. Many fleshy red algal blades (figs. 47, 48) were observed, but these 

were not collected because of strong currents. These fleshy red algae 

were abundant on hard substrates over the entire depth range surveyed. 

Large scattered clumps of the brown alga Spatoglossum macrodentum

were also observed on hard substrate. Small filaments and clumps of the 

filamentous green alga Cladophora sericea were observed floating in 

the current and being transported across the bottom. A dense Halimeda

kanaloana meadow (fig. 49) was discovered starting at 86 m (284 ft) 

depths in soft sediments, and it appeared to continue into shallower water. 

Overall, the deep-water macroalgal community is very rich 

and abundant, likely forming unique habitats and acting as a food 

source for numerous organisms. We did not observe any invasive 

or introduced macroalgae in the deep-water environment, although 

the invasive green alga Avrainvillea amadelpha has been observed 

in similar habitats in deep water off of Oÿahu. Exploration of these 

unstudied environments is likely to yield the discovery of many species of algae, invertebrates, and even fishes that are new to science. 

The West Shore 

Very little is known about the benthic communities off the west shore 

of Molokaÿi. The southwest section of coast consists of a few small bays 

with black sand beaches and basalt points, whereas the northwest coast 

is largely made up of extensive carbonate sand beaches and occasional 

basalt outcroppings. Judging from a limited amount of field work, the 

intertidal communities in these areas appear to be rich with seaweeds and 

consist of species mentioned previously, including Sargassum, Turbinaria, 

Padina, Galaxaura, Tricleocarpa, a brown alga Dictyopteris (limu lïpoa) 

(fig. 50), several species of the red algal genus Laurencia (fig. 51), and a 

surprisingly high abundance of the small calcified green alga Neomeris

annulata (fig. 52). Overall, these communities appear to be quite diverse. 

Figure 47. Unidentified red algal blades growing intermixed 

with the green alga Ulva in the rubble zone at 70-m (230 ft) 

depth.

Figure 46. Halimeda spp. found growing on hard substrate 

at 67-m (221 ft) depth. These species often form clumps or 

large mounds in these deep-water habitats.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

76

Figure 52. A dense clump of the calcified green alga 

Neomeris growing along the west shore of Molokaÿi. This 

species usually grows alone or in clumps consisting of 

a few individuals less than 2 cm (0.8 in) tall; however, 

this photograph shows more than 50 individuals growing 

together.

Summary 

The marine environment of Molokaÿi is diverse and includes boulder 

fields, basalt tidepools, manmade fishponds (which were not surveyed 

here), shallow carbonate benches, extensive mud flats, coral-dominated 

reef slopes, macroalgal-dominated deep reefs, and various combinations of all of the above. Each of these different habitat types supports a 

unique assemblage of marine algae. Both the east and the west coasts are 

exposed to large waves, have little human impact, and support diverse 

algal communities in the shallow intertidal and reef-flat areas. The south 

shore is quite different because it is sheltered from wave action and has 

an extensive reef flat and many other unique types of habitats. The wide, 

Figure 51. The red alga Laurencia is a common component of intertidal communities on rocky or basalt shorelines. 

There are more than 10 species of Laurencia in Hawaiÿi, 

most of which are less than 10 cm (4 in) tall, and species 

identification can be quite difficult without a microscope.

Figure 50. The brown alga Dictyopteris (limu lïpoa) can 

be quite common in the shallow subtidal communities in 

Hawaiÿi, where it often forms extensive beds. This genus 

is similar to Dictyota but has a rib that extends through the 

middle of each branch, rather than being completely flat 

like Dictyota (see fig. 16). Most species of Dictyopteris can 

grow up to 30 cm (12 in) in height.

shallow reef flat is one of the largest continuous shallow-water habitats 

for the endemic seagrass Halophila hawaiiana in Hawaiÿi. These areas 

have been historically productive and have been invaluable collecting 

areas for numerous species of seaweed or “limu,” which are regularly 

consumed by the people of Molokaÿi. There is an indication, however, 

that large portions of these habitats are being overgrown by invasive 

macroalgal species. The actual cause of this overgrowth remains unclear, 

but it could be a result of increased nutrients, sedimentation, or perhaps 

a lack of herbivores in these areas. The deep-water reefs of Molokaÿi 

harbor numerous seaweed species, many of which are likely new to science. Every effort should be made to preserve the unique marine environments on Molokaÿi and the diversity of species that inhabit them.

Figure 48. An unidentified red algal blade with frilly or ruffled 

red edges at 73-m (240 ft) depth off southwest Molokaÿi.

Figure 49. An expansive meadow of the green alga Halimeda kanaloana. Such meadows typically occur in soft, 

sandy sediments and span depths from 10 m (33 ft) to 

more than 90 m (295 ft).
Mean high tide line

Spring low tide line

Shoreline/

Intertidal

Reef flat Reef

crest Fore reef

Shelf

Shelf

escarpment

Character of the South Molokaÿi

CHAPTER 9

 Fringing Reef

Benthic Habitat Maps of the South Molokaÿi Coral Reef

Susan A. Cochran1

To document evidence of change in any ecosystem, one must first 

have a starting point—a “baseline” inventory of resources. Thematic 

maps providing this baseline inventory are an important tool for 

assessing changes in coral reef ecosystems, allowing scientists to spatially 

document changes in coral location, percentage of cover, and relative overall health of the system. In the past decade, scientists and managers have 

recognized the lack of thematic maps for coral reefs worldwide. In 1998, 

the White House issued Executive Order 13089 establishing the U.S. Coral 

Reef Task Force (CRTF). Composed of several Federal agencies, including the U.S. Geological Survey (USGS), the CRTF has the primary duty of 

mapping and monitoring the coral reefs in the United States and U.S. Trust 

Territories.

One of the strategic goals of the U.S. Geological Survey is to assist 

land-use managers by establishing the geologic framework for ecosystem 

structure and function. Provide here are detailed high-resolution maps 

and habitat characterization of the reef off the south shore of Molokaÿi, a 

critical coral reef in Hawaiÿi that is of concern to resource managers and 

the public. The threats to this reef, including excessive sedimentation and 

pollution, were some of the reasons for selecting this site to develop procedures for producing such maps. The USGS is in a position to provide highresolution benthic habitat maps of the Molokaÿi reef because it is not under 

the 1-acre Minimum Mapping Unit (MMU) and 1:6,000 scale restrictions 

self-imposed on other agencies. These maps can be used as stand-alone 

static documents or in a geographic information system (GIS) and provide 

useful information to scientists, managers, and the general public.

Mapping on Molokaÿi 

Before 1998, the University of Hawaiÿi Marine Options Program 

undertook the only mapping effort of the south Molokaÿi reef, funded by 

the U.S. Army Corps of Engineers (Manoa Mapworks, 1984). Qualitative 

field data were collected over a two-week period using scuba and snorkel, 

and maps were plotted using 1975 black and white aerial photography at 

1:6,000 and 1:24,000 scales as a base layer. These maps provide a useful 

background to the reef ecosystem; however, the 1975 aerial photographs 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

were not georectified, and thus no true quantitative measurements of scale 

and distribution can be made.

In 1999, in response to the mandate set forth by Executive Order 

13089, The National Oceanic and Atmospheric Administration (NOAA) 

National Ocean Service (NOS) implemented a program to provide digital 

maps of our Nation’s coral reefs for use in a GIS. The south Molokaÿi reef 

was mapped using orthorectified aerial photography, hyperspectral remotely 

sensed images, and quantitative field observations (Coyne and others, 

2003). The classification scheme used to produce these maps documented 9 

out of a possible 23 distinct benthic habitat types on the south Molokaÿi reef 

and 10 out of a possible 14 morphological reef zones. However, under the 

restrictions of a 1-acre (64 m by 64 m) MMU and at no finer resolution than 

a 1:6,000 scale, many smaller details of the reef system were overlooked. 

Using additional photography and bathymetric data, the USGS has now 

mapped the south Molokaÿi reef at a much higher resolution.

How the Maps Were Made

The Molokaÿi benthic habitat classification maps in this report were 

created from visual interpretation of georectified, color aerial photography 

and SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey) 

bathymetric data (see Field and others, this vol., chap. 2). In addition to the 

remotely sensed imagery, we used knowledge from field data collected over 

a period of four years, from 1999 to 2003. These field data include underwater photographs, video, and visual observations and were collected using 

towed instruments, scuba and snorkel, and on foot. Mapping was accomplished using a GIS, and a statistical analysis of accuracy was performed. 

See Cochran-Marquez (2005) for a complete description of background 

layers, mapping methodology, and statistical accuracy assessment.

The classification scheme used was based on a scheme established by 

NOAA’s biogeography program in 2002 and subsequently revised in 2004 

(NOAA National Centers for Coastal Ocean Science, 2005). Our maps 

use NOAA’s scheme as a starting point to provide some continuity to the 

coral-reef scientific community. However, modifications were made to the 

original scheme in order to better reflect the benthic habitats, geologic substrates, and historical features (such as fishponds) found on Molokaÿi. 

More than 4,200 polygons covering more than 120 km2

 were digitized 

by interpreting features seen in both the aerial photographs and SHOALS 

bathymetry, with additional input from underwater video footage, photographs, and field observations. A minimum mapping unit (MMU) of 100 

m2 was used, but smaller features were mapped if they carried habitat significance (for example, an individual coral colony 2 m in diameter located 

in an otherwise uncolonized area). The classification scheme uses four 

basic attributes to describe each polygon on the benthic habitat map: (1) 

the dominant geomorphic structure or underlying substrate; (2) the major 

biologic cover found on the substrate; (3) the percent biologic coverage; 

and (4) the geographic zone indicating the location of the habitat (table 1). 

Each combination of a geomorphic structure with an overlying biologic 

cover may be described as a separate habitat. A geographic zone describes 

the cross-shelf location of a habitat, using terminology common in current 

coral-reef literature (fig. 1).

Figure 1. Schematic diagram showing the generalized cross-shelf coral reef 

zonation of the south Molokaÿi reef. Not shown are the zones land, channel, 

dredged, or vertical wall (modified from Kendall and others, 2004).
Dominant structure/substrate Area (m2) Area (acres) Percent

Aggregate reef 5,718,383 1,413.043 4.69

Aggregated patch reef 683,658 168.938 0.56

Artificial 119,770 29.597 0.10

Artificial/historical 116,252 28.730 0.10

Individual patch reef 38,325 9.471 0.03

Land 2,192,241 541.715 1.80

Mud 5,142,276 1,270.678 4.22

Pavement 38,531,126 9,521.240 31.59

Pavement with sand channels 5,029,773 1,242.888 4.12

Rubble 2,428,875 600.188 1.99

Sand 47,047,502 11,625.692 38.57

Scattered coral/rock 1,387,821 342.932 1.14

Spur-and-groove 12,030,578 2,972.823 9.86

Unknown 1,511,423 373.479 1.24

Grand total 121,978,003 30,141.414 100.00%

Major biological cover Area (m2) Area (acres) Percent

Coral 34,031,767 8,409.427 27.90

Coralline algae 179,632 44.387 0.15

Emergent vegetation 150,170 37.111 0.12

Macroalgae 16,615,684 4,105.825 13.62

Mangrove trees 2,967,857 733.366 2.43

Unclassified 234,028 57.834 0.19 

Uncolonized 66,087,550 16,330.591 54.18

Unknown 1,711,315 422.873 1.40

Grand total 121,978,003 30,141.414 100.00%

Relative percent coverage

80

100

60

40

20

0

Location along shore

MolokaØi Inner Reef Zones

Major Biological Coverage

Mangroves

Unclassified

Uncolonized

Unknown

Coral 

Coralline algae

Emergent vegetation

Macroalgae

West East

Hale O LonoWai a KnePlØauKaunakakai

KamiloloaKawela

KamalPkoØoKmimi

Figure 2. Graph showing the major biological coverage on the inner reef zones 

(shoreline/intertidal, reef flat, and reef crest) for each geographic area of the south 

Molokaÿi reef. The geographic locations alongshore relate to the map sections shown 

in the following pages. The percent coverages are relative to each map section.

Geomorphic structure/substrate Biological cover Geographic zonation

Hardbottom Coral Land

 Aggregate reef Coralline algae Shoreline/intertidal

 Spur-and-groove reef Emergent vegetation Reef flat

 Aggregated patch reef Macroalgae Reef crest

 Individual patch reef Mangrove trees Fore reef

 Pavement Uncolonized Shelf

 Pavement with sand channels Unclassified Shelf escarpment

 Scattered coral/rock (land or artificial) Channel

 Reef rubble Unknown Dredged

Softbottom Vertical wall

 Sand Percent biological cover

 Mud 10% to <50%

Other 50% to <90%

 Artificial 90% to 100%

 (for example, wharves)

 Artificial/historical 

 (for example, fishponds)

 Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

78

The Maps—What They Tell Us

Dominant Structure/Substrate

The south Molokaÿi reef is classified into 14 different structures/substrates 

that compose the reef morphology. The maps on the following pages show the 

reef in nine different geographic subsets. The dominant structure/substrate and 

major biological cover maps have been overlaid on aerial photographs for viewing purposes. Nearly 66 km2

 (54 percent of the total area mapped) consists of 

combined reef and hard-bottom substrates; soft-bottom sand and mud make up 

just over 52 km2

 (43 percent) (table 2).

A major feature of the South Molokaÿi reef is the broad, shallow pavement 

platform of the reef flat, which extends nearly 1.5 km offshore in the Päläÿau 

and Kamiloloa areas (see Storlazzi and others, this vol., chap. 3). A mud veneer 

and ancient fishponds lie close to shore on the reef flat, while sand patches and 

coral-covered pavement dominate the seaward edge of the reef flat. 

Seaward of the reef crest, the fore reef alternates between abundant reef 

and barren, hard pavement. Aggregate reef growth is predominantly in the 

Wai a Käne/Päläÿau and Kawela/Kamalö regions. Spur-and-groove formations are more common to the west. The areas off Kaunakakai/Kamiloloa 

and Pükoÿo, however, are barren “dead zones” of hard pavement with little or 

no active reef development.

The base of the fore reef is around 27-m (90 ft) water depth, where it 

drops off into a gently sloping sand-covered plain of the shelf zone. Reef 

development pinches out near the east and west ends of the island, where high 

wave energy limits growth (see Storlazzi and others, this vol., chap. 11).

Additional structure/substrate descriptions for each geographic subset area 

may be found on the following pages.

Major Biological Cover

The 14 different structures/substrates are covered with 8 different 

biological classes. Coral covers 51 percent of the suitable hard substrate 

on the Molokaÿi reef (more than 34 km2 or 28 percent of the overall study 

area) (table 3). 

Major biological cover varies geographically along the inner reef zones 

(shoreline/intertidal, reef flat, and reef crest). Macroalgae is most abundant on 

the reef flat in the Kaunakakai (eastern half), Kamiloloa and Pükoÿo areas. The 

majority of the mangroves may be found along the Päläÿau and Kaunakakai 

shorelines (see D’Iorio, this vol., chap. 16). Wai a Käne, Päläÿau and Kamalö 

have large areas of uncolonized sand or barren pavement (fig. 2). 

The coral-covered pavement at the seaward edge of the reef flat is 

dominated by Porites lobata colonies. These shallow colonies appear 

as knobby flat-topped mesas, with live tissue on the sides of the coral 

mounds. Surface exposure, wave influences, and/or relative sea-level 

change prevent vertical growth of these coral mounds. These flat-topped 

coral mounds can be exposed at low tides, making it difficult for live coral 

tissue to grow. The irregular surface of the knobby mounds sometimes has 

macroalgae or coralline algae growing on it, and it may contain sediment 

in the depressions (pukas).

Of the areas of suitable hardbottom available for coral growth on 

the Molokaÿi fore reef, the highest percentages of coral are found in the 

Wai a Käne and Päläÿau areas near the west end of the island and in the 

Kawela and Kamalö (western half) areas along the center of the island. 

Studies have shown these areas to have the highest coral coverage in the 

Table 2. Areas and percentages for each class of dominant structure/substrate 

on the south Molokaÿi reef.

Table 3. Areas and percentages for each class of major biological cover on 

the south Molokaÿi reef.

Table 1. Categories of dominant structure/substrate, major biological cover, percent 

cover, and geographic zones used in the Molokaÿi benthic habitat classification 

scheme (modified from NOAA National Centers for Coastal Ocean Science, 2005).

[Less than 10 percent biological cover is considered to be uncolonized; therefore, <10 percent is not used 

as a percent biological cover category.]
Character of the South Molokaÿi Fringing Reef

Relative percent coverage

80

100

60

40

20

0

Location alongshore

West East

Hale O LonoWai a KnePlØauKaunakakai

Kamiloloa

Kawela

KamalPkoØo

Kmimi

MolokaØi Fore Reef

Major Biological Coverage on Hard Substrate

Mangroves

Unclassified

Uncolonized

Unknown

Coral 

Coralline algae

Emergent vegetation

Macroalgae

0 20 40 60 80 100

40

35

30

25

20

15

10

5

0

Depth, in meters

Coverage, in percent

Mangroves

Uncolonized

Unknown

Coral 

Coralline algae

Macroalgae

Major Biological Covers

Figure 4. Graph showing the percentages of major biological covers at various depths 

along the south Molokaÿi reef. See text for description.

Figure 5. Three-dimensional perspective view, looking northwest, of the Päläÿau 

region of the south Molokaÿi reef showing major biological cover. The natural 

channel leading from the mangrove-dominated shoreline (blue-green) is visible 

in the left-center portion of the image. Aggregate coral (blue) dominates the fore 

reef; the reef flat is a mix of uncolonized sand and pavement (tan), macroalgae 

(light green), and coral-covered pavement (blue). Approximate distance across 

the bottom of the image is 4.5 km (2.8 mi).

Mangrove-dominated shoreline

Aggregate coral

Uncolonized sand

and pavement

Macroalgae Coral-covered pavement

Benthic Habitat Maps of the South Molokaÿi Reef

79 

Figure 3. Graph showing the major biological coverage on hardbottom substrates for each geographic area of the south Molokaÿi fore reef. The geographic 

locations alongshore relate to the map sections shown in the following pages. 

The percent coverages are relative to each map section.

according to the field check. The greatest amount of error was found in the 

uncolonized class. In this instance, what was originally mapped as uncolonized sand on the fore reef at depths of approximately 27 m (90 ft) was 

found many times to be sand with >10 percent macroalgae (Halimeda sp.). 

After accuracy assessment calculations were performed, any misinterpreted polygons on the Molokaÿi reef maps were corrected using the fieldcheck data, thus increasing the accuracy of the final map to greater than 

86.27 percent.

Using the Maps

The detailed high-resolution maps provided here document habitat 

characterization of a critical coral reef in Hawaiÿi and are for use by managers, scientists, and the general public. These maps can be used by themselves 

(as shown in this volume), or the digital files may be used in a GIS or with 

other types of visualization software (fig. 5). Integration of the aerial imagery, SHOALS bathymetry, and field observations made it possible to create 

detailed thematic map layers reaching depths of 35 m (120 ft), encompassing 

the base of the Molokaÿi fore reef, which is deeper than can be mapped with 

standard optical remote sensing instruments. 

The maps are based on an interpretation of conditions at the time the 

imagery was collected (1993, 1999, and 2000) and are constrained in positional accuracy by use of a standard GPS receiver (±3 m). Hence, they are 

not to be used for navigation and are limited in their use for recognizing 

small-scale changes in the ecosystem. For example, it would be difficult to 

discern if bleaching occurs, or if there is a loss of coral colonies due to storm 

waves or dredging. However, large-scale shifts in the dominant bottom type, 

such as a change from uncolonized pavement to a predominantly algal-covered or coral-covered environment, could be detected.

main Hawaiian Islands (Jokiel and others, 2001). The fore reef in the 

Kaunakakai (eastern half), Kamiloloa (western half), Kamalö (eastern 

half), and Pükoÿo areas have large “dead zones” where most hardbottom 

substrate suitable for coral growth is either barren or covered with macroalgae (for example, Halimeda sp.) (fig. 3).

Aggregate reefs and spur-and-groove formations on the fore reef 

are dominated mostly by Porites sp. and Montipora sp. Large patches of the 

macroalgae Halimeda sp. may be found in the sand at the base of the fore 

reef. These calcareous algae are a major contributor to the sand supply on the 

reef shelf (see Field and others, this vol., chap. 17).

The highest percentages of coral are found at depths between 5 m and 

15m (15–50 ft) across the entire reef (fig. 4). However, abundant coral is also 

found at depths between 20 m and 25 m (65–85 ft) in some localized areas. 

Additional biological cover descriptions of each subset area may be 

found on the following pages.

Accuracy of Maps

The validity of map classifications is determined with actual on-site 

checks at random locations and an accuracy assessment. For this project, 

a total of 816 map points were checked in the field. The overall accuracy 

of 86.27 percent (with a 95-percent confidence interval of ±2.36 percent) 

indicates the number of points on the map which were classified correctly 
21°

06' 

 N

21°

05'

21°

04'

157°15' W 157°14' 157°13' 157°12'

Hale O Lono

Harbor

Kolo Wharf

Hlen

Gulch

Hlena

K

Hinawai Gulch

One

o

Hilo

Gulch

Ono

p

ala

ni Gulch

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Hale O Lono

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

80

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

06' N

21°

05'

21°

04'

157°15' W 157°14' 157°13' 157°12'

Hale O Lono

Harbor

Kolo Wharf

Hlena

Gulch

Hlena

K Hinawai

Gulch

One o Hilo

Gulch

Onopalani Gulch

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Coral

Coralline algae

Emergent vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Hale O Lono

Major Biological Cover

Benthic Habitat Maps of the South Moloka

ÿi Reef

81 

This area marks the western extent of active 

coral-reef growth along the south shore of 

Molokaÿi. Coral-covered aggregate reef and the 

spur-and-groove structure of the fore reef trend 

progressively closer to the shoreline from east 

to west, before finally pinching out near Hale 

O Lono. Reef growth further west is limited by 

high-energy waves from the North Pacific Swell 

that wrap around the end of the island near 

Läÿau Point (see Storlazzi and others, this vol., 

chap. 11). The reef flat here is home to a few 

scattered individual heads of coral and is steeper 

than along the central portion of the island. 

There is no discernable reef crest in this region. 

BIOLOGICAL COVER
21°

06' 

 N

21°

05'

157°11' W 157°10' 157°09' 157°08'

Wai a Kne

Kolo

Gulch

Hikauhi Gulch

Wai a

Kne

Gulch

Naninanikukui Gulch

K

kk

Gulch

P

u

nako

u

Gulch

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Wai a Kne

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

82

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

06' N

21°

05'

157°11' W 157°10' 157°09' 157°08'

Wai a Kne

Kolo Gulch

Hikauhi Gulch

Wai a Kne Gulch

Naninanikukui Gulch

Kkk Gulch

Punakou Gulch

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Coral

Coralline algae

Emergent vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Wai a Kne

Major Biological Cover

Benthic Habitat Maps of the South Moloka

ÿi Reef

83 

Westward from Wai a Käne Gulch the reef crest 

disappears and the coral-covered aggregate reef 

and spur-and-groove structure begin to trend 

progressively closer to the shoreline (see previous 

section). East of Wai a Käne Gulch, coral-covered 

ridges are found along the seaward edge of the 

reef flat, which is relatively deeper here than in 

the central portion of the island. Several isolated 

coral-covered mounds are located offshore from 

the main aggregate fore reef on the deeper portion 

of the sand-covered shelf.

BIOLOGICAL COVER
21° 

06'

21° 

05'

157°07' W 157°06' 157°05' 21° 

07' 

N

PlØau

Pkanaka Fishpond

Kamanakai Gulch

Manawainui Gulch

0 0.5 1 Kilometer

0 0.25 0.5 Mile

PlØau

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement 

with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

84

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21° 

06'

21° 

05'

21° 157°07' W 157°06' 157°05' 

07' 

 N

PlØau

Pkanaka Fishpond Kamanakai Gulch

Manawainui

Gulch

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

PlØau

Major Biological Cover

Benthic Habitat Maps of the South Moloka

ÿi Reef

85 

Mangrove trees, planted in an attempt to trap sediment 

runoff from the island, dominate the shoreline at Päläÿau 

(see D’Iorio, this vol., chap. 16). However, large amounts 

of sediment still reach the coast and are deposited on 

the reef flat. A major offshore feature in this region is a 

channel that bisects the reef flat, reef crest, and fore reef. 

The vertical walls of the channel host a number of coral 

species, except in the turbid, muddy waters close to shore.

West of this Päläÿau channel, the reef flat is mostly 

uncolonized terrigenous mud. Live coral is found along 

the seaward edge of the reef flat and on the portion of 

the reef crest near the channel. West of the channel and 

seaward of the coral-covered fore reef, the sand-covered 

shelf is dotted with isolated coral-covered mounds.

The reef flat is shallower and broader on the east 

side of the Päläÿau channel. It is dominated by a 

large area of uncolonized, white carbonate sand. Live 

coral is found along the seaward edge of the reef 

flat, and beyond the reef crest on the fore reef.

Two large “blue holes,” or pits, are found on the eastern 

edge of the Päläÿau fore reef. These features were pos-

sibly formed by dissolution of the limestone reef by the 

subsurface percolation of fresh groundwater through 

the system (see Grossman and others, this vol., chap. 

13). The vertical walls of these pits are covered with 

live coral, and the bottoms are covered with sand.

BIOLOGICAL COVER
21°

06'

N 

21°

05'

21°

04'

157°04' W 157°03' 157°02' 157°01'

Kaunakakai

Kapuiwa 

Coconut Grove

ØUmipaØa

Kaluaapuhi

Fishpond

Kaunakakai

Harbor

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kaunakakai

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement 

with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

86

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

06'

N 

21°

05'

21°

04'

157°04' W 157°03' 157°02' 157°01'

Kaunakakai

Kapuiwa 

Coconut Grove

ØUmipaØa

Kaluaapuhi

Fishpond

Kaunakakai

Harbor

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Kaunakakai

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Benthic Habitat Maps of the South Moloka

ÿi Reef

87 

Kaunakakai is the population center of Molokaÿi. The 

impermeable Kaunakakai Wharf extends 0.85 km 

(0.5 mi) offshore, blocking the east to west transport 

of water and sediment. The natural offshore exten-

sion of Kaunakakai Gulch, which bisects the reef 

flat, reef crest, and fore reef, has been enhanced by 

dredging to create a deep harbor next to the wharf. 

West of Kaunakakai Wharf, mangrove trees are found 

along most of the shoreline. A broad band of terrestrial 

mud covers the inner reef flat; the middle portion of the 

reef flat is dominated by algae-covered pavement; coralcovered ridges interspersed with sand are found on the 

seaward edge of the reef flat. Beyond the reef crest, the 

fore reef is dominated by healthy coral-covered aggre-

gate reef and spur-and-groove structures, which have 

high coral coverage offshore of the Kapuäiwa Coconut 

Grove area. However, the fore reef just west of the chan-

nel is barren, possibly owing to dredging activities.

East of Kaunakakai Wharf, the reef flat is dominated 

by algae and terrestrial mud on hard pavement. A 

few live corals grow on ridges near the seaward edge 

of the reef flat. Beyond the reef crest, the fore reef 

is mostly barren pavement; however, some aggre-

gate reef growth may be found at deeper depths.

BIOLOGICAL COVER
21°

05'

N

21°

04' 

21°

03' 

157° W 156°59' 156°58' 156°57'

One AliØi Park

Moku

Kamiloloa

KalokoØeli

Fishpond

AliØi

Fishpond

Knoa

Fishpond

Ønini Gulch

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kamiloloa

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

88

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

05' N

21°

04' 

21°

03' 

157° W 156°59' 156°58' 156°57'

One AliØi Park

Moku

Kamiloloa

KalokoØeli

Fishpond

AliØi

Fishpond

Knoa

Fishpond

Ønini

Gulch

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Kamiloloa

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

BIOLOGICAL COVER

Benthic Habitat Maps of the South Moloka

ÿi Reef

89 

A narrow band of terrestrial mud is found 

along the shoreline of Kamiloloa, tran-

sitioning into the predominately algaecovered platform of the middle reef flat. 

A few live corals grow on ridges near the 

seaward edge of the reef flat. The barren 

fore-reef platform of Kaunakakai (see pre-

vious pages) continues eastward through 

Kamiloloa to One Aliÿi (center of this 

area), with coral-covered spur-and-groove 

structures found only at greater depths. 

East of One Aliÿi, the shallower areas of 

the fore reef have increasing coral growth 

continuing toward the Kawela area (see 

next pages).
21°

04'

 N

21°

03'

21°

02'

156°57' W 156°56' 156°55' 156°54'

Kawela

Kpapa

East Fork Kawela

Gulch

Waiakuilani

Gulch

Pnhh

Fishpond

KawiØu

Fishpond

Kanukuawa

Fishpond Pahiomu

Fishpond

KamhuØehuØe

Fishpond

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kawela

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

90

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

04'

 N

21°

03'

21°

02'

156°57' W 156°56' 156°55' 156°54'

Kawela

Kpapa

East Fork Kawela

Gulch

Waiakuilani

Gulch

Pnhh

Fishpond

KawiØu

Fishpond

Kanukuawa

Fishpond

Pahiomu

Fishpond

KamhuØehuØe

Fishpond

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kawela

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Benthic Habitat Maps of the South Molokaÿi Reef

91 

The inner and middle portions of the reef flat at 

Kawela are a mixture of terrestrial mud, algaecovered platform, and uncolonized sand. Live 

corals grow on ridges near the seaward edge of 

the reef flat. The reef crest is discontinuous in this 

region. The fore reef is dominated by coral covered 

spur-and-groove structures. The deep embayment 

southeast of Kawela was possibly created from a 

major drainage during a period of lower sea-level 

(see Grossman and others, this vol., chap. 10, for 

discussion of historical sea-level rise).

BIOLOGICAL COVER
21°

04'

N

21°

03'

21°

02'

156°53' W 156°52' 156°51' 156°50'

Ka

m

al

Gulch

Kamal

Harbor

KamhuØehuØe

Fishpond

Blue holes

KinØohe

Fishpond

Keawa Nui

Fishpond

Kalaeloa

Harbor

Phloa

Fishpond

ØUalapuØe

Fishpond

Keawa

Nui

Gulch

KaØpah

u

Rid

ge

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kamal

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

92

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

04' N

21°

03'

21°

02'

156°53' W 156°52' 156°51' 156°50'

Kamal

Blue holes

Kamal

Gulch

Kamal

Harbor

KamhuØehuØe

Fishpond

KinØohe

Fishpond Keawa Nui

Fishpond

Kalaeloa

Harbor

Phloa

Fishpond

ØUalapuØe

Fishpond

Keawa

Nui

Gulch

KaØpahu Ridge

0 0.5 1 Kilometer 0 0.25 0.5 Mile

Kamal

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Benthic Habitat Maps of the South Moloka

ÿi Reef

93 

The Kamalö region marks a transition in the shoreline 

orientation along the south shore of Molokaÿi. East of 

Kamalö, the southeast-facing shoreline and reef flat are 

more susceptible to constant wave action from the daily 

trade winds. Siltation from a dredging operation in the 

1960s (the rectangular outline of which is still visible) 

resulted in damage to the reef flat between Kamalö and 

Kalaeloa and to the downstream Kamalö fore reef (Capt. 

Joe Reich, oral commun., 2000). Although the fore reef 

has since regenerated, the reef flat is still mainly uncolo-

nized, bare pavement with some algae-covered areas. 

A unique feature of the reef flat in this area is a series of 

“blue holes.” The alignment of the blue holes suggests 

that they may be old stream channels, formed during 

a time of lower sea level, that were subsequently seg-

mented by new coral growth after sea level attained its 

present position. An alternative hypothesis is that they 

were formed by dissolution of the limestone reef by the 

subsurface percolation of fresh ground water through the 

system (see Grossman and others, this vol., chap. 13). 

The vertical walls of the blue holes are covered with live 

coral, and the bottoms are covered with carbonate sand.

There is no reef crest in this region, possibly because of 

the higher wave activity experienced on this part of the 

island. The algae-covered pavement of the deep fore reef 

directly off Kamalö is a remnant of damage from the 

dredging operations. The shallower portions of the fore 

reef have regenerated and show substantial aggregated 

coral; however, this coral growth ends abruptly just 

east of Kalaeloa, where the fore reef transitions into an 

algae-covered pavement.

BIOLOGICAL COVER
21°

05'

 N

21°

04'

21°

03'

156°50' W 156°49' 156°48' 156°47'

PkoØo

KaluaØaha

PkoØo

KaluaØaha

PkoØo

Harbor

KaØopea Hina

Fishpond

NØaupala

Fishpond

Kpeke

Fishpond

ØUalapuØe

Fishpond

No high resolution aerial imagery available

0 0.5 1 Kilometer

0 0.25 0.5 Mile

PkoØo

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

94

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

05'

 N

21°

04'

21°

03'

156°50' W 156°49' 156°48' 156°47'

PkoØo

KaluaØaha

PkoØo

KaluaØaha

PkoØo

Harbor

KaØopea Hina

Fishpond

NØaupala

Fishpond

Kpeke

Fishpond

ØUalapuØe

Fishpond

No high resolution aerial imagery available

0 0.5 1 Kilometer

0 0.25 0.5 Mile

PkoØo

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Benthic Habitat Maps of the South Molokaÿi Reef

95 

The reef flat west of Pükoÿo Harbor is a mixture of 

algae-covered and sand-covered pavement with rare bits 

of live coral on scattered rocks. Ridges of aggregated 

patch reef are found along the seaward edge of the reef 

flat east of the harbor; however, the actual fore reef is 

only an algae-covered pavement. Several deep channels 

that correspond to onshore drainages bisect the reef flat, 

reef crest, and fore reef, and live coral is found on the 

vertical walls of the channels.

The reef crest is nonexistent west of Pükoÿo Harbor, possibly owing to higher wave activity; however, it is present 

east of the Harbor. East of Pükoÿo Harbor, the reef crest 

and fore reef begin to trend closer to the shoreline as they 

approach the eastern extent of aggregate coral growth 

along the southeast shore of the island.

BIOLOGICAL COVER
21°

08'

 N

21°

07'

21°

06'

156°46' W 156°45' 156°44' 156°43'

Kmimi

Waialua Waialua

Phakuloa

Kahinaphaku

Fishpond

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kmimi

Dominant Geomorphic Structure

Aggregate reef

Spur-and-groove

Aggregated patch reef

Individual patch reef

Pavement

Pavement with sand channels

Scattered coral/rock

Rubble

Sand

Mud

Artificial

Artificial/historical

Land

Unknown

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

96

STRUCTURE
Character of the South Molokaÿi Fringing Reef

21°

08'

 N

21°

07'

21°

06'

156°46' W 156°45' 156°44' 156°43'

Kmimi

Kahinaphaku

Fishpond

Kmimi

Phakuloa

Waialua Waialua

Kahinaphaku

Fishpond

0 0.5 1 Kilometer

0 0.25 0.5 Mile

Kmimi

Major Biological Cover

Coral

Coralline Algae

Emergent Vegetation

Macroalgae

Mangrove trees

Uncolonized

Unclassified

Unknown

Benthic Habitat Maps of the South Molokaÿi Reef

97 

Also known locally as “Rock Point” or “20 Mile Beach,” 

Kümimi Point marks the eastern extent of the fringing 

coral reef off the south shore of Molokaÿi. Although 

scattered areas of live coral are found northeast of here, 

aggregate reef growth is limited by high-energy waves 

from the North Pacific swell that wrap around the end 

of the island near Halawa (see Storlazzi and others, this 

vol., chap. 11).

West of Waialua, the reef flat consists of sand and algaecovered pavement with little or no live coral. However, 

the reef flat from Waialua to Kümimi consists of many 

live coral heads and ridges. The reef crest is found fairly 

near to the shore in this region and provides protection 

from offshore waves to snorkelers enjoying the reef flat 

environment. The fore reef consists solely of a barren, 

sloping pavement.

BIOLOGICAL COVER
Päläÿau channel on the south-central coast of Molokaÿi.
Natural Change to the Molokaÿi Reef

All coral reefs are susceptible to change from natural processes. Storm waves, rapid rises in sea level, and flushing 

of nutrient-rich ground water are just a few of the natural 

forces that shape reefs. Identifying these processes and their 

influence on the south Molokaÿi reef is crucial for understanding those changes brought about by human activities.

View of south Molokaÿi, looking eastward from Hale O Lono harbor.
Kahinapöhaku fishpond on the east end of Molokaÿi.
Eric E. Grossman1

The position of sea level, and changes in that position over time, is a primary force that influences reef initiation and development. Sea level can 

vary from millimeters to meters as a result of winds, tides, and waves, 

and over thousands of years sea level may rise and fall more than 100 m (330 ft) 

as continental ice sheets and glaciers expand and recede. In many locations, sea 

level rises or falls relative to the coastline and nearby reefs because of tectonic 

movements that make the land uplift or subside. As Earth’s climate warmed following the last glacial period, sea level rose along island flanks and over coastal 

shelves around the world, enabling coral-reef animals and plants to settle, grow, 

and ultimately create the structural features we observe as reefs today. Most 

modern tropical reefs originated between 14,000 and 8,000 years ago, when sea 

level was 30 to 90 m (100–300 ft) below its present level.

In recent decades scientists have discovered that climate change and the rate 

of sea-level rise and fall have fluctuated widely (Alley and others, 1996; Bard 

and others, 1990, 1996; Blanchon and Shaw, 1995). Consequences for reefs have 

been equally variable. Studies of modern and Holocene reefs in Hawaiÿi have 

shown that under rising and stable sea level, corals generally grow at rates of 

1–2 cm/yr (0.4–0.8 in/yr) (Grigg, 1983; Grossman and Fletcher, 2004). Similar 

studies around the world reveal that where corals could not grow fast enough to 

keep up with sea-level rise, they “drowned.” In other cases, corals grew vertically 

at the same rate that sea level was rising, thus keeping pace. Occasionally, reef 

communities have outpaced the rate of sea-level rise and spread laterally, building 

wide shelves. Because some species of reef-building animals and plants are better 

suited to specific depth and wave-energy settings, they may form assemblages 

unique to discrete subenvironments within a reef. These assemblages naturally 

become stratified into zones as reefs build and respond to the changes in sea-level 

position (and wave energy), often recording these in the structure of the reef. 

Coral skeleton also provides a unique record of ocean surface-water properties, 

including temperature, salinity, and ocean-water volume, because its formation is 

partly controlled by the temperature and chemistry of the surrounding seawater. 

Late Quaternary and Postglacial Eustatic 

Sea-Level History

Sea-level changes result primarily from four mechanisms: (1) eustatic or 

global sea-level change, (2) tectonic movements of the land or sea floor (for 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060.

example, subsidence or uplift), (3) tides, and (4) storms and waves. Of these 

mechanisms, eustatic sea-level rise has had the most profound effect on the 

origin and development of Hawaiian coral reefs. 

Eustatic sea-level change results from change in the volume or mass of 

water within the ocean basins. The two principal mechanisms that control eustatic sea-level change are (1) glacio-eustatic changes resulting from the growth 

and decay of high-latitude and alpine glaciers that add or remove water to and 

from the oceans and (2) thermal expansion or contraction of the ocean water as 

it warms or cools. On very long timescales, change in the volume of the ocean 

basins that hold ocean water also influences eustatic sea level. Our knowledge 

of eustatic sea level is derived from geologic and paleoenvironmental reconstructions of glaciers and their moraines, lake sediment, deep-sea sediment, and 

coral reefs and through the use of tide gages and radar altimetry from satellites. 

It is now widely accepted from paleoenvironmental reconstructions that 

glacio-eustatic sea-level changes have ranged over approximately 120 m 

(400 ft) about every 100,000 years (fig. 1) during the late Quaternary (Lea 

and others, 2002). This resulted from high-latitude ice sheets expanding 

up to 1 to 3 km (0.6 to 1.9 mi) in thickness and later melting in response to 

Figure 1. Late Quaternary sea level (after Lea and others, 2002) is marked by 

100,000-year oscillations that on average range through 120 m (394 ft). Periods 

of warm climate like that of today are characterized by high sea level, while during glacial conditions like those of the last glacial maximum, sea level was 120

m 

lower than today. Sea level generally falls slowly over approximately 80,000 years 

toward lowstands, yet sea-level rise following full glacial conditions usually occurs rapidly in about 20,000 years (as indicated by the arrows).

Figure 2. Curves of postglacial eustatic sea level derived from coral reef studies from Tahiti (orange symbols; Bard and others, 1996) and Barbados (blue 

symbols; Bard and others, 1990). The curves are marked by three periods 

of abrupt rise at approximately 14,500, 11,500, and 8,000 years ago. These 

reconstructions are supported by studies of glacier retreat and marine sediment 

that suggest rapid climate warming and ice melt at those times. Global sea-level 

reconstructions including the ICE-4G model (brown line; Peltier, 1994, 1999) also 

match the general character of these sea-level oscillations.

variations in Earth’s climate associated with changes in its orbit around the 

Sun. As sea level rose and fell as much as 120 m (394 ft) around the world, 

it alternately flooded and exposed large areas of coasts, continental shelves, 

and slopes, causing great environmental change.

Detailed reconstructions and modeling show that periodic falls in glacioeustatic sea level span approximately 80,000 years and are marked by rapid 

oscillations that sequentially fall deeper as climate approaches full glacial 

conditions. In contrast, eustatic sea level rises approximately 120 m very 

abruptly after the culmination of glacial conditions; these rapid fluctuations 

occur on average in only 10 to 20 thousand years. More importantly, recent 

studies show that the rise in eustatic sea level following the last glacial maximum (about 21,000 years ago) was characterized by episodic and abrupt jumps 

in position (fig. 2). During periods of steady ice decay, sea level rose between 

1 and 10 mm/yr (0.04–0.4 in/yr); however, two (perhaps three) marked periods 

120

80

40

0

Sea-level position relative to present, in meters

0 5 10 15 20

Time, in thousands of years ago

Abrupt sea-level rise events

Sea-level

Barbados

Tahiti

Global sea-level 

reconstruction (ICE-4G model)

0 100

Last glacial

maximum

Rapid Slow fall

rise

200 300

 120

 80

Sea-level position

relative to present, in meters  40

0

Time, in thousands of years ago

Natural Change to the Molokaÿi Reef

CHAPTER 10

Sea Level and its Effects on Reefs in Hawaiÿi
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

102

of rapid sea-level rise >20 to 40 mm/yr (>0.8 to 1.6 in/yr) occurred (Bard and 

others, 1990; Blanchon and Shaw, 1995; Edwards and others, 1993; Fairbanks, 

1989). These abrupt jumps in sea level are thought to have been associated 

with rapid melting of ice sheets or catastrophic release of glacial lake water 

(water impounded in temporary lakes behind retreating ice sheets). The history 

of postglacial sea level is supported by numerical models of Earth’s response 

to deglaciation that calculate the volume transfer of ice to ocean water through 

time (for example, ICE-4G, Peltier 1994, 1999). Such models take into 

account how climate evolved and how the land surface responded to the loading and unloading of massive ice sheets and the equivalent weight of ~120 m 

of ocean water added to the ocean floors (Peltier, 1999).

An important aspect of eustatic sea-level rise today is thermal expansion of seawater, also known as the “steric effect.” Volumetric expansion of 

surface ocean waters occurs in response to increased temperature associated with global warming and/or changes in surface ocean currents. Thermal expansion is thought to be a major cause of present sea-level rise and 

may continue for centuries. It can affect sea level on timescales of days 

to years as it influences large-scale currents, such as the Gulf Stream in 

the Atlantic, as well as climate regimes, such as ENSO (El Niño-Southern 

Oscillation), that can temporarily warm the water and raise sea level by 

10 –30 cm (4 –12 in). Sea-level rise and its interaction with other oceanographic and climate processes will affect areas regionally and across the 

entire globe.

Although eustatic sea level is considered a global phenomenon driven 

by additions of water to the ocean basins, regional to local variations in 

the rate of sea-level change occur as a result of tectonic movements, which 

change the position of the sea floor and/or land level. Tectonic movements 

of the land are observed in many areas around the world and are especially 

common where Earth’s lithospheric plates interact and faults are active. 

Tectonic movements also occur in plate interiors, where enormous amounts 

of sediment discharging from large rivers like the Mississippi River depress 

the coast and shelf, and in settings like Tahiti and Hawaiÿi, where growing 

volcanoes place a large load on the sea floor. 

Tides and waves, especially those associated with storms (or tsunamis), also alter sea-level position, although these generally influence reefs 

temporarily on timescales ranging from days to years, and reefs can usually 

recover from the damage they cause. More important to reefs than waterlevel changes associated with tides and waves are the destructive effects of 

scour and shear resulting directly from wave and current impacts. In areas of 

high tidal range or settings protected from waves and storms, often it is the 

low tide levels that restrict upward accretion of reefs.

Because of these different influences on sea level, coastal and reef 

scientists working on particular coastlines are generally most concerned 

with local or regional relative sea-level change. Satellite monitoring and 

tide-gauge records from around the world now show that global eustatic sea 

level is rising at ~2 –3 mm/yr, or about one inch per decade, as a result of 

thermal expansion and glacial meltwater additions to the oceans. Because of 

tectonics, regional relative sea-level change can vary as much as 5–10 mm/

yr (0.2– 0.4 in/yr) faster or slower than that rate.

Figure 3. Least median of squares regression of tide-gauge data from Näwiliwili 

(Kauaÿi), Honolulu (Oÿahu), Kahului (Maui), and Hilo (Hawaiÿi), showing that the 

rate of relative sea-level rise increases toward the Big Island of Hawaiÿi. The difference in the rates is thought to be related to differential island subsidence associated with volcano building, which is centered today near the southern coast of 

the Big Island. Tide-gauge data from the Permanent Service for Mean Sea Level 

(http://http.pol.ac.uk/psmsl/, last accessed April 29, 2008).

In Hawaiÿi the most important influence on relative sea-level rise is the 

variation in island stability due to tectonics. Tide gauge records measure the 

change in sea-level position owing to movements of the tides, winds, waves, 

and decadal-scale climate change. Tide gauges on Kauaÿi, Oÿahu, Maui, 

and the Island of Hawaiÿi show considerably different rates of sea-level rise 

after averaging out the effects of tides, winds, and waves (fig. 3). The global 

eustatic component of sea-level rise should be the same across the relatively 

small area spanning the main Hawaiian Islands, and the difference in the 

relative rates observed in the tide gauges is best explained by vertical island 

movements. Studies of fossil reefs and shorelines on the main Hawaiian 

Islands and modeling studies support the notion that the lithosphere (Earth’s 

crust and upper mantle) is flexing under the weight of the Island of Hawaiÿi’s 

young and massive Mauna Loa and Kïlauea volcanoes (fig. 4). Lithospheric 

flexure, also observed in Tahiti and other island chains, creates maximum 

subsidence where the greatest load is placed on the sea floor and uplift at 

some distance away from the load. The amount of flexure is governed by the 

size of the load and the rigidity of the sea floor and lithosphere. In the main 

Hawaiian Islands, flexure has resulted in rates of relative sea-level rise that 

vary from 1.4 and 1.6 mm/yr on Kauaÿi and Oÿahu, respectively, to 2.3 mm/

yr at Kahului, Maui, and 3.5 mm/yr at Hilo, Hawaiÿi. 

Hawaiian records of postglacial sea-level history have been primarily derived from studies of drill cores and samples from Kauaÿi, Oÿahu, and 

Figure 4. Lithospheric flexure is a hypothesis that is largely validated by tidegauge measurements as shown in figure 3, studies of fossil shorelines that indicate uplift near Oÿahu and subsidence near the Big Island of Hawaiÿi, and modeling 

of plate interactions at hot spots. Lithospheric flexure leads to rapid subsidence 

near the recent volcanic load emplaced on the sea floor near Mauna Loa and 

Kïlauea on Hawaiÿi and uplift at some distance away from the load (for example, 

near Oÿahu). A similar phenomenon occurs in Tahiti, where a young volcano is 

building and inducing higher rates of relative sea-level rise than on neighboring 

islands. Lithospheric flexure, through its influence on relative sea-level rise, affects 

coral reefs in different ways on each island. For example, an early postglacial reef 

on the Big Island of Hawaiÿi that formed when sea level was ~90 m (295 ft) below 

present terminated coral growth about 14,000 years ago and is now found 150 m 

(492 ft) below sea level, capped by deep-water coralline algae (see text).

1950 1970 1990 2010

6,800

6,900

7,000

7,100

7,200

1.40 mm/yr

1900 1940 1980 2020

6,800

6,900

7,000

7,100

7,200

1940 1960 1980 2000

6,800

6,900

7,000

7,100

7,200

2.29 mm/yr

1940 1960 1980

Year

2000

6,800

6,900

7,000

7,100

7,200

Relative sea level, in millimeters

Nwiliwili

Honolulu

Kahului

Hilo

3.53 mm/yr

1.62 mm/yr

KauaØi

Mantle

Lithosphere Uplift

Sea level

Subsidence

Crust

OØahu

50 km

Maui HawaiØi
Sea Level and its Effects on Reefs in Hawaiÿi

103 

Molokaÿi (Easton and Olson, 1976; Engels and others, 2004; Grossman and 

Fletcher, 2004; Rooney and others, 2004). Samples have been collected from 

depths between 50 m (164 ft) and mean sea level, with cores ranging from 

1 to 18 m (3.3 to 59 ft) in length. The texture, composition, age, and diagenetic (alteration) history of the sediment in the cores have been analyzed, and 

interpretations have been made of the depths and environments in which they 

were formed and subsequently altered. Radiometric dating of the calcium 

carbonate skeletal remains within the reef (coral, coralline algae, Halimeda, 

echinoderms, and molluscs), using the isotopic decay rates of radiocarbon 

(

14C) and/or uranium series (230Th/234U), enables reef accretion histories to be 

developed. Corrections to these reef accretion records accounting for differential island movements and adjustments for their habitat (depth) zonation 

allows estimation of past sea-level positions and construction of a history of 

sea-level rise.

Postglacial Sea Level and its Impact on Hawaiian 

Reef Development

The interaction of sea-level rise, wave exposure, and the morphology 

and composition of the sea floor strongly control the location, timing, and 

eventual development of Hawaiian reefs. This has been especially true during the Holocene on Oÿahu and Molokaÿi, as postglacial sea level rose to its 

present position over relatively stable and mature shelf systems. The Oÿahu 

shelf is marked by steep walls at 55 m and 24 m (180 ft and 79 ft) below 

modern sea level, and it is now thought that the shelf at these depths is composed of fossil limestone formed before the Holocene (Fletcher and Sherman, 1995; Sherman and others, 1999). The depths at which notches have 

been cut into these walls correspond to depths at which hypothesized abrupt 

eustatic sea-level rise events began at ~11,500 and ~8,000 years ago (fig. 2), 

though the exact origin of the notches is difficult to conclusively document. 

Rapid sea-level rise over this stepped topography resulted in varied reef 

response. Some reefs were likely abandoned offshore of these walls, where 

corals could not keep pace with sea-level rise. Other locations witnessed the 

formation of sequences of back-stepping reef tracts where the walls were 

more gradually sloping, such as at Hale O Lono (Engels and others, 2004; 

Engels and Fletcher, this vol., chap. 4). Radiometric dating of drill-core 

samples indicate that the fringing reefs we observe today above the wall at 

24-m depth initiated coral growth between 9,000 and 8,000 years ago, with 

greater vertical development where wave exposure is low.

The longer reef-accretion records developed so far in Hawaiÿi show that 

between ~8,400 and 8,000 years ago, an abrupt transition in coral community occurred, with shallow-water corals and coralline algae rapidly replaced 

by deep, calm-water corals (fig. 5). The difference in the representative 

depth zones of the shallow-water assemblages (water depths of ~2–3 m or 

~6.6–10 ft) and the deep-water assemblages (water depths of ~14 m or 46 ft) 

suggests a rapid drowning of as much as ~11–12 m (~36–39.3 ft). A rapid 

sea-level jump is also noted in Caribbean coral-accretion records (Blanchon 

and Shaw, 1995) and may be related to the preservation of an erosional notch 

Figure 5. Model of Holocene reef development (after Grossman and Fletcher 2004; 

Grossman and others, 2006) in relation to accommodation space. Accommodation 

space for reef accretion is controlled by erosive forces, including wave scour, and 

occurred throughout the Holocene largely below the rising sea level (Grossman 

and Fletcher, 1998, 2004; Peltier, 1999). Blue band is Hawaiian sea-level curve of 

Grossman and Fletcher (1998, 2004). Black dashed line is predicted sea level from 

the ICE-4G model of Earth’s lithospheric and sea-level response to deglaciation 

(Peltier, 1999). Blue arrows in time plots of reef accretion show the depths at which 

wave scour controls encrusting coral/bindstone (E) and branching coral framestone 

(Br) accretion based on modern coral ecology (Grigg, 1983; Grossman, 2001). 

Early Holocene reef accretion began between 9,000 and 8,000 years ago with 

encrusting bindstone and early massive coral framestone development. At 6,000 

years ago, massive and branching coral framestones were rapidly accreting following modest rates of sea-level rise. At 4,000 years ago, as sea level stabilized above 

present, branching coral framestone development on open coasts slowed as it filled 

the accommodation space set by sea level and wave scour (approximately ~14 m 

or ~46 ft below sea level). Vertical reef accretion was largely replaced by lateral reef 

development. At 2,000 years ago, as sea level fell from the mid-Holocene sea level 

highstand (~3,500 years ago), reef tops were eroded, which furnished debris that 

accumulated across the reef as rudstone and became bound by cement and algae. 

In the most recent time period with sea level rise, encrusting coral bindstones 

formed thin veneers on top of rudstones or older fossil reef. 

circling Oÿahu at a depth of 24 m (79 ft) (Fletcher and Sherman, 1995). The 

full range of impacts of this abrupt sea-level rise event on reef development 

around the entire Hawaiian Islands has yet to be determined. Recently, a 

study of a reef tract at 150-m depth off the Island of Hawaiÿi showed that a 

shallow-water coral community was abruptly replaced by a deep-water coralline algae community between 15,000 and 14,000 years ago (Webster and 

others, 2004). This history is consistent with rapid drowning during Meltwater Pulse 1A (14,500 years ago, see fig. 2). It also illustrates the importance 

of the combined effect of subsidence and episodic sea-level rise on limiting 

the ability of corals to maintain their position within the photic zone and 

continue accretion.

From analyses of the current inventory of coral core samples in Hawaiÿi, 

we have determined that modern reefs began to develop with encrusting 

and massive framestone accretion between 9,000 and 8,000 years ago (fig. 5) 

as sea level inundated the insular shelves (framestone is limestone that is 

organically bound by coral skeletal growth). These encrusting and massive 

framestone facies (body of rock with specific characteristics) are dominated 

by Porites lobata and Monitpora capitata corals and Porolithon onkodes

? 0

5

 5

 10

 15

 20

 25

Elevation relative to present sea level, in meters

0 2,000 4,000 6,000 8,000 10,000

Time, in years ago

Sea-level curve

(Grossman and Fletcher,

1998, 2004)

ICE-4G

(Peltier, 1999)

HOLOCENE

Massive corals/framestone

Branching corals/framestone

Rudstone (debris-cemented)

Encrusting corals/bindstone

ACTIVE GROWTH

Massive corals/framestone

Branching corals/framestone

Rudstone (debris-cemented)

Encrusting corals/bindstone

Limestone

PLEISTOCENE

Present

sea level

Sea level

Encrusting and massive

corals begin accreting as

accommodation space forms

Massive then branching corals

accrete as accommodation

space increases rapidly

Present 2,000 years ago 4,000 years ago 6,000 years ago 8,000 years ago

Branching and massive corals

try to catch up to sea level

Rudstone (debris) form as reefs

catch up and sea level falls

E

Br

E

Br E

Br

E

Br

Wave scour

Wave scour

Wave scour Wave scour Wave scour

E

Br

Encrusting (binding) corals

develop thin veneers below

wave scour

Natural Change to the Molokaÿi Reef
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

104

and P. gardineri coralline algae. Rapid rates of eustatic sea-level rise between 

8,000 and 6,000 years ago allowed extensive units of branching coral framestone to accrete, outpacing massive and encrusting coral forms, as the reef 

community strove to catch up with sea level position. During this time, thick 

sequences of branching Porites compressa created reef strata between 3 and 

10 m thick, generally in areas protected from waves. P. compressa also grew 

in wave-exposed settings because rapid sea-level rise expanded accommodation space (space for corals to grow below the depth of wave scour). In shallower areas, massive and encrusting coral framestones accumulated at rates 

lower than for branching coral framestone because of wave-induced bottom 

shear stress.

As the melting of the last great ice sheets ended about 6,000 years ago, 

the rise of eustatic sea level slowed and sea level eventually stabilized. It is 

thought that eustatic sea level reached its present position between 6,000 and 

5,000 years ago and that it has remained relatively static since. This stabilization of sea level is thought to have altered the environmental conditions for 

reef growth and, consequently, the types of reef facies that developed. Coral 

sequences identified in core samples show a transition from deeper, calmer 

water corals to shallower, more wave-tolerant corals between 5,000 and 4,000 

years ago, consistent with the interpretation that reef growth and sea-level 

stabilization brought about a change to shallower, more energetic conditions as 

reef tops caught up to sea level and shoaled into the zone of active wave scour. 

By about 4,000 years ago vertical reef framestone accretion nearly terminated, 

and since 3,500 years ago lateral framestone accretion has outpaced vertical 

framestone accretion. Vertical accretion since 3,500 years ago has been largely 

restricted to the development of coral gravels (rudstones), through the accumulation and cementation of eroded reef fragments and sediment, and the formation of thin veneers of encrusting corals (Porites lobata, Montipora capitata, 

M. patula) and coralline algae (Porolithon onkodes) that form bound layers 

of thin-bedded carbonate (bindstone). Exceptions to this pattern occur where 

reefs have developed in settings protected from large open-ocean swell.

The depths and times of formation of these reef facies transitions are 

consistent with the influence of wave scour during the sea-level history of 

Hawaiÿi. Large waves that reach Hawaiÿi’s shores are generated during the 

year from north and south Pacific swell and from tropical storms (see Storlazzi 

and others, this vol., chap. 11). These waves induce erosive forces that break 

coral, transport sediment, and alter reef development and community structure 

at depths reaching 10–14 m (Grigg, 1983; Storlazzi and others, 2003; Grossman and Fletcher, 2004; Storlazzi and others, 2005). As a result, on the stable 

islands of Kauaÿi, Oÿahu, and Molokaÿi reef framework development slowed 

and, where exposed to long-period, large open ocean swell, reef framework 

accretion terminated altogether ~4,000 years ago as reefs caught up with sea 

level and the destructive forces of wave energy limited their vertical accretion 

(Engels and others, 2004; Grossman and Fletcher, 2004). In central southern 

Molokaÿi, however, drill core studies show that framestone continues accreting today (Engels and others, 2004; Jokiel and others, this vol., chap. 5). This 

is likely because of dissipation of wave energy by wave-ray divergence and/or 

sheltering by the nearby islands of Lanaÿi and Kahoÿolawe. (see Storlazzi and 

others, this vol., chap. 11, for a further discussion about waves on Molokaÿi). 

An additional factor that may have caused reefs to stop accreting may be a 

change in the climate regime about the same time (5,000 years ago) that led to 

increased north Pacific swell associated with especially strong El Niño years 

(Rooney and others, 2004).

A unique aspect of middle to late Holocene sea level (5,000 to 2,000 

years ago) was a lowering of equatorial sea level in the Pacific (Fletcher and 

Jones, 1996; Grossman and Fletcher, 1998; Grossman and others, 1998) 

associated with shifts in Earth’s gravity field following the removal of glacial 

ice loads in the high latitudes. Sea level fell 1 to 2 m around Pacific islands, 

contributing to the shoaling of reef flat environments and causing a reduction in water circulation and wave energy landward of the reef crest. Fringing reefs throughout Hawaiÿi experienced shallowing and the shift of coral 

growth to deeper water on the fore-reef slopes. Back-reef environments 

tended to collect fine sediment throughout the late Holocene as a result, 

while active coral growth shifted offshore.

Historical tide-gauge measurements show that global sea-level rise over 

the past 100 years has ranged from 1 to 3 mm/yr and may be accelerating 

(Douglas, 1991; Douglas and others, 2000). These rates are an order of magnitude greater than rates characterizing the past 5,000 years. This rise is thought 

be forced by global warming and the associated thermal expansion of surface 

waters (“steric” effect). Recent studies suggest, however, that retreat of alpine 

glaciers, especially because of disproportionate warming of the high latitudes, 

is adding to the observed rise. For coral reefs this would seem beneficial, 

adding more vertical accommodation space under the water’s surface for new 

growth. This may in fact be the case in Hawaiÿi, where we often observe high 

coral cover in shallow wave-exposed environments, although most of this 

reef cover is composed of encrusting corals and coralline algae that are better 

suited to high wave stresses. The realization that these are thin covers on fossil 

(5,000 to 3,000 years old) substrates suggests that they are transient features 

and perhaps of only recent (<100 years) origin owing to the last century of sealevel rise. 

Summary

Sea level directly influences reef development, and in Hawaiÿi it has 

played a primary role in the initiation, structural development, and community 

composition of Holocene reefs. Significant reef development occurred around 

Kauaÿi, Oÿahu, and Molokaÿi beginning between 9,000 and 8,000 years ago as 

postglacial sea-level rise inundated island flanks. Evidence of abrupt rises in 

sea level, including one at ~8,000 years ago, is preserved in the reef as rapid 

transitions from shallow-water coral and coralline-algal communities to deepwater coral assemblages. The vertical accretion of deep-water assemblages 

slowed and ultimately terminated about ~5,000 to ~3,000 years ago as sea 

level stabilized, and the reefs themselves altered their growth habits as they 

grew into the shallow depths zones above 14 to 10 m. At these depths, waves 

influenced reef community composition toward more wave-tolerant species 

and significantly limited new vertical reef development. This has resulted in 

a narrow depth window for Holocene reef growth and has limited major reef 

accretion to protected settings in deeper water and sheltered embayments. 

The possibility of increased wave impact associated with a change in climate 

~5,000 years ago is being tested to explore its role on reef development along 

with sea-level history. Modern thin encrusting reef communities appear to be 

transient features less than 100 years old that are scoured off the sea floor periodically by massive swell events, leaving a fossil shelf exposed at the surface. 

Reef development history may be significantly different on Maui and Hawaiÿi 

in response to higher rates of relative sea-level rise and the continued addition 

of accommodation space as those islands actively subside.
Natural Change to the Molokaÿi Reef

CHAPTER 11

Waves and Their Impact on Reef Growth

Curt D. Storlazzi1, Michael E. Field1, Eric K. Brown2, and Paul L. Jokiel3

I

t has been known for some time that there are strong qualitative correlations between wave energy and coral distribution (Rosen, 1975; Geister, 

1977; Vosburgh, 1977; Dollar, 1982; Done, 1983; Massel and Done, 

1993; Rogers, 1993; Blanchon and Jones, 1997). Grigg (1998) has most 

recently discussed the interplay between wave energy and reef properties in 

the Hawaiian Islands, but as in most past studies, wave-energy is classified in 

terms of the loosely divided categories of “low,” “medium,” and “high” wave 

energy regimes. Jokiel and others (2004) show that maximum wave height in 

Hawaiÿi is negatively correlated with coral cover, diversity, and species richness. There has yet to be, however, a large-scale quantitative investigation of 

these relationships that compares the motions exerted by waves upon the reef 

to the distribution of different stony coral species. Our goal here is to better 

quantify the interplay between wave-induced forces, reefs, and corals.

Waves

Wave Observations and Modeling

In order to provide accurate wave data for the entire south shore of 

Molokaÿi, we used the U.S. Naval Oceanographic Office’s Spectral Wave 

Prediction System (SWAPS) version 4.0 wave model (Storlazzi and others, 

2005). This model generated a gridded field of wave height, wave period, 

and wave direction for analysis. Real-time output of wave height and wave 

period were compared to concurrent observations of wave height and wave 

period made at two U.S. National Oceanographic and Atmospheric Administration (NOAA) offshore deep-water buoys (#51001 and #51002; National 

Data Buoy Center, 2002) and five wave/tide gauges that we deployed in 

11-m (36 ft) mean water depth off the south shore of Molokaÿi (fig. 1). Wave 

heights and wave periods calculated from the wave model differed from the 

measurements by less than 10 percent and verified that the model’s estimation of energy loss due to refraction and shoaling worked properly. The 

modeled wave heights and wave periods produced by the wave model for 

the shallowest grid cells that were closest to the south shore of Molokaÿi 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

2

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744;

current address: Kalaupapa National Historical Park, P.O. Box 2222, Kalaupapa, HI 96742

3

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneÿohe, HI 96744

were then used for input into the coral force-balance model. When the orbital 

motion of the water as a wave passes interacts with the sea floor, it imparts a 

force or stress (stress = force/area) on the sea floor; this stress is better known 

as a wave-induced shear stress. Table 1 gives examples of how variations in 

wave height, wave period, and water depth affect wave-induced shear stresses 

on the sea floor. These peak wave-induced near-bed shear stresses were calculated following the methodology presented by Jonsson (1966).

The wave events modeled corresponded to the largest waves observed 

for each wave-energy regime over the previous 15 years for which the 

boundary-condition data were of a high enough quality to run the wave 

model (table 2). We chose to use these events because 15 years approaches 

the recovery time for the dominant coral species observed off Molokaÿi 

(Dollar, 1982; Grigg, 1983) and corals and their resulting reefs generally 

develop under the environmental conditions observed over the time necessary for their recovery (Graus and others, 1984; Rogers, 1993). The North 

Pacific swell model run displayed a strong shadowing effect by the island of 

Molokaÿi along its south coast (fig. 2). The north, east, and west sides of the 

island were exposed to the brunt of this large swell, which could generate 

peak wave-induced near-bed shear stresses greater than 1.75 N/m2

 (newtons 

per square meter) along most of Molokaÿi’s shoreline. The area off the south 

Figure 1. U.S. Geological Survey (USGS) wave data for south coast of Molokaÿi. 

A., Location of USGS wave and tide gauges deployed along the 11-m (36-ft) 

isobath. B, Calculated peak wave-induced near-bed shear stresses from the 

USGS wave and tide gauges. Gaps in the individual gauges’ records are a result 

of instrument failures.

Hale o Lono PlØau

Kamiloloa

Kamal

PkoØo

0 4 8 Kilometers

0 2 4 Miles

2

3

Wave-induced shear stress, in newtons per square meter

1

0

2

3

1

0

2

3

1

0

2

3

1

0

2

3

1

0

Hale o Lono

PlØau

Kamiloloa

Kamal

PkoØo

A

B

July 1

2001 2002

Apr 1 Oct 1 Jan 1

Variable Wave height 

(m)

Wave period 

(s)

Water depth 

(m)

Resulting shear stress 

(N/m2)

1 10 15 0.42

Wave height 2 10 15 1.36

3 10 15 2.72

1 5 15 0.04

Wave period 1 10 15 0.42

1 15 15 0.54

1 10 5 1.38

Water depth 1 10 15 0.42

1 10 25 0.20

Table 1. Examples of variations in wave-induced shear stresses caused by 

variations in wave height, wave period, and water depth.

[Peak wave-induced near-bed shear stresses calculated as per Jonsson (1966)]
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

106

shore, which contains the large fringing reef, is a shadow zone for North 

Pacific swell, with modeled peak wave-induced near-bed shear stresses less 

than 0.25 N/m2. Along most of the fringing reef, these peak stresses were less 

than 0.10 N/m2 in 10 m (33 ft) of water. At both the east and west ends of the 

island, where the reef narrows, peak wave-induced near-bed shear stresses 

rapidly increase fivefold to more than 0.50 N/m2

 as some wave energy 

refracts around the ends of the island (fig. 3).

Kona storm waves (not shown), which were modeled to observe how 

the south shore of Molokaÿi would be affected by an unrefracted swell 

from the south, did not generate high near-bed wave orbital velocities (peak 

wave-induced near-bed shear stresses = 0.08 N/m2) anywhere along the 

island. The Southern Ocean swell, on the other hand, produced intermediate 

values of peak wave-induced near-bed shear stresses (~0.25 N/m2) along 

almost the entire length of Molokaÿi’s fringing reef, and these waves have 

been observed to resuspend high quantities of fine-grained terrestrial sediment on the reef flat (Storlazzi and others, 2000). The modeled northeast 

trade-wind waves appear to affect primarily the east and southeast coast 

of the island. The coastline between the southernmost point of the island 

at Kamalö and the east end of the island is directly exposed to trade-wind 

waves. This resulted in peak wave-induced near-bed shear stresses being 

50 percent to 100 percent higher than to the west of Kamalö, where the 

coast is protected from direct impact of trade-wind waves. The short wave 

period characteristic of trade-wind waves is reflected in the greater disparity between the peak wave-induced near-bed shear stresses for the different 

isobaths east of Kamalö than to the west of Kamalö, where the coastline is 

not directly affected by these waves (fig. 4). These waves produce rather low 

values of peak wave-induced near-bed shear stresses to the west of Kamalö, 

on the order of 0.15 N/m2

. However, the modeling suggests that trade-wind 

waves, in combination with a typical North Pacific swell, can generate 

much higher peak wave-induced near-bed shear stresses along the east and 

southeast shore of Molokaÿi, on the order of 0.40 – 0.50 N/m2, in the area 

where the reef starts to pinch out.

Figure 2. Wave model runs for 

three of the main wave climates 

for Molokaÿi. Note how the different combinations of wave 

heights, wave periods, and wave 

directions result in different areas being affected by high peak 

wave-induced near-bed shear 

stresses. A, North Pacific winter 

swell. B, Trade-wind waves. 

C, Summer south swell. 

Figure 3. Aerial photograph looking north over Molokaÿi’s west end, showing 

North Pacific winter swell waves wrapping around Läÿau Point and propagating 

east along the southwest shore. 

Gordon Tribble, USGS

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

Moloka`i

Maui

Lna`i

1

0

2

3

4

5

6

8

10

12

14

16

0

0.25

0.50

0.75

1.00

20.8

21.0

21.2

21.4

20.8

21.0

21.2

21.4

157.4 157.2 157 156.8 156.6 156.4

20.8

21.0

21.2

21.4

A North Pacific winter swell

1

0

2

3

4

5

6

8

10

12

14

16

0

0.25

0.50

0.75

1.00

20.8

21.0

21.2

21.4

20.8

21.0

21.2

21.4

157.4 157.2 157 156.8 156.6 156.4

20.8

21.0

21.2

21.4

B Trade-wind waves

1

0

2

3

4

5

6

8

10

12

14

16

0

0.25

0.50

0.75

1.00

20.8

21.0

21.2

21.4

20.8

21.0

21.2

21.4

157.4 157.2 157 156.8 156.6 156.4

20.8

21.0

21.2

21.4

C Summer south swell

Wave height,

in meters

Wave period,

in seconds Wave-induced shear stress, in newtons per square meter

Latitude, in degrees north

Longitude, in degrees west

Wave height,

in meters

Wave period,

in seconds Wave-induced shear stress, in newtons per square meter

Latitude, in degrees north

Longitude, in degrees west

Wave height,

in meters

Wave period,

in seconds Wave-induced shear stress, in newtons per square meter

Latitude, in degrees north

Longitude, in degrees west

Wave regime Date

(month/day/year)

Wave 

height 

(m)

Wave 

period 

(s)

Wave 

direction 

from (deg)

Wind 

speed 

(m/s)

North Pacific swell 01/28/1998 8.4 20 320 6.1

Trade-wind waves 06/23/1998 3.2 8 50 13.8

Southern Ocean swell 06/08/2000 2.5 20 180 6.2

Table 2. Modeled wave conditions.

[Based on data from the National Data Buoy Center (2002)]
Waves and Their Impact on Reef Growth

107 

Natural Change to the Molokaÿi Reef

Relationship Between Waves and Reef Morphology

Overall Reef Morphology

A strong inverse exponential relationship (r 2 >0.81) exists between the 

modeled peak wave-induced near-bed shear stresses and the width of the 

reef flat at different locations along the south shore of Molokaÿi (fig. 5). This 

correlation implies that reef-flat width, and thus very likely the overall reef 

width, decreases exponentially with increasing wave energy.

Spur-and-Groove Morphology

Computed mean spur heights and spur widths for the south Molokaÿi 

reef were compared to the modeled wave-induced peak bed shear stresses to 

determine if there are any relations between spur-and-groove morphology 

and wave forces. Mean spur height along the 5-m, 10-m, 15-m and 20-m 

(16-ft, 33-ft, 49-ft, and 66-ft) isobaths was shown to decrease logarithmically 

with increasing modeled peak wave-induced near-bed shear stresses (fig. 

6A). Mean spur width along the same four isobaths showed a similar relation to wave energy, decreasing logarithmically with increasing peak waveinduced near-bed shear stresses (fig. 6B). However, at shallow depths (5 m), 

spur height and spur width both displayed a trend opposite to the data for the 

four isobaths, with a slight increase in height or width with increasing peak 

wave-induced near-bed shear stresses (not shown).

The ratio of spur height to spur width decreased logarithmically with 

increasing peak wave-induced near-bed shear stresses (fig. 6C); this relation 

is significant above the 1-percent level. The low percentages of variability 

indicated by these correlations between waves and spur-and-groove morphology are likely due to a number of factors. These include natural variability in a complex biogeomorphological system, the disproportionate spatial 

resolutions of the SHOALS data (on the order of meters) and the wave 

model output (on the order of kilometers), and other important controls like 

variations in light availability for photosynthesis due to turbidity.

Spur-and-groove features appeared to occur in two distinctly different 

morphological groups correlated with different wave energies (fig. 7). Overall, spur-and-groove structures tended to be more than twice as high (1.1 m 

or 3.6 ft) and slightly wider (93 m or 305 ft) in lower energy environments as 

compared to those in higher energy environments (0.5 m and 87 m, or 1.6ft 

and 285 ft, respectively). In regions of low wave energy, numerous short, 

narrow spur-and-groove structures are observed in shallow water depths (fig. 

7B), whereas in mid water depths (fig. 7C), fewer, broader, and taller spurs 

dominated, followed by a reversal back to more numerous, short and narrow 

spurs lower on the fore reef along the 20-m isobath. The numerous short, narrow spur-and-groove structures either truncated or merge together to form the 

Figure 4. Modeled peak wave-induced near-bed shear stresses at different fewer, broader and taller spur-and-groove structures along the 15-m isobath.

depths off Molokaÿi’s south shore. Also noted are the different types of waves 

that dominate each section of coastline.

Figure 5. Plot showing the relation between peak wave-induced near-bed shear 

stress and the width of the reef flat off Molokaÿi’s south shore. The exponential 

curve fit is significant above the 0.1-percent level. This trend demonstrates that 

the reef flat is widest where the wave energy is lowest.

Figure 6. Relations between peak wave-induced near-bed shear stress and 

shape parameters of coral spurs: spur height (A), spur width (B), and ratio of 

spur height to spur width (C ). The error bars denote ±one standard deviation. 

The exponential curve fits in A, B, and C are significant above the 0.1-percent, 

0.1-percent, and 2-percent levels, respectively. Spur height and width are variable, but spurs tend to be higher and wider at low wave-induced shear stresses 

and become shorter and narrower at high shear stresses. The data suggest 

that waves are the dominant factor controlling spur-and-groove morphology, 

but they explain only a relatively low percent of the variability. Natural variability 

in the system and other factors, such as light availability for photosynthesis, 

turbidity, and the high resolution of the measurements as compared to the low 

spatial resolution of the wave model, are likely other sources of variability.

0

0.5

1.0

1.5

2.0

157 156.75

5

10

15

20

 Wave-induced shear stress, in newtons per square meter

Longitude, in degrees west

157.25

Water depth,

in meters

2.5

Kamal N o r t h P a c i f i c s w e l l Southern ocean swell or Kona storm waves

T r a d

e - w i n d

w a v e s

North Pacific swell

500

0

1,000

1,500

2,000

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Width of reef flat, in meters

Wave-induced shear stress, in newtons per square meter

y = 2503.28x-5.54x

R = 0.902

0 0.2 0.4 0.6 0.8 1.0

y = 0.3569  1.1826 log(x)

R = 0.42 

A

B

C

y = 62.67 39.59 log(x)

R = 0.51 

y = 0.0078  0.0069 log(x)

R = 0.27 

Spur height, in meters

 Wave-induced shear stress, in newtons per square meter

0

1.0

2.0

3.0

4.0

Height/width ratio

0

0.01

0.02

0.03 Spur width, in meters

40

80

120

160
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

108

In higher energy environments, however, spur-and-groove structures 

tended to be roughly similar in height and width regardless of water depth, 

with the tallest spurs typically being observed in the shallowest water depths 

(fig. 7D–E). The spur-and-groove measurements in shallow water (5 m) 

agree with the observations made by Munk and Sargent (1954) along atolls 

in the western Pacific Ocean and by Roberts (1974) along Grand Cayman in 

the Caribbean Sea. At greater depths (15–20 m or 49 – 66 feet) off Molokaÿi, 

the relation of spur-and-groove morphology to wave energy was the exact 

opposite of that found by Roberts and others (1980) and Blanchon and Jones 

(1997) along the deeper shelf-edge reef off Grand Cayman. The population 

of reef-forming corals in both the Caribbean and western Pacific is distinctly 

different from those of Hawaiÿi. It is not clear what role different species 

may play in influencing the morphology of spur-and-groove structures.

Implications of Results to Understanding 

Processes of Reef Formation

The relations shown here between modeled wave parameters and reef 

morphology suggest that peak wave-induced near-bed shear stresses are a 

dominant control on the distribution and morphology of the reef off southern 

Molokaÿi. The central portion of the reef is protected from the direct impact by 

large waves, and thus the reef framework is able to develop into an extensive 

shallow reef flat and broad fore reef. The east and west ends of the island are 

exposed to larger, longer period waves that generate high orbital wave velocities and thus strong near-bed shear stresses. This appears to limit substantial 

reef development because of physical breakage of the coral and high abrasion 

by both bedload and suspended sediment (Dollar, 1982; Grigg, 1998).

In order to explain the variability in spur-and-groove morphology at a 

given water depth, where light availability remains relatively constant, other 

physical factors must come into play (fig. 7). The slightly higher and more 

widely spaced spurs in higher energy shallow areas (fig. 7D) relative to 

lower energy shallow regions (fig. 7B) may be explained by the interaction 

between sediment and coral. The spurs in the shallow, high-energy portions 

of the reef are likely constructed in part by the binding of coral rubble by 

diagenetic cementation or coralline algae as described by Shinn and others 

(1981), Macintyre (1997) and Rasser and Riegl (2002). These studies and 

others (Tribble and others, 1990, 1992; Brainthwaite and others, 2000) have 

shown that such processes are dominant along low-slope energetic shallow reef crests and upper fore reefs but are absent along the more quiescent 

deeper portions of the steep fore reef. The large amount of coral rubble and 

coarse-grained sediment resulting from the weathering of broken coral may 

effectively increase the abrasion experienced by corals closer to the sea bed. 

Consequently only those spurs with enough height to avoid this abrasion 

likely promote coral growth. Thus, higher wave energy in more exposed 

shallow areas may advance spur development through cementation and 

binding of coral rubble, whereas at depth intact coral development may be 

retarded by wave stresses where these processes are absent and in-place coral 

growth dominates. 

Another significant factor shown to control reef development is antecedent topography. While a lower gradient shelf would result in a greater crossshore extent of the shelf being situated in the zone optimal for coral growth 

in terms of light availability and wave energy, reef development in this space 

would still be contingent on low enough wave energy to allow reef growth. 

This is why well-developed active reefs are not observed off northern Oÿahu 

or Maui. These areas have similar coastal configurations to south Molokaÿi but 

are directly impacted by the large North Pacific swell that generates extremely 

high (>1.5 N/m2

) peak wave-induced near-bed shear stresses (Grigg, 1998).

These analyses clearly show that waves are the primary control on both 

large-scale reef morphology and smaller scale spur-and-groove morphology 

where high peak wave-induced near-bed shear stresses are present. In these 

areas light, water depth, and antecedent topography are subordinate. The 

higher morphologic variability at lower peak wave-induced near-bed shear 

stresses demonstrates that when wave-induced water motions are low enough 

as to not dominate reef and spur-and-groove morphology, other factors such 

as light availability for in-place coral growth and the ability to bind or cement 

rubble increase and may become the dominant factors.

Figure 7. Diagrams showing the relations among spur 

morphology, wave energy, and water depth off south 

Molokaÿi. A, Plot of spur width against spur height in different conditions. The error bars denote ±one standard 

deviation. B–E, Cross-sectional profiles of typical spurs 

at different water depths and levels of wave energy. In 

low-energy conditions, the spurs are short and narrow 

in shallow water (B) but are very tall and wide in deeper 

water (C). In high-energy areas, by contrast, the spurs do 

not vary much in shape (D–E), suggesting that different 

processes might be more important in defining coral-spur 

morphology in calm areas versus high-energy areas. Spur height, in meters

Spur width, in meters

Spur height, in meters

Spur width, in meters

Low Energy

High Energy

5

10

15

15

Depth,

in meters

Shallow

Low energy (<0.3 N/m2)

Deep

Low energy (<0.2 N/m2

)

Shallow

High energy (>0.4 N/m2

)

0 40 80 120

0 40 80 120

0 40 80 120

0 40 80 120

B

A

D

C

E

Deep

High energy (>0.3 N/m2)

0.2

0.6

1.4

1.8

2.2

1.0

0

1

2

0

1

2

0

1

2

0

1

2

B

C

E

D

60 70 80 90 100 110 120
Natural Change to the Molokaÿi Reef

CHAPTER 12

The Freshwater Cycle on Molokaÿi

Gordon Tribble1 and Delwyn S. Oki1

Rain is the source of all freshwater on Molokaÿi. Most of the rain falls 

in the upland (mauka) areas at higher elevations, generally above 

600 m (2,000 ft). Figure 1 shows the average amount of daily rainfall per square mile in four zones of the island; also shown is the amount 

of rain that runs off as surface water, evaporates and transpires back to the 

atmosphere, and recharges the aquifer. The wettest part of the island, the 

upper slopes of East Molokaÿi Volcano, receives an average of nearly 4 m 

(more than 150 in) of rain per year (fig. 2). The driest parts of the island, 

such as coastal areas of West Molokaÿi, receive an average of less than 

0.4 m (15 in) per year (Giambelluca and others, 1986; fig 3). 

On an average day, the entire island receives about about 2 billion liters 

(552 million gallons) of rain. Of this amount of rainwater, about 50 percent 

evaporates or is transpired by plants, about 16 percent runs off to streams, 

and the remaining 34 percent infiltrates into the ground and becomes ground 

water. Drier areas of the island (such as the West Aquifer Sector; fig. 1) tend 

to have a higher percentage of rainwater going to evaporation and transpiration and a lower percentage going to runoff and recharge, whereas the wetter 

areas (such as the Northeast Aquifer Sector; fig. 1) tend to have a higher 

percentage of rainwater going to runoff and recharge and a lower percentage 

going to evaporation and transpiration (Shade, 1997). 

Figure 2. View of Pelekunu Valley, on the northeast coast of Molokaÿi, showing 

lush vegetation and deep erosion in the valleys resulting from the wet climate in 

the northeastsern part of the island.

Figure 3. View of the south coast of Molokaÿi from Kamiloloa to Läÿau Point as 

seen from Kawela, showing the subdued landscape and sparse vegetation that 

result from the arid climate in the southwestern part of the island. The fringing 

coral reef, coastal fishponds, and mudflats at Päläÿau are also visible. 

Figure 1. Map of Molokaÿi showing 

average annual rainfall per square 

mile in different areas of the island. 

The percentage distribution of the 

rainwater into runoff, evaporation 

and transpiration, and ground-water 

recharge is shown in pie charts. The 

size of each pie chart indicates the 

total amount of rainwater. The map 

also shows the locations of stream 

(continuous-record and peak-flow) 

and spring-discharge monitoring stations operated by the U.S. Geological 

Survey in 2007. Data from these stations can be found at http://hi.water.

usgs.gov/ (last accessed April 29, 

2008).

1,381 ft

1 million gallons rainfall

per square mile per day

5 million gallons rainfall

per square mile per day

Amount and

distribution of rainfall

Evapotranspiration

Runoff

Recharge

KakaØako

Kaunala

KluapeØelua

Manawainui

Kawela Gl

Pu naØula

P hakupili

Kamiloloa

WwØia

Mna ol

Pphaku Gl

Hakina

Onopalani

Wa ai kne

Waiahewahewa Gl

Waialua Str

Kahawaiiki Str

Hlawa

W

Str ailau Str

Pelekunu Str Waiohookalo

Waikolu Str

Wai aØ leØia Str

Waihnau

Str

Kawainui Str

Ppio

H el na

Kaunakakai

Coconut Grove

Pelekunu

Bay

MoØomomi

Hlawa Bay

Kalaupapa

KalaØe Hlawa KualapuØu

Maunaloa

Kaunakakai Kamiloloa

Kamal

ØUalapuØe

ØUmipaØa

Kawela

PlØau

PkoØo

Hale o Lono Kkk

Cape

Hlawa

KahiØu Point

Kalaupapa

Peninsula

Øìlio Point

LØau 

Kapuiwa

Coconut Grove 

Point

KualapuØu

Reservoir

0

0

2 4 Miles

2 4 Kilometers

4,961 ft

FishpondÑmodified from Hawaii Office of Planning (2006)

Aquifer-sector boundary

Molokai Irrigation System pipeline

Molokai Irrigation System tunnel

Streamflow-gaging station, continuous

Streamflow-gaging station, peak flow

Spring measurement site

WEST AQUIFER SECTOR

NORTHEAST

CENTRAL AQUIFER SECTOR

AQUIFER SECTOR SOUTHEAST

AQUIFER SECTOR

Mapulehu
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

110

Recharge is important because it replenishes underground aquifers with 

freshwater. This fresh water flows slowly through the aquifers and eventually 

discharges from springs and seeps into streams or the ocean. Under natural 

conditions, the average rate of recharge to an aquifer balances the amount of 

discharge. Water removed from aquifers by pumping is no longer available to 

discharge to streams or the ocean. Such ground water pumped from aquifers 

supplies most of the freshwater for drinking and other uses on Molokaÿi. For 

the period 2000 to 2002, about 4.2 million gallons (about 16 million liters) 

a day was pumped for domestic use on the island. About 50 percent of this 

water is pumped from the Kualapuÿu area. To assist in managing the supply of 

ground water, computer models are used to simulate the flow of ground water 

and evaluate the effects of pumping (Oki, 1997, 2000, 2006). Water use on the 

island of Molokaÿi is regulated by the State of Hawaiÿi Commission on Water 

Resource Management, which designated the entire island as a Ground Water 

Management Area in 1992.

Ground-water discharge along the south shore of Molokaÿi is much 

higher on the east because of the greater amount of rain that falls on that 

part of the island. For example, between Kawela and Pükoÿo the discharge 

of freshwater is estimated to be 

approximately 4.0 million gallons per 

day per mile (about 9.4 million liters 

per day per kilometer) of coastline. On 

the west side, between Hale O Lono 

and Kükükü Gulch, the discharge of 

freshwater is only one-tenth as much, 

approximately 0.39 million gallons per 

day per mile (about 0.9 million liters 

per day per kilometer) of coastline. 

Water in streams comes both from 

the direct runoff of rainfall and from 

the discharge of ground water from 

springs and seeps (which is called base flow). Streams on the northeast side 

of the island flow continuously (perennial streams) and are sustained by 

base flow because their valleys have eroded deeply enough over time to cut 

into the water table (fig. 2). Hälawa Valley, on northeast Molokaÿi, contains 

a good example of a perennial stream. Streamflow in Hälawa Valley has 

been measured for most of the time since 1917 at a site where the stream 

drains an area of about 11.9 km2

 (about 4.6 mi2) that receives an average 

of 2 m (80 in) per year of rainfall. Since 1917, the median flow at that site 

has been about 8.4 million gallons (about 32 million liters) per day. However, flow has varied from a daily low of 0.56 million gallons per day to a 

peak flow of 17,400 million gallons per day (about 2.1 million to 65,900 

million liters per day). Figure 4 shows an example of how flow can change 

very rapidly. Such extremes in the amount of flow are typical of streams in 

Hawaiÿi, and a rapid increase in flow can be very dangerous if people are 

crossing or playing in the stream. Great caution should be exercised around 

any stream during periods of rain.

Flow can also change over a period of many years. For example, from 

1918 to 2002, the average annual base flow in Hälawa Valley has decreased 

by an average of 0.53 percent per year; this equals a cumulative decline of 

44 percent (fig. 5). This decline is a result of lower rainfall in the second 

half of the 20th century (Oki, 2004), but it is not known if this trend is part 

of a long-term cycle or represents a distinct change in climate. 

Most streams on Molokaÿi have no base flow and are ephemeral; they 

flow to the ocean only when there is enough rain to generate runoff. Runoff 

from watersheds carries sediment and other materials from the land to the 

ocean. In 2004 a gage was built to measure the amount of water and sediment flowing in Kawela Stream about 600 m (about 2,000 ft) inland from 

the coast. The flow at the gage is from a drainage area of about 13.7 km2

(about 5.3 mi2

) with an average annual rainfall of about 1.1 m (45 in). The 

amounts of water flow and sediment load measured by the gage are shown 

in figure 6. 

For the one-year period from October 1, 2004, to September 30, 2005, 

water flowed in the stream 29 percent of the time. The median flow, during 

times when the stream was flowing, was 2.2 million gallons (about 8.3 

million liters) per day, and the peak flow was 345 million gallons (about 

1,300 million liters) per day. The concentration of suspended sediment 

ranged from 1 to 1,700 mg/L. The 

total amount of sediment carried 

towards the ocean at this site during 

this period was 2,789 tons, and the 

highest amount for a single day was 

1,030 tons on March 9, 2005. The five 

days with the highest sediment load 

contributed more than 90 percent of 

the total sediment load for the year. 

Information from this site will be used 

to estimate the amount of sediment 

reaching the coral reefs along the 

south coast of Molokaÿi and provide 

information on how the coral reefs are being affected by processes in the 

watershed. This information can also be used to assess the effectiveness of 

watershed management plans because it will be possible to measure reductions in the rate of erosion over time.

Figure 4. Flow in Hälawa Stream during January 1–4, 2005, showing rapid 

changes in flow from less than 10 million to more than 1,000 million gallons per day. 

The high rates of flow are caused by direct runoff of rainfall, whereas the sustained 

low rates are caused by the base-flow discharge of ground water into the stream.

Figure 5. Average annual base flow in Hälawa Stream since 1918. The solid brown 

line shows the long-term trend of declining flow (from Oki, 2004). Base flow is estimated from daily records of total flow using computerized methods that separate the 

measurement of total flow into direct runoff and base flow. Note: data are missing for 

the years of 1920 and 1932 to 1938.

Figure 6. Graphs of daily flow (top) and sediment load (bottom) for Kawela Stream 

during the one-year period from October 1, 2004, to September 30, 2005. Streamflow occurred only intermittently, and most of the sediment load occurred during a 

few periods of wet weather. Instantaneous values can differ substantially from daily 

values because streamflow can vary greatly even during one day.

January 1 January 2 January 3

Peak flow of 1,200 million gallons

per day at 5:35 A.M. on

January 2, 2005

10

1

100

Flow,

in million gallons per day 1,000

10,000

2005

1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

2

0

4

Flow,

in million gallons per day 6

8

10

20

0

40

Flow,

in million gallons per day

60

Sept Oct Nov

2004 2005

Dec Jan Feb Mar April May June July Aug Sept

Sediment load,

in tons per day

0

200

400

600

800

1,000

The U.S. Geological Survey (USGS) has measured flow in both perennial and ephemeral streams on Molokaÿi since 1917. At one time 

(1968), 9 continuous and 11 peak-flow stream gages were in operation on the island, but as of 2007 only 3 continuous and 4 peak-flow 

stream gages were in operation. The locations of these sites are 

shown in figure 1. Data from most of these sites are available on the 

internet at www://hi.water.usgs.gov/ (last accessed July 11, 2007). 

Funding to operate these stations currently comes from the State 

of Hawaiÿi Department of Land and Natural Resources, the State of 

Hawaiÿi Department of Transportation, Maui County Department of 

Water Supply, and the USGS Cooperative Water Program.
Natural Change to the Molokaÿi Reef

CHAPTER 13

Ground Water and its Influence on Reef Evolution

Eric E. Grossman1, Joshua B. Logan1, Joseph H. Street2, Adina Paytan2, and Pat S. Chavez, Jr.3

Ground water—water that is stored in and transported through the 

rocks and sediment below the Earth’s surface—is an important 

resource of water as well as a site for biologic, geologic, and chemical processes (fig. 1). The influence of human activities on this resource 

is of particular concern because such activities can produce nutrients and 

contaminants that may infiltrate the ground water and move with it to areas 

of discharge at the coast. Unlike streams and lakes, ground-water discharge 

is much more difficult to access and measure, and therefore our understanding of its quantity, character, fate, and ultimate role on coastal environments 

is still limited.

Because water flowing through rocks often moves slowly, it can have 

a long residence time within the rocks. As a result, the water can acquire 

a unique chemical signature, which can be used to distinguish its source. 

Where land-use activities (industry, agriculture, urbanization) are intense, 

ground water may contain concentrated levels of terrestrially derived nutrients, trace metals, and other organic and inorganic contaminants, including 

many synthetic compounds and those associated with human waste.

Ground water commonly plays a vital role in structuring coastal habitats and ecosystems. When either ground-water composition or groundwater discharge rate changes, it is likely to induce alteration of the environmental conditions at the coast. Submarine ground-water discharge (SGD) 

is that portion of ground water that enters the ocean below sea level and 

may be composed of fresh ground water or a mixture of fresh ground water 

and seawater (for example, brackish water). As a result, studying SGD and 

determining its effects on coastal ecosystems is very challenging.

Exciting advances in the technology of airborne sensors, submersible 

instruments, and marine chemistry are enabling better mapping of SGD 

along the coast and providing means to calculate the quantity, composition, 

and fate of SGD into the coastal ocean. These tools can help differentiate 

processes and pathways of contamination associated with land-use activities 

from those of natural phenomena by examining the chemistry of SGD. Analyzing compounds collected in the annual growth bands of coral skeleton 

may help determine past levels of discharge and temporal changes in them. 

Freshwater from surface or ground water has long been known to be more 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

2

 Stanford University, Department of Geological and Environmental Sciences, Stanford, CA 94305;

current address: University of California, Institute of Marine Science, Santa Cruz, CA 95064

3

 U.S. Geological Survey Flagstaff Science Center, 2255 North Gemini Dr., Flagstaff, AZ 86001; 

current address: Northern Arizona University, Department of Geography, PO Box 15016, Flagstaff, AZ 86001

acidic than seawater and, as a result, is notable for dissolving coral reefs on 

many scales: on the microscopic scale, submillimeter cements form within 

coral skeleton, and over tens to hundreds of kilometers, entire reef systems 

dissolve to produce karst landscapes. 

Determining the role of ground water on Hawaiian reefs, and the south 

Molokaÿi reef in particular, is important for understanding the reef’s current 

health and future fate. A growing concern is the impact of excess nutrients 

derived from land-use activities, especially inorganic nitrogen in the form of 

nitrate and nitrite. These chemicals are often found in higher concentration 

in ground water and are considered a limiting nutrient in coastal waters. The 

addition of excessive amounts of nitrogen to relatively nutrient-poor coastal 

waters surrounding coral reefs therefore can have significant ecosystem 

impacts. In many reef settings, excess inputs of nutrient have led to a shift 

from coral-rich communities to excessive algal and plant growth, which can 

smother and kill reefs (Hughes and others, 2003b; Szmant, 2002). 

Understanding Submarine Ground-Water Discharge 

in Coastal and Reef Settings

Our understanding of ground-water discharge and its effects on coastal 

and reef settings is derived in four primary ways: (1) local knowledge and 

observation, (2) physical property surveys of the water column, (3) chemical analyses of water samples, and (4) hydrologic modeling. Specific sites of 

SGD in the shallow coastal zone of Hawaiÿi, and of Molokaÿi specifically, as 

in many island settings, are often known from local observations of fisherman 

and coastal residents. Many legends are also validated today by observation of 

cold and/or fresh water emanating from the ground (fig. 2A–D) and through 

the shallow sea floor at sites known in Hawaiÿi as “water holes” (lua wai), like 

the many cold-water springs around Kona on the Big Island of Hawaiÿi and in 

Waikïkï (which means “spouting-water” in Hawaiian) on the island of Oÿahu.

Modern instruments, including airborne thermal infrared (thermal IR) 

imaging systems and CTDs (submersible instruments that measure water 

conductivity, temperature, and depth) are now sensitive enough to map and 

quantify physical properties of the water, including temperature and salinity (the dissolved salt content of water) at specific depths and over large 

areas rapidly. Thermal IR systems image Earth’s surface temperatures (in 

soil, water, and vegetation) by measuring the amount of radiation reflecting 

back to space in the thermal portion of the spectrum. Thermal IR imaging 

systems are typically flown on small aircraft and measure only the uppermost layer of the water column, whereas CTDs are often deployed from a 

boat to measure temperature and salinity through the entire water column. 

CDTs can also be fastened to a tripod on the sea floor to record temperature 

and salinity variations at a single point over time. These types of data allow 

discrete water masses to be mapped in order to examine pathways of freshwater into and through the coastal ocean, along with characteristics of its 

mixing. In Hawaiÿi the shallow sea floor is commonly composed of porous 

coral reefs, and CTD data can be especially useful to help identify where 

SGD, characterized by being colder and of lower salinity, flows out to the 

sea through the reef.

Figure 1. Diagram of the hydrologic cycle showing how upland precipitation 

and streams infiltrate into the soil and recharge ground water. Ground water 

is composed of fresh, brackish, and saline types, and their zonation fluctuates 

with changes in recharge and pumping through wells. Many land-use activities 

can add nutrients and contaminants to ground water, which eventually discharges to the coast. 

Bedrock

Saline ground water

Submarine

ground-water

discharge Brackish ground water

Fresh Ground-water

Recharge Contaminants Nutrients

Ground-water

discharge

Stream discharge

Recharge
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

112

numbers of neutrons in their nuclei) that radioactively decay at specific rates. 

Knowing the rate of decay and activity of these isotopes in water samples 

enables calculations that help identify the source and mixing rate of fresh 

ground water (using Rn) and brackish ground water (using Ra). In Hawaiÿi, Rn 

and Ra are often concentrated in ground water and are associated with nutrient 

concentrations, especially nitrogen as nitrate and nitrite (Garrison and others, 

2003; Soicher and Peterson, 1997; Paytan and others, 2006). Other chemical 

methods include the use of stable isotopes of carbon (13C) and nitrogen (15N). 

These can be used to identify a wide range of sources and processes that influence the fate of nutrients, including terrestrial plant or organic material, fertilizers, sewage, and waste products, and to track the mediation of those nutrients 

through biologic uptake and geochemical cycling (Leichter and others, 2003; 

Umezawa and others, 2002; Yamamuro and others, 2003). Using a combination of stable isotopes, radioactive isotopes, and concentrations of various 

nutrients and elements, scientists can determine and in many cases quantify 

(1) SGD at the coast, (2) nutrient fluxes, and (3) nutrient sources.

Hydrologists and geologists use the rock properties and structure of 

coastal environments, rainfall patterns, and water levels in wells to map the 

distribution of ground water below the surface. By the use of tracers and a 

knowledge of ground-water behavior, the rate of ground-water flow from areas 

of recharge (where rainfall and infiltration are greatest) to the ocean can be 

determined (fig. 1). Using these data, models are constructed to estimate and 

describe flow patterns and locate suitable areas for ground-water development. 

In island settings like Hawaiÿi, these models and direct observations reveal that 

within the island mass a freshwater lens commonly floats above more saline 

ground water and seawater. In times of high rainfall and recharge, freshwater 

discharge to the ocean increases. During times of drought and/or in areas of 

heavy well-water use, saline waters from the ocean intrude into the freshwater 

reservoirs, sometimes contaminating freshwater resources needed for local 

consumption. Understanding these processes and the nutrient and contaminant 

concentrations associated with freshwater discharging across coastal ecosystems is of critical importance as human activity increasingly alters the quality 

and composition of freshwater and coastal ocean water.

Results—Submarine Ground-Water Discharge 

Across the Molokaÿi Reef

There are multiple indicators of submarine ground-water discharge 

(SGD) to the Molokaÿi reef. These include morphologic evidence, remote 

sensing data (thermal IR), direct measurements of temperature and salinity, 

studies of chemical radioactive (Ra and Rn) and stable (C and N) isotopes, 

and measurements of nutrients.

Morphologic Evidence of Submarine Ground-Water Discharge

The “blue holes” of Kamalö (fig. 2B) are characteristic of karst topography, sculpted limestone terraines formed by dissolution by freshwater. They 

probably formed during periods of subaerial exposure (lower sea level). It 

remains uncertain if the Kamalö blue holes are indeed karst, but significant freshwater outflow along the coast there, and at other sites such as the 

Kapuäiwa coconut grove, 2.5 km (1.5 mi) west of Kaunakakai, and Pälä’au 

where similar features occur, points to ground-water dissolution as a likely 

cause for erosion of the holes. Subsequent reef accretion around the edges 

may add reefal material to the perimeter of the blue holes in time. Dissolution of carbonate reefs by freshwater occurs much as chalk is dissolved when 

dropped in a carbonated beverage, which is slightly acidic. Freshwater is 

relatively acidic compared to seawater and will dissolve calcium carbonate, 

which is the dominant mineral in reefal limestone. Karst features are found 

across the entire Molokaÿi reef in the form of isolated sand-filled circular 

basins. A prominent blue hole referred to herein as “The Pit” occurs near 

the reef crest between Pälä’au and the Kapuäiwa coconut grove (Field and 

other, this vol., chap. 2). Local knowledge of its longtime existence suggests 

that this feature was naturally formed. Several other large karstlike features 

occur along the reef crest offshore of Kawela (Storlazzi and others, this 

vol., chap. 3). Drilling through the outer reef flat at the Kapuäiwa coconut 

grove (Engels and Fletcher, this vol., chap. 4) revealed a cavity or cave just 

below the sea floor extending to nearly 20 m (60 ft) below the reef surface. 

This type of cavity is uncommon in living reefs and is likely due to dissolution, probably during times of lower sea level. The blue holes of Molokaÿi 

offshore of the Kapuäiwa coconut grove and Kamalö have been the focus of 

conductivity, temperature, and depth (CTD) measurements and water analyses to determine if they are active conduits of SGD escaping through the 

reef. The reef flat and fore reef offshore of Kamiloloa also has been studied 

to examine SGD and potential nutrient flux where the reef shows signs of 

extensive deterioration and high algal growth.

Spatial Patterns of Submarine Ground-Water Discharge

Thermal IR images collected by scientists from the U.S. Geological 

Survey and University of Hawaiÿi in 2001 using the Airborne Hyperspectral Imager (AHI) show the large-scale pattern of cool surface waters along 

the Molokaÿi south shore relative to further offshore (fig. 3). This version 

of the AHI data is qualitative and shows relative temperature variations of 

the surface water in the top 1 cm (0.4 in) across the reef. In the absence 

of surface-water runoff through rivers and streams during most of the 

year, cool surface waters along the coast suggest extensive SGD. Thermal 

IR data also show the cross-shore influence of fresh surface water near 

stream mouths, where active subsurface flow is likely concentrated because 

ground-water levels are higher than the elevation of the incised stream 

channel. On the island of Molokaÿi, modest rainfall in the upper elevations 

recharges ground-water levels sufficiently to maintain ground-water flow 

at the coast (fig. 4).

Measurements of surface-water temperature, conductivity, and salinity from 325 CTD casts and 50 point measurements using a temperature 

and conductivity probe across the Molokaÿi reef made during two different 

surveys (May and August, 2002) reveal a persistent pattern of freshwater 

discharge and mixing across the reef flat and inner reef crest (fig. 5). These 

Figure 2. Photographs of landscape features associated with ground-water discharge. A, Ground water discharging through springs at the shoreline of Kapuäiwa coconut grove. B, The “blue holes” off Kamalö, characteristic of karst (dissolution) features. C, Ground water discharging through the beachface at Kamiloloa 

during low tide. D, Fleshy brown and/or green algae commonly found in coastal 

waters with high nutrient concentrations. 

New chemical methods that determine the concentration of specific 

compounds and elements are used to determine the composition of water. 

Many chemicals and compounds can be used as tracers to differentiate water 

masses and sources, including SGD, in the coastal ocean. Several constituents, 

including barium, silica, lead, and iron, are indicative of freshwater and/or 

ground water, and recently the naturally occurring radioactive elements radon 

(Rn) and radium (Ra) have proven of excellent utility in identifying SGD to 

the coast (Moore, 1996). Rn and Ra occur in several isotopes (different forms 

of the same element having slightly different masses resulting from different 

C

A

B

D

Brown algae

Brown algae

Ground-water

discharge
Ground Water and its Influence on Reef Evolution

113 

Natural Change to the Molokaÿi Reef

results are similar to what the thermal IR data show and suggest that freshwater is discharging along the south Molokaÿi shoreline. In the absence of 

inflowing surface waters via streams, these data are a strong indication that 

ground water is the source of this freshwater.

At Kapuäiwa coconut grove, the influence of freshwater from springs 

and SGD is dramatic based on surveys collected in August 2004 (fig. 6). Cold 

freshwater emanates from the springs and extends more than 0.5 km (0.3 mi) 

offshore at the surface. This ground water rapidly warms over the shallow reef 

flat. In addition, ground water can be seen discharging through the reef (black 

arrow, fig. 6B) 0.5 km (0.3 mi) offshore of the coconut grove. Interestingly, 

however, cooler temperatures are not always indicative of freshwater, because 

marine water associated with the incoming tide may be cooler than freshwater 

that warms rapidly near shore on the reef flat (fig. 6C, D). 

Across the reef offshore of Kamiloloa, sensors were mounted on tripods and left recording at the same location for several years. The resulting measurements show that water temperature and salinity vary both daily 

and seasonally. Although temperature varies by only a few degrees, salinity 

varies as much as ~10 practical salinity units (psu), whereas marine waters 

commonly have values around 35 psu (Ogston and others, this vol, chap. 20). 

Each day, as the tide rises it brings slightly cooler, more saline marine waters 

toward shore. They mix with surface waters across the shallow reef flats, 

which become warmer throughout the day as they are heated by the sun. 

Episodic decreases in temperature and salinity, however, may be related to 

discrete periods of high rainfall. Whether these variable conditions adversely 

affect coral directly remains uncertain. 

Radiochemical Signatures of Submarine Ground-Water Flux

Elevated activities of radium isotopes measured in nearshore waters 

provide strong evidence of widespread SGD to the fringing reef along south 

Molokaÿi. Water samples were collected on July 12, 2003, approximately 

1 m (3 ft) from the shoreline at 12 sites between Kapuäiwa coconut grove 

and ÿUalapuÿe fishpond east of Kamalö (fig. 5). 223Ra and 224Ra activities 

measured in these samples ranged from 0.3 to 2.5 and 9 to 47 decays per 

minute per 100 liters (dpm/100 L), respectively, showing a high degree of 

variability. Because these isotopes are evenly present in very low concentration in seawater, this likely reflects an uneven distribution of SGD along the 

coast. Peak Ra activities were measured just west of the Kaunakakai Wharf 

in high-salinity waters, and Ra activities at all sites were 10 to 100 times 

greater than those measured in the open ocean. 

Samples were also collected along cross-shore transects at three sites 

along the south coast: Kapuäiwa coconut grove, Kamiloloa, and near 

ÿUalapuÿe fishpond (two are shown in figure 7). Elevated 224Ra activities 

(17–34 dpm/100 L) were detected in samples as much as 60 m offshore 

on the reef flat. Assuming slow mixing rates and water residence times of 

2–3days (several tide cycles) in the protected back-reef areas (Ogston and 

others, 2004; Presto and others, 2006), the persistence and elevated activities 

of 223Ra and 224Ra offshore suggest high discharge of brackish ground water. 

The discharge may be occurring through the sea floor over a broad area of 

Figure 3. Map showing uncalibrated 

thermal infrared imagery collected March 

8, 2001, across the south shore Molokaÿi 

reef. The rainbow color codes represent 

relative temperature variations, where 

red is warm and blue is cool. Offshore of 

Kapuäiwa coconut grove several streams 

of cool water are observed flowing out 

across the reef flat (blue arrows). Along 

the Kamiloloa coast, several areas of 

cooler water are found flowing as streams 

across the reef flat (blue arrows) and are 

commonly associated with fishponds. 

In the absence of surface runoff from 

streams, these cooler water masses are 

best explained by ground-water discharge 

along the shore. 

Figure 4. Map showing rainfall isopleths (contours of equal rainfall, blue lines) 

from direct measurements and the predicted model ground-water flow patterns 

(brown arrows) on Molokaÿi (after Oki, 1997). Many streams in East and North 

Molokaÿi flow perennially (solid blue lines), but many along the west and south 

shores are only intermittent (dash-dotted blue lines). 

Relative

temperature

Warm

Cooler-water streams 

Fishpond

Fishpond

Cool

FIGURE 6 

FIGURE 5 

Kapuiwa

coconut grove

Kamiloloa

Degraded reef

0

0

0.5 1 Mile

0.5 1 Kilometer

N

0

0

2 4 Miles

2 4 Kilometers

Rainfall isoplethÑannual average in inches (millimeters)

Model-calculated direction of ground-water flow

Ground-water divide

Topographic divide

Stream

Intermittent stream

FIGURE 3

Cape

Hlawa

KahiØu Point

Kalaupapa Peninsula

Øìlio

Point

LØau

Point 

HØupu

Bay Hlawa

Bay

157

(4000)

118

(3000)

78.7

(2000)

78.7

(2000)

15.7

(400)

23.6

(600)

23.6

(600)

39.4

(1000)

39.4

(1000)

31.5

(800)

15.7

(400)

157

(4000)

118

(3000)

78.7

(2000)

78.7

(2000)

15.7

(400)

15.7

(400)

23.6

(600)

23.6

(600)

39.4

(1000)

39.4

(1000)

31.5

(800)

ØUmipaØa

ØUalapuØe

Kaunakakai

Kamiloloa

Kawela

Kamal

Kalaupapa

Hlawa

Mapulehu
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

114

Figure 5. Diagrams showing depth-averaged water temperature and salinity 

(measured in practical salinity units, or psu) across the Kamiloloa reef flat from 

direct measurements during high tide. Sample locations include conductivity, 

temperature, and depth (CTD) casts (crosses) and water samples (triangles, see 

also figure 7). High-tide temperatures in August 2002 (A) are generally cooler 

along the shoreline than farther offshore on the reef flat and reef crest and are 

considerably higher near the Kaunakakai Wharf than to the east near One Ali’i. 

Similarly, salinities in August (B) and May (C) are significantly lower along the 

coast than offshore, consistent with a persistent input of freshwater at the shore. 

Figure 6. Diagrams showing seawater salinity (measured in practical salinity units, or psu) and temperature on the reef flat offshore 

of Kapuäiwa coconut grove in August 2004 during the initial stages 

of a rising tide. Sample locations include conductivity, temperature, 

and depth (CTD) casts (crosses) and water samples (triangles, see 

also figure 7). A, Sea-surface salinity shows a tongue of freshwater 

extending across more than 0.5 km (0.3 mi) of the inner reef flat, 

with values as low as 5 psu at the spring along the shore. B, Bottom 

salinity (~0.5 m or 1.6 ft above the sea floor) shows that freshwater 

in the form of submarine ground-water discharge (SGD) emanates 

from the springs along the shore as well as from a spring out in the 

middle of the study area (white arrow). C, Sea-surface temperatures 

reveal that the plume of freshwater is cool near the spring but quickly 

warms over the shallow inner reef flat. D, Bottom temperatures reveal that water is warmest in the shallowest parts of the reef flat and 

coldest near the offshore spring.

Figure 7. Plots of surface salinities, radium activities, and 

nutrients along transects offshore of Kapuäiwa coconut grove 

and Kamiloloa measured on July 10, 2003 (see figures 5 and 6 

for locations). Salinity (A, B) generally increases away from the 

shore as expected with increasing influence of marine water 

(psu, practical salinity units). Activities of 223Ra and 224Ra (C, 

D) are higher near shore and correlated with each other as 

they decrease offshore with increasing salinity, suggesting a 

terrestrial source of Ra, likely from uranium and thorium decay 

in basalt source rocks. An exception at Kapuäiwa coconut 

grove, where Ra is lower in the fresh coastal spring, is consistent with a fresher source of ground water there and a more 

brackish source emanating from the offshore spring. Similarly, 

offshore of Kamiloloa a slight increase in Ra activities 20 m 

offshore may indicate submarine ground-water discharge 

(SGD) through the reef flat. Unit of activity is (radioactive) decays per minute per 100 liters (dpm/100 L).All nutrients measured (E, F) are significantly higher close to shore and, taken 

with the Ra results, indicate that the source of these nutrients 

is ground water. Dissolved organic carbon (DOC) is relatively 

uniformly distributed across the reef flat. 

C Salinity, May, 2002

B Salinity, August, 2002

A Temperature, August, 2002

CTD location

Water sample

Kamiloloa

Transect

0 0.5 Mile

0 0.5 1 Kilometer

Temperature (°C)

26.2

28.7

Salinity (psu)

30.92

35.18

Salinity (psu)

30.92

35.18

Transect

Kapuiwa coconut groveÑAugust 12, 2004

A B

C D

Spring

Spring

Surface salinity

(psu)

0 0.25 Mile

0 0.25 0.5 Kilometer

4.94

34.67

Surface

temperature (°C)

24.79

28.25

Bottom

temperature (°C)

24.79

28.25

Bottom salinity

(psu)

4.94

34.67

CTD location

Water sample

A

Salinity (psu)

0

20

40

0 0

20

40

60

10

20

0

1

2

Salinity

Salinity

B

C D

E F Salinity (psu)

33

34

35

0

20

40

60

80

Distance offshore (m)

0 20 40 60

Distance offshore (m)

0 10 3020 40

0

1

2

0

20

40

Kapuiwa coconut grove transect (A, C, E) Kamiloloa transect (B, D, F)

0

1

2

223Ra (dpm/100L)

224Ra (dpm/100L)

224Ra (dpm/100L)

223Ra (dpm/100L)

0

1

2

PO4, DOC (μ

M)

NO3+NH4, SiO4 (μ

M)

NO3, NH4, PO4, DOC (μ

M)

SiO4 (μ

M)

224Ra 224Ra

223Ra

NO3+NH4

NH4

NO3

PO4

PO4

DOC

223Ra

SiO4

SiO4
Ground Water and its Influence on Reef Evolution

115 

Natural Change to the Molokaÿi Reef

the reef flat, rather than from a discrete, onshore source. This observation is 

consistent with a subterranean source of ground water composed of a mixture of both terrestrial ground water and recirculated seawater discharging 

offshore, like the situation observed in figures 5 and 6. 

A brackish to saline mixing zone is usually present within groundwater systems of many islands. This mixing zone migrates back and forth in 

a cross-shore direction under the influence of the tides and rainfall. These 

factors modulate the gradient of hydraulic head (equal to water level in 

unconfined systems) that drives ground-water discharge. At low tide, when 

ground-water flow across the reef flat is strongest (greatest water-level 

gradient), the brackish zone migrates offshore. Conversely, at high tide, or 

during dry periods when ground-water flow is weaker, the brackish zone 

moves inland. Because desorption (leaching) of radium from rocks and sediment is suppressed in freshwater, Ra isotopes are primarily tracers of saline 

ground water. They thus can show the location of the discharge of brackish 

mixing-zone water. This model of coastal aquifer dynamics on Molokaÿi 

is consistent with the observation that Ra activities in nearshore waters are 

much higher than in onshore samples taken from the coastal aquifer at low 

tide. The spring at Kapuäiwa coconut grove (fig. 2A) was sampled at low tide 

and was found to have 223Ra and 224Ra activities of only 0.26 dpm/100 L and 

3.19 dpm/100 L, respectively, and a salinity of only 2.9 psu. Just a few tens 

of meters offshore, where the saline ground water discharged, Ra activities 

and salinities on the reef flat were four to seven times greater.

Nutrient Concentrations Along the South Molokaÿi Reef

Ground water discharging along the south Molokaÿi shoreline is a major 

source of new nutrients, including inorganic nitrogen (N), phosphorus (P), 

and silica (SiO2) to the Molokaÿi reef (fig. 7). The mean molar concentration of inorganic nutrients in three low-tide samples of undiluted ground 

water collected from springs at Kapuäiwa coconut grove were 63.7±3.3 μM, 

2.20±0.18 μM, and 823±32.6 μM, for nitrate, orthophosphate, and silicate, 

respectively. These concentrations are 10 to 1,000 times greater than those 

measured in open-ocean samples. Elevated concentrations were also measured in the surface pool sampled near ÿUalapuÿe fishpond (7.20 μM NO3

-

, 

4.03 μM PO4

3-, 593 μM Si(OH)4), although nutrient concentrations of the 

original ground water there are likely modified by internal cycling and/or 

inputs and uptake by nearby vegetation. In the coconut grove ground-water 

samples, the N/P ratio is approximately 29, whereas in the ÿUalapuÿe fishpond the ratio is less than 2, suggesting that biologic uptake of N within the 

pond is depleting the nitrogen. 

Submarine Ground-Water and Nutrient Flux to the 

South Molokaÿi Reef

We calculate submarine ground-water discharge (SGD) and nutrient 

fluxes to the Molokaÿi reef at Kamiloloa and Kapuäiwa coconut grove using 

Ra isotope-tracer and salinity mass-balance methods adapted from Moore 

(1996) and our understanding of residence time of water on the reef, the 

water ranges from 5.9 to 255 g/d of N. Again, the TIN fluxes based on current measurements are higher than those from the other two methods, reflecting the more rapid SGD flux estimated by the process measurements. The 

measured concentrations of N found here are comparable to nutrient concentrations found in ground water in other areas of the Hawaiian coastal zone, 

which range from 13.2 to 63.6 μM (at Kahana; Garrison and others, 2003), 

60 to 95 μM (at Waikoloa and Keahou; Dollar and Atkinson, 1992), and 0.45 

to 1.7 μM (at West Maui; Soicher and Peterson, 1997). More importantly, the 

nutrient concentrations found in ground water at Kamiloloa and Kapuäiwa 

coconut grove are 10 to 100 times higher than concentrations found in open 

ocean water. The higher levels of nutrients discharging from the spring at 

Kapuäiwa coconut grove are likely a result of being associated with fresh 

ground water, while at Kamiloloa nutrients discharging in brackish to saline 

ground water may be diluted by recirculated seawater. 

Dissolved Carbon and Carbon-Isotope Signatures 

of Water Source and Mixing

Ground-water discharge appears to be only a minor contributor of dissolved organic carbon (DOC) to the Molokaÿi reef (fig. 7E, F). The average 

DOC concentration and standard deviation for all nearshore and back-reef 

samples collected during July 2003 are 0.66±0.24 ppm (54.6±20.2 μM, 

n=30). This places the Molokaÿi reef within the lower to middle portions 

Figure 8. Map showing calculated submarine ground-water discharge (SGD) 

and total inorganic nitrogen (TIN) flux at the Kapuäiwa coconut grove and 

Kamiloloa. Although ranges of SGD are higher for Kamiloloa, nitrogen flux to 

the reef is higher at Kapuäiwa because of higher nitrogen concentration in the 

fresh ground water there. Estimates range considerably because of our limited 

knowledge of residence times of water on the reef and because of the complex 

mixing of fresh and brackish ground water with recirculated seawater in the 

coastal aquifer.

time required to replace water on the reef through tidal exchange or currents. Given the measurement uncertainties in the activities of 224Ra and 

223Ra, it would take at least 1.5 days to detect a change in the ratio of 224Ra 

to 223Ra. Because we do not observe these ratios to vary across shore, it 

is assumed that there is negligible Ra decay within the residence time of 

water on the reef, and that the decrease in activities offshore is due to dilution with seawater. The dilution occurs with mixing through tidal exchange 

and currents. Because of uncertainties in our understanding of currents and 

water exchange in the shallow reef flat, we calculate SGD on the basis of 

residence times of water at two sites where we measured Ra activities across 

the inner reef flat—a box at Kamiloloa 45 m (148 ft) long (across shore) by 

1 m (3.3 ft) wide (along shore) by 1 m (3.3 ft) deep and a box at Kapuäiwa 

coconut grove 60 m (197 ft) long, 1 m (3.3 ft) wide, 1 m (3.3 ft) deep (fig. 

7). The residence times for these boxes range from 1.5 days (based on our Ra 

measurements) to 37 minutes (based on net cross-shore currents of 2 cm/sec) 

(Presto and others, 2006).

Calculations of SGD based on Ra decay and salinity mass balance range 

from 8.8 to 638 L/min/m (liters per minute per meter of shoreline) (2.3 to 

169 gal/min/m) at Kamiloloa and from 4.7 to 204 L/min/m (1.2 to 54 gal/

min/m) at Kapuäiwa coconut grove (fig. 8). A key difference between the 

sites is the presence of natural springs at Kapuäiwa coconut grove, which can 

be observed to discharge freshwater directly to shore. The SGD estimates 

derived from using current measurements of Presto and others (2006) are 

significantly higher than those derived from the Ra activity and salinity mass 

balance. This may result from the fact that the current measurements, like 

most instantaneous observations, are biased toward higher rates (for example, higher flushing rates, lower residence time across reef). In contrast, the 

extrapolations of residence time across larger portions of the reef via the Ra 

activity and salinity mass-balance methods tend to average out instantaneous 

rates over longer time and space scales. It is probable that the residence time 

of water across the reef used in our Ra activity and salinity mass-balance 

calculations is closer to 1.5 days because of incomplete exchange within one 

tidal cycle. However, during periods of strong currents caused by high trade 

winds and/or large waves, the residence time of water across the reef may be 

shorter and on the order of hours, as reflected by Presto and others (2006). 

The results here based on tidal exchange are in close agreement with estimates based on ground-water models, which predict regional trends (fig.

4). 

The model predictions of SGD range from 1.7 cubic feet per second per mile 

of shoreline (1.9 L/min/m) at Kamiloloa to 3.2 cubic feet per second per 

mile (3.4 L/min/m) at Kapuäiwa coconut grove (Oki, 1997; D.S. Oki, written 

commun., 2007). 

We calculate nutrient fluxes to the South Molokaÿi reef by multiplying 

the concentration of nutrient in SGD by the SGD flux rates noted above. For 

example, the nitrogen flux to the reef is the product of total inorganic nitrogen (TIN, nitrate plus ammonium) concentration in ground water emanating 

to the reef (fig. 7) and the SGD rate converted to grams of elemental nitrogen 

flux per day to the reef. At Kamiloloa, we calculate a range of TIN between 

0.7 and 50.2 grams of nitrogen per day (g/d of N) entering the reef with 

ground water. At Kapuäiwa coconut grove, TIN flux to the reef in ground 

Kapuiwa

coconut grove

Kamiloloa

SGD: 5Ð204

TIN: 6Ð255

SGD: 9Ð638

TIN: 1Ð50

Flux

SGD (L minÐ1 mÐ1)

TIN (g N dayÐ1 mÐ1)

0 0.5 Mile

0 0.5 1 Kilometer

N
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

116

of the range of DOC (9 to 290 μM in coral-reef sites globally; Atkinson and 

Falter, 2003. This result is consistent with the semiarid climate of the Hawaiian region. Samples collected within a few meters of the Molokaÿi shore are on 

average slightly enriched in DOC (0.73±0.30, 60.6±25.8 μM, n=15) relative 

to samples from farther offshore, likely reflecting carbon from mangroves and 

terrestrial plant debris, although the springs at the Kapuäiwa coconut grove 

contain a lower average concentration of DOC (0.35 ppm, 29.0 μM, n=3). 

These data suggest that some other source, such as export of DOC from the 

fore reef and reef crest, are necessary to support the higher observed DOC concentrations in the nearshore and on the reef flat. 

Measurements of stable-isotope ratios (δ13C) of DOC also suggest that 

ground water is not a dominant source of carbon to the Molokaÿi reef. The 

vast majority of nearshore samples have δ13C signatures of DOC in the range 

of –17 to –24 permil (parts per thousand), consistent with the δ13C signatures of marine phytoplankton (–18 to –24 permil) and marine algae (–15 

to –22permil) (Michener and Schell, 1994). These values fall within the 

broad range of published values for marine and estuarine systems (see, for 

example, Coffin and others, 1994), the DOC of which is commonly enriched 

in 13C relative to rivers and freshwater. Exceptions occur within the ÿUalapuÿe 

fishpond (–26.77permil) and in a shoreline sample collected between Kawela 

and Kamalö (–30.92 permil), where an abundance of emergent terrestrial 

vegetation (mangroves) that have low δ13C values (–22 to –30 permil) is 

likely exporting carbon to the reef flat (Michener and Schell, 1994). The 

δ13C values in spring-water samples from Kapuäiwa coconut grove average 

–21.46±0.99 permil (n=3), a signature not distinguishable from most inner 

reef-flat samples. 

Gaps in our Knowledge of SGD Flux, Phasing, and Impact 

to the Molokaÿi Reef

It is evident from the foregoing that submarine ground water does discharge on the south shore of Molokaÿi and that the discharge contains high 

concentrations of nutrients. It may also contain contaminants, but this is 

not yet documented. Understanding the variability of ground-water composition, flux, and circulation processes that determine its fate is important 

to predict impacts to coastal ecosystems. A particularly challenging issue 

is being able to identify (1) the origin of ground water and contaminant 

additions, (2) transport pathways, and (3) the timing of discharge relative to 

rainfall and recharge. To these ends, scientists are actively pursuing experiments to quantify these processes. With suitable measurements, adequate 

models can be developed and applied to societally relevant issues concerning 

ground-water effects on coastal ecosystems. For example, ground-water flow 

models can predict travel times, discharge rates, and nutrient fluxes to the 

coast that are important parameters to resource managers tasked with balancing the needs of human water supplies and coastal ecosystems.

Summary

Submarine ground-water discharge (SGD) to the coast is highly variable 

along the shore and through time because of the complex nature of subsurface 

flow through a variety of different rock and sediment types. Along the Molokaÿi 

reef, evidence in the form of legends, reef morphology, thermal infrared imaging, CTD (conductivity, temperature, and depth) studies, water chemistry, and 

modeling studies indicate that a significant amount of freshwater discharges 

at the shoreline or below sea level on the nearby reef flat. Although springs 

like those at the Kapuäiwa coconut grove have only a localized effect on the 

reef, their persistent nature may shape nearshore reef communities and at the 

same time dissolve the reef below, as has been observed in drill cores. Localized areas of SGD occur across the reef, although the timing and amount of 

this flow remains uncertain. Further inquiry into the range of nutrient sources 

to ground water and nutrient concentrations reaching the coast in ground-water 

discharge will aid in future planning and resource management.

Kapuäiwa coconut grove.

Mike Field, USGS
Kamiloloa

REEF FLAT

REEF CREST

FORE REEF

ReefProbe

Patch reefs (2000)

Ambient seawater

measurements (2001)

Sand bottom (2000)

Sand bottom (2001) Patch reefs (2001)

Coral rubble (2000)

Platform site

0 500 1000 Feet

0 150 300 Meters

Natural Change to the Molokaÿi Reef

CHAPTER 14

Light, Calcification, and Carbonate Sediment Production on the Molokaÿi Reef Flat

Robert B. Halley1 and Kimberly K. Yates1

There are many variations in how people perceive and define what 

are commonly called coral reefs. One widely accepted definition of 

coral reefs is that they are geological structures built by the accumulated skeletal remains of corals and other organisms that live on the 

reef surface. The reef on south Molokaÿi is an excellent example of this 

definition and is discussed in this chapter. Another definition for coral 

reefs focuses on the marine biological communities that have reef-building 

potential, regardless of whether or not those communities are presently 

building reefs. The coralline algae and coral-rich areas on basalt boulders 

along portions of the Kona coast of Hawaiÿi are an example of this biological definition. However, these coral areas are not considered reefs in the 

geological sense. Grigg (1998) and Engels and others (2004) have pointed 

out areas of Oÿahu and Molokaÿi where reef accumulation is limited by 

wave action. To further complicate things, coral reefs may be considered 

relict, meaning the structure remains long after the animals and plants that 

built them were replaced by marine communities that are not reef building. 

Portions of the reef in Kailua Bay, Oÿahu, are examples of this third type 

(Grossman and Fletcher, 2004).

These three types of reefs may sometimes be differentiated on the 

basis of geomorphic observations. Often, however, it may be difficult to 

determine when a reef has reached senility and stopped generating sediment. The ability of coral reef communities to accumulate sediments and 

build reefs depends on the rate of sediment production versus the rates at 

which these sediments are transported or dissolved from the reef. Although 

reef building may seem obvious (the very presence of a coral reef attests 

to the dominance of deposition over erosion and dissolution), perceiving 

what is actually occurring now may be difficult. It is easier to evaluate the 

cumulative results of these processes over millennia than it is to measure 

short-term rates—those on time scales of decades to centuries. These shortterm rates have become more important in light of the present-day crisis of 

coral reef ecosystems (Bellwood and others, 2004). Here we describe some 

short-term measurements and observations that focus on the ability of the 

benthic marine community of the Molokaÿi reef flat to continue producing 

reef structure.

1 U.S. Geological Survey Center for Coastal and Watershed Studies, 600 Fourth Street South, St. 

Petersburg, FL 33701

Setting

The fringing reef on the south shore of Molokaÿi is characterized by 

several gradients across the kilometer-wide reef flat. Seaward from the shore, 

water depth increases from 0 to about 2.5 m, wave energy increases, and 

turbidity decreases toward the reef crest (Ogston and others, this vol., chap. 20; 

Ogston and others, 2004; Presto and others, 2006). One of the most important 

gradients is the percentage of coral cover. Coral cover increases from 0 percent 

along the shore to as much as 40 percent at the reef crest, although it is patchy 

along the outer margin of the reef flat. Seaward of the reef crest, coral cover 

in the fore reef may be as great as 50 to 80 percent in water depths of 5–10m 

(Rodgers and others, 2005). The increase in coral cover is inversely related to 

the amount and type of suspended sediment, which in turn affects turbidity, 

water clarity, and irradiance. The silt and clay fractions of inner reef-flat sediments contain as much as 80 percent terrigenous sediment and are typically 

dark red-brown in color. The terrigenous sediment is preferentially resuspended 

Figure 1. Aerial photograph of a portion 

of the Molokaÿi reef flat at Kamiloloa 

showing SHARQ chamber sampling 

locations (red dots) for this study in 

2000 and 2001. The samples were 

collected in an area approximately 200 

m landward of the reef crest and 300 

m seaward of the ReefProbe (yellow 

triangle), a small instrumented tripod 

(see Ogston and others, this vol., chap. 

21, for more information on the ReefProbe). Instruments to measure ambient seawater were housed on the field 

laboratory platform. 

because it is finer grained than carbonate sediment (Field and others, this vol., 

chap. 17; Bothner and others, 2006). Several hundred meters offshore, the 

sediments are almost 100 percent carbonate, white or slightly tan in color. This 

color change is apparent on aerial photographs of the reef (fig. 1). Also apparent 

in figure 1 are dark streaks across the reef flat, which are composed of skeletal 

sand, gravel, pebbles, and cobbles (Wentworth, 1935), collectively referred to as 

rubble, much of which is colonized by benthic algae and coral. A gradient exists 

in colonization, the nearshore rubble being dominated by fleshy macroalgae and 

that on the seaward flat dominated by turf algae, live coral, and coralline algae.

On the outer reef flat, corals, calcareous algae, and other photosynthetic 

organisms fix carbon into organic tissues and calcium carbonate skeletons. 

The metabolism of bottom organisms can be monitored by carefully measuring chemical changes in seawater that result from photosynthesis, respiration, 

and calcification. These changes can be readily detected in seawater on the reef 

flat, but quantitative evaluation of metabolism rates requires measurements in a 

known volume of water. For this purpose, a clear plastic chamber, the SHARQ 

(Submersible Habitat for Analyses of Reef Quality) was deployed on the reef 

flat (Yates and Halley, 2003).

Methods

In February 2000 and July 2001, using platforms as field laboratories, we 

made chemical measurements in water from the SHARQ chamber that was 

deployed over areas of the reef flat and in ambient reef-flat water on the outer 

reef flat (fig. 2). The chemical measurements were used to calculate metabolic 

activity of the reef. Details of measurement parameters and intervals may be 

found in Yates and Halley (2003, 2006a). Measures of reef-flat productivity 

were collected over four bottom types: sand, rubble, 10-percent coral cover, and 

22-percent coral cover. These areas typify the bulk of the outer reef flat (Rodgers 

and others, 2005). The platforms served as 24-hour observatories from which to 

measure metabolic activity on the reef flat in general (ambient measurements) 

and of the community of organisms living within the SHARQ in particular.

Results

In general, the very shallow nearshore water exhibits more extreme temperatures near the sea floor than deeper water located farther offshore near the 

MAP AREA
118

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Temperature, in degrees Celsius

Tidal elevation, in meters

Date and time

Tidal elevation 

Offshore water

temperature

Reef crest water

temperature

18

20

22

24

26

28

30

32

0

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Date and time

Tidal elevation 

Sea floor light Surface light

Ð500

0

500

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PAR, in microEinsteins per square meter per second

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

the platform site (fig. 1) in about 2.2 m of water. At this location, afternoon 

light is reduced by as much as 35 percent. However, even with a 35-percent 

reduction through the water, sufficient light for photosynthesis reaches the 

bottom (Yates and Halley, 2000). 

The plants and animals of the reef derive their energy through photosynthesis and respiration. During the day, the Molokaÿi reef is dominated by 

photosynthesis, a process that consumes carbon dioxide and produces oxygen. The daytime rate of photosynthesis is greater than the rate of respiration 

by nonphotosynthetic organisms, so oxygen accumulates in the water. At 

night, photosynthesis stops and the underlying rate of respiration is revealed 

by the consumption of oxygen and the production of carbon dioxide. This 

cycle repeats every 24 hours, and these two processes are roughly in balance 

over weeks and years (fig. 6A).

The increased carbon dioxide at night reacts, in turn, with seawater to 

make an acid, called carbonic acid, and lowers the pH of the seawater. The 

nightly decrease in pH is not perfectly correlated with decreases in dissolved 

oxygen, but the daily cycle in pH is apparent on the outer reef flat (fig. 6B). 

Although the illustrated cycles were measured in the SHARQ, similar pH 

shifts were observed in ambient water at the platform site. These shifts seem 

small, about half a pH unit, but they are sufficient to cause the dissolution of 

carbonate minerals at night (Yates and Halley, 2006a)

Algae, in one form or another, occupy the entire surface of the Molokaÿi 

reef flat (Smith and others, this vol., chap. 8). There are (1) symbiotic algae 

(termed “zooxanthellae”) in the tissues of corals, foraminifera, and some mollusks; (2) calcareous macroalgae (such as Halimeda sp. and Porolithon sp.); 

(3) fleshy (noncalcareous) macroaglae (such as Dictyosphaeria sp. and Caulerpa sp.); (4) the ubiquitous turf algae, a community of microscopic species 

creating a fuzz on hard surfaces; and (5) endolithic algae, microscopic forms 

that bore into and live below the surface of rocks and sand. The distribution of 

these photosynthetic organisms, together with the amount of light that reaches 

them during the day, controls the amount of oxygen produced on the reef flat. 

Higher plants, such as seagrass, also contribute to photosynthesis, but they are 

sparse on the Molokaÿi reef flat and do not form the extensive meadows that 

are characteristic of reef areas in the Caribbean.

Bacteria and animals respire, consuming oxygen both day and night. 

Researchers who have studied reef productivity assume that respiration is constant both day and night (Marsh and Smith, 1978; Carpenter, 1985). This may 

not be the case, because some animals are more active during the day, some at 

night. However, the assumption may be a good approximation because (1) day/

night animal activity and respiration offset each other; or (2) the system may 

be dominated by bacterial respiration that is little affected by light and dark. 

Unfortunately, there are very few studies of respiration in carbonate sediments, 

and the existing generalizations are predominately based on clastic coastal 

sediments (Middelburg and others, 2005). In practical terms, respiration is 

often measured in the field as the rate of oxygen consumption at night. Daytime measurements of oxygen production reflect oxygen produced by photosynthesis minus that consumed by respiration. Thus, daytime measurements 

are net measures of photosynthesis. For gross photosynthesis rates, the amount 

of oxygen consumed by respiration at night is added to the net (day) rate, with 

Figure 2. Platforms were erected on the reef flat and used for SHARQ support 

systems and as a field laboratory. A, Scaffolding with a plywood top used during 

February 2000. B, The University of Hawaiÿi drill platform used during July 2001.

Figure 3. A 48-hour record (October 15 to 17, 2001) of tide and temperature on 

the inner and outer Molokaÿi reef flat. Tide range (blue line) is about 0.7 m during 

this period. Water temperature 50 m offshore (green line) shows a daily cycle, 

with a range of 4°C and maximum temperatures in the afternoon. Water temperatures at the reef crest (orange line) have a range of 2°C, with a maximum in the 

afternoon and a secondary maximum at high tide.

reef crest—colder at night and warmer in the afternoon (fig. 3). High tides 

have a secondary effect, bringing warmer surface seawater from offshore 

to the reef crest area at night but having little effect inshore. Although these 

temperature differences are small, only a degree or two, the difference can 

become critical for coral bleaching, which can be induced by small temperature increases (Jokiel and Brown, 2004). 

Light measurements taken at the ReefProbe (a small instrumented 

tripod; see Ogston and others, this vol., chap. 20, for more information) site 

on the reef flat (fig. 1) indicate that about 87 percent of the available surface 

light reaches the sea floor at this location (fig. 4). At this shallow site (water 

depth 1.2 m), neither the turbidity nor tidal fluctuations affect light penetration enough to inhibit photosynthesis. Figure 5 illustrates light penetration at 

B

A

Figure 4. A 48-hour light and tide record (October 14 to 16, 2001) at the ReefProbe location. Light measurements are of photosynthetically active radiation 

(PAR) measured in microEinsteins. Light reaching the sea floor (orange line) is 

only slightly reduced from surface light (green line) at this shallow water depth 

(1.2 m). During this period, tide range (blue line) was 0.7 m.
119 

Natural Change to the Molokaÿi Reef

0

250

500

750

1,000

1,250

1,500

1,750

2,000

2/12/00

10:00

2/12/00

18:00

2/13/00

15:00

2/13/00

18:00

Surface

Sea floor

PAR, in microEinsteins

per square meter per second

Date and time

Max. attenuation = 35%

Figure 5. Partial 48-hour light record from coral-rubble SHARQ site in February 2000. Light (PAR, photosynthetically active radiation) reaching the sea floor 

(brown) is more reduced from surface light (blue) in the deeper water (2.2 m) 

at this study site. Windy conditions and increased turbidity on the afternoon of 

February 12, 2000, reduced surface light by as much as 35 percent. The calmer 

afternoon of February 13 reduced sea-floor light by only about 10 percent.

PAR, in microEinsteins per square meter per second

Oxygen, in grams per square meter per fifteen minutes

0

 0.1

 0.05

0.05

0.1

0

 0.1

 0.05

0.05

0.1 A Sand B Rubble

C 10-percent 

 coral cover

D 22-percent 

 coral cover

0 1,000 2,000 0 1,000 2,000

Light, Calcification, and Carbonate Sediment Production on the Molokaÿi Reef Flat

the assumption that respiration rates during the day are similar to respiration 

rates at night.

Photosynthesis rates, as a function of sea-floor light, for four bottom types on the outer reef flat (sand, rubble, 10-percent coral cover, and 

22-percent coral cover) are shown in figure 7. These rates are determined 

from within the SHARQ enclosure, which excludes gain and loss of oxygen 

and carbon dioxide from the atmosphere or from water flow across the reef 

flat. As expected, photosynthesis was detected in all four bottom types and 

increases in the order: sand, rubble, 10-percent coral cover, and 22-percent 

coral cover. Linear approximations show that for a given increase in light, 

the increase in photosynthesis in 22-percent coral cover is four times that in 

sand. It is also interesting to note that the rate of photosynthesis on rubble 

bottom is three times greater than on sand. This occurs because the rubble 

is more stable than sand and supports a more abundant growth of macroand microalgae. 

The data presented in figure 7 also indicate that photosynthetic saturation is not reached on the Molokaÿi reef flat. Saturation occurs when photosynthesis remains constant with increased light intensity. Maximum photosynthetic rates (Pmax) are well known for corals and algae, but it is also 

known that these organisms can adapt to changed light conditions within 

days and weeks (Anthony and Huegh-Gulberg, 2003). The community-level 

measurements of this study do not show evidence for saturation, nor do they 

suggest any deviation from linearity. These communities become productive (net oxygen production) at about two to four times the light intensity 

required by corals elsewhere in the Pacific. For example, net oxygen production begins at about 500 microEinsteins on the outer reef flat, compared to 

125 to 250 microEinsteins for corals along the Great Barrier Reef (Barnes 

and Chalker, 1990).

Calcification in corals and calcareous algae is enhanced by photosynthesis (Barnes and Chalker, 1990). In reef settings, calcification rates are 

indicated by alkalinity changes in seawater (Smith and Key, 1975). Alkalinity decreases during calcification and increases during calcite or aragonite 

dissolution. As with many other parameters, alkalinity exhibits daily cycles 

on the reef flat, decreasing during the day and increasing at night. Summing alkalinity changes through a 24-hour cycle provides an estimate of net 

calcification or dissolution. On the basis of our limited data, the four bottom types all show evidence of calcification during the day and dissolution 

at night. Sand and rubble areas show net dissolution. Sandy sea floor with 

10-percent live coral cover shows net dissolution or net sediment production, 

depending on location and time of year. Sandy sea floor with 22-percent live 

coral cover shows net sediment production (Yates and Halley, 2006a). 

Discussion

Suspended Sediment: Light Versus Loading

As pointed out by Rogers (1990) and Ogston and others (2004), corals 

are generally not found in areas where suspended sediments exceed 10 

mg/L, and greater concentrations are injurious to corals. The measurements 

made at the ReefProbe site, 300 m landward of the platform site, indicate 

that suspended sediment in excess of 10 mg/L is common during high 

tides on windy afternoons and evenings (Ogston and others, 2004; Presto 

and others, 2006). Storlazzi and others (2004) found that hourly mean 

concentrations often exceeded 5 mg/L and some individual measurements 

exceeded 40 mg/L. Rodgers and others (2005) found no coral growing 

shoreward of this position.

It appears that the ReefProbe location represents the approximate level of 

sediment stress that excludes coral growth. Yet even at this highly turbid site, 

the shallow water allows more than 500 microEinsteins to reach the bottom 

for most of the day (fig. 4). Therefore, it seems unlikely that light attenuation 

is preventing coral growth. More likely, the physical loading of coral surfaces 

with fine sediment, and the energy required to keep these surfaces clean, is 

beyond the capability of these photosynthetic organisms to overcome. In this 

way, turbidity limits coral growth to 400 m offshore and beyond, the distance 

that Rodgers and others (2005) found coral growth. It is interesting to note 

that even on the outer reef flat, living corals must continually shed significant 

amounts of sediment. Living corals are the only sediment-free surfaces on the 

reef flat. Dead coral and algae are quickly covered with a thin veneer of brown 

Figure 6. Daily cycles of dissolved oxygen and pH record for the Molokaÿi reef 

flat, July 20–28, 2001. The daily metabolic activities on the reef flat induce 

chemical changes in the overlying water. Erratic nature of these cycles is partially 

due to variations in light, wind, and other naturally occurring conditions. Breaks 

in data are due to equipment failure during inclement weather. A, Daily variation 

in dissolved oxygen dominated by daytime photosynthesis and nighttime respiration. B, Daily variation of pH dominated by photosynthetic consumption and 

respiratory production of carbon dioxide. 

7.6

7.8

8.0

8.2

8.4

pH

Dissolved oxygen

2.0

4.0

6.0

8.0

July 21 July 22 July 23 July 24 July 25 July 26 July 27

Dissolved oxygen,

in milligrams per liter pH

2001

A

B

Figure 7. Photosynthetically active radiation (PAR) and oxygen (O2) production 

on the Molokaÿi reef flat. These graphs show the relationships between available 

light and photosynthesis or respiration for sand (A), rubble (B), 10-percent coral 

cover (C), and 22-percent coral cover (D). Note that net respiration (negative 

values) changes to production (positive values) at 0 on the left (vertical) scale.
120

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

sediment, giving most of the reef a drab and dirty appearance (Field and others, this vol., chap. 17, fig. 10).

Coral Cover and Productivity

Aerial photos show the patchiness of coral growth on the Molokaÿi 

outer reef flat. The patchy distribution is reflected in the transect data 

of Rodgers and others (2005), who recorded coral cover from 1 percent 

to over 40 percent at the outer (550 m and 700 m) stations of their survey transects across the reef flat. These coral patches, along with areas 

of rubble and sand, are organized into fuzzy elongate stripes, meters to 

tens of meters wide and hundreds of meters long, roughly perpendicular 

to the shore. Storlazzi and others (2003) and Storlazzi and others (2004) 

recognize these features as “ridge and runnel” structures, similar to the 

“ridge and furrow” structures of Grand Cayman as described by Blanchon 

and Jones (1995). However, the ridges and furrows of Grand Cayman are 

erosional features as much as 10 m deep in sculptured bedrock (Blanchon 

and Jones, 1995, 1997). On Molokaÿi, ridges are depositional, formed by 

in-place growth of coral and subsequent binding by algae and other organisms. As seen from above, the dark stripes are irregular and anastomizing, 

consisting of coarse sand and rubble covered by algae landward, but of 

increasing amounts of live coral seaward. From the SHARQ measurements, patches with 10 to 20 percent or greater coral cover are net sediment producers. Although dissolution may be occurring at night, there is 

excess sediment being produced that may be transported across and along 

the reef flat, to the beach, or, in the opposite direction, to the fore reef. 

Areas with less than approximately 10 percent coral cover are losing carbonate sediment through dissolution. Rodgers and others (2005) estimate 

overall coral cover on the outer reef flat to be less than 10 percent. From 

these estimates we suggest that the reef flat as a whole is not producing 

sediment sufficient for net accumulation or transport out of the system. 

Rather, the flat is largely a relict feature, with only a thin layer of modern 

sediment. Modern sediments on the reef flat and crest may have accumulated during a brief but significant sea-level highstand that occurred in 

Hawaiÿi during the late Holocene (Grossman and Fletcher, 1998; Grossman 

and others, 1998). These sediments may overlie an older, possibly Pleistocene, edifice similar to the reef flat along southern Oÿahu that was cored 

and dated at Mamala Bay (Grigg, 1998). 

Reef-Flat Extension

Although much of the Molokaÿi reef flat may be relict, two areas are 

actively accreting. First, within the reef flat proper, large patches of coral 

growth must be net producers (>20 percent coral cover). In addition to the 

larger patches of coral, the “blue holes” on the reef flat are surrounded by 

coral fringes actively growing into these solution features. This active coral 

growth is similar to that on the fore reef and creates steep (45°) walls, from 

which colonies tumble down slope to form an apron of talus and sediment. 

The blue holes are sedimenting rapidly (geologically speaking) and adding 

to the reef flat as the depressions fill in.

Secondly, the fore reef (5 to 20 m deep) is another area of spectacular 

coral growth. Although coral growth is sparse immediately off our fore-reef 

study site, elsewhere the fore reef is characterized by meadows of 50 to 

80 percent coral cover. Accretion has been studied by coring at two areas 

on the western end of the Molokaÿi reef (Engels and others, 2004). At the 

extreme western end of the island accretion has been prevented since the mid 

Holocene by high wave energy associated with northern swells. At Hikauhi 

farther east, where the island protects from those swells, Engles and others (2004) documented vertical accretion rates of 3 to 5 mm/yr. Given the 

fore-reef slope of about 5° (1:11), this accretion rate translates to an addition 

of 30 to 50 m of reef flat per 1,000 years. These rates are comparable to reef 

growth in the equatorial Pacific and Indian Oceans, for example at Mauritius, 

where Montaggioni and Faure (1997) documented rates of 4.3 mm/yr. The 

entire fore reef of south Molokaÿi may not be accreting as quickly as the 

Hikauhi area, but much of it is characterized by more than 50 percent coral 

cover and is actively building seaward.

Relating measures of reef growth and sediment production over different spatial and temporal scales remains a challenge for biologists and 

geologists. On small spatial scales, rates of dissolution and calcification can 

be measured almost instantaneously, on time scales of minutes to weeks. 

Using repetitive measurements, these geochemical measurements can be 

scaled to describe seasonal changes (Atkinson and Grigg, 1984). Addressing 

longer timescales in this way has proven prohibitively expensive, but a new 

generation of sensors may allow repetitive monitoring over years to decades 

(Berkelmans and others, 2002).

Sediment production and accumulation in reefs and related carbonate 

sedimentary environments continues to be an active area of research, often 

approached within the framework of a sediment budget (Hubbard and others 

1990; Harney and Fletcher, 2003; Yates and Halley, 2006b). These studies, 

typically with a timescale of a few thousand years, allow some estimates to 

be made of long-term sedimentation rates. More often than not, however, 

important aspects of these sediment budgets remain unconstrained—dissolution or transport for example—and closure of the budget remains out of 

reach. It continues to be a challenge to relate short-term measurements of 

sediment production to larger scales of time and space. Examples of studies 

approaching this challenge are the summaries by Bosscher (1992) and Smith 

(1995) that expand carbonate-sediment production estimates to a global scale 

and throughout geologic time, respectively. It seemed intuitive to Darwin 

(1842) that his short-term observations of coral growth and the geometry 

of reefs could explain their growth and accumulation over geologic time. 

However, 150 years of research have left us with an incomplete understanding of the structural make-up of reefs (Braithwaite and others, 2000). Continued studies of reefs like the Molokaÿi fringing reef bring us closer to a more 

complete knowledge of coral-reef growth and the factors that affect them.

Summary

The reef flat along the south shore of Molokaÿi is an example of a reef 

area that might be said to have reached senility. By “senility” we do not 

mean that the reef is entirely dead, or relict. Rather, it supports some coral 

growth but not enough to accumulate sediment across broad areas of the flat. 

Inhibited by an influx of sediment from the island, live coral cover now is 

insufficient to maintain reef structure on the reef flat. Sufficient light is available for coral growth, but calcium carbonate production is likely limited by 

other factors, such as sediment loading and diminished settlement of young 

corals. Although those areas with more than about 10-percent coral cover 

(larger patches of coral on the flat) are net sediment producers, the flat as a 

whole maintains less than 10-percent coral cover. 

In contrast, large areas of the fore reef are extremely productive, with 

coral cover approaching or exceeding 50 percent. These areas are rapidly 

producing reef structure, and in some locations vertical accretion rates of 3 

to 5 mm/yr have been verified by drilling. These rates, together with forereef geometry, indicate that the reef flat is growing seaward on the scale of a 

few meters per century. The reef flat as a whole is growing outward, but not 

upward. It will slowly widen, but sediment thickness will not deepen significantly if major changes do not alter current patterns of coral growth.
Ridge-to-Reef: Sediment Shed from the Island

Erosion of volcanic islands is a natural process that 

begins almost immediately upon their formation. 

Weathering of basaltic rock and ash flows leads to 

the transport of sediment through the watershed 

and onto the reef. The arrival of people on Molokaÿi, 

as on all islands, led to changes in the vegetation 

cover, resulting in increased rates of erosion and 

sediment production.

Molokaÿi shoreline just east of Kamalö.
Boats in Kaunakakai Harbor.
Kalaupapa

National

Historical Park

Hlawa

Point

PlØau

Hlawa

Kmimi

One AliØi PkoØo

Kawela

Mauna Loa

Mauna Loa

(West Volcano)

Wailau

Wai a Kne (East Volcano)

HoØolehua

MolokaØi Forest Reserve

Øìlio Point

LØau

Point 

157°10' 156°50'

21°

10' 

N 

21°

05' 

157°

Kalohi Channel Pailolo Channel

Kaiwi Channel

0

0

2 4 Miles

2 4 Kilometers

Kaunakakai

Kamal

Kapualei

Phloa Fishpond

ØUalapuØe

Kihaloko Fishpond 

KulaØalamihi Fishpond 

Kaunakakai Gulch

Kawela Gulches

Kamalo

100

100

100

100

500

500

300

300

300

Gulch

Ridge-to-Reef: Sediment Shed from the Island

CHAPTER 15

People, Land, and the Reefs of South Molokaÿi

Lucile M. Roberts1 and Michael E. Field2

Waters along the southern shore of Molokaÿi are frequently turbid, a result of large quantities of suspended sediment. The most 

dominant factor contributing to this condition is the legacy of 

several hundred years of human activities on the island. We have examined 

archaeological and historical evidence to understand the impact that evolving societies had on Molokaÿi, from the watersheds to the reefs. This chapter 

takes a look at activities that have influenced the rate, volume, and extent of 

soil erosion and deposition and the overall changes to the south-facing slopes 

and coast. These activities include land settlement and clearing, construction of shoreline fishponds, interisland war, livestock introduction, farming 

practices, and the effects of fire and feral grazers. 

Impacts of Human Occupation on Molokaÿi 

Early Human Activities

The first people arrived on Molokaÿi 1,400 years ago and through their 

settlement began to alter the surface of the land. For much of early Hawaiian 

history, the southeastern coastal plain or kona district (fig. 1) carried most 

of the population (de Loach, 1975). Hawaiians eventually settled other parts 

of the island, including the Hoÿolehua Saddle (the area between the east and 

west volcanoes), the upper reaches of the south-facing slopes, and much of 

the south-central coastal plain.

Although no written records were kept during the early period of human 

habitation, there are indicators of how settlement proceeded. We can combine knowledge of the social organization in the Pacific Islands where the 

settlers lived before traveling to Molokaÿi, and archaeological evidence 

found on Molokaÿi (Weisler and Kirch, 1985), to provide a sense of conditions and processes during the time of the early settlers. 

The earliest Polynesian voyagers to settle on Molokaÿi arrived around the 

year 600 C.E.3 in the area now known as Hälawa, at the eastern tip of the island. 

As settlers began to explore the island, they discovered the gentler slopes and 

broad reef areas to the south and west. Here, long-term stream erosion had 

1

University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Käneÿohe, HI 96744

2

U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

3

C.E. (Common Era) and B.C.E. (Before Common Era) are often used by social scientists to refer to 

what others call A.D. (Anno Domini or in the year of the Lord) and B.C. (before Christ), but without 

religious connotation.

produced low-lying coastal plains and alluvial fans mantled with loose, rich soil 

that was easy to cultivate. The resources of the adjacent reef were also valued, 

and settlements gradually spread westward along the south coast. 

The early Hawaiian settlers brought with them their customs and traditions, including a system of land use and organization still used today in 

many Pacific island nations. In this feudal scheme, an aliÿi (chief) assigned 

the use of an ahupuaÿa (a portion of the land) to a small group or family unit 

(fig. 2). The ahupuaÿa commonly included a valley and the adjoining hillsides, running from the top of the watershed to the nearshore waters and the 

adjacent reef. Each ahupuaÿa provided access to everything the people might 

need: water from a stream or springs, wood from the higher elevations, forage on the lower slopes, land along the coast for cultivating, and access to 

the full resources of the reef flats and shallows. 

Early clearing and farming activities centered on planting traditional 

staples the settlers brought from their previous homes in Polynesia: taro and 

sweet potatoes. They also introduced many other plants: sugar cane, bananas, 

coconut, breadfruit, ginger, pepper, and mulberry (Bryan, 1954). Little evidence of formal villages has been found from this very early period, only signs 

of occasional small huts and perhaps a community heiau (altar or sacred site). 

Although the clearing process may have included some burning, with few 

people and a broadly scattered population, the impact of the people on the land 

was likely minimal for the first four centuries of human habitation. 

Building on the Edge of the Sea

During the next 400 years, starting about 1,000 C.E., changes in the 

evolving society began to leave visible changes on the landscape. Early in 

this period the aliÿi nui (high chiefs) of the larger Hawaiian islands seem 

to have ignored the small, lightly populated island of Molokaÿi as they 

battled over larger prizes elsewhere in the islands. This was the period 

when many stone works were completed on Molokaÿi, including terraces 

on the hillslopes to keep soil and water in place for growing taro and other 

Figure 1. Map of Molokaÿi, showing locations mentioned in the text. Bathymetric contour interval, 100 m.
2

1

3 4

5

6

7

Figure 2. Drawing of an ahupuaÿa 

(modified from HawaiiHistory.org, http://

www.hawaiihistory.org, last accessed 

April 29, 2008). The ahupuaÿa is based 

on a traditional Polynesian style of land 

management that divides an island into 

segments running from the mountainous 

interior to the edge of the reef, cutting 

through all the ecological zones (Weisler 

and Kirch, 1985, p. 129). Each ahupuaÿa 

provides for all the needs of a family or 

small group of people. 1, Streams carry 

mountain rainwater from ridges down 

steep slopes. 2, Stream water is diverted 

in narrow taro fields in the upper valley. 

3, Coconut trees provide food and drink. 

4, Broader taro fields fill the wider valley 

mouth. 5, Coastal fishpond is open to the 

sea. 6, Fishing village heiau (sacred altar). 

7, Rock marker at ahupuaÿa boundary.

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

124

crops (de Loach, 1975). More than 50 fishponds for raising fish were built 

on reef flats along the south coast, most of them completed by the early 

15th century.

Although other islands also had fishponds, Molokaÿi had perhaps the 

greatest concentration, in large part because of the extensive shallow reef 

flat along the south shore (fig. 3). Some fishponds occupied several hundred 

acres, with enclosing walls averaging 1.5 m (5 feet) high and 3 m (10 feet) 

thick (Kepler and Kepler, 1991). Most fishponds were relatively shallow (1 

m or 3 feet) but some were as deep as 10 m (33 feet). The presence of fishponds on the reef flat altered the natural pathways for water and sediment. 

Sediment flushed to the coast during rainy periods was deposited in the 

ponds, requiring constant tending to prevent them from filling and becoming 

unusable for raising fish (as in fig. 3A) for the island aliÿi.

A Time of Extremes—Quiet Prosperity or Invasion

During the time between the first fishponds and the arrival of outsiders 

from Europe and America in the late 1700s, the people of Molokaÿi developed a comfortable level of prosperity. Crops were plentiful and the ponds 

filled with fish. However, in this same period the aliÿi nui of the larger 

islands fought many battles. Because it was located between the warring 

factions of Hawaiÿi, Maui, and Oÿahu, Molokaÿi served as both a refuge 

and a source of provisions. The island was conquered, perhaps many 

times, by aliÿi nui of other islands. One of the conquering aliÿi nui was 

Kapiÿiohokalani, who ruled Oÿahu in the 1730s. According to de Loach 

(1975, p. 27), “Kapiÿiohokalani invaded Molokaÿi somewhere around 

1736. He attacked with a large force, and his army broke down sea walls 

and destroyed many fishponds.” Conflict among the aliÿi nui continued for 

at least 30 years after the outsiders came. Then, in the late 1700s, Kamehameha I used Western firearms and small, fast boats to conquer his opponents and declare his power over all the islands. It is said of his taking of 

Molokaÿi that, “Kamehameha’s descent upon it had desolated the country” 

(Menzies, 1920, p. 118). Robert Louis Stevenson, visiting Molokaÿi several 

years after the battles ended, painted a vivid word picture of the devastation: “...upon this island a battle was fought in which it has been computed 

that thousands were engaged; and he who made the computation, though 

Figure 3. Aerial photographs of fishponds on the southeastern coast of Molokaÿi. 

Constructed in the 11th through 15th centuries, these fishponds have great cultural and historical value to the people of Hawaiÿi. A, Pühäloa fishpond between 

Kalaeloa and ÿUalapuÿe, showing the large amount of sediment infilling that has 

occurred here. As vegetation takes hold, trapping of sediment and infilling rates 

increase. B, Kihaloko (left) and Kulaÿalamihi fishponds east of Pükoÿo link together to form a nearly continuous armored shoreline. This photograph shows the 

lower part of several ahupuaÿa as well. C, Submerged remains of fishpond walls 

are shown by arrows.

National Oceanic and Atmospheric Administration, 1993 Bruce Richmond, USGS Bruce Richmond, USGS

B

A

C
Sugar cane fields

Fishpond

Reef flat

People, Land, and the Reefs of South Molokaÿi

125 

Ridge-To-Reef: Sediment Shed from the Island

he lived long after, has seen and counted, when the wind blew aside the 

sands, the multitude of bones and skulls” (Stevenson, 1973, p. 75).

A Faster Rate of Change

While the aliÿi nui were battling for power, Europeans, beginning with 

Cook and Vancouver in the 1770s, entered the islands, bringing with them 

irrevocable change to the people and the habitat. Along with firearms and 

fast ships, the explorers from Europe and America introduced grazing animals and commercial farming. Both ideas caused rapid changes to the hillsides and reefs of Molokaÿi over the next 100 years. Animals were initially 

introduced by explorers to be used as a food source on their return. The 

aliÿi quickly realized that they could also raise animals to use and to sell. 

Goats, swine, sheep, cattle, and horses were introduced and began to thrive 

on Molokaÿi by the end of the 1800s. In 1870, Kamehameha V sent his 

small herd of deer to Molokaÿi, where they also flourished (Judd, 1936). 

Hawaiian vegetation evolved without native grazing animals. Trees, 

ferns, and bushes developed soft bark, tender greenery, and lacked 

thorns—and were therefore defenseless against grazing animals (Bryan, 

1954). As the number of animals increased on Molokaÿi, the distribution 

of native vegetation was altered, particularly in the cooler upper reaches 

of the island’s watersheds. Goats, sheep, and cattle are known to graze 

heavily on dense vegetation wherever it is available, leading to increased 

susceptibility of exposed areas to the effects of wind and rain. This is 

consistent with conditions noted on the higher slopes of Molokaÿi. Increasing amounts of soil were transported and deposited on the low southern 

slopes and, eventually, on the reef flat. The loss of soil in the uplands led to 

further habitat destruction and changes in local microclimates. Without the 

heavy vegetation to capture the moist, rising winds, rainfall decreased in 

upland areas. Many springs dried up (Kepler and Kepler, 1991), and people 

abandoned efforts to raise grazing animals. Pigs and goats turned feral 

(wild) and migrated to the rugged, hard-to-reach areas of the high slopes, 

where they, along with deer, fed on small saplings and grasses and dug 

for roots, further contributing to soil loss. As the animal population grew, 

the human population declined. From early estimates of 10,500 people on 

Molokaÿi in 1779 (Summers, 1971), the population dropped to 2,307 by 

1896 (Bottenfield, 1958), leaving fewer people to hunt the animals and 

keep them in check.

Two other early activities adversely affected the south slopes of 

Molokaÿi: the sandalwood trade and plantation-style agriculture. Early in 

the 1800s, sandalwood—highly prized in the Orient—became a great trade 

commodity. Molokaÿi stands of this tree were smaller than those found on 

other islands but considered large enough to warrant logging, as evidenced 

by the digging of at least one ‘sandalwood pit’ (a hole in the ground the 

size of a ship’s hold, used for measuring the amount of wood needed to 

provide a full cargo; see fig. 4). To support this venture, the aliÿi directed 

the men to leave their fields and focus on harvesting the trees (Bottenfield, 

1958). According to Cooke (1949, p. 61), “It is said that the extinction 

of sandalwood was caused by the natives destroying young sandalwood 

plants. They did not wish their descendants to undergo the hardship of 

obtaining the sandalwood from the cold, wet mountains and having to 

transport it to the shore.” The removal of sandalwood and exposure of soil 

Figure 4. A historical lua moku ÿiliahi, or sandalwood measuring pit (outlined by 

dashed line), on the Molokaÿi high slopes. Approximately 23 m (75 ft) long, the pit 

was dug in a size and shape that mimicked the size of the hold of sailing ships 

that carried the sandalwood back to New England. Sandalwood was cut and 

stacked in the pits, and then hauled to ships waiting at anchor.

Susan Cochran, USGS

for agriculture, especially in the area of Hoÿolehua Saddle, contributed to 

additional erosion. 

Soil loss likely accelerated in the mid 1800s when cattle ranching intensified on the upper reaches of Hoÿolehua Saddle. Low-quality cattle, sold for 

hides and tallow, were raised in large numbers and driven south to Päläÿau 

for shipment. The damage done by the trampling of driven herds added to 

the damage done by grazing (fig. 5). By the end of the 1800s, sediment was 

deposited in the central lowland plain at a rate of one foot (30 cm) every six 

years (Cooke, 1949; D’Iorio, this vol., chap. 16). 

Plantation monoculture was also introduced during the 1800s. Sugar 

cane was one of the first such crops and was grown in several areas along 

the southeastern coastal plain (Jackson, 1882; fig. 6), “but the holdings were 

abandoned when it was found that the artesian water being used was too 

salty” (Keesing, 1936, p. 27). This system left lasting problems. Between the 

destruction of native vegetation necessary for plantations and the insects and 

diseases associated with the introduced species, even taro and some vegetables could no longer be grown in certain areas. Changes in land-use patterns 

were visible long after the sugar plantations disappeared (Gast, 1982). Thus 

Figure 5. Characteristic severe erosion on west Molokaÿi, just west of the 

Hoÿolehua Saddle. The red-brown soils are eroded during heavy rainfall and 

transported to the coast through gulleys and by overland flow. This type of erosion is typical of areas that have had the forest canopy removed.

E. Trimpert

Figure 6. Map of Kamaloÿo (as it was called then; today it is Kamalö) showing 

the shape of the coast in 1882 and the sugar cane fields near the shoreline (from 

Jackson, 1882). Sediment derived from the erosion of the exposed sugar cane 

fields was transported to the coast, resulting in a change in shape of the shoreline.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

126

the clearing necessary for such monoculture heightened the susceptibility of 

the soils to erosion in the lower coastal areas as well.

Modern Actions, For Better or Worse

By the twentieth century, the damage to the landscape and soil cover 

(fig. 7) caused by ranching and sugar cane farming had become evident, and 

some corrective measures were introduced. At the same time, some damaging activities continued and new ones were introduced. Repairing centuries 

of damage has proved challenging: 

• Between 1898 and 1905, Molokaÿi Ranch, the largest landholder 

on the island, shifted from open country grazing to a paddock 

system in an effort to limit the impact of cattle on the vegetation 

in selected areas (Henke, 1929). This measure was only margintive effort of government and local landowners. Fencing and 

volunteer restraint were used to give the upper slopes of the 

south-draining watersheds a chance to recover from the livestock 

damage. Effectiveness of conservation and remediation efforts 

varied with the dedication of those involved.

• In the 1920s, much of the Hoÿolehua Saddle was set aside as 

Hawaiian Homestead lands and was opened to farming. Clearing 

the land for farming made the loose, red soil more susceptible to 

the eroding effect of the prevailing northeast trade winds, while 

the planting of windbreaks was only marginally successful in 

mitigating the damage. Ironically, the homesteaders found it 

more lucrative to lease their parcels to the big pineapple producers than to farm them individually, leading to the additional 

environmental damage associated with extensive monoculture.

• By 1930, much of the island was devoted to either pineapple 

cultivation or to grazing. Both activities continued to alter the 

land of the Hoÿolehua Saddle and the coastal plain. As Handy 

(1931) noted, most fishponds by this time had been abandoned 

and were filled with mud.

• In 1934, the modern pier at Kaunakakai was built (Kepler and 

Kepler, 1991). This large-rock structure had a base substantial 

enough to carry a two-lane road, and it supported an expanded 

landing facility. Its position, jutting as it did at least 800 m 

(nearly 0.5 mi) south across the reef flat along the eastern edge

of a natural channel in the reef, disrupted the along-shore flow 

of water. 

• In 1948, the Molokaÿi-Länaÿi Soil Conservation District was 

formed by the U.S. Department of Agriculture to help local 

residents reverse the environmental damage and improve the 

management of their lands. A Molokaÿi field office was established to disseminate information and assistance to landowners, 

farmers, and tenants to promote their conservation efforts by a 

broad variety of means (S. Cox, oral commun., 2000).

• The Nature Conservancy established an office on Molokaÿi in 

1983 (E. Misaki, oral commun., 2000), adding their efforts to 

those of others attempting to reverse centuries of environmental 

damage and halt its progress. Since that time they have been 

working with local hunters and State and Federal agencies to try 

to control the feral goat population.

The Late Twentieth Century

Although agriculture has been the main land use influencing erosion 

rates on Molokaÿi, increased construction and road building have added to 

the runoff of water and sediment. Road cuts expose layers of loose soil that 

ally successful, and erosion caused by destruction of overlying 

vegetation can still be seen today (fig. 5). 

• In 1902, mangroves (Rhizophora mangle) were introduced by 

the American Sugar Company in an effort to stabilize the coastal 

mudflats at Päläÿau in south-central Molokaÿi and hold back the 

soil washed down by heavy rains into the sea (Allen, 1998, p. 62). 

• By 1907, the sheep population on Molokaÿi reached 17,000 

animals, grazing mostly on the grasses of the Hoÿolehua Saddle 

and denuding large areas of pasture. Sheep-raising was finally 

discontinued “only because of reduced grazing areas and a poor 

wool market” (Henke, 1929, p. 18).

• In 1912, the Molokaÿi Forest Reserve was created, covering 

a large part of the eastern uplands of the island as a cooperaFigure 7. Views of characteristic severe 

erosion or weathering of the underlying volcanic rock on West Molokaÿi. The rock produces iron-rich soils that are characteristically red-brown in color and range in grain 

size from sand to clay. Open exposures 

such as these lead to increased sediment 

runoff and siltation on the adjacent reef.

S. Peckham, USGS

A
Pre-1800 1800 1850 1900 1950 2000

1

1 2

2 

3

4

5

6

7 8

3

1 Deforestation 2 Sugar cane and pineapple farming

4 Feral pigs and goats/slope denudation

3 Ranching/overgrazing

5 Wharf construction

6 Coastal dredging

7 Sewage release

8 Fire and hillside development

 ?

People, Land, and the Reefs of South Molokaÿi

127 

Ridge-To-Reef: Sediment Shed from the Island

is easily eroded and transported to the coast, often via the sloped, paved 

surfaces or drainage ditches cut alongside the roads. 

In the second half of the 1900s, residents and visitors alike increased in 

number, resulting in the construction of hotels, condos, and housing developments. In the early 1970s, sections of the reef flat at Kamalö and Pükoÿo 

were dredged for marina development (fig. 8). Some of the excavated material was used to alter the shape of the remains of a large fishpond, and some 

of it was discharged onto the reef. After completion of the dredging and 

reshaping of the coast, these marina projects were abandoned. 

Additional development along the southern shoreline began with 

small-scale residential units, including low-rise condominiums and private 

homes. This was followed by upslope development involving large-scale 

clearing and grading and the building of roads, houses, walls, parking lots, 

and driveways. These activities altered the runoff patterns. Less water was 

absorbed into the ground, and surface flows became larger, more frequent, 

and more erosive.

Population growth on Molokaÿi also led to an increase in sewage discharge, another potential impact to the reef. The county sewage treatment 

plant serves only the town of Kaunakakai (J. Souza, oral commun., 2000) 

and uses injection wells on the plant site west of town and a quarter mile 

north of town, in Kaunakakai Gulch. Newer residences outside the town 

have septic systems, while the older ones have cesspools. Condominiums 

have their own small primary treatment systems with on-site injection wells. 

There is a potential—as yet undocumented—impact on the reef from the 

migration of sewage through ground-water seepage. Nutrients in the sewage 

may contribute to blooms of undesirable algae and cyanobacteria along the 

inner reef flat near residential areas.

The Twenty-First Century 

Centuries of human activity have clearly had an impact on water quality and reef condition along the Molokaÿi south coast. Human activities that 

have adversely affected much of the south coast include coastal dredging at 

Pükoÿo in the east, wharf construction and sugar cane farming in the central 

part of the coast, and heavy cattle grazing on the west end. Although the 

impact from these activities has been severe, evidence indicates that once 

the activities ended, water quality improved and the adjacent reef began to 

recover. Assessment will continue for many years in an effort to discern the 

degree of improvement in both the water and the reef. Coastal waters off 

Päläÿau, for example, are relatively clear compared to those off Kawela, and 

the coral cover on the reefs is increasing. 

Many activities that occur in watersheds have a lingering effect, or 

legacy. Muddy sediment introduced into gullies, stream channels, and the 

coastal plain remains there for many years, but ultimately it is eroded and 

transported to the reefs. The greatest threats to reefs and to water quality 

are activities that expose upland soils to accelerated erosion and transport. 

Two of the principal causes of current soil loss from south Molokaÿi watersheds are the stripping of vegetation by fires and the destructive grazing of 

feral goats (fig. 9). 

Figure 8. Aerial photographs of the reef flat east of Kamalö showing impact of 

coastal development. Dredging of coastal materials and reef deposits produces 

high levels of turbidity and sedimentation locally. These two projects generated 

tons of sediment that washed across the reef, and according to local accounts, 

killed acres of corals. A, 1993 aerial photograph shows an irregular pit in the reef 

flat (straight-edged area of deeper, dark blue water) that was the site of excavation in the 1950s for a proposed marina. B, Oblique 1998 photograph of Pükoÿo 

Harbor is an example of shoreline modification. Construction of the harbor 

resulted in a large volume of fine sediment being released onto adjacent coral 

covered areas.

Figure 9. This map and chart summarize the human activities that have influenced watershed character and soil loss on Molokaÿi over several centuries. The 

locations are somewhat generalized; although the locations of some activities 

are well known (farming in the Hoÿolehua Saddle, for example), the locations 

of others, such as fires and feral grazers, are widespread and change through 

time. The time line illustrates the approximate timing for each activity that has 

caused soil loss and altered conditions on the Molokaÿi reef. Although some past 

activities seem to have had a severe impact, the reef appears to be recovering now that the activity has ceased or diminished. Some activities, in particular 

feral grazing, fire, and hillside and coastal construction still have the potential to 

continue degradation of the reef. Ongoing studies by U.S. Geological Survey scientists show that in some upland areas the damage to the terrain will be difficult 

to repair. Loss of vegetation leads to erosive overland flows and soil loss, making 

it difficult for vegetation to become reestablished. Note that the impact from 

sewage release into the subsurface is essentially unknown, and that at present, 

damage from feral goats is a leading contributor to soil loss and reef degradation.

B

A

Bruce Richmond, USGS National Oceanic and Atmospheric Administration, 1993
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

128

Federal, State, and local organizations have recognized the threat to 

Hawaiian coral reefs from land-derived sedimentation. Together they have 

developed a Local Action Strategy (LAS) under the auspices of the U.S. 

Coral Reef Task Force to address land-based pollution. The LAS focuses on 

three areas in Hawaiÿi, one of which is the Kawela to Kapualei area of east 

Molokaÿi. The LAS is watershed based and incorporates the native management system for lands and natural resources, the ahupuaÿa. The objectives of 

the LAS are threefold: (1) reduce pollutant load to surface water and ground 

water, (2) improve understanding of the link between land-based pollution 

and coral-reef health, and (3) increase awareness of pollution prevention. 

The LAS proposes specific positive actions to address the problems of soil 

erosion in the Kawela-Kapualei area and the consequent damage to coastal 

water quality and the reef: 

• fencing and other procedures to control feral grazers and reduce 

the impact of grazing, 

• educating and training local residents about fire management, 

• restoring the watersheds with native vegetation, and 

• constructing sediment basins to capture land-derived silt before 

it reaches the ocean.

Construction of sediment basins is costly, and thus it is uncertain 

whether they will be built. The other measures, along with a program to 

reduce the total number of goats, will likely reduce sediment runoff.

Summary

Although landscape change is a continuous and anticipated part of all 

natural systems, the rate of erosion, deposition, and coastal change in South 

Molokaÿi has been altered significantly by human activities that range from 

farming and ranching to the introduction of feral grazers. These activities 

were not confined to any one part of the island, nor was any single activity 

responsible for the changes to the watersheds. The changes visible today 

result from the cumulative effects of many different activities over decades 

or, in some cases, centuries (fig. 10). 

During the past few years, the residents of Molokaÿi have recognized the 

extent to which they affect the rate of change in their environment and have 

initiated efforts to understand and possibly reverse the effects. Today scientists continue to study the upland regions and the processes that erode and 

transport sediment to the reef. On the reef, they examine physical evidence 

of the impact of upland watershed change on the coral community. They 

seek to identify and quantify the rate of change and learn the causes of both 

past and present change. The goal is a worthy one: knowledge of Molokaÿi’s 

past will help the residents deal more effectively with present and future 

threats to their coral reefs.

Figure 10. The denuded hill slopes of eastern Molokaÿi upslope from Kamalö. 

Feral goats strip the grasses and young shoots before they can develop sufficient 

root structures to hold soil or a canopy to protect against direct rain action. Infrequent fires exacerbate the problem of vegetation loss.

G. Tribble, USGS
Volcanic bedrock

Coral reef

 Mudflat

Terrestrial 

influence

Marine 

influence

Mangrove environment

Ocean

~20 cm

KauaØi

OØahu

MolokaØi

Maui

LnaØi

KahoØolawe

NiØihau

Mangrove colonization

HawaiØi

25 50 Miles

0 50 100 Kilometers 

0

Ridge-to-Reef: Sediment Shed from the Island

CHAPTER 16

Invasive Mangroves and Coastal Change on Molokaÿi

Mimi D’Iorio1

Mangroves are salt-tolerant, woody plants that grow on island shores 

and along low-relief coastlines throughout the world’s tropical and 

subtropical oceans (fig. 1). Often found in association with coral 

reef environments, mangroves form coastal forests that exist in a dynamic 

equilibrium between the ocean and the land (fig. 2). The term “mangrove” 

refers to an individual mangrove plant, but is more generally used in an ecological context to describe an entire 

plant community, including approximately 70 different species of trees 

and shrubs, that inhabits the brackish 

land-sea margin along low-latitude 

coastlines (Duke, 1992). Not native 

to the Hawaiian Archipelago, mangroves were introduced to Molokaÿi 

in the early 20th century to help 

stabilize eroding coastal mudflats on 

the south coast near Päläÿau (MacCaughy, 1917; Wester, 1981; Allen, 

1998). Their subsequent colonization 

of the island’s south shore has altered 

the structure of the coastal margin, 

influencing a variety of changes to 

surrounding nearshore habitats and 

coastal infrastructure.

Regarded as land builders and 

known for their ability to promote 

sedimentation in coastal settings, 

mangroves play a valuable role in 

protecting and stabilizing eroding 

coastlines worldwide. They are most 

prolific in sediment-rich areas where 

freshwater and terrestrial runoff are 

abundant, such as near the mouths of 

coastal rivers and streams and in coastal wetlands. Mangroves trap sediment 

being transported seaward, thereby reducing water turbidity and decreasing 

the amount of sediment flowing into adjacent coastal ecosystems. Water flow 

1

 University of California, Department of Earth Sciences, Santa Cruz, CA, 95064; current address: 

NOAA Marine Protected Areas Science Center, 99 Pacific St., Suite 100F, Monterey, CA 93940, 110 

Shaffer Rd., Santa Cruz, CA, 95060

is decreased in the vicinity of the extensive aerial root systems, allowing 

silt (fine-grained terrigenous sediment) to deposit. The high abundance of 

microbial communities within mangrove forests also plays an important role 

in trapping and binding sediment, as fine particles settling out from reduced 

water velocity are aggregated and mineralized by biological activity (Alongi 

and McKinnon, 2005).

Mangroves are unique plants that have evolved special physiological and 

morphological adaptations which allow them to thrive in the relatively harsh 

environmental conditions that exist within the intertidal zone. Increased 

water salinity, the presence of anaerobic substrates, high-energy wave action, 

and frequent tidal inundation are all factors that contribute to the severe 

nature of the mangrove environment. Their ability to survive in these relatively adverse conditions sets mangroves apart from other plant species and 

allows them to occupy a niche within the coastal zone where they have little 

to no competition among other plants. 

History of Mangroves on Molokaÿi

Although no mangrove species are native to the Hawaiian Islands, 25 

species exist naturally elsewhere in the Pacific, with 9 species identified 

on Guam and 12 on both the Republic of Palau and the Federated States 

of Micronesia (Ellison, 1999). Because of the relatively young geological 

age of the Hawaiian Islands and their geographic and oceanographic isolation from large continental land masses, the various mangrove species that 

evolved and radiated throughout the Indo-Pacific during the Cretaceous 

Period and have since colonized many island coasts throughout the South 

Pacific, Atlantic, and Indian Oceans did not naturally disperse to the Hawaiian Islands. 

In 1902, however, the Florida red mangrove, Rhizophora mangle, was 

introduced to the south coast of Molokaÿi by the American Sugar Company. 

Young mangrove trees were planted on the Päläÿau mudflats to help protect 

the shoreline and adjacent coral reef environment from the increased agricultural runoff associated with the cultivation of the uplands for sugar cane 

(Allen, 1998). Since its introduction, Rhizophora mangle has become well 

established on Molokaÿi, migrating eastward along the coastline, colonizing 

tidal mud flats, shallow embayments, and the rock walls and muddy interiors 

of coastal fishponds. The species has also spread to other nearby islands in 

the Hawaiian chain but is most prolific on Molokaÿi. It now dominates the 

Figure 1. Red mangrove (Rhizophora 

mangle) juvenile tree growing on the 

reef flat off Molokaÿi’s south coast near 

Päläÿau. 

Figure 2. Schematic diagram illustrating the dynamic relation that exists between 

coral reef and mangrove environments because of the complex interaction of 

marine and terrestrial processes at the land-sea margin.

Figure 3. Map showing the approximate sites (blue dots) of known mangrove 

colonization throughout the eight main Hawaiian Islands. Sites updated from Allen (1998) and Wester (1981). 
~30 cm

A

B

C

D

B

C

D

~ 10 cm

~ 20 cm

~ 20 cm

~ 20 cm

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

130

island’s south coastline at Päläÿau, and localized coastal colonies are identified as far as Pauwalu on the island’s east end. 

In most of the world, native mangrove systems are declining as a result 

of overharvesting, land reclamation, and aquaculture (Spalding and others, 

1997; Alongi, 2002), and large-scale efforts focus on restoring degraded 

mangroves and preserving remaining natural mangrove forests (Saenger 

and Siddiqi, 1993; Islam and Wahab, 2005). In Hawaiÿi, however, the alien 

mangrove is not threatened, but is spreading, colonizing coastal property 

on at least six of the eight main islands (fig. 3; Allen, 1998). The proliferation of introduced mangroves has contributed to loss of habitat for four 

endemic waterbirds, including the endangered Hawaiian stilt (Himantopus mexicanus knudseni), damage to sensitive archaeological sites, premature infilling of historic fishponds, and restructuring of the native coastal 

wetland ecosystem. Management of R. mangle is of concern in many of 

the national parks and wildlife refuges on the islands, where native habitats, nesting grounds, and important archaeological sites are threatened by 

encroaching mangroves. These effects of alien mangroves on the native 

systems have encouraged local agencies to consider and implement alienspecies management strategies. Mangrove removal projects have already 

been initiated in the Kaloko-Honoköhau National Historic Park on the 

Island of Hawaiÿi (Pratt, 1998) and in the Nuÿupia Ponds Wildlife Management Area (NPWMA) within the Käneÿohe Bay Marine Corps Base on 

Oÿahu (Cox and Allen, 1999).

Characteristics of Mangroves on Molokaÿi

All mangrove species are considered facultative halophytes, meaning 

that they can survive in both saltwater and freshwater environments. In native 

mangrove environments, where multiple species exist and compete for space, 

the mangrove habitat exhibits species zonation in response to local cross-shore 

environmental gradients of salinity, substrate composition, and elevation. Each 

zone is dominated by the mangrove species that is best adapted to survive in 

the given conditions. On Molokaÿi, however, there is a monospecific distribution of R. mangle, a species that is characteristically found at the water’s edge, 

along the turbulent boundary of the intertidal zone. As a result, the species 

zonation evident in native systems is not observed on Molokaÿi, and R. mangle

has been able to successfully migrate and colonize outside its traditional niche.

Rhizophora mangle, or red mangroves, are easily distinguished from 

other mangroves throughout their reproductive cycle by their prominent 

aerial prop roots, their thick, waxy leaves, and their pencil-shaped fruit or 

propagules, which grow in dense, low-hanging clusters. These propagules, 

which include embryonic root structures, germinate while still attached to 

the parent tree and grow into long greenish-brown seedlings that drop into 

the water, where they float upright and are dispersed by currents. After an 

often lengthy water-borne dispersal period (that can be as long as a year), 

during which they continue to grow and develop, the germinated seedlings 

eventually encounter land or are stranded on tidal flats, where they plant 

their roots, grow leaves, and flower (fig. 4). The dense system of prop roots, 

extending up to a meter above the soil surface, provide stability from wind 

and waves and are equipped with pores called lenticels that promote respiraFigure 4. Large mature red mangrove tree growing on the reef flat at Päläÿau. 

Note the dense matrix of prop roots that extend from the tree’s main stem onto 

the sediment-covered reef flat. 

Figure 5. Features of red mangrove (Rhizophora 

mangle) on Molokaÿi’s south coast. A, Propagules 

hanging from mature red mangrove tree at the coastal 

margin near Päläÿau. B, Young mangrove seedlings 

at various stages of root development colonizing the 

reef flat just offshore of the Kapuäiwa coconut grove. 

C, Mangrove seedling that has taken root on the reef 

flat near the Kapuäiwa coconut grove. Notice the aerial 

prop roots that have begun to grow laterally away from 

the main stem toward the substrate. D, Red mangrove 

tree establishing a stronghold on the reef flat at Päläÿau. 

Note the young seedling in the foreground that has yet 

to grow any prop roots. The prop roots collect algae 

and sediment around their base, forming a unique and 

protected environment for marine microfauna.
~350 m

Mangroves

Reef Flat

Mangroves ~300 m

Reef Flat

Reef Flat

Fishpond

Shoreline

1940 1940 1975 1975 2000 2000

Invasive Mangroves and Coastal Change on Molokaÿi

131 

Ridge-To-Reef: Sediment Shed from the Island

(T-sheets), published by the National Oceanic and Atmospheric Administration’s (NOAA) Coastal Services Center (and previously by NOAA’s National 

Ocean Service), are the oldest reliable representation of the shoreline, providing a valuable point of reference for defining historical coastal features 

and shoreline positions. Quantification of coastal change involves the acquisition and digitization of these historical resources, their rectification and/

or georegistration, digital extraction of shoreline position, and the calculation and interpretation of change rates. Additionally, satellite data and aerial 

imagery were processed to extract species coverage and distribution information for assessing the spatial extent of Rhizophora mangle as observed from 

space and airborne imaging sensors.

In the year 2000, mangroves occupied 2.4 km2 of coastal land on 

Molokaÿi, with the Päläÿau and ÿUmipaÿa regions exhibiting the most extensive coverage of dense mangrove canopy. The western portion of the island’s 

south coast exhibits greater overall coverage, whereas mangroves in the east 

preferentially colonized in coastal fishponds. The geographic boundaries of 

mangrove habitat alongshore are from Wai a Käne in the west to Pauwalu in 

the east (fig. 7).

The distribution of R. mangle may be both naturally and anthropogenically influenced. The red mangrove prefers muddy, protected settings for 

colonization, but it is likely that mangrove colonization has been prevented 

on some suitable substrates by clearing of the propagules as they begin 

to encroach onto private property. In support of this, anecdotal evidence 

through discussion with local residents indicates that at least some fishponds 

are routinely cleared of mangrove pods to maintain operability and prevent 

overgrowth and excess sedimentation. 

Mangrove propagules are dispersed into the water column and are transported by winds and surface currents, influencing the coverage of mangroves 

along the south coast of Molokaÿi. With predominantly east-to-west surface 

flow and northeasterly trade winds, material and seedlings deposited at the 

coast are generally transported westward (see Ogston and others, this vol., 

chap. 20, for further discussion about surface flow and transport). This may 

account for the greater mangrove coverage at the west end of the island. 

Additionally, observations at the Kaunakakai Wharf show a substantial accumulation of pods along the east side of the causeway, suggesting that further 

westward transport is blocked by the presence of this long structure that 

extends across the shallow reef flat.

Shoreline change rates are highly variable both alongshore and over 

time. From 1915 to 2000, mangrove progradation in regions of the south 

coast accounted for shoreline change rates in excess of 14 m (45.9 ft) per 

year (fig. 8). Long-term trends of shoreline change rates show an overall 

decrease in progradation from west to east, which appears to be related to 

the distribution of coastal fishponds alongshore. Interval rates for the Päläÿau 

and ÿUmipaÿa regions suggest that there was an early period of rapid mangrove expansion that occurred from 1915 to 1940, during which a majority 

of the currently existing mangrove habitat was established (fig 9). Maximum rates calculated over this period are found in Päläÿau, where shoreline 

progradation rates exceeded 44 m (144.4 ft) per year. Interval change trends 

tion in anerobic (low-oxygen) soils by diffusing oxygen down to the buried 

roots when exposed at low tide. Figure 5 shows the features of R. mangle

trees in various stages of development.

Fishponds are easy targets for mangrove encroachment, providing root 

support within rock walls and protection from surface currents and northeasterly trade winds. Shoreline change associated with mangroves and 

fishponds is most evident in the ÿUmipaÿa region, where a number of previously shoreline-bound fishponds (loko ÿume iki) are now separated from the 

coastline by hundreds of meters of mangrove forest (fig. 6).

Mangroves and Shoreline Change

To understand how the invasion of Rhizophora mangle influenced 

coastal change on Molokaÿi, rates of shoreline progradation were calculated 

across various time scales to assess the temporal and spatial variability in 

mangrove colonization throughout the 20th century. Remote sensing and 

geographic information systems (GIS) techniques were used to interpret satellite data, aerial photographs, historical maps, and coastal surveys in order 

to identify the position of the south coast shoreline and to map its seaward 

migration since the introduction of R. mangle in 1902. 

Historical maps and aerial photography offer a perspective into the 

past that can be digitally recreated in a GIS environment to assess spatiotemporal patterns of change in coastal systems. Topographic coastal surveys 

Figure 6. Historical aerial photographs showing the relative location of the shoreline (orange dotted line) to the Kaluaapuhi 

Fishpond (yellow dotted line) near ÿUmipaÿa in 1940, 1975, and 2000. Note the lack of mangroves on the tidal reef flat in 1940. 
PlØau

Wai a Kne

Kaunakakai

ØUmipaØa Kamililoa

Kawela

Kamal

PkoØo

Pauwalu

A B C D E

F

A B C

D E F

0 0.5 1 Mile

0 1 2 Kilometers

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

132

Figure 7. Maps showing the distribution of Rhizophora mangle (shown in red) 

on the south coast of Molokaÿi in the year 2000. The left image is a composite of 

Landsat 7 data showing the mangrove distribution for the entire south coast of 

Molokaÿi, with rectangles outlining the extents of the six insets presented above 

(A–F). Each red pixel represents an area 10 m by 10 m (32.8 ft by 32.8 ft) that is 

predominantly occupied by the red mangrove. The total coverage for R. mangle in 

the year 2000 was approximately 240 hectares (2.4 km2).
0 2 4 6 8 10

16

12

8

4

0

Distance along shore, in kilometers

Shoreline change rate, in meters per year

12 14 16 18 20 22 24 26

KawiØu

Pkanaka

ØØØia

Kaluaapuhi

KalokoØeli AliØi

Knoa Pnhh

Kanukuawa

Pahiomu

KamhuØehuØe

Kpapa

KluapeØelua Manawainui KalamaØula Kaunakakai Kamililoa Kawela Kamal

Kolo

Wai a Hewahewa

Erosion

Progradation 

1915

Shoreline

2000

Watershed boundary

Invasive Mangroves and Coastal Change on Molokaÿi

133 

Ridge-To-Reef: Sediment Shed from the Island

Figure 8. Landsat composite image of a 27-km (16.7 mi) section of coastline of 

southeastern Molokaÿi from the Päkanaka Fishpond (west) to the Kamalö Harbor 

(east), showing the position of the shoreline in 1915 and 2000. Below the image is 

a histogram of long-term shoreline change rates (1915–2000) along this stretch of 

coast. White dotted lines on the image represent boundaries of coastal watersheds 

named along the margin of the image (watershed boundaries from Hawaiÿi State 

GIS Program, http://www.state.hi.us/dbedt/gis/, last accessed April 29, 2008). The 

arrows on the plot identify the locations of coastal fishponds alongshore.
Distance along shore, in kilometers

0 1 2 3 4 5 6 7 8 9 10

10

0

10

0

10

0

Shoreline change rate, in meters per year

10

0

30

20

40

B 1915Ð1961

A 1915Ð1961

C 1961Ð1975

D 1975Ð2000

E 1915Ð2000

Erosion

Progradation 

1915Ð1940

1940Ð1961

Pl`au

1915

1940

1961

1975

2000

Shoreline

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

134

show an overall decrease in shoreline progradation rates and an increase in 

rates of erosion of mangrove habitat (fig. 9). From 1915 to 1961, a majority 

of the shoreline showed progradation, with a maximum rate of 27 m (88.6 

ft) per year near the channel at Päläÿau. Erosional trends were more evident 

between 1961 and 1975, when approximately 35 percent of the shoreline 

studied showed varying rates of retreat as great as 4 m (13.1 ft) per year. 

Average rates of change decayed exponentially through time, starting high 

in the early 20th century at 4.1 m (13.4 ft) and decreasing to modern rates of 

less than 0.1 m (0.3 ft) per year. 

Summary 

No mangroves were present on the Hawaiian Islands before their introduction (the species Rhizophora mangle) in 1902. Since that time, shoreline 

change along the leeward (south) coast has been dominated by seaward 

growth and progradation at specific locations, most notably the Päläÿau and 

ÿUmipaÿa regions of the south-central coastline. Over the 20th century, R.

mangle has colonized vast areas of the coastal interface, leading to seaward 

progradation of the shoreline across the reef flat, filling ancient fishponds, 

and encroaching onto private beachfront property. These effects are typical 

of invasive species, which often have devastating impacts on native habitats 

and species’ distributions, phenomena that are often exacerbated on remote 

islands with already low species diversity.

The invasive mangroves on Molokaÿi have both positive and negative 

feedbacks on the dynamics of the coastal system. They reduce the area of 

wetland habitat important for endemic waterbirds and encroach onto the reef 

flat, coastal beaches, and fishponds, yet at the same time they stabilize coastal 

land, create new fishery and aviary habitat, and inhibit cross-shore sediment 

transport onto the reef. With so many intertwined potential effects on the 

coastal system, understanding how the patterns of this invasive species migration, colonization, and habitat conversion proceed is an essential part of developing effective alien- species policy and coastal management practices. 

Figure 9. Shoreline change rates for 

the Päläÿau region from 1915 to 2000. 

A, Orthophotomosaic of the 2000 aerial 

imagery overlaid with the five historical 

shorelines used for change analysis. 

B–E, Plots of the end-point change rates 

measured at 10-m increments alongshore 

in the defined time intervals. Note that 

the vertical scale varies between plots. 

The blue and orange lines on B represent 

the end-point rates calculated across a 

limited area of Päläÿau from 1915 to 1940 

and 1940 to 1961, respectively.
Impact of Sedimentation on the Molokaÿi Reef

The volume of sediment shed from island slopes to the south Molokaÿi reef 

has greatly increased since the mid 

19th century. The character of the 

sediment, and its ultimate fate on the 

reef, remain key scientific fields of endeavor—the results are of paramount 

importance to the future of the reef.

Bleached and sediment-covered corals on the Molokaÿi reef.
View of suspended sediment on the Molokaÿi reef flat as seen from Kawela.
CHAPTER 17

Sediment on the Molokaÿi Reef

Impact of Sedimentation on the Molokaÿi Reef

Michael E. Field1, R. Scott Calhoun2, Curt D. Storlazzi1, Joshua B. Logan1, and Susan A. Cochran1

Sediment is an important, natural component of coral reef systems. 

Sand and silt particles derived from organic growth and erosion are a 

major constituent of reefs, filling in framework voids, channels, and 

holes. Aggregations of particles constitute microhabitats for microfauna. 

In excessive amounts, however, sediment can lead to degradation of a 

reef. Fine particles that are suspended in the water column block light and 

decrease levels of photosynthesis. Particles settling on coral surfaces interrupt feeding patterns and cause corals to expend valuable energy to slough 

off the sediment. More importantly, even a thin deposit of sediment on 

hard surfaces eliminates potential recruitment sites for larvae. When mobilized, sediment can abrade coral tissues. The net result of excessive sediment on reefs is reduced calcification, reduced recruitment, altered species 

composition, shallower depth distribution limits, and a loss of biodiversity 

(Fabricius, 2005).

As noted in the preface to this volume, the south Molokaÿi coral reef 

has been recognized as an area of excessive sedimentation for many years; 

indeed, mangroves were introduced at Päläÿau in 1902 to combat that problem. This chapter describes the sources, composition, and distribution of 

sediment on the reef and discusses its transport and ultimate fate. Companion chapters by Bothner and others and Ogston and others (this vol., chaps. 

19 and 20, respectively) address related aspects of sediment accumulation 

and suspension on the reef.

Sources of Sediment to the Reef

Sediment accumulates on Hawaiian coral reefs, usually in minor 

amounts, through two natural processes: (1) erosion of the adjacent island 

surface and transport onto the reef and (2) production of sediment on the 

reef itself by breakdown of organic materials and bioerosion. Materials 

from the adjacent island are derived by physical and chemical breakdown of 

parent basaltic lava flows into particles. These land-derived, or terrigenous, 

grains typically form in a variety of sizes from sand (2.0 to 0.063 mm) to 

silt (0.063 to 0.004 mm) to clay (<0.004 mm). The weathered grains are 

commonly red-brown in color from the oxidation of iron in minerals, and 

organic matter incorporated with the grains also contributes to the red-brown 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

2

 Associated Earth Sciences, Inc., 911 Fifth Ave., Suite 100, Kirkland, WA 98033

color of the island’s soils. Silt and clay particles can adsorb chemical compounds, including nutrients and pollutants, onto their surfaces and thereby 

transport them to the reef environment.

The weathered products in soils vary in size from fractions of a millimeter to as much as a meter. Under natural conditions, soil particles are transported to the reef by a two-step process, starting with infilling of hillside gullies by water flow and landslides. Particles are subsequently carried to stream 

mouths and small coastal deltas by channelized and overland flows during 

heavy rains. Slope, soil thickness, and climate are important natural factors 

influencing the volume of water and the amount and character of sediment 

flowing onto the reef, but there is ample evidence that human activities also 

have a strong influence on water and sediment yield (Guilcher, 1985; Bryant 

and others, 1998; Hughes, 2003). For example, loss of vegetation and root 

structures, which hold the soil in place, because of fires, grazing, agriculture, 

and other activities can markedly increase stream flows and the amount of 

sediment that is eroded from the land surface (Van Vuren and others, 2001; 

Warne and others, 2005; see also Roberts and others, this vol., chap. 14).

On the reef itself (fig. 1), sediment is produced by the breakdown of coral, 

coralline algae, and other marine organisms. Sediment here is also produced in 

a variety of sizes from rubble (>2 mm) to sand to silt and clay, and the grains 

are almost uniformly light in color—white, cream, or tan. All of the sediment is produced initially by organic growth, some of which converts rapidly 

to sediment particles (for example, one-celled animals, such as foraminifers, 

and small algae, such as Halimeda spp.) Shells of macroinvertebrates (such as 

snails) and the hard limestone structure of the reef are slowly converted to sand 

and smaller particles by wave forces and by grazing fish and invertebrates.

Figure 1. Cross section of the south Molokaÿi 

reef system showing the transition from the 

sediment-covered inner reef flat to the coral-rich 

fore reef. The relative abundances of rubble, 

sand, and mud are shown schematically from 

the beach across the reef flat and reef crest onto 

the fore reef. A, B, and C mark the approximate 

locations of samples described in figure 2. The 

age of the ancestral reef is late Pleistocene (oxygen isotope stage 5e, approximately 120,000 

years before present).

ModernÐlate Holocene

Ancestral reef

Coastal facies Terrigenous mud

with carbonate sand

Carbonate-rich sandy mud

Reef flat Reef crest Fore reef

A B C

Beach

Modern

 coral

 growth

Silt/Clay

Sand

Rubble

? ?
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

138

Composition of Sediment on the Reef 

and its Significance

Particle Sizes and Their Origin

Sediment on the south Molokaÿi reef consists of rubble, sand, silt, and 

clay. Because of their small mass, both silt and clay are easily suspended 

and transported by water; together they are referred to as mud (see Folk, 

1974, and Folk and Ward, 1957, for definitions of particle sizes and their 

measurement). Rubble, sand, and mud occur in changing abundances 

across the reef in response to available sources as well as to wave and 

current forces that transport and sort the materials. The south Molokaÿi 

beaches are in a low- to medium-energy setting, and most beach deposits 

incorporate only minor amounts of mud in a dominantly sand environment. The inner reef flat is dominated by mud, with significant quantities 

of sand. Reef-flat sediment becomes coarser in a seaward direction, with 

increasing amounts of sand and rubble and decreasing amounts of mud 

toward the reef crest. Sediment on the fore reef is dominantly sand, with 

only very minor amounts of mud present. 

The composition of sediment similarly changes in a cross-reef fashion. 

Sand and rubble throughout the reef are largely reef-derived, whereas mud 

is clearly derived from both the reef and the land. These cross-reef trends 

are illustrated in the diagram in figure 1. With few exceptions, sand is 

composed of mostly calcium carbonate (reef-derived) grains. Values are 80 

to 95 percent on the reef flat and consistently higher than 90 percent on the 

fore reef. The mud (silt and clay) fractions are quite different. On the inner 

reef flat, the mud is mostly land derived (only 10 to 20 percent calcium 

carbonate) and the balance between land-derived and reef-derived gradually and persistently shifts towards the outer reef flat (fig. 2).

Calhoun and Field (2008) evaluated cross-reef changes in calcium 

carbonate by analyzing sediment samples from the beach to the fore reef 

just east of the Kaunakakai Wharf (fig. 3). All their analyses show a clear 

trend of decreasing land-derived content and increasing reef-derived 

content, as indicated by increasing calcium carbonate content, in sediment 

across the reef. The changes are approximately linear for both silt and 

clay. The reef-derived content steadily increases seaward across the reef 

and stabilizes at its maximum value at 500 to 650 m (1,640 to 2,133 ft) 

from the shoreline. The reef-derived content of the sand fraction likewise 

increases with distance from the shoreline, reaching its maximum value at 

approximately 400 m (1,312 ft) from shore. Overall, there is an unmistakable pattern of reef-derived content of sediment increasing with distance 

from shore until a maximum is reached between 400 and 650 m offshore. 

As terrigenous sediment enters the reef at point sources, the consistency of 

this pattern for many kilometers along the shoreline suggests that sediment transport occurs in a shore-parallel direction as well as an onshoreoffshore direction.

Figure 2. Size and composition of three typical sediment samples from the south 

Molokaÿi reef flat; the rubble fraction is not shown. The land-derived (terrigenous) 

and reef-derived (carbonate) fractions were determined by HCl acid solution, a 

standard laboratory procedure. Note: the x-axis is a decreasing logarithmic scale, 

converted to millimeters from the phi (φ) sizes commonly used by geologists. 

Figure 3. Calcium carbonate content of sand, silt, and clay fraction. The calcium 

carbonate contents increase with distance from the shoreline, reaching their forereef levels at approximately 400 m (1,312 ft) for sand, 500 m (1,640 ft) for clay, 

and 650 m (2,133 ft) for silt.

Sand Silt

15

10

5

0

Clay

20

15

10

5

0

Weight percent

Grain size, in millimeters

20

15

10

5

0

2.000 0.063 0.004

Sample A

Sample B

Sample C

A

B

C

Reef-derived

Land-derived

Location

of samples

Kamiloloa

0 500 1,000 1,500 2,000

0

20

40

60

80

Inside the reef crest

Y = 0.056x + 11.068

R2

 = 0.739

Distance from shore, in meters

0 500 1,000 1,500 2,000 0

20

40

60

80

100

Inside the reef crest

Y = 0.086 x + 16.400

R2

 = 0.87

Calcium carbonate content, in percent

0 500 1,000 1,500 2,000

40

60

80

100

Inside 400 m from shore

Y = 0.06x + 72.27

R2

 = 0.54

Inside 225 m from shore

Y = 0.078x + 74.051

R2

 = 0.68

Reef

crest

Reef

crest

Reef

crest

Sand

Silt

Clay
Sediment on the Molokaÿi Reef

Impact of Sedimentation on the Molokaÿi Reef

139 

Figure 4. Photomicrographs of sand grains of various 

origins from sediment on the south Molokaÿi reef (Calhoun and Field, 2008).

A sand grain consisting of the shell of the common onecelled organism called a foraminifer (foram for short). 

Forams are abundant throughout the reef habitat, from the 

reef flat to sand channels to fore reef. On the Molokaÿi reef, 

forams account for approximately 4 percent of the sand 

along the beach, 1 percent on the reef flat, and 4 percent on 

the fore reef.

A sand grain derived from an echinoid shell (probably an 

urchin). Urchins are another common type of invertebrate 

on coral reefs. Their thin calcium carbonate shells are easily 

broken into sand-size particles after death, and their distinctive shell structure makes them readily identifiable with a 

microscope. On the Molokaÿi reef, echinoid grains account 

for approximately 1 percent of the sand along the beach, 

1percent on the reef flat, and 2 percent on the fore reef.

A sand grain (occupying the entire field of view) derived from 

volcanic deposits on land. These types of grains are formed 

by weathering of volcanic rocks and transport to the reef by 

streams. They are the only type of sand grain on the reef not 

derived from the reef itself. On the Molokaÿi reef, the abundance of volcanic grains along the beach and reef flat varies 

markedly, being approximately 13 percent of the sand along 

the beach, 22 percent on the reef flat, and only 1 percent on 

the fore reef.

 A sand grain derived from a fragment of a Halimeda plate. 

Halimeda are algae having a calcium carbonate center that 

is released to the reef as a sand-size particle when the algae 

die. On the Molokaÿi reef, Halimeda grains account for approximately 6 percent of the sand along the beach, 4 percent 

on the reef flat, and 9 percent on the fore reef. 

A sand grain derived from a fragment of calcium carbonate coralline algae. Coralline algae, like coral, are a major 

contributor to the reef framework, and their high abundance 

leads to a corresponding high abundance of derived sand 

grains. On the Molokaÿi reef, grains derived from coralline 

algae account for approximately 34 percent of the sand 

along the beach, 18 percent on the reef flat, and 22 percent 

on the fore reef.

A sand grain derived from a fragment of coral. The calcium 

carbonate (coral) part of the grain is the light-colored areas; 

the darker material is sediment that has infilled holes in the 

grain. Corals are eroded (most commonly when the colony 

dies) by both physical processes, such as wave forces, and 

biologic processes such as grazing by fish and boring by 

invertebrates. On the Molokaÿi reef, calcium carbonate coral 

fragments account for approximately 11 percent of the sand 

along the beach, 12 percent on the reef flat, and 16 percent 

on the fore reef. 

A sand grain (left center) derived from a fragment of a calcium 

carbonate mollusc shell. Molluscs (for example clams and 

snails) make up a significant portion of the invertebrate fauna 

on reefs. On the Molokaÿi reef, molluscan fragments account 

for approximately 6 percent of the sand along the beach, 

4percent on the reef flat, and 4 percent on the fore reef.

1 mm 1 mm 1 mm 1 mm 1 mm 1 mm 1 mm

1 mm 1 mm 1 mm 1 mm 1 mm 1 mm
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

140

Figure 5. Composition of beach sand on south Molokaÿi. The principal components 

are coralline algae, chemically altered (micritized) carbonate grains, terrigenous 

grains, and coral. Foraminifers and Halimeda are locally abundant along west 

Molokaÿi. Similarities between these samples and those found on the reef flat (fig.6) 

suggest sediment exchange between the two environments (see text for explanation). Streams draining to the south side of Molokaÿi shown in blue.

Figure 6. Composition of reef-flat sand on the south Molokaÿi reef. The most impor -

tant components are chemically altered (micritized) carbonate grains, terrigenous 

grains, coralline algae, and coral. Streams draining to the south side of Molokaÿi 

shown in blue.

Kaunakakai

Wharf

Averages

34%

11%

23%

13%

Unknown

Chemically altered (micritized)

Terrigenous (land-derived)

Halimeda spp. (algae)

Mollusc, echinoid, foraminifers (forams) 

Coral

Coralline algae

Types of sediment grains

forams

forams

Drainage divide

Kaunakakai

Wharf

Unknown

Chemically altered (micritized)

Terrigenous (land-derived)

Halimeda spp. (algae)

Mollusc, echinoid, foraminifers (forams) 

Coral

Coralline algae

Types of sediment grains

Drainage divide

Averages

12%

22%

31%

18%
Sediment on the Molokaÿi Reef

Impact of Sedimentation on the Molokaÿi Reef

141 

Sand Composition as an Indicator of Source

Studies examining the composition and distribution of individual sand 

grains elsewhere in Hawaiÿi have been useful for understanding the major 

sources of sand and providing information about long-term transport history 

(Harney and others, 2000; Calhoun and others, 2002; Harney and Fletcher, 

2003). The sand-size fraction of south Molokaÿi beach and reef sediment, 

recently studied by Calhoun and Field (2008), is composed of both marine 

biogenic-carbonate grains and terrigenous grains from the Molokaÿi volcanoes. As described above, contribution of sand from land sources is evident 

in beach and inner reef-flat deposits, but less so in outer reef-flat deposits 

and particularly fore-reef deposits. Calhoun and Field (2008) noted that 

coralline algae, coral, chemically altered (micritized) carbonate grains, and 

terrigenous grains are the most common components of sand along the 

south Molokaÿi fringing reef, and their proportions vary depending on location. Other important contributors, usually in minor amounts, are molluscs,

Halimeda spp., foraminifers, and echinoderms. Photomicrographs of most of 

these grain types are shown in figure 4. 

The distribution of the principal components of the sand in beach, 

reef-flat, and fore-reef deposits is shown in figures 5, 6, and 7. On south 

Molokaÿi beaches, the most abundant sand grains are coralline algae (34±11 

percent), chemically altered carbonate grains (23±10 percent), terrigenous 

grains (13±13 percent), and coral (11±7 percent), as shown in figure 5. 

The higher abundance of terrigenous grains on the eastern beaches (20±13 

percent) compared to the western beaches (5±5 percent) probably results 

from the higher rates of sediment influx to the coast from east Molokaÿi relative to west Molokaÿi because of higher rainfall and steeper gradients (Field 

and others, this vol., Introduction). On the reef flat, the most abundant sand 

grains are chemically altered carbonate grains (31±16 percent), terrigenous 

grains (22±26 percent), coralline algae (18±13 percent), and coral (12±4 

percent) (fig. 6). Note that sand is a dominant grain type of the beaches 

and fore reef, but it makes up less than 50 percent of the sediment on the 

reef flat. Thus the total sediment composition may be quite different from 

that illustrated in figure 6, for only the sand-size fraction was used in those 

analyses. Seaward of the reef crest on the fore reef, chemically altered grains 

are the dominant component (40±10 percent), and particles of the calcareous 

green algae Halimeda (9±6 percent) become the fourth principle grain type 

(fig. 7). Terrigenous grains are largely absent from the fore reef, with only 

one location having more than 5 percent.

Halimeda spp., foraminifers, and molluscs are locally important contributors to the sand-fraction components in all environments. In some areas, 

Halimeda spp. reaches almost 50 percent of the total sample, foraminifers 

just over 20 percent, and molluscs just over 10 percent. Small amounts of 

echinoderm fragments (<4 percent) and bryozoans (<1 percent) are also 

found along south Molokaÿi.

Figure 7. Composition of fore-reef sand on the south Molokaÿi reef. The four principal components are coralline algae, coral, chemically altered (micritized) carbonate 

grains, and Halimeda. The scarceness of terrigenous grains indicates that there is 

little sediment exchange between the reef-flat and fore-reef environments. Streams 

draining to the south side of Molokaÿi shown in blue.

Kaunakakai

Wharf

Unknown

Chemically altered (micritized)

Terrigenous (land-derived)

Halimeda spp. (algae)

Mollusc, echinoid, foraminifers (forams) 

Coral

Coralline algae

Types of sediment grains

Drainage divide

18Ð19 m

23Ð27 m

9Ð14m

Water depth

of sample sites

Averages

40%

22%

16%

9%
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

142

Figure 8. Sediment thickness on the Molokaÿi reef flat. Red lines on map show 

locations of sampling transects. Red lines on graphs are best-fit averages. 

Sediment thickness off Kakahaiÿa: The thick sediment within 100 m (328 ft) 

of the shoreline is a reflection of the proximity of Kawela Gulch and the large 

quantities of land-derived mud that are transported to the coast during Kona 

storms. This location is one of the few places where the average sediment 

thickness is consistently greater than 10 cm (3.94 in). The lack of very high 

values on the central reef flat (400 – 600m/1,312 –1,969 ft from shore) is due 

to a general lack of holes for the sediment to accumulate in. Sediment thickness off West Kawela: Terrigenous mud derived from Kawela Gulch, mixed 

with reef sand, accumulates in deposits 10 – 20 cm (3.94–7.87 in) thick on the 

inner reef flat. The deposits thin in a seaward direction to their lowest values 

about 220 m (722 ft) from the coast, then farther seaward show high variation 

caused by irregular reef topography. Sediment thickness off Kamiloloa: Here, 

as at locations farther east, the inner reef flat is covered by sandy mud several 

centimeters to 20 cm in thickness. The deposits rapidly thin seaward, reaching 

their lowest values between 100 and 200 m (656 ft) from shore. Variable thicknesses farther seaward reflect the irregular reef topography. Sediment thickness off Kapaÿakea: This area is the “downstream” portion of the Molokaÿi reef 

flat, in that prevailing currents transport sediment from Kawela and Kamalö 

regions west to this area, where further transport is effectively blocked by the 

Kaunakakai Wharf (Ogston and others, this vol., chap. 20). Sediment thickness 

is highly variable, but overall average values are the highest (>15 cm/5.91 in) 

on the reef flat 400 – 600 m from shore. Sediment thickness off Kapuäiwa coconut grove: This section of the coast has undergone erosion and is embayed; 

consequently, the inner reef flat is very broad. The reef flat is generally protected from large waves and strong currents from the east by the Kaunakakai 

Wharf. The inner reef flat is exposed at low tide; consequently there is little 

space available for additional sediment deposition. Thickness values near the 

shoreline range from 5 to >90 cm (35.43 in) and show the greatest range of any 

location. Between 200 and 350 m (1,148 ft) offshore, the sediment is generally 

<10 cm thick but farther seaward is again highly variable because of irregular 

reef topography. Sediment thickness off ÿUmipaÿa: The ÿUmipaÿa reef flat (just 

east of Päläÿau) has greater wave exposure and limited sediment sources at 

present relative to areas farther east, consequently sediment deposits on the 

inner reef flat are typically less than 15 cm thick. An abundance of reef-derived 

sediment here provides for consistent thickness values of several centimeters to 

15 cm for a distance of 700 m (2,297 ft) offshore. Further seaward, the values 

become highly variable because of the irregular reef topography.

5

5

0 1 2 Miles

0 2 Kilometers

Isobath = 5 meters

Map area

KakahaiØa

0

20

40

60

0 200 400 600

Distance offshore, in meters

0 200 400 600 0 200 400 600

Sediment thickness, in centimeters

Kamiloloa West Kawela

KapaØakea

Distance offshore, in meters

Kapuiwa

0

20

40

60

80

100

0 400 800 0 200 400 600 800 0 200 400 600

Sediment thickness,in centimeters

ØUmipaØa
Sediment on the Molokaÿi Reef

Impact of Sedimentation on the Molokaÿi Reef

143 

Sediment Patterns on the Reef Flat

Sediment is nearly ubiquitous on the Molokaÿi reef flat. Because of the 

apparent degradation of coral habitats on the reef flat, emphasis was placed on 

measuring both the sediment cover and the particles in suspension across the 

reef. The type of sediment and its distribution across the reef provides important 

information about the processes at work and the recent history of the reef. To 

measure sediment thickness on the reef, a marked steel rod was pushed into the 

substrate along transects between Kamalö on the east and the old Kolo Wharf 

on the west. Data were collected roughly every 20 m (65 ft) on each of 38 

shore-normal transects from the beach offshore for distances of 600 to 1,200m 

(1,969 to 3,937 ft), depending on water depth. The results of the systematic 

profiling have been grouped into six areas between Kamalö and Päläÿau; each 

area includes measurements made along 3 to 5 transects (fig. 8).

The thickness of sediment on the reef flat is generally less than 1.0 m 

(3.3ft), less than 0.5 m (1.6 ft) in most places. These low values reflect two 

important characteristics about the Molokaÿi reef flat: the shallow depth of 

holes and channels in which sediment can accumulate and the overall shallow 

nature (<2 m/6.6 ft) of water depth on the reef flat, which inhibits accumulation of thick sediment bodies. The sediment that has accumulated, and that 

continues to accumulate, on the south Molokaÿi reef flat is highly variable in 

thickness. The variations on the mid to outer reef flat are due in large part to 

the irregularities of the subsurface ancestral reef pavement—sediment collects in holes and low areas between coral ridges and is swept away from high 

areas. On the inner reef flat, sediment thickness is influenced by the availability of terrigenous mud and the presence (or absence) of wave stresses sufficient 

to remove the sediment. Thus the inner reef flat in small embayments and 

between large native Hawaiian fish ponds along the coast between Kamalö and 

Päläÿau is typically buried by tens of centimeters of sandy terrigenous mud, 

whereas less protected areas are mantled by only 2 to 5 cm (0.79 to 1.97 in). 

The only locations where sediment is absent are (1) broad, shore-parallel rock 

surfaces on the inner reef flat and (2) elongate, shore-normal coral ridges on 

the outer reef flat. The rock surfaces on the inner reef flat are smooth, slightly 

shallower than adjacent areas, and mostly algae covered (see Cochran, this 

vol., chpt. 9). On the mid to outer reef flat, raised shore-normal ridges support 

coral growth and accretion at low (<10 percent) and moderate (10–50 percent) 

coverage levels (Rodgers and others, 2005).

The distribution of sediment across the reef flat is variable, but there are 

three noteworthy trends:

• Sediment on the innermost reef flat (1–100 m/3.3–328 ft from 

shore) is commonly on the order of 20 cm (7.87 in) thick, and 

these deposits are dominantly terrigenous mud. Actual thicknesses 

are typically 10 to 20 cm (3.94 to 7.87 in). Some of the greatest 

thicknesses (40 cm/15.75 in) occur in depocenters between fish 

ponds (where low energy inhibits removal processes) or inside of 

partially destroyed fish ponds (for example, Kakahaiÿa).

• Sediment deposits tend to thin seaward from the shoreline to values of 5 cm or less several hundred meters offshore and are only a 

few cm thick—or absent entirely—on top of smooth, raised rock 

surfaces. 

• Sediment thicknesses are highly variable on the mid and outer reef 

flat (50 cm/19.69 in variations are not uncommon). Much of this 

variation is due to admixtures of terrigenous mud and reef-derived 

sand and rubble infilling low areas.

Transport and Fate of Sediment on the Molokaÿi Reef

The variations in composition indicate that the origin and transport of 

sand on beach and inner reef-flat environments are effectively separated from 

production and transport on the fore reef. The fore reef is the zone of high 

biological productivity and therefore also of sand production. Despite this 

productivity, and because of the physical isolation of the live reef created by 

the broad, shallow reef flat (see images in Field and others, this vol., chap. 2), 

the narrow to nonexistent beaches of south-central Molokaÿi have very limited 

sources available for natural replenishment.

On the south shore of Molokaÿi, beach sand and reef-flat sand are 

similar in both composition and relative abundance of coralline algae, 

chemically altered (micritized) carbonate grains, terrigenous grains, and 

coral. The similarity in composition and abundance suggests that sediment 

is exchanged between the beach and the reef flat and that beaches therefore 

are locally supplied by adjacent reef-flat productivity. Sand on the fore 

reef is distinctly different in composition in several regards, including the 

absence or near absence of land-derived terrigenous grains. Sand appears 

to be locally generated on the fore reef, seaward of the reef crest. Locally 

derived calcareous green algae, Halimeda spp., are the fourth most abundant sand component on the fore reef. The near-absence of land-derived 

sand grains on the fore reef indicates that sand is not being transported 

from the shoreline and across the reef flat in significant quantities to the 

fore reef. The silt and clay fraction of fore-reef sediment similarly lacks a 

significant land-derived component.

The reef-derived portion of the sand, silt, and clay fractions increases 

with distance from shore, reaching maximum value midway across the 

reef flat (fig. 3). The land-derived fraction is larger close to shore, perhaps indicating sediment trapping. Low productivity of reef sediment on 

the inner reef flat also partially explains why the relative portion of landderived sediment is high there. On the Molokaÿi inner reef flat, only minor 

production of grains by corals and coralline algae occurs (fig. 4), probably 

because of the combined effects of low coral cover and chronic turbidity 

from entrained mud. Living coral colonies and encrusting coralline algae 

on the outer reef flat and reef crest make up 10 percent to 30 percent of the 

substrate, thus leading to higher production of calcium carbonate grains in 

these areas than on the relatively barren inner reef flat. Equal abundances 

of reef-derived sand grains on both sides of the reef crest indicate that sediment exchange occurs across the reef crest. Thus the wide reef flat is more 

effective than the reef crest itself as a physical barrier to sand movement. 

The Mud Factor

Our studies on the Molokaÿi reef clearly indicate that different size fractions have different transport patterns. Mud is more mobile than sand and is 

resuspended frequently (often daily; see Ogston and others, this vol. chap. 20). 

The observations of sediment thickness, combined with the measurements of 

particle size and carbonate content, show that land-derived mud is a dominant component on the inner reef flat, decreasing in content seaward across 

the reef flat. Little land-derived mud is present on the reef crest or fore reef, 

indicating either that little mud is transported to these environments or that it 

is not retained there. Bothner and others (2006; this vol., chap. 19) discuss the 

amounts and composition of mud collected on the fore reef and conclude that 

fine-grained sediment is moving through the system but only minor amounts 

are retained on the fore reef. Whether the mud particles bypass the outer reef 

entirely, or are deposited for a short time is not clear. What is evident is that the 

Figure 9. Diagram of suspended sediment concentrations across the reef flat 

between Kamalö and the Kaunakakai Wharf measured at 1-mile intervals along 

the shore in April 2005. All of the profiles show high levels of suspended sediment 

(typically 10 to 40 mg/L and higher). The values steadily decrease to about 5 to 

10mg/L at a distance of 400 m (1,312 ft) from the shore. One profile is consistently higher than other profiles; its position is just west of the mouth of Kawela Gulch, 

where large amounts of terrestrial sediment are deposited during storms. A 

concentration of 10 mg/L, based on the work of Rogers (1990), is commonly accepted as sufficient to affect corals, although many factors, such as land-derived 

content, duration, and particle size, also affect coral growth and survivability.

0

10

20

30

40

50

60

70

50 100 250 400 550 700

Suspended sediment concentration, in milligrams per liter

Suspended sediment concentration

on the MolokaØi reef flat,

April 6-8, 2005

Critical concentration

(Rogers, 1990)

Limited impact

on coral growth

Increasing impact on coral 

growth and sustainability

Distance of sampling sites from shore, in meters

80
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

144

residence time of land-derived mud on the fore reef is minimal under presentday conditions. Changes to the reef due to increased sediment yield from the 

land or increased storage capacity because of local sea-level change or other 

factors could alter pathways such that land-derived mud could become more, 

or less, abundant on the coral-rich fore reef in coming years.

More important at present is the excessive amount of land-derived mud 

on the inner reef flat. Sediment is 5 to 10 cm thick across tens of square 

kilometers. That sediment is at least half mud in many places, and most of 

that is land-derived in origin (fig. 2). The persistency of the mud over years 

of monitoring the reef indicates a long residence time, probably resulting 

from a slow flushing process. Although the mud is very mobile and results 

in high turbidity levels on a daily basis, the amount that is transported across 

the reef appears to be minor relative to along-reef transport and residency on 

the inner reef flat. Fine-grained sediment is introduced only episodically to 

the Molokaÿi reef (Field and others, this vol., chap. 21). Once it is deposited, 

however, the relatively weak wave stresses and the coast-parallel transport 

that characterizes the reef are not efficient at dispersing the mud.

Mud in Suspension

As pointed out by Ogston and others (2004) and Storlazzi and others 

(2004), the combination of waves generated by trade winds and tide levels 

above median height generate wave stresses that are sufficient to resuspend 

mud particles on the shallow reef flat. These conditions exist on most days of 

the year, leading to chronic conditions of high turbidity. In 2005, a study of 

turbidity was conducted on the Molokaÿi reef flat to examine the concentration 

of suspended sediment across the reef flat and along the coast between Kamalö 

and the Kaunakakai Wharf. Some of the preliminary results, shown in figure 9, 

follow the findings of Presto and others (2006). Turbidity levels are consistently 

high (typically 20 to 40 mg/L and higher) near the coast and decrease markedly 

to concentrations of 5 to 15 mg/L mid-way across the reef flat (400 m/1,312 ft 

from shore). Further, the content of the suspended sediment changes. Presto and 

others (2006) show that the sediment is dominantly land-derived (>90 percent) 

close to shore; these values decline steadily across the reef flat to values of ~75 

percent at distances of 500 m. On the outermost reef flat, the amount of landderived sediment in suspension is only about 10 percent. 

Mud Fallout and Algal Trapping

One interesting result of the daily resuspension of mud on the reef flat is 

the resulting daily deposition when winds abate and/or the water level drops at 

low tide. The sediment is deposited in thin layers over hard surfaces (thereby 

limiting recruitment) and over live coral and macroalgae (fig. 10). A study by 

Stamski and Field (2006) showed that macroalgae trapped an average of 1.26 g 

of sediment per gram of dry weight biomass. Individual macroalgae weigh on 

average about 3 g and are very abundant on the inner reef flat. The sediment 

trapped by macroalgae is dominantly land-derived mud (nearly 60 percent). 

Stamski and Field estimate that about 310 metric tons of sediment were 

retained by macroalgae across 5.75 km2

 (spread evenly, that equates to roughly 

54 g/m2

). It seems likely that most of the particles attached to the macroalgae 

remain affixed until decay of the algae, at which time the particles reenter the 

sediment reservoir on the inner reef flat. Thus macroalgae are an important factor in sediment retention and the overall sediment budget for the reef.

Summary

Sediment is a natural part of coral reefs, especially those bordering high 

islands such as those of the Hawaiian archipelago. However, it is clear from 

even a cursory examination of photographs that the amount of sediment, and 

of mud in particular, on the south Molokaÿi coral reef is large compared to 

similar reefs elsewhere.

At present, the fore reef exists in a near-natural state with regard to 

sediment. Nearly all of the sediment is sand size and locally derived from 

the reef by natural processes. The sand is concentrated in pukas (holes) in 

the reef, in channels and grooves, and within the framework of the reef. 

The only exception to this overview is the area east of Kaunakakai Wharf, 

where coral is essentially absent in a small zone that extends from the reef 

crest (3-m water depth) to the base of the fore reef (28-m water depth). 

This zone is mantled by thin layers of sand that are periodically mobilized 

by the long-period swell, and this movement likely inhibits coral recruitment and recovery. There is limited evidence that mud from the land and 

reef flat is deposited only temporarily on the fore reef. However, continued 

sedimentation in excessive amounts, now largely confined to the reef flat, 

has the potential to reach the fore reef in larger amounts and impact reef 

health and sustainability.

Sediment quantities derived from adjacent island slopes increased 

substantially in the late 1800’s, largely a result of the changes that occurred 

in the watersheds, as described by Roberts and others (this vol., chap. 14); 

changes in the watersheds continue today. Much of the land-derived terrigenous sediment is stored on the Molokaÿi reef flat between Kamalö on the 

east and the Päläÿau embayment on the west. Farther east and farther west 

from those locations, reef-flat sediment generally contains markedly less 

mud, and most of the sediment is reef-derived. This is a result of two factors: 

less mud is entering the reef in those locations, and the processes of mud 

removal are more efficient because of oceanic swell (south and west) and 

direct exposure to deep-water trade wind waves (east).

Between Kamalö and Päläÿau the reef flat is mantled with sediment that 

ranges from 0 to >50 cm in thickness. Most of the inner reef flat is covered 

by 5 to 15 cm (2.0 to 5.91 in) of sediment, in which land-derived mud is 

a dominant component. Sediment is resuspended on almost a daily basis 

and through this process blocks light, covers recruitment sites, and mantles 

coral. Large amounts are trapped by the abundant macroalgae (nearly 4 g per 

clump of macroalgae). Thickness of the mud has not changed significantly 

since at least 1999, indicating that despite the daily resuspension and transport events, much of the mud fraction is retained on the inner reef flat.

Figure 10. Photographs from the Molokaÿi reef flat (Kakahaiÿa and Kamililoa) showing the abundance of sediment-covered 

macroalgae overgrowing—and in some instances smothering—live colonies of Pocillopora meandrina, Porites spp., and 

Pocillopora damicornis. These photos were taken in May 2002 following a winter in which large amounts of sediment were 

washed onto the reef. The sediment seemed to trigger an algal bloom, which in turn led to more sediment trapping. Each 

individual macroalgae clump on the Molokaÿi reef flat traps and holds nearly 4 g of sediment (Stamski and Field, 2006). Few 

live colonies (a) are evident in the photographs; most are covered by algae and sediment (b) or bleached and dead (c).

M. Field and R. Stamski-MacKnight, USGS

a

a a

a

b

b

b

b

b b

b

c

c

b

a

a a

a

b

b

b

b

b b

b

c

c

b
Impact of Sedimentation on the Molokaÿi Reef

CHAPTER 18

Measurement of Toxicity in Reef Sediments

R. Scott Carr1 and Marion Nipper2

The shoreline of the south coast of Molokaÿi is composed of narrow 

sandy beach deposits and muddy mangrove habitats that border the 

wide, low-energy reef flat. Such a setting is prone to the deposition 

of fine-grained particles, and increased siltation has occurred on the reef in 

historical times as a result of several factors (see Roberts and others, this 

vol., chap. 15, for a discussion of historical siltation). Terrigenous sediment runoff and deposition on coral reefs is recognized as a hazard to coral 

health. In addition to the smothering effect, terrigenous runoff can contain 

contaminants from urban and agricultural activities (fig. 1) that have the 

potential to affect the health and survival of the coral reef with its associated 

flora and fauna.

The probable biological effects of sediment-associated contaminants 

can be determined using porewater toxicity tests, which allow an assessment 

of the presence of bioavailable contaminants in toxic amounts (Carr, 1998; 

Carr and Nipper, 1998; Carr and others, 2003; Nipper and Carr, 2001). A 

sediment quality survey was conducted to assess the presence of bioavailable 

contaminants in toxic amounts in surficial sediments off the southern coast 

of Molokaÿi, Hawaiÿi. The specific objectives of this study were to measure 

porewater toxicity by analyzing fertilization and embryological development success using the sea urchin Arbacia punctulata in conjunction with 

measurements of water-quality parameters (salinity, dissolved oxygen, pH, 

sulfide, temperature, ammonia, and dissolved organic carbon).

Materials and Methods

The toxicity of sediment collected from 10 stations was assessed using 

porewater tests during 2001, and a more focused study was conducted in 

2002. Pore water was extracted on site using a diver-held syringe with an 

aquarium airstone attached (fig. 2). The syringe was inserted approximately 

6–8 cm (2–3 in) below the sediment surface. Toxicity of the sediment pore 

water was determined using the fertilization and the embryological development tests with the sea urchin Arbacia punctulata (Carr and others, 2003). 

Complete methodology may be found in Carr and Nipper (2001, 2003).

1

 U.S.Geological Survey Marine Ecotoxicology Research Station, Unit 5863, 6300 Ocean Dr., Corpus 

Christi, TX 78412

2

 Texas A&M University-Corpus Christi, Center for Coastal Studies, Unit 5866, 6300 Ocean Dr., 

Corpus Christi, TX 78412

What the Tests Showed

The primary objective of the initial study in 2001 was to determine if 

erosion problems in Molokaÿi have led to the deposition of contaminants 

in toxic amounts in the vicinity of the extensive coral reef system along the 

south shore. Porewater toxicity test results suggest that station 9 on the fore 

reef off Kamiloloa had the highest amount of bioavailable contaminants, 

with toxic effects in the sea urchin fertilization and embryological development tests at 50-percent and 25-percent porewater concentrations, respectively (fig. 3). If the contaminants occurring at station 9 originated from 

direct runoff from land-based activities, as opposed to an underground spring 

exiting offshore, the same chemicals would be expected to occur at stations 

10 and 1, closer to shore. However, stations 10 and 1 were much less toxic. 

The higher toxicity at station 9 could be due to higher bioavailability of contaminants as a result of the apparent coarser nature of the sand at that station. 

Finer sediment was observed at stations 10 and 1. The role of both total and 

dissolved organic carbon (DOC) in binding organic contaminants (Di Toro 

and others, 1991; Haitzer and others, 1998) as well as some metals (Mahony 

Toxicity Test Results

The toxicity test results for the 2001 survey are presented in figure 3. Porewater samples from stations 2, 4, 6, 9, and 10 were significantly toxic in the 

sea urchin fertilization test at a concentration of 100 percent, and samples from 

stations 2 and 9 were also toxic at a 50-percent dilution, denoting a higher concentration of bioavailable contaminants. Only the sample from station 9, on the 

fore reef off Kamiloloa, was significantly toxic even when diluted to 25 percent 

pore water, indicating a very high concentration of bioavailable contaminants.

The toxicity test results for the more focused 2002 survey are presented 

in figure 4. Only two samples from this survey were toxic to sea urchin early 

life stages. The sample from station 1 near Kapuäiwa coconut grove was 

marginally toxic to sea urchin fertilization at 100 percent pore water, and the 

sample from station 3 just east of Kaunakakai Wharf was significantly toxic 

to sea urchin embryological development at a concentration of 100 percent. 

Water-quality data and dissolved oxygen concentrations for the porewater 

samples have been reported elsewhere (Carr and Nipper, 2001, 2003) and 

were well within acceptable ranges.

Figure 1. Aerial view, looking south, of agricultural activities in south-central 

Molokaÿi.

Figure 2. Diver conducting porewater extraction on the sea floor using a syringe 

with an aquarium airstone attached.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

146

and others, 1996) and reducing their bioavailablity is well recognized (Nipper and others, 2002). For the three stations mentioned above, the porewater 

DOC concentration was highest at station 1 (not toxic), and lowest at station 

9 (highly toxic), with an intermediate value at station 10 (moderately toxic 

to sea urchin fertilization). This supports the suggestion that organic carbon content may have been at least partially responsible for the toxicity test 

results. Chemical analyses for the assessment of specific contaminants and 

their concentrations were not performed in this study.

Nearshore stations 4 (at One Aliÿi Park) and 6 (near Hale O Lono) were 

moderately toxic to sea urchin fertilization, and station 2 (at Pükoÿo) was 

also toxic at the 50 percent porewater dilution. The reason for the toxicity 

to the fertilization test only, and not to sea urchin embryological development, is likely to be related to the type of contaminants present at the sites. 

For instance, the sea urchin fertilization test is more sensitive than urchin 

embryos to a variety of metals, including copper, mercury, zinc, and silver 

(Nacci and others, 1986; Carr and others, 1996). It was not the objective of 

this initial survey to establish the category of contaminants present in the 

marine sediment tested.

The primary objective of the followup study in 2002 was to conduct 

a more focused survey in the region exhibiting toxicity previously and 

to attempt to identify the contaminants at a site that was highly toxic in 

2001. Porewater toxicity test results for samples collected in August 2001 

suggested that station 9 (offshore Kamiloloa) had the highest amount of 

bioavailable contaminants. That sample exhibited toxic effects in the sea 

urchin fertilization and embryological development tests at 50-percent 

and 25-percent porewater concentrations, respectively. However, a new 

porewater sample collected in August 2002 at this station (fig. 4, station 19) did not exhibit any toxic effects in either test. These data suggest that the contaminants that caused toxicity in the previous year were 

from a transient source and had been dispersed a year later. Dispersion 

of contaminants from such a coarse sand substrate, where the porewater 

DOC concentration was among the lowest measured from all the stations 

sampled on Molokaÿi, is not particularly surprising. Toxicity would only 

be expected to persist in such an environment if a continuous source of 

contaminants existed in the area, which apparently is not the case. Nearshore stations 11 and 13 were toxic to urchin fertilization and development, respectively, in 2002 (fig. 4). Station 11 was near a freshwater seep 

at Kapuäiwa Park, which might have brought land-based or ground-water 

contaminants to the area. Station 13 was on the eastern side of Kaunakakai 

Wharf, and the observed toxicity could be the result of runoff of contaminants generated by urban activity.

In conclusion, this study did not demonstrate the consistent presence 

of bioavailable contaminants in toxic amounts in most areas off southern 

Molokaÿi. Therefore, it appears that the degraded reefs are not the result 

of persistent contaminants or chronic contaminant inputs. These results, in 

conjunction with previous studies, suggest that transient events from ground 

water or runoff can introduce contaminants at toxic concentrations at some 

locations, but these inputs do not appear to be continuous.

Figure 3. Toxicity test results on sea urchin 

fertilization and embryo development for 

sampling sites 1 to 10 on south coast of 

Molokaÿi in 2001 survey.

Figure 4. Toxicity test results on sea urchin 

fertilization and embryo development for 

sampling sites 11 to 20 on central south 

coast of Molokaÿi in 2002 survey.

100

100

100

100

300

300

500

500

100

300

Kaiwi Channel

Kalohi Channel

P A C I F I C O C E A N

0 2 4 Miles

0 4 Kilometers

Pailolo Channel

9

1

10

4

8

3

6

5

7

2

Fertilization

Embryo

development

FIGURE 4

Not toxic

Toxic at 100%

Toxic at r50%

Toxic at r25%

TYPES OF TEST RESULTS 

Isobath interval 100 m 

25

25

25

125

100

200

* N.D.

100

PlØau

Kaunakakai

Kamiloloa

Kamal

0 1 2 Miles

0 2 4 Kilometers

Isobath interval 25 m 

*N.D.Ñno bathymetry data

12

13

20

18

14 15

19 16

17

11

Not toxic

Toxic at 100%

Toxic at r50%

Toxic at r25%

TYPES OF TEST RESULTS 

Fertilization

Embryo

development
Kaunakakai

Kawela

Kalaupapa

LØAU POINT

HLAWA POINT

Mauna Loa

Kmimi

One AliØi

Wai a Kne

Site 1: Hale O Lono Site 3: PlØau

Non-storm

Sediment trap sites

1

2, 3

4, 5, 6

8, 9

10

7

Storm

Site 5: Kamiloloa Site 7: Kamiloloa Reef Flat Site 8: Kamal Site 10: PkoØo

Water depth: 12.8 m Water depth: 9.6 m Water depth: 10.1 m Water depth: 0.9 m

Above bottom: 0.3 m

Water Depth: 9.5 m Water depth: 10.4 m

2000 2001 2002 2000 2001 2002

Year

2000 2001 2002 2000 2001 2002 2000 2001 2002 2000 2001 2002

1

10

100

1,000

10,000

Collection rate, in grams

per square meter per day

0.0

0.2

0.4

0.6

0.8

Average wave stress,

in newtons per square meter

Impact of Sedimentation on the Molokaÿi Reef

CHAPTER 19

Sediment Mobility Along Molokaÿi’s Fringing Coral Reef: Evidence From Sediment Traps

Michael H. Bothner1, Richard L. Reynolds2, Michael A. Casso1, Curt D. Storlazzi3, and Michael E. Field3

There is concern about the potential negative impact of suspended sediment and sedimentation on the health of the coral reef off Molokaÿi 

(Roberts, 2001; Jokiel and others, 2004). To gain information about 

the composition and quantity of sediment that was being transported within 

the reef system, we deployed a series of sediment collection tubes on the 

reef (fig. 1) as described in Bothner and others (2006). We were interested 

in characterizing the material collected in traps during and after flood events 

associated with Kona storms to determine how much land-derived material was transported to the fore reef where coral growth was most active. 

By analyzing sediment collected in traps and wave-stress data, we sought 

to improve understanding of the frequency, cause, and relative intensity of 

1

 U.S. Geological Survey Woods Hole Science Center, 384 Woods Hole Rd., Woods Hole, MA 02543

2

 U.S. Geological Survey, P.O. Box 25046, Denver Federal Center, MS980, Denver, CO 80225

3

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

sediment mobility/resuspension events within the Molokaÿi fringing reef 

system. We also analyzed potentially toxic metals and magnetic properties of 

the trapped sediment.

We found evidence of land-derived sediment moving throughout the reef 

system. This material had low concentrations of toxic metals and, in some 

areas, had distinctive magnetic properties that may reflect different sources 

of the land-derived sediment. Time-series traps with a temporal resolution of 

4.5 days showed that during Kona storms and significant floods, the sediment 

collection rate increased by as much as 1,000 times compared to nonstorm 

intervals. The sediment collection rate increase was related to resuspension of 

existing sediment by Kona storm waves rather than higher fluxes of floodderived sediment. The similar composition of trapped sediment collected 

before, during, and after the Kona storms indicated that when a major flood is 

accompanied by high waves, the land-derived sediment has a low potential for 

burying coral on the fore reef. 

Trapping Sediment

Two types of sediment traps were used. The simpler tube trap (fig. 2) consisted of a 60-cm-long (23.6 in), clear plastic tube having an internal diameter 

of 6.7 cm (2.6 in). A baffle of “honeycomb-like” cells (diameter 0.5 cm/0.2 in, 

length 7.6 cm/3 in), consisting of resin-coated fiber and treated with anti-fouling paint, was placed in the top of the sediment traps to reduce turbulence and 

to prevent fish occupation. The tube trap openings were at 0.6 meters above bottom (mab) (2 ft), except on the shallow reef flat where they were 0.3 mab (1 ft). 

Reproducibility of the collection-rate measurement by tube traps was evaluated 

by setting duplicate traps on seven occasions. The average difference in collection rate between pairs was 11 percent.

The programmable time-series trap (fig. 3) had a 20-cm (8 in) internaldiameter collection cylinder with an overall length of 75 cm (30 in). The 

cylinder opening was 1.3 mab (4 ft). A funnel, within the cylinder, directed 

Figure 1. Location of sediment traps 

along the south shore Molokaÿi reef, along 

with collection rates (bars, log scale) 

during successive roughly three-month 

periods. Collection rates from nonstorm 

periods are shown in blue, and data from 

the deployment period containing Kona 

storms are shown in orange (scale at 

left). Average wave-induced bottom stress 

is shown in corresponding colored dots 

alongside the graphs (scale at right); 

bars show plus and minus one standard 

deviation. Higher bottom stress at the 

wave-exposed east and west ends of 

the reef, and lower bottom stress in the 

more protected central area, explain the 

regional differences in collection rate. 

High bottom stress during Kona storms 

generated higher collection rates at most 

locations. See text for details.
Figure 2. Tube trap at site 

3 on coral ridge at start of 

deployment. Water depth 

is 9.6 m (31.5 ft).

A Site 1: Hale O Lono

 Water depth: 12.8 m

 Above bottom: 0.6 m

MKT56 11/11/01–2/14/02

0

10

20

30

40

50

Depth Split trap X-ray Texture Carbonate Depth

6%

1%

1%

13%

4%

4%

85% 94%

92% 87%

13%

(centimeters)

4%

4%

2%

2%

B Site 6: Kamiloloa

 Water depth: 4.9 m

 Above bottom: 0.6 m

MKT62 11/14/01–2/14/02

0

10

20

30

40

50

Split trap X-ray Texture Carbonate

9%

7%

84% 89%

11%

60% 73%

92% 90%

10%

8%

96% 73%

27% 12% 28%

Flood

material

C Site 7: Kamiloloa Reef Flat

 Water depth: 0.9 m

 Above bottom: 0.3 m

MKT55 11/13/01–2/16/02

0

3

6

9

12

Depth X-ray Texture Carbonate Depth

13%

4%

25%

62% 70%

30%

14%

20%

66% 77%

23%

29%

67% 77%

22%

(centimeters)

(centimeters) (centimeters)

2%

1%

D Site 10: PukoÕo

 Water depth: 10.4 m

 Above bottom: 0.6 m

MKT59 11/13/01–2/15/02

0

10

20

30

40

50

Split trap X-ray Texture Carbonate

8%

97% 92%

6%

3%

11%

91% 89%

1% 2%

2%

1%

9%

7%

78%

97%

19%

91%

93%

Texture Carbonate

Gravel

Sand

Silt

Clay

CaCO3

Non-CaCO3

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

148

using sediment traps in this environment as “collection rates” rather than 

“sedimentation rates.” The term “collection rate” is used because particles 

settling into a well-designed trap encounter significantly lower turbulence 

than particles falling onto the reef surface, where they may be resuspended 

and removed as the next wave passes. For these reasons traps do not measure 

net sediment accumulation in this setting.

In addition to the relative measure of sediment mobility, the traps provided sufficient material for analysis of temporal variability in sediment 

properties. Some tube traps were X-rayed at the Kaunakakai Hospital to 

identify layers of different sediment density and grain size. Concentrations of 

heavy metals in trap samples were determined by using inductively coupled 

plasma mass spectrometry after the sediments were completely dissolved in 

strong acids (Lamothe and others, 1999). Magnetic properties of the sediment

Figure 3. Time-series sediment 

trap at 11.5-m (37.7 ft) water depth 

off Kamiloloa.

Figure 4. Characteristics of sediment trapped 

during the deployment in which Kona storms 

occurred. Traps were X-rayed, split, and photographed. Selected layers were analyzed for 

sediment grain size and for calcium carbonate content. Traps shown in A, B, and D were 

full when recovered. A, The exposure to large 

waves at this location accounts for collection 

of sandy sediment composed predominantly 

of reef-derived carbonate. B, The appearance 

and composition of the layer at 31–34 cm 

(12–13 in) (sampled parallel to layer boundaries) indicates that land-derived material 

was transported in a pulse to this fore-reef 

location. C, The trapped sediment from the 

reef flat contained the highest noncarbonate 

fraction and the highest silt and clay percentages observed. Floods accompanying the 

Kona storms increased the inventory of landderived fine-grained sediments on the reef 

flat. D, No increases in land-derived sediment 

were observed at this location. The consistently high percentage of coarse-grained 

carbonate sediment reflects the frequent 

exposure to high waves.

settling particulate material into one of 21 plastic bottles (500 ml/17 oz 

each). A carousel rotated a new sampling bottle under the funnel after a 

period of about 4.5 days (McLane Research Laboratories, Inc., 2004), 

providing a detailed temporal resolution of collection-rate variability. Timeseries traps were positioned at Kamiloloa (site 4) and Päläÿau (site 2) at 

about 12-m (39ft) water depth.

Self-contained wave gauges were deployed at six locations, which were 

also occupied by sediment traps, to provide co-located measurements of 

wave heights and wave periods for estimating wave-induced near-bed shear 

stress. Details of sampling and analytical techniques are provided in Bothner 

and others (2006).

We emphasize that sediment traps in this study provide only a relative 

measure of sediment mobility/resuspension. This limitation exists because 

the efficiency of traps for collecting particles and the degree of biasing in 

favor of the more rapidly settling particles have not been quantified in areas 

influenced by large surface waves. We also define the rates determined by 

samples also were measured following the methods of Thompson and Oldfield (1986) in order to evaluate the characteristics and abundance of magnetic 

minerals, such as magnetite, a common component of island rocks and soils.

Sediment Collection Rate: How it Varies 

Along the Reef

Tube Trap Collection Rates

The three-month average sediment-collection rates indicate regional 

variations in sediment mobility (fig. 1). The orange bar in each graph 

shows collection rates during the period that included the Kona storms 
B

A

Bottle 3

30 2

1

0

0

2

4

6

8

20

10

0

Bottle 12

Bottle 17

Nov. Dec.

2001 2002

Jan. Feb.

Noncarbonate in sediment trap, in percent

Rainfall, in centimeters per day

0

0.4

0.8

1.2

Wave-induced stress,

in newtons per square meter

Trap collection rate,

in kilograms per square meter per day

Sediment Mobility Along Molokaÿi’s Fringing Reef: Evidence from Sediment Traps

Impact of Sedimentation on the Molokaÿi Reef

149 

of November 2001 and January 2002. Those storms brought exceptionally 

high waves and wave-induced bottom stresses (orange dots and error bars) to 

most areas along the fore reef and flooding rains to the normally dry south 

side of Molokaÿi.

During the tube trap deployment that included the period of Kona 

storms, the three-month collection rate along the fore reef averaged about 10 

times higher than collection rates during prestorm deployment periods but 

showed considerable variability (fig. 1). Tube trap collection rates influenced 

by Kona storms increased from 1.3 times higher at Päläÿau (site 3) to 39 

times higher at Kamalö. The tube traps at Hale O Lono (site 1) and Pükoÿo 

(site 10) were full at the time of recovery, so the sediment collection rates 

during the storms were minimum estimates, yet they represent the highest 

rates observed (>7,400 g/m2

/day or 1.5 lbs/ft2/day and >6,400 g/m2/day or 

1.3 lbs/ft2

/day, respectively; fig. 1). 

A comparison of sediment collection rates and wave-induced bottom 

stress during deployment periods without Kona storms (blue dots and error 

bars, fig. 1) supports our assumption that collection rates are influenced 

primarily by waves. The average collection rates in tube traps were highest at 

Hale O Lono and Pükoÿo, on the wave-exposed western and eastern ends of 

the reef, and lowest at Kamalö. Sediment collection rates were intermediate 

at Kamiloloa and Päläÿau. Variations in average bottom stress have the same 

spatial pattern. 

The influence of wave-induced bottom stress on sediment collection rate 

is evident in the comparison of results from two tube traps at different depths 

(4.9 m/16 ft and 10.1 m/33.1 ft) on the same type of bottom at Kamiloloa. 

The shallower tube trap collected sediment at a rate 28 times higher than the 

collection rate of the deeper tube trap, reflecting wave-induced bottom stress 

that was about three times higher than at the deeper site.

The collection rate also depends on sediment availability. At Päläÿau, 

one tube trap was on a coral spur (site 3) at 9.6-m (31.5 ft) water depth 

that was covered by 95 percent living coral with, at most, a dusting of fine 

sediment, except in deep crevices of the reef surface. A second tube trap 

was approximately 12 m (39.4 ft) away from the first tube trap, at 12.6-m 

(41.3 ft) water depth, in a channel (site 2) floored by freshly rippled sand, an 

indication of active sediment movement. Although the bottom stress on the 

coral spur is considerably higher because of shallower water, the collection 

rates were only one third of those measured in the sand channel where there 

is an abundant near-by source of sediment. 

On the reef flat (fig. 1, site 5), high collection rates are related to the 

combination of a large inventory of fine-grained sediment, shallow water, 

trade-wind waves, and occasional long-period ocean swell, particularly at 

high tide (see Ogston and others, this vol., chap. 20, for further discussion 

about trade-wind waves and transport). 

Composition of Material in Tube Traps

The Kona storms completely filled some of the tube traps, providing 

sufficient sediment volume to identify and sample individual layers by using 

X-rays of the whole tube trap and photographs of split tube-trap samples. 

Figure 5. Sediment collection at Päläÿau at 12.6 m (41.3 ft) water depth. A, Photograph of sample 

bottles with various amounts of gray sediment collected at Päläÿau during successive 4.5-day intervals 

between November 15, 2001, and February 14, 2002. B, Plots of trap-collection rate, rainfall, waveinduced bottom stress, and percentage of noncarbonate sediment trapped during the deployment off 

Päläÿau between November 15, 2001, and February 14, 2002. 
Site 4

KAMILOLOA

0

Jan Feb Mar Apr May June

2001

Collection rate, in grams per square meter per day

2002

July Aug Sept Oct Nov Dec Jan Feb

200

400

600

800

Site 2

PLØAU

0

400

800

1,200

1,600

Deployment 1 Deployment 2

Funnel plugged

Trap failed

to advance

815

716

Kona

storm

Kona

storm

Kona

storm

Deployment 3 Deployment 4

12 ± 40.053 ± .04833 ± 1810 ± 63 ± 20.023 ± .00717 ± 50.10 ± .2831 ± 31

Percent of ERM

0.1

1

10

35

3.5

Arsenic

Cadmium

Chromium

Copper

Lead

Mercury

Nickel

Silver

Zinc

Figure 7. Histogram showing the average metal concentration (in parts per million, mean ±one standard deviation) in sediments trapped off south Molokaÿi as a 

percentage of the Effects Range-Median (ERM), a guideline for sediment toxicity 

(Long and others, 1995).

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

150

out, however, that a causal linkage between offshore sediment transport 

and the low coverage of living coral in this region has not been confirmed. 

The compositional tube trap results suggest that sediment from the reef flat 

can be transported across the reef crest to the fore reef in this region and 

may impact coral health. 

Tube traps from Hale O Lono and Pükoÿo were completely filled during the deployment that included periods of Kona storms, and therefore the 

collection-rate estimates constitute a lower limit (at least 7,400 g/m2

/day and 

6,400 g/m2/day, respectively). The texture of the trapped sediment at these 

sites is >78 percent sand, and the carbonate contents within both tube traps 

fall between 87 percent and 94 percent, indicating a consistent predominance of reef-derived material in each horizon selected. There was also no 

evidence of increased land-derived material from visual or X-ray assessment 

of trapped sediment. At both sites gravel-size particles were collected in the 

tube traps, indicating periods with significant resuspension.

How Sediment Collection Varies with Time

The storm of November 27–28, 2001, generated the highest rainfall 

(>4 cm/day or 1.6 in/day) and the highest wave-induced bottom stress during the 14-month study (fig. 5B). The sediment collection rate at Päläÿau 

during the 4.5-day period that included the November storm (fig. 5A, 

bottle 3) was also the highest recorded. We used percent noncarbonate to 

determine the relative importance of reef and land as sources of material 

trapped before, during, and after the storm. At Päläÿau we found that the 

percent noncarbonate in storm and poststorm samples was equivalent to, or 

Figure 6. Collection rates determined in 

time-series traps were consistently higher 

at Päläÿau (top) than at Kamiloloa (bottom) in response to higher wave-induced 

bottom stress. Orange bars represent 

intervals during which Kona storms occurred. The similarities in timing of many 

peaks in collection rate at these two locations indicate that wide areas of the reef 

are affected by the same wave events.

Selected horizons were analyzed for texture (grain size) and carbonate 

content (fig. 4). 

On the reef flat off Kamiloloa, the sediment trapped during the Kona 

storm period had the highest noncarbonate fraction (76 percent), indicating 

a land source, and the highest percentage of silt and clay (90 percent) in 

this study (fig. 4C). The 76-percent noncarbonate value was significantly 

above the 60-percent prestorm average and the 58-percent single poststorm 

value. These percentages are consistent with the hypothesis that landderived mud is delivered to the south Molokaÿi reef flat during the floods 

that typically accompany Kona storms (Ogston and others, 2004; Field and 

others, this vol., chap. 21). 

Farther offshore of Kamiloloa, the flood signal also was measurable. The sediment tube trap on the fore reef at 4.9-m (16 ft) water depth, 

100 m (328 ft) seaward of the reef crest (site 6), had a distinct horizon at 

31–34 cm (12–13 in) that was visible in the trap X-ray and in the photograph of the split-trap sample (fig. 4B). The steep dip of this horizon, and 

other horizons that were nearly parallel to it, may result from a slight tilt 

of the trap from vertical (typically <10°) during deployment that would 

allow particles to preferentially accumulate along one side of the tube trap. 

The 31–34-cm horizon had significantly higher amounts of silt and clay 

(40 percent), noncarbonate particles (27 percent), and magnetite particles 

than other horizons in the trap. Although it is not possible to assign dates 

to deposits within the tube trap, we interpreted this horizon as a pulse of 

land-derived material that was transported over the reef crest and deposited 

in the trap during or after the storm (Field and others, this vol., chap. 21). 

Compared to a horizon deposited at 45– 48 cm (18 –19 in), percentages of 

both mud and noncarbonate particles in the 31–34-cm horizon increased by 

a factor of 2.5, and the percentage of magnetite particles (another indicator of land-derived material) increased by a factor of 4.5. This is the best 

example of presumed flood material that accumulated in tube traps beyond 

the reef crest. 

The tube trap at site 6 is in an anomalous region, about 6 km (3.7 mi) 

in length, along the reef crest where live coral coverage is <12 percent, as 

compared to 80–90 percent elsewhere (Jokiel and others, this vol., chap. 5). 

Ogston and others (2004) and Storlazzi and others (2004) note that this 

anomalous region is exposed to elevated suspended-sediment concentrations resulting from resuspension and offshore transport of reef-flat material by trade-wind waves, swell, and local circulation. These authors point 
Copper content, in parts per million

Iron content, in percent

25

20

15

10

5

0

0 1 2 3 4 5

Kamiloloa

PlØau

r 2

 = 0.96

Flood

r 2

 = 0.99

6 7 8

Nonflood

Carbonate content, in percent

IRM, in Am2 per kilogram

90

80

70

60

0.00 0.02 0.04

Kamiloloa

PlØau

r 2

 = 0.69

r 2

 = 0.89

0.06 0.08

Flood

Nonflood

Figure 9. The different regression lines for isothermal remanent magnetization 

(IRM in units of ampere meter squared per kilogram) versus percent calcium carbonate in trap samples from Päläÿau and Kamiloloa indicate a higher magnetite 

content in the land-derived fraction of Kamiloloa traps. Data from flood periods 

(orange) plot with the same slope as data from non-flood periods (blue). These 

trends provide further evidence that floods did not significantly alter the composition of trapped sediment.

Figure 8. Concentrations of iron (percent dry weight) and copper (parts per 

million = μg/g) are highly correlated within trap samples from Päläÿau and 

Kamiloloa. Samples collected during flood periods (orange) plot on the same 

line as those collected during non-flood periods (blue). These data suggest that 

the source of land-derived material in traps is different for each site but was not 

changed by the floods.

Sediment Mobility Along Molokaÿi’s Fringing Reef: Evidence from Sediment Traps

Impact of Sedimentation on the Molokaÿi Reef

151 

lower than, the prestorm average of 24 ± 2 percent (fig. 5B). We concluded, 

therefore, that the high sediment collection rates during this period were 

due primarily to wave-induced resuspension of bottom sediment at the trap 

site and that the composition of mobile sediment collected in this trap was 

not changed as a result of the flood. 

At the Kamiloloa fore-reef site (site 4, water depth 11.5 m/37.7 ft), the 

percent noncarbonate in time-series trap samples increased from 34±2 percent during prestorm periods to 39±2 percent during poststorm periods of 

comparable wave-induced bottom stress (data not shown). This magnitude of 

increase is considerably less than observed in tube traps from the Kamiloloa 

reef flat (fig. 4C) or from the 31–34-cm horizon in the tube trap at 4.9 m 

(fig. 4B), implying that material introduced during the flood period was not 

easily identified beyond the reef crest. 

The low variability in percent noncarbonate in sediment collected by 

traps suggests that there is a low potential for flood-derived sediment to bury 

or smother coral on the fore reef off Molokaÿi when floods are accompanied 

by high waves. The potential for settling particles to be retained in sediment 

traps is much greater than on the exposed surface of the coral reef where 

wave-induced turbulence may cause settling particles to be removed by successive waves. The observation that flood-derived material does not accumulate in fore-reef traps in sufficient quantity to significantly change composition indicates that this material is unlikely to accumulate on the adjacent 

surface of living corals. However, the consistent presence of land-derived 

material in all traps is evidence that terrestrial sediment is passing through 

the system, thus possibly having an influence on reef health.

The time-series sediment traps at Päläÿau and Kamiloloa (fig. 6) provide two pieces of new information. First, the collection rates are consistently higher at Päläÿau than at Kamiloloa—about 10 times higher during 

deployments 1–3 and about 2 times higher during deployment 4. There are 

consistent relative differences in wave-induced bottom stress at the two 

locations. Average bottom stress at Päläÿau is higher than at Kamiloloa by 

about 2 times during deployments 1–3, but higher by only 20 percent during 

deployment 4. 

Second, the timing of collection events is similar at Päläÿau and 

Kamiloloa, which indicates that wide areas of the reef are impacted by the 

same wave events. There were 16 peaks in the collection-rate record at 

Kamiloloa (fig. 6) when time-series traps at both locations were operating 

properly. Of these 16 peaks, the timing of 8 peaks matches exactly with the 

timing of peaks at Päläÿau. For 6 additional peaks, the timing at the two locations agrees within one interval.

Concentrations of Metal Collected in Traps

The concentrations of metals in trapped sediment were low (fig. 7) 

compared to sediment-quality guidelines that are often used to assess 

potential toxicity of coastal sediments. These guidelines, known as the 

Effects Range-Median (ERM), are the metal concentrations above which 

toxic effects in marine organisms were observed in approximately 50 

percent of reviewed studies (Long and others, 1995). Low concentrations 

of heavy metals are not surprising, because there are no mining or metalrelated industries on Molokaÿi, and there are no ocean outfalls for sewage. 

Sewage from the town of Kaunakakai (940 m3

/day from a population of 

2,700) undergoes secondary treatment, with disposal of dry sludge to a 

landfill and discharge of treated effluent into injection wells. Impacts from 

potential migration of ground water from these injection wells or from 

others used by private septic systems outside of Kaunakakai have not been 

observed (John Souza, Department of Waste Water Reclamation, Molokaÿi, 

oral commun., 2004). 

The concentrations of most heavy metals correlate with concentrations 

of iron, one of the main elements in Molokaÿi’s red soils, indicating they 

are associated with the land-derived fraction of the trapped sediment. For 

example, a strong correlation of copper and iron exists for tube trap samples 

from Päläÿau and Kamiloloa (fig. 8). The different slopes suggest a different composition and source of the land-derived material collected at the two 

trap sites. The range of metal concentrations reflects variable dilution by 

reef-derived carbonate sediment. In figure 8, flood-period data are shown in 

orange, preflood and postflood data are shown in blue. The indistinguishable 

copper/iron ratio during flood and nonflood periods indicates that the landderived material had a consistent composition (and source) throughout the 

study period that was not changed by the floods. 

Magnetic Properties of Trapped Sediment

The magnetic properties of the trapped sediment provide additional evidence of land-derived material at all locations along the reef, with no change 

in composition following Kona storms. Magnetite, a mineral present in rocks 

and soils of the Hawaiian Islands, is detected by using isothermal remanent 

magnetization (IRM; Thompson and Oldfield, 1986) and was found in all the 

samples of trapped sediment. The IRM findings indicate that land-derived 

magnetite makes up a measurable fraction of the actively mobile sediment 

throughout the reef system. IRM increases linearly as calcium carbonate 

decreases at both Kamiloloa and Päläÿau fore-reef sites, but with different slopes (fig. 9). These linear but different relations are interpreted to be 

mixing lines between the reef-derived carbonate sediment and two different 

land-derived sources. The source at Kamiloloa has a higher concentration of 

magnetite than the source at Päläÿau. 

The linear relations in figure 9 include storm and nonstorm samples 

on the fore reef. There are no significant or consistent increases in IRM 

(magnetite) relative to carbonate in trap samples collected during or after 

storms. This indicates that the Kona storms did not markedly increase 

the land-derived component of suspended sediment relative to the reefderived component captured by traps on the fore reef. This conclusion is 

supported by the nearly uniform noncarbonate percentage in prestorm and 

poststorm samples.
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

152

A Summary of What the Sediment Traps Show

Land-derived sediment was a measurable component of sediment-trap 

material at all trap sites, but it was not collected in large amounts on the 

fore reef. The consistent distribution of land-derived sediment indicates 

that fine-grained terrestrial material is constantly moving through the 

Molokaÿi reef system. 

Following the Kona storms and floods beginning in late November 

2001, we measured a significant increase in the land-derived fraction of 

trapped sediment on the reef flat at Kamiloloa, and a much weaker “flood 

signal” at sites on the fore reef at 4.9-m and 11.5-m water depth. At other 

sites along the fore reef (10 –13 m water depth) there was an increase in 

the quantity of trapped sediment during Kona storms, but no corresponding 

increase in the land-derived fraction. 

These patterns indicate that when a major flood is accompanied by 

high waves, the land-derived sediment has a low potential for burying or 

smothering coral on the fore reef. However, one potential impact of floodderived sediment could be periodic high turbidity and reduction in light 

needed for photosynthesis by algae in coral. 

We observed consistent regional differences in the relative magnitude 

of sediment-collection rates that correlated with spatial variations in waveinduced bottom stress. Regional differences in sediment-resuspension 

intensity reflect the greater wave exposure on the western and eastern ends 

of the reef, and the more tranquil central region that is sheltered by neighboring islands and by Molokaÿi itself. 

The time-series traps with 4.5-day resolution revealed that Kona 

storms increased the collection rate by more than 1,000 times compared to 

nonstorm intervals. Good agreement was observed in the timing of peak 

resuspension events measured in time-series traps at two locations 12.3 km 

apart. This observation indicates that the same wave events cause resuspension along wide areas of the fore reef.

The concentrations of heavy metals in the trapped sediment within 

the reef system off Molokaÿi are well below Effects Range-Median (ERM) 

sediment guidelines for toxicity (Long and others, 1995). Specific guidelines for metal concentrations in sediments keyed to health of coral, however, do not yet exist. The copper/iron ratios in trapped sediment may be 

another index, similar to magnetic signatures, which may identify different 

source areas of land-derived material to the reef system.
Impact of Sedimentation on the Molokaÿi Reef

CHAPTER 20

Causes of Turbidity on the Molokaÿi Reef Flat and Resulting Sediment-Transport Patterns

Andrea S. Ogston1, M. Katherine Presto2, Curt D. Storlazzi2, and Michael E. Field2

Turbidity and sedimentation are generally recognized as important 

factors that can impair coral health (Acevedo and others, 1989; 

Fortes, 2000; Rogers, 1990; Buddemeier and Hopley, 1988). Turbidity on reefs is a difficult parameter to characterize, because it concerns 

not only the amount of sediment suspended in the overlying water column 

(suspended-sediment concentration), but also the frequency and duration of resuspension and the types of particles in suspension. In general, 

the physical environment controls the factors that regulate turbidity on 

reefs. Four important factors that control the sedimentary regime are tides, 

waves, currents, and amount of available sediment, all of which vary over 

time on hourly, daily, seasonal, and longer scales.

Between January 2000 and August 2002, we studied these processes 

and their influence on turbidity on the Molokaÿi reef flat (fig. 1), a topographically rough, nearly horizontal surface ranging from 0 to 2 m ( 0 

to 6.5 ft) in depth, that extends almost 1 km (0.6 mi) from the shoreline 

outward to the reef crest. The reef crest itself is a shallow, irregular barrier 

between the reef flat and the open ocean. Our studies identified a suite of 

processes and their temporal changes, using a small instrumented tripod 

(ReefProbe, fig. 2) deployed at 1-m (3.3 ft) water depth approximately 

midway across the reef flat. The instruments collected a 2-year record of 

water-surface elevation, water properties (temperature and salinity), currents (due to waves, tides, and winds), and suspended-sediment concentration. To complement the single-point time-series data, in May and August 

2002 we used a portable instrument package (Backpack, fig. 3) to measure 

currents and water properties for a few minutes at multiple sites along 

transects spanning the reef flat and reef crest between One Aliÿi Park and 

Kaunakakai Wharf (fig. 1). 

In this chapter, we summarize the results of these investigations to 

illustrate the processes that contribute to turbidity on the Molokaÿi reef flat 

and the resulting sediment-transport patterns. For a more detailed analysis, 

see reports by Ogston and others (2004), Storlazzi and others (2004), and 

Presto and others (2006). 

1

 University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195

2

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

Transport of Sediment and Water on the Reef Flat

Typical Environmental Conditions

The amount of sediment suspended in the water column and how rapidly 

sediment transport occurs depend on environmental conditions related to the 

weather. The most conspicuous feature of the weather pattern in the tropical Pacific Ocean is the persistent trade winds that blow generally northeast 

to southwest. Their effect is greatest across Hawaiÿi during the trade-wind 

season (May-November), when they are prevalent 80–95 percent of the 

time; during the non-trade-wind season (November-May), trade winds still 

blow across the islands much of the time, though with decreased frequency. 

Storms bring heavy rains and strong winds to the Hawaiian Islands, generally during the non-trade-wind season. In particular, Kona storms from the 

south generate large waves offshore and rainfall that brings freshwater and 

sediment from the uplands to the reef on southern Molokaÿi. Storms over the 

open ocean to the south of the Hawaiian Islands generate large waves that 

can propagate as swell to the south coast of Molokaÿi. 

 To illustrate the effect of environmental conditions over the year, we 

classified the data from the ReefProbe according to the four environmental 

conditions that occur most frequently on the Molokaÿi reef flat: (1) trade 

winds, (2) storms, (3) swell, or (4) light winds of varying direction without swell (referred to simply as light winds). We compared the magnitude, 

direction, and duration of winds, as well as offshore wave conditions, during 

the period May 2001–May 2002 to evaluate which environmental condition 

dominated. We divided the periods over which each of these environmental 

conditions occurs by season (fig. 4). For example, during the trade-wind 

season, the trade-winds environmental condition occurred 83 percent of the 

time, while the swell environmental condition occurred only 13 percent of 

Figure 1. Molokaÿi reef flat, showing general location of study area. Data were 

obtained from two sources: (1) a small instrumented tripod, the ReefProbe, 

placed on the mid reef flat collected time-series data for almost 2 years; and 

(2) a portable instrument package, the Backpack, was used to collect measurements at multiple sites along transects (dashed lines) during high and low tides 

when trade winds were active.

Figure 2. Time-series data on the 

Molokaÿi reef flat were obtained by using 

the ReefProbe deployed on the mid reef 

flat at 1-m (3.3 ft) water depth. The ReefProbe contained sensors that measured 

current (electromagnetic current meter), 

tidal elevation and wave height (pressure sensor), turbidity or suspendedsediment concentration (optical-backscatter sensor and transmissometer), 

and salinity and water temperature 

(conductivity and temperature sensor).

Instrument Height above 

bed (cm)

Electromagnetic current 

meter

20

Pressure sensor 30

Optical-backscatter sensor 20

Transmissometer 20

Conductivity and temperature 

sensor

55

+

157°01' W

21°

05' 

N 

21°

04' 

157° 156°59'

One Ali`i

Park

Kamiloloa

Kaunakakai Wharf

Reef Probe

Reef crest

Fore reef

Reef flat Fishpond

Fishpond
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

154

Molokaÿi reef flat throughout the year, as is also true on other shallow-reefflat environments where tidal currents and wave energy are relatively small 

(Pickard, 1986; Andrews and Pickard, 1990). 

Currents and What Drives Them

Currents provide the stress (force per unit area) required to erode sediment from the seabed, create turbidity in the water column, and move the 

sediment elsewhere. On the Molokaÿi reef flat, currents are principally driven 

by winds, tides, and waves, each producing a different sediment-transport 

pattern. 

Wind-Driven Currents

In general, wind-driven currents are the strongest unidirectional currents on the reef flat. Throughout the year, these currents are driven by 

trade winds (Nov. 16 –26, 2000, figs. 5A, B), with flow toward the west 

and offshore at the ReefProbe site (fig. 1). The offshore component is not 

a persistent feature along the reef flat but can be explained by the position 

of the instruments relative to the orientation of the shoreline, as indicated 

in the Backpack studies discussed below. During the trade-winds environmental condition (fig. 5A), mean currents at the ReefProbe site were 

influenced by the speed and duration of the winds, resulting in a flow (>10 

cm/s) to the west and slightly (~2 cm/s) offshore on the reef flat, regardless 

of tidal cycle. These currents occurred predominantly during the afternoon 

for approximately 70 percent of the year, during both the trade-wind and 

the non-trade-wind seasons. Under the light-winds environmental condition, slow (1–3 cm/s) currents were observed. During Kona storms, wind 

speeds were high, and currents were strong (>10 cm/s) but varied in direcFigure 3. The Backpack, used for 

spatial studies, provided a perspective on sediment-transport processes 

along and across the Molokaÿi reef flat 

under specific conditions. The instrument package contained sensors that 

measured the same properties as the 

ReefProbe (fig. 2).

the time, although historically the swell environmental condition is much 

more frequent (Andrews and Pickard, 1990). Sheltering of the south-central 

coast of Molokaÿi by other islands (Maui and Lanaÿi) partly accounts for the 

limited occurrence of the swell environmental contribution. The light-winds 

and storms environmental conditions rarely occurred (2 percent each). 

During the non-trade-wind season, the light-winds environmental condition dominated the record (51 percent), while the trade-winds environmental condition occurred 37 percent of the time and the swell environmental 

condition 7 percent of the time. The storms environmental condition, which 

occurred only 5 percent of the time, was more prevalent during this season; 

a strong Kona storm struck Molokaÿi in November 2001, and smaller storms 

in December 2001 and January 2002. Although their strength varies, trade 

winds are the principal factor controlling sediment-transport processes on the 

Figure 4. The distribution of four environmental conditions as measured by the 

ReefProbe on the Molokaÿi reef flat in a typical 1-year period (2001-2002) during 

trade-wind (May-November) and non-trade-wind (November-May) seasons. Environmental conditions were determined by examining the direction, speed, and 

duration of winds as measured by a sensor at the end of Kaunakakai Wharf, as 

well as by offshore wave conditions.

Figure 5. Environmental factors that contribute to sediment resuspension on 

the Molokaÿi reef flat include winds and tides. In this example from a time series 

in late November 2000, both trade-winds and light-winds environmental conditions are prevalent. During the trade-winds environmental condition, winds 

were strong and varied on a daily cycle, peaking in the afternoon (A), and were 

in a direction along the reef flat of ~110 ̊ (B). During light-winds environmental condition, the winds were weaker and had no specific direction. Tides are 

defined as microtidal and have a range of ~0.6 m (~2 ft) with a mixed semidiurnal signal, varying on a fortnightly cycle between neap and spring tides 

(C ). Response of water on the reef flat to trade winds and tides is complex. 

Trade winds generate along-reef currents with a small offshore component (D ). 

During light winds, currents are weak and vary in direction. Winds also create 

waves on the sea surface that, in turn, cause wave-orbital motions at the sea 

bed; wave-orbital velocities (E) are controlled by both winds and water depth. Wave-orbital velocity, in centimeters per second

0

5

10

15

Wind speed,

in meters per second

0

5

10

15

2000

E

Tidal elevation,

in meters

0

0.5

1 C

Wind direction (from),

in degrees

0

180

360 B

A

Current, in centimeters

per second

-5

0

5 D Trade-wind-dominated currents Tidal currents

Neap tide Spring tide Neap tide

Nov. 16 Nov. 20 Nov. 24 Nov. 28 Dec. 2 Dec. 6

Strong trade winds

Light winds

N

E

S

W

Trade-wind season

(MayÐNovember) 

Non-trade-wind season

(NovemberÐMay)

Environmental

conditions

Trade winds

Light winds

Storms

&IG Swell

Instrument

Height

above bed

(cm)

Electromagnetic

current meter

20

Pressure sensor 50

Optical-backscatter

sensor

20, 50

Transmissometer 70

Conductivity and

temperature sensor profiling

Instrument

Height 

above bed 

(cm)

Electromagnetic current meter 20

Pressure sensor 50

Optical-backscatter sensor 20, 50

Transmissometer 70

Conductivity and temperature 

sensor

Profiling
Causes of Turbidity on the Molokaÿi Reef Flat and Resulting Sediment-Transport Patterns

Impact of Sedimentation on the Molokaÿi Reef

155 

against the wharf. A more detailed analysis of these data was presented by 

Presto and others (2006).

The spatial maps of currents in figure 6 allow an approximation of a 

water-transport rate for various sections of the Molokaÿi reef flat by moving water at the speed and in the direction measured at one transect to a 

position on the next transect. For a pathway approximately halfway across 

the reef flat, water is transported from One Aliÿi Park to the Kaunakakai 

Wharf (fig. 1), a distance of ~5 km (3.1 mi), in 13 hours (average speed, 

9.7 cm/s) during high-tide and high-wind conditions. Typically the tradewinds environmental condition occurs, on average, 6 hours per day. Thus, 

a parcel of water flowing on the mid reef flat would take about 2 days to 

travel the 5 km from One Aliÿi Park to the wharf. For a pathway on the 

inner reef flat, water is transported during high tide and under the tradewinds environmental condition in about twice the time (> 4 days), owing 

to the much slower currents (average speed 6.4 cm/s) and the slightly 

longer pathlength due to obstructions (fishponds). Because water moves 

slower on the inner reef flat than on the mid reef to outer reef flat and the 

distance traveled in the pathway around the fishponds is greater, alongshore flushing rates are significantly slower on the inner than on the outer 

reef flat. 

Tidal Currents

The tides in Hawaiÿi are defined as microtidal (tidal range, <2 m or 

6.5 ft) and have a mixed semidiurnal signal, with two daily highs and 

lows, dominated by one extreme high and low each day (fig. 5C). Though 

small in range, the changing tidal elevation is particularly important 

because of its influence on waves and currents on the Molokaÿi reef flat, 

owing to the reef’s broad, shallow shape. The observed tidal range on the 

reef flat was typically 0.6 m (2 ft), with a maximum of ~1.0 m (3.3 ft). 

Although the tidal range off southern Molokaÿi is small relative to elsewhere in the world, it represents more than a doubling in water depth over 

much of the reef flat. This tidal variation over the reef flat is important 

for sediment resuspension and transport because wave heights are limited by the depth of water. At higher tides, the effects of offshore waves 

are greater on the reef flat because more offshore wave energy can be 

transferred through the deeper water over the reef crest. The greater water 

depth also allows for larger local wave formation on the reef flat. 

Tidal currents on the reef flat have both semidiurnal (12.4 h) and diurnal (24.8 h) components. We evaluated data recorded at the ReefProbe site 

(fig. 1) during light-winds environmental conditions to estimate the tidal 

currents. Tidal currents on the reef flat were generally slow (3 –5cm/s) 

relative to wind-driven currents (max 15 cm/s; fig. 5D). 

Wave-Orbital Currents

The waves on the Molokaÿi reef flat are small and result from offshore waves that propagate over the reef crest, from waves that are generated locally by wind stresses, or from some combination of both factors. 

Figure 6. Observed nearbed (20 cm or 7.9 in above seabed) mean currents on 

the Molokaÿi reef flat during trade-winds environmental condition vary spatially and 

with tidal stage, as seen in measurements at low tide (A) and high tide (B). Dominant current direction is alongshore to the west at most survey points on the reef 

flat, with a slight offshore flow on the east (upwind) side of fishponds and onshore 

flow on the west (downwind) side at both tidal stages. All measurements were 

taken in <2-m (<6.6 ft) water depth.

Figure 7. Photographs taken from an autonomous camera mounted above 

Kamiloloa show the difference in waves breaking on the Molokaÿi reef crest at 

low (A) and high (B) tides. At low tide, waves break on the reef crest, and little 

wave energy from offshore waves reaches the reef flat, whereas at high tide, 

some offshore wave energy can propagate through the reef crest to generate 

wave-orbital velocities on the reef flat.

Wave heights in the open North Pacific Ocean can be large because they 

are not influenced by the landmasses of the islands, and significant wave 

heights average 2 to 3 m (6.6 to 9.8 ft) throughout the year. The waves 

offshore of the south-central coast of Molokaÿi are much smaller, typically 

averaging ~0.5 m (~1.6 ft), because the surrounding islands of Lanaÿi and 

Maui protect south Molokaÿi from large, open-ocean waves. As waves off 

the coast approach the reef, they significantly decrease in height as they 

tion as the storms passed, resulting in relatively minor net currents in both 

the alongshore and cross-shore directions. Under the swell environmental 

condition without the presence of trade winds, wind-driven currents were 

slow (<5 cm/s), with flow slightly eastward and shoreward. 

From Backpack measurements, the pattern of trade-wind-driven currents over the Molokaÿi reef flat was observed to be strongly alongshore to 

the west, following the coastline in the direction of the trade winds, with 

a minor offshore component at the fishponds and wharf (fig. 6). Currents 

ranged from ~1 to 25 cm/s, faster where the reef flat is narrow and deep, 

slower over the broader, shallower parts of the reef flat, and significantly 

slower near Kaunakakai Wharf, indicating some stagnation of water 

0 3,000 Feet

0 1,000 Meters

Map area

1.0Ð6.0

6.0Ð9.25

9.25Ð12.5

12.5Ð16.0

16.0Ð21.5

Nearbed current,

in centimeters

per second

A Low tide

KalokoØeli fishpond

AliØi fishpond

Kaunakakai Wharf

Trade winds

Trade winds

Reef crest

Reef crest

B High tide

Reef Probe

Reef Probe

KalokoØeli fishpond

AliØi fishpond

Kaunakakai Wharf

A Low tide

B High tide
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

156

break on the reef crest. The waves that we observed on the reef flat were 

small (~0.1– 0.2 m or ~0.3 – 0.7 ft during high tide), and their height was 

more influenced by tidal elevation than offshore wave height (fig. 7).

Waves cause the water to move in oscillating (or orbital) patterns, 

which were measured on the Molokaÿi reef flat as the wave-orbital velocity. 

Nearbed wave-orbital velocities varied on a daily basis at tidal periods (fig. 

5E), with a maximum of ~15 cm/s. At low tidal elevations, the wave-orbital 

velocity was low (<5 cm/s), whereas at high tidal elevations, it was higher 

(max 10–15 cm/s) and less predictable. The high degree of correlation 

between tidal elevation and wave-orbital velocity implies that the waveorbital velocity on the reef flat is strongly controlled by water depth, irrespective of whether the waves are generated offshore or locally on the reef 

flat by trade winds. 

The combination of high water elevation (due to tides and storm 

surge—an increase in water elevation caused by wind and low atmospheric pressure) and high offshore waves caused the highest wave-orbital 

velocity at the seabed of the reef flat; however, this combination of factors 

occurred only over relatively short periods. The trade-winds environmental condition, with lower wave-orbital velocities on the reef flat, occurs 

persistently during the trade-wind season and intermittently during the 

non-trade-wind season.

Suspended-Sediment Concentrations

The amount of sediment suspended in the water column (suspendedsediment concentration) can be attributed to resuspension of sediment from 

the seabed or its advection (transport) from another area, or a combination of 

both these processes. Suspended-sediment concentration, which is expressed 

in units of weight per volume of seawater (milligrams per liter), increases in 

response to stresses on the seabed caused by wave-orbital velocities and/or 

mean currents. Currents measured on the reef flat are generally insufficient 

alone to resuspend even fine silt from the seabed. Wave-orbital velocities 

observed on the Molokaÿi reef flat, in the absence of currents, also are generally insufficient to resuspend the finest seabed sediment. When waves and 

currents act together (wave-current interaction), however, their combined 

stress is sufficient to resuspend sediment on the reef flat. Thus, the daily 

resuspension of sediment observed on the reef flat is typically generated by 

wave-current interaction. 

Suspended-Sediment Concentrations Under Typical 

Environmental Conditions

Suspended-sediment concentrations on the Molokaÿi reef flat are 

responsive to the daily variations in winds, tides, and waves under the 

four typical environmental conditions. Thus, during the trade-winds 

environmental condition, sustained strong winds generate currents of 

>10cm/s that flow westward alongshore, regardless of the tidal cycle. 

Peak suspended-sediment concentrations during the trade-wind season 

were measured at high tide, when the greater water depth over the reef 

flat allowed the generation of substantial wind-driven waves in addition to 

strong alongshore wind-driven currents. A typical daily peak in suspendedsediment concentration during the trade-wind season was ~20 mg/L on the 

mid reef flat (fig. 8).

Storms environmental conditions on the reef flat were infrequent, but 

the large storm waves that propagated onto it during high tide resuspended 

large amounts of sediment from the seabed, with peak suspended-sediment 

concentrations of 100 mg/L on the mid reef flat. Similarly, suspendedsediment concentrations under the swell environmental condition were also 

high (~40 mg/L) because wave-orbital velocities were high (>10cm/s), 

especially during high tides, when larger deep-water waves are able to 

propagate over the reef crest and onto the reef flat. The nearbed mean 

currents under the swell environmental condition, in the absence of trade 

winds, were small and driven primarily by tides. Under the light-winds 

environmental condition, mean and tidal currents and wind-generated 

wave-orbital velocities were minimal. Although some sediment was still 

suspended, concentrations were quite low (<10 mg/L) and generally fluctuated with the tidal cycle (fig. 9). 

Patterns of Suspended-Sediment Concentration

Backpack measurements at low and high tides under the trade-wind 

environmental conditions show the spatial relation between tidal stage and 

suspended-sediment concentration over the study area (fig. 10). During low 

tide (fig. 10A), the suspended-sediment concentration measured on the reef 

flat ranged from <5 to >70 mg/L, generally decreasing with distance offFigure 8. During trade-winds environmental condition, sediment is resuspended 

from the seabed, owing to a combination of wave-orbital velocities and wind-driven currents. During the period shown here, the water-surface elevation varies because of the tides (A) and strong trade winds (B), which control the wave-orbital 

velocity and wind-driven current (not shown). Resulting suspended-sediment 

concentrations (C) measured at the ReefProbe site on the Molokaÿi reef flat vary 

on a daily basis, with peak suspended-sediment concentrations resulting from 

strong winds and high tidal elevation. 

Figure 9. Photographs taken from same camera as in figure 7 show the difference in suspended-sediment concentration on the Molokaÿi reef flat at low tide 

under calm-wind conditions (A) and at high tide under trade-wind conditions (B). 

At low tide, turbidity of water is low, and the reef-flat surface is visible, whereas 

at high tide, wave-current interaction causes sediment to be suspended from the 

seabed, and turbidity of water is high. 

2001

Suspended sediment,

in milligrams per liter

0

10

20 C

Spring tide

Trade winds

Daily resuspension of sediment

Neap tide

Windspeed, in

meters per second

0

0.5

1 B

Tidal elevation,

in meters

0

1

2 A

Aug.7 Aug. 9 Aug. 11 Aug. 13 Aug. 15 Aug. 17

A Low tide

B High tide
Causes of Turbidity on the Molokaÿi Reef Flat and Resulting Sediment-Transport Patterns

Impact of Sedimentation on the Molokaÿi Reef

157 

shore. The highest suspended-sediment concentrations were measured near 

shore and to the east (upwind) of Kaunakakai Wharf.

During high tide and under the trade-winds environmental condition 

(fig. 9B), the suspended-sediment concentration ranged from <5 mg/L 

offshore to >130 mg/L near shore and to the east (upwind) of the fishponds 

and wharf (fig. 10B). Higher suspended-sediment concentrations during high 

tide are due to greater water depths over the reef flat that allow deep-water 

waves to propagate over the reef crest, trade-wind-driven waves to be generated on the reef flat, and trade-wind-driven currents to speed up over the reef 

flat. Most suspended sediment is confined between the coastline and the mid 

reef flat (~600 m or 2,000 ft from shore), owing to the shallow, broad, lowrelief morphology of the reef flat and the alongshore wind-driven currents. 

Suspended-sediment concentrations decreased to <10 mg/L near the reef 

crest, indicating that only small amounts of sediment were in suspension and 

possibly being transported to the fore reef during the study period. The only 

area on the reef flat where suspended-sediment concentrations >20 mg/L 

extended farther offshore was just to the east (upwind) of the wharf. 

Sediment Flux: Where is the Sediment Going?

We evaluated the sediment flux (or sediment-transport rate) along and 

across the Molokaÿi reef flat by multiplying the current and suspendedsediment concentration together over a specific period. These two factors 

must be correlated over time to contribute to the sediment flux (for example, 

suspended-sediment concentration may be high, but if there is no water 

movement, the sediment flux is zero). Hourly records of sediment flux were 

calculated, then summed over the year-long data record, to get the net sediment flux for the year.

The data on net sediment flux were classified according to the four 

environmental conditions for the period May 2001–May 2002 (fig. 11). The 

trade-winds environmental condition is prevalent for most of the year (fig.4), 

and the ability of trade winds to suspend moderate amounts of sediment, 

Map area

0 3,000 Feet

0 1,000 Meters

KalokoØeli fishpond

Observed sediment flux

AliØi fishpond

Kaunakakai Wharf

Trade winds

= 0.15 mg/cm2/s

drive strong wind-driven currents, and persist over the study period resulted 

in their dominant contribution to the total sediment flux. The trade-winds 

environmental condition was subdivided into two parts: normal trade winds 

with no new sediment added, and trade winds following storms that introduce significant amounts of sediment onto the reef flat. Although the sediment flux on the Molokaÿi reef flat during the November 2001 Kona storm 

was small, under the trade-winds environmental condition in the subsequent 

days (and with subsequent sediment-input events), large amounts of sediment were transported westward, owing to the high sustained suspendedsediment concentrations (>50 mg/L) and strong currents (Field and others, 

this vol., chap. 21), illustrating the effect of readily available fine sediment 

on the sediment flux.

Environmental conditions other than trade winds contribute less to the 

total sediment flux on the Molokaÿi reef flat. The sediment flux associated 

with the storms environmental condition (to the west and offshore) was 

relatively small during the study period, owing to its infrequent occurrence, 

and although windspeeds were high, wind directions shifted over the event, 

causing current directions to vary during periods of high suspended-sediment 

concentrations and resulting in little net sediment flux. During the swell 

environmental condition, the net sediment flux was relatively low, owing to 

weak, varying tidal currents and resulting in net sediment flux to the east, 

moving sediment opposite to the prevailing trade-wind direction. The infrequent occurrence of this environmental condition on the reef flat (fig. 4), in 

combination with minimal sediment flux, leads to a low contribution to the 

Figure 10. Amount of sediment resuspended under trade-winds environmental 

condition depends on location on the Molokaÿi reef flat and tidal stage, as shown 

by the spatial measurements at low tide during the period May 11–16, 2002 (A), 

and at high tide during the period August 18–22, 2002 (B). Peak suspended-sediment concentrations (>70 mg/L at low tide and >80 mg/L at high tide) observed at 

20 cm (7.9 in) above seabed are shown in a nearshore band that widens around 

fishponds and to the east of Kaunakakai Wharf. Suspended-sediment concentration is relatively uniform throughout water column and rapidly decreases with 

distance offshore, indicating that most resuspendable fine sediment is contained 

on the inner reef flat. All measurements were taken in <2-m (<6.6 ft) water depth.

Figure 11. Dominant sediment-transport direction on the Molokaÿi reef flat is 

southwestward under trade-winds environmental condition at the ReefProbe site, 

consistent with mainly along-reef transport shown by Backpack measurements. 

Lengths of vectors, shown for various environmental conditions, are scaled to 

sediment flux through a square centimeter of water column summed over the time 

period of the condition from the May 2001–May 2002 dataset. Trade-wind sediment flux was further divided to show sediment flux following a large Kona storm 

(Field and others, this vol., chap. 21). Although swell and light-winds environmental conditions contributed only a minor amount to total sediment flux, they are 

important in that they can move sediment in directions opposite to trade winds. 

Figure 12. Observed sediment flux on the Molokaÿi reef flat under trade-winds 

environmental condition is dominantly within an inner-reef band, as indicated 

by measurements made at high tide. Sediment flux is calculated from current 

and suspended-sediment concentration at each survey point on the reef flat. 

Dominant direction of sediment flux is alongshore to the west for most survey 

points. Peak sediment fluxes were calculated for areas just seaward of fishponds, where current velocities are high, as well as for areas on the inner reef 

flat to the east and west of fishponds, where suspended-sediment concentrations are high.

0 3,000 Feet

0 1,000 Meters

Map area KalokoØeli fishpond

AliØi fishpond

Kaunakakai Wharf

KalokoØeli fishpond

AliØi fishpond

Kaunakakai Wharf

Suspended-sediment

concentration, 

in milligrams per liter

<10

10Ð20

20Ð30

30Ð40

40Ð50

50Ð60

60Ð70

>70

A Low tide

B High tide

0 3,000 Feet

0 1,000 Meters

Map area

KalokoØeli fishpond

AliØi fishpond

Kaunakakai Wharf

Trade winds 

Swell

Light winds

Trade winds (poststorm) Storms

= 100 g/cm2

Dominant sediment-transport direction
THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

158

total annual sediment flux. The south-central part of the Molokaÿi reef flat 

is generally blocked from deep-water ocean waves, minimizing the effect 

of offshore waves relative to many other reefs in Hawaiÿi. The light-winds 

environmental condition, though frequent on the reef flat, resulted in minimal sediment flux, owing to low suspended-sediment concentrations and 

weak nearbed currents (generally due only to tidal ebb and flood). Thus, over 

the study period, trade-wind-driven processes were the dominant control on 

reef-flat sediment transport. 

Distribution of Sediment Flux on the Reef

Backpack measurements of sediment flux under the trade-winds environmental condition indicate that much of the terrigenous sediment resuspended on the Molokaÿi reef flat is transported predominantly alongshore 

and is confined to the inner to middle reef flat (fig. 12), mainly owing to the 

higher suspended-sediment concentrations on the inner reef flat and the relatively constant windspeeds across the mid reef flat. Little crossreef mixing 

of water and sediment by the dominantly alongshore wind-driven currents 

occurs under the trade-winds environmental condition, as evidenced by the 

marked cross-shore gradients in suspended-sediment concentration and composition. In areas where flow is impeded by obstacles (east of the fishponds 

and wharf, fig. 1), currents are very slow, resulting in trapping of water and 

sediment there. 

Sediment-Deposition Rate Over a Tidal Day

During the trade-wind season, sediment-suspension events occur on the 

Molokaÿi reef flat on a daily basis. Most of the sediment in the water column 

settles out of suspension when the winds and wind-driven waves subside, 

generally in the late afternoon. A sediment-deposition rate can be estimated 

over a 24-hour period to simulate the amount of sediment that would settle 

out of suspension and be deposited on the reef flat (fig. 13). Using an average daily suspended-sediment concentration of 20 mg/L on the mid reef flat, 

the mean daily sediment-deposition rate on the reef flat is estimated at 

2mg/cm2 per day. Using the maximum measured suspended-sediment concentration of 100 mg/L, the sediment-deposition rate during these times is 

estimated at 10 mg/cm2

 per day, approaching the limit of acceptable rates for 

coral growth, as suggested by Rogers (1990).

An important point is that the sediment deposited in one day at the 

above rates may get resuspended the next day—it cannot be viewed as a 

long-term accumulation that results in burial of individual corals. Fine sediment has been shown to be trapped temporarily by macroalgae (Stamski and 

Field, 2006), and presumably, some of the sediment deposited on the reef flat 

gets trapped in selected areas, resulting in net accumulation over time. This 

analysis also highlights the difficulty in interpreting sediment-trapping data 

as a net flux (Bothner and others, 2006). 

Sediment Recharge to the Reef Flat

If sediment were constantly being resuspended and transported along 

the Molokaÿi reef flat without new sediment being injected, over time the 

reef flat would clear out and become a dominantly carbonate system, with 

sediment originating only from the offshore reef system. Terrestrial sediment 

however, is recharged into the system, and local sinks store sediment on the 

reef flat (for example, near shoreline structures). Although only a few storms 

occurred during the study period, the runoff from these storms transported 

significant amounts of sediment from the land to the reef flat (Field and others, this vol., chap. 21). Thus, as the upland sources discharge sediment at 

point sites onto the inner reef under the storms environmental condition, the 

subsequent trade winds act over longer periods to redistribute the sediment 

westward in a thin band constrained relatively near shore. The sediment on 

the reef flat, though resuspended and transported on a daily basis, is replenished by episodic storms. 

Implications for Reef Flat Management

Rogers (1983, 1990) demonstrated that variations in suspended-sediment concentration affect coral growth and health. Our study adds to that 

understanding by documenting that suspended-sediment concentration varies across a reef flat and that a single measurement in both space and time 

is inadequate to describe conditions on a reef flat where, on a daily basis, 

seasonally varying winds and waves resuspend sediment. Thus, the effect of 

a discharge of even minor amounts of sediment onto the reef flat may be significant because this sediment can be resuspended many times before finally 

leaving the reef-flat environment.

Figure 13. In each daily tidal cycle on the Molokaÿi reef flat, sediment is suspended in the water column and redeposited back onto the seabed, as illustrated by 

the variation in suspended-sediment concentration over a single day (for example, 

one day highlighted in blue). For a typical suspended-sediment concentration in 

the water column and a typical water depth of 1 m (3.3 ft), an average of ~20mg/L 

of sediment settles out of suspension, and the short-term sediment-deposition 

rate is 2 mg/cm2 per day. For higher suspended-sediment concentrations, a 

maximum of ~100 mg/L of sediment settles out of suspension and the sedimentdeposition rate increases to 10 mg/cm2 per day. Note that sediment deposited in a 

daily cycle will likely be resuspended in the following daily cycle.

1.0 m

Suspended-sediment

concentration, in

milligrams per liter

Particle

settling

21 22 23 24 25

October 2000

0

20

40
Figure 2. Rainfall amounts recorded hourly during the November 2001 storm by 

the rain gauge at Kaunakakai, Molokaÿi (source: National Weather Service Hydronet System, http://www.prh.noaa.gov/hnl/hydro/hydronet/hydronet-data.php, 

last accessed April 29, 2008).

0

0.2

0.4

0.6

0.8

0:00 12:00 0:00 12:00 0:00 12:00 0:00

Nov 26

17:00Ð18:00

Rainfall, in inches per hour

Rainfall, in centimeters per hour

Date and local time

Nov. 26, 2001 Nov. 27, 2001 Nov. 28, 2001

0.0

0.5

1.0

1.5

2.0

Nov 27

04:00Ð05:00

Nov 27

11:00Ð12:00

Rainfall, in inches per hour

Rainfall, in centimeters per hour

0

1994 1996 1998 2000 2002 2004

4

8

12

16

Year

0

10

20

30

40 Kaunakakai

Kamal

Inferred

Dry period

21°

05' 

 N

157° 156° 157 55' °05' W

PlØau

Kaunakakai

Kamiloloa

Kawela

Kamal

Kawela

Gulch

Kamal

Gulch

Clouds

25

50

25

125

100

150 no data

200

200

Isobath interval 25 m 

0 1 2 Miles

0 2 4 Kilometers

Map area

USGS land-based cameras

USGS underwater camera

NOAA rain gauges

USGS weather station

USGS rotating sediment traps

USGS ReefProbe tripod

Impact of Sedimentation on the Molokaÿi Reef

CHAPTER 21

The Effects of a Kona Storm on the Molokaÿi Reef: November and December 2001

Michael E. Field1, Michael E. Bothner2, Pat S. Chavez, Jr.3, Susan A. Cochran1, Paul L. Jokiel4, Andrea S. Ogston5, M. Katherine Presto1, and Curt D. Storlazzi1

On the average, heavy rains affect the southern slopes of Molokaÿi once 

or twice each winter, usually as part of a Kona storm, a seasonal, 

tropical storm with winds coming from the “kona” (or leeward) side 

of the islands. In November 2001, such a storm occurred, the first in a fouryear period. Runoff of sediment was heavier than normal, as evidenced by road 

closures. Large quantities of plant debris and soil that had accumulated on the 

hillsides through normal weathering processes were washed down through 

gulches to the coast. Culverts carrying water and debris beneath Highway 450 

(King Kamehameha V Highway) quickly became clogged, as often occurs 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

2

 U.S. Geological Survey Woods Hole Science Center, 384 Woods Hole Road, Woods Hole, MA 02543

3

 U.S. Geological Survey Flagstaff Science Center, 2255 North Gemini Dr., Flagstaff, AZ 86001; 

current address: Northern Arizona University, Department of Geography, PO Box 15016, Flagstaff, AZ 86001

4

 University of Hawaiÿi, Hawaiÿi Institute of Marine Biology, P.O. Box 1346, Kaneohe, HI 96744

5

 University of Washington, School of Oceanography, Box 357940, Seattle, WA 98195-7940

Figure 1. Aerial photograph of central south Molokaÿi and the adjacent reef, 

showing the locations of the U.S. Geological Survey instruments and cameras and 

the National Oceanic and Atmospheric Administration rain gauges that recorded 

information during the November 27, 2001, Kona storm. Much of the sediment delivered to the coast in the storm was discharged from Kawela and Kamalö Gulches 

and other drainages.

Figure 3. Monthly rainfall amounts for Molokaÿi between July 1994 and September 

2005 as recorded by the rain gauges at Kaunakakai and Kamalö (source: National 

Weather Service Hydronet System, http://www.prh.noaa.gov/hnl/hydro/hydronet/hydronet-data.php, last accessed April 29, 2008). The rain gauge at Kamalö stopped 

working at times, including during the November 2001 storm. As in the December 

2001 storms, rainfall at Kamalö is inferred to be nearly twice that at Kaunakakai. 

Note that, on average, each winter has a period of heavy rainfall (shown by gray 

bars), except for the four winters preceding the November 2001 storm. The lack of 

heavy rain for four years may have led to a heavier-than-normal soil buildup, and 

consequently to heavier-than-normal sediment runoff.

during a Kona storm, and gravel, sand, and mud washed onto the road. Muddy 

sediment poured over the coastal terrace, into the water, and onto the reef. The 

U.S. Geological Survey (USGS) had a set of instruments on the reef, as well 

as two cameras on land to photograph changes in ocean turbidity (fig. 1). The 

USGS conducted follow-up high-resolution aerial surveys on November 29 and 

acquired satellite images (Landsat TM) from that same day. What follows is our 

analysis of the storm and its short-term effect on the reef and some speculation 

about how such storms affect the reef over longer time periods.

The Storm of November 27, 2001

 Rain began falling on Molokaÿi late on November 26, 2001, reaching 

a maximum rate of more than 2 cm/hr (0.8 in/hr) between 4 and 5 a.m. on 

November 27 and tapering off that afternoon (fig. 2). Total rainfall for the storm 

was nearly 10 cm (4 in) at Kaunakakai (National Weather Service Hydronet 

System, http://www.prh.noaa.gov/hnl/hydro/hydronet/hydronet-data.php, last 

accessed April 29, 2008). The rain gauge at Kamalö was not functioning during 

the storm, but by comparison with other storms (fig. 3), rainfall there was likely 

twice as high as at Kaunakakai, probably reaching levels near 20 cm (8 in). The 

previous three years had been relatively dry with little rainfall (fig. 3), and the 

high rainfall rate in the November 2001 storm overwhelmed possible infiltration 

rates and resulted in rapid transport of gravel, sand, and mud in large quantities 

down the steep slopes of south-central Molokaÿi. Overland flow and massive 

deposition on Highway 450 resulted in road closures and direct flow of muddy 

water onto the central Molokaÿi reef flat (fig. 4).

 Long-term rainfall records at both Kaunakakai and Kamalö show that 

these areas typically experience one or two months of heavy rainfall each 

winter, usually from Kona storms such as the one on November 27, 2001 

(fig. 3). The previous wet winter was 1996 –97, although the Kaunakakai 

gauge also recorded more than 15 cm (6 in) of rain in the summer of 1998. 

Wind and Waves During the Following Month

USGS instruments recorded wind speed and direction, as well as wave 

heights and periods (calculated as wave stress) on south Molokaÿi throughout 

the fall and winter of 2001; these values are shown in figure 5. The winds 

show a pattern that is typical for the period during and following a Kona 

storm (fig. 5A). Before and during the storm, winds blew towards the north 
Wave stress, in newtons

per square meter 

Wind speed,

in meters per second

0.4

0

0.8

10

0

10

Nov. 30 Dec. 5 Dec. 10

2001

Nov. 20 Nov. 25 Dec. 15 Dec. 20 Dec. 25

Measured wind speed

Wind direction and proportional speed

A

B

Kona storm with winds

from the southeast

Trade winds from the east

N

E

S

W

ReefProbe

Kaunakakai

Kawela

Kamal

Kamiloloa

Underwater

camera

MAP AREA

0 5 Kilometers

0 5 Miles

Figure 6. The Landsat satellite passed over Molokaÿi on November 29, two days 

after the heavy rainfall, and provided this image of highly turbid waters on the 

fringing reefs of south Molokaÿi. Note that at this time the muddy waters were 

concentrated on the reef flat from Kamalö to Kawela and farther west but had not 

yet reached the ReefProbe instrument site at Kamiloloa.

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

160

A

B

and northwest at speeds greater than 10 m/s (22.4 mph). Immediately after 

the storm, on November 28, the winds decreased in speed and shifted to a 

typical trade-wind pattern, with winds blowing dominantly toward the west. 

In the days that followed, from November 28 to about December 1, trade 

winds were variable in intensity. From about December 1 through December 

15, trade winds were consistently strong, as is typical during the trade-wind 

season, attaining speeds of 10 m/s (22.4 mph) to 15 m/s (33.6 mph) during 

the day and relaxing at night. This pattern of wind speeds and directions is 

important to the fate of sediment on the reef.

Wave stresses for November and December 2001 (fig. 5B) were calculated from wave heights as recorded by a wave gauge deployed at 10-m 

(33 ft) water depth on the fore reef off Kamiloloa (see Storlazzi and others, 

this vol., chap. 11). The measurements show high stresses starting before 

November 27, peaking that day, and steadily decreasing for the followFigure 4. Sediment transport and deposition resulting from the November 2001 

storm on Molokaÿi. A, Highway 450 near mile marker 7, showing sediment deposits 

on roadbed. Closures such as this are common along this stretch of road and typically occur about once a winter. B, Overland flow of sediment across Highway 450 

and onto the reef flat near mile marker 6. 

A

B

Figure 5. Physical forcing during the November and December 2001 storms on Molokaÿi. 

A, Wind speed and direction for the months of 

November and December 2001, as recorded 

by the U.S. Geological Survey weather station 

on the Kaunakakai Wharf. The brown lines 

point in the direction that the wind was blowing, and their lengths gives the proportional 

wind speed. Measured wind speed is shown 

in orange. Note the Kona wind pattern (blowing strongly north and northwest) on November 26 and 27, followed by trade winds blowing to the west that increase in strength after 

the storm. B, Wave stress (a measure of force 

per unit area) as recorded by the wave gauge 

at 10-m (33 ft) depth offshore Kamiloloa. Note 

that waves are strong (high stress values) during the storm on November 27 and subside 

significantly following the storm’s passage.

ing two days. Short bursts of high stress occurred sporadically throughout 

December, but never to the same level as recorded on November 27 and 28. 

This, too, is important to the fate of sediment on the reef.

Sediment Plumes on the Reef Flat

Critical information about the timing and extent of sediment plumes 

on the reef flat was provided by comparing satellite images (fig. 6), turbidity measurements (fig. 7), land-based photographs (fig. 8), and aerial 

photographs for the days before, during, and following the storm. The 

ReefProbe (see Ogston and others, this vol., chap. 20, for a description of 

this oceanographic instrument) was positioned on the reef flat about 5 km 

(3 mi) west of Kawela Gulch, one of the major sources of flood sediment 

to the coast. Salinity measurements at the ReefProbe show a large decrease 

in salinity on November 27 (fig. 7), immediately after the storm, as fresh 

water flowed across the reef flat and downwind to the west. Interestingly, 

the fresh-water pulse was not accompanied by a maximum in turbidity 

levels. Previous studies by Ogston and others (2004), detailed in chapter 

20 of this volume, found a strong correspondence between tide stage and 

turbidity. During high tides, water on the reef flat becomes deep enough 

for waves to form there and to resuspend fine-grained sediment; thus there 

exists a strong diurnal signal to the values of suspended sediment on the 

reef flat. During the five days following the heavy rains of November 27, 

turbidity rose sporadically, or spiked, several times because of the coincidence of large offshore wave energy and high tide. Turbidity values (fig. 7) 

during those five days were equal to or slightly higher than values that typically occur on the reef flat during normal (nonstorm) periods of high tides 

and moderate trade winds (Ogston and others, 2004; Storlazzi and others, 

2004; Presto and others, 2006). This indicates that much of the sediment 

discharged in the 2-day storm was deposited near the discharge points at 

Kamalö and Kawela to form deposits of terrigenous (land-derived) sediment along the shoreline and on the inner reef flat.

Following the Kona winds (blowing towards the northwest and north) that 

accompanied the rain on November 27, 2001, the winds gradually returned 

to a trade-wind-dominated pattern on November 28, 29, and 30 (fig. 5A). 

Satellite imagery, instrument data, and cameras mounted on a water tower on 

land captured this pattern convincingly (figs. 6, 7, and 8). Photographs taken 

from November 27 through 30 show that at the ReefProbe site at Kamiloloa 

the turbidity levels were apparently low (relative to the Kawela site) and 

that winds were also variable, as evidenced by the palm tree fronds hanging 

Lyndon Dela Cruz
Salinity,

in parts per thousand 

Turbidity,

in milligrams per liter 

Wave orbital veloctiy, in

centimeters per second

0

30

35

0

10

20

Relative sea level height,

in meters

0

0.5

1.0

1.5

0

50

100

Along-reef currents, in

centimeters per second  10

0

East

West

10

November 2001 December 2001

26 28 30

Figure

8A

Figure

8B

Figure

8C

2 4 6 8 10 12 14 16

D

C

B

A

E

Maximum instrument reading of sediment in the water column

Kawela Kamiloloa

Nov. 27

Nov. 29

Dec. 2

A

B

C

Figure 8. Photographs looking south taken from cameras mounted on water towers on the south coast of 

Molokaÿi show the westward movement of sediment-laden water under the influence of strong trade winds. A pair 

of photographs is shown, one from Kawela and one farther to the west at Kamiloloa, for each of the three mornings of November 27 (A, the day of the rain), November 29 (B), and December 2 (C), 2001. The reddish-brown 

mud is present in the water at Kawela on November 29 and finally at Kamiloloa on December 2 after several days 

of strong westward-blowing trade winds.

The Effects of a Kona Storm on the Molokaÿi Reef: November and December 2001

Impact of Sedimentation on the Molokaÿi Reef

161 

Figure 7. Tides, waves, currents, turbidity, and salinity for the period of November 25 to December 18, 2001, 

as recorded at the ReefProbe on the reef flat off Kamiloloa, south Molokaÿi. Note the marked drop in salinity 

that occurred shortly after the rainfall on November 27, and again on December 13. Of particular interest is the 

lag in turbidity after the November storm. Typical daily values of turbidity were seen in the 4 days following the 

flood event until about December 2; at that time strong along-reef currents appear to have transported large 

quantities of sediment in the water column, resulting in maximum values of turbidity. Note that on December 

14 there is another rain period, marked by the low salinity, yet trade winds were strong and there was no lag in 

turbidity. Photographs representing the time periods marked by the gray bars can be seen in figure 8. A, Sea 

surface height, or tidal stage. B, Wave orbital velocities. C, Along-reef (parallel to shore) currents (negative 

values show flow to the west.) Currents were measured at 0.2 m above the bed. D, Turbidity. E, Salinity. 
Gridded block

Nov 15, 2001 04:00 Nov 26, 2001 04:00 Nov 27, 2001 04:00 Nov 27, 2001 16:00 Nov 28 2001 00:00 

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

162

Was the Fore Reef Affected by the Sediment Discharge?

The fore reef off south Molokaÿi is a magnificent reef with luxuriant coral 

cover hundreds to thousands of acres in extent (see chapters 5 and 9 of this 

volume for a discussion of the coral and benthic habitats, respectively, found 

along the south Molokaÿi reef). An important question is how much of the 

high load of sediment that washes onto the Molokaÿi reef flat is transported 

across the reef crest and onto the fore reef. Again, the storm of November 27, 

2001, provided us an opportunity to learn more about that process during a 

large Kona storm, the typical kind of event that causes heavy rains on south 

Molokaÿi. Before the storm, instruments had been placed on the fore reef, as 

well as on the reef flat. The instruments on the fore reef included a rotating 

sediment collector with a wave gauge attached (see Bothner and others, this 

vol., chap. 19, for details about the instrument) and an underwater camera station (fig. 9) that recorded images of sediment collecting on a flat surface and 

the seabed six times daily. The data from these two systems show that sediment deposition on the fore reef is a complicated process and is not simply the 

result of plume fallout after heavy rainfall onshore.

Using a time series of photographs of the sea floor on the fore reef just 

west of the Kaunakakai Wharf, it was possible to document the timing of 

Figure 9. Bottom-mounted underwater camera 

on the fore reef west of Kaunakakai Harbor, 

Molokaÿi. The camera was located at a water 

depth of 12 m in a groove between two coral 

spurs (one visible in background). The camera 

took an image every four hours of the colony of 

Porites lobata (lobe coral) and a gridded concrete block, both beneath the frame. Scale is indicated by the size of the block (20.3

× 40.6cm; 

8× 16 in).

straight. Trade-wind strength gradually increased from November 29 through 

December 1 and reached maximum speeds on the afternoon of December 2. 

The steady and strong winds that started at the very end of November likely 

resulted in erosion of flood sediment that had been deposited near the shoreline 

at Kawela and other source areas farther east. The trade-wind-induced waves 

and currents then resuspended and transported the muddy flood sediment 

westward toward Kaunakakai. The pulse of muddy sediment moving westward 

along the reef flat is clearly shown by the maximum turbidity levels that were 

recorded at the ReefProbe site starting December 2 and by the land-camera 

images at Kamiloloa (fig. 8).

Satellite images (Landsat Thematic Mapper, or TM) with 30-m resolution 

and aerial images with 0.6-m resolution were collected on November 29, 2001, 

to detect and map the distribution, concentration, and total amount of suspended sediment over the south Molokaÿi reef. Two large suspended-sediment 

plumes were evident on November 29 (fig. 6), one on the west end of the south 

coast and a much larger one along the south-central coast, and their suspendedsediment concentrations (SSC), determined from water samples collected 

from the upper 0.5 m, ranged from 30 to 410 mg/L. These values were used to 

develop a model relating digital values from the satellite images to SSC, and 

the amount of suspended sediment in the upper 0.5 meter of water was then 

computed. Calculations using the TM images show that the total amount of 

suspended sediment in the upper 0.5 m of water on November 29, 2001, was 

approximately 1,500 tons (Chavez and others, 2004).

The significance of this series of events to the health of the reef is 

twofold. First, it is now evident that even a moderate amount of rain results 

in substantial transport of mud, sand, gravel, and associated debris directly 

from land to the reef flat. Second, the mud component is not moved 

entirely or even mostly in a plume across the reef flat and into deeper 

water. Rather, most of the mud is stored along the shore and inner reef flat 

near its on-land source. There it becomes a long-lasting source of turbidity 

as trade-wind waves resuspend the deposits nearly every day and slowly 

move the particles westward by means of trade-wind-driven currents.

high-turbidity events and thus relate them to the events that were occurring 

along the coast. The pictures in figure 10 show that, starting on November 27 at 4 a.m. and continuing for the next 24 to 36 hours, the amount of 

suspended sediment was so high that visibility was greatly reduced in water 

that is normally quite clear, probably because of local sediment resuspension. Importantly, the water began to clear late on November 28. The satellite image obtained on November 29 shows that even by that date sediment 

plumes had not moved a significant distance along the reef flat, let alone 

onto the fore reef. Thus the high level of turbidity here was related to waves 

and not to plume fallout. However, at other locations along the reef, closer to 

discharge points, plume fallout may have generated turbid water.
Nov 28 2001 12:00 Nov 29 2001 00:00 Nov 29 2001 04:00 Nov 29 2001 08:00 Nov 29 2001 12:00

The Effects of a Kona Storm on the Molokaÿi Reef: November and December 2001

Impact of Sedimentation on the Molokaÿi Reef

163 

If sediment plumes had not reached the fore reef by November 27 and 

28, then what caused the distinctive high levels of turbidity observed by 

the underwater camera? One possible, and likely, source is wave action. On 

November 26 through 28, large waves hit the south Molokaÿi coast. Bottom shear stresses from waves such as occurred on those dates are capable 

of resuspending sand and mud particles that lie in areas normally too deep 

or protected from wave disturbance. Thus it is likely that the large southerly swell arriving at south Molokaÿi on November 26 was responsible for 

resuspending large volumes of sediment and causing high levels of water 

turbidity before the products of the on-land rainfall could be dispersed to 

the fore reef.

Additional evidence supporting such a scenario was provided by sediment collected in traps that had been placed on the reef flat and the fore 

reef before the storm. In addition to the rotating sediment trap described 

above, single 0.5-m tube traps (10-cm diameter) were placed at various 

locations on the reef for a three-month period (November 15, 2001, to 

February 14, 2002). Single tube traps from both ends of the island (Hale 

O Lono and Pükoÿo) were completely filled during this three month fallwinter period, but analysis of the sediment in the traps indicates that its 

source was chiefly from local reef areas, with only a small (~10 percent) 

amount contributed from land (Bothner and others, 2006). The evidence 

for this is that the material filling the traps is about 90 percent calcium 

Figure 10. This series of bottom photographs (taken with the underwater camera near Kaunakakai Harbor, see 

figure 9) shows the timing of periods of turbidity on the fore reef that can be compared with the land-camera images of the reef flat shown in figure 8. The images show clear water and a sediment–free block surface at the time 

of deployment in mid-November 2001. Black lines on the concrete block are 2 cm apart. Before the rain event in late 

November, the block had become covered by sediment. The water became very turbid beginning at 4 a.m. on November 27, as rains just began onshore, and remained turbid for approximately the next 24 hours. This interval coincides 

with the period of large waves that accompanied the storm. By November 29, the water is relatively clear again, there 

is less sediment on the block surface, and what sediment is present appears coarser than before the storm.

carbonate, which can only be derived from the reef, and is coarse (nearly 

80 percent sand).

The time-series trap sample number 3 at Päläÿau (fig. 11) was collected during the storm period and was full of sediment. Because the 

sampling period was 4.5 days, and because the runoff and large waves 

occurred at nearly the same time within this period, which process was 

most responsible for the high sample volume is difficult to determine. That 

is, how much of the sediment collected was contributed by wave-induced 

resuspension of sediment on the fore reef, and how much was contributed 

by direct settling from sediment plumes emanating from the coast?

The nature of the sediment filling the tubes provides some answers. 

Trap sample number 3, which included the storm period, had about the 

same (or lower) amount of terrigenous sediment as prestorm samples 

(about 24 percent). At Kamiloloa, another rotating sediment trap showed 

that the terrigenous sediment there increased from approximately 34 percent in prestorm samples to 39 percent in poststorm samples during periods 

with similar wave conditions. This increase is relatively small and does not 

indicate massive sedimentation from plume fallout. The sediment tube trap 

on the fore reef off Kamiloloa had a distinctive layer corresponding to the 

storm, but even that layer is unlikely to result solely from plume fallout. It 

is composed of only 27 percent terrigenous sediment, and therefore more 

than two-thirds of it was derived from the reef itself. Furthermore, 60 percent of the sediment in that layer is sand, which is not easily transported by 

surface currents. The composition of sediment caught in these tube traps is 

persuasive evidence that wave resuspension, not sediment discharge from 

streams, is the key factor causing turbidity on the reef. Recent work (Bothner and others, 2006) shows a strong relation between sediment collection 

and wave stresses, even in periods of no runoff.

How Kona Storms Generate Turbidity 

on the Molokaÿi Reef

Our study of the November 27, 2001, Kona storm on south Molokaÿi has 

demonstrated that such events contribute sediment and turbidity to the reef 

flat and fore reef, but they do it in a complex manner. The waves that accompany the weather fronts are a major factor in resuspending both terrigenous 

and reef-based sediment and transporting it on the reef, where it deposits on 

corals and other benthic organisms and inhibits photosynthesis. Heavy rains 

produce floods in intermittent streams and gullies as well as overland flow at 

the coast. In the November 2001 storm, large amounts of terrigenous sediment were deposited along the coast and on the reef flat, where they became 

available as a source for later resuspension. These events and resulting conditions are summarized in figure 12. 

The sequence of natural processes that occurred before, during, and following the November 27, 2001, Kona storm provide a solid basis for understanding the natural cycle of ridge-to-reef sedimentation in this environment. These processes are neither unusual nor extreme, and they most likely 

represent events that happen one or more times annually. The conditions that 

led to high sediment mobility and high turbidity on the reef in November and 

December 2001 were as follows:

• On land: A high rate of rainfall (10 cm, or 4 in, within 24 hours) 

led to high water and sediment runoff onto the reef. The high 
November 2001 December 2001

Winds

Waves

Onshore

events

Reef-flat events

downwind of

discharge

sites at shore

Fore-reef

events

Alongshore trade winds

Strong onshore

Kona winds

Low

salinity

Resuspension of sediment by trade winds

Clear

water

Maximum

turbidity

High turbidity

from storm runoff;

full sediment traps

Heavy rain

and runoff

Rapid deposition

27 28 29 30 1

8

6

4

2

0

2

1

0

30

20

10

0

1.2

0.8

0.4

0.0

Rainfall, in centimeters per day

Sediment-trap collection rate, in killograms

per square meter per day

 Terrigenous sediment in trap, in percent

Wave-induced stress, in newtons per square meter

11 15 19 23 26 30 4 8 12 16 20 

November 2001 December 2001

Bottle 1 Bottle 5 Bottle 2 Bottle 4

Bottle 3

Bottle 6 Bottle 7 Bottle 8

A B

C

75 cm

Sediment trap collection

Rainfall

Wave-induced stress

Terrigenous sediment

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

164

Figure 12. Schematic diagram summarizing the series of events in late November 

2001 that influenced turbidity and sediment transport on the south Molokaÿi reef off 

Kamiloloa. The timing of observations of turbidity on the reef flat and the adjacent fore 

reef are important, as they document that the two settings are somewhat independent, or decoupled, in the short term. Rainfall and wave stress were both at maximum 

values on November 27. Although it took several days for terrestrial sediment to have 

an impact on the reef flat, wave-induced turbidity was immediate on both the reef flat 

and the deeper fore reef. On the reef flat, sedimentation and maximum turbidity came 

later than on the fore reef, as terrigenous flood deposits were gradually and increasingly resuspended by trade-wind waves, transporting sediment west along the reef flat 

by means of trade-wind currents.

sion and high turbidity. Macro algae and silt increased markedly for at least 

six months after the storm (Stamski and Field, 2006). 

On the fore reef, the storm was characterized by high wave stresses and 

onshore winds, resulting in temporary high turbidity and temporary deposition. Because periodic high wave stresses subsequently resuspended and 

transported the sediment, there was apparently no significant long-term storage of fine-grained terrestrial sediment on the fore reef.

Figure 11. Sediment-collection data from the rotary sediment trap at Päläÿau, Molokaÿi. Each tube from the rotating sediment 

trap on the fore reef represents 4.5 days of sediment collection (see Bothner and others, this vol., chap. 19). A, The sediment 

trap in place on the sea floor (photograph taken on the fore reef off Kamiloloa). B, The Päläÿau sediment-collection tubes 

aligned in chronological order. Tube number 3 represents the time period of the Kona storm. C, Sediment-collection rate for 

each tube plotted against rainfall (blue), wave stresses (black), and percent noncarbonate in the collected sediment (orange). 

Although the high rate of sediment collection (as shown by the filled tube number 3) corresponds with the heavy rainfall, it also 

correlates with the period of high wave stress. The lack of significant change in the amount of terrestrial sediment (noncarbonate) in the tubes indicates that wave resuspension was the significant factor in filling the tubes, not fallout from flood plumes. 

sediment runoff was likely exacerbated by land-use practices 

and a preceding dry period (~3 years). The sediment reached the 

coast and inner reef by means of gullies and overland flow, and 

much of it was deposited near the discharge points at the coast.

• On the reef: Waves were high before and during the heavy 

rainfall, then dropped back to normal levels. During the storm, 

large storm waves caused resuspension of previously deposited sediment, leading to turbidity on the fore reef. Winds were 

onshore; trade winds were mild to nonexistent and only slowly 

became reestablished in the days following the storm. The return 

to trade-wind conditions following the storm caused renewed 

resuspension and along-reef transport of flood sediment.

These conditions led to a decoupling of sedimentation on the reef flat 

and on the fore reef off central Molokaÿi. The reef flat was characterized by 

initial high levels of deposition at coastal deltas and adjacent areas. Following the storm, trade winds became reestablished, leading to daily resuspen-
Summary

Sunset from south Molokaÿi, with a rain cloud over the island of Lanai.
Spyridia filamentosa red algae on the Molokaÿi reef flat.
Evolution of the South Molkaÿi Fringing Reef Summary

SUMMARY

The Molokaÿi Coral Reef Today, and Alternatives for the Future

Michael E. Field1, Susan A. Cochran1, Joshua B. Logan1, and Curt D. Storlazzi1

From the contributions collected in this publication have emerged two 

important observations that have significance locally, nationally, and 

internationally. First, the fringing coral reef along the south coast of 

Molokaÿi is one of the most extensive and luxuriant reefs in the eight main 

Hawaiian Islands. It is longer and more continuous and has denser coral cover 

than reefs at any of the other islands—this alone makes it a state and national 

treasure worthy of study and protection. The second observation is more 

sobering: sections of the south Molokaÿi reef have been damaged in the past 

by sedimentation ultimately caused by human activities in adjacent watersheds. Although some of those activities are no longer taking place, their 

lingering effects are still being felt in the form of excess sediment runoff. We 

here review the basis for each of these observations, discuss how sedimentation affects the reef, and summarize the choices that are faced by the people 

of Molokaÿi and the State of Hawaiÿi with regard to the reef’s future.

The Magnificent Coral Reef of South Molokaÿi

The early chapters of this book describe the expansiveness, diversity, 

and beauty of the fringing reef off south Molokaÿi. Except for Molokaÿi 

locals, infrequent scuba divers, and occasional visiting marine scientists, 

few people have understood the nature and importance of the reef. As a 

result of recent studies, we now know a great deal more about the reef—

from lidar bathymetric surveys, aerial photographs, diver inventories, and 

instrument measurements. The south Molokaÿi reef is a living, growing 

structure stretching along most of the island’s south coast, from Hale O 

Lono on the west to Kamalö on the east, a distance of 40 km (25 mi). This 

stretch of coast is sheltered from the North Pacific swell by the island 

itself. Observations and mapping presented in this publication (see in particular Jokiel and others, this vol., chap. 5, and Cochran, this vol., chap. 9) 

document that much of the reef is an exceedingly rich and diverse marine 

benthic habitat, as exemplified by figure 1. Nearly all of the coral species 

known from the Hawaiian Islands, as well as several soft corals and an 

octocoral, are present on the Molokaÿi reef. 

The Molokaÿi reef is a classic fringing coral reef with a wide, shallow 

reef flat; a well-developed reef crest at the seaward edge of the reef flat; 

and a biologically rich fore reef reaching to depths of 27 to 32 m (90 to 

1

 U.S. Geological Survey Pacific Science Center, 400 Natural Bridges Dr., Santa Cruz, CA 95060

105 ft). As noted by Jokiel and others (this vol., chap. 5), the Molokaÿi coral 

reef hosts a rich and diverse marine benthic community dominated by five 

reef-forming corals: Montipora capitata, Montipora patula, Porites compressa, Porites lobata, and Pocillopora meandrina. The fore reef consists of 

coral ridges, or “spurs,” separated by sand channels, or “grooves,” In areas 

of dense coral coverage, the tops of the spurs are dominated by a Porites

compressa (finger coral) community and the steep sides are dominated by a 

Montipora spp. community. Live coral is abundant on the fore reef, and the 

highest percentages, commonly in excess of 70 percent, are found at depths 

between 5 and 15 m (16 to 50 ft) along the entire reef and locally at depths 

between 20 and 25 m (66 to 82 ft). 

The “blue holes” that characterize the Molokaÿi reef east of Kamalö 

appear to be unique within the main Hawaiian Islands. Some of these blue 

holes are open on one side toward the deep ocean, and others occur in the 

middle of the reef and thus are isolated from direct oceanic flow. Many blue 

holes, which are rimmed at the surface by Porites compressa, extend from 

near the water surface to depths of 25 m (83 ft) or more. The steep walls that 

form the perimeter of the blue holes are commonly covered by a lush coral 

growth and a vertically zoned habitat unique to these formations.

Many fishes observed in residence on Hawaiian reefs are important to the 

overall health of the coral reefs and ultimately of the islands. As Friedlander 

and Rodgers (this vol., chap. 7) point out, “fish provide food, cultural identity, 

and commerce to a broad majority of the local population of Molokaÿi and are 

an integral component of the marine environment.” Both the herbivorous fish 

that feed on algae and the predatory fish that feed on other fishes are critical to 

the overall health of the Molokaÿi coral reef. The herbivores help in the prevention of algal overgrowth on corals and the spread of alien and harmful algae. 

Predators play an important role in maintaining turnover rates and overall 

productivity. Finally, a comment by Friedlander and Rodgers (this vol., chap. 

7) is particularly relevant to the present state of the reef: 

The broad reef fringing the south shore of Molokaÿi is probably the most 

productive reef flat in the main Hawaiian Islands for the harvest of reef 

fishes and invertebrates. The fish assemblage represents a diverse fauna that 

is healthier than those in more heavily exploited urban areas around the state, 

but it has shown signs of decline in overall health and quality over time.

There are many factors that can cause fish declines on coral reefs. For the 

Molokaÿi reef, one such factor may be the decline in the overall condition of 

the reef because of sedimentation.

Sediment as a Primary Threat to the Reef

As with most coral reefs around the world, the long-term health of the 

reef off south Molokaÿi faces numerous threats, most of which are induced by 

humans. On many Pacific and Caribbean coral reefs, rising sea-surface temperatures cause bleaching and enhance disease, overfishing alters the food web 

and allows competing algae to thrive and smother coral, and nutrification from 

runoff and from sewage-contaminated ground water also supports algal growth 

at the expense of coral growth (see, for example, Rogers, 1990; Hughes and 

others, 2003; Pandolfi and others, 2003; Richmond and others, 2007). 

Degradation of coral reefs, as pointed out by Pollnac (2007), Pandolfi 

and others (2003), and other authors, is often related directly to human habitation (fig. 2). Progressive degradation has occurred throughout reefs in the 

Pacific Ocean, Red Sea, and Caribbean Sea as nearby societies have shifted 

from subsistence agriculture to colonial development to modern development. In the analysis by Pandolfi and others (2003), reefs of the eight main 

Hawaiian Islands are identified as being nearly 60 percent degraded (fig. 3).

Figure 1. Underwater photograph taken off Kamalö, south Molokaÿi, showing a 

healthy fore reef. The coral colonies are multicolored (a sign that they are alive), 

and algae and fine sediment are noticeably absent.
168

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

of the reef at depths of about 27 m (90 ft) up to shallow depths near the reef 

crest. Between the Kaunakakai Wharf and One Aliÿi, however, the pattern is 

distinctly different: coral was severely degraded or absent at all water depths 

in a 1-km swath off the wharf; and for the entire distance from the wharf to 

One Aliÿi, coral cover was absent or nearly absent on the upper fore reef at 

depths of 5 to 10 m (16 to 33 ft) (fig. 4A). Recent observations in 2007 (fig. 

4B) established that at even shallower depths (< 5 m) coral was very sparse 

or absent, the bottom consisted of bare rock, reef rubble, and sand, and the 

water was chronically turbid. Observations west of the Kaunakakai Wharf 

and east of One Aliÿi show that coral is relatively abundant and the water 

clear at those same depths. The coral-poor area between the Kaunakakai 

Wharf and One Aliÿi coincides with the movement of chronically turbid 

waters westward and off the reef flat (Ogston and others, this vol., chap 20; 

Ogston and others 2004; Presto and others, 2006). 

The link between coral health and sedimentation is more evident on the 

expansive reef flat bordering south Molokaÿi. The entire reef flat between 

Päläÿau and Kamalö bears evidence of damage from sedimentation. West of 

Päläÿau, the reef flat has healthy coral growth within 50 m of the shoreline 

and, on the very west end of the island, the limiting factors appear to be 

wave stresses and abrasion from the Southern swell and North Pacific swell. 

East of Kamalö, the coast and reef are more exposed to waves, and the less 

extensive coral cover appears to be a consequence of waves breaking on the 

reef flat. (Storlazzi and others, this vol., chap.11).

The evidence for sediment damage on the central Molokaÿi reef flat 

between Päläÿau and Kamalö is both circumstantial and direct. The circumstantial evidence consists of observations of relatively thick deposits 

of muddy sand along the inner reef flat, photographs and measurements of 

turbid water, and an apparent lack of coral. The connection between these 

characteristics seems clear, but without direct documentation of poor coral 

condition caused by excess sediment, it remains circumstantial. The direct 

evidence comes from measurements of those factors, specifically coral cover 

and suspended sediment concentrations, made in April 2005 between the 

Kaunakakai Wharf and Kamalö (fig. 5A,B). Within 300 m of the shoreline, 

sediment concentrations greatly exceeded 10 mg/L, a number commonly 

used as the upper limit for turbidity for maintaining healthy corals (Rogers, 

1990). Concentrations at many of the inshore sites exceed 20 mg/L, a level 

found to be lethal at other reefs (Hodgson, 1997; Hodgson and Dixon, 1992). 

Another factor is time—even low concentrations of suspended sediment 

over extended time periods can cause coral death (Hubbard, 1997). Virtually 

no coral is present within 300 m of the shoreline between the Kaunakakai 

Wharf and Kamalö. The first measurable coral cover, about 2 percent, 

appears about 400 m from shore, and coral cover gradually increases to 

about 10 percent (still a low level) on the central to outer reef flat 600 m 

from shore (Rodgers and others, 2005).

The low to absent coral on the inner reef flat between the Kaunakakai 

Wharf and Kamalö is striking. Evidence presented in earlier chapters on the 

abundance of algae and the low abundance of fish, when combined with the 

information on low coral cover and chronic high levels of sedimentation, 

makes it clear that this magnificent reef has been degraded and faces ongoing threats to its well-being as an ecosystem.

Why the Well-Being of the Reef Matters

Our comments above have sought to recognize that the Molokaÿi coral 

reef has major significance in terms of its intrinsic beauty, biodiversity, 

cultural importance, and as a local food resource. We have also endeavored 

to summarize findings that we and our colleagues interpret as evidence that 

parts of the reef are impacted by sediment, and that other parts (conceivably 

the entire reef) are threatened by sediment runoff from the land. Degradation 

of coral reefs is not limited to Molokaÿi, nor is it limited to Hawaiÿi— it is 

occurring on a global scale (Brown, 1997; Hughes and others, 2003a). Sediment runoff is one of the major causes of global reef decline (Pandolfi and 

others, 2003; Bellwood and others, 2004; Fabricius, 2005; Richmond and 

others, 2007). Our comments here are intended to place the potential loss 

of the Molokaÿi reef in perspective, for loss of the living reef would forever 

alter Molokaÿi. Coral constructs a complex reef that hosts a diverse biota, 

protects the shoreline from erosive waves, and provides food for locals. 

When coral dies in significant quantities, the food web shrinks, erosion of 

the old reef reduces its effectiveness as a barrier, and what once was a thriving complex ecosystem becomes a surface dominated by alga and sediment.

Although no one would argue that the survival of people on Molokaÿi is 

dependent upon actions taken in the watershed and on the reef, the situation 

Figure 3. Trends in ecosystem conditions over the past five centuries for coral reefs 

in the Pacific Ocean, Caribbean Sea, and Red Sea (modified from Pandolfi and 

others, 2005). The arrow shows the general trend, from 20 to 30 percent degraded 

in the 16th century to 30 to 70 percent degraded at present. Note that the main 

Hawaiian Islands are now summarized as being nearly 60 percent degraded. 

Figure 2. This graphic diagrammatically portrays the interplay between human 

civilization and coral reefs through time. As societies have grown and spread 

throughout tropical regions, there has been a corresponding decline in reef 

health, including depletion of larger fish stock from overfishing, and degradation 

from runoff of sediment and other anthropogenic pollutants has occurred. The 

path to recovery is uncertain. Few truly pristine coral reefs remain in the world 

today, some of which reside in the Northwestern Hawaiian Islands (from Pandolfi 

and others, 2005; image credit Mary Parrish). 

At Molokaÿi, the damage from sediment-laden runoff from the land 

ranks as a first-order cause of reef degradation. By blocking light needed for 

photosynthesis, covering sites of potential coral growth, providing nutrients 

for harmful algae, and directly smothering coral polyps, sediment inhibits or 

prevents coral growth. However, even on this reef, where turbid red-brown 

water along the central coast of south Molokaÿi is considered the norm by 

locals and visitors alike, the linkage to declining coral health on the fore reef 

is not easily established. On the reef flat, though, the linkage is better defined.

Monitoring on the fore reef at depths of 3 m (10 ft) and 10 m (33 ft) for 

the past five years has produced confusing results. Monitoring sites on the 

reef along the Molokaÿi south shore have both some of the highest and some 

of the lowest levels of coral cover found in Hawaiÿi. Brown and others (this 

vol., chap. 6) and Jokiel and others (2004) document that sections of the reef 

around Kamalö contain some of the most densely packed coral communities 

in Hawaiÿi. However, they also point out that the Molokaÿi reef, judging from 

the sites that were surveyed, may not be faring as well as other sites in the 

State. The six Coral Reef Assessment and Monitoring Program (CRAMP) 

stations at Molokaÿi experienced the largest decline in coral cover (by 

island) between 2000 and 2002, compared to the other 54 CRAMP stations 

on Kauaÿi, Oÿahu, Kahoÿolawe, Maui, and Hawaiÿi (Brown and others, this 

vol., chap. 6; Jokiel and others, 2004). Declines on other Hawaiian Islands 

can be roughly correlated with rising human population, but on Molokaÿi 

the population is relatively low and stable. Here, the impacts to watersheds 

from farming, ranching, feral grazers, and other causes of soil loss (Roberts 

and Field, this vol., chap. 15) are a strong indication that sediment yield is a 

cause of coral decline at selected locations on the fore reef.

Visual surveys of the reef condition and estimates of coral cover were 

made along the fore reef by Jokiel and others (this vol., chap. 5) and are 

reproduced here in figure 4 because of the importance of this survey to 

understanding the condition of the reef. Previous work has determined that in 

most places on the Molokaÿi reef, coral growth is continuous from the base 

100 Degradation, in percent

80

60

40

20

0

Year

Agricultural Colonial occupation Colonial

development

Early

modern

Late

modern

The

future

1500 1600 1700 1800 1900 2000

Main Hawaiian Islands

Major world coral reefs

?
The Molokaÿi Coral Reef Today, and Alternatives for the Future

169 

Evolution of the South Molkaÿi Fringing Reef Summary

Figure 4. Variation of coral abundance along the south Molokaÿi reef. A, Coral abundance on the fore reef along the south shore 

of Molokaÿi in 2001, based on single point observations (dots in upper panel) in about 10 m (33 ft) of water every 2 km (1.24 mi) 

along the length of the reef (Jokiel and others, this vol., chap. 5). Note the very high total coral abundance from Hale O Lono on 

the west end of the island to east of Kamalö, with the pronounced drop in coral cover between the Kaunakakai Wharf and Kawela. 

B, An aerial photograph overlaid on shaded topography gives a 3D perspective view of the Molokaÿi reef between Kaunakakai and 

One Aliÿi and shows results of detailed follow-up surveys in 2002 and 2007. Observations made on scuba dives in the area of low 

coral cover from the 2001 survey showed that the entire upper fore reef at depths of 10 m (33 ft) in this area has low coral cover of 

~15 percent (shown in red on the image). Coral cover in deeper water is high (>80 percent), except off the 3-km reef segment east 

of the wharf at Kaunakakai, where coral is degraded or absent. Distance along bottom of image is about 6.5 km (about 4 mi).

might be different if the island were isolated geographically, culturally, and 

economically from other human populations. Molokaÿi is part of the county 

of Maui, the State of Hawaiÿi, and the United States. As such, even if the reef 

were to be totally destroyed, the Molokaÿi community would remain intact, 

at a reasonable standard of living. But the community of Molokaÿi and all 

of Hawaiÿi would be far poorer—culturally and biologically—from the loss 

of this reef. The most intractable issue is the question of habitat damage and 

potential loss of a resource that has immeasurable intrinsic and cultural value.

In his seminal book, "Collapse: How Societies Choose to Fail or Succeed," Jared Diamond (2005) identifies four ways that societies fail to 

respond to ecological damage:

First of all, a group may fail to anticipate a problem before that problem 

actually arrives. Second, when the problem does arrive, the group may fail 

to perceive it. Then, after they perceive it, they may fail even to try to solve 

it. Finally, they may even try to solve it but may not succeed. (p. 421)

How then do Diamond’s four categories apply to Molokaÿi? Gradual 

landscape change is difficult to detect, and it is likely that early Hawaiians did not anticipate or recognize changes in watershed processes and the 

downstream effects that resulted from cutting hardwood trees and cultivating crops. It may be that the problem was perceived during the last century, 

but that few actions were taken to remediate the effects from these activities 

(planting of mangroves at Päläÿau was one such action). Finally, it may be 

that some actions were undertaken to offset soil erosion and loss, but that 

these actions largely failed because of the complex nature of the problem.

The key issue for avoiding environmental disasters is the recognition that 

change is occurring. Such recognition is often difficult—perceptions by residents of an area tend to be distorted by “shifting baselines” (Pauly and others, 

1998) or “creeping normalcy” (Diamond, 2005), slow trends that are masked 

by large and irregular year-to-year variations. For example, a 75-year-old resident of Molokaÿi may recall reef waters off Kamiloloa as being clearer in his 

youth, but 45-year-old and 15-year-old residents will probably disagree with 

each other and with the older resident about what is “normal.” Over time and 

through generations, humans tend to develop “landscape amnesia” (Diamond, 

2005), forgetting original environmental conditions because change is gradual. 

Thus the potential exists for people to accept the degraded coral, depleted fishery, abundant algae, and turbid water as normal.

It need not be so, and there is encouraging evidence that, with enough will 

and local support, the condition of the reef can improve. The introduction of mangroves at Molokaÿi in 1902—itself an action with its own environmental consequences—is direct evidence that successful actions can be taken to preserve and 

protect the reef. What future actions will be taken, and will they be successful?

What’s Next for the Molokaÿi Coral Reef?

Alternative scenarios are possible for the future of the reef, besides continued decline. The inhabitants of Molokaÿi, along with resource managers at the 

State and Federal levels, share responsibility for the protection and restoration 

of the reef. The sheltered setting of south Molokaÿi and the variety and abundance of bottom types—pinnacles, fore reef, channels, blue holes, reef flat—

Coral cover, in percent

Longitude

0

40

80

157¡10' 157¡ 156¡50'

Total cover

Porites lobata

Porites compressa

Montipora spp.

Pocillopora meandrina 

21°

 10' 

N 

 21°

 05' 

100

100

100

100

300

300

500

500

100

300

Kaiwi Channel

Kalohi Channel

Kaunakakai

Wharf

Kamiloloa

Kamal

Kalaupapa

Hale o Lono

ØìLIO POINT

LØAU

POINT

HLAWA

POINT

Mauna Loa

PkoØo

Kmimi

One AliØi

Wai a Kne

PlØau

Kawela

FIGURE 4B

P A C I F I C O C E A N

0 2 4 Miles

0 4 Kilometers

157°10' 157° 156°50'

Pailolo Channel

Vertical exaggeration =  5

Contour interval = 10 m

B

A

Kamiloloa

Kaunakakai

One Ali`i
170

THE CORAL REEF OF SOUTH MOLOKAÿI, HAWAIÿI—PORTRAIT OF A SEDIMENT-THREATENED FRINGING REEF

The ultimate fate of the coral reef off south Molokaÿi has not yet been determined (fig. 6). It is for the people of Molokaÿi, along with appropriate support at 

the county, State, and Federal levels, to take the necessary measures to safeguard 

this very special place—the reef of south Molokaÿi—for future generations. 

First steps could include support of efforts by nongovernmental and government groups to control fires, goats, and other factors that contribute to excess 

sediment runoff. Preserving the reef for generations of Molokaÿi and Hawaiÿi 

residents can ultimately be best achieved by designating a large section of the 

reef as a marine protected area, one that is shielded from abuse by overfishing, 

sediment runoff, and other impacts. 

The fate of reefs receiving large quantities of sediment from adjacent 

landmasses is well studied. Woolfe and Larcombe (1998, 1999) discuss how 

reefs transform from healthy, accreting reefs to ones marked by declining 

health because of increased sediment input. The positions of the Molokaÿi 

reef flat and fore reef in such a scenario are well known at present, but the 

future pathway will depend on whether sedimentation continues unchecked 

or whether local actions are successful in significantly reducing the amount 

of land-derived sediment reaching the reef.

As long as large quantities of sediment are delivered annually onto the 

Molokaÿi reef flat, and as long as the reef is unprotected from overfishing, anchor 

damage, and other local stresses, it will be at risk. Climate change will likely place 

additional stresses on the coral communities through more acidic and warmer 

ocean waters (Hoegh-Guldberg and others, 2007), but that is a factor that cannot 

be controlled locally. The other stressors can be controlled locally, through strong 

sediment-management methods and establishment of large marine protected 

areas on the reef. As Hoegh-Guldberg and others (2007) point out, one important 

approach for offsetting the impacts of climate change on reefs is to reduce the 

influence of local stressors, such as suspended sediment in overlying waters. 

Some steps are being taken locally to limit the impact of sediment on the 

reef. The Molokaÿi Watershed Advisory Group, The Nature Conservancy, the 

Hawaiÿi Local Action Strategy of the U.S. Coral Reef Task Force, the U.S. Geological Survey, and other groups and agencies have all made concerted efforts 

to understand the factors that cause significant sediment runoff to the reef and to 

implement measures to limit the runoff. Eliminating feral goats, limiting wildfires, 

and guarding against harmful coastal and watershed construction activities all 

help to improve the situation, but these measures by themselves are not enough.

Figure 6. Two photographs of coral habitats that represent alternative pathways 

for the future of the Molokaÿi reef. A, Sediment and algae covering the surface 

have led to a decline in coral growth and its demise in places such as shown 

here, most notably on the reef flat and upper fore reef off central Molokaÿi. If land 

use and sediment runoff are not controlled, this may be the fate of large portions 

of the reef. B, Much of the Molokaÿi reef today remains healthy, as seen in this 

photograph. Reduction in sediment loads to the reef will help preserve this type 

of setting—and perhaps allow damaged areas to recover to a healthier state.

Figure 5. Turbidity of water on the south Molokaÿi reef flat and its relation 

to coral cover. A, Aerial mosaic of the central south Molokaÿi reef flat, 

between the Kaunakakai Wharf and Kamalö, with superimposed images 

of round filter papers containing the residue from filtering 1 to 2 liters (~1 

to 2 quarts) of water. Samples were collected on April 7, 2005, at distances of 50, 100, 250, 400, 550, and 700 m from the shoreline. The filter 

papers are arranged on the image from nearshore to offshore along the 

sampling transects, but are shown non-overlapping for visual purposes. 

The darker brown color indicates that the nearshore waters contain relatively high levels of silt particles, which are derived from the island slopes 

(as confirmed by chemical tests). The highest concentrations of landderived particles are on the inner reef flat. B, Concentrations of sediment 

in water on the Molokaÿi reef flat between the Kaunakakai Wharf and 

Kamalö, and their relation to coral cover, from field surveys and samples 

collected in April 2005. Suspended sediment concentrations are high 

near the shoreline (typically 20 to 70 mg/L) and decrease to 10 mg/L 

or less about 400 m from the shoreline. A concentration of 10 mg/L is 

considered the upper limit at which coral can grow successfully (Rogers, 

1990). Also shown on this figure is the average coral cover on the reef 

flat, as mapped by Rodgers and others (2005). No appreciable coral exists within ~400 m of the shoreline, and at 400 m it is less than 5 percent. 

Coral cover gradually increases seaward to values of ~10 percent, which 

are still low compared with most other places on the reef.

have given rise to a rich coral setting. As Jokiel and others (this vol., chap 5) 

observe, diverse habitats in this area support rich coral reef communities.

The reef of south Molokaÿi has long served as a food, cultural, and 

recreational resource for the people of Molokaÿi, but its ability to do so in the 

future is not guaranteed. Many residents have noted that the nearshore waters 

are browner and the food resources reduced compared to earlier years. Friedlander and Rodgers (this vol., chap. 7) noted that fish resources are fewer and 

that management starting at the very local level was required:

In addition, the men and women of the fishing community have expressed 

concern about the declining size and abundance of several important 

resource species such as kümü and moana kale (blue goatfish, Parupeneus cyclostomus).

The coral reef off south Molokaÿi has been damaged—in some places 

severely—by excess sediment runoff from the land, particularly between the 

Kaunakakai Wharf and Kawela. Without protection of the reef from overuse 

and from excess sedimentation, its future is in jeopardy. Brown and others 

(this vol., chap. 6) point out that only 5 percent of Hawaiian reefs are in a 

threatened stage at this time but that portions of the Molokaÿi reef make up 

a significant part of the threatened group. Continued overuse of the reef for 

food sources, along with sediment influx from the land, will lead to continued degradation of the Molokaÿi coral reef ecosystem.

0

20

40

80

100 200 300 400 500 600 700

0

5

10

15

Distance offshore, in meters

Average suspended sediment

concentration, in milligrams per liter

Coral cover, in percent

Suspended sediment Coral cover

Kaunakakai

Kamal

0 4 Kilometers

0 2 Miles

B A

C

D

E

F

H G

I

A

B

A

B
171 

Glossaries

Acronyms

ANOVA analysis of variance (a statistical analysis procedure)

CCA crustose coralline algae

CRAMP Coral Reef Assessment and Monitoring Program

CRTF United States Coral Reef Task Force

CTD instrument that measures water conductivity (salinity), temperature, 

and depth

DAR State of Hawaiÿi Division of Aquatic Resources

DEM digital elevation model

DLNR State of Hawaiÿi Department of Land and Natural Resources

DOC dissolved organic carbon

DOI United States Department of the Interior

ENSO El Niño Southern Oscillation

ERM effects range-median; a measurement of sediment toxicity

GIS geographic information system

GLM general linear model; a statistical analysis procedure

GPS global positioning system

IRM isothermal remanent magnetization; a technique to measure the 

magnetic properties of rock or sediment

LIDAR light detection and ranging

MAB meters above bottom

MMU minimum mapping unit

NOAA National Oceanic and Atmospheric Administration

NOS National Ocean Service

NPWMA Nuÿupia Ponds Wildlife Management Area (Oÿahu)

PAR photosynthetically active radiation

PSU practical salinity units

SAG spur-and-groove

SGD submarine ground-water discharge

SHOALS scanning hydrographic operational airborne lidar survey

SSC suspended sediment concentration 

SWAPS spectral wave prediction system

TIN total inorganic nitrogen

USACE United States Army Corps of Engineers

USGS United States Geological Survey

Hawaiian Words

ÿaÿama crab

äholehole Hawaiian flagtail

ahupua'a smaller subdivision of moku which define watersheds

aku skipjack tuna

akule bigeye scad

aliÿi royalty

ÿamaÿama mullet

enenue chub 

häÿukeÿuke shingle urchin

halalü smaller sized akule

heÿe octopus

hieau sacred worship site or altar

hïnälea lauwili saddle wrasse

hooÿilo cooler and wetter weather season when winds are inconsistent

humuhumu hiÿu kole pink durgeon

humuhumu lei lei triggerfish

humuhumu nukunuku äpuaÿa reef triggerfish 

humuhumuÿeleÿele black durgeon

kähala amberjack

kala bluespine unicornfish

kapu ancient Hawaiian system of laws and regulations; forbidden

kau warmer and drier weather season when winds blow steadily

kawakawa wavy-back tuna

kole goldring surgeonfish

kümü whitesaddle goatfish

laenihi razorfish

limu seaweed

limu ÿele ÿele a type of seaweed

limu kala a type of seaweed

limu kohu a type of seaweed

limu lïpoa a brown algae

limu manauea a type of seaweed

limu pälahalaha a type of green seaweed

limu waiwaiÿole a green algae

loko iÿa fishpond

loko kuapä type of shoreline fishpond, fully enclosed except for a makaha 

loko ÿume iki low-walled fishpond with walled lanes leading in and out

lüÿau special occasion feast

lua wai water holes

mäÿiÿiÿi brown surgeonfish

mahimahi dolphinfish

makaha slotted gate in fishpond

makai toward the sea

manauea a type of seaweed

manini convict tang

mauka towards the mountains

moana kale blue goatfish

moana ukali a type of goatfish

moi Pacific threadfin

moku wedge-shaped parcels of land from mountain crest to shore

nabeta razorfish

nui large, big

ÿoama juvenile goatfishes

öÿio bonefish

ogo a type of seaweed

onaga long-tailed red snapper, also known as ÿulaÿula koaÿe

ono wahoo, delicious

ÿöpakapaka pink snapper

ÿopihi limpets

palani eyestripe surgeonfish

pali cliff

päpio juvenile jacks

pualu yellowfin surgeonfish

puka a depression, hole

roi peacock grouper

taÿape bluestripe snapper

taro staple starchy plant

toÿau blacktail snapper

uhu parrotfish

uhu uliuli spectacled parrotfish

uku green jobfish

ula spiny lobster

ula papa slipper lobster

ÿulaÿula red snapper

ÿulaÿula koaÿe long-tailed red snapper, also known as onaga

ulua jacks

uma-uma lei orangespine unicornfish

ÿüÿü soldierfishes and squirrelfishes

weke goatfish
172

Fieldwork Activities

Fieldwork activities for the research reported in this volume were assigned identification numbers, to which all data are tied. InfoBank 

(http://walrus.wr.usgs.gov/infobank/) is the U.S. Geological Survey Coastal and Marine Geology Team’s online data catalog of these 

activities and more. Fieldwork activity IDs for USGS work on the Molokaÿi reef and associated volume chapters are shown below. 

Activity ID# URL Date (month/year) Chapter(s)

S-22-00-HW http://walrus.wr.usgs.gov/infobank/s/s2200hw/html/s-22-00-hw.meta.html January 2000 9, 17, 19, 20

A-14-01-HW http://walrus.wr.usgs.gov/infobank/a/a1401hw/html/a-14-01-hw.meta.html January 2001 19, 20

A-2-01-MK http://walrus.wr.usgs.gov/infobank/a/a201mk/html/a-2-01-mk.meta.html April 2001 19, 20

A-1-01-HW http://walrus.wr.usgs.gov/infobank/a/a101hw/html/a-1-01-hw.meta.html July 2001 19, 20

W-1-01-HW http://walrus.wr.usgs.gov/infobank/w/w101hw/html/w-1-01-hw.meta.html September 2001 1

A-1-01-MK http://walrus.wr.usgs.gov/infobank/a/a101mk/html/a-1-01-mk.meta.html November 2001 19, 20

A-1-02-HW http://walrus.wr.usgs.gov/infobank/a/a102hw/html/a-1-02-hw.meta.html February 2002 19, 20

A-2-02-HW http://walrus.wr.usgs.gov/infobank/a/a202hw/html/a-2-02-hw.meta.html July 2002 9

A-L-02-HW http://walrus.wr.usgs.gov/infobank/a/al02hw/html/a-l-02-hw.meta.html August 2002 13, 20

A-1-03-HW http://walrus.wr.usgs.gov/infobank/a/a103hw/html/a-1-03-hw.meta.html February 2003 9

H-1-03-HW http://walrus.wr.usgs.gov/infobank/h/h103hw/html/h-1-03-hw.meta.html July 2003 9

A-5-03-HW http://walrus.wr.usgs.gov/infobank/a/a503hw/html/a-5-03-hw.meta.html July 2003 13

A-6-04-HW http://walrus.wr.usgs.gov/infobank/a/a604hw/html/a-6-04-hw.meta.html August 2004 13
173 

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