A rich diversity of life inhabits Hawai‘i’s coral reefs, complex ecosystems that are being affected

by both natural processes and human activities. The U.S. Geological Survey is using a combination of techniques to map these reefs in a step toward evaluating their overall health.

M

New Mapping Techniques Help Assess the Health of

Hawai‘i’s Coral Reefs

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 084-01

2001

PROMOTING THE HEALTH AND SUSTAINABILITY OF CORAL REEF HABITATS

More than 5,000 square miles (14,000

km2

) of coral reef habitat is found in the

Hawaiian Islands from beyond Midway

to the Island of Hawai‘i, constituting

more than 60% of coral reefs found in

U.S. waters. Hawai‘i’s reefs are of great

importance both environmentally and

economically to those who visit or live

on the islands. These reefs shelter and

provide nursery grounds for many

commercially and culturally important

species of fish and invertebrates, they

protect the islands’ harbors, beaches,

and shorelines from erosion and wave

damage by storms, and they are vital to

Hawai‘i’s marine tourism industry.

The world’s coral reefs are home to 25%

of all marine species, yet it is estimated that

75% of these reefs will be destroyed or

significantly damaged in the next 20 years.

Hawai‘i’s coral reefs still appear to be

relatively healthy, but some areas of dead

and dying coral have been found in recent

years. The causes of this degradation are

poorly known, but are probably in part

related to human activities.

The U.S. Geological Survey (USGS) is

working closely with academic institutions and state and Federal agencies to

assess the factors that affect the health of

Hawai‘i’s and our Nation’s coral reefs. In

order to establish a basis from which

scientists can objectively detect changes

in reef health, the USGS and its cooperators are applying many new techniques

 ore than 60% of coral

 reefs in U.S. waters are

found in the extended Hawaiian

Island chain. These complex

and diverse marine ecosystems

are not only ecologically important but also provide hundreds

of millions of dollars annually to

Hawai‘i’s economy. Elsewhere in

the world, corals are dying at

unprecedented rates, and the

reefs of Hawai‘i may also be at

risk. To monitor and protect

these reefs and to help understand what is threatening coralreef habitats worldwide, U.S.

Geological Survey (USGS) and

other scientists are using new

techniques to create detailed

maps of Hawai‘i’s coral reefs.

© 1998, Ed Robinson/Hawaiian Watercolors
MAPPING CORAL REEF LANDSCAPES

Many different types of information are gathered to assist in mapping coral reefs. The

map (above) shows a portion of the coral reef off the south-central shore of the island

of Moloka’i. Very detailed bathymetry (see color scale) was obtained using lidar, a

laser-ranging technique that can penetrate shallow water. The oblique perspective

view of the same portion of reef (below) was created by digitally draping an aerial

photograph on the bathymetric model. By using such detailed maps and images that

show prominent features, including channels and large holes, scientists can better

understand the structure and development of coral reefs.

New Mapping Techniques

Lidar (light detection and ranging) is a

laser-ranging technique that can penetrate

shallow water to depths as great as 120

feet (36 m) to create precise bathymetric

maps that are accurate to within a few

inches (15± cm). These maps show

details of reef structures and zonations

that cannot be revealed using traditional

sonar technology, which requires deeper

water for successful operations.

Lidar data from the island of Moloka‘i,

digitally combined with aerial photographs, are resulting in a better understanding of shallow-water coral reef

development. Moloka‘i’s reefs, like many

reefs elsewhere in the world, consist of

nearshore inner reef flats that slope to

deeper water fore reefs farther offshore.

The reef crest, between the inner reef flat

and outer fore reef, lies in extremely

shallow water and may be exposed during

the lowest tides. Waves commonly crash

against or break on the reef crest.

Lidar also reveals details of channels

that cross Moloka‘i’s reefs. These

channels were originally valleys formed

by streams that cut across and eroded the

reefs when sea level was much lower

about 18,000 years ago. The channels

across the reefs typically contain sediment

that is a mixture of calcium carbonate

(CaCO3

) debris (derived from the

breakdown of reef material) and silt and

other sediment eroded from the island.

The role of these channels in transporting

sediment and their relation to coral reef

health are poorly understood and are one

focus of USGS investigations in Hawai‘i.

Other Mapping and Monitoring

Techniques

Although lidar maps provide “snapshots”

that reveal the large-scale structures of a

coral reef, other techniques provide greater

detail on the biological and physical

composition of a reef and can document

changes on a scale of months to decades.

For example, large-scale maps showing

changes in the distribution of coral, algae,

and sediment are compiled from sequences

of aerial photographs taken over decades.

Such maps may show changes that identify

groups of coral colonies stressed by

to the mapping and monitoring of coral

reefs in Hawai‘i.

Mapping of coral reefs provides

important information about a number of

reef characteristics, such as overall

structure and morphology, abundance and

distribution of living coral, and distribution and types of sediment. No single

approach is effective for evaluating the

overall health of a reef. It is only through

combining techniques that scientists can

establish the most complete view of a

reef, one that can be used for evaluating

reef health and for future monitoring.

Map of the eight main Hawaiian Islands.

More than 60% of coral reefs in U.S. waters

are found in the extended Hawaiian Island

chain, which stretches another 950 miles

(1,500 km) northwestward from the island of

Ni‘ihau to beyond Midway Island.

Honolulu

P a c i f i c O c e a n

Hawai‘i

Maui

Moloka‘i

Kaho‘olawe

Lana‘i

O‘ahu

Kaua‘i

Ni‘ihau

Honolulu

0 60 Miles

60 Kilometers
A sediment plume obscures the coral reef in this aerial photograph of the

western coast of Maui. The sediment was transported by stream runoff from

fields where soil is exposed. (Image courtesy of National Oceanic and

Atmospheric Administration.)

sediment influx or encrusting algal MAPPING CORAL AND SEDIMENT DISTRIBUTION

overgrowth.

Hyperspectral imagery, collected from

aircraft, is being used in Hawai‘i to map

coral reefs and potentially identify areas

of dead coral. Hyperspectral imagers

measure reflected sunlight in many

narrow wavelengths, ranging from

ultraviolet to near infrared. This spectral

information allows for detection of subtle

variations in the reef substrate that are not

visible to the naked eye. To calibrate and

help interpret the remotely collected

imagery, a hand-held hyperspectral sensor

specially outfitted for underwater data

collection is used by divers. The divers

collect spectra from healthy coral colonies, stressed coral colonies, and other

sea-floor substrates.

Another technique being used by

scientists to map coral reefs in Hawai‘i is

acoustic mapping. Dual-frequency sonar

technology is used to map areas of coral

reefs in water depths ranging from 25 feet

(8 m) to 120 feet (35 m). By processing

data from two frequencies, it is possible to

identify the acoustic signatures of

different types of material, such as soft

sediment deposits or hard coral. Observations from scuba dives are used to help

confirm these interpretations.

Growing urbanization and development of coastal areas are a concern for the

long-term sustainability of coral reefs. This aerial photograph of Kailua Bay,

O’ahu, shows an area of well-developed and healthy coral reefs. University of

Hawai’i and U.S. Geological Survey scientists are studying these reefs for their

role in producing calcareous sand and in protecting beaches from erosion.

(Image courtesy of National Oceanic and Atmospheric Administration.)

Scuba and snorkel dives also allow

scientists to directly identify coral species

and make estimates of living coral coverage

and biodiversity. Underwater photography

and video transects made by divers provide

a record that can be used to map and

understand subtle changes in the reef

ecosystem. For example, University of

Hawai‘i scientists are taking photographs of

1-square-meter plots at selected locations on

Hawai‘i’s reefs to assess changes over time

within individual colonies of coral.

90% live coral

Carbonate mud and sand

Land-derived mud and sand

Mixed carbonate and land-derived sand

Coralline algae and 5 to 60% live coral

Carbonate sand

upland area coastal plain

fish pond mangroves

Inner Reef (Reef Flat)

Outer Reef (Fore Reef)

blue hole

Reef Crest

MAP EXPLANATION

sand channel

stream

mouth

coral reef

coral reef

beach

This oblique aerial photograph (above)

shows a portion of the reef tract on

Moloka’i. The map of the same area (right)

shows the percentage of live coral and

types of sediment. This style of mapping

requires direct observations from snorkel

and scuba dives. The shoreline in the photo

is about 0.7 mile long.
Michael E. Field, Pat S. Chavez, Jr.,

Kevin R. Evans, and Susan A. Cochran

Graphic design by

Kevin R. Evans and Stephen L. Scott

Layout by Stephen L. Scott

COOPERATING ORGANIZATIONS

University of Hawai’i

Hawai’i Department of Land and Natural Resources

National Oceanic and Atmospheric Administration

National Park Service

University of California Santa Cruz

University of Washington

U.S. Fish and Wildlife Service

For more information contact:

Coral Reefs Program

U.S. Geological Survey

Pacific Science Center

1156 High Street

Santa Cruz, CA 95064

email: coralreefs@octopus.wr.usgs.gov

This Fact Sheet and any updates to it are available online

at http://geopubs.wr.usgs.gov/fact-sheet/fs084-01/

Detailed mapping of coral reefs requires direct observations in the water for identifying

coral species and collecting samples. This scientist is documenting changes through time in

a group of coral colonies (inset), visited a year earlier, by rephotographing them using a

precisely positioned frame. (Inset photograph courtesy of University of Hawai‘i.)

These broad, healthy thickets

of the coral Porites

compressa thrive in unpolluted, sediment-free water off

the southern coast of

Moloka’i. The health of

Moloka’i’s reefs varies

locally—other areas appear

to show signs of degradation

from both natural and

manmade causes. Inset

shows an unhealthy coral

colony off the coast of

Moloka’i that is entangled in

an abandoned fishing net.

This colony’s productivity is

also impaired by suspended

sediment that is making the

water cloudy or turbid.

Historical patterns of sedimentation are

important for determining the role of human

activities and their impact on a coral reef.

On shallow reef-flat areas in Hawai‘i,

scientists are using sediment probes

(graduated stainless steel rods) to measure

sediment thickness. Maps of sediment

thickness made using these measurements,

combined with sediment ages and compositional data, provide a detailed record of the

history of deposition.

Maps of coral reefs help scientists better

understand changes brought about by

activities on land. Sediment eroded from

land as a result of both natural processes and

human activities can pose a serious threat to

coral reef health by (1) reducing the area of

sea floor suitable for growth of new corals,

(2) creating cloudy or turbid water that

diminishes the amount of light available

for photosynthesis by symbiotic algae that

live within individual coral animals, and

(3) in extreme cases, burying coral

colonies. For example, in the past 100

years, a large amount of sediment has

accumulated on some inner-reef areas of

Moloka‘i, and ongoing studies indicate

that this has had an effect on reef health.

Many of the world’s coral reefs already

have been severely damaged by human

activities. In order to protect and preserve

the health of our Nation’s coral reefs, it is

essential to monitor these treasured and

important ecosystems. Mapping and

understanding the history of the coral

reefs of Hawai‘i is a large step toward this

goal. The work of USGS scientists

studying reefs in Hawai‘i is only part of

the ongoing efforts of the USGS to help

protect people’s lives and property from

geologic and environmental hazards in the

coastal zones of the United States.