### Tampa Bay Estuary Program Technical Publication # 03-04



# SEAGRASS COMMUNITIES of the GULF COAST of FLORIDA: STATUS and ECOLOGY

FINAL REPORT

AUGUST 2004

# CLINTON J. DAWES RO NALD C. PHILLIPS GE ROLD MORRISON A ugust 2004

# CLINTON J. DAWES

Univer sity of South Florida Ta mpa, Florida, USA 

# RO NALD C. PHILLIPS

In stitute of Biology of the Southern Seas Sevastopol, Crimea, Ukraine 

# GE ROLD MORRISON Environmental Protection Commission of Hillsborough County Ta mpa, Florida, USA



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ugust 2004

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# CITATION Dawes, C.J., R.C. Phillips, and G. Morrison. 2004. Seagrass Communities of the Gulf Coast of Florida: Status and Ecology. Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research I nstitute and the Tampa Bay Estuary Program. St. Petersburg, FL. iv + 74 pp. 

# COPIES This document may be obtained from the following agencies:
Ta mpa Bay Estuary Program FWC Fish and Wildlife Research Institute 100 8th Avenue SE 100 8th Avenue SE Ma il Station I-1/NEP ATTN: Librarian St. Petersburg, FL 33701-5020 St. Petersburg, FL 33701-5020 Te l 727-893-2765 Fax 727-893-2767 Tel 727-896-8626 Fax 727-823-0166 www.tbep.org http://research.MyFWC.com



# AUTHORS
Clinton J. Dawes, Ph.D. Distinguished University Research Professor Univer sity of South Florida Department of Biology Ta mpa, FL 33620 cd awes@cas.usf.edu

Ro nald C. Phillips, Ph.D. Associate I nstitute of Biology of the Southern Seas 2, Nakhimov Ave. Sevastopol 99011 Cr imea, Ukraine

ro nphillips67@hotmail.com

Gerold Morrison, Ph.D. Director, Environmental Resource Management Environmental Protection Commission of Hillsborough County 3629 Queen Palm Drive Ta mpa, FL 33619 813-272-5960 ext 1025 morrisong@epchc.org

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# TA BLE of CONTENTS

iv Fo rewo rdand Acknowledgements



## 1 Introd uction



## 6 Distribution, Status, and Trends



## 15 Aute co logy and Population Genetics



## 28 Ecological Roles



## 42 Natural and Anthropogenic Effects



## 49 Ap pendix: Taxonomy of Florida Seagrasses



## 55 Re ferences

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# FOREWORD The waters along Florida’s Gulf of Mexico coastline, which stretches from the tropical Florida Keys in the south to the temperate Panhandle in the north, contain the most extensive and diverse seagrass meadows in the United States. Seagrass meadows rival or exceed most kinds of agriculture in their productivity and also provide unique aesthetic and recreational opport unities. The importance of seagrasses as food, shelte r, and essential nursery habitats for commercial- and recreational-fishery species and for the many other organisms that live and feed in seagrass beds is well known. A single acre of seagrass can produce over 10
to ns of leaves per year and can support as many as 40 thousand fish and 50 million invertebrates. This high level of production and biodiversity has led to the view that seagrass communities are the marine equivalent of tropical rainforests.The importance of seagrasses to society has become fully recognized by government agencies. Seagr asses are now receiving focused attention from environmental managers, who require integrated science
to aid in developing seagrass-protection programs. Studies concerning the ecology, biology, and management of Gulf-coast seagrasses are increasingly diverse and complex; yet a synthesis of this research has not been prepared since the late 1980s. The need for an up-to-date synthesis has resulted in the production ofthis document, which compiles and organizes the many diverse research efforts that have been accomplished for this region since that time.

Ho lly Greening David W. Crewz Senior Scientist Research Scientist Ta mpa Bay Estuary Program Florida Fish and Wildlife Conservation Commission Fish and Wildlife Research Institute A

ugust 2004



# ACKNOWLEDGEMENTS

Ma nyindividuals contributed their time and expertise towards the completion of this document. Foremost among them are the three authors, Clinton J. Dawes (University of South Florida), Ronald C. Phillips (now at the Institute of Biology of the Southern Seas, Crimea, Ukraine), and Gerold Morrison (now at the Environmental Protection Commission of Hillsborough County). David W. Crewz (FloridaFish and Wildlife Conservation Commission, Fish and Wildlife Research Institute [FWC–FWRI]) provi ded outstanding contributions to content, editorial review, and document design. A dedicated group of seagrass scientists and resource managers located throughout the Gulf coast of Florida and elsewhere provided documentation (published and unpublished) and helpful comments on earlier drafts of this document. We appreciate the comments provided by Diane Altsman, Walt Avery, SethBlitch, Catherine Corbett, Frank Courtney, Mike Crane, Tom Cuba, Kellie Dixon, Mark Flock, Mark Fonseca, Tom Frazer, Roger Johansson, Alice Ketron, George Kish, Eric Lesnett, Jeannine Lessman, Graham Lew is, Robin Lewis, Kevin Madley, Rob Mattson, Gary Raulerson, Bill Sargent, Andy Squires, Betty Staugl er, Larinda Tervelt, Dave Tomasko, Tom Ries, Bob Virnstein, and Kim Yates. Llyn French (FWC–FWRI) provided an invaluable service by designing and composing the document for publication. This document is a joint product of the Gulf of Mexico Program (Larinda Tervelt, project lead), the Florida Fish and Wildlife Conservation Commission, Fish and W ildlife Research Institute ([formerly the Florida Marine Research Institute] Kevin Madley, project lead), the United States Geological Survey (J immy Johnson, project lead), and the Tampa Bay Estuary Program (Holly Greening, project manager).

# NTRODUCTION

The goals of this review are to update what hasbecome known about seagrasses since the publication of Zieman and Zieman’s review in 1989 and to assess the current status of seagrass habitats on the Gulf coast of Florida. Published studies about seagrasses have in cr eased substantially since the 1960s. Seag rass losses on F lorida’s Gulf coast, documented in the 1970s and 1980s, became thebasis for state and local government involvement in developing management a ppr oaches.
W hether continuous or patchy in plant coverag e, all seagrass communities should be viewed as having the same valued functions, such as

ser ving as nu r series. At least thirteen ecological roles have been assigned to seagrass communities, including

ro les as primary pr oducers, as habitat for animals and plants, and as suppor t f or f ood w ebs.Florida Gulf-coast seagrass communities suppor t multimillion-dollar commercial and recrea tional industries, especially with regard to fi shing. Tu r tlegrass, Thalassia testudinum

Ron Phillips photoGULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY 

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# VI SION and SCOPE
During the 1930s, most seagrass studies in the northern hemisphere focused on the decline and demise of Zostera marina L. (Eelgrass) on both sides of the Atlantic Ocean. During this period,only the work by Setchell addressed the distribution, taxonomy, and ecology of seagrasses (Setchell 1920, 1929, 1934, 1935). In the late 1950s, the Florida State Board of Conservation (now the Fish and Wildlife Research Institute [FWRI], a part ofthe Florida Fish and Wildlife Conservation Commission) initiated research efforts dealing with the ecology and distribution of Florida’s seagrasses and their roles in coastal and estuarine processes, especially

re garding food webs and habitat

characters supporting recreationally and commercially important fish and shellfish. Part of thiseffort culminated in a publication by Phillips (1960a), one of the few at that time concerning seagrass ecology or any aspect of seagrass biology (Zieman 1987). In1978, a bibliography compiled by the Seagrass Ecosystem Study listed over 1,400 titles worldwide (Bridges et al. 1978), and by 1982, a community profile of south Florida seagrasses

co ntained over 550 references (Zieman 1982). Ina summary of seagrass studies published

over a period of 25 years, Zieman (1987) found that initially nearly all seagrass literature wasdescriptive and qualitative. By 1970, most published works were quantitative, and developmentof conceptual models had begun. By 1980, increasingly robust models of the mechanisms by whichseagrass systems develop and maintain their productivity were being proposed and used as guides for developing proposed research ( e.g., McMillan 1978, 1980). By 1982, scientists, resource managers, and agency personnel monitoring and managing bays and estuaries on the Atlantic and Gulf of Mexico

co astlines of Florida, such as for the Indian River Lagoon, Charlotte Harbor, Sarasota Bay, Tampa Bay, and Pensacola Bay, noted dramatic seagrasslosses. Starting around 1950, those areas experiencing large population increases also experienced seagrass losses, probably as a result of increasingdevelopment pressure (Figure 1.1). To address seagr ass losses in the State of Florida, management programs were initiated between 1985 and 1995 to

co nserve and restore seagrass communities. TheState’s Surface Water Improvement and Management (SWIM) programs, within the Water Management Districts, address seagrass conservation issues statewide. Federally sponsored National Estuary Programs (NEPs) were designated for four specific estuaries: Tampa Bay National Estuary Pr ogram (TBNEP, now TBEP), Sarasota Bay National Estuary Program (SBNEP), Charlotte Ha rb or National Estuary Program (CHNEP), and I ndian River Lagoon Program (IRLP). Since that time, considerable research, particularly regarding the light requirements of different seagrass species, has been conducted. This work was stimulated by the development of goals and targets established by the SWIM Districts and NEPs with respect to

re ducing eutrophication and nutrient loadings in Figure 1.1 Sa tellite

image of Florida, sho wing population centers. Graph shows decline of

seag rass as population

increases. LANDSAT 2000 imag e processed by FWC–FWRIFWC–FWRIthe major bays and estuaries. In several of these estuaries, the large seagrass losses observed in theearly 1980s have halted, and moderate gains in seagr ass extents have been occurring since about 1988 in some areas.Although many recent studies concerning seagr ass ecology and biology along Florida’s Gulf

co ast have been and are being conducted, a synthesis of this information has not been compiled since the 1989 publication of “Ecology of the Seagrass Me adows of the West Coast of Florida: A Community Profile” by Zieman and Zieman. Several

r esource-management and science programs haveidentified the need for an updated synthesis of seagr ass information: The FWC Fish and Wildlife Research Institutehas developed a framework for a statewide Seagr ass Conservation Plan (Morrison et al. 2003a).
In A ugust 2000, over 70 seagrass scientists and managers attended a Seagrass Management Symposium convened by the Tampa Bay Estuary Program (Greening 2002a). Workshop participants identified, as a first and critical step inseagrass management, the need for a compilation of scientific information on Florida’s seagr asses published since the review by Zieman and Zieman (1989). The US Geological Survey’s (USGS) Coastal and Marine Geology Program initiated a “Gulf of Mexico Estuaries Assessment” in 2001, using Ta mpa Bay as the subject of a pilot study. Aninitial element of the study is to develop a “synthesis report” and web-based information bank that would link directly to the USGS National Estuaries Assessment. The Gulf of Mexico Program (GMP) made a

co mmitment that “By 2004, the GMP will complete development of an updated gulf-wide

characterization of the status and trends of seagr asses and coastal wetlands” (Gulf of Mexico Pr ogram 2003).
Kn owledge of seagrass ecology and distribution

w ithin Florida has progressed substantially in the last 20 years. The objective of this publication is to summarize available data and information about seagrass research performed along the Gulf coastof Florida since 1985. Literature and studies published prior to 1985 are summarized in Zieman and Zieman (1989), which is available from the FWC Fish and Wildlife Research Institute Web site (www.floridamarine.org). The geographical scope of this document extends from Florida Bay andthe Florida Keys at the southern extreme northward and westward through the Florida Panhandle
to the Alabama border (Figure 1.2). Although the distance is only about 700 km (435 miles) from Florida Bay to Apalachicola Bay (extending over6.5° of latitude), the aquatic climate changes dramatically. In Florida Bay, conditions are tropical, wh ereas in the Panhandle region, conditions are
te mperate and delimit the northern distribution in the Gulf for several Florida seagrass species, including Thalassia testudinum (Turtlegrass; Figure 1.3) and Sy r ingodium filiforme (Manateegrass; Figure 1.4).



## 3 CHAPTER 1 | INTRODUCTION Figure 1.2 Demarca ted watersheds of Florida’s Gulf coast.

Figure 1.3 Thalassia testudinum (T urtle g rass)

Figure 1.4 Syringodium filiforme (Manateegrass)Graphic by FWC–FWRI adapted fromNOAA CoastalAssessment FrameworkRon PhillipsRon Phillips

# DEFINITION of SEAGRASS HABITAT

S imply put, habitat is where an organism resides. Ne ve r theless, habitat is not only where organisms live but also includes how they live there. Seagrasshabitat is an ecological function provided by seagr asses. It is the location where certain organisms can thrive (survive, grow, and reproduce). Seagrass habitat is defined in this document as a physical space containing seagrasses in sufficient quantity and pattern to produce the appropriatestructural and physiological characteristics to support organisms typical of seagrass communities. These characteristics include food webs based onorganic-matter production, nutrient cycling, detritus production, shelter, and sediment formation. Co ntinuous-coverage beds as well as patchybeds of seagrasses provide critical and valued habitat functions. Fonseca et al. (1998) found that scattered orpatchy Z. marina beds covered many thousands of acres of estuarine seafloor in NorthCarolina, had shoot densities and primary production equivalent to those of continuous-coveragebeds, had significantly greater below-ground biomass than did continuous-coverage beds, and often supported densities of economically valuable animal species, e.g., pink shrimp, similar to thoseof continuous-coverage seagrass beds. They concluded that seagrass habitat must be recognized as indicating not only continuous-coverage seagrass beds, but also chronically patchy areas, therein

co nsidering the unvegetated spaces between vegetation as seagrass habitat as well (Figure 1.5). Thus, the value of seagrass habitat should notbe judged on the basis of seagrass densities or patte rn s, but upon the provided functions. In thisdocument, any reference to areas covered by seagr ass means seagrass habitat, as long as valued functions are present and measurable. One should

co nsider that patchy seagrass beds perhaps represent areas in the process of recovering from past disturbances, or they may be areas held in a patchypattern because of the characteristics of the present physical environment. In any event, patchy seagr asses support valued animals and plants and display typical seagrass functions. 

# ECOLOGICAL and ECONOMIC IMPORTANCE of

SEAGRASS COMMUNITIES Seagrasses are a vital component of Florida’s

co astal ecology and economy. They provide nutrition and shelter to animals that are important to marine fisheries, provide critical habitat for many other animals ( e.g., wading birds, manatees, and sea turtles), and improve water quality (Thayer et al. 1997,1999; Livingston 1990; Kenworthy et al. 1988b; McMichael and Peters 1989; Stedman and Ha nson 1997; Valentine et al. 1997). For example, Heck et al. (2003) found a strong link between seagr ass abundance and those of juvenile finfish and shellfish that was related to habitat structure. Insystems where seagrasses occur, nearly all ofthe commercially and recreationally valuable estuarine and marine animals depend on seagrass beds as refuge or habitat for parts or all of their life cycles (Kikuchi and Peres 1977; Thayer et al. 1978, 1984; Kikuchi 1980; Ogden 1980; Thayer and

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY 

## 4 Figure 1.5 A mixture of patchyand continuous seagrasses along a mangroveshoreline in Tampa Bay, Florida.Gandy Photography,2001Ustach 1981; Phillips 1984). As reported by W ingrove (1999) in the Florida Keys, hundreds of fish species, including many of commercial value,

re ly on seagrass habitats during some parts of theirlife cycles. Seagrasses help support a thriving, multimillion-dollar recreational fishery including, as an example, the shallow-water seagrass flats fishery seeking bonefish and tarpon. In addition, over 30 species of tropical invertebrates that depend onseagrasses are collected in the Florida Keys annually for the marine aquarium industry. Short et al. (2000) list ecological services provi ded by seagrasses (modified here):

Pr imary production (food for animals and support for fisheries and wildlife)Canopy structure (habitat, refuge, nursery, settlement and support of fisheries)
E pibenthic and benthic production (support of food webs and fishery support)
Nutr ient and contaminant filtration (improvedwater quality, support of adjacent habitats, support of fisheries) Sediment filtration and trapping (improved water quality, countered sea-level rise, support of adjacent habitats)
E piphytic substratum (support of secondary production, production of carbonate sediment, support of fisheries) Oxygen production (improved water quality,support of adjacent habitats, support of fisheries)Organic-matter production and export (support of estuarine and offshore food webs, support of adjacent habitats, support of fisheries)
Nutr ient regeneration and recycling (support ofprimary production, support of adjacent habitats, support of fisheries) Organic-matter accumulation (support of food

we bs, countered sea-level rise, support of fisheries) Dampening of waves and currents (prevention of erosion/resuspension, support of adjacent habitats, increased sedimentation)Seed production/vegetative expansion (selfmaintenance of habitat, support of wildlife)Self-sustaining ecosystem (recreation, education, landscape-level biodiversity) C ostanza et al. (1997) and Costanza (1999) stated that, for the entire biosphere, the economic value of all ecosystem services for 16 biomes is in the range of 16 to 54 trillion US $ y –1 , w ith an average of 33 trillion US $ y –1 . T hey considered this tobe a minimum estimate. The value of coastal enviro nments, including estuaries, coastal wetlands (mangroves and salt marshes), seagrass beds and algae, coral reefs, and continental shelves, is of a disproportionately high value. These communities

cove r only 6.4% of the world’s surface, but they are

r esponsible for 43% of the estimated value of the

wo rld’s ecological services. InFlorida, seagrass beds are directly responsible for bringing in millions of dollars annually from out-of-state and resident recreational boaters and fishermen and commercial fishermen (Bell 1993; M ilon and Thunberg 1993; Virnstein and Morris 1996; Virnstein 1999; Wingrove 1999; Thomas and Stratis 2001). Seagrass beds on the Gulf coast of Florida are important not only for the ecological services they provide, but for the economic health of the state and region.



# DOCUMENT CONTENT The focus of this review is the biology and ecology of seagrasses and of seagrass communities on Florida’s Gulf coast. Chapter 2 considers distribution of the Florida Gulf coast seagrasses and reports the trends inareal extents of seagrass beds, as recorded by monitoring efforts of various local and regional progr ams. Chapter 3 synthesizes new information

re garding autecology and presents what is known

re garding genetic analyses of Florida seagrasses.These genetic techniques were not applied to seagr asses prior to the 1990s. Chapter 4 addresses the ecological roles of seagrass communities, of their macroalgal components (epiphytic and drift), and of adjacent coastal communities (mangroves and salt marshes). Chapter 5 focuses on the natural and anthropogenic effects on Florida seagrasses. The Appendix presents keys to the Florida seagrass species and presents brief taxonomic descriptionsfor the families, genera, and species (includes figures). A companion document entitled “The Florida Seagrass Manager’s Toolkit” (Morrison et al. 2003b) addresses seagrass management in Florida and is available at www.floridamarine.org.



## 5 CHAPTER 1 | INTRODUCTION

# IST RIBUTION, STATUS, and TRENDS

F lorida’s Gulf coast can be divided into four re gions—South F lorida, Gulf Peninsula, Big Bend, and Panhandle—for the purpose of assessing near-shore seagrass community sta tus and trends. Aerial photography taken during the 1990s revealed that the South Florida region contained the majority (65%) of the Gulf coast’s seagrass cov erage, followed by the Big Bend (28%), Gulf Peninsula (5%), and Panhandle (2%) re gions. The most abundant seagrass species on the Florida Gulf coast are Thalassia testudinum, Syringodium filiforme, and Halodule wrightii, each of which principally has a tropical to subtropical distribution. Two other species ( Halophila engelmannii and H. decipiens) also occur in the area, in near -shore meadows dominated by T. testudinum and S. filiforme and in deeper waters where the latter two species

ar e absent. A substantial decline in seagrass cov erage has occurred in the South Florida

re gion over the past 15 years, following a drama tic “die-off” that began in Florida Ba y during 1987. In the Gulf Peninsula region between ca. 1959 and 1982, mapping efforts in T ampa Bay, Sarasota Bay and Greater Charlotte Harbor revealed r eductions in seagrass coverage. However, diffi culty in obtaining accurate cov erage estimates from the 1950s-era maps has complicated attempts to quantify these declines, pa r ticular ly in the Greater Charlotte Harbor area.
Incr easing cov erage tr ends have occurred in Tampa Bay and Sarasota Bay since 1982 in response to improved management of nitrogen l oadings and increasing water clarity. Increased rainfall, stormwater r unoff, and nutrient loadings associat ed with the 1997–1998 El Niño ev ent interrupted the trends in seagrass cov erage g ains, but they appear to have r esumed in r ecent years. Se veral other areas within the Gulf P eninsula region—including Charlotte Harbor Pr oper and Lemon Bay—have been mapped approximately biennially since 1988. No significant seagrass coverage tr ends have been repor ted from these r ecent
ma pping efforts. The Big Bend region is a unique “zer o-energy” coastline and contains F lorida’s second-largest near-shore seagrass bed. The region has r eceived rela ti ve ly little

r esearch and management attention. The only long-term seagrass cov erage changes reported have been localized losses a ttributed to the effects of an industrial facility that discharges to the Fenho lloway River and Apalachee Bay.In portions of the Panhandle region, which is also poorly studied, seagrass coverag e may be increasing in some low-salinity areas and declining in some areas of higher salinity. In addition to these near-shore seagrass resources, recent work on the West F lorida Shelf indicates the presence of ext ensive, seasonal, deep-water Halophila beds, which may ex ceed four hundred thousand hectares (one million acres).

Shoalgrass, Halodule wrightii

Ron Phillips photo

## 7 CHAPTER 2 | DISTRIBUTION, STATUS, AND TRENDS 

# DIST RIBUTION Seagrasses are a relatively small group of flowering plants that have adapted to survive and reproduce in the marine environment. They are present in all

co astal states of the U.S., with the exception of Georgia and South Carolina, where a combination of freshwater inflows, high turbidity, and large tidal amplitude restricts their occurrence (Thayer
et al. 1997). The most abundant taxa in Florida’s near-shore
waters are T. testudinum, S. filiforme, and H. wrig htii, each of which principally has a tropical to subtropical distribution (Zieman and Zieman 1989). Thalassia testudinum (T urtlegrass) is the largest of these species, with long strap-shapedleaves and robust rhizomes (see Appendix for taxonomic keys, descriptions, and illustrations). Extensive seagrass beds are usually dominated by this species, either alone or in combination with other species, such as Sy r ingodium filiforme. Sy r ingodium filiforme (Manateegrass) can be distinguished by its cylindrical (terete) leaves that,because they are brittle and buoyant, are frequently broken off from the parent plant and dispersed widely by winds and currents. H alodule wrig htii (Shoalgrass) has flat, narrow leaves and a shallow root system. It is thought to be an early successional species in the development of seagrass beds in the Gulf of Mexico and Caribbean Sea. Three other species, H alophila engelmannii (Stargrass), H. decipiens (Paddlegrass), and H. jo hnsonii (J ohnson’s Seagrass), are also found in Florida’s coastal waters. In the Big Bend region, H.

engelmannii and H. decipiens are scattered throughout beds dominated by T. t estudinum and S. filiforme but also occur in deeper water wherethese latter two species are absent (Iverson and Bittaker 1986). H alophila decipiens has been found in the Big Bend and Tampa Bay regions and at depths
to 90 m near the Dry Tortugas (Zieman 1982), and it forms single-species stands in depths of 20 m or more, beyond the deep edge of the extensive T. testudinum/S. filiforme beds (Zieman and Zieman 1989, Dawes and Lawrence 1990). Halophila johns onii is a relatively newly described species and is morphologically similar to H. decipiens (Eiseman and McMillan 1980). H alophila johnsonii is now listed as a threatened species by the National Ma r ine Fisheries Service (NMFS; 2002) and is apparently an endemic whose range is restricted to the lagoon systems of Florida’s southeastern (Atlantic) coast. It has not been documented to occur on the Gulf coast (NMFS 2002), and recent

ev idence suggests it is genetically indistinguishable from H. ovalis, a species of the Indo-Pacific region (W ayc ott et al. 2002). A seventh species, Ruppiamaritima (Widgeongr ass), is a euryhaline plant that is often encounte re d in the waters of Florida’s Gulf coast, particularly in estuaries such as Homosassa Bay (Koch and Dawes 1991a, b) and Tampa Bay (Lazar and Dawes 1991). This species can form dense beds, as found in upper Tampa Bay (Lazar and Dawes 1991). In recognition of its broad salinity
t olerance, some authors have suggested that R. maritima may be thought of as a freshwater speciesthat is also capable of living in saline environments, rather than a seagrass in the strictest sense ( e.g., Zieman 1982, Kuo and den Hartog 2001). Inaddition to seagrasses, drift and attached seaweeds also make up an important component of the total submerged aquatic vegetation (SAV) in many areas of Florida (Dawes et al. 1987, Dawes1986). In the Big Bend region, for example, benthic green algae in the order Caulerpales— including H alimeda incrassata, seven species of Caulerpa, and two species each of Udotea, Penicillus, and Co dium —are important associates in the region’s seagrass beds, with standing crops exceeding those of seagrasses in some areas (Mattson 2000). Depth-related zonation patterns of Gulf-coast seagrass beds (Figure 2.1) have been described by Lew is et al. (1985a), Iverson and Bittaker (1986), Zieman and Zieman (1989), and Mattson (2000). Asa general rule, H. wrightii and R. maritima tend
to be more abundant in shallow inshore areas because they tolerate frequent tidal exposure and low salinities. Thalassia testudinum and S. filiforme

Fig 2.1 An e xample of an inshore-offshore seagrass zonation profile on Florida’s Gulf coast (from McNulty et al. 1972).

## 8 re ach their highest abundance and biomass inslightly deeper areas, often forming dense singleor mixed-species stands (Figure 2.2). In some
areas, H. wrightii exhibits a second abundance peak along the deep-water edge of T. testudinum/S. filiforme meadows (Iverson and Bittaker 1986, Zieman and Zieman 1989). H alophila spp. formsparse beds in deeper waters outside these meadows (Zieman and Zieman 1989, Fonseca et al. 2001). In the Big Bend region and Tampa Bay, H.

engelmannii also occurs in low-salinity areas (<5 ppt) within 1–2 km of river mouths, where anability to tolerate low light levels in waters of relatively high color may be an important factor

explaining its persistence (Dawes 1967, Zimmerman and Livingston 1976, Mattson 2000). Wh en considering the distribution of seagrasses on Florida’s Gulf coast, dividing the area into four

re gi ons is helpful. Regions defined by Sargent et al. (1995) are summarized in Table 2.1 and Figure 2.3: South Florida (Florida Keys and Florida Bay to Estero Bay) Gulf Peninsula (Estero Bay to Anclote Key) B ig Bend (Anclote Key to Ochlockonee Point) P anhandle (Ochlockonee Point to the FloridaAlabama border)The spatial distribution and areal extent of seagr asses vary substantially between these regions (Zieman 1982, Iverson and Bittaker 1986, Sargent et al. 1995). Recent status and trends in seagrass

cove ra ge at this geographic scale are summarized in the following section. 

# REGIONAL STATUS and TRENDS
Sargent et al. (1995) estimated that, on a statewide basis, Florida’s near-shore coastal waters support approximately 1.1 million ha (2.7 million acres) ofseagrass. This statewide estimate includes 0.8 million ha (1.9 million acres) of dense and relativelyeasily mapped seagrasses in state waters where visibility allowed interpretation of bottom communities (within 14.4 km [9 miles] of shore along the Gulf coast). It also includes, in portions of the South Florida region, an estimated 0.3 million ha (0.8 million acres) of sparse and incompletely mapped seagrass beds that are interspersed with hard-bottom communities and are thus difficult to map accurately. This estimate does not include thesparse beds that occur in deeper waters on portions of the West Florida Shelf (Sargent et al. 1995). Along the state’s Gulf coast, the coverage of the sparse deep-water beds of the West Florida Shelf and the small, patchy mixed-species beds that occur intermixed with hard bottom outside the main seagrass beds in Florida Bay remain the largest question marks in the effort to develop accurate estimates of overall seagrass coverage. For

example, recent assessments indicate that the total area of deep-water beds in the Gulf-coast region may be on the order of 0.4 million ha (1 million acres), which would place them, on a worldwide basis, among the most extensive seagrass habitats currently known (Fonseca et al. 2001). During the 1990s, aerial photographs were used
to produce digitized maps of seagrass coverage for

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Tab le 2.1 Extent of seagrass coverage and aerial photo gr aphy dates in four regions of Florida’s Gulf coast (Madley et al. 2003). Seagrass Seagrass Region (hectares) (acres)

P anhandle (1992) 17,474 43,178 B ig Bend (1992) 247,598 611,815 Gulf Peninsula (1999) 43,323 107,051 South Florida (1992, 1995) 574,875 1,420,517
Gulf coast total 883,270 2,182,561 Fig 2.2 Dense seagrass beds begin as patches that coalesce into

a lar ge r, mor e g enetically diverse meadow.

## 9 CHAPTER 2 | DISTRIBUTION, STATUS, AND TRENDS each of the regions shown in Figure 2.3. The datesof these mapping efforts and their results are summarized in Table 2.1. The South Florida region

co ntained the majority (65%) of the Gulf coast’s seagrass coverage, followed by the Big Bend (28%), Gulf Peninsula (5%), and Panhandle (2%) regions.



# SOUTH FL ORIDA The South Florida region (Figure 2.3) includes the

co astal waters of Collier, Monroe, and MiamiDade counties (Table 2.1). The portion that lies immediately south of Cape Romano includes the Te n Thousand Islands, an area that is dominated by mangrove islands and tidal channels but that also contains patches of seagrasses and some large seagrass beds ( e.g., as reported from the Lostmans River area by Dawes et al. 1995). The Florida Bay portion, which lies south of Cape Sable and west of the Florida Keys, is a carbonate-sediment-based system that supports extensive seagrass beds. Based on monitoring data collected annually from 1974 through 1980, Iverson and Bittaker (1986) noted that, in addition to their greater

exte nt, the 0.5 million-ha (1.4 million-acre) Florida Bay seagrass meadows also had about two
to four times the short-shoot densities of T. testudinum and S. filiforme as occurred in the 0.3 million ha (741,000 acre) Big Bend meadows. They hypothesized that the density differences observed in the two areas may be a consequence of greater seasonal variations in solar radiation and water
te mperature in the Big Bend, which is at the northern limits of tropical American seagrasses (Iverson and Bittaker 1986). Seagrass coverage and condition in the South Fig 2.3 County-based regions used to describe seagrass distributions along the Gulf coast of Florida (after Sargent et al. 1995) and containing locations r eferred to in the text.

## 10 Florida region have changed since the 1974–1980 period documented by Iverson and Bittaker (1986). A dramatic decline in coverage began in

western Florida Bay during the summer of 1987 (Hall et al. 1999). Seagrasses in the bay were apparently subjected to decreased light availability

re sulting from resuspended sediments and widespread, persistent microalgal and cyanobacterialblooms. Bay-wide surveys in 1984 and 1994 indicated that the biomass of T. testudinum, S. filiforme, and H. wrightii declined by 28%, 88%, and 92%, respectively, during that 10-year period. The spatial patterns of seagrass losses suggested that ch ro nic light reductions, which affected all species, and “die-off” (rapid, unexplained plant mortality), which also affected T. t estudinum, most likely caused the overall decline. Although the loss rate has slowed considerably in recent years, die-off and regression of seagrasses are still occurring in parts of the bay (see also Chapter 5).


# GU LF PENINSULA The Gulf Peninsula region lies between Estero Bay and Anclote Key (Figure 2.3) and includes the

co astal waters of Lee, Charlotte, Sarasota, Manatee, Hil lsborough and Pinellas counties. This region is a moderate-energy coastline, with extensive sand beaches and barrier islands that enclose two large estuarine embayments (Tampa Bay and Charlotte Ha rb or) and many smaller lagoons ( e.g., Estero Bay, San Carlos Bay, Matlacha Pass, Pine Island Sound, Lemon Bay, Sarasota Bay, and St. JosephSound) containing the majority of the region’s seagr ass beds. Recent seagrass-coverage trends in this

re gi on appear somewhat irregular, apparently

r esponding to site-specific situations within the different estuary and lagoon systems. Fo r Tampa Bay, Sarasota Bay, and the GreaterCharlotte Harbor system, the earliest photography-based seagrass coverage maps for the region

we redeveloped by the FWRI, using aerial photogr aphs taken in the late 1940s and early 1950s and again in 1982 (Harris et al. 1983, Tampa Bay Re gi onal Planning Council [TBRPC] 1984, Janicki
et al. 1994). Maps were subsequently prepared by the Southwest Florida Water Management District (SWFWMD) for Tampa Bay (for the years 1988,

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Figure 2.4 Time series of mapped seagrass coverage within major coastal water bodies of the Gulf Peninsula region (GCH = Gr

ea ter Charlotte Harbor; TB = Tampa Bay; CHP = Charlotte Harbor Proper; SJS = St. Joseph Sound; SB = Sarasota Bay; LB = Lemon Bay) from ca. 1950 through 2002. The 1950 values were developed using mapping methods that differ from those currently in use and should be viewed as approximations. (Data sources: FWRI, SWFWMD)

## 11 CHAPTER 2 | DISTRIBUTION, STATUS, AND TRENDS 1990, 1992, 1994, 1996, 1999, and 2002), Charlotte Ha rb or (1988, 1992, 1994, 1996, 1999, and 2002), Sarasota Bay (1988, 1994, 1996, 1999, and 2002), Le mon Bay (1988, 1994, 1996, 1999, and 2002), and St. Joseph Sound (1999 and 2002) (TBNEP 1996, Kurz et al. 2000, D. Tomasko pers. comm. ). T ime series of seagrass coverages based on these maps are shown in Figure 2.4. InTampa Bay, approximately 46% (7,452 ha or 18,400 acres) of the existing seagrass coverage was lost between ca. 1950 and 1982 (Figure 2.4) from the combined effects of dredging and reductions in water clarity (Haddad 1989). Coverage losses in Ta mpa Bay over longer time periods are difficult toestimate with any accuracy, because of the sparseness of data and absence of aerial photography prior to ca. 1950. Indirect methods suggest, however, that as much as 81% of predevelopment coverage may have been lost during the years 1879 through 1982 (Lewis et al. 1991). Between 1982 and 1996 Tampa Bay regained approximately 2,090 ha (5,160 acres) of seagrass, apparently in response to management efforts that led to reduced nutrient loadings and increased water clarity. Reduced nutrient loadings and increased seagrass coverage were also observed in Sarasota Bay during the same period (Tomasko et al. 1996). Seagrass coverage then declined in bothestuaries, apparently in response to the heavy rainfall and increased stormwater runoff that occurred during the 1997–1998 El Niño event (Johansson 2002a). Seagrass coverage increased once again in Ta mpa Bay during the 1999–2002 mapping period, as water clarity improved during the relatively dry years that occurred following the cessation of the 1997–1998 El Niño event. During this period the
t otal mapped coverage in the bay increased by 501 ha (1,237 acres), to 10,561 ha (26,078 acres). In Sarasota Bay, on the other hand, the total mapped

cove ra ge declined slightly between 1999 and 2002, from 4,799 ha (11,850 acres) to 4,740 ha (11,703acres). Seagrass coalescence occurred in both estuaries during the 1999–2002 mapping period,through a net increase in the coverage of continuous (as opposed to patchy) seagrass beds (D. To masko pers. comm. ). The documented increases in seagrass coverage in Tampa Bay and the slight increase in Sarasota Bay between 1982 and 2002 apparently occurred in r esponse to improved management of anthropogenic nitrogen loads to both estuaries (Tomasko
et al. 1996). Through the Grizzle-Figg Act (403.086 Florida Statutes), the Florida Legislature required that all sewage treatment plants discharging to the two estuaries and their tributaries must provide advanced wastewater treatment (AWT) prior to discharge. The City of Tampa upgraded its sewage
tr eatment plant to AWT in 1979, greatly reducing the amount of nitrogen entering Tampa Bay fromthat source. The City of St. Petersburg implemented a wastewater reuse program which almost eliminated its direct wastewater discharges to Ta mpa Bay. Similar improvements to sewage treatment plants in Pinellas, Hillsborough, Manatee, and Sarasota counties also helped improve water quality in the receiving estuaries. By the early 1990s, water clarity in some of the most degraded portions of Tampa Bay had already begun to improve (Johansson 1991, TBNEP 1996). Im proved management of seagrass communities has been identified as a priority issue in Tampa Bay and Sarasota Bay and is being addressed through broad-based stakeholder groups. Both systems are part of the U.S. EPA’s National Estuary Pr ogram, which helps to coordinate the management activities of local, state, and federal agencies and the private sector. The Tampa Bay Estuary Pr ogram is pursuing a resource-based management strategy that seeks to limit anthropogenic nitrogen loadings at levels needed to achieve its seagrass-coverage goal of 15,400 ha (38,000 acre),

re presenting 95% of the seagrass coverage that wasmapped in the Bay in 1950 (TBNEP 1996, Johansson and Greening 2000). The Sarasota Bay National Estuary Program has adopted a technology-based strategy that seeks to control nitrogen loadings through the adoption of “best available
te chnology” for anthropogenic nitrogen sources in the contributing watershed (SBNEP 1995). Nitr ogen-management strategies are effectiveseagrass restoration tools in Tampa Bay and Sarasota Bay. Because phytoplankton are important sources of light attenuation in both estuaries, by

co ntrolling nitrogen inputs, managers can reduce phytoplankton biomass, increase water clarity, andincrease the bay-bottom area that receives sufficient sunlight to support seagrasses ( e.g., J ohansson and Greening 2000). A similar situation appears to exist in Lemon Bay, where phytoplank-

## 12 to n are also a major source of light attenuation (T

omasko et al. 2001). In Charlotte Harbor, however, light attenuation is affected more by water “c olor”—naturally elevated levels of dissolved organic matter discharged from extensive wetlands in the Peace and Myakka river systems—than by phytoplankton abundance (McPherson and Miller 1994). Because of its large watershed, Charlotte Ha rb or also experiences large seasonal and annual variations in fresh-water inflow, producing largefluctuations in salinity that can be stressful to seagr asses (Tomasko and Hall 1999). The nitrogenbased management strategies developed for TampaBay and Sarasota Bay may thus have limited applicability for Charlotte Harbor (D. Tomasko pers.

comm. ). The time series of mapped seagrass coverage in portions of the Charlotte Harbor system is shown in Figure 2.4. Much of the reduction in coverage in Greater Charlotte Harbor (which includes San Carlos Bay, Matlacha Pass, and Pine Island Sound, in addition to Charlotte Harbor Proper) between ca. 1950 and 1982 occurred in the southern portion of the system, particularly in Pine Island Sound andSan Carlos Bay. This reduction has been linked, circumstantially, to a series of large-scale anthropogenic activities that occurred in the area during the 1960s, including dredging of the Intracoastal Wate rway , construction of the Sanibel causeway, and installation of dam and lock structures in the lower Caloosahatchee River (Harris et al. 1983). In addition to direct destruction of seagrass habitats,these projects have been postulated to have indire ctly altered the water clarity, salinity, hydrodynamics and flushing characteristics of the area in ways that made it less conducive to seagrass growth and survival (Harris et al. 1983). Due to difficulties encountered in interpreting and digitizing the aerial photographs that were taken in the area during the

ca. 1950 period, however, the coverage values shown for the Greater Charlotte Harbor system in that period in Figure 2.4 should be viewed as rough approximations (Harris et al. 1983). No geographically and technically consistent mapping of seagrass coverage throughout the Gr eater Charlotte Harbor system has been conducted since 1982. The northern portion of the system—which falls largely within the SWFWMD and is referred to locally as Charlotte Harbor Pr oper—has been mapped approximately biennially since 1988 by the SWFWMD (Kurz et al. 2000). From 1988 through 2002, seagrass coverage in this area fluctuated between 7,200 ha (17,800acres) and 7,800 ha (19,300 acres), with no apparent upward or downward trends (Figure 2.4). Le mon Bay has been mapped over the same time period and has shown relatively small fluctuations around an average value of 1,058 ha (2,600 acres),

w ith no apparent trends (Figure 2.4). Mapping data from St. Joseph Sound near Clearwater are available only for the years 1999 and 2002, when an average of 5,840 ha (14,400 acres) were recorded (Figure 2.4).


# BIG BEND The Big Bend region extends from Anclote Keynorthwestward to Ochlockonee Point in the Panhandle region (Figure 2.3) and includes the coastal waters of Pasco, Hernando, Citrus, Levy, Dixie, Ta

ylor, Jefferson, and Wakulla counties. Zieman and Zieman (1989) note that this portion of the

co ast is unique in that it is an extensive area, with no offshore barrier islands, where a number of rive rs, creeks, and marshes discharge directly intothe Gulf of Mexico. It is also one of the few examples of a “zero-energy” coastline, with average breaker heights of 3–4 cm or less and little littoral
tr ansport of sand (Murali 1982). Factors contr ibuting to the low-energy characteristics of thearea include a wide and gently sloping shelf, divergence of approaching wave trains into a large

co astal concavity, the location of the coast in agenerally upwind direction, and the wave dampening effects of old submerged beaches and seagrass meadows (Murali 1982). The region is an environmentally diverse area that can be divided into five subregions (Mattson 2000). The Springs Coast subregion, which extends from Anclote Key northward to the mouth of the W ithlacoochee River, is dominated by flows from a series of short, spring-fed river systems: the Weeki Wa ch ee, Chassahowitzka, Homosassa, and Crystal rivers. Concentrations of nitrate nitrogen have been increasing steadily in these rivers in recent decades, due to increasing anthropogenic nitrogen discharges in their highly karstic watersheds and spring recharge areas (Katz e t al. 1997). The limestone bedrock and sediments of this portion of the

co ast are rich in carbonates, however, and tend to

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY

## 13 CHAPTER 2 | DISTRIBUTION, STATUS, AND TRENDSbind inorganic phosphorus from the water column. Because inorganic phosphorus is less available in the water column, primary production ofnear-shore aquatic ecosystems are tilted from Nlimitation toward P-limitation (Hauxwell et al. 2001). The four subregions north of the Springs Coast subregion are Waccasassa Bay, Suwannee Sound and adjacent coastal waters, Deadman Bay, and Apalachee Bay (Mattson 2000). Discharges from rive r systems in these subregions tend to be high in

co lor during periods of high flow, a factor that apparently contributes to relatively low seagrass

cove ra ge in the vicinity of the river mouths (Mattson 2000). Although the inshore and offshore seagrass beds of the Big Bend are among the largest in the eastern Gulf of Mexico (Iverson and Bittaker1986), the region has received relatively little management attention (Mattson 2000). Several mapping surveys have been conducted, but most have

cove re d only a limited portion of the region and have produced highly variable coverage estimates (Mattson 2000). The most extensive, region-wide mapping efforts have been carried out by Iverson and Bittaker (1986) and Sargent et al. (1995), producing coverage estimates of 300,000 ha (741,000 acres) and 334,842 ha (827,000 acres), respectively. Ne ither of these estimates includes the sparse, deep-water seagrass beds that are located offshore. Fo r the entire West Florida Shelf, Fonseca et al. (2001) estimated the areal coverage of deep-water H. decipiens beds at 0.4 million ha (1 million acres), which would place them among the largest seagrass communities in the world. Ina general sense, the Big Bend area has been described as one of the least polluted coastal

re gi ons of the continental United States (Livingston 1990). However, the Fenholloway River—a
tr ibutary to Apalachee Bay—is an exception to thisgeneralization, receiving discharges from an industr ial facility that are high in color and contain elevated levels of sulfate, BOD, suspended solids and

nutr ients (Mattson 2000). These discharges have apparently caused localized reductions in water clarity and seagrass coverage in portions of Apalachee Bay (Livingston 1993, Livingston et al. 1998). 

# PANHANDLE The Panhandle region extends from Ochlockonee Po int westward to the Florida-Alabama border (Figure 2.3) and includes the coastal waters of Fr anklin, Gulf, Bay, Walton, Okaloosa, Santa Rosa, and Escambia counties. It resembles the Gulf Pe ninsula region in being a moderate-energy

co astline with extensive sand beaches and barrier islands enclosing protected estuaries and lagoons ( e.g., Apalachicola Bay, St. Joseph Bay, St. Andrew Bay, Choctawhatchee Bay, Santa Rosa Sound, Escambia Bay, and Pensacola Bay) that contain the

re gi on’s seagrass beds. Like the Big Bend region, itis the subject of limited recent research and management activity. Based on aerial photography taken in 1992–1993, Sargent et al. (1995) estimated that 19,509 ha (48,170 acres) of seagrasses were present in the coastal waters of the Panhandle region. No regio n-wide coverage estimates have apparently been produced since that time. Lores et al. (2000) assessed recent coverage trends in seagrasses and other SAV in the Escambia-Pensacola Bay portionof the region, and provided the following summary: SAV in the Escambia-Pensacola Bay System underwent a substantial decline in the late 1940s through the early 1970s;Although scientific documentation of SAV distr ibution since that time is lacking, some observations suggest SAV growth in the oligohaline

re gi ons of estuaries in northwestern Florida has shown recent improvements ( e.g., in Mobile Bay [Alabama], Escambia Bay, and Perdido Bay);
Ev idence suggests these same areas are also losing SAV in the euryhaline regions;
Im provements in water quality of the upper bay

re gi ons is thought to be leading to recovery of low-salinity seagrasses; and Co ntinuing increases in coastal development in the lower bay region, with resulting increased

nutr ient input and sediment loading/resuspension, may be having an adverse impact on thehealth and productivity of high-salinity seagr asses such as T. testudinum.

# UT ECOLOGY and POPULATION GENETICS

Fl ow ering is known in all sixspecies of seagrasses on the Gulf coast of Florida, but Thalassia testudinum pr oduces fewer fruits at higher latitudes (north of Tampa Bay). Genetic information for Florida seagrasses exists only for T. testudinum, with lower genetic diversity observed in its northernpopulations. This may be a result of the lower level of seed production and, therefore, more dependence on vegetative expansion for reproduction.Genetic data are needed for more populations and for other seag rass species. The clonal nature of seagrasses allows transport of soluble carbohydrates, proteins, and nitrog en (as glutami ne) to stressed shor t-shoot and long-shoot rhizome meristems.
De pth distribution of T. testudinum is influenced by a variety of f actors, in cluding water transparency, epiphyte load of the leaf blades, and water movement. Salinity variations affect the local distributions of seagrasses. T. testudinum does not survive if held in culture for over 6 weeks in 6 ppt seawater. In the field, ot her factor s will raise that thr eshold. More data are needed regarding osmoregulation in

eur yhaline species such as Halodule wrightii. Higher sulfide levels in the sediment are toxic to T. testudinum, occur in areas of eutr ophica tion in a number of Gulf-coast estuaries, and may play a role in the patchy nature of seagrass comm unities. Moderate grazing by invertebra tes (especially sea urchins), fish, sea turtles, and manatees ma y enhance seagrass-bed development and species di versity.
Sta ble isotope ratios can be used to trace the flow of organic components in seagrass beds, alt hough macroalgae may have similar isotopic signatures. Carbon fixation by most seagrasses occurs via the C 3 pa thway. How ev er , it is debatable whether a modified form of the C4 pathway exists in some species. Entir e-plant carbon budg ets of T. testudinum and H. wrightii show that below-ground components account for over 50% of a plant’s respiration.

Wi dgeongrass, Ruppia maritima

Ron Phillips photo

## 15 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICS 

# SEAGRASS MORPHOLOGY and ANATOMY
Detailed descriptions of seagrass morphology and anatomy can be found in den Hartog (1970), Ancibar (1979), Tomlinson (1980, 1982), and Kuoand McComb (1989). Also see the Appendix, Taxonomy of Florida Seagrasses, in this document. The basic construction of most seagrasses (Arber1920) is an indeterminate horizontal stem (plag iotropic rhizome or long shoot) that periodicallyproduces determinate erect stems (orthotropic rhizo mes or short shoots) having leaves and flowers. Adve ntitious roots develop from both types of rhizo me. Rhizomes are usually cylindrical and belowthe sediment surface in species with robust morphologies. In contrast, rhizomes of species with more delicate morphologies ( e.g., H. decipiens and H. johnsonii ) often occur above the sediment surface. Rhizome growth is either sympodial ( e.g., H. wrig htii and R. maritima ) or monopodial ( e.g., T. testudinum and S. filiforme ). Leaves of the Gulfco ast Florida species differ greatly in morphology, being long, wide and thick in T. testudinum; long, narrow, and thin in H. wrightii and R. maritima; long, rigid, and cylindrical in S. filiforme; and short, thin, and membranaceous in species of H

alophila.

The rhizomes of Florida seagrasses are herbace ous, with little fiber tissue. In seagrasses similar
to T. t estudinum, the vascular stele and fiber bundles in the cortexes of the blades, short shoots, and

r hizomes are poorly lignified (Dawes 1986). Epidermal cells of seagrass blades lack stomata and associated guard cells, contain most of the blades’

chloroplasts, and have a thick outer cell wall covered by a thin, porous cuticle. As determined by uptake studies (Larkum et al. 1989), the cuticle apparently does not prevent absorption ( e.g., of CO 2 , c admium, or manganese) by the blade. As

w ith those of T. t estudinum, the epidermal cells of R. maritima may be involved in osmoregulation. E pidermal cells of R. maritima blades that are gr ow n in higher salinities ( e.g., 32 ppt) develop masses of gelatinous polysaccharides and form

ce ll-wall ingrowths that may be involved in ion binding and exchange between the seawater and cytoplasm (Kruzcynski 1994).


# SEXUAL REPRODUCTION Although sexual reproduction is known in all the seagrass species of Florida, with the possible exceptionof H. johnsonii (see Appendix), data on flowering and seed production and survival are limited (Ferguson et al. 1993). Leaf and inflorescence scars(Figure 3.1) have been used to determine shortshoot age, sex, and frequency of flowering in T. testudinum (Cox and Tomlinson 1988; van T

ussenbroek 1994; Witz and Dawes 1995), with short shoots that produce female flowers oftenhaving narrower leaves than short shoots that produce male flowers (Durako and Moffler 1985). In florescence scars on short shoots of T. testudinum (Fig 3.1) suggest that abundant flowering occurs in northern areas along Florida’s Gulf coast ( e.g., St. Joseph Bay and Apalachee Bay) and throughout the Florida Keys (C. Dawes pers. obs. ), ye t flowering and seed production have not been documented for most T. t estudinum beds along the northern Gulf coast. In contrast, in May or June of each year, thousands of T. testudinum seeds may occur in the beach wrack at Mathieson Hammock near Miami and in the wrack lines in the Florida Keys (C. Dawes pers. obs.; Lew is and Phillips 1980), and countless more are eaten by birds (Fishman and Orth 1996). Seagrass flowering and fruiting occur annually between February and August along the Gulf coast of Florida (Phillips 1960c; Durako and Moffler 1987; Witz and Dawes 1995) and on the Caribbean

co ast of Mexico (van Tussenbroek 1994). Photoperi od does not appear to influence the onset of flowering in T. testudinum (Moffler and Durako 1987), S. filiforme, or H. wrightii (McMillan 1982), as these three species flowered under continuous light. Instead, water temperature influences flower development (Moffler and Durako 1987), and all species in Florida flower within a temperature

r ange of 20°–26°C (McMillan 1982). Lower water
te mperatures (10° to 18°C) in the winter in the Figure 3.1 Inflorescence, leaf, and root scars on short shoots of Thalassia testudinum (modified from Witz 1994).

## 16 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Ta mpa Bay area may cause T. testudinum to flower later in the spring than it does in the Florida Keys, causing immature fruits to dehisce early in

r esponse to rapidly rising water temperatures in Ma y and June (Witz and Dawes 1995). Seed germination has not been studied in most Florida Gulf-coast seagrass species. Ruppiamaritima seed germination is influenced by salinity and temperature but not by photoperiod. Seeds

co llected from the sediment at the mouth of the We eki Wachee River germinated at all temperatures (17°, 23°, and 39°C) and in 0 and 15 ppt but not 30 ppt salinities. In contrast, seeds from NorthCarolina germinated in all temperature and salinity combinations (Koch and Dawes 1991a). Studies on fish predation of R. maritima seeds demonstrated that they can pass through the gut and thusbe dispersed (Agami and Waisel 1988). The importance of seed reserves as an early source of carbohydrate energy for seedlings of T. testudinum was proposed by Durako and Sackett (1993). Seedlings had a lower carbon isotopic fractionation ( δ 13 C plant

– δ 13 C
source = 15.4 ppt) than did one-year-old plants (21.0 ppt), possibly indicating a shift from using carbon reserves in the seed to taking up CO 2 from the water column. However, the one-year-old plants were cultured under artificial conditions and were preconditioned, which may have altered their fractionation level. 

# ASEXUAL REPRODUCTION and CLONAL BIOLOGY

Ve getative growth by the long-shoot rhizome is thought to be the principal means of expansion for G ulf-coast seagrasses, in light of the curtailment of sexual reproduction explained above. Seagrass short shoots can be regarded as ramets, and the clonal group of ramets connected by long-shoot

r hizomes derived from a single propagule is considered to be the genet (see Harper 1990). Rhizo me extension is the basis for vegetative

expansion in all seagrasses (Tomlinson 1974; Dawes 1998a; Andorfer and Dawes 2002; Dawes and Andorfer 2002). Two possible advantages in being clonal are theability to carry out vegetative expansion or foraging (Cain 1994) and the ability to share resources between ramets of the same genet growing in areas of stress or low nutrients (Tomasko and Dawes 1989; Wijesinghe and Handel 1994; Andorfer 2000). Short shoots of T. testudinum near Mullet Ke y in Tampa Bay were isolated from others by severing the long-shoot rhizome on each side of the short-shoots that were then shaded. These short shoots had significantly lower blade growth than did shaded short shoots that were connected
to other short shoots (Tomasko and Dawes 1989). Rhizomes adjacent to shaded short shoots were depleted of soluble carbohydrates and proteins, while the rhizome portion beyond the severed short shoots was not, suggesting that sugars and amino acids stored in the adjacent rhizomes could be mobilized. Laboratory culture studies using the stable nitrogen isotope 15 NO 3 demonstrated that nitrogen in the form of glutamine was translocated from the fourth-oldest ramet of T. testudinum to the gr ow ing long-shoot rhizome meristem (Andorfer 2000). Further, leaf regrowth, after the four y

oungest short shoots were clipped, was supported bycarbon fixed in the fifth short shoot and transported to it via the rhizome. This high degree of short-shoot integration, over a distance of five

ra mets, demonstrates the importance of the physiological integration that allows T. testudinum to

expand through regions of low light or nutrientsand to tolerate periods of intense grazing (Andorfer 2000). These studies support earlier findings for T. testudinum subjected to weekly clipping of blades (Dawes and Lawrence 1979) and for S. filiforme in the Indian River Lagoon subjected to shading of connected and severed short shoots(Rey and Stephens 1996). In the latter study, soluble-carbohydrate levels declined from 26.5% to 18.4% and then stabilized in rhizomes connected
to shaded short shoots; this suggests movement of soluble carbohydrate from adjacent non-shadedshort shoots, thus showing physiological integration of the genet. Pr oduction of new long-shoot rhizome branches by T. testudinum occurs principally at the long-shoot meristem and rarely occurs through branching of the older long-shoot rhizome. Dawes and Andorfer (2002) found that older short shoots

we remore likely to produce a new long-shoot rhizo me than were younger ones and that initiation of rhizomes was suppressed unless the existing long-shoot meristem was removed. The presenceof “inactive” or dormant short shoots and rhizo mes on T. t estudinum suggests that a meristem bank may exist in T. testudinum beds, as was found

## 17 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICSon the Caribbean coast of Mexico (van Tussenbroek 1996a). Just what role these inactive short shoots play in vegetative expansion in a seagrassbed is yet unknown. Perhaps, under certain conditions, the dormant short shoots will again resume gr owth and produce new blades or rhizome meriste ms. Further, seagrasses differ in rhizome-branch frequency. For example, H. wrightii r hizomes branch frequently and grow rapidly, whereas those of T. testudinum do not. Species that exhibit more aggressive growth are often chosen for restoration in Florida. Thus H. wrightii is more often selected for restoration projects than T. testudinum, although the latter species forms the dominant beds in Florida and contains the greatest diversity of species (Zieman and Zieman 1989). The types of growth (foraging) strategies of seagrasses arethe same as those of terrestrial clonal plants. Rhizo mes of H. wrightii show a “guerrilla”-type gr owth by branching frequently and spreading

r apidly. In contrast, T. testudinum r hizomes show a “p halanx”-type growth, growing linearly and with few lateral axes (Dawes 1998a). 

# EVOLUTION and GENETICS
The discovery of fossil seagrasses in the Avon Park formation in Florida (Lumbert et al. 1984) indicates that species of Thalassodendron and Cymo docea occurred with T. testudinum in the late M iddle Eocene ( ca. 40 million ybp). These fossils

we represerved as carbonized imprints within the bedding planes of a micritic dolomitic limestone in a rock outcrop of the Ocala Arch in central Florida, which contains the oldest exposed rocks in the state. Generic determinations were certain,

exce pt for one of the fossils, and species identifications were questionable for two of the fossils. The fossils were identified as Thalassodendron auriculaleporis den Hartog, Cy modocea floridana den Hartog, T. testudinum, Cymodocea sp., H alodule sp., and an unknown Zosteroid. Species of Thalassodendron and Cymo docea presently occur only in the Old World tropics (Indo-Pacific region), indicating that a more diverse seagrass flora existed in the Caribbean Seain the past. One hypothesis suggests that they disappeared from the Caribbean region when the Central American isthmus elevated and separatedthe Atlantic and Pacific oceans, changing dominant circulation patterns, which caused Caribbean water temperatures to fall. Evidentially, an extensive and diverse seagrass flora existed in central Florida in the past, judging by the abundance and

exte nt of the fossilized seagrasses and by the diversity of less well-preserved animals from seagrass beds (two families of foraminifera, some bivalves,a bryozoan, a crab carapace, and possibly an ostraco d). The fossil findings support the vicariance model of Heck and McCoy (1979), who proposed that the present-day distribution of seagrass species is a product of continental drift, speciation, and extinction. Hypotheses regarding the origin ofseagrasses include evolution from salt-tolerant terr estrial shrubs (den Hartog 1970) or from freshwate r hydrophilous ancestors (Cox and Humphries 1993). Co mparison of isozymes of various enzymes and molecular-DNA techniques have been used to

examine genetic diversity within and between present-day seagrass populations and species in Florida and the Caribbean region. Early isozyme studies

examined genetic differences between Caribbean Sea and Gulf of Mexico populations of T. testudinum, S. filiforme, and H. wrightii but found little intraspecific variation (McMillan 1980). The studies supported McMillan’s (1978) earlier culture

experiments, which revealed that leaf width was influenced by the immediate environment and not by genetic differences between plants. Being the dominant seagrass in Florida and the Caribbean (Zieman and Zieman 1989), T. testudinum has been the principal species to have its genetic diversity measured using isozymes(Schlueter and Guttman 1998) and molecularDNA approaches (Kirsten et al. 1998; Davis et al. 1999; Waycott and Barnes 2001). Allozyme loci of 14 enzymes in 18 populations in the lower Florida Ke ys indicated that asexual reproduction is probably the basis for the low genetic diversity there and for a trend towards genetic uniformity (Schlueter and Guttman 1998). Allozyme and Amplified Fr agment-Length Polymorphism (AFLP) analyses

we reused to compare genetic diversities of T. testudinum from two sites in Panama and from another in Bermuda (Waycott and Barnes 2001).The authors found high levels of genetic uniformity, and they suggested that it is due to vegetative-fragment dispersal, even over a distance of 2,700 km. Incontrast to isozyme and allozyme studies, two other molecular DNA studies found a higher

## 18 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY level of genetic diversity within populations of T. testudinum, rather than between geographically distinct ones. Randomly Amplified PolymorphicDNA (RAPD) analysis showed high genetic diversity within two proximal Florida Keys populations ( e.g., Fiesta Key and Craig Key) and within another northern population off the Steinhatchee River (as Apalachicola Bay in Kirsten et al. 1998). Further,almost all samples from an outlier Jamaican population and the two Florida Keys sites were distinct genetic individuals within and between the three populations. This is in contrast to the northern population that had the fewest RAPD phenotypes (Figure 3.2). The lower within-community genetic diversity of northern populations of T. t estudinum may reflect the limited introduction of new genets ( e.g., drift plants) or a low level of flowering andseed production, perhaps because of less-thanoptimum water temperatures or water transparencies. A second study on T. t estudinum, using DNA-fingerprinting techniques, concentrated on clonal variation in populations in Florida Bay andfound discrete beds that were not genetically uniform (Davis et al. 1999), again emphasizing the

r ole of sexual reproduction in maintaining population genetic variation. Thus, beds of T. testudinum in more tropical regions contain a greater

nu mber of distinct genets than do beds at higher latitudes. The difference in genetic diversity between tropical and subtropical beds may reflect a lower rate of seed production in more northern sites, as shown for seagrasses in Tampa Bay (Witz 1994; Witz and Dawes 1995). This may explain the lower genetic diversity for the northern population of T. testudinum found off the Steinhatchee River (Kirsten et al. 1998). The concept that sexual

re production is less likely to be successful when anorganism encounters less-than-optimal temperatures was described by Gessner (1970). However, little is known about flowering and the production of viable seeds on most of the Gulf coast of Florida, with the exception of some data for Ta mpa Bay and the Florida Keys. Another study (Angel 2002) using RAPD analysis compared three populations of H. wrightii from Texas (Christmas Bay, Corpus Christi) and from Florida (Florida Bay). All individuals appeared to have unique genotypes, with plants from Corpus Cr isti more closely clustered to those of Florida Bay, which Angel suggested was due to similarhabitats that acted as a selective force. The molecular phylogeny of 11 species of H alophila found that H. decipiens and H. engelmannii we redistinct species and that there was a 100% overlap betweenpopulations of the former species between populations from Australia, the Caribbean, and Florida (W ayc ott et al. 2002). Further, the internal transcribed spacer (ITS) region of the ribosomal DNA indicated that H. johnsonii from the Indian River in Florida could not be distinguished from H. ovalis and needs further study.


# PHYSIOLOGICAL ECOLOGY

Atleast five abiotic properties influence the morphological and anatomical adaptations of seagr asses (Dawes 1998a), including the (1) osmotic effects of salt water; (2) availability of dissolved CO 2 and nutrients; (3) intensity and quality of illumination; (4) density (greater than air) and mechanical drag of an aqueous medium; and (5) effects of an aquatic medium on the dispersal of pollen and seeds. The general physical requirements of five Florida seagrass species were discussed in Phillips (1960a). Seagrasses show physiological adaptations
to a variety of abiotic and biotic conditions, including salinity, temperature, water motion, anoxia, nutrient limitation, epiphytes, irradiance, infection, and herbivory (Kuo and den Hartog1989). The most-studied factor has been the relationship between light and the depth distributionof seagrasses. Nevertheless, other physical, geological, and chemical characters must be examined to

explain the patchy distribution of seagrasses on the Gulf coast of Florida (Koch 2001). Koch identified Figure 3.2 Randomly amplified pol ymorphic DNA (RAPD) phenotypes within four populations of Thalassia testudinum. (Modified from Kirsten et al. 1998)

## 19 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICStidal amplitude (Figure 3.3) and wave energy (Figure 3.4) as two factors that influence seagrass depth distribution. Tolerances to common abiotic factors may differ between populations of a species from different environments, as shown for R. maritima gr own from seed (Koch and Dawes 1991b). Different strains grown from seeds collected in the We eki Wachee River estuary and from North Carolina had growth rates that were similar under different combinations of temperature and photoperiod but differed with regard to salinity. This was also reported in earlier studies for other species (McMillan 1979, 1984). In those studies, different strains of T. testudinum, S. filiforme, and H. wrightii showed different tolerances to temperature, and these tolerances were often related todistinct environmental origins. These findings suggest that transplanting from one area to a different site at greater distances ( e.g., seeds from Biscayne Bay to Tampa Bay) may not be as successful as when using local plants. A large global-level literature base shows the critical effects of water quality on the presence, gr owth, and vitality of seagrasses. Adequate light
tr ansmission is vital to the presence of seagrasses and affects the density and biomass of seagrasses at all depths (Dixon and Leverone 1991; Duarte 1991; Johansson 1991; Kenworthy and Haunert 1991; Lapointe and Clark 1992; Tomasko 1992, 2002; Lapointe et al. 1994; Fletcher and Fletcher 1995; Kenworthy and Fonseca 1996; Kurz et al.1999, 2000; Dixon 2000; Neely 2000). Depth determinations (±10 cm) can be obtained with a mapping-grade differential Global Positioning System,carrier-phase equipment and surveyed benchmarks (Johansson 2002b). In Tampa Bay, the deep edges for H. wrightii r anged from –0.30 to –0.76 m below mean low water (MLW), from –1.19 to –1.96 m for S. filiforme, and from –0.57 to –1.73 m for T. testudinum. The maximum depth of T. testudinum at Port Manatee in Tampa Bay was –1.6 m, in agreement with the above determinations carried out before the 1997–1998 El Niño event (Dixon and Leverone 1995). The shallowest deepedges for all three species occurred at the northernmost sites, and the deepest deep edges were near the mouth of Tampa Bay, where more oceanic (clear water) conditions exist. Neely (2000) demonstrated an inverse relationship between light reduction and short-shoot numbers for H. wrightii in Tampa Bay. Leaves on plants

exposed to a 43% light reduction were longer thanthose on shoots in control plots or in other lightre duction treatments. Her experiments showed that loss of biomass was a result of winter die-back and also occurred with a >60% reduction in light.The data showed that epiphytes and surface scattering of light must be accounted for when calculating carbon budgets. Durako and Hall (1992) found that for T. t estudinum the carbon supply became nonlimiting when light was reduced to levels that limited photosynthetic rates. Tomasko (1992) demonstrated that H. wrightii, when growing under a canopy of T. testudinum leaves, had longer

rhizome internodes, less frequent branching, and lower above-ground to below-ground biomass

ratios than when growing alone. Studies by SandJe nsen (1977, 1990), Cambridge et al. (1986), and To masko and Lapointe (1991) showed that heavy epiphyte loads on seagrass leaves are indicative of

nutr ient over-loading in the water column and, therefore, eutrophication, i.e., poor water quality (see Chapter 5). In addition to the adverse effects of

nutr ient-enriched water on photosynthetic functioning of seagrass leaves, increased levels of nutrients can result in high epiphyte loads that shade and weigh down the leaves, further lowering them in the water column. Figure 3.3 Influence of tidal range on seagrass depth distribution. Minimum depth is deeper with greater tidal range (Koch2001). Reprinted with permission from Estuarine Research Federa tion. Figure 3.4 Influence of wave energy on seagrass depth distribution. Grea ter wave energy and deeper wave-m ixing depths

r esult in deeper minimum depths (Koch 2001). Reprinted with permission from Estuarine Research Federation.

## 20 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY The relationship between light, both quantity and quality, and the depth at which seagrasses gr ow was the subject of two workshops in Florida in the 1990s, one sponsored by NOAA (Kenworthy and Haunert 1991) and the other by the Indian River Lagoon National Estuary Program (Morris and Tomasko 1993). Even though they can grow under high irradiences, seagrasses are regarded as shade plants (Hillman et al. 1989), with compensation values (I
c ) of 25 to 50 μmole-photons m –2 s –1 and saturation irradiances (I k ) of <200 μmolephotons m –2 s –1 . S tudies comparing photosynthetic

r esponses of seagrasses to different irradiance levels ( i.e., I
c , I k , a nd P
max levels) often show a relationship to temperature and salinity, as seen for two species of H alophila (Dawes et al. 1989). For example, the estuarine, intertidal populations of H. jo hnsonii in the Indian River Lagoon tolerate a wi de range of temperatures (10°, 20°, and 30°C) and salinities (15, 25, and 35 ppt) and did not become photoinhibited by high photosynthetically active radiation (PAR; ca. 500 μmole-photons m –2

s –1 ). In contrast, H. decipiens from deeper, nonestuarine Gulf-coast populations were intolerant of a 10°C temperature, 15 and 25 ppt salinity, and PAR above 300 μmole-photons m –2 s –1 (Dawes et al. 1989). Depth distributions of Florida’s seagrasses are governed by PAR levels in Tampa Bay (Miller and M cPherson 1995; Dixon and Leverone 1995, 1997;Dixon 2002) and the Indian River Lagoon (Gallegos and Kenworthy 1996). Recent work has demonstrated more specific light requirements forthe dominant seagrass species (Table 3.1). PAR levels measured at the deep edges of T. testudinum beds in Tampa Bay were significantly higher than the required average light level of 11% (of surface value) proposed for seagrasses by Duarte (1991). Also, T. t estudinum in Florida Bay (Fourqurean and Zieman 1991) and Caribbean seagrasses (Onuf 1994) required at least 22% of surface PAR levels, when whole-plant requirements and average daily irradiances were considered. Continuous in situ measurements of underwater PAR levels for one year at the shallow and deep edges of four T. testudinum beds in Tampa Bay also indicated the Ta ble 3.1 Light requirements of Florida seagrasses.

M inimum Light Requirement Species and Sites (percentage of total surface irradiance) Reference
Thalassia testudinum B ig Bend area, FL 15.3% Iverson and Bittaker (1986), as

re ported by Duarte (1991) and by

Dennison et al

. (1993) No rthwest Cuba 23.5% Duarte (1991) Pu erto Rico 24.4% Duarte (1991) Charlotte Harbor, FL 20% Tomasko and Hall (1999) Ta mpa Bay, FL 22.5% Dixon and Leverone (1995) Ta mpa Bay, FL 20.1% (annual mean) Dixon (2000) Co r pus Christi Bay, TX 14% (4% survival after 12 months) Czerny and Dunton (1995) Sy r ingodium filiforme Florida Bay, FL 18.3% Iverson and Bittaker (1986) as

re ported by Duarte (1991) I ndian River Lagoon, FL 17.2% Duarte (1991) pers. comm. from J. Ke nworthy (1990) I ndian River Lagoon, FL 24%–37% Kenworthy and Fonseca (1996) (southern region)
H

alodule wrightii I ndian River Lagoon, FL 17.2% Dennison et al . ( 1993) pers. comm. from J. Kenworthy (1990) Florida Bay, FL 10%–20% Fourqurean and Zieman (1991) Laguna Madre, TX 7%–23% Tomasko and Dunton (1991); Dunton (1994); Czerny and Dunton (1995)

## 21 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICS maximum depth limit in the bay occurred at about 20.5% of subsurface PAR (Dixon and Leverone 1995, 1997). Another study on irradiance in the more oceanic waters at the mouth of Tampa Bay showed that, using 20% light penetration as the threshold, sufficient light was available for T. testudinum beds to extend to two meters of depth(Miller and McPherson 1995). That depth is similar to measured seagrass-bed depth limits, which

r anged from 1.98 to 2.37 m in estuaries (Dixon and Leverone 1995). Evaluating short-shootdensity, Neely (2000) found that shallow, monospecific beds of H. wrightii showed a direct relationship between density and irradiance level at Mullet Key in Tampa Bay (Figure 3.5). The same level of density reduction with shading occurred in the cool and warm seasons, although summer short-shoot densities were almost twice those of the winter densities, indicating the critical effectthat underwater irradiance has on seagrasses. Similar declines in leaf growth, short-shoot density, biomass and primary production have been caused by shading T. testudinum (Lee and Dunton 1997; Kr aemer and Hanisak 2000) and H. wrightii (Czerny and Dunton 1995).Light attenuation that limits seagrass depth distr ibution can result from a variety of sources (Kenwo rthy and Haunert 1991; Morris and Tomasko1993). One factor governing light effects on seagr ass depth distribution is the level of epiphyticbiomass. The average annual attenuation by epiphytes on water-column PAR levels available to T. testudinum blades was from 32%–47% in Tampa Bay, 40%–56% in Sarasota Bay, and 21%–44% in Charlotte Harbor (Dixon and Leverone 1997). Var iation in epiphyte loads on T. testudinum blades resulted in a 7%–67% seasonal range in P

AR attenuation in Tampa Bay, 13%–99% in Sarasota Bay, and 6%-89% in Charlotte Harbor (Dixon and Leverone 1997). Thus, in determining seagrass light requirements, seasonal levels of epiphyte loads should be considered in addition to the actual PAR levels (Dixon 2002). Other factors shown to have critical effects onlight available to seagrass, in a light model developed for the Indian River Lagoon, were total suspended solids ( e.g., tr ipton or non-algal particulate matter) and chlorophyll a concentrat ion, which

we rethe dominant causes of light attenuation affecting seagrass depth distribution (Christian and Sheng 2002). Attenuation of light by turbidity is a critical abiotic factor governing availability of light to seagrasses, as shown by the effects of dredging in the Laguna Madre of Texas that

re sulted in the decline of H. wrightii (Onuf 1994). Depth distribution, biomass, and photosynthetic

r esponses were compared for shallow and deep populations of T. t estudinum on the Gulf coast of Florida (Dawes and Tomasko 1988; Dawes 1998b). Ingeneral, biomass decreased while chlorophyll a increased with greater depth (Table 3.2), and the

ratio of above- to below-ground biomass

re mained similar. Also, light quality ( i.e., wave-length distribution) and PAR affect seagrass growth. Exposure of H. wrightii, H. engelmannii, and S. filiforme toUV-A and PAR irradiation resulted in photosynthesis inhibition in the latter species (Trocine et al. 1982). A low ratio of red:far-red wave lengths

re duced branching and node formation in R. maritima, a response that may allow plants shaded by larger plants to escape by emphasizing horizontal

r hizome elongation (Rose and Durako 1994). A similar response by H. wrightii, when shaded by T. testudinum, was noted in the Florida Keys (T

omasko 1992). Photosynthetic responses have been used to

evaluate seagrass tolerances to salinity and temperature variations (Dawes et al. 1987, 1989). Estuarine populations of H. engelmannii and H. johnsonii from the Gulf coast and Indian River Lagoon of Florida, respectively, had broader tolerances to a

r ange of temperatures and salinities when compared to Gulf-coast oceanic populations of H.

engelmannii and H. decipiens. Differences in phot osynthetic factors ( e.g., P
max , α , I k ) were evident

w ith freshly collected plants and after culturing Figure 3.5 Average density (short shoots m –2 ) of Halodule wrightii at control and three increasing levels of shading(30, 50, 80%) in Tampa Bay seagrass beds (± 1 S.D.). (Modifi ed from Neely 2000)

## 22 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY plants under common garden conditions for a

nu mber of weeks using various combinations of
te mperature and salinity. Salinity tolerance of T. testudinum from San Carlos Bay in the Caloosahatchee River Estuary in southwest Florida was assessed using long-shoot rhizomes bearing two to four short shoots (Doering and Chamberlain 2000). After a four-week acclimatization period inambient seawater (35 ppt) in the laboratory, salinity was changed daily in 1.5 ppt increments. Re gardless of other variables, blade length decreased (Figure 3.6) and blade production ended after 43 days in the lowest (6 ppt) salinity. Blade length was positively correlated with salinity after 43 days of exposure. Also, the number of blades per short shoot and blade biomass were similar in 12, 18, 25, and 35 ppt salinities after 43 days of exposure (data not shown). Salinity affects the local distribution of seagrass species on the Gulf coast of Florida (Phillips 1960b, c), with euryhaline species such as H. wrig htii able to grow in a broad range of salinities ( e.g., 5 to 55 ppt; Dunton 1996). However, the effect on seagrass distribution due to salinity aloneis difficult to separate from the effects of other facto rs, as shown in the general reduction of seagrass beds throughout Tampa Bay after the 1997–1998 El Niño event. The 1998 rainfall increase resulted in higher levels of dissolved inorganic nitrogen,

chlorophyll a (phytoplankton), and color content (tannins) from extensive runoff into the bay (J ohansson 2002a), which in turn reduced PAR penetration in the water column. In addition, the increase in rainfall directly lowered salinities, with M iddle Tampa Bay salinities declining from a winte r high of 30 ppt to 10 ppt by March and then to less than 5 ppt over a two-week period in April,1998 (Dawes, unpublished data). Therefore, comFigure 3.6 Blade length and mean blade elonga tion rates for Thalassia testudinum cultured for 43 days in salinities ranging from 6 to 35 ppt. Letters indicate statistical differences between trea tments and means, different letters are statistically different (P < 0.05). (Modified from Doering and Chamberlain 2000) Ta ble 3.2 Photosynthetic characters (P
max , alpha, I
c , t otal chlorophyll), primary production, and leaf growth (± S.D.) of Thalassia testudinum in deep- and shallow-water populations at two Florida Gulf-coast sites.

L eaf Relative Primary Gr owth Rate Production Total Chl Site (mg dw · g dw –1 · d–1 ) (g dw · m –2 · d–1 ) (mg · g dw –1 )
Egmo nt Key

Shallow (–0.4 m) 28.7 (5.4) 11.71 4.86 (0.99) Deep (–2.5 m) 27.4 (6.3) 8.17 3.92 (1.28)
Anclote Key

Shallow (–1.0 m) 31.1 (9.3) 1.39 3.00 (0.84) Deep (–2.5 m) 20.5 (0.3)* 1.00 2.39 (0.70)

P max I c Alpha Site (mg O 2 · g dw –1 · h –1 ) (μE · m –2 · s –1 ) (P
max ⁄ I c )
Egmo nt Key

Shallow 4.54 (0.70) 39 0.063 Deep 6.31 (1.21) 45 0.069
Anclote Key Shallow 6.08 (0.92) 32 0.103 Deep 2.60 (0.33)* 24 0.097

Mo dified from Dawes and Tomasko (1988). * = probability <0.01

## 23 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICSbined with higher nutrient loadings and turbiditycaused light attenuation, lower salinities resulted in a die-back of T. testudinum in Cockroach Bay by June 1998 and a reduction in seagrass acreage throughout west-central Florida coastal areas (see Chapter 2). Seagrass osmoregulatory responses to changing salinities involve structural and physiological

changes. The epidermal cells of T. testudinum and R. maritima show cell-wall and or ganelle modifications when grown or collected in salinities above 30 ppt (Jagels and Barnabas 1989). In R. maritima, clones of a single genetic strain cultured in 32 ppt had convoluted cell membranes, smaller vacuoles, and numerous mitochondria associated with the

ce ll membrane (Table 3.3). Also, plasmodesmata

we renot evident between the epidermal and cortical cells, and a significant increase occurred in acidic mucopolysaccharides associated with the inner cell wall (Kruzcynski 1994). Clones from the same genetic strain of R. maritima did not show any of the above features when cultured at a lower salinity ( ca. 10 ppt). The increase in cell membranesurface area and associated mitochondria are characteristic of cells active in osmoregulation, andacidic carbohydrates may be involved in ion transport and binding (Dawes 1998a). In addition, osmotic roles of the amino acids proline, alanine, and glutamate have been identified in seagrasses (Pulich 1986; Adams and Bate 1994). Pulich (1986) reported that proline functions as an organic osmoticum in R. maritima, T. testudinum, and H. wrightii, and alanine serves that role in H. engelmannii. Data on temperature effects on seagrasses are limited to studies examining flowering and seed germination (see earlier discussion) and from astudy of leaf-blade production near an electricitygenerating plant (Barber and Behrens 1985). In the latter study, the productivity of S. filiforme increased in cooler months and decreased in warmer months in areas where water temperatures

we reenhanced by thermal pollution, whereas T. testudinum showed little or no response. Wate r movement influences the spatial distribution of seagrass beds, in addition to the effects on depth distribution from tidal and wave activity (seealso Chapter 5). Water velocities, degree of exposure to waves, and relative water depths strongly affect the distribution of H. wrightii and Z. marina beds near Beaufort, North Carolina (Fonseca andBell 1998). Water movement, expressed as bladefriction velocities (u * = cm s –1 ), reduced boundarylayer effects around the leaf, and photosynthetic

rateincreased in T. t estudinum, up to a current

ve locity of 0.25 cm s –1 (Koch 2001). In addition, increased water movement causes a decrease in

co ncentration of sediment sulfide, a phytotoxin. Fur ther, seedlings of T. t estudinum collecte d in theFlorida Keys showed optimum growth under intermediate flow rates (u * = 0.3 cm s –1 ), whereas stagnant water (u * = 0.0) contributed to lower biomass possibly because of increased sulfide levels, and high flow rates (u * = 1.0 cm s –1 ) reduced nutrient

availability in the sediment pore water (Koch 1999a). Back-and-forth wave motion increases

nutr ient exchange between blades and the water

co lumn, in contrast to unidirectional movement or currents (skimming) that flow over T. t estudinum meadows in the Florida Keys (Koch and Gust 1999). When the flow rate (u * ) equaled 25 cm s –1 , the water current began to erode the sand; this caused a 50% reduction in seagrass coverage. Re peated intertidal exposure, and thus the development of tolerance to desiccation, has been suggested as a causal factor in the ability of Ta ble 3.3 Structural features of Ru ppia maritima epidermal cells from a single monoculture grown in 10 ppt and 32 ppt salinity.

Co mponent of Mean (± SD) Mean (± SD) Significance Cell Volume at 10 ppt at 32 ppt Level

Va cuolar volume 28.4% (± 19.8) 9.5% (± 9.3) P = 0.03* Cell membrane 0.5 μm (± 0.7) 5.0 μm (± 4.2) P = 0.001** invagination
Po lysaccharide volume 6.7% (± 3.5) 25.0% (± 13.4) P = 0.03* Chloroplast volume 8.6% (± 4.8) 13.3% (± 5.1) P = 0.02* Mito ch ondria number 12.2 (± 8.9) 17.3 (± 5.5) P = 0.03*

Mo dified from Kruzcynski (1994). * = significant; ** = highly significant.

## 24 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY H. wrightii to grow intertidally, where it may form

exte nsive beds, in contrast to the subtidal distributionof T. testudinum (Phillips 1960a). However, factors other than desiccation tolerance may play a more critical role in terms of the higher vertical

zo nation for H. wrightii, for example, a greater tolerance to high irradiances and benefits from higher

nutr ient input from the shore (Björk et al. 1999). T

olerance of exposure to high light levels in shallow or intertidal waters suggests that seagrasses may have evolved photosynthetic mechanisms ( e.g., C 4 photosynthesis) that reduce damage byUV radiation or intense PAR. However, photosynthetic mechanisms of all seagrass studied thus farappear to involve carbon-fixation reactions characteristic only of the Calvin cycle (C 3 plants) (Beer
et al. 1977, 1980; Andrews and Abel 1977; Benedict
et al. 1980; Durako 1993), with the possible exception of the Mediterranean species, Cymo docea nodosa (Beer et al. 1980). Carbon fixation in C 3 plants occurs via the Calvin Cycle, in which theenzyme ribulose-1,5-bisphosphate carboxylaseoxygenase (rubisco) initially produces a three-carbon (C 3 ) compound, phosphoglycerate (Abel and Drew 1989; Beer 1996). However, an earlier study (Benedict and Scott 1976) reported that high levels of labeled carbon ( 14 C-bicarbonate) were incorporate d into malate (30%) and aspartate (33%) in T. testudinum. These two four-carbon acids are commonly associated with the initial fixation step of the Hatch-Slack pathway of C 4 plants. The process described by Benedict and Scott (1976) for T. testudinum is similar to Crassulacean Acid Metabolism (CAM), which prevents loss of fixed carbon (as CO 2 ) resulting from photorespiration suffered byplants under high light irradiance. Because seagr ass lacunae store respired CO 2 , d etermining thelevel of photorespiration in seagrass blades is difficult, although it appears to be lower than in susce ptible terrestrial plants (Abel and Drew 1989). Mo restudies are needed to determine whether any of the six Gulf-coast species have modified C 4 carbon-fixation pathways. Durako (1993) demonstrated that T. testudinum can utilize both bicarbonate (HCO 3 – ) and carbon dioxide, with relatively efficient HCO 3 – utilization, despite an apparently low affinity for this form of carbon. Plants with C 4 photosynthesis have higher (less negative) δ 13 C values than those of plants using only the C 3 pathway. It has been shown that seagr asses have δ 13 C values similar to those of C 4 plants. The similarity in δ 13 C values may be due to diffusional restrictions on carbon movement intothe chloroplast in a relatively closed carbon-fixation system rather than reflecting their mode ( e.g., C 3 or C 4 ) of carbon fixation (O’Leary 1988). Overall, average seagrass δ 13 C values range between –10and –11 ppt, showing stable-isotope-ratio signatures that are usually less depleted in 13 C than theyare in other aquatic primary producers (Hemminga and Mateo 1996). Thus, δ 13 C values can beused to trace carbon flow in short-term carbonallocation and production studies. In contrast tothe overall averages for seagrasses given by Hemminga and Mateo, the δ 13 C values for T. t estudinum leaves in south Florida, when growing adjacent to mangroves (Figure 3.7), ranged from –7.3 to –16.3

w ith stable-isotope ratios of carbon that were similar to those of mangrove detritus (Lin et al. 1991). The difference in δ 13 C values suggests influence of mangrove detritus on nearby seagrass beds. Ma croalgae may have nitrogen stable-isotope

ratios similar to those of seagrasses, as shown for δ 15 N. Using δ 15 N values, Dillon et al. (2002) determined the sources of nitrogen in Florida. In Sarasota Bay, macroalgae had δ 15 N values that averaged +4 per mil greater than those of seagrasses, the higher values being positively correlated with δ 15 N values of wastewater discharge and being similar to macroalgae δ 15 N values near populated areas in the Florida Keys. Seagrasses facilitate the transport of inorganic carbon from seawater to the chloroplasts of their Figure 3.7 Sta ble-carbon isotope values of seagrasses are influenced by adjacent mangroves.

## 25 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICS epidermal cells to make up for slow CO 2 diffusion

ra t es and low CO 2 levels in water, as well as for the low affinity of rubisco for CO 2 when the lacunae have high concentrations of O 2 (Beer 1996). Mo re importantly, seagrasses can use HCO 3 – fromseawater via carbonic anhydrase-catalyzed extrace llular conversions. Unlike seaweeds, which havephotosynthetic rates that are saturated by present-day CO 2 levels, seagrasses continue to be CO 2 -limited (Beer and Koch 1996). The authors suggest that when seagrasses colonized the sea in the Cretaceous, CO 2 levels were higher. With atmospheric CO 2 levels again increasing, the authors suggest that nearshore beds may expand (but see Beer et al. 2003). However, CO 2 concentr ation in seawater need not be a limiting factor
to seagrass growth under low irradiance conditions, as shown by Durako and Hall (1992). Using δ 13 C values, they reported that CO 2 concentration in natural seawater was not limiting for T. testudinum under low light when photosynthetic

ra t es were low, a relationship that may be relatively common in turbid water.Entire-plant carbon budgets have been measured for T. testudinum in Florida Bay (Fourqurean and Zieman 1991) and for H. wrightii in Tampa Bay (Neely 2000). Below-ground biomass (roots,

r hizomes, and short shoots) of T. testudinum accounted for 85% of the standing stock and 57.5% of total-plant respiration (Fourqurean and Zieman 1991). In contrast, below-ground biomass of H. wrightii r anged from 94% (winter) to 69% (summer) in Tampa Bay, with an above-ground productivity of 0.25 g C m –2 d–1 in September (Neely 2000). To masko and Dunton (1995) compared four methods (blade clip-and-reharvest, above-ground biomass values, leaf-production turnover rates, and below-ground:above-ground-biomass ratios) for estimating diel rates of primary production in H. wrightii populations of Texas. The whole-plant biomass and leaf-turnover approaches appeared to be useful tools for estimating productivity in H. wrig htii . T he blade clip-and-reharvest methoddevised by Virnstein (1982) for determining primary production in narrow-bladed seagrasses such

as H. wrightii, underestimated primary productivity. Further, they concluded that the H sat model that calculates total PAR for a photoperiod was

re liable and accurate in the prediction of daily carbon gain. However, they stated that although the in situ measurement of underwater light levels and P vs. I parameters, as used in Zimmerman et al. (1994), is a more rigorous approach for calculating primary production, it requires a large investment of time and resources.Because of their conspicuous roles in productivity and biomass generation, both the organic

co mposition and the caloric values of seagrasses have been studied (Dawes 1998a). Annual mean caloric values per gram of dry weight (g dry wt) for leaves, short shoots, and rhizomes of T. testudinum we re10–13, 8–11, and 12–14 kJ, respectively. Similar values were obtained for S. filiforme (10–13, 10–14, and 15–16 kJ, respectively) and H. wrig htii (13–15, 12–13, and 14–16 kJ, respectively). Another study in seagrass beds between Tarpon Springs and Cedar Key estimated that available kilocalories ranged from a 16-month low of 344 to a high of 1,837 kcal–1 m –2 , w ith the highest SAV biomass and caloric values occurring in the late spring and summer (Dawes et al. 1987). Dawes (1998a) found that lipid levels for T. testudinum, S. filiforme, and H. wrightii were low,

r anging from <0.1% to 6.3% of the dry weight for various organs. Soluble protein ranged from 5% to 22% of the dry weight, with leaves having thehighest content in all three species. Soluble carbohydrates ranged from 6% to 54% of the dry

we ight, with rhizomes having the highest levels. Soluble protein and soluble carbohydrate levels show pronounced seasonal and species variations, as summarized by Zieman and Zieman (1989) and Dawes (1998a). Rhizomes, followed by short shoots, are the principal storage organs for soluble carbohydrates and proteins in T. testudinum, S. filiforme, and H. wrig htii (Dawes and Lawrence 1980) and in H.

en

gelmannii (Dawes et al. 1987). Further, rhizomesare the source of soluble carbohydrates and prote ins for regrowth of cropped blades and for initiation of spring blade growth in T. t estudinum (Dawes and Lawrence 1979, 1980; Tomasko and Dawes 1989). In a review of carbon, nitrogen, and phosphorus content in the leaves of 27 seagrass species, Duarte (1990) found that they accountedfor 34%, 2%, and 0.2% of the dry weight, respectively, with a mean C:N:P ratio of 474:24:1. Although the cell walls of seagrasses contain lignin,

ce lluloses, and hemicelluloses, they are in lower

co ncentrations than in terrestrial monocots (Table 3.4). Soluble, nonstructural carbohydrates (hemi-

## 26 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY ce lluloses) can account for 1%–16% of cell-wall dry weight in seagrass blades, whereas structural

ce llulose and lignin account for 18%–40% and 0.2%–5% respectively. By comparison, tropical gr asses such as tall fescue ( F estuca arundinacea Schreb.) and Bermudagrass ( Cy nodon dactylon (L.) Pe rs.) have 37% and 40% hemicellulose, 32% and 27% cellulose, and 10% and 6% lignin per g dry
wt –1 , r espectively (Dawes 1986). The adaptations of seagrasses to a hydrophytic environment are seen in their flexible blades ( i.e., few fiber bundles) and low amounts of lignin. Perhaps the brief life spans of leaves of many seagrass species, ca. 15 days in T. testudinum (W itz and Dawes 1995; van Tussenbroek1996b), may not allow time for significant production of lignin to occur, compared to the long leaflife spans (months) of terrestrial plants, thus making seagrass blades more edible to grazers.


# RHIZOSPHERE and SEDIMENT DYNAMICSSeagrasses serve as sediment traps by acting as baffles, causing sediment-laden water to slow and drop its sediment as shown for seagrasses of Florida Bay (Prager and Halley 1999). Even species

w ith diminutive morphologies, such as H. decipie ns, can reduce sediment movement (Fonseca 1989). However, studies in T. testudinum beds in Ta mpa Bay show that sediment resuspension doesoccur because of tidal currents, so these communities are both sinks and sources of suspended matte r (Koch 1999b). Seagrasses modify the anaerobic sediment inwhich they grow. Roots and rhizomes release oxygen translocated from the shoot, cause bioturbation of the soil, and add organic matter viadecomposition (Moriarty and Boon 1989). Chemical changes wrought by the below-ground components are critical for plant survival because high levels of sulfides in anoxic reducing sediments are
toxic to plants (Erskine and Koch 2000; Koch and Erskine 2001; Carlson et al. 2002). The above-cited

experiments created sediment sulfide concentrations up to 13.0 mM; natural sulfide levels of healthy seagrass beds in Florida Bay average <2mM, whereas in areas of seagrass die-off, the phyto toxin averages 4–10 mM (Carlson et al. 2002). Some evidence exists that die-off of overdeveloped beds of T. testudinum in Florida Bay may be duepartly to lowering of plant resistance to the parasite Labyrinthula sp. during periods of increased environmental stresses such as high salinity, high or low temperatures, hypoxia, and high sediment sulfides (Durako 1994; Blakesley et al. 2002; see Chapter 5). Evidence for sulfide toxicity was found wh en sulfide addition (6 mM) in tank cultures was

co mbined with high salinity and temperature over a period longer than 28 days (Koch and Erskine 2001), thus mimicking die-off conditions in Florida Bay. Carlson et al. 2002 suggested that the higher sensitivity of T. testudinum to s ulfide levels than that of H. wrightii may be due to the former species having a higher below- to above-groundbiomass ratio. They also reported high sedimentsulfide levels in other estuaries, including SarasotaBay, which may help explain the patchy distribution of seagrass beds (Figure 3.8). This suggests that carbonate sediments, as found in Florida Bay, are not the only type of substrate that contain high sulfide concentrations and that seagrass losses (but not die-off) in other areas on Florida’s Gulf coast may also reflect sediment conditions. The release of oxygen into its rhizosphere by T. testudinum andthe resultant decline in sulfides (converted to sulfates) was shown in seagrass beds in Laguna Ma dre, Texas (Lee and Dunton 2000a). Ta ble 3.4 Cell-wall constituents of seagrass leaves, including lignin, cellulose, and soluble carbohydrates. Species Site (Ref.) Lignin Cellulose Sol. Carb.
Thalassia testudinum Florida (1) 0.2–2% 18–32% 1–9% Sy r ingodium filiforme Florida (1) 1–3% 19–26% 10–16% H

alodule wrightii Florida (1) 2–4% 25–33% 4–14% H. uninervis Gulf of Acaba (2) <1% 41% – H

alophila stipulacea Gulf of Acaba (2) 2% 36% – H. ovalis Gulf of Acaba (2) <1% 33% – He terozostera tasmanica Philip Bay (3) 5% 20% –

Mo dified from Dawes (1986). l. Dawes (1986); 2. Baydoun and Brett (1985); 3. Webster and Stone (1994).

## 27 CHAPTER 3 | A UTECOLOGY AND POPULATION GENETICS Mu chof the sulfide in anoxic sediments derivesfrom the activities of sulfate-reducing microorganisms. Many microbial associations in the bulk soil and with the plant’s roots can provide a means for obtaining scarce nutrients. For example, fungalassociations (mycorrhizal) might provide phosphorus or micronutrients. Nielsen et al. (1999) hypothesized that the nature of seagrass sediments

r estricts the development of vesicular-arbuscular myco rr hizae (VAM) because they could not find the association in the rhizosphere of T. testudinum,in contrast to their finding that VAM are associated with submergent and emergent freshwater angiosperms. Because they did not find any (V

AM) associations with the roots of T. testudinum, Nie lson et al. (1999) suggested that formation of the association was restricted by the comb ined effects of highly anaerobic nature and high salinity of seagrass sediments. S ince the classic studies of Patriquin and Kn owles (1972), who used acetylene reduction by the seagrass rhizosphere to infer nitrogen fixation,a number of studies have described nitrogen fixation and nutrient uptake by marine angiosperms in greater detail. Based on acetylene-reduction

experiments, Kenworthy et al. (1987) determinedthat bacteria associated with the roots and rhizo mes of T. testudinum fixed nitrogen. Nitrogen fixation occurs in Ly ngbya wollei (Phlips et al. 1992), a freshwater cyanobacterium. Perhaps L. majescula, a common mat and epiphytic species in seagrass beds of Florida’s Gulf coast, has a similar capability. Seagrasses take up nutrients principally from sediment pore water ( e.g., ammonium and phosphorus; Fourqurean et al. 1992) but also from the water column ( e.g., nitrate and ammonium; T

ouchette and Burkholder 2000). However, porewater nutrient concentrations in seagrass beds can vary widely, as shown in Florida Bay, where median values of 0.34 μM for soluble reactive phosphorus and 78.6 μM for ammonium occur (Fourqurean et al. 1992). Because of rapid growth

rat es, seagrasses require large amounts of fixed nitrogen (10 to 450 mg N m –2 d–1 ), with ammonium being the preferred form (Moriarty andBoon 1989). By contrast, nitrate and nitrite concentratio ns are usually low (<5 μM) in anaerobic sediments, presumably because they are rapidly used by denitrifying and anaerobic bacteria. Seagrasses show a variety of responses when

nutr ients become limiting. Phosphorus is usually the most limiting nutrient in seagrass sediments of Florida Bay, where calcium carbonate rapidly binds any free PO 4 –3 (Powell et al. 1989; Short et al.1990; Touchette and Burkholder 2000), and its levels have been correlated with seagrass-bed development (Fourqurean et al. 1992). In Florida Bay, phosphorus apparently controls the successional sequence between early-stage H. wrightii beds and late-stage T. t estudinum (Fourqurean et al. 1995).The former species tolerates higher nutrients bette r than T. testudinum does and will replace it in areas of natural or anthropogenic eutrophication. Ne ve r theless, factors other than nutrient levels may contribute to the replacement of T. testudinum by H. wrightii. Studies in Florida Bay (Carlson et al. 2002) reported that higher sulfide levels in the root zone, resulting from enhancedseagrass nutrition, can act as a phytotoxin in sediments and limit T. t estudinum. Bycontrast, ammonium deficiency limits growth of T. t estudinum in Laguna Madre, Texas (Lee and Dunton 2000b) and of S. filiforme in the Indian River Lagoon (Short et al. 1993). Figure 3.8 Fr equency distribution of pore-wa ter sulfide concentrations in carbonate sediments of Florida Bay (Rabbit Key Basin) and in siliceous, terrigen ous sediments of Sarasota Ba

y (Carlson et al. 2002).Manateegrass, Syringodium filiforme, with drift algae 

# COLOGICAL

ROLES

Biomass production of the three Florida seagrass species with lar ger statures is highly varia ble , with highest values reported for each in summer and early fall. Below-ground biomass accounts for 50%–90% of total plant biomass. Thalassia testudinum pr oduces 10–19 leaf blades per short shoot per year, with lower turnover rates in more northern latitudes. Determina tions of leaf pr oduction rates for other species are needed. Although vegetative growth is the primary method for seagrass expansion, little is known about rhizome extension rates for most species. Seag rass communities serve as habitats and nurseries for over 170 species of inv er tebrates and more than 100 species of fish. About 60 fish species using seagrass beds as ha bitat have noncommercial values and have poorly known requirements. Epiphytic biota are often seasonal, serve as food, and enhance seagrass community str ucture but may also nega ti vely influence seagrass growth by a ttenuatin g light. A positive relationship exists between epiphytic load on seagrass blades and nutrient enrichment of their environment, whether the source of the nutrients is natural (e.g., bir d r ookeries) or human (e.g., se ptic tanks or stormwater runoff). Drift macroalgae deposited on seagrasses can r esult in seagrass die-back but can also serve as ha bitat, as nutrient sinks, as f ood, and as a transport v ehicle for inver tebrates and fish. Unlike overgrazing, which has been documented twice in Fl orida waters, moderate grazing may enhance the producti vity and expansion of T. testudinum . The role of secondary metabolites that may reduce grazing in seagrasses is not known.
Da ta re ga r ding competition between seagrass species or with their associated macroalgae are limited. Animal species using seagrass habitats also often use nearby salt marshes, mangrove forests, and coral r eefs. The export of in sh or e seagrass de tritus offshore into deep wateris poorly documented for Florida waters but may be an important source of organic matter in deep marine systems.

FWC–FWRI photo

## 29 CHAPTER 4 | ECOLOGICAL ROLES 

# SEAGRASS COMMUNITY ECOLOGY
Like Florida’s coastal mangrove and salt-marsh

co mmunities, seagrass communities are important primary producers, stabilize mobile sediments, serve as habitats and nurseries, and are direct and indirect food for diverse fauna (Dawes 1998a). Fu r ther, the characteristics of seagrasses ( e.g. short-shoot densities, presence of inflorescencescars, levels of storage products) can aid in determining whether seagrass beds are stressed, thereby

co ntributing to the monitoring of conditions in

co astal and estuarine communities (Dennison et al. 1993; Dawes 1998a). Ina study of seagrass distributions and conditions on the northeastern Gulf of Mexico, Livingston et al. (1998) listed four significant predictors of seagrass community health. These predictors are photic depth, light quality, water quality (color, dissolved organic carbon, and

chlorophyll a ), and sediment characteristics. The principal predictor is light quantity (duration andintensity) in relation to seagrass depth distribution, which is related to water transparency, as shown in the review on seagrass depth limits by Duarte (1991). In this regard, water transparency was found to be the principal factor influencing the depth distributions of T. testudinum in Tampa Bay (Dixon and Leverone 1995, 1997) and growth form of H. wrightii below a canopy of T. testudinum in the Florida Keys (Tomasko 1992). Howe ve r, light quality influences seagrass morphology as shown by altering the red to far-red

ratios in cultures of Ruppiamaritima (Rose and Durako 1994). Seagrass communities have been measured for standing stock biomass (g dry wt m –2 ) and shortshoot densities (numbers m –2 ). Biomass data are

co mmonly limited to above-ground material, mostly the photosynthetic blade tissue. Such data are available for the larger-stature seagrass species, including T. testudinum, H. wrightii, R. maritima, and S. filiforme (Tab le 4.1), but data are usually lacking for the smaller-stature species ( i.e., species of H alophila ). In general, the below-ground biomass (roots and rhizomes) constitutes 50% to 90% of standing stock for the larger-stature species (Zieman and Zieman 1989). For example, total biomass estimates (above and below ground) for T. testudinum in Florida Bay ranged from 195 to 2,254 g dry wt m –2 , and 85% of this was below gr ound (Fourqurean and Zieman 1991). Beds of S. filiforme in the Indian River Lagoon have an annual mean above-ground leaf biomass of 87 g dry wt m –2 (± 151) or 43% of the total, whereas below-ground roots and rhizomes accounted for 117 g dry wt m –2 (± 94) or 57% of the total plant biomass (Short et al. 1993). Dunton (1990) found that in the Laguna Madre of Texas below-ground biomass for H. wrightii was 50% to 85% of the
t otal and for R. maritima was 20% to 70% of the
t otal biomass. On Florida’s Gulf coast, the greatest leaf biomass for T. t estudinum occurs in spring and early summer, whereas below-ground biomass (roots and rhizomes) is greatest in the fall (Dawes
et al. 1985). Unlike below-ground biomass, leaf biomass in T. testudinum can show a four-fold seasonal fluctuation, with above- to below-ground

ratios of 0.17 to 0.4 in December and 0.4 to 1.0 in Ap r il in Tampa Bay (Dixon and Leverone 1995). L eaf turnover rates were reviewed by Short and Duarte (2001), and techniques to measure leaf gr owth were compared by Tomasko and Dunton(1995) and van Tussenbroek (1996b). Leaf production (g dry wt shoot–1 y –1 ) in T. t estudinum was measured by puncturing leaf clusters at their bases and measuring new leaf production after a few days (Tomasko and Dawes 1989). In a review of leaf turnover rates for T. t estudinum, Marbá et al. (1994) reported that 14 to 19 leaves are produced annually by each short shoot. Leaf longevity on short shoots of T. t estudinum in the Tampa Bay area using leaf-scar chronology was 14.6 days at S unset Park in Tarpon Springs and 19.2 days in south Tampa Bay (Witz and Dawes 1995) and 24 days in a lagoon in the Yucatán, Mexico (van T

ussenbroek 1996b). Leaf production generally

r anged between 10 and 19 leaves short shoot–1 y –1 in Florida Bay (Zieman et al. 1989), the Indian River Lagoon (Gacia 1999), Tampa Bay (Witz and Dawes 1995), Charlotte Harbor (Tomasko and Hall 1999), Lower Laguna Madre, Texas (Kaldy et al. 1999), and the Caribbean coast of Mexico (Marbá et al. 1994; van Tussenbroek 1996b). Populations growing in cooler waters had lower annual

rat es of leaf production (10–15 leaves shoot–1 ) for the Indian River Lagoon and the Lower Laguna Ma dre in Texas compared to those in Tampa Bay. Determination of leaf turnover in narrow-bladed seagrasses, such as H. wrightii (T omasko and Dun-

## 30 to n 1995) and S. filiforme (Fry and Virnstein 1988), requires using an approach different from the leaf-punch technique. Leaf productivity by S. filiforme, determined using leaf clipping and photo gr aphy, was 1.8 g dry wt m –2 d–1 (Virnstein 1982; see Short and Duarte 2001 for a detailed review of
te chniques). Little is known about rhizome growth rates of Florida’s seagrasses, yet this is the sole method for

ve getative expansion. Reviews indicate that vegetative growth differs among species, rhizome elongation rates being higher for smaller-stature seagrass species (Marbá and Duarte 1998). In their review, mean rhizome elongation rates were highest for H. wrightii, the smallest stature of the larger Caribbean seagrasses (223, range; 81–365 cm yr –1 ), followed by S. filiforme (123, range: 52–182 cm yr–1 ) and then the largest stature species T. testudinum (69, range: 22–152 cm yr –1 ). Short et al. (1993) found that S. filiforme r hizome elongation exceeded 100 cm y –1 in the Indian River. At Mullet Key in Tampa Bay, rhizome internode length and increase in seagrass patch size for H. wrightii were po sitively correlated; the larger the internodes of H. wrightii , the greater the increase in seagrass coverage at the edges of patches (Jensen and Bell 2001). The authors also found that a 100-fold increase in phosphorus, but not nitrogen, resulted in a significant increase in

r hizome internode length. Jensen and Bell (2001) suggested that the observed patch-size patterns in seagrass beds ( e.g. larger landscape features) may be determined by small-scale factors ( e.g. nutr ient

availability).

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Ta ble 4.1 Standing stock (biomass) and productivity of south eastern United States seagrasses and closely related species (congeners) from other areas. Procedures differ gr eatly and influence the ranges in biomass and productivity.

Bi omass Productivity Species Locality (g dry wt · m –2 ) (g C · m –2 · d–1 )

H

alophila H. ovalis I ndia 48 W

estern Australia 40–60 H. engelmannii Texas (Gulf coast) 1.6 Thalassia

T. testudinum Florida (general) 20–8,100 Florida (Atlantic coast) 20–1,800 0.9–16.0 Florida (Gulf coast) 75–8,100 SW Florida Bay (Gulf coast) 60–125 In dian River, FL (Atlantic coast) 890 Co r pus Christi, TX (Gulf coast) 454–885 Mexico, lagoon (Gulf coast) 420–1,418 Mexico, reef (Gulf coast) 573–811 Cuba (Caribbean Sea) 20–800 9.3–12.5 T. hemp richii Queensland, Australia 70 Sy r ingodium

S. filiforme Florida (general) 15–200 In dian River, FL (Atlantic coast) 27–81 S. isoetifolium New Guinea (East Indies) 327 H

alodule H. wrightii Mullet Key, FL (Gulf coast) 120 0.25 Florida (general) 0.8 No r th Carolina (Atlantic coast) 105–200 0.5–2.0 Laguna Madre, TX (Gulf coast) 70–300 M ississippi Sound (Gulf coast) 256 H. uninervis New Guinea (East Indies) 150 Ru ppia maritima Texas (Gulf coast) 0–200

Mo dified from McRoy and McMillan (1977), Zieman and Wetzel (1980), and Hillman et al. (1989).

## 31 CHAPTER 4 | ECOLOGICAL ROLES 

# STRUCTURE, SHELTER, AND PREDATION

Seagrass beds function both as nurseries (Lewisand Stoner 1981; Lewis 1984; Virnstein 1987; Zieman and Zieman 1989; Fonseca et al. 1996b; Bell et al. 2001; Heck et al. 2003) and as habitats for invertebrates (Greening and Livingston 1982; Vi r nstein 1987; Valentine and Heck 1993), fish (Stoner 1983; Gilmore 1987), sea turtles (Williams 1988a), and manatees (Lefebvre et al. 1989). The

r ole of seagrass meadows as nurseries was reviewed by Heck et al. (2003) who reported astrong link between seagrass abundance and abundances of juvenile fin fish and shell fish. The principal factor in this positive correlation was the structure that seagrasses added to the habitat. In ve rt ebrates add structure and serve as food, thus supporting increased diversity of other forms. For

example, high densities of the mussel Mo diolus americanus in St. Joseph Bay reached 2000 individuals m –2 , which enhanced invertebrate diversity and secondary production (Valentine and Heck 1993). The high variation of animal diversities among seagrass sites may be due in part to wave and current activity (Bell et al. 1994). The authorsfound that the mean numbers of fish and copepods and mean numbers of fish species were higher in low-energy seagrass beds ( e.g., minimum

exposure to wind direction, speed, and effectivefetch in Tampa Bay and Onslow Bay, North Carolina). Densities of macroinvertebrates in seagrass beds on Florida’s Gulf coast vary widely among sites (Table 4.2), ranging from 910 individuals m –2 (Rookery Bay) to 33,485 individuals m –2 (T ampa Ta ble 4.2 Densities of macroinvertebrates at various sites in Florida, arranged geographically from the Atlantic coast
to the northern Gulf of Mexico. Single-density values are means; ranges generally represent several sampling sites. Several values are derived indirectly.
Mesh Number Seagrass Faunal Size Density of Study Site Species* Group (mm) (indiv. m –2 ) S pecies Reference
Card Sound Tha t otal 0.8 1,085 – Brook (1977) Bahia Honda Tha Am phipoda 1.0 102 5 Nelson (1980) Florida Bay Tha t otal 0.8 2,794 52 Brook (1978) Rooker y Bay Hal Am phipoda 1.0 910 3 Nelson (1980) Tam pa Bay Hal Po lychaeta 0.62 13,313 44 Santos and Simon (1974) Tha Po lychaeta 0.62 33,485 Santos and Simon (1974) Anclote Anchorage Syr t otal 1.0 2,347 68 Mahadevan and Patton (1979) Syr t otal 0.5 9,538 58 Mahadevan and Patton (1979) Tha t otal 1.0 3,724 63 Mahadevan and Patton (1979) Tha t otal 0.5 18,916 50 Mahadevan and Patton (1979) Apalachee Bay Tha/Syr t otal 1.0 1,782 to 2,424 72 Lewis and Stoner (1981) Tha/Syr t otal 0.5 3,154 to 4,754 86 Lewis and Stoner (1981)

Tha t otal 0.5 16,108 80 Lewis and Stoner (1983) Tha/Hal Cr ustacea 0.5 6,716 46 Lewis (1984) Tha/Syr/Hph t otal 0.5 2,827 170 Stoner (1980b) Apalachicola Bay Hal t otal 0.5 38,780 58 Sheridan and Livingston ( 1983) St. Andrew Bay Tha t otal 0.7 7,567 89 Saloman et al. (1982) Hal t otal 0.7 3,370 67 Saloman et al. (1982) Tha/Hal t otal 0.7 4,192 86 Saloman et al. (1982) Santa Rosa Sound Tha t otal 0.5 13,260 144 Morton et al. (1986) H

al, co re t otal 0.5 9,020 41 Stoner et al. (1983) H

al, suction total 0.5 6,077 37 Stoner et al. (1983)

Mo dified from Virnstein 1987. * H al = Halodule wrightii, Tha = Thalassia testudinum, Syr = Syringodium filiforme, Hph = Halophila

engelmannii.

## 32 Bay). Invertebrate communities of seagrass beds in Florida are highly regional, and overlap occursbetween subtropical, tropical, and warm-temperate faunas (Virnstein 1987). Holmquist et al. (1989) found that seagrass beds in Florida Bay playa secondary role as habitat for decapods and stomatopods compared with their reactions to winter

c old fronts as a function of their positions on mud banks. Decapod crustaceans, such as carideanshrimps, usually dominate the larger fauna numerically, similar species of shrimp being widespread in Florida’s Gulf-coast seagrass beds (Holmquist et al. 1989). The macrofauna varies widely in species

co mposition and animal density over distance and time, including those of amphipods, gastropod molluscs, and polychaete worms (Virnstein 1987).Because infaunal, epibenthic, and epiphytic animals of seagrass beds are prey for larger animals,fish are abundant there (Gilmore 1987). Principally, the larger-sized fish ( e.g., spotted seatrout, Cy noscion nebulosus , and pinfish, Lagodon rhomboides ) have been studied. Smaller nonfishery species ( e.g., gobiids, syngnathids), which account for most of the resident species in seagrass beds, are less well known but include over 100 species (Gilmore 1987). As with invertebrates, fish faunas vary with geographic regions on Florida’s Gulf

co ast and include warm-temperate species, eurythermal tropical species, and stenothermal tropical species (Gilmore 1987). Four subenvironments in Florida Bay had an overall average density of 11 fish m –2 and a total of 56 species (Sogard et al. 1987). The two-year study found seagrass standingcrop and litter to be the most important determinates of fish numbers. Mo nitoring animal densities and diversity inplanted seagrass beds is a useful method for determining habitat restoration success. A study of a three-year-old planted seagrass bed in Tampa Bay (Fonseca et al. 1996a, 1996b) found that shrimp, fish, and invertebrate densities were equivalent tothose of natural communities, although shortshoot density was only one-third those of natural beds. Another study in Tampa Bay found thathigher numbers of polychaetes occurred in twoyear-old planted sites than in natural H. wrightii beds (Bell et al. 1993). The rapid increase of animals in planted beds suggests that the threshold value of habitat structure may be much lower than previously thought and that the lack of correlation between patch size and animal abundance suggests other factors are involved in maintaining animal populations. Clearly, more studies are needed to determine animal population relationships in

r estored, created, and patchy seagrass beds.Studies on seasonal animal-population fluctuations and patch formation in seagrass meadows suggest that they are not static, even under “stable” environmental conditions. For example, Tampa Bay seagrass beds experiencing more rapid water movement have a lower number of fish and harpacticoid copepods (Bell et al. 2001), which may also be influenced by organic matter contentin the sediment. In St. Croix, Virgin Islands, tropical seagrass beds fluctuate in response to seasonal

co nditions (Williams 1988b). In addition to seasonal changes, severe plant die-off has occurred inFlorida Bay, probably hastened by spatial variations in environmental stresses (Durako 1994; Zieman et al. 1999; see also Chapter 5). Water-column

nutr ient increases resulting in eutrophication have been cited as important environmental stressors that can shift species dominance in seagrass beds and influence sulfide production (Carlson et al. 2002). Williams (1987) found that T. testudinum

c ould shade and replace S. filiforme beds in St. Cr oix. She also reported that the successionalsequence from bare sediment began with rhizophytic (psammophytic) algae, then proceeded to S. filiforme, and ended with T. t estudinum, as sediment nutrient concentrations increased over time (Williams 1990). In this regard, considering nutrient increases emanating from bird perches installed in Florida Bay, Fourquean et al. (1995) proposed that H. wrightii might replace T. testudinum in Florida Bay as sediment nutrient levels

r ise. However, this study did not consider the effect of increased sulfide levels in the sediment (Carlson
et al. 2002) or the effect of phosphate and nitrate on seagrass growth (Powell et al. 1989). The competition for space or the “space race” is

ev ident by the numbers of macroalgae, cyanobacte r ia and microalgae (Hall 1988; Harris 1997) that gr owon seagrass blades and that show strong seasonal changes. At Egmont Key in Tampa Bay, thefive species of cyanobacteria and 20 species of epiphytic macroalgae varied in dominance (by algal division) during a 15-month study of T. testudinum. Br own algae dominated during the winte r and spring months and cyanobacteria in the

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY

## 33 CHAPTER 4 | ECOLOGICAL ROLES summer months (Figure 4.1). Hall (1988) divided the algae into those that (1) are characteristically epiphytic on seagrasses; (2) settle and reproduce on seagrass blades, but are commonly epilithic; and (3) germinate on blades and then break free before reproduction. A study of T. testudinum beds near Leffis Key and Siesta Key in Sarasota Bay (T

able 4.3) found many species of centric and pennate diatoms, six species of cyanobacteria, and 21species of macroalgae (Harris 1997). Algae epiphytic on H. wrightii and T. testudinum in Tampa Bay, Sarasota Bay, and Charlotte Harbor (Dixon and Leverone 1995; Dixon and Kirkpatrick 1995, 1999) and Santa Rosa Sound in Pensacola Bay (Macauley et al. 1988) change seasonally, species diversity being negatively correlated with water
te mperature. Gacia et al. (1999) found that sheetlike and filamentous species of green-algae genera ( En teromorpha, Cladophora ) were less resistant to gr azing fish than were species of more coarsely branched red-algae genera ( Hy p nea, Chondria, A canthophora ) in the Indian River Lagoon. Algae epiphytic on seagrass blades can account for 46%of the total biomass and 60% of the benthic primary production, as found using stable-isotope analysis, in H. wrightii beds in Mississippi Sound (Moncreiff and Sullivan 2001).Studies have shown that epiphytes, in combination with PAR attenuation and seasonal lowgr owth periods, depress blade growth and primary production and reduce the depth of T. t estudinum in Tampa Bay (Dixon and Leverone 1995, 1997), Florida Bay (Frankovich and Fourqurean 1997),the Florida Keys and Keys in Belize, Central America (Tomasko and Lapointe 1991), and Perdido Ke y in northwest Florida (Wear et al. 1999). To masko and Lapointe (1991) found water-column dissolved inorganic nitrogen (DIN) concentr ation was high near Big Pine Key (3.96 μM), an island in the Florida Keys with over 2,000 septictanks. This DIN level is similar to that of a mangr ove island in Belize (Man-O-War Key) having 75pairs of nesting frigate birds (3.80 μM). In contr ast, DIN levels were low around Cutoe Key (0.50 μM), a remote mangrove island in the Florida Ke ys. The seagrass beds near Big Pine Key and M an-O-War Key had higher epiphytism, lower short-shoot densities, lower leaf-area indices, and lower biomass in contrast to those near Cutoe Key (T

omasko and Lapointe 1991). Wear et al. (1999)

re ported similar results from an experiment usingslow-release fertilizer (Osmocote®) in northwestern Florida at Perdido Key. Nutrification caused significant increases of diatoms, as well as red and brown macroalgae, on blades of T. testudinum, H. wrig htii, and S. filiforme over a 12-month period. Fr ankovich and Fourqurean (1997) reported that epiphytic loads on T. t estudinum, along a nutrientenrichment gradient in Florida Bay, gave similar

re sults to the previous studies, and this study indicated that the effect is localized. The authors concluded that epiphyte levels alone are not as

r esponsive to moderate nutrient enrichment as are other seagrass characters, such as leaf tissue N:C

ratios. As noted in Chapter 3, epiphytes of seagr asses reduce water-column PAR by as much as 99% seasonally. The question of whether organiccarbon, nitrogen, or phosphorus can be transferred from seagrass blades to the epiphytic comm unity or vice versa is not clear (see Lobban and Ha rr ison 1994). Some of the most abundant invertebrates in Figure 4.1 Seasonal variations in the mean numbers of cyanobacteria and macroalgae, grouped by divisions, epiphytic

on Thalassia testudinum blades at Egmont Key in Tampa Bay

over a 15-month period (Hall 1988).

## 34seagrass beds are the meiofauna, including crustaceans (Lewis 1987) and copepods (Hall 1988). Ha rp acticoid copepods are often the most common animal group associated with seagrasses (Hall and Bell 1988, 1993; Walters and Bell 1986; Walters 1991). One study reported that more than 20,000 harpacticoid copepods migrated h –1 m –2 from the sediment during the night into the water column,

re presenting over 50% of the total benthic harpacticoid community (Walters and Bell 1986). Fu r ther, this postsunset entry into the water column from the sediment involved over 30 species in 15 families (Walters 1991). At Mullet Key in Ta mpa Bay, artificial blades (green ribbon) withartificial epiphytes (cotton fibers) attached had significantly higher densities, compared to green ribbons alone, of copepods, polychaetes, and nematodes associated with them after three days. This shows that epiphytic biota enhance structural

co mplexity of seagrass blades, thereby increasing habitat values (Hall and Bell 1988). In a similar study, Hall and Bell (1993) showed that the most abundant meiofaunal groups on seagrass blades at Eg mont Key were adult harpacticoid copepods,

co pepod nauplii, and nematodes. They also determined that meiofauna density was most signifiGULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Ta ble 4.3 Epiphytic algae on blades of Thalassia testudinum at Leffis Key and Siesta Key in Sarasota Bay (modified from Ha rr is 1997) and at Egmont Key in the mouth of Tampa Bay (modified from Hall 1988). Species Leffis Key Siesta Key Egmont Key Cyanobacteria

A nabaena oscillarioides x xx A nacystis sp. x x x Calothrix crustacea x xx En tophysalis conferta x Microcoleus lyngbyaceus x xx Po rp hy rosiphon notarisii x Chlorophyta

B oodleopsis pusilla x Chaetomorpha minima x x Cladophora spp. x x x En teromorpha chaetomorphoides x Ulva lactuca x P haeophyta

Cladosiphon occidentalis x x Ectocarpus elachistaeformis x Ectocarpus rallsiae (as Giffordia rallsiae )x x x Ectocarpus sp. (as E. rhodochortonoides )x x x H

incksia mitchellae (as Giffordia mitchellae )x x x H

ummia onusta x xx My r ionema magnusii (as M. orbiculare )x x x My ri otrichia occidentalis x xx Rhodophyta

Acrochaetium spp. x x x Centrocer as clavatum x xx Ce r amium cimbricum (as C. fastigiatum )x x x C. flaccidum (as C. byssoideum )x x x Chondria dasyphylla x xx Chondrophycus papillosus (as Laurencia papillosa ) x Ery throtrichia carnea x Hydrolithon farinosum (as F osliella farinosa )x x x Hy p nea musciformis x x Po lysiphonia subtilissima x x St

ylonema alsidii (as Goniotrichum alsidii ) x

## 35 CHAPTER 4 | ECOLOGICAL ROLEScantly correlated with percentage cover of filament ous algae. A 15-month study of T. t estudinum blades at Egmont Key at the mouth of Tampa Bay(Hall 1988) showed seasonal shifts in the abundance of nematodes (highest in winter) but notthat of harpacticoid copepods and nauplii (abundant throughout the year). Epiphytes trap detritus, which attracts significantly higher numbers of

co pepods that feed on it (Meyer and Bell 1989). S mall grazers ( e.g., co pepods) can play a role in epiphyte control (Virnstein 1987), as shown for H. wrig htii plants in the Indian River Lagoon (Howard and Short 1986). Along with drift macroalgae, rhizophytic(psammophytic) algae may equal or surpass seagr ass biomass in Florida Gulf-coast seagrass beds (Dawes et al. 1985). With the exception of some
tr opical coenocytic green algae, macroalgae usually cannot attach to sand or mud. Coenocytic green algae are a diverse group of siphonaceous (lacking internal cell walls) algae placed in the order Caulerpales. They form extensive rhizoid holdfasts and include species of calcified ( Halimeda, Penicillus, Udotea ) and noncalcified ( Caulerpa, Riphilia, Avr ainvillea ) genera. Even though rhizophytic

co enocytic algae may compete for space and light

w ith seagrasses, they may also serve as pioneer plants, which then may be replaced by seagrasses, as shown in a coral-reef lagoon in St. Croix (W illiams 1990). Over 230 species of unattached macroalgae (drift algae) are known throughout the world (Norton and Mathieson 1983). Drift algae are usually noncalcified, remain infertile, and reproduce vege tatively (Collado-Vides et al. 1994). Two studies, one in Tampa Bay, Boca Ciega Bay, and Anclote An ch orage (Phillips 1960b) and the second in Cr ystal Bay (Phillips 1960c), listed 195 and 46 taxa,

r espectively, of algae that could not be distinguished as either drift or attached. Drift algae are

co ncentrated in seagrass beds where water-current

ve locities are reduced and blades will entangle the algae (Figure 4.2). To some extent, seagrass blade size may influence entanglement (Bell et al. 1995). Seagrass beds on the Gulf coast of Florida are known to contain from 8 to 65 species of drift algae with the total biomass ranging from 0.002 g
to 930 g dry wt m –2 (T able 4.4). Brown (2001) found that algal biomass differed for three sites on the Gulf coast of Florida, both by season and by site. Annual means of 6.01, 3.83 g, and 2.25 g dry
wt m –2 we reobtained for Tampa Bay in Cockroach Bay, Tarpon Bay, and Sunset Beach respectively. Although biomass varies with season and site on the Gulf coast, the types and numbers of species are similar (Table 4.4).Drift algae are an important ecological component of seagrass beds because they serve as habitat, provide transportation, and provide a food source for invertebrates (Ballantine et al. 1994; Holmquist 1994; Greenway 1995; Knowles and Bell 1998; Ma ciá 1999; Brooks and Bell 2001; Rydene and M atheson 2003). In Florida Bay, Holmquist (1994) found 61 invertebrate taxa in drift algae mats in Figure 4.2 Drift algae are entangled by seagrass leaves, as in this Thalassia testudinum bed.FWC–FWRI

## 36 seagrass beds. Average drift rate was 0.5 km d–1 , indicating that drift algae are an effective mode of
tr ansport for animals. Ballantine et al. (1994)

re ported that balls of algae in Venezuela contained 12 species of invertebrates. The numbers of amphipods, isopods, and tanaeids in Venezuela

we resignificantly higher on two species of red algae than were associated with blades of S. filiforme, as similarly found in other previous studies (Knowles and Bell 1998). Virnstein and Howard (1987) noted that drift algae serve as habitat and food sources for gammaridean amphipods. Brooks and Bell (2001) found that drift clumps of the red alga Hy p nea cervicornis, as they moved through seagrasses and into sandy areas, had significantly higher amphipod densities; they concluded that drift algae clumps provide a mobile corridor foranimals. In Tampa Bay, drift algae form an important alternative habitat for 12 of the 20 most abundant juvenile and small adult fish (Rydene and M atheson 2003). However, drift algae deposited inseagrass beds can also affect these habitats by causing a seagrass die-back as a consequence of light and oxygen reduction (Norkko et al. 2000). A

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Ta ble 4.4 Nu mber of drift-algae species and dominant species, biomass, and presence/absence (by division) in seagr ass beds on the Gulf coast of Florida. Locations include (1) Anclote Estuary near Tarpon Springs (Hamm and Humm 1976), (2) Hillsborough Bay including studies 2a (Kelly 1995) and 2b (Avery 1997), (3) Sunset Beach at Tarpon Springs (Brown 2001), (4) Cockroach Bay in Tampa Bay (Brown 2001), and (4) Tarpon Bay at Sanibel Island (Brown 2001).

Locations 1 2a 2b 3 4 5

Nu mber of species 65 19 18 9 9 8 Nu mber of dominant species 4 5 9 9 9 8 M ean biomass (g dry wt · m –2 ) low – – 80 0.07 0.0002 0.14 high – – 930 6.34 9.45 9.32 Cyanophyta

Ly ngbya majescula x x Chlorophyta

Caulerpa prolifera x Chaetomorpha crassa x C. gracilis x Chaetomorpha sp. x En teromorpha spp. x En teromorpha clathrata x Ulva lactuca xx x Phaeophyta

Sargassum pteropleuron x Sargassum spp. x Rho dophyta

A canthophora spicifera x xx Chondrophycus papillosus (as Laurencia papillosa ) x Digen ia simplex x Gr

acilaria armata x x G. caudata xxx G. cornea (as G. debilis ) x Gr

acilaria spp. x x Hy p nea musciformis xxx Laurencia intricata (as Laurencia obtusa )x x L. poiteaui x Po lysiphonia ramentacea x P. subtilissima x Solieria filiformis (as Ag ardhiella tenera ) x xxxx Sp

yr idia filamentosa xx xxx

## 37 CHAPTER 4 | ECOLOGICAL ROLES co mbination of algae deposition and sea-urchin gr azing increased the die-back of T. t estudinum inFlorida Bay (Maciá 1999). The amount of driftalgae biomass can also have a negative correlation

w ith seagrass bed expansion if dissolved nutrients are high, as shown for Hillsborough Bay (Avery 1997). In a 10-year study, Avery (2000) found that

w ith improved water quality the 18 species of dominant drift algae declined in average biomass from 164 g dry wt m –2 in 1987 to 0.0029 g dry wt

m –2 in 1998 in Tampa Bay. The green coenocytic alga Caulerpa prolifera had colonized Hillsborough Bay in 1986 with an initial coverage of almost 220 ha (543 acres) but had declined to zero in 1997. In

co ntrast, H. wrightii cove ra ge increased from 0.2 ha (0.5 acres) in 1986 to nearly 57 ha (141 acres) in 1998 (Avery 2000). A few studies exist concerning competition between seagrasses or between seagrasses and attached macroalgae, with two reports on T. testudinum, S. filiforme, and H. wrightii and rhizophytic algae in the Caribbean (Williams 1987, 1990). Williams (1987) removed the leaf canopy of T. testudinum in St. Croix, resulting in increased irradiance reaching the S. filiforme understory, and its biomass doubled within nine months. A study in Tampa Bay indicated that competition occurred between T. t estudinum and H. wrightii (Rose and Dawes 1999), where T. t estudinum had a lower biomass when growing with H. wrightii, suggesting interspecific competition. Further, leaf biomass and growth were significantly lower in dense, monotypic beds of T. testudinum co mpared to lessdense beds, suggesting light was reduced via shading in dense beds (Rose and Dawes 1999). There are also reports regarding competition between H. wrightii and the coenocytic green macroalga C. prolifera (Bottone and Mattson 1987; Bottone and Savercool 1993). Although the data are not concl usive, C. prolifera may act as a weak competito r for space with H. wrightii, co existing with it asan opportunistic species under stable environmental conditions in Tampa Bay (Bottone and Saverco ol 1993). This result is similar to those found in studies in the Indian River Lagoon (White and Sn odgrass 1990). Further, C. prolifera, although susceptible to sudden environmental changes ( e.g., low salinity), can rapidly colonize areas devoid of

ve getation that may occur following seagrass dieback (Bottone and Mattson 1987). Overgrazing of seagrass beds is a rarity but has occurred on the Florida Gulf coast. Overgrazing by Lytechinus variegatus (purple sea urchin) resulted in large-scale denudation of seagrass beds. Over20% of an area 26 km by up to 9.5 km were consumed near the Pepperfish Keys south of the Steinhatchee River in the northern peninsula (Camp et al. 1973). Density at the front of a single aggregate

averaged 636 sea urchins m –2 , w ith individuals piled upon one another 2–8 individuals deep that

cove re d the substrate. Dense aggregations of the same urchin species overgrazed about 0.81 km 2 of T. t estudinum in outer Florida Bay; mean densities

r anged from 364 to <1 individual m –2 (Rose et al. 1999). The origins of the urchin feeding fronts arenot understood. In Florida Bay, high urchin densities may reflect unusually high recruitment or a

re lease from predation pressure because of overharvesting of spiny lobsters and stone crabs (Rose
et al. 1999). Moderate grazi ng pressures on T. t estudinum blades were correlated with specific leaf growth

rate s (0.024 mg dry wt. d–1 ) in the Yucatán, Mexico;this showed that sporadic grazing had little detrimental effect on T. t estudinum (Cebrián et al. 1998). Studies on T. t estudinum beds in northwest Florida and a review of the literature indicated thatherbivory plays a major role in stimulating seagr ass growth by removing macroalgae (Heck and Valentine 1995; Valentine and Heck 1999). Also, long-term effects of grazers have been shown in a

re lationship between L. variegatus and T. testudinum in the Gulf of Mexico (Heck and Valentine 1995), where urchins and seagrass coexist in balance. In these cases, a balance developed among intensive grazing, loss of habitat, and predation byfish on the urchins. In the 1995 experiment, protection from predator fish by enclosures resulted in intense grazing by the urchins, which became most destructive in winter when T. t estudinum cou ld not

re cove r as rapidly. The more intensively grazed beds showed significant reductions in above- and below-ground biomass, which was apparent even 3.5 years after grazing had ceased. Varying the number of L. variegatus in experimental plots in St. Joseph Bay, Valentine et al. (1997) found an increase in shoot density and productivity of T. testudinum under moderate grazing pressure. In a later study, Valentine et al. (2000) demonstrated that the effect of grazing in Florida Bay was related

## 38 to season and water depth, with grazing having a stronger effect in the winter or in seagrass beds at gr eater depths. Thus, repeating experiments in environments with differing physical conditions and in different seasons is critical in order to determine the effects of blade loss and seagrass bed maintenance. Few studies have been carried out on predation

w ithin seagrass beds since the reviews by Orth et al. (1984) and Zieman and Zieman (1989). Basedon feeding and caging studies, predation, especially by pinfish and pink shrimp, is probably amajor biological interaction influencing invertebrate community structure (Virnstein 1987). In a study in St. Joseph Bay, increased biomass of a

m ussel ( Mo diolus americanus ) in T. testudinum beds resulted in greater abundance, biomass, andannual production of other invertebrates (Valentine and Heck 1993).


# TROPHIC DYNAMICS
The ultimate fate of plant biomass is mineralization to simple inorganic compounds, with a portion of seagrass leaf decomposition occurring while it is still attached to the plant. Nevertheless, most biomass decomposition occurs on and in the sediment of seagrass meadows (Hemminga and Duarte 2000). A long-term study (419 days) in Laguna Ma dre, Texas of H. wrightii senescent blades suspended in the water column revealed that decomposition resulted in a loss of 36% of the organicmatter within the first 24 days (Opsahl and Benner 1993). By the end of the study, decomposition had resulted in a 76% loss of the organic matter. Ne utral sugars were the most abundant in the starting material and also the most rapidly lost. In contrast, the most persistent sugars in thesenescent blades were xylose and glucose, reflecting the more stable polymers of cellulose and xylan from which the sugars are derived. Near St. Cr oix, at depths between 14 and 32 m, the decomposition rates of a more delicate seagrass, H. decipiens, differed between litter bags that

we re buried in the sediment and those tethered at the surface of the sediment (Kenworthy et al. 1989). After 6.5 days, buried leaves lost 56% of the original ash-free dry weight compared to only28% lost by those left on the surface. These findings were similar to two 7-day studies made intwo consecutive years that included photosynthetic measurements in the same area and using the same species (Josselyn et al. 1986), this earlier

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Ta ble 4.5 δ 15 N and δ 13 C values of seagrasses and marine macroalgae from various locations.
Plants δ 15 N δ 13 C Seagrasses

Thalassodendron ciliatum (Australia) +3.5 –9.3 Sy r ingodium isoetifolium (Australia) +5.0 –4.0 Thalassia testudinum (Nicaragua) +3.5 –13.2 H

alodule wrightii (T exas) +3.9 –10.3 Ru ppia maritima (T exas) +3.6 –7.9 H

alophila engelmannii (T exas) +3.8 –7.9 Thalassia testudinum (T exas) +3.9 –8.9 Thalassia testudinum (J amaica) +4.3

– 11.1

– x = 3.9 ± 0.5 –9.1 ± 2.7 Ma r ine Macroalgae

Ulva fasciata (T exas) +8.1 –14.6 Ulva lactuca (T exas) +8.1 –14.6 Ge lidium crinale (T exas) +7.9 –14.3 As cophyllum nodosum (Maine) +8.0 –16.9 F

ucus vesiculosis (Maine) +8.1 –16.5 Acet abularia kilnori (Australia) +6.5

– 9.5

– x = 7.8 ± 0.6 –14.4 ± 2.6

Ma cko (1981) cited in Fry et al. (1987).

## 39 CHAPTER 4 | ECOLOGICAL ROLES study reporting greater than 50% decomposition of all buried seagrass material.Seagrass epiphytes appear to be a more significant source of direct food for many invertebrates than are seagrass detritus or living tissue. Virnstein (1987) has proposed the following sequence for a south Florida seagrass food web: (1) the important primary producers are epiphytic algae, which have

r apid turnover rates; (2) epiphytic algae are preferentially grazed by most species of small invertebrates associated with seagrass blades; (3) smallinvertebrates are preyed upon by decapod crustaceans or small fishes that include resident adults and juvenile seasonal visitors; and (4) decapods and small fishes are preyed on by larger, mostly nonresident fishes. Of the 154 grazers listed by Mc Roy and Helfferich (1980) that directly feed on seagrass blades, the most important invertebrates are echinoderms, molluscs, and crustaceans. For example, seagrass blades constitute up to 50% of the total diet for crustaceans (Klumpp et al. 1989). Cr abs, isopods, and sea urchins are the principaldirect grazers on seagrasses, whereas other invertebrates consume seagrass detritus and various algal epiphytes. Fry and Parker (1979) and Fry (1984), using stable carbon isotopes ( δ 13 C), showed that microalgae on the sediment and seagrass blades are a major source of carbon for grazers. In someseagrass meadows, micro- and macroalgae predominate in food webs (Virnstein 1987). In a

re vi ew of stable-isotope studies, Fry et al. (1987)

co ncluded that algae are nutritionally more important for consumers than are seagrasses but that δ 13 C data alone can be misleading because benthic algae have values similar to those of seagrasses. Thus, they recommended using other elements

w ith stable isotopes, including nitrogen and sulfur (T

ables 4.5 and 4.6). Ta ble 4.6 δ 34 S values for sulfur sources and plants, Redfish Bay, Texas, November 1980–February 1981 (from Fry, 1981).

Sources δ 34 S Seawater Sulfate +19.7 to +20.0 Sediment Sulfide –23.4 Po rewater Sulfate +15.0 to +17.0 Rain Sulfate +6.3 a
Leaves Ro ots After After T

otal Washing Total Washing Algae, Submerged Plants

Digen ia simplex +18.7 +17.2 Gr

acilaria verrucosa +18.6 Gr

acilaria debilis +18.9 +17.5 Thalassia epiphytesb +17.4 +15.2 Seagrasses, Submerged Plants

Thalassia testudinum Sample 1 (November 29) c +15.2 –12.3 –15.9 Sample 2 (November 29) +12.9 d Sample 3 (January 3) 6.9 d –17.3 H

alodule wrightii +10.4 –9.3 Sy r ingodium filiforme c +11.5 –3.3 H

alophila engelmannii +11.2 +11.5 In te rt idal, Emergent Plants

Sp artina alterniflora e +3.5 –9.4 Av icennia germinans e (as A. nitida ) –0.2 –3.2

a Fr om Jensen and Nakai (1961).

b E piphytic community collected after freeze drying. Contained some small animals and mud.

c, e Plants marked with the same letter were growing intertwined in the same sediments.

d E piphyte-free seagrass.

## 40 

# RELATIONSHIPS with OTHER COMMUNITIES

Because seagrass beds are mostly found in areas of low wave energy, they often occur next to tidal-flat, salt-marsh, and mangrove communities along Florida’s Gulf coast and are influenced by these intertidal communities. Salt marshes are estimated
to cover about 170,000 ha (419,900 acres) of Florida’s coasts; about half of the marsh area

exte nds from Tampa Bay north to the Alabama border on the Gulf coast (Montague and Wiegert1990). The estimated salt-marsh area is substantially less than the estimate of McNulty et al. (1972) of 214,000 ha (528,580 acres) for the Gulf

co ast of Florida. The largest salt marshes on the Gulf coast of Florida are in the Big Bend area, from Aripeka to Apalachicola Bay, a coast with low wave energy and extensive seagrass beds. About60% of northwest Florida salt marshes are monotypic stands of J uncus roemerianus Scheele (Needle R

ush), which often extend down to 0.2 to 0.5 m above MLW and thus are adjacent to near-shore seagrass beds. Above-ground primary production

r anges from 250 to 950 g dry wt m –2 yr–1 for J. roemerianus and 130 to 700 g dry wt m –2 yr–1 for Sp artina alterniflora Loisel. (Smooth Cordgrass). (Montague and Wiegert 1990), the difference probably due to the higher density of J. roemerianus culms. The large biomass of salt-marsh plants and low direct consumption by herbivores (10%) means that most of the biomass decomposes andbecomes part of the salt-marsh detritus (Montague et al. 1987). Apparently little of the detritus is exported to near-shore seagrass beds because of the relatively high elevation of Florida’s salt marshes and often the presence of a berm that

re tains the detritus within the salt marsh (Montague et al. 1987). Incontrast to the low level of detrital export, a diverse fauna is shared between salt marshes and adjacent seagrass beds, for example, mullet, spot,blue crabs, oysters, and penaeid shrimps (Montague and Wiegert 1990). In addition, tarpon ( Me galops atlanticus ), snook ( Ce ntropomus undecimalis ), red drum ( Sciaenops ocellatus ), seatrout ( Cy noscion spp.), and kingfish ( Mentic irrhus spp.)move from near-shore seagrass beds into saltmarsh tidal streams (Lewis et al. 1985b). The three Caribbean mangrove species, Rhizophora mangle L. (Red Mangrove), Av icennia germinans (L.) L. (Black Mangrove), and Laguncularia racemosa (L.) C. F. Gaertn. (White M angrove) are estimated to cover 189,725 ha (468,620 acres) along Florida’s coasts (Lewis et al.1985b). About 90% of mangrove forests (or mangals) occur in the most southern four counties ofFlorida (Lee: 14,275 ha, Collier: 29,126 ha, Monro e: 94,810 ha; Dade: 32,931 ha), a region of low wave energy and consequently abundant seagrassbeds. Because of the lack of severe freezing temperatures since 1989, mangroves extend intermitte ntly northward of Tampa Bay to the western panhandle on the Gulf coast and north to the To moka River on the Atlantic coast (Odum and Mc Ivor 1990; D. Crewz pers. obs. ). Export of mangr ove-leaf detritus is highest in riverine (1.2–2.7 g carbon m –2 d–1 ) and fringing forests (0.5–0.7 g carbon m –2 d–1 ), and these contribute to detrital-based food webs in near-shore seagrass beds (Odum and Mc Ivor 1990). Stable carbon isotope levels ( δ 13 C) of seagrass blades and mollusk shells taken from seagrass beds near south Florida mangrove forests had significantly lower mean values (–12.8 and –2.3 ppt, respectively) than did blades and shells far from the coastal forests (–8.3 and +0.6). The lower δ 13 C values near mangrove forests indicate

re lease of carbon dioxide by mangrove-detritus mineralization, conversion to bicarbonate, and subsequent uptake of bicarbonate by the adjacent seagrass beds (Lin et al. 1991). The habitat values of mangals, adjacent seagrass beds, and nonvegetated open water were compared for densities of fish, shrimp, and crabs in Rookery Bay near Naples (Sheridan 1992). Fish densities

we rehighest in nonvegetated areas (74% of the
to tal caught), while shrimp and crab densities were highest in seagrass beds (74% and 47% of the total capture respectively). However, flooded R. mangle forests had resident and transient fish and crab

nu mbers equal to those of other areas on some occasions. The data suggest that seagrass beds are a principal habitat for shrimp and crabs, whereas mangrove prop roots and pneumatophores may serve an ancillary role for a variety of fish and invertebrates, contrary to the conclusion drawn in earlier studies ( e.g., Thayer et al. 1978, 1987). South Florida’s shallow-water coral reefs areunique in North America. Florida’s reefs are simiGULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY

## 41 CHAPTER 4 | ECOLOGICAL ROLESlar in species composition and physiographic features to those in the Caribbean Sea and Bahama I slands (Dawes 1998a). Occasionally, hard corals occur as far north as Jupiter Island on the Atlantic

co ast (27°N lat.). Coral reefs form extensive threedimensional structures south and west of Cape Florida and along the Florida Keys archipelago (J aap and Hallock 1990). Seagrass beds cover 80% of the sea bottom between Cape Sable and throughout Florida Bay, along the Florida Keys,

exte nding westward to the Dry Tortugas, and north on the Atlantic coast into Biscayne Bay, thus

c o-occurring with the Florida reef tract (Jaap and Hallock 1990). Aerial photography of Biscayne National Park showed that moderately dense to highly dense seagrass beds covered 25,445 ha (62,849 acres) of the 55,000 ha (135,850 acres) in the park (Lewis et al. 2002) and were closely associated with coral patch reefs. The relationship of seagrass beds to patch and
tr act reefs is considered to be in danger due to

nutr ient-rich runoff from canals in Biscayne Bay (Lewis et al. 2002) and from septic tanks in theFlorida Keys (Jaap and Hallock 1990). A key feature of coral-reef dependence on adjacent seagrass beds is their sensitivity to nutrient enrichment, which enhances macroalgae overgrowth on the

co r al structures (Dawes 1998a). Thus, nutrient

re moval from the water column by adjacent seagr ass beds may play a major role in protecting co r al reefs. In addition, the ecological role of seagr asses as habitat and nursery in the life cycle of the spiny lobster ( Pa nulirus argus ), whose juveniles settle in algae associated with seagrass beds, has been noted (Marx and Herrnkind 1985). Seagrass detritus and vegetative parts are

exported into deep-water habitats, especially on the Atlantic coast. Deep water (>200 m) is farther offshore on the Gulf coast of Florida than along the Atlantic, but seagrass beds are more developed in the Gulf. The larger-stature seagrass species

exte nd to about 9 m (Dawes 1974; Iverson and Bittaker 1986). In contrast, the smaller-stature H

alophila engelmannii gr ows in 1–5 m in Tampa Bay (Dawes 1967), in 1.4 m off Tarpon Springs (Phillips 1960b), and to 73 m off the Gulf coast (Dawes and van Breedveld 1969; Dawes and Lawrence 1990) and the Dry Tortugas (Taylor 1928). The more diminutive H. decipiens appears to be limited to deep water on the Gulf coast and occurs in 20 to 90 m (Dawes and van Breedveld 1969; Zieman 1982; Dawes and Lawrence 1990; Fo nseca et al. 2001; see also Chapter 4). In the summer and early fall on the Gulf coast, H. decipiens often forms extensive beds in 20–30 m of water(Dawes and Lawrence 1990). However, no information currently exists regarding the export or

r ole of detritus from inshore seagrass beds to the 200-m deep continental shelf west of Florida (Florida Institute of Oceanography 1994).

# ATURAL and ANTHROPOGENIC EFFECTS

Seagrass re gre ssion and die-off observed in Florida Bay in the la te 1980s resulted from a combination of natural and human causes, including overproduction of seagrasses that r esulted in dense shoots; anoxia that increased sediment sulfide lev els;

n utrient inputs that caused algal blooms, reducing light lev els; lack of freshwater inputs that resulted in hypersalinity; and infection by La b yrinthula, a pathogen of Thalassia testudinum. The rela tionship between La b yrinthula infection and die-off of T. testudinum is not clear, as the pa thogen has been f ound in a

n umber of Florida Gulf-coast estuaries in seagrass beds that ha ve not experienced die-off.
Storms a ppear to have little direct impact on most seagrass comm unities, except for erosion and deposition of sediment in some

ar ea s, as found following Hurricane Andrew. The 1997–1998 El Niño ev ent caused a general die-back of seag rass communities because of low salinities and increased n utrient loads and turbidity fr om runoff. The importance of long-shore sand bars is not adequately understood, but they may protect seagrass beds from waves and erosion in some locations. Mechanical damage to seagrass beds continues, with propeller scarring being common in shallow-w a ter communities. The effects of small-scale seagrass-bed damage on local animalpopulations is not adequately understood; for example, modera tel y scarred beds show little change in shrimp and pinfish densities. Nutrient pollution continues to be a serious problem, with 8 of the 15 Florida Gulf-coast estuaries regarded as being eutrophic. Nutrient enrichment from natural or human sour ces results in in cr eased phytoplankton density and resultant chlorophyll levels
, r eduction in light penetration, increased seagrass loss, in cr eased epiphytism, increased sediment hypoxia, and shifts in seagrass species dominance. The effects of sediment contaminants on seagrass communities

ar e not clear. The highest contaminant levels of metals and pesticides are associated with fine muds, limited PAR, and low salinities.

Stargrass, Halophila engelmannii

Ron Phillips photo

## 43 CHAPTER 5 | NA TURAL AND ANTHROPOGENIC EFFECTS any studies, including those by resource management programs of the National Estuary Programs for Tampa Bay, Sarasota Bay,and Charlotte Harbor and by the National Estuarine Research Reserves at Apalachicola and Rookery bays, have emphasized the importance of Florida’s G ulf-coast seagrass communities. Understandingof natural- and human-induced seagrass disturbances has increased substantially, as shown in a

re vi ew by Durako (1988), Short and Wyllie-Echeverria (1996) and Duarte (2002). Since 1989, manypublications have examined the condition of seagr ass communities on Florida’s Gulf coast. These include studies of the north-central Gulf coast (Mattson 1995, 2000), Tampa Bay (Lewis et al. 1991, 1998; Johansson and Ries 1997; Kurz et al. 1999; Pribble et al. 1999; Greening 2002a; Johansson 2002a), and Sarasota Bay, Lemon Bay, and Charlotte Harbor (Kurz et al. 1999; Staugler and Ott 2000; Tomasko et al. 2001). Declines in seagr ass coverage in various estuaries on the Gulf

co ast (including Florida Bay) have often been attributed to interactions between natural and anthropogenic factors. In Florida Bay, T. t estudinum die-off was noted in late 1987 by Robblee et al. (1991) and has been studied by many others since then (Carlson et al. 1994; Durako 1994; Durako and Kuss 1994; Thayer
et al. 1994; Carlson and Yarbro 2001; Fourqurean and Robblee 1999; Hall et al. 1999; Zieman et al. 1999). Often, natural processes can result in increased damage to seagrass beds if they had been previously stressed by anthropogenic impacts, as appears to have happened regarding the seagrass die-off in Florida Bay. Despite the acknowledgedinteractions of natural and anthropogenic influences in these and many other systems, however, these categories are addressed separately below.


# NATURAL PROCESSES

Natural processes have biological and nonbiological sources and include such influences as storms, sea urchin population increases, phytoplankton blooms, and macroalgae overgrowth. Such sources can cause direct or indirect damage that include shading, overgrazing, or disease. Although moderate grazing is common and may even affect seagrass communities positively,

overgrazing by urchins, and to a lesser extent by manatees or sea turtles, can result in completedenudation in some areas. For example, overgrazing by Lytechinus variegatus (purple sea urchin)

re sulted in severe seagrass denudation near the Pepperfi sh Keys (Camp et al. 1973) and in outer Florida Bay (Maciá and Lirman 1999; Rose et al. 1999). Assummarized in Chapter 4, epiphytes on seagr ass blades have both positive and negative effects, with most damage being linked to shading and reductions in photosynthesis. In Tampa Bay, shading by epiphytes of T. t estudinum blades is animportant controlling factor in its depth distribution (Dixon and Leverone 1997). Hi ghlevels of drift algal biomass suppress seagr ass-bed expansion and, similar to seagrass epiphytism, are usually linked to nutrient enrichment. InHillsborough Bay, Avery (2000) reported in a 10-year study (1986–1994) that the 18 species of dominant drift algae (see Table 4.6 and Chapter 4) declined from an average biomass of 930 to 80 g dry wt m –2 and continued to decline through 1997. Incontrast, due to reductions of water-column

nutr ients in Hillsborough Bay, H. wrightii beds

expanded from 0.2 ha to 40 ha (Avery 2000). On the positive side, drift and epiphytic algae probably serve as nutrient sinks and as food for a variety of invertebrates and fish (Kharlamenko et al. 2001; Mo ncreiff and Sullivan 2001). The decline in the past 20 years of seagrass

co mmunities in Florida Bay is a conspicuous

example of the effects of natural and possibly hu man influences on seagrass populations on the Gulf coast. In 1987, vast areas of T. testudinum began dying rapidly. Short-shoot densities dropped by 22% and standing crop by 28% at 107 sampling stations (Figure 5.1) between 1984 and 1994 (Hall et al. 1999). Losses were highest in

western Florida Bay in areas with high standing crops. Similar or greater declines in standing crops were evi dent for H. wrightii (92%) and S. filiforme (93%) between 1984 and 1994. Hall et al. (1999) suggested that the most likely cause was chronic light reduction due to increased water turbidity. A pathogen, Labyrinthula, was also suggested as being involved in the die-off of T. t estudinum in Florida Bay (Porter and Muehlstein 1989; Robblee
et al. 1991). It is similar to one that was implicated in the wasting disease of Z. marina over the past 50 years in the Atlantic (Burdick et al. 1993). Porter 

# M

## 44 and Muehlstein (1989) reported the presence of an undescribed species of Labyrinthula in blackened, necrotic lesions on the leaves of T. testudinum collected in seagrass die-off areas of Florida Bay.Durako and Kuss (1994) found that photosynthesis in necrotic lesions on T. testudium blades was impaired. They suggested that this may have

re duced oxygen available for transport via the blade lacunae to belowground organs, resulting in increased sediment hypoxia and leading to sulfide
toxicity. The pathogen has also been found to be infecting from 0% to 100% of T. testudinum blades in at least 10 sites on the Gulf coast (Figure 5.2) from the Chandeleur Islands (LA) to Florida Bay (Blakesley et al. 2002). The authors found that 60% to 90% of the sites sampled in Tampa Bay had infected short shoots of T. testudinum. Thayer et al. (1994) ascribed the die-off in Florida Bay to “as yet unknown environmental stresses.” Studies now suggest that the die-off arisesfrom a combination of human-derived and natur al factors that may collectively stress T. testudinum and thereby enhance infectivity by the pathogen (Blakesley et al. 2002). The factors involved include hypersalinity (Fourqurean and Robblee 1999); persistent microalgal and cyanobacterial bloomsand resuspended sediments that reduced illumination (Hall et al. 1999); anoxia that increased levels of sediment sulfide (Carlson et al. 1994, 2002; Carlson and Yarbro 2001); and enhanced growth that resulted in high biomass (Zieman et al. 1999). Hypersaline areas of Florida Bay probably resulted in part from a lack of flushing in the absence of periodic hurricanes. This condition may also be

exacerbated by a reduction in freshwater flow from the Everglades because of surface-water diversion. Ne ve r theless, data covering 1989–1995 suggest that seagrasses were once again growing rapidly in Florida Bay and that the loss of T. testudinum appeared to be slowing (Zieman et al. 1999). Little information is available regarding the effects of storms on Florida’s Gulf-coast seagrass

co mmunities, except for that following Hurricane An drew, which swept across southern Florida on A

ugust 24, 1992, and exited into Florida Bay at Lo stmans River (Ogden 1992; Dawes et al. 1995). Although coastal mangrove forests were destroyed, seagrass beds in Lostmans Bay showed no signs of alteration or depletion of the meiofauna when sampled two and nine months after the storm (Dawes et al. 1995). To be sure, during hurricanes large volumes of water fall simultaneously uponbroad landscapes, leaching large amounts of nutrients and other compounds from soils. Together

w ith the lowering of salinities, this short-term pulse of pollutants may alter community dynamics in seagrass beds. Also, excessive rainfall over an extended period can damage seagrass communities. An El Niño

event between December 1997 and March 1998

re sulted in over 52 cm (approximately 20 inches) of rain, which triggered widespread, persistentphytoplankton blooms, decreasing the light available to seagrasses, along the Gulf coast of Florida (Carlson et al. 2003). In 1997, 1998, and 1999, sampling near the Homosassa and Anclote rivers and in Tampa Bay and Charlotte Harbor foundthat short-shoot densities, blade widths and numbers, leaf-area indices, rhizome-apex densities, and stored carbohydrates of T. testudinum declined (Carlson et al. 2003) in response to the 1997–1998 El Niño. Long-shore sand bars often occur seaward of seagrass communities and have been suggested to play a role in protecting seagrasses from strong waves and currents (Lewis 2002). Thus, reductions in seagrass coverage in some areas of Tampa Bay may reflect historical loss of these bars, similar to those in North Carolina (Fonseca and Bell 1998). Inthis regard to this idea, Fonseca et al. (2002)employed a wave-exposure model (Relative ExpoGULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Fig 5.1 Standing crop (±SD) of Thalassia testudinum in six ecological regions of Florida Bay 1984 and 1994 (Hall et al. 1999). MA = Mainland, NE = Northeast, EC = East Central, IN = Interior, AT = Atlantic, GU = Gulf.

## 45 CHAPTER 5 | NA TURAL AND ANTHROPOGENIC EFFECTSsure Index—REI) to evaluate the influence of longshore sandbars on seagrass coverage in Tampa Bay. Their results hindcasted that erosion or loss of long-shore sand bars on the southeastern shore of the bay, where wave exposure is relatively high, had caused the loss of seagrass originally located between the shore and the historical sand bars. They recommended that no restoration efforts be

co nducted in areas of strong wave exposure without including engineering to reduce REIs.


# ANTHROPOGENIC EFFECTS Livingston (1987) summarized anthropogenic (human-derived) effects on seagrass meadows (T

able also presented in Zieman and Zieman1989), including direct physical damage and pollution, with an emphasis on nutrient enrichment. S ince that review, various studies have described anthropogenic effects and also include estimates of seagrass losses (see Chapter 4). Nutrification and sediment loading from maintenance dredging ofshipping channels and vessel-generated resuspension are cited as important causes of seagrass declines (Schoellhamer 1991; Tomasko andLapointe 1991; Culter and Leverone 1993; Quammen and Onuf 1993; Lapointe et al. 1994). Dredging, although now generally restricted, continuesin all estuaries on the Gulf coast to maintain channels and create new harbors. Certainly, such activities cause temporary resuspension of

co ntaminated sediments and contribute to reduction of water transparency (Godcharles 1971; Schoellhamer 1991; Ailstock et al. 2002). Me chanical damage is also done to seagrass beds by erosive effects following boat (and ship) gr oundings, by propeller wash, and by ship and boat wakes in shallow waters. Kenworthy et al. (1988a) found that boat wakes substantiallyincreased bottom shear stress along edges of shallow seagrass beds. Even though nets used for baitshrimp trawling in Tampa Bay do not appear to cause much direct damage to seagrass beds (Meyer
et al. 1999) a large diversity of animals are killed as by-catch, plus trawling may suspend sediments which decreases water clarity. Pr opeller scarring is another type of physical damage to seagrass beds (Figures 5.3, 5.4) that

co ntinues to increase in all areas of coastal Florida (Sargent et al. 1995). Many shallow flats and mud banks are now severely eroded due to constant scarring, ship groundings, chronic wave action from boats, and water-current scouring (Kruer 1994). Culter and Leverone (1993) stated that prop scarring was visible in almost all seagrass beds in Sarasota Bay. Sargent et al. (1995) determined that 6.2% (over 70,000 ha or 183,000 acres) of Florida’s 1.1 million ha (2.7 million acres) of seagrass bedshave been scarred by boat propellers (prop-dredging, prop cuts) and by similar causes, principallyin coastal waters less than 2 m deep. They concluded that scarring in seagrass beds has become acute because of increasing human populationdensities, increasing popularities of boating, fishFigure 5.2 Mean percentage of Thalassia testudinum shortshoots infected with a pathogenic species of La byrinthula at ten sites along the Gulf coast. Numbers over bars are sample sizes. NE = not e xamined. (modified from Blakesley et al. 2002).

## 46 ing, diving, and other water sports, and increasing
t ourism. Scarring level was divided among three categories: 1) light scarring, in which less than 5% of the seagrasses in a particular map polygon were scarred; 2) moderate scarring, in which 5% to 20% of the seagrasses within the polygon were scarred; and 3) severe scarring, in which more than 20% of the seagrasses in the polygon were scarred (Sargent
et al. 1995). The highest level of scarring was found in Monroe and Citrus counties, which also

co ntained the highest amount of seagrass coverage (688,259 ha; 1.6 million acres). Of this total, they calculated that 23,332 ha (57,630 acres) were scarred with 17.3% of the seagrasses being in Mo nroe County and 15.8% in Citrus County.The influences of prop scars on seagrass comm unity production have been studied in Tampa Bay and Charlotte Harbor (Durako et al. 1992; Clark 1995; Dawes et al. 1997; Bell et al. 2002), Sarasota Bay (Folit and Morris 1992), and the Florida Keys (Matthews et al. 1991). Fragmentation of seagrass beds in Tampa Bay due to propeller cuts did not appear to have any consistenteffects on some animal populations over a oneyear period, as long as seagrass patch sizes were gr eater than 1 m 2 (Bell et al. 2002). The numbers of pinfish ( L. rhomboides ), pipefish ( Sy ngnathus

scovelli ), and eight species of epibenthic shrimp

we resimilar in moderately scarred (6% to 31% loss of the beds) and nonscarred seagrass beds in Ta mpa Bay. The results of these studies suggest that propeller scars that fragment seagrass beds may enhance certain faunal development caused by edge effects along the cuts as long as they arenot too severe. Nevertheless, a recent study of scarr ing in a T. testudinum bed in Puerto Rico revealed a negative effect of scarring on crabs and molluscs up to 5 m from the scar. Also, shrimp species

w ithin the scar differed from those in the nonscarred seagrasses. Fish populations did not show an effect from the scarring. Further studies are clearly needed to define the effects of moderate scarring compared to those of severe scarring on seagrass productivity. Mo rerecent studies have confirmed previous observations by Jones (1968), Godcharles (1971), and Zieman (1976) for the long time period

re quired for scars to heal (Eleuterius 1987; Durako
et al. 1992; Dawes et al. 1997). When seagrass rhizo mes and roots are completely removed by scarr ing, sediment is destabilized and resuspension occurs, thereby lowering water transparency and

re tarding seagrass regrowth into the scar. Regrowth of T. t estudinum into prop scars in Ta mpa Bay was estimated to require an average of 3.5 to 4.1 years for existing propeller scars and up
to 7.6 years in newly made ones (Dawes et al. 1997). Shorter recovery periods in existing scars probably reflect their older ages and shallower depths. Based on studies at Weedon Island (Durako et al. 1992) and Cockroach Bay (Dawes et al. 1997) in Tampa Bay, the authors recommended that shallow-water seagrass beds be protected from propeller scaring. Criteria for the protection of seagrass beds have been proposed and adopted for Ta mpa Bay (Clark 1995; Stowers et al. 2002). Another source of physical impacts that degrade seagrass communities is docks, which shade seagrass beds, as shown in Perdido Bay (Shafer 1999), Charlotte Harbor (Loflin 1995), and the Indian River Lagoon (Beal and Schmit 2000). S mith and Mezich (1999) surveyed 200 out of the 3,592 permit-exempt single- and multi-family docks that were deemed to have the potential to

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY Figure 5.3 Propeller scarring from recreational boating in a Thalassia testudinum bed near P anama City , Florida.FWC–FWRI photo

## 47 CHAPTER 5 | NA TURAL AND ANTHROPOGENIC EFFECTS damage seagrass beds in Palm Beach County. Theyfound that 16% of the docks were larger than regulations allowed, owing to additions, and that approximately 1,491 single-family docks in the

c ounty had affected seagrass negatively. They estimated that, in total, docks eliminated 20.4 ha (50.4 acres) of seagrass beds, which is equal to 2.1% of the seagrass extents in Palm Beach County. Their

re co mmendations were that all docks in Florida be

co nsidered within the jurisdiction of FDEP permitting programs. Currently, environmental

r esource permits are not required within designated Aquatic Preserves if the docks are less than 46.5 m 2 (500 ft 2 ) or if they are less than 92.9 m 2 (1,000 ft. 2 ) when outside Aquatic Preserves. Smith (1998) concluded that strong enforcement of the

re gulations protecting sovereign submerged lands (253.04–05, 380.05, F.S. and Chapter 18-14, F.A.C.)

would act as a powerful deterrent against boat and dock damage to seagrass systems. Direct physical damage of seagrasses can resultfrom changes in freshwater inputs. The importance to watershed management and reduction of freshwater removal from tributaries flowing into G ulf-coast estuaries has been reviewed for Tampa Bay (Zarbock 1991) and for Charlotte Harbor (Kurz et al. 1999). Estevez (2000) found that

changes in the amount, timing, and location offreshwater inflow are primary stressors to estuarine and oceanic seagrass communities, with salinity changes often being a first-order stressor (Montague and Ley 1993). The present estimated daily freshwater inflows into Tampa Bay (Zarbock 1991) range from 152.0 to 214.6 m 3 s –1 (1,792 to 2,530 ft 3 s –1 ) compared to historical estimates of 193.4 m 3 s –1 (2,280 ft 3 s –1 ). Although timing and duration of riverine inflow, along with nutrient loading and changes in water color, clearly affect seagrass communities in Charlotte Harbor, the

re lationships between these factors is more complex in Tampa Bay and Sarasota Bay (Kurz et al. 1999). Similarly, diversion of fresh water before it enters the Everglades, coupled with a decrease in hu rr icane-induced flushing (Thayer et al. 1994) and rising salinities and sediment anoxia (Zieman
et al. 1999), has been proposed as influencing facto rs in the precipitous decline of Florida Bay seagr asses. Short and Neckles (1999) hypothesized that gl obal warming will probably influence seagrass distribution by causing increases in sea level, in storm frequency and intensity, in disease from higher water temperatures, and in turbidity from eutrophication. The authors also predicted an increase in ultraviolet radiation that will damage seagrasses, as has been shown for macroalgae (Larkum and Wood 1993) and terrestrial plants (Caldwell et al. 1989). In addition, the shallow

co astal and estuarine waters and low elevations on the Gulf coast of Florida will certainly be affected by any eustatic sea-level rise. A potential positive effect on seagrass distribution may occur from anincrease in dissolved inorganic carbon (Ci) composition in seawater, as seagrasses are presently CO 2 limited and have an affinity for Ci (Beer and Ko ch1996). Nutr ient enrichment, probably the most common human effect in Gulf-coast estuaries, can

re sult in a variety of changes in seagrass communities. A number of these changes have been presented in Chapter 4. These include an increase in water-column chlorophyll levels (Johansson 1991; J anicki et al. 1999; Tomasko et al. 1996, 2001; Morr ison et al. 1997), in algal epiphytism and shading Figure 5.4 Propeller scarring from commercial fishing in a seagrass bed in Pine Island Sound, Florida.FWC–FWRI photo

## 48 of seagrass blades (Tomasko and Lapointe 1991; To masko et al. 1996; Dixon 2002), in shifts in do minant seagrass species (Duarte 1995; F ourqurean et al. 1995), and in sediment hypoxia (Carlson et al. 2002). A review of U.S. estuaries (Bricker et al. 1999) cited 15 estuaries on the Gulf coast of Florida that had various levels of eutrophic conditions. Eight of these estuaries (Florida Bay, the southern Ten Thousand Islands, Caloosahatchee River, Charlotte Ha rb or, Sarasota Bay, Tampa Bay, Choctawhatchee Bay, and Perdido Bay) had high levels of eutrophicsymptoms. More detailed data are available regarding nutrient loadings and damage to seagrasses in Ta mpa Bay (Treat and Clark 1991; Zarbock et al. 1994, 1996; Treat 1997; Lewis et al. 1998; Pribble et al. 1999; Greening 2002b), Sarasota Bay (Haddad 1989; Tomasko et al. 1992, 1996), and Lemon Bay (T

omasko et al. 2001). Tampa Bay has been the subject of several reviews (Treat and Clark 1991; Tr eat 1997; Greening 2002a) that include data

re garding point- and nonpoint-source pollution, domestic and industrial effluents, accumulation of
toxic substances in the sediment, and potential brine damage from desalination facilities. Ma nystudies worldwide support the conte ntion that seagrass distribution, survival, and gr owth are principally determined by water clarity (Batiuk et al. 1992; Dennison et al. 1993; Duarte 1991; Kenworthy and Haunert 1991; Kenworthy 1993; Koch and Beer 1996; Morris and Tomasko 1993; and Tomasko et al. 2001; also individual papers contained in the publications of Bortone 2000 and Greening 2002a). However, other factorssuch as waves and water currents can affect seagr ass distribution as well (Koch 2001; see Chapter 2). Water clarity is influenced by nutrient inputs,

which stimulate the growth of phytoplankton (particularly by nitrogen), and by total dissolvedand suspended solids. An overabundance of nutrients leads to eutrophication that supportsincreased levels of phytoplankton, drift algae biomass, and coverage by epiphytes on seagrass blades, all of which result in shading of seagrass leaves (Sand-Jensen 1977, 1990; Cambridge et al. 1986). In all cases a decrease in water clarity overseagrass beds was correlated with a decline in seagr ass standing stock (Orth and Moore 1983; Short
et al. 1996; Short and Wyllie-Echeverria 1996; Fitzpatrick and Kirkman 1995; Lewis et al. 1985a).The influence of possible sediment contaminants on seagrass beds is not understood. The conce ntration of metals, nutrients, petroleum,

chlorinated hydrocarbons, and radionuclides corre late with the smaller particle-size sediments (muds; <63μm in diam.) in Tampa Bay (Brooks and Doyle 1991) and with sediment-quality factors (Zarbock et al. 1997). Thus, concentrations of all

co ntaminants are highest in west-central Hillsborough Bay and Old Tampa Bay (Brooks and Doyle 1991) for total organic matter (to 16%), total hydrocarbons (>40 μg g–1 ), total organic nitrogen (0.1%), and total phosphorus (0.4%). Using the
tr iad method of weighting sediment chemistry,
toxicity, and benthic samples, Zarbock et al. (1997) determined that upper and middle Hillsborough Bay, parts of Old Tampa Bay, Boca Ciega Bay, and

western Middle Tampa Bay had the most contaminated sediments. In a review of sediment quality in Ta mpa Bay, Grabe (1999) found that approximately 1% of its sediments were subnominal and had a high probability of being toxic. However, at the levels known for Tampa Bay, evidence that theheavy metals and pesticides directly affect seagr asses does not exist. Inconclusion, it is apparent that a mix of natur al and human-induced events has negatively influenced the seagrass meadows on the Gulf coast of Florida. Because of limited historical data, determining the extent of seagrass decline is often difficult, although detailed historical analyses are possible in some areas, such as for Tampa Bay(Greening 2002a). Anthropogenic influences, particularly those that result in a decline in waterquality, are considered the principal causes of seagr ass losses throughout the world (Short and Wyllie-Echeverria 1996) and in the estuaries of Florida’s Gulf coast (Lewis et al. 1985a). The loss of seagrass communities continues worldwide (Duarte 2002) as well as on Florida’s Gulf coast (see Chapter 2). However, restoration management can result in the expansion and enhancement ofseagrass meadows, as seen in Tampa Bay and Sarasota Bay (Greening 2002b). Future studies are needed to distinguish secondary and primary stressors that result in seagrass declines, such as the modification of watershed-level stormwater runoff and direct nutrient input.

GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY

### ix seagrass species, known throughout the Caribbean Sea and Gulf of Mexico , occur on the Gulf coast of Florida. A seventh species, Halophila johnsonii, is kn o wn on ly fr om the Atlantic coast of Flori da. Worldwide, sixty species of seagrasses are recognized. They ar e currently p laced by Tomlinson (1982) in 12 genera and f our familie s in a single class of monocots, the Liliopsida (Kuo and McComb 1989) and one subclass, Helobiae (Tomlinson 1982). The inclusion of Ruppia maritima as a seagrass (Dawes 1998a; Littler and Littler 2000) brings the number of species to 61 and families to 5. The family and genus descriptions below are modified from Dawson (1966), Tomlinson (1982), and Kuo and den Hartog (2001). A ke y based on vegeta ti ve characters is followed by s pecies descriptions modified from den Hartog (1970), Littler and Littler (2000), and Kuo and den Hartog (2001). 

# S AXONOMY of 

# F LORIDA 

# S EAGRASSES

APPENDIX

Charles Seaborn photo

## 50 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY 

# ARTIFICIAL KEY to SPECIES, BASED on VEGETATIVE FEATURES
(see also Florida Department of Environmental Protection: http://www.fiu.edu/~seagrass/key/seagrasskey.html) 1. Leaf blades cylindrical; terete in cross section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sy r ingodium filiforme. 1. Leaf blades flat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. 2. Leaves paddle-shaped, associated with scale-like leaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3. 2. Leaves strap-like or linear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5. 3. Leaves in a pseudo-whorl; each shoot with 2 scales at the base and 2 scales halfway up the shoot . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H alophila engelmannii. 3. Leaves paired, with 2 scales at their base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4. 4. Leaves linear-elliptical, margins entire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . H alophila johnsonii. 4. Leaves oblong to elliptical, margins minutely serrate . . . . . . . . . . . . . . . . . . . . . . . . . . . H alophila decipiens. 5. Leaves greater than 3 mm wide; rhizomes scaley . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thalassia testudinum. 5. Leaves usually less than 3 mm wide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6. 6. Leaf tips blunt with dentate points; rhizomes usually straight . . . . . . . . . . . . . . . . . . . . H alodule wrightii. 6. Leaf tips with fine points; rhizomes often in a zig-zag pattern . . . . . . . . . . . . . . . . . . . . . Ruppiamaritima.



# HY DROCHARITACEAE Jussieu

Fa mily of 15 genera, diverse and cosmopolitan, of which 3 are seagrasses ( Enhalus, Halophila, Thalassia ). Plants aquatic herbs, leaves either large, linear
to strap-shaped and with sheathing base ( Thalassia ) or small, usually nonlinear, with distinct petiole and without sheathing base ( H alophila ). Rhizomes with scale-like leaves, and branching via apical bifurcation. Flowers unisexualand anthers either sessile or on long slender filaments. Female flowers usually with long hypanthium and inferior ovary. Fruits fleshy and often indehiscent.

THALASSIA Banks and Solander ex König. 1805. Ann. Bot. (König et Sims ) 2: 96 (Gr. thalassa: marine, of the sea). Rhizomes creeping, horizontal, thick, fleshy, and indeterminate; bearing scale leaves and erect short-shoots (rhizomes) at irregular intervals that arise from rhizome meristem. Erect short shoots determinate, encased in leaf sheaths, producing foliate leaves and flowers. Roots unbranched, fleshy, arising at nodes of short shoots. Leaves strap-shaped, with sheathing bases, and growingfrom basal meristem. Short shoots with conspicuous leaf and inflorescence scars. Plants are dioecious. Male flowers on short stalks, in clusters of 1–3. Female flowers solitary. Fruits globose and opening by irregular splitting, containing several large angular “seeds” (seedlings).
Thalassia testudinum Banks and Solander ex König. 1805. Ann. Bot. (König et Si ms) 2: 96 (L. testudinis: a tortoise). Common name: Turtlegrass (Figure A.1.4). Rhizomes creeping, 3–6 mm in diameter, 4–7 mm long between nodes, with scale leaves. Roots single, unbranched, arising at short-shoot nodes, producing delicate root hairs at tips. Erect short shoots with clusters of 3–7 leaves; leaves to 2 cm wi de, with persistent colorless basal sheath, 9–17 parallel veins, and lacunae that are continuous throughout the plant. Plants dioecious. Flowers on short peduncles, with single whorl of white tepals. M ale flowers with 3 to 12 stamens, pollen grains in

m ucilage, forming moniliform chains. Female flowers bearing one ovary with 6–8 carpels; style divided into 2 filiform stigmata. Fruits buoyant, 15–20 mm long, rough, fleshy, tips pointed, with one to few “seeds” (seedlings).

HALOPHILA Du Petit-Thouars. 1806. Nova Ma dag. 2 (L. halo: salt + philus: loving). Rhizomes horizontal, usually on surface andbearing two scales at each node, producing a lateral shoot and one unbranched root, short erect shoots either with paired leaves on long petioles, distichously arranged along the axes, or in a pseudo-whorl at top of shoot. Leaves oval, ellipti-

## 51 APPENDIX | TA X

ONOMY OF FLORIDA SEAGRASSES cal, lanceolate or linear, with mid and marginal

ve ins. Plants monoecious or dioecious. Flowers

cove re d by 2 bracts; sepals small, petals absent. M ale flowers stalked, with 3 stamens having sessile anthers. Female flowers sessile, with 3–6 styles. Fr uits ovoid capsules, bearing many seeds. Halophila decipiens Ostenfeld. 1902. Bot. T ildsskr. 24: 260 (L. decipio: beguiling, deceptive). Co mmon name: Paddlegrass (Figure A.1.1). Rhizomes thin, with transparent, hairy scales, internodes 1.0–4.5 cm long. Petioles 3–15 mm long, bearing one pair of leaves; leaves oblong to elliptical, 10–25 mm long, 3–6 mm wide, with short rigid unicellular hairs on one or both sides of blades, margins finely serrate. Plants monoecious, male and female flowers on same stalk, covered by spathe, stalks to 1.5 mm long. Male flowers with 2 anthers to 1 mm long. Female flowers with 3 styles 1.5–2.5 mm long; single ovary to 1 mm long. Fr uits 2.5 mm long, 1.5 mm wide, elliptical, bearing up to 30 seeds; seeds oval, to 0.2 mm long. Halophila engelmannii Ascherson in Neumayer. 1875. Anl. Wiss. Beobeibet Reiser. p. 368 (Named for George Engelmann [1809–1885], an American physician and German botanist who settled in St. Louis). Common name: Stargrass (Figure A.1.3).Rhizomes indeterminate, narrow, with internodes 2–4 cm long. Determinate, erect shoots 20–40 mm tall at each node. Leaves with petioles, 2 scalesat base and another pair half-way up shoot, petioles to 2 mm long, bearing 2–4 pairs of leaves in pseudo-whorls. Blades oblong to linear-oblong, 10–30 mm long, 3–6 mm wide, tips pointed and margins serrate. Plants dioecious. Male flowers

w ith 3 imbricate tepals and 3 stamens. Female flowers with one sessile, inferior ovary, ovaries 3–4 mm long, ovoid, with 1–3 styles; styles to 30 mm long. Fruit globose, fleshy capsule, 3–4 mm in diam. with several minute subspherical seeds. Halophila johnsonii Eiseman. 1980. In Eiseman and McMillan, Aquatic Bot. 9: 16 (Named for J. Seward Johnson, cofounder of Harbor BranchOceanographic Institute, where N. Eiseman studied). Common Name: Johnson’s Seagrass. (Figure A.1.2). Rhizomes thin, 1 mm in diam., nodes bearingscale leaves on upper and lower surfaces, internodes 1–2 cm long. Petioles 10–20 mm long, bearing pair of linear to spatulate leaves; leaves slightlyasymmetrical, 5–25 mm long, 1–4 mm wide, marg ins entire and surfaces glabrous. Plants dioecious(?), only female plants found. Female flowers sessile, with three styles 4–6 mm long; seeds unknown. occurring only in Atlantic Florida (Sebastian Inlet to Biscayne Bay) and classified as a threatened species. May not be distinct from H. ovalis (R. Brown) J.D. Hooker f ., 1858. 

# CYMODOCEACEAE N. Taylor

Fa mily of 5 morphologically distinct genera and about 20 species, all seagrasses. Plants perennialherbs. Rhizomes creeping, leafy, herbaceous or distinctly woody, scale-bearing. Leaves distichous

w ith distinct sheath and blade; blades linear, flat or
te re te , l eaves with numerous tannin cells. Plantsdioecious. Flowers solitary or in cymose infloresce nces and usually terminal. Male flowers subsessile or stalked with 2 anthers producing filiform pollen. Female flowers with 2 free carpels, each

w ith 1 style. Fruits 1-seeded nuts, indehiscent.

HALODULE Endlicher. 1841. Gen. 1368 (L. halo: salt + dule: loving). Rhizomes monopodially branched; each node producing one to several unbranched roots and erect short shoot with 1–4 leaves. Leaves narrow, linear, with 3 longitudinal veins and short basalsheaths. Plants are dioecious. Flowers solitary, terminal, enclosed by leaf. Male flowers stalked, with 2 anthers. Female flowers with 2 free carpels, withlong undivided styles. Fruits oval to slightly flatte ned, with a stony pericarp. Halodule wrightii Ascherson. 1868. Bot. Zeitung (Berlin) 26: 511 (Named for Charles Wright [1811–1885], who collected extensively in the southwest U.S. and Cuba and sent specimens to As ch erson). Common names: Shoalgrass, Cuban Shoalgrass, or Shoalweed (Figure A.2.3). Rhizomes slender, bearing 2–4 roots and a short shoot at each node; internodes 0.75–3.5 cm long, with elliptic scale leaves 5–10 mm long. Leaf sheaths 1.5–6.0 cm long; blades 5–12 (–20) cm long, 0.50–1.5 (–2.0) mm wide, with 3 parallel

ve ins; blade tips with 2–3 short horn-like points. Plants are dioecious. Flowers lack tepals; male flowers on peduncles 1.5–2.0 cm long, anthers slender, 3.5–5.0 mm long. Female flowers with one

## 52 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY oval to elliptic ovary, 1.5–2 mm wide, with 10–28 mm long style. Fruits about 1.5–2.0 mm in diam.

w ith stony pericarp and stylar beak.

SYRINGODIUM K ützing. 1860. In Hohenacker, Alg. Marin. Sicc. 9: 426 (Gr. Sy r ingx: a tube + odium: similar to; thus like Syr inga, a genus of flowering plants). Rhizomes indeterminate, with monopodial branching and scale leaves; each node bearing 1–3branching roots and erect, unbranched, determinate short shoot. Short shoots producing 2–3 cylindrical leaves with basal sheaths. Plants dioecious. Flowers in erect cymose infloresences; male flowers stalked, with 2 anthers. Female flowers with 2 free carpels, each with short style. Fruits with stony, smooth pericarp. Syringodium filiforme K ützing. 1860. In H ohenacker, Alg. Marin. Sicc. 9: 426 (L. filiforme: long and thin). Common name: Manateegrass (Figure A.2.2). Rhizomes 2–4 mm in diameter, nodes bearing 2–4 roots with abundant root hairs, scale leaves, and erect short shoot. Short shoots with 2–3 leaves; leaves to 30 cm long, 1–2 mm in diam.,cylindrical, with extensive basal sheathing, cylindrical. Plants dioecious. Male flowers on short peduncles, with 2 anthers. Female flowers sessile,

w ith leaf sheath and 2 free carpels. Fruits oval to oblique, to 6 mm long, 3–5 mm in diam. 

# RUPPIACEAE Hutchins

Fa mily of one genus and 2–3 species, all seagrasses, closely related to the Potomogetonaceae. Plants g labrous, submerged perennial herbs found in alkaline lakes and brackish to oceanic waters. Rhizo mes creeping and little differentiated from erect axes, with monopodial branching. Leaves opposite or alternate, narrowly linear, with single medial

ve in and short basal sheath. Plants monoecious.Flowers perfect, on spikes subtended by 2 subopposite involucral leaves with slightly inflated sheaths; two flowers per spike, each with 2 stamens and few to 4 carpels with short, stout to finelyattenuate styles and peltate stigmas. Fruits indehisce nt achenes, the outer layers soft, decaying and inner pericarp stony.

R UPPIA Linnaeus. 1753. Species Plantarum, p. 127 (Named for Heinrich Reinhard Ruppius [1688–1719], a German botanist who wrote the Flora of Jena). Genus with characters of the family.

Ruppia maritima Linnaeus. 1753. Species Plantarum, p. 127 (L. maritima: of the ocean, marine). Co mmon names: Ditchgrass; Widgeongrass (Figure A.2.1). Rhizomes thin, 1–3 mm in diameter, profusely branched, with growth forms; often geniculate. Le aves produced at nodes, sheath 6–10 mm long. Re productive axes on erect stalks 60–90 cm long. Le aves to 1 dm long, 0.3 mm wide, 1 veined, with pointed tips and appearing to arise directly from

r hizome. Plants monoecious. Flowers on short (>6 cm) peduncles elongating after anthesis, becoming loosely coiled spiral. Flowers with 2 sessile anthers 2-celled, elliptical, 0.6–0.7 mm long; carpels oval,
to 2 mm long, style short and stout to attenuate. Fr uitlets (nutlets) 2–3 mm long, on short stipe, often curved.

## 53 APPENDIX | TA X

ONOMY OF FLORIDA SEAGRASSES Figure A.1 HYDROCHARITACEAE: 1 Halophila decipiens, Paddlegrass ( × 2

⁄

3 ); 2 H. johnsonii, Johnson’s Seagrass ( × 3

⁄ 4 ); 3 H. engelmannii, Stargrass ( × 3

⁄ 4 ); 4 Thalassia testudinum , Turtlegrass ( × 1

⁄

3 ). Scale approximate. Smaller phenotypes are common. 

## 1

3

4 2Figure A.2 R UPPIACEAE: 1 Ruppia maritima, W idg e ongrass ( × 1

⁄ 4 ). CYMODOCEA CEAE: 2 Syringodium filiforme; Manateegrass ( × 1

⁄ 4 ); 3 Halodule wrightii, Shoalgrass ( × 1

⁄

3 ). Scale approximate. Smaller phenotypes are common.



## 54 GULF COAST FLORIDA SEAGRASS: STATUS AND ECOLOGY 

## 1

2

3

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