1

Seagrass distribution in south Florida: a multi-agency coordinated

monitoring program

James W. Fourqurean

Department of Biological Sciences and Southeast Environmental Research Program, Florida

International University, Miami, FL 33199

Michael J. Durako

Departments of Biology and Center for Marine Science Research, The University of North

Carolina at Wilmington, 7205 Wrightsville Ave., Wilmington, NC 28403

Margaret O. Hall

Florida Marine Research Institute, Florida Department of Environmental Protection, 100 8 Ave. th

S.E., St. Petersburg, FL 33701

Lee N. Hefty

Miami-Dade Department of Environmental Resources Management, 33 S.W. 2 Ave., Miami, nd

FL, 33130

A manuscript intended for publication in the book: Linkages between ecosystems in the south

Florida hydroscape: the river of grass continues, edited by J.W. Porter and K.G. Porter.

Introduction

Seagrass beds are a vitally important component of the nearshore marine environment. 

Seagrasses provide habitat for commercially and economically important fish and invertebrates,

and feeding grounds for wading and diving birds. Seagrasses enhance sediment stability, decrease

wave energy and increase water clarity (see reviews by Phillips and McRoy 1980, McRoy and

Helfferich 1977). Seagrass beds are very sensitive to changes in their environment, and are

particularly vulnerable to any decrease in the transmission of light through the water column and

dredging of the sandy and muddy bottoms on which they grow. Much human activity in the
2

coastal zone has the potential to deleteriously effect seagrasses. Dredging and filling of coastal

areas for navigation and development can directly remove potential seagrass habitat (Zieman et al.

1989b); alter hydrological conditions that lead to erosion (Giesen et al. 1990; Larkum and West

1990); as well as cause a reduction in light available to seagrasses by increasing turbidity (Onuf

1994). Increasing human population density in coastal regions has often led to eutrophication

which can reduce light available for seagrasses; eutrophication has been implicated in the loss of

seagrasses from many areas of the world (e.g. Cambridge et al. 1986; Orth and Moore 1983). 

Recreational and commercial use of seagrass beds also can damage them. For example, contact

of the bottom by outboard motors can cause scars that can take years to recover (Zieman 1976);

the cumulative impacts of frequent such events can lead to complete loss of seagrass beds from

heavily trafficked areas (Sargent et al. 1995). Commercial harvesting of shellfish can also have

severe effects on seagrass beds (Thayer et al. 1984).

Seagrasses are a dominant component of the hydroscape of south Florida, and they

occupy the position between the freshwater environments of the mainland and the deep ocean. 

Seagrass communities are found from the mangrove-lined estuaries of Florida Bay, the Shark

River drainage and the Ten Thousand Islands out to back-reef environments and open continental

shelf waters. Six species of rooted vascular plants, or seagrasses, are commonly found in south

Florida: Thalassia testudinum Banks ex König (turtle grass), Syringodium filiforme Kützing

(manatee grass), Halodule wrightii Ascherson (shoal grass), Halophila decipiens Ostenfeld,

Halophila engelmannii Ascherson, and Ruppia maritima L. (widgeon grass). One additional

species, Halophila johnsonii Eiseman, occurs in Florida, but its distribution is limited to the

Indian River Lagoon and extreme northern Biscayne Bay (Eiseman and McMillan 1980), which is
3

outside of the geographic scope of this paper. The general patterns of the distribution and relative

abundance of these species is well-understood (see Zieman 1982; Zieman et al. 1989b for review),

but specific information on the areal extent of seagrass species in south Florida is lacking. In

general, R. maritima is restricted to areas near freshwater sources. In areas of stable salinity,

stable sediments and high light availability, T. testudinum is often dominant. In slightly deeper or

more frequently disturbed areas, H. wrightii and/or S. filiforme are often found. The Halophila

species generally are restricted to low light environments such as deep waters where <15% of

surface light penetrates to the bottom or to shallow turbid waters. 

Previous surveys have documented the widespread occurrence of seagrasses in the south

Florida region. In the area of Florida Bay within Everglades National Park, there are ca. 2000 km2

of seagrasses, mostly dominated by Thalassia testudinum (Zieman et al. 1989a). Using diver

surveys, Iverson and Bittaker (Iverson and Bittaker 1986) estimated that there were an additional

2900 km of seagrass beds in outer Florida Bay (defined as water depths > 2m); these beds were 2

a mixture of T. testudinum, Syringodium filiforme, Halodule wrightii and Halophila decipiens. 

A more intensive in situ and aerial survey of the entire southeastern Gulf of Mexico region

documented 16,600 km of seagrass beds in the area north of the Florida Keys and south of Cape 2

Romano (Continental Shelf Associates 1991). By far, the most common seagrass encountered in

this large area was H. decipiens. On the Atlantic Ocean side of the Florida Keys there are at least

an additional 1029 km of seagrass beds (Klein and Orlando 1994); this brings the estimate of 2

total seagrass habitat in the south Florida region to at least 17,629 km of semi-continuous beds. 2

The near-shore marine and estuarine habitats of south Florida are managed by a diverse

group of governmental agencies at local, state and federal levels (Figure 1). At the local level,
4

county agencies are charged with protection of biotic resources; three counties occupy the

shoreline of our study area: Monroe, Miami-Dade, and Collier. The State of Florida’s

Department of Environmental Protection (FDEP) has jurisdiction on biotic resources in state

waters; i.e. within three nautical miles of the shoreline. Some of the marine area controlled by the

state is further managed by sub-agencies of FDEP. For example, John Pennekamp Coral Reef

State Park occupies a sizeable portion of the potential seagrass habitat in south Florida; the state

parks are administered by their own agency (FDEP Division of Parks and Recreation). The South

Florida Water Management District, a Florida state agency, is charged with environmental

protection of state waters in addition to its primary goals of flood control and water supply. Many

agencies of the Federal government also exercise control over marine waters of the area. Within

the Department of the Interior, the National Park Service (NPS) and the Fish and Wildlife Service

(FWS) each control large areas in south Florida. Everglades National Park and Dry Tortugas

National Park are largely marine parks. The FWS operates a number of Wildlife Sanctuaries in

the region that have large areas of seagrass habitat within their boundaries. The U.S. Department

of Commerce is also involved in management of the region; the Key Largo, Looe Key and Florida

Keys National Marine Sanctuaries are operated by the National Oceanographic and Atmospheric

Administration, an agency of the Department of Commerce. The Environmental Protection

Agency also has regulatory authority over the marine waters in south Florida. Each agency that

has some administrative authority over the marine environment has its own mission; these

missions sometimes conflict. This myriad of overlapping agencies is also a regulatory gauntlet for

people who wish to exploit the resource (e.g. tourism operators, fishermen) as well as for

scientists doing research in the area.
5

While the details of each agency’s mission vary, they all have the same goal: a healthy,

stable, and sustainable environment. All of the agencies have also recognized the need for proper

resource assessment and monitoring of the seagrass communities of south Florida. While

regional-scale deterioration of the seagrass beds has yet to occur in south Florida, there have been

many reports of smaller-scale seagrass decline and potential threats to regional water quality. In

the south Florida case, the regulatory agencies have taken the opportunity to act in a coordinated

effort before large-scale degradation, in the hopes that we will be able to detect, and possibly

avert, regional-scale seagrass loss.

Monitoring programs have been implemented in response to three major seagrass-related

concerns in south Florida: the relationship of seagrass communities to water quality in the Florida

Keys National Marine Sanctuary (FKNMS); changing freshwater runoff in northeast Florida Bay;

and a poorly understood seagrass die-off event that began in Florida Bay in 1987. 

Communication amongst scientists and resource managers in south Florida has led to the

complementary design of these three monitoring programs, so that the programs not only are

providing data to address the original question of concern, but they are also providing data that

can be combined to give a comprehensive view of the distribution and status of seagrass

communities in the region as a whole. The goal of this paper is not to address any of the

questions that led to the original creation of the monitoring efforts, but to use the data to develop

an integrated description of the distribution, relative abundance and species composition of the

seagrass communities from the entire south Florida region.

Three seagrass monitoring programs in south Florida.
6

Seagrass communities as an indicator of water quality in the Florida Keys National Marine

Sanctuary. The FKNMS was established by the Florida Keys National Marine Sanctuary and

Protection Act of 1990 to “preserve and protect the physical and biological components of the

south Florida estuarine and marine ecosystem to ensure its viability for the use and enjoyment of

present and future generations” (NOAA, 1995). Seagrasses are an important biological

component of the FKNMS. Water quality and the health of seagrass communities have been

linked in many locations around the world; as water quality has deteriorated, seagrass

communities have been lost (e.g. Cambridge et al. 1986; Orth and Moore 1983). Concern has

been raised over eutrophication and the status of seagrass communities in the waters of the

FKNMS (Lapointe and Clark 1992; Lapointe et al. 1990; Lapointe et al. 1994; Tomasko and

Lapointe 1991). Because of these concerns, the U.S. EPA established a monitoring program in

1995 designed to define the status and trends of seagrass communities as a part of its

comprehensive Water Quality Protection Plan for the FKNMS (Figure 2). This program was

designed to determine regional-scale gradients in the status of the seagrass communities of the

Sanctuary.

Gaging the effects of changing freshwater flow on benthic communities of Florida Bay. Much of

the historic freshwater inflow to Florida Bay has been severely altered by canalization of the

Everglades ecosystem directly to the north, altering the pattern of salinity in Florida Bay (Light

and Dineen 1994; McIvor et al. 1994; Smith et al. 1989). The present system is one in which

hypersalinity is common (Fourqurean et al. 1993; Tabb et al. 1962). It has been hypothesized that
7

changes in the freshwater flow into Florida Bay has led to changes in benthic communities, such

that Thalassia testudinum is more prevalent in northeast Florida Bay today than historically, when

Halodule wrightii was more common (Zieman 1982). Salinity plays a very important role in

controlling benthic plant communities in the upper estuaries of Florida Bay; areas of high

variability in salinity have low biomass of submerged plants (Montague and Ley 1993). Currently,

water managers are attempting to restore much of the historic flow of freshwater to the

northeastern part of Florida Bay by engineering manipulations of the C-111 canal system. If these

changes have an effect on salinity in Florida Bay, it is probable that benthic communities in Florida

Bay will respond to the changes. The South Florida Water Management District (SFWMD) and

Miami-Dade County Department of Environmental Resources Management (DERM) began a

monitoring program in 1993 to assess the effects of changing freshwater flows on the macrophyte

communities of northeast Florida Bay (Figure 3).

Determining the causes and extent of seagrass dieoff in Florida Bay. Florida Bay is currently

undergoing an unprecedented modification of its ecosystems. The mass mortality of seagrasses

within Florida Bay (Robblee et al. 1991) and the more recent widespread algal blooms (Butler et

al. 1995; Phlips and Badylak 1996; Phlips et al. 1995) may have far-reaching consequences on the

habitat quality and restoration potential of this important ecosystem. Causes of the mortality of

seagrasses have yet to be fully described, but it is clear that a pathogen (Durako and Kuss 1994), 

sulfide toxicity (Carlson et al. 1994) and salinity (Zieman et al. in press) all play some role in the

mortality of the dominant seagrass in Florida Bay, Thalassia testudinum. In 1995, the Florida

DEP initiated a monitoring and research program designed to provide spatially comprehensive
8

status and trends information on the benthic communities of Florida Bay while continuing to

provide physiological information on seagrass species in the Bay (Figure 3). 

Methods

Plant ecologists have worked for many years to devise the best metric for describing the structural

characteristics of plant communities. Each question that may be asked about community structure

has its own optimal method for assessment. Moreover, the scale at which a study is being

conducted also influences the sampling methods. The prime questions motivating the seagrass

monitoring programs in south Florida are: 1) what species make up the seagrass beds?; 2) what

are the relative abundances of the species?; 3) are there spatial trends in the structure of seagrass

communities?; and 4) are there temporal trends in the structure of the seagrass communities? 

Given that the area to be assessed is ca. 19,000 km , the methods adopted for these projects 2

required rapidity and precision; sometimes at the expense of detail. Hence, we chose to utilize a

rapid, visual assessment technique developed early in the 20 century by the plant sociologist th

Braun-Blanquet (Braun-Blanquet 1972). This method is very quick, requiring only minutes at

each sampling site; yet it is robust and highly repeatable, thereby minimizing among-observer

differences. In this method, a series of quadrats are randomly placed on the bottom at a given

location. Each quadrat is examined by a scientist using SCUBA apparatus. All species occurring

in the quadrat are listed, and a ranking based on abundance of the species in that quadrat is

assigned for each species. We have adopted a modified Braun-Blanquet scale for our work in

south Florida (Table 1). Cover, as defined for this purpose, is the fraction of the total quadrat
9

area that is obscured by a particular taxon when viewed from directly above. The only allowable

scores for each taxon in each quadrat are listed in Table 1. The choice of quadrat size is also very

important for this technique; it is important that the quadrat be of sufficient size to accurately

represent the make-up of the community, yet small enough so that they may be rapidly assessed,

sometime under very turbid conditions. We have found that quadrats 0.5 m on a side (0.25 m )

2

work well in south Florida seagrass communities.

Slightly different methods are used to ensure an unbiased placement of sampling quadrats

in the three monitoring programs. In the FKNMS program, 10 quadrats are placed at each site by

locating the quadrats at pre-determined random distances along a 50 m transect placed in a N-S

direction at each site. In the water management and seagrass dieoff monitoring programs, 4

sample quadrats are haphazardly placed at each site. In the SFWMD/DERM, the quadrats are

placed off of the port, starboard, bow and stern of the small boat used as a research vessel,

resulting in a spacing of about 5 m between quadrats. In the seagrass dieoff program, the

quadrats are placed a few meters N, S, E and W of the site location, resulting in a similar layout of

quadrats as the water management program.

From the raw observations of species cover in each quadrat at a site, a single density

estimate is calculated for each plant taxon encountered in the quadrats at a site. Density is

calculated as D = 3S /N; where D = Density of taxon i; j = quadrat number from 1 to N, the total i ij i

number of quadrats sampled at a site, and S = the Braun-Blanquet score for taxon i in quadrat j. ij

For any taxon, D can range between 0 and 5, the maximum Braun-Blanquet score. At a site,

however, the sum of all taxa D values can be greater than 5, because of the relatively broad cover

ranges for each Braun-Blanquet value and the fact that seagrass canopies are three dimensional. 
10

It should also be noted that a taxon may be observed at a site by the sample collector, but unless

the taxa falls within one of the randomly-placed observation quadrats, the taxon receives a D = 0. 

For this reason, our methods underestimate the true areal distribution of individual taxa by

defining a lower density limit for inclusion in the area. In addition, species richness S is calculated

for each site by summing the number of taxa for which D > 0.

When attempting to describe the distribution of habitat types in a landscape, it is important

to sample in a way that allows for unbiased interpolation of the actual sample points to produce

the distribution maps. This means that all points within the landscape must have an equal

probability of being sampled, and that sampling effort be quasi-evenly distributed across the

landscape. Yet, pure random distribution of sampling points often leads to clumped and nonuniformly distributed data points. To meet both of these requirements, we have used the

systematic random method of hexagonal tesselation, developed by the USEPA’s EMAP program,

to locate our sampling locations. The entire region to be sampled was defined, and based on the

number of samples to be collected, the region was divided into hexagonal subunits. One random

location was then chosen as a sample site from within each hexagonal sub-unit. These randomlychosen sites are located in the field using Differential Global Positioning Systems (DGPS); we

have found by repeat visits to marked sites that DGPS is reproducible in our area to ± 5 m.

Sites within the boundaries of the FKNMS (Figure 2) were sampled during the summer

months of 1996 and 1997. Additionally, 100 sites were sampled in a roughly triangular area north

of the FKNMS defined by Cape Romano, Key West, and Florida Bay during August 1998 as part

of the FKNMS program. Data within Florida Bay was all collected in the summer of 1998
11

(Figure 3): the seagrass dieoff program sampled 378 sites, and the SFWMD/DERM program

sampled 228 sites within Florida Bay.

Point data on species density was used to produce continuous maps of the density of

seagrass species, as well as maps of species richness. A krigging algorithm (Watson 1992) was

used to interpolate between the random point data. A spatial analysis program (SURFER, Golden

Software, Golden, CO, USA) was used to compute areas of seagrass coverage from these

interpolated surfaces.

Since none of the species density data were normally distributed, correlations between

densities of species, and between species densities and depth were tested using the nonparametric

Spearman’s D; significances of correlations were assessed using 2-tailed tests.

Results

We assessed the seagrass species composition and density of 1207 sites distributed across

19,402 km of nearshore marine and estuarine environments in south Florida (Figure 1). At these 2

sites, a total of 8434 quadrats (0.25 m ) were sampled, covering an area of 2108.5 m . At least 2 2

one species of seagrass was common enough to be counted in our quadrats at 1056 of the 1207

sites, or 87.5 percent of all sampling sites (Table 2). Thalassia testudinum was the most

commonly encountered species; it was found at 898 sites. Halodule wrightii was the second most

commonly encountered species, occurring at 459 sites; followed by Syringodium filiforme (239

sites), Halophila decipiens (96 sites), Ruppia maritima (41 sites) and Halophila engelmannii (28

sites).
12

Differing morphology and life history characteristics are apparent in the comparison of the

relative densities of the species (Table 2). With 2 exceptions, only Thalassia testudinum and

Syringodium filiforme were found to occur at very high density (D > 4; 4 / 50 - 75% cover,

Table 1); 6.0% of all 1207 sites sampled had very dense cover of T. testudinum, and 1.8% of all

sites had very dense beds of S. filiforme. Since seagrass beds in the region often contain more

than one seagrass species, very dense beds of total seagrass cover were found at 18.1% of the

sites sampled. Density greater than 4 was very rare for the seagrass species of smaller stature

than T. testudinum and S. filiforme. Even the two larger species were most often found to have

moderate density at most sites.

There were species-specific differences in density tendency (Table 2). Restricting the

analysis to only those sites where a species was found, Thalassia testudinum and Syringodium

filiforme were most frequently encountered at D between 1 and 2, although D was higher and

lower than this mode at a significant number of sites. The other species were almost always found

at lower D: Halodule wrightii, Halophila decipiens, Halophila engelmannii and Ruppia

maritima were most commonly found to have D between 0.1 and 0.5. This lower mean D may

have multiple causes. Some species, like H. wrightii and H. engelmannii, are often found as

understory plants beneath a canopy of T. testudinum or S. filiforme. Other species, like H.

decipiens and R. maritima, tend to occur at the extremes of the available habitat for seagrasses,

and their D may be limited by the environment. Because more than one species may contribute to

the overall seagrass D, sites with seagrass were most frequently observed in the 2 to 3 density

class.
13

The density of one species was frequently correlated with densities of other seagrasses

(Table 3). There was no relationship between the density of Thalassia testudinum and

Syringodium filiforme, but T. testudinum density was positively correlated to Halodule wrightii

density, and negatively correlated to the densities of Halophila engelmannii, Halophila decipiens

and Ruppia maritima. Syringodium filiforme density was not correlated to the densities of H.

wrightii or H. decipiens, but was positively correlated to H. engelmannii density and negatively

correlated to R. maritima density. Halodule wrightii density was negatively correlated with H.

decipiens density, and positively correlated with the density of H. engelmannii and R. maritima.

Halophila decipiens and R. maritima densities were negatively correlated, while there was no

significant relationship between the densities of the two congeners of Halophila. No significant

relationship was present between H. engelmannii and R. maritima, most likely due to the small

number of stations where either species occurred.

Water depth was significantly related to the densities of all seagrass species except for

Halophila engelmannii (Table 3). Densities of Thalassia testudinum, Halodule wrightii and

Ruppia maritima were higher in shallow water, while Syringodium filiforme and Halophila

decipiens densities were higher in deeper water. Owing to the coastal nature of the region

surveyed, shallow sites were much more common than deep sites. 43% of all of the sites fell

within the depth range of 0 - 2 m; fewer than 10% of the sites were deeper than 10 m (Table 4). 

The likelihood of finding T. testudinum at a site decreased as site depth increased. More than

80% of sites shallower than 4 m supported T. testudinum. While H. wrightii was most likely to

be encountered at the shallowest sites, a significant number of relatively deep stations also

supported this species. Ruppia maritima was restricted to only those sites shallower than 2 m. 
14

Syringodium filiforme was much less common at the shallowest sites than at mid-depth sites; it

was particulary common in the depth range 6-8 m; 45.6% of all sites in this depth class supported

S. filiforme. Halophila decipiens, in contrast, was absent from the shallowest sites, but was

found at over 50% of all sites sampled that were deeper than 18 m. Halophila engelmannii

presence showed no clear relationship with water depth.

With the exception of Ruppia maritima, the seagrass species had similar ranges of depth

of occurrence, but there were clear differences in the median depth at which each species was

recorded (Table 5). Ruppia maritima was never found deeper than 1.4 m, with a median depth of

0.9 m. Thalassia testudinum and Syringodium filiforme were found to have the same maximum

depth of 18.0 m, but the median depth for T. testudinum, 2.1 m, was shallower than the median

depth for S. filiforme, 4.6 m. Halodule wrightii penetrated slightly deeper in the water column,

with a maximum depth of 18.6 m, but the median depth of 1.4 m illustrates the fact that it was

most commonly found in shallow water. Halophila engelmannii was similar to H. wrightii in

maximum and median depth. Halophila decipiens showed a much different pattern with respect

to depth. It was found as deep as 26.5 m, with a median depth of 6.2 m.

Many (47.6%) of the 1207 sampled sites supported more than one species of seagrass

(Figure 4). Even though it was relatively common for seagrass species to co-occur, a slim

plurality (40.0%) of the 1207 sites supported only 1 seagrass species. Two seagrasses were found

at 37.8% of all sites. Higher species richness was uncommon: 3 species were found at 8.6% of

sites, and only 1.2% of sites had 4 or more species. There was no clear spatial pattern in species

richness; relatively diverse (>3 species) seagrass beds were found on both the Atlantic ocean and

Gulf of Mexico sides of the Florida Keys (Figure 5). The only 2 sites with 5 species (Thalassia
15

testudinum, Syringodium filiforme, Halodule wrightii, Halophila decipiens and Halophila

engelmannii) were found within the Dry Tortugas National Park.

Because sampling intensity varied spatially due to different goals of the three monitoring

programs, frequency of occurrence data (Table 2) can not be used directly to calculate the relative

importance, in terms of area, of the six seagrass species in south Florida. Instead, maps of the

occurrence of each species were analyzed for the areal extent of the species. In areal extent,

Thalassia testudinum was the most common seagrass in the sampling region. Density of T.

testudinum was highest in Florida Bay, in the area between the upper Florida Keys and the reef

tract, and in the shallow, protected waters north and west of Key West (Figure 6). In all, 8482

km of T. testudinum beds were mapped, which was 43.7% of the 19,402 km survey area. 2 2

Roughly half of this total area was made up of very sparse T. testudinum cover: 3927 km of the 2

T. testudinum area had D > 1 (Table 6). 

Second to Thalassia testudinum in terms of areal extent was Halophila decipiens, which

was found to cover 7410 km ; or 38.2% of the survey area (Table 6). In contrast to T. 2

testudinum, however, H. decipiens was found predominantly in the waters of the southwest

Florida Shelf, to the west of the Florida Mainland and to the north of the FKNMS (Figure 7). 

Most of this coverage consists of low density seagrass beds: of the 7410 km of total area, 4652 2

km consisted of areas where D < 1. Only rarely did H. decipiens form very dense beds; the area 2

for which D > 3 was less than 1% of the total area surveyed. 

Syringodium filiforme was also commonly encountered and was found to cover 4879 km . 

2

While Thalassia testudinum was the most dense species immediately adjacent to the Florida Keys

and in Florida Bay (Figure 6), S. filiforme density generally increased in a offshore direction until
16

reaching the reef tract (Figure 8). A very dense bed of S. filiforme dominated the area to the

north of the middle Florida Keys, north of Marathon and west of Florida Bay, encompassing

about 350 km . Most of the area that supported S. filiforme had sparse cover; 3537 km2 of the 2

total area of S. filiforme had D < 1 (Table 6). 

The only other species of seagrass that covered a large proportion of the surveyed area

was Halodule wrightii; it occupied 3540 km , or 18.2% of the surveyed area. While H. wrightii 2

was found sporadically throughout the region, it was most common in Florida Bay, on the Gulf of

Mexico side of the Florida Keys, and in an area west of Key West known as the Quicksands

(Figure 9). 83 % of all of the area supporting H. wrightii had D < 1. The other two species

encountered, Halophila engelmannii and Ruppia maritima, were found to be very limited in

spatial extent. In the extreme upper estuaries of Florida Bay, R. maritima occupied 73 km2

(Figure 10). Halophila engelmannii was occasionally observed, found in 143 km scattered 2

around the survey area (Figure 10). 

The individual species distributions combine to produce a very large area of almost

continual seagrass cover (Figure 11). 75.4% of the total surveyed area supported seagrasses,

resulting in a total area of seagrass beds in the region of 14,622 km (Table 6). Of this total area, 2

5197 km was very sparse, with D < 1. Most of these sparse areas were dominated by Halophila 2

decipiens, like the southwest Florida Shelf area north of Key West and the relatively deep water

between the Quicksands and Dry Tortugas National Park. The most dense areas of seagrass were

generally on the Gulf of Mexico side of the Florida Keys. On the Atlantic Ocean side of the Keys,

seagrass beds were more dense in the Upper Keys than further west. 
17

Discussion

The 14,622 km of seagrasses in south Florida ranks this area among the most expansive 2

documented seagrass beds on earth, comparable to the back-reef environment of the Great Barrier

Reef in Australia (Lee Long et al. 1996) and the Miskito Bank of Nicaragua (Phillips et al. 1982). 

Accordingly, the economic impact and ecological importance of the south Florida seagrass beds

are large (Zieman 1982): fisheries landings in the Florida Keys total over 12 @ 10 kg annually of 6

mostly seagrass-associated organisms (Bohnsack et al. 1994), and over half of all employment in

the Florida Keys is dependent on outdoor recreation (NOAA 1995). For the larger part, these

outdoor activities are reliant on the clear waters and healthy marine habitats of the marine

environment.

Proper environmental stewardship requires accurate data on the present state of resources. 

Prior to the initiation of the three monitoring programs that supplied data for this paper, there was

only general understanding of the magnitude and composition of the seagrass beds of south

Florida. Our work has provided baseline data that will be required for assessing the efficacy of

management of the marine environment in south Florida. In terms of areal extent, seagrasses are,

by far, the most commonly encountered habitat type in the survey area. At 87.5 % of randomly

selected stations, at least one species of seagrass was present; on an areal basis, this translated to

seagrass present over 75.4% of the surveyed area. The remaining area was predominantly

unvegetated soft-bottom communities. Coral reef communities, while in many respects the most

valued and visible benthic habitat type in the region, make up only a small percentage of the total

bottom cover in the survey area (Porter et al. This volume).
18

Analyses of the spatial scope required for this assessment are often impossible because of

the magnitude of the task of collecting the data and because of overlapping jurisdictional

boundaries. Careful coordination between management agencies and research groups ensured

that data collected by different principle investigators, for different goals funded by different

agencies, could be pooled and analyzed as a whole. This type of cooperation should serve as a

model to other groups embarking on the assessment of resources over large geographic ranges.

Very near shore in the survey area, Thalassia testudinum was the dominant seagrass. T.

testudinum may be limited to shallow water because of its high light requirement. This

requirement is a consequence of its relatively low proportion of leaves to roots and rhizomes

compared to the other seagrass species found in the area (Fourqurean and Zieman 1991). 

Nutrient availability may also play a role in T. testudinum distribution. This species is the

competitive dominant in the high-light, low-nutrient environment of Florida Bay (Fourqurean et

al. 1995). Phosphorus availability, which limits the biomass of Thalassia testudinum, increases

from east to west in Florida Bay (Fourqurean et al. 1993; Fourqurean et al. 1992); it also

increases from onshore to offshore on the ocean side of the Florida Keys (Szmant and Forrester

1996). Experimental increases in P availability have resulted in other seagrasses outcompeting T.

testudinum and become dominant (Fourqurean et al. 1995). We hypothesize that the increase in

the abundance of Syringodium filiforme with distance from shore, as well as the very dense bed of

S. filiforme north of Marathon, may be partially a response to relatively high P availability: only in

areas of relatively high P availability can S. filiforme outcompete T. testudinum. This hypothesis

needs to be confirmed by experimental manipulation, however.
19

Interspecific differences in light requirements allow some species of seagrasses to grow in

deeper water than others. Most seagrass genera have a minium light requirement of >10% of

surface irradiance (Duarte 1991). Species in the genus Halophila, however, are often found in

waters deeper than species of other genera (e.g. Lee Long et al. 1996), suggesting that Halophila

spp. have lower light requirements. The median depth of sites that supported H. decipiens was

6.2 m, compared to 4.6 m for S. filiforme and 2.1 m for Thalassia testudinum. This lower light

requirement of Halophila spp. is probably the factor responsible for the expansive beds of H.

decipiens that we documented in the deeper water areas of our survey area. These areas are deep

enough to prevent adequate light from reaching the bottom to support the larger species

Thalassia testudinum and Syringodium filiforme. Of interest is the observation that H. decipiens

was completely absent from shallow ( < 2.4 m) areas. Without experimental evidence, we can

only hypothesize that H. decipiens is competitively displaced from higher light environments by

other seagrass species. In contrast to H. decipiens, median depth for H. engelmannii was a

relatively shallow 1.9 m. We never found extensive meadows dominated by H. engelmannii;

instead it was encountered as a sparse understory species, generally associated with denser beds

of Syringodium filiforme and Halodule wrightii. It is probable that the generally low light

requirements of Halophila spp. allows H. engelmannii to exists as an understory plant, but what

is not clear are the life history differences between H. decipiens and H. engelmannii that allow H.

engelmannii to be a successful understory species, while its congener H. decipiens rarely occurs

as an understory. Also, the minimum light requirements for H. engelmannii do not appear to be

any greater than those for H. decipiens, since H. engelmannii has been documented growing at
20

90m depth within the study area (den Hartog 1970), so it is unclear why H. decipiens is a

meadow-former in deep water, while H. engelmannii is not.

While Halophila species were restricted to areas of truly marine, near-constant salinity,

the other seagrass species were also found in Florida Bay, where salinity is strongly influenced by

runoff from mainland Florida and by exchange of oceanic water with the Gulf of Mexico. Florida

Bay can be either hypo- or hypersaline, depending on location, season and year. Deviations from

normal seawater salinity are deleterious to most seagrasses, but there is apparently a range in

tolerances of species to salinity variation. Of the non-Halophila species, Ruppia maritima is the

most tolerant of hyposalinity events; it is so tolerant of freshwater that it is often found growing in

completely fresh water. This fact has led some authors (e.g. (den Hartog 1970) to exclude R.

maritima from membership within the polyphyletic group of seagrasses. Of the remaining species

encountered in our surveys, Halodule wrightii is the most tolerant of salinity fluctuation,

Thalassia testudinum has intermediate tolerance, and Syringodium filiforme the least tolerant

(McMillan and Moseley 1967). The extreme northeastern portions of Florida Bay are subject to

very large salinity variability; the salinity range for the period 1991-1994 for northeast Florida Bay

was 50 ‰ (Frankovich and Fourqurean 1997). It is likely that this salinity variation limits the

ability of all species but R. maritima to flourish in the extreme northeastern parts of Florida Bay. 

It is not clear, however, why R. maritima is not often found in other parts of the survey area. 

From distributional evidence around point sources of nutrients in Florida Bay, nutrient availability

may have a role in determining R. maritima distribution. Adjacent to point-sources of P, R.

maritima dominates the benthic flora; further from the point sources, H. wrightii and T.

testudinum dominate (Powell et al. 1991). These authors interpreted these observations as
21

evidence that R. maritima can only compete with other seagrass species in high-nutrient areas or

where salinity variability limits the other species. 

It has been suggested that changing water management practices on mainland Florida have

led to changes in distribution of seagrasses in Florida Bay. Surveys of Florida Bay from the mid

1970's recorded large areas in central and eastern Florida Bay that were dominated by Halodule

wrightii (Schmidt 1979), yet these areas were reported to be dominated by Thalassia testudinum

in the 1980's (Zieman et al. 1989a), and were dominated by T. testudinum in our surveys. Zieman

(Zieman 1982) speculated that these changes were the result of changes in timing and amount of

freshwater runoff. 

Concerns for the state of the seagrass beds of south Florida are well-founded. While

presently the seagrass beds are nearly continuous and apparently healthy, there is cause for alarm. 

Localized cases of coastal eutrophication have led to loss of seagrasses in the study area

(Lapointe and Clark 1992; Lapointe et al. 1990; Lapointe et al. 1994; Tomasko and Lapointe

1991). We now have the baseline data against which to measure future changes in these

communities.

The present distribution and species composition of seagrasses in south Florida is a result

of the interaction of many factors, the most important being water depth, water clarity, and

nutrient availability. Changes in the movement and quality of water in the region, whether natural

or anthropogenic, are likely to cause changes in the large-scale patterns in abundance and

composition of these seagrass beds. Since near-shore oceanic water quality is determined by the

interaction of coastal influences, marine influences, and man’s activities, it is clear that proper

management of seagrass beds in south Florida requires holistic knowledge of the entire
22

hydroscape of south Florida. Timing and amounts of freshwater runoff can change coastal

salinity. Degradation of water quality of the freshwater runoff can directly effect nutrient

availability and water clarity. Restriction of water exchange with the open ocean can alter salinity

patterns and nutrient availability. Anthropogenic actions both in the marine and mainland realms

can change nutrient availability and water clarity. Since any of these actions has the potential to

alter the seagrasses of south Florida, all of these activities must be managed to ensure the

continued existence of the seagrass communities in their present state. It is also likely that the

first symptoms of a changing coastal environment will be a change in species composition of

seagrass beds, not a wholesale loss of seagrass cover. For this reason, accurate data on the

species composition of the seagrass communities needs to be collected periodically as a measure

of the state of the coastal environment. 

Conclusions

Seagrass beds are an important, often dominant component in many coastal marine environments,

however, there are few locations in the world where seagrasses are as dominant in the hydroscape

as south Florida. Because of the close proximity between human development activities and

historic seagrass distribution, seagrass beds are being increasingly threatened in many locations

worldwide. Seagrass beds are being lost due to the combined effects of dredging, filling and

water quality degradation throughout their range. Often, habitat degradation is only recognized

after a vital resource is lost or severely altered. In south Florida, the importance of seagrasses to

the economic vitality and ecological integrity of the region has long been recognized; this
23

recognition has led to the development of coordinated seagrass monitoring programs involving

government agencies from Federal, State and Local levels, university scientists and private sector

environmental groups. These monitoring programs have been largely implemented before

regional-scale habitat degradation has severely affected the distribution of seagrasses. The data

from these monitoring efforts provide a view of the distribution and abundance of seagrasses of

the region that is without precedent.

Clear jurisdictional boundaries in the seagrass-supporting marine areas of south Florida provide

both a help and a hindrance to the development of an integrated seagrass monitoring effort. 

These jurisdictional boundaries - National Marine Sanctuaries, National Parks, National Wildlife

Refuges, State waters, State Parks, County Parks, etc - clearly define the entity in government

that is responsible for proper environmental stewardship and set up clear areas of responsibility. 

Delineation can also be to the detriment of a coordinated effort, since governmental agencies have

independent staffs and differing mandates, often leading to disparities among science and

monitoring programs. Since the components of the hydroscape do not respect political

boundaries, many resources occur across multiple jurisdictions. Further, the environmental

factors controlling the distribution of resources also do not respect jurisdictional boundaries. The

regional, cross-ecosystem nature of environmental phenomena make a coordinated effort

paramount if proper data are to be collected to address questions of environmental sustainability.

Funding agencies, management groups and university scientists in south Florida have recognized

the need for complementary monitoring of seagrass ecosystems. There are three major seagrass
24

monitoring efforts ongoing: a U.S. Environmental Protection Agency-funded program addressing

status and trends of seagrasses within the Florida Keys National Marine Sanctuary; a State of

Florida-U.S. Department of Interior (US Geological Survey and Park Service) funded program

assessing the seagrass communities of Florida Bay, and a South Florida Water Management

District and Miami-Dade County funded program concentrating on seagrass distribution in the

upper estuaries of Florida Bay. Together, these programs are producing regional scale maps of

the distribution of benthic marine habitats over a 19,402 km area. Seagrasses were found to 2

occur in 75.4% of this total area, or 14,622 km . 

2

 

Acknowledgments

Monitoring projects of this size would not be possible if not for the hard field and laboratory work

of a legion of people. On the FKNMS project, Braxton Davis, Cassie Furst, Leanne MillerRutten, Brad Peterson, Craig Rose and Alan Willsie did the lion’s share of the field work. The

project was greatly facilitated by Captain Dave Ward and the R/V Magic. The U.S.

Environmental Protection Agency provided funding for the FKNMS through a cooperative

agreement with Florida International University. The Florida Bay Fish-Habitat Assessment

Program (FHAP) has benefitted from field and lab assistance from Donna Berns, Scott Daeschner,

Nancy Diersing, Scott Fears, Jeff Hall, Manuel Merello, Erica Moulton, and Leanne MillerRutten. Financial and logistical support for FHAP was provided by the Florida Department of

Environmental Protection (#3720-2060-204 D1), United States Geological Survey

(#98HQAG2186) and Everglades National Park. For the SFWMD/DERM program, Jason
25

Bacon, Forrest Shaw, Kenneth Liddell, Susan Kim, and Martin Roch conduct all of the field work

and assist with data management. They are recognized for their commitment and hard work in

conducting this program. Cecelia Weaver is SFWMD’s project manager and is recognized for her

continued guidance and support of this program. This is contribution number # of the Southeast

Environmental Research program at Florida International University.

References

Bohnsack, J.A., D.E. Harper, and D.B. McClellan. 1994. Fisheries trends from Monroe County,

FL. Bull. Mar. Sci. 54: 982-1018.

Braun-Blanquet, J. 1972. Plant sociology: the study of plant communities. Hafner Publishing

Company.

Butler, M.J., IV, J.H. Hunt, W.F. Herrnkind, M.J. Childress, R. Bertelsen, W. Sharp, T.

Matthews, J.M. Field, and H.G. Marshall. 1995. Cascading disturbances in Florida Bay,

USA: cyanobacterial blooms, sponge mortality, and implications for juvenile spiny lobsters

Panulirus argus. Mar. Ecol. Prog. Ser. 129: 119-125.

Cambridge, M.L., A.W. Chiffings, C. Brittan, L. Moore, and A.J. McComb. 1986. The loss of

seagrass in Cockburn Sound, Western Australia. II. Possible causes of seagrass decline.

Aquat. Bot. 24: 269-285.

Carlson, P.R., L.A. Yarbro, and T.R. Barber. 1994. Relationship of sediment sulfide to mortality

of Thalassia testudinum in Florida Bay. Bull. Mar. Sci. 54: 733-746.

Continental Shelf Associates. 1991 Southwest Florida nearshore benthic habitat study narrative

report. U.S. Department of the Interior, Minerals Management Service, Gulf of Mexico

Regional Office 89-0080.

den Hartog, C. 1970. Sea-grasses of the world. North-Holland Publishing Company.

Duarte, C.M. 1991. Seagrass depth limits. Aquat. Bot. 40: 363-377.

Durako, M.D., and K.M. Kuss. 1994. Effects of Labyrinthula infection on the photosynthetic

capacity of Thalassia testudinum. Bull. Mar. Sci. 54: 727-732.
26

Eiseman, N.J., and C. McMillan. 1980. A new species of seagrass, Halophila johnsonii, from the

Atlantic coast of Florida. Aquat. Bot. 9: 15-19.

Fourqurean, J.W., R.D. Jones, and J.C. Zieman. 1993. Processes influencing water column

nutrient characteristics and phosphorus limitation of phytoplankton biomass in Florida

Bay, FL, USA: inferences from spatial distributions. Estuarine Coastal Shelf Sci. 36: 295-

314.

Fourqurean, J.W., G.V.N. Powell, W.J. Kenworthy, and J.C. Zieman. 1995. The effects of longterm manipulation of nutrient supply on competition between the seagrasses Thalassia

testudinum and Halodule wrightii in Florida Bay. Oikos. 72: 349-358.

Fourqurean, J.W., and J.C. Zieman. 1991. Photosynthesis, respiration and whole plant carbon

budget of the seagrass Thalassia testudinum. Mar. Ecol. Prog. Ser. 69: 161-170.

Fourqurean, J.W., J.C. Zieman, and G.V.N. Powell. 1992. Phosphorus limitation of primary

production in Florida Bay: Evidence from the C:N:P ratios of the dominant seagrass

Thalassia testudinum. Limnol. Oceanogr. 37: 162-171.

Frankovich, T.A., and J.W. Fourqurean. 1997. Seagrass epiphyte loads along a nutrient

availability gradient, Florida Bay, USA. Mar. Ecol. Prog. Ser. 159: 37-50.

Giesen, W.B.J.T., M.M. van Katwijk, and C. Den Hartog. 1990. Eelgrass condition and the

turbidity in the Dutch Wadden Sea. Aquat. Bot. 37: 71-85.

Iverson, R.L., and H.F. Bittaker. 1986. Seagrass distribution and abundance in eastern Gulf of

Mexico coastal waters. Estuarine Coastal Shelf Sci. 22: 577-602.

Klein, C.J.I., and S.P.J. Orlando. 1994. A spatial framework for water-quality management in the

Florida Keys National Marine Sanctuary. Bull. Mar. Sci. 54: 1036-1044.

Lapointe, B.E., and M.W. Clark. 1992. Nutrient inputs from the watershed and coastal

eutrophication in the Florida Keys. Estuaries. 15: 465-476.

Lapointe, B.E., J.D. O'Connell, and G.S. Garrett. 1990. Nutrient couplings between on-site

sewage disposal systems, groundwaters, and nearshore surface waters of the Florida Keys.

Biogeochem. 10: 289-307.

Lapointe, B.E., D.A. Tomasko, and W.R. Matzie. 1994. Eutrophication and trophic state

classification of seagrass communities in the Florida Keys. Bull. Mar. Sci. 54: 696-717.

Larkum, A.W.D., and R.J. West. 1990. Long-term changes of seagrass meadows in Botany Bay,

Australia. Aquat. Bot. 37: 55-70.
27

Lee Long, W.J., R.G. Coles, and L.J. McKenzie. 1996. Deepwater seagrasses in northeastern

Australia - how deep, how meaningful? pp. 41-50. In J. Kuo, R. C. Phillips, D. I. Walker

and H. Kirkman [eds.], Seagrass Biology: proceedings of an international workshop.

Faculty of sciences, University of Western Australia.

Light, S.S., and J.W. Dineen. 1994. Water control in the Everglades: a historical prospective. pp.

47-84. In S. M. Davis and J. C. Ogden [eds.], Everglades: the ecosystem and its

restoration. St. Lucie Press.

McIvor, C.C., J.A. Ley, and R.D. Bjork. 1994. Changes in freshwater inflow from the Everglades

to Florida Bay including effects on biota and biotic processes: a review. pp. 117-146. In

S. M. Davis and J. C. Ogden [eds.], Everglades: the system and its restoration. St. Lucie

Press.

McMillan, C., and F.N. Moseley. 1967. Salinity tolerances of five marine spermatophytes of

Redfish Bay, Texas. Ecology. 48: 503-506.

McRoy, C.P., and C. Helfferich. 1977. Seagrass ecosystems : a scientific perspective. M. Dekker.

Montague, C.L., and J.A. Ley. 1993. A possible effect of salinity fluctuation on abundance of

benthic vegetation and associated fauna in northeastern Florida Bay. Estuaries. 16: 703-

717.

Onuf, C.P. 1994. Seagrasses, dredging and light in Laguna Madre, Texas, USA. Estuarine Coastal

Shelf Sci. 39: 75-91.

Orth, R.J., and K.A. Moore. 1983. Chesapeake Bay: an unprecedented decline in submerged

aquatic vegetation. Science. 222: 51-53.

Phillips, R.C., and C.P. McRoy. 1980. Handbook of seagrass biology : an ecosystem perspective.

Garland STPM Press.

Phillips, R.C., R.L. Vadas, and N. Ogden. 1982. The marine algae and seagrasses of the Miskito

Bank, Nicaragua. Aquat. Bot. 13: 187-195.

Phlips, E.J., and S. Badylak. 1996. Spatial variability in phytoplankton standing crop and

composition in a shallow inner-shelf lagoon, Florida Bay, Florida. Bull. Mar. Sci. 58: 203-

216.

Phlips, E.J., T.C. Lynch, and S. Badylak. 1995. Chlorophyll a, tripton, color, and light availability

in a shallow tropical inner-shelf lagoon, Florida Bay, USA. Mar. Ecol. Prog. Ser. 127:

223-234.
28

Powell, G.V.N., J.W. Fourqurean, W.J. Kenworthy, and J.C. Zieman. 1991. Bird colonies cause

seagrass enrichment in a subtropical estuary: observational and experimental evidence.

Estuarine Coastal Shelf Sci. 32: 567-579.

Robblee, M.B., T.R. Barber, P.R. Carlson, M.J. Durako, J.W. Fourqurean, L.K. Muehlstein, D.

Porter, L.A. Yarbro, R.T. Zieman, and J.C. Zieman. 1991. Mass mortality of the tropical

seagrass Thalassia testudinum in Florida Bay (USA). Mar. Ecol. Prog. Ser. 71: 297-299.

Sargent, F.J., T.J. Leary, D.W. Crewz, and C.R. Kruer. 1995 Scarring of Florida's seagrasses:

assessment and management options. Florida Marine Research Institute TR-1.

Schmidt, T.W. 1979 Ecological study of the fishes and water quality characteristics of Florida

Bay, Everglades National Park, Florida. South Florida Research Center, Everglades

National Park RSP-EVER N-36.

Smith, T.J., III, J.H. Hudson, M.B. Robblee, G.V.N. Powell, and P.J. Isdale. 1989. Freshwater

flow from the Everglades to Florida Bay: a historical reconstruction based on fluorescent

banding in the coral Solenastrea bournoni. Bull. Mar. Sci. 44: 274-282.

Szmant, A.M., and A. Forrester. 1996. Water column and sediment nitrogen and phosphorus

distribution patterns in the Florida Keys, USA. Coral Reefs. 15: 21-41.

Tabb, D.C., D.L. Dubrow, and R.B. Manning. 1962. The ecology of northern Florida Bay and

adjacent estuaries. Technical Series #39, State of Florida Board of Conservation, Miami,

FL. 81 pp. .

Thayer, G.W., W.J. Kenworthy, and M.S. Fonseca. 1984 The ecology of eelgrass meadows of the

Atlantic coast: a community profile. U.S. Fish and Wildlife Service 84/02.

Tomasko, D.A., and B.E. Lapointe. 1991. Productivity and biomass of Thalassia testudinum as

related to water column nutrient availability and epiphyte levels: field observations and

experimental studies. Mar. Ecol. Prog. Ser. 75: 9-17.

Watson, D.F. 1992. Contouring: a guide to the analysis and display of spatial data, vol. 10.

Pergammon Press.

Zieman, J.C. 1976. The ecological effects of physical damage from motor boats on turtle grass

beds in southern Florida. Aquat. Bot. 2: 127-139.

Zieman, J.C. 1982 The ecology of the seagrasses of south Florida: a community profile. U.S. Fish

and Wildlife Service 82/25.
29

Zieman, J.C., J.W. Fourqurean, and T.A. Frankovich. in press. Seagrass dieoff in Florida Bay

(USA): long-term trends in abundance and growth of turtle grass, Thalassia testudinum.

Estuaries.

Zieman, J.C., J.W. Fourqurean, and R.L. Iverson. 1989a. Distribution, abundance and

productivity of seagrasses and macroalgae in Florida Bay. Bull. Mar. Sci. 44: 292-311.

Zieman, J.C., R.T. Zieman, E.C. Pendleton, National Wetlands Research Center (U.S.), and

United States. Minerals Management Service. 1989b. The ecology of the seagrass

meadows of the west coast of Florida : a community profile. U.S. Dept. of the Interior

Fish and Wildlife Service Research and Development.
30

Table 1. Braun-Blanquet abundance scale used to assess seagrass density. Cover is defined as the

fraction of the bottom that is obscured by the species when viewed by a diver from directly above.

Cover Class Description

0 Absent

0.1 Solitary individual ramet, less than 5 % cover

0.5 Few individual ramets, less than 5% cover

1 Many individual ramets, less than 5% cover

2 5% - 25 % cover

3 25 % - 50 % cover

4 50 % - 75 % cover

5 75 % - 100 % cover
31

Table 2. Distribution of seagrass density D at the 1207 sampling sites.

Species Density class (D)

0 0<D#0.1 0.1<D#0.5 0.5<D#1 1<D#2 2<D#3 3<D#4 4<D#5 D>5

Number of sites

Thalassia testudinum 309 33 116 111 240 201 124 73 0

Syringodium filiforme 968 24 32 44 70 25 22 22 0

Halodule wrightii 748 56 162 76 109 43 12 1 0

Halophila decipiens 1111 27 26 11 13 11 7 1 0

Halophila engelmannii 1179 11 12 3 2 0 0 0 0

Ruppia maritima 1166 2 18 10 9 2 0 0 0

3 D for all seagrasses 151 32 80 79 223 233 190 155 64

Fraction of all sites sampled (%)

Thalassia testudinum 25.6 2.7 9.6 9.2 19.9 16.7 10.3 6.0 0.0

Syringodium filiforme 80.2 2.0 2.7 3.6 5.8 2.1 1.8 1.8 0.0

Halodule wrightii 62.0 4.6 13.4 6.3 9.0 3.6 1.0 0.1 0.0

Halophila decipiens 92.0 2.2 2.2 0.9 1.1 0.9 0.6 0.1 0.0

Halophila engelmannii 97.7 0.9 1.0 0.2 0.2 0.0 0.0 0.0 0.0

Ruppia maritima 96.6 0.2 1.5 0.8 0.7 0.2 0.0 0.0 0.0

3 D for all seagrasses 12.5 2.7 6.6 6.5 18.5 19.3 15.7 12.8 5.3

Fraction of sites where species occurs (%)

Thalassia testudinum 3.7 12.9 12.4 26.7 22.4 13.8 8.1 0.0

Syringodium filiforme 10.0 13.4 18.4 29.3 10.5 9.2 9.2 0.0

Halodule wrightii 12.2 35.3 16.6 23.7 9.4 2.6 0.2 0.0

Halophila decipiens 28.1 27.1 11.5 13.5 11.5 7.3 1.0 0.0

Halophila engelmannii 39.3 42.9 10.7 7.1 0.0 0.0 0.0 0.0

Ruppia maritima 4.9 43.9 24.4 22.0 4.9 0.0 0.0 0.0

3 D for all seagrasses 3.0 7.6 7.5 21.1 22.1 18.0 14.7 6.1
32

Table 3. Correlations (nonparametric Spearman’s D) between densities of seagrass species, and seagrass species density and water depth, from

the 1207 seagrass sampling sites. Correlation coefficients are below the diagonal; 2-tailed significances are above the diagonal. Significant

correlations ( P # 0.05) are in boldface type.

Depth T. testudinum S. filiforme H. wrightii H. decipiens H. engelmannii R. maritima

Water depth < 0.001 < 0.001 < 0.001 < 0.001 0.734 < 0.001

Thalassia testudinum -0.350 0.580 0.005 < 0.001 0.014 < 0.001

Syringodium filiforme 0.291 0.016 0.092 0.953 0.034 0.001

Halodule wrightii -0.451 0.080 -0.049 0.006 < 0.001 < 0.001

Halophila decipiens 0.317 -0.314 -0.002 -0.079 .0251 0.050

Halophila engelmannii 0.010 -0.071 0.061 0.176 0.033 0.316

Ruppia maritima -0.262 -0.129 -0.092 0.226 -0.058 -0.029
33

Table 4. Frequency of encountering seagrass species as a function of the depth of the sample site.

Percent of sites occupied by

Depth interval Number of Thalassia Syringodium Halodule Halophila Halophila Ruppia Any

 (m) Sites testudinum filiforme wrightii decipiens engelmannii maritima Species

0-2 518 83.0 6.4 60.2 0.0 2.9 7.1 95.0

2-4 301 89.0 24.3 29.6 6.0 2.7 0.0 96.0

4-6 121 61.2 37.2 17.4 21.5 0.8 0.0 81.8

6-8 114 68.4 45.6 21.1 11.4 0.0 0.0 78.9

8-10 64 50.0 31.3 6.3 14.1 0.0 0.0 64.1

10-12 36 27.8 27.8 16.7 25.0 5.6 0.0 55.6

12-14 18 11.1 11.1 0.0 16.7 0.0 0.0 27.8

14-16 12 25.0 16.7 8.3 41.7 8.3 0.0 58.3

16-18 10 10.0 20.0 10.0 40.0 0.0 0.0 40.0

18-20 7 0.0 0.0 14.3 57.1 14.3 0.0 57.1

20-22 1 0.0 0.0 0.0 100.0 0.0 0.0 100.0

22-24 0 nd nd nd nd nd nd nd

24-26 4 0.0 0.0 0.0 75.0 0.0 0.0 75.0

26-28 1 0.0 0.0 0.0 100.0 0.0 0.0 100.0
34

Table 5. Depth range of sample sites where the six seagrass species were collected.

Species n min depth max depth mean depth median depth

Thalassia testudinum 898 0.2 18.0 3.0 2.1

Syringodium filiforme 239 0.9 18.0 5.1 4.6

Halodule wrightii 460 0.2 18.6 2.3 1.4

Halophila decipiens 96 2.4 26.5 8.7 6.2

Halophila engelmannii 28 1.4 18.3 3.9 1.9

Ruppia maritima 41 0.4 1.4 0.9 0.9
35

Table 6. Area inventory of seagrass species in the surveyed region. Total area of the survey was 19,402 km (Figure 1) 2

Species Density class (D)

 

0<D#0.1 0.1<D#0.5 0.5<D#1 1<D#2 2<D#3 3<D#4 D > 4 D $ 0.1

Area in a density class (km )

2

Thalassia testudinum 10920 2193 1734 2657 1370 472 55 8482

Syringodium filiforme 14523 2421 1116 718 249 196 179 4879

Halodule wrightii 15862 2163 772 554 49 1 0 3540

Halophila decipiens 11992 2984 1668 1838 780 138 2 7410

Halophila engelmannii 19259 132 10 1 0 0 0 143

Ruppia maritima 19329 43 20 10 0 0 0 73

3 D for all seagrasses 4780 3052 2145 4183 3112 1473 657 14622

Fraction of surveyed area (%)

Thalassia testudinum 56.3 11.3 8.9 13.7 7.1 2.4 0.3 43.7

Syringodium filiforme 74.9 12.5 5.8 3.7 1.3 1.0 0.9 25.1

Halodule wrightii 81.8 11.1 4.0 2.9 0.3 0.0 0.0 18.2

Halophila decipiens 61.8 15.4 8.6 9.5 4.0 0.7 0.0 38.2

Halophila engelmannii 99.3 0.7 0.0 0.0 0.0 0.0 0.0 0.7

Ruppia maritima 99.6 0.2 0.1 0.1 0.0 0.0 0.0 0.4

3 D for all seagrasses 24.6 15.7 11.1 21.6 16.0 7.6 3.4 75.4
36

Figure Captions

Figure 1. Area of seagrass surveys in south Florida. The geographic extent of surveys is

delineated by the solid gray line. Management jurisdictional boundaries are given

for the major management areas in the region.

Figure 2. Station locations for the U.S. Environmental Protection Agency funded monitoring

program for determining water quality within the Florida Keys National Marine

Sanctuary (FKNMS).

Figure 3. Station locations in Florida Bay within Everglades National Park. Filled circles are

sites sampled in the Florida Department of Environmental Protection’s FishHabitat Assessment Program (FHAP), funded by Florida DEP, the U.S. Geological

Survey - Biological Resources Division , and Everglades National Park, designed

to investigate the causes and consequences of seagrass die-off in Florida Bay. 

Crosses indicate Miami-Dade County Department of Environmental Resources

Management (DERM) sampling locations in the South Florida Water Management

District/DERM funded project investigating the consequences of changing

freshwater discharge into Florida Bay on benthic communities.

Figure 4. Frequency histogram of the Species Richness, S, at sampling locations. S is

defined as the number of seagrass species occurring at a station (see text).

Figure 5. Spatial distribution of species richness of seagrass beds across the south Florida

hydroscape. Small crosses indicate sampling points.

Figure 6. Spatial distribution of the density of Thalassia testudinum across the south Florida

hydroscape. Small crosses indicate sampling points. Density scale is in BraunBlanquet density units (see text and Table 1).

Figure 7. Spatial distribution of the density of Halophila decipiens across the south Florida

hydroscape. Small crosses indicate sampling points. Density scale is in BraunBlanquet density units (see text and Table 1).

Figure 8. Spatial distribution of the density of Syringodium filiforme across the south

Florida hydroscape. Small crosses indicate sampling points. Density scale is in

Braun-Blanquet density units (see text and Table 1).

Figure 9. Spatial distribution of the density of Halodule wrightii across the south Florida

hydroscape. Small crosses indicate sampling points. Density scale is in BraunBlanquet density units (see text and Table 1).

Figure 10. Spatial distribution of the density of Halophila engelmannii (main map) and

Ruppia maritima (inset) across the south Florida hydroscape. Small crosses
37

indicate sampling points. Density scale is in Braun-Blanquet density units (see text

and Table 1).

Figure 11. Spatial distribution of the sum of the density scores for all seagrass species across

the south Florida hydroscape. Small crosses indicate sampling points. Density

scale is in Braun-Blanquet density units (see text and Table 1).