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Introduction

Question 4—What are the causes and mechanisms for the

observed changes in the seagrass community of Florida

Bay? What is the effect of changing salinity, light, and nutrient regimes on these communities?

Seagrass beds are an important component in many

coastal marine environments; however, there are few

locations in the world where seagrasses are as dominant in the hydroscape as they are in south Florida

(Fourqurean et al., 2002). Seagrasses are the dominant

biological community in Florida Bay, historically covering more than 90% of the 180,000 ha of subtidal mudbanks and basins within the bay (Zieman et al., 1989).

By comparison, mangrove islands cover only about 7%

of Florida Bay. Because of the shallow nature of Florida

Bay (mean depth less than 2 m; Schomer and Drew,

1982), seagrasses are also the dominant physical feature

of the bay, and their presence greatly affects physical,

chemical, geological, and biological processes in this system (Zieman, 1982). Seagrass communities are also important to the economy of south Florida because they

provide food and shelter to numerous fish and invertebrate species, many of which are commercially important to the region (Powell et al., 1989; Thayer and

Chester, 1989; Tilmant, 1989; Chester and Thayer, 1990).

In addition, the waters of western Florida Bay form

a hydrodynamic link between the Everglades and the

coastal waters of the southwestern Florida peninsula

and eastern Gulf of Mexico to the north and the Florida

Keys reef tract and the Atlantic Ocean to the south

(Schomer and Drew, 1982).The seagrass communities

of this region form an important buffer by intercepting the flow of water and reducing nutrient and particulate loads in the waters reaching the reef tract

(Kenworthy et al., 1998). Continued loss of seagrasses

along this margin and proposed increases in water

flow from the Everglades could result in greater exports

of nutrients and particulates out of Florida Bay and onto

the reef tract (Kenworthy et al., 1998). Resource managers will need to consider actions that might aid in the

reestablishment of continuous seagrass cover in western Florida Bay.This would be an important step in reducing sediment-resuspension-induced turbidity along

this boundary that could reverse the cascading declines that characterize the present system.

Seagrasses act as integrators of net changes in

water-quality variables, which tend to show rapid and

wide fluctuations when measured directly.The shallow

distribution of seagrasses places them in close proximity to the land–sea interface, a region experiencing

a rapid growth of human populations.This coastal distribution also places seagrass communities at the end

of the watershed pipe, so the health of these communities reflects not only direct coastal influences but

larger-scale, landscape-to-regional influences as well.

Because most seagrasses are benthic-perennial plants,

they are continuously subjected to stresses and disturbances that are associated with changes in water

quality along the land–sea interface.To a large extent,

seagrass abundance determines public perception regarding the “health”of the coastal waters of Florida (Goerte, 1994; Boesch et al., 1993, 1995). Thus, the recent

changes in the distribution and abundance of seagrasses within Florida Bay have been perceived as a

change in the health of the bay. For these reasons, seagrasses may be the best indicators of changes in the

Florida Bay ecosystem (Fourqurean et al., 1992).

Although the initial die-off originated in the interior basins of the bay (Robblee et al., 1991), the greatest changes in seagrass abundance in the present

system are occurring far from the Everglades-Florida

Bay land–sea interface (Durako et al., 2002).The spatial

patterns of changes in abundance from 1995 to 2000

suggest that the most perturbed environment in Florida

Bay with respect to seagrasses is currently along the

western and southern margins of the bay. Much of the

focus of management and restoration efforts in south

Florida have been directed toward landscape-scale

modifications and an extensive flood-control system to

increase the quantity of freshwater delivered to northeastern Florida Bay and, more recently, to Shark River

slough.

Seagrass Communities in Florida Bay

Seagrasses continue to be the dominant biological

Chapter 7

Seagrass Ecology

Team Leaders Michael Durako (University of North Carolina at Wilmington), 

Joseph Zieman (University of Virginia), Michael Robblee (USGS, PMC)
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FWRI Technical Report TR-11 93

community in Florida Bay. Of the more than 14,000

Braun-Blanquet samples (each 0.25 m2) taken in the bay

from 1995 to 2000 by the Fisheries Habitat Assessment

Program (FHAP; Durako et al., 2002), approximately

97% contained seagrass. In northeastern Florida Bay,

Thalassia was present at 75.9% and Halodule was present at 69.0% of the 762 randomly selected stations

sampled from May 1999 to May 2000 (Bacon et al.,

2001).The entire south Florida coastal zone, including

the areas west of Florida Bay and within the Florida

Keys National Marine Sanctuary (FKNMS), is dominated by seagrass habitats. Fourqurean et al., (2002) assessed seagrass species composition and density at

1,207 sites distributed across 19,402 km2 of nearshore

marine and estuarine environments in south Florida.

At these sites, a total of 8,434 quadrats (each 0.25 m2 total

area = 2,108.5 m2) were sampled from 1996 to 1998. At

least one species of seagrass was observed at 1,056 of

the 1,207 sites, or 87.5% of all sampling sites. Thalassia

testudinum (turtlegrass) was the most commonly encountered species, being found at 898 sites. Halodule

wrightii (shoalgrass) was the second most commonly

encountered species, occurring at 459 sites, followed

by Syringodium filiforme (manateegrass, 239 sites),

Halophila decipiens (paddlegrass, 96 sites),Ruppia maritima (widgeongrass, 41 sites) and Halophila engelmannii (stargrass, 28 sites).

Recent and Historical Changes

A widespread die-off of seagrasses within Florida Bay

began in 1987 (Robblee et al., 1991; Table 7.1; http://people.uncw.edu/durakom/seagrass/seagrass.htm). This

event was first observed by backcountry fishing guides

who reported the occurrence of “potholes”in the seagrass beds in the bights along the north-central part

of Florida Bay. Extensive areas of Thalassia began dying

rapidly in central and western basins, and by 1990,

4,000 ha were completely lost and 24,000 ha were affected by the die-off (Robblee et al., 1991).

The patterns of changes in seagrass abundance in

Florida Bay have recently undergone four phases: 1) primary die-off; 2) secondary mortality, with mortality

primarily due to light attenuation; 3) seagrass recovery associated with improving water clarity; and 4) renewed primary die-off in areas where die-off has not

been previously observed.The initial phase of primary

seagrass die-off occurred only in Florida Bay, which is

the most continentally influenced and least oceanically flushed of all of the extensive south Florida seagrass beds, and it occurred only in the most densely

developed beds within the bay. To this day, no occurrence of primary seagrass die-off has been found outside of the densest seagrass beds within Florida Bay,

and no primary seagrass die-off has occurred in sparseor medium-density beds within the bay.

PRIMARY SEAGRASS DIE–OFF

The first phase of primary seagrass die-off occurred

during the relatively dry and clear period of 1987 to

early 1991. During this period, density, standing crop,

and productivity of Thalassia were high but then declined as the die-off progressed (Durako, 1995; Zieman

et al., 1999). Some stations had shown large increases

in seagrass standing crop in the period prior to the dieoff (Zieman et al., 1999). In addition, Batophora oerstedi

and Halodule spread rapidly into die-off patches (Thayer

et al., 1994). At this time, Florida Bay waters as a whole

remained as clear as they had been in the years just

prior to the die-off, with limited turbidity from suspended sediments in the immediate vicinity of the

denuded die-off patches.

SECONDARY MORTALITY

Several years after the initiation of the seagrass die-off,

Florida Bay began exhibiting widespread and chronic

turbidity with a concomitant decline in Thalassia and

Halodule in Johnson Key Basin and Rankin Lake

(Stumpf et al., 1999). The increase in turbidity, which

began during the fall of 1991, was principally due to

cyanobacteria-dominated microalgal blooms and resuspended sediments, which were associated with the

loss of seagrasses on the western banks and was most

severe in the western and central bay (Phlips and

Badylak, 1996).These blooms may have been initiated

by the nutrients liberated by the die-off of seagrasses

(Butler et al., 1995). Loss of seagrass cover was the

major factor of the increases in sediment resuspension

in the bay (Prager, 1998). This resulted in a negative

feedback loop in which loss of seagrass cover from

die-off led to exposed, easily resuspended sediments

and more widespread losses of seagrasses due to turbidity. Sponge mortality, changes in juvenile lobster

population dynamics (Butler et al., 1995), and indications of cascading effects on plant and animal communities in adjacent systems (e.g., sea urchin

population explosions and unbalanced growth of Syringodium filiforme in the waters of the Florida Keys National Marine Sanctuary southwest of Florida Bay;

Kenworthy et al., 1998; Rose et al., 1999) were also observed. From 1992 to 1995, salinities showed a progressive decline throughout the bay (Boyer et al., 1999).

During this time, there was speculation regarding

how much seagrass had been lost, with as much as

100,000 acres (40,000 ha) estimated with no supporting

data. With the decline in water clarity, aerial photography became useless in determining seagrass distribution over much of Florida Bay. Because of the

Seagrass Ecology
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94 FWRI Technical Report TR-11

Table 7.1 Florida Bay seagrass die-off milestones.

Event or Observation Date Significant Publication/Research Initiated

SPOT image shows no die-off patch in Rabbit Feb 2,

Key Basin, Johnson Key Basin, or Rankin Lake. 1987

“Potholes”seen in Thalassia beds in Rankin Lake Summer FIU/SERP water-quality monitoring initiated.

and Cross Bank by back-country fishing guides. 1987

Heat stress kills tagged conch in Hawk Channel Jul 1987

(Berg); coral bleaching at Looe Key (Causey);

fish dying on Cross Bank (Holmquist, Robblee).

Robblee observes die-off patches Late Fall

along north shore of Johnson Key. 1987

Guides contact Susan Bell about dying seagrasses. Mar 1988

Guides meet Mike Robblee and Jim Tilmant at Apr 1988 Robblee and Zieman visit Rankin Lake and

Little Rabbit Key and observe extensive die-off. Johnson Key Basin.

ENP consults Jay Zieman.

May 1988 Zieman provides ENP a trip report on die-off,

indicating its uniqueness and stressing need to

monitor and conduct research. Carlson-Durako

survey Johnson Key Basin.

Zieman observes lesions were not apparent. Spring–

Robblee and Holmquist observe phytoplankton Summer

bloom NE of Long Key. 1988

SPOT imagery shows large die-off patch in Jun 1988

Rabbit Key Basin.

Sep 1988 Robblee provides memo on seagrass die-off to 

Superintendent Finley. Finley provides funds for 

researchers to evaluate die-off.

Nov 1988 ENP-sponsored field trip: Robblee, Carlson,

Durako,Fourqurean, Muehlstein, Porter, and

Zieman visit die-off areas. Labyrinthula detected.

Apr 1989 Durako initiates studies on morphometric changes,

Braun-Blanquet abundance, and demography.

Carlson begins studies on hypoxia/sulfide-ADH,

ethylene, H2S, conducts bucket experiments.

May 1989 Robblee initiates studies of seagrass associated

animal responses to die-off in Rankin Lake,

Johnson and Rabbit Key basins. Monitoring of 

plant community response to Thalassia die-off 

initiated.

Jun 1989 Zieman completes version 1 of his conceptual

model of seagrass die-off.

Zieman initiates long-term Thalassia productivity

measurements near four core die-off areas.

Jul 1990 Thayer et al. investigate plant community response

to die-off in Johnson and Rabbit Key basins.

Halodule begins to colonize die-off patches. 1990–1991

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FWRI Technical Report TR-11 95

Earliest observation of persistent turbidity Apr 1991 Robblee et al., 1991: Marine Ecology 

caused by tidally resuspended sediments Progress Series 71: 297–299. First published

observed in Johnson Key Basin. Kuss, observations of the die-off: clear, warm,

DiDomenico, Robblee observers. hypersaline conditions.

Early conceptual models of die-off Fall 1991 Durako-Kuss: Labyrinthula effects on Thalassia

developed: Zieman, Carlson-Durako. photosynthesis.

Cyanobacteria bloom begins in Rabbit Key basin. Nov 1991

Hurricane Andrew passes over south Florida; Aug 24,

has little direct impact on Florida Bay. 1992

High rainfall occurring the week after Andrew Approx.

washes mangrove “rot”into Florida Bay. Sep 1,

Zieman observes that this event began the 1992

severe turbidity events in the bay.

Major turbidity plume from western Dec 22,

Florida Bay reaches Big Pine Key. 1992

Apr 1993 Zieman et al. look at light-reduction effect on

Thalassia in Rabbit Key basin and Sunset Cove.

Oct 1993 Hefty et al.: DERM initiates seagrass monitoring

of northeastern basins.

Jun 1994 Hall-Durako: 107 stations previously sampled

in 1983–84 are resampled for decadal comparison

of seagrass abundance.

Oct 1994 Series of faunal studies conducted in mid-1980s

are repeated as a part of the decadal comparison

program: Thayer, Matheson and Camp, Robblee.

Nov Durako-Hall: Preliminary test of EMAP-based 

1994 Braun-Blanquet sampling.

Very wet spring and summer; true estuarine Spring Durako-Hall: Fish Habitat Assessment

conditions exist throughout Florida Bay. 1995 Program (FHAP) initiated.

Landsberg-Blakesley: Labyrinthula spatial and 

temporal dynamics.

Oct Montague initiates field studies in northern 

1995 Florida Bay; starts construction of a mesocosm 

system to look at effects of salinity fluctuations.

Halophila engelmannii first observed in Johnson Oct 1996

Key basin, suggests a shade-adapted community.

Aug 1997 Stumpf examines satellite imagery to detect

changes in seagrass abundance from 1986–1997.

Rudnick: Florida Bay ecosystem conceptual model. 1997

Oct 1997 Carlson-Blakesley initiate banktop die-off 

studies.

Nov 1997 Fourqurean et al. sample two areas in Rabbit Key

Basin to examine scale-based variability in 

potential Thalassia ecoindicators.

Flowering of Thalassia and Halodule widespread. May 1998

Table 7.1 Florida Bay seagrass die-off milestones. (continued)

Event or Observation Date Significant Publication/Research Initiated

Seagrass Ecology
Florida Bay Synthesis Hunt and Nuttle

96 FWRI Technical Report TR-11

continuing concern regarding the extent of seagrass

changes within Florida Bay and the need to monitor the

effects on seagrass communities of proposed watermanagement alterations for the restoration of the Everglades-Florida Bay ecosystem, the FHAP was initiated

during spring 1995. The turbid conditions in western

Florida Bay from 1995 to 1997 made measuring and interpreting seagrass losses and changes in species distributions more complicated because of the primary

die-off versus secondary effects attributable to light limitation. Early FHAP data (1995–1997) indicated that

seagrass decline in the western basins was primarily

due to degraded water quality because mortality was

signified by a general thinning of shoots, rather than

the occurrence of distinct die-off patches surrounded

by dense beds. A comparison of seagrass distributions

in Florida Bay between 1984 and 1994 (Hall et al., 1999a)

and between 1995 and 1998 (Durako et al., 2002) also indicated that the chronically turbid regions had exhibited the most significant losses of T. testudinum.

STABILIZATION AND RECOVERY

Following the initial die-off and period of widespread

turbidity, seagrass abundance has shown three distinct

phases (Zieman et al., 1989, 1999): at stations associated

with primary seagrass die-off, standing crop declined

from 1989 to 1995; standing crop remained stable from

1995 through 1997; and standing crop has increased

during the past several years (1998–2001). Thalassia

has shown little net change in abundance (±8% of the

mean) at the bay scale from 1995 to 2001 (Figure 7.1),

although at the basin-scale, abundance has varied by

an average of ±30% (losses then gains in the west, contrasted with stability or gains in the middle and eastern basins). Most of the increase in seagrass abundance

has resulted from expanding coverage by Halodule

(Figure 7.2; + 200% baywide, + 450% in Johnson Key and

Rabbit Key Basins; Durako and Hall, 2000). Thus, the

dominance of Thalassia is declining, and mixed turtlegrass and shoalgrass beds are becoming more common.

In the past five years, relative Thalassia abundance has

dropped from being more than five times that of Halodule to being less than three times more abundant; in

spring 1997, Halodule replaced Thalassia as the most

abundant seagrass in Johnson Key Basin (Durako et al.,

2002). During fall 1996, the small-bodied, low-lightadapted seagrass Halophila engelmannii was observed

at one station in Johnson Key Basin. By spring 1998, this

species was present at 15 of the 32 stations in this

basin.

The passage of Hurricane Georges west of the bay

in the fall of 1998 uprooted much of the Halophila in

Johnson Key Basin (it was observed only at five stations

nine days after the storm); reduced the cover of Thalassia (especially in areas where it had been sparse); and

removed much of the litter layer on the bottom. However, by spring 1999, Halophila cover increased in Johnson Key Basin, and this species was observed in Rankin

Lake, Whipray Basin, and Twin Key Basin. This rapid

increase in spatial distribution suggests the hurricane

may have played a role in distributing Halophila

propagules (Durako et al., 2002). Longer internodes

and deep apical meristems on short shoots of Thalassia at Barnes Key were observed shortly after Hurricane

Hurricane Georges passes west of Florida Bay. Sep 1998

“Classic”die-off patches present on bank edge Oct 1998

east of Big Rabbit Key.

Barnes Key eastern bank and basin dense 

Thalassia, crystal-clear water.

Fall 1998 Koch: Sulfide phytotoxicity studies on Thalassia.

New “classic”die-off observed in Barnes Key Jan 1999 Hall-Blakesley monitor changes in Thalassia

area and in Sunset Cove. characteristics and Labyrinthula abundance.

Die-off patches present on bank to east of Apr 1999

Barnes Key and in basin north of key.

Flowering of Thalassia and Halodule again May 1999

observed in core samples from several stations.

Hurricane Irene passes west of Florida Bay. Oct 1999

Flowering of Thalassia observed in field, May 2000

recruiting seedlings observed in cores.

Table 7.1 Florida Bay seagrass die-off milestones. (continued)

Event or Observation Date Significant Publication/Research Initiated

Seagrass Ecology
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FWRI Technical Report TR-11 97

Georges, suggesting the hurricane resuspended sediments in Florida Bay. Hurricane Irene also reduced seagrass cover in areas of sparse Thalassia and reduced

Halophila distribution as determined by before-andafter sampling by FHAP.

Another recent dramatic change in the ecology of

seagrasses in Florida Bay is the widespread occurrence of flowering in both Thalassia and Halodule. In

spring 1999, reproductive short-shoots of Thalassia

were present at 19 sites in 7 basins across the bay; reproductive short-shoots of Halodule were present at 24

sites in 4 basins in the western part of the bay (Durako

et al., 2002). In spring 2000, flowering Thalassia was observed at 19 sites in 6 basins. The recent increases in

seagrass cover and the dramatic increase in flowering

may reflect improvements in water quality, although

shifts from turtlegrass to shoalgrass and the appearance and spread of Halophila engelmannii are often associated with declining light availability.

Since 1998, there has been an upturn in abundance,

Figure 7.2 Changes in relative abundance, as determined by Braun-Blanquet sampling, of Halodule from spring 1995 to spring 2001.

Seagrass Ecology

Figure 7.1 Changes in relative abundance, as determined by Braun-Blanquet sampling, of Thalassia from spring 1995 to spring 2001.
Florida Bay Synthesis Hunt and Nuttle

98 FWRI Technical Report TR-11

productivity, standing crop, and flowering of Thalassia

and Halodule that seems to be associated with improving water clarity (Zieman et al., 1999; Durako et al., 2002).

Some of this improvement in water clarity may be due

to a decrease in unvegetated bottom (Durako et al.,

2002). However, as recovery begins to occur at the original die-off sites, a new instance of primary seagrass dieoff that began in January 1999 has been observed north

of Barnes Key.This new primary die-off is occurring in

an area where die-off has not been previously observed,

but it has some characteristics very similar to those observed during the initial seagrass die-off in 1987.The area

has excessively dense beds of T.testudinum, with die-off

patches interspersed among the dense beds, and the

same unusual seagrass morphologies (“twinning”) have

been observed in surviving shoots (Zieman et al., 1999).

Seagrasses affected by the new die-off have symptoms

like those of the 1987 event; i.e., the short-shoot meristem tissue appeared to be the tissue most immediately

affected. Meristems seemed mushy and smelled like

“mustard,”whereas the rest of the blade looked green

and healthy (Carlson et al., 1990).

PALEOECOLOGICAL INVESTIGATIONS 

Although not part of the seagrass research program,

paleoecological investigations add relevant information

for understanding how recent seagrass changes fit

into the matrix of historical expansion and contractions

of seagrass cover. At present, it appears that seagrass

coverage, as estimated both by the abundance of seagrass-associated microfossils and by chemical signatures in the sediments, has shown repeated cycles of

presence and absence at the core sites.Thus, the recent

changes in seagrass cover are apparently not unprecedented. However, the paleoecological data come

from only a few selected locations, limiting the confidence that can be placed in their general applicability. Therefore, it would be extremely valuable to have

additional paleoecological information from more sites

throughout the bay.

Conceptual Models for Seagrass Die-Off

Early in the die-off studies, conceptual models were developed of hypothesized die-off mechanisms and the

conditions and processes leading up to the die-off.

Initially, the model developed by Zieman et al. was a

mostly process-oriented model and placed strong emphasis on the historic conditions leading up to the dieoff. The other model was developed by Carlson and

Durako and was a more mechanistic model with more

emphasis on the die-off process. These conceptual

models are shown in the accompanying figures.

The Zieman et al. (1999) model (Figure 7.3), has

three major phases: A Developmental Phase (A–C), in

which a combination of natural and anthropogenic

processes contributed to an extensively developed

(overdeveloped, actually) Thalassia ecosystem; an Initiation Phase (D–G), in which the heavily developed

system interacted with a suite of environmental stresses

to produce the initial die-off episodes; and a Maintenance Phase (G–L–G and repeat), in which the process

became self-sustaining. In this model, the interactions

of the dense Thalassia and the environmental stresses

formed the primary trigger to the initial die-off

episodes.

The Carlson and Durako model (Figure 7.4) included over-developed Thalassia as a component but

focused more on the role of physiological stress, especially hypoxia and sulfide toxicity, as major drivers.

In addition, this model gives a much larger role to the

effects of a slime mold, Labyrinthula, as a causative element. Although both of these models have matured

as research has progressed, the process has been one

of small refinements, and both have changed little in

the past decade.

As research progressed and the conceptual models matured, seagrass researchers found much common

Figure 7.3 Zieman et al. model for seagrass die-off.

Seagrass Ecology
Hunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 99

ground on which to agree. Among these points of

agreement are the following:

• Primary seagrass die-off is species-specific, restricted

to Thalassia testudinum.

• Primary die-off occurs only in very dense beds;

sparse and medium-density beds have never been

shown to have primary die-off.

• Primary die-off has occurred in Florida Bay only. It has

occurred in continentally influenced regions of the bay

with reduced circulation and reduced flushing.

• Primary die-off occurs principally during late summer–fall or fall–early winter. Both the Sunset Cove

and Barnes Key die-off events were first observed

during January–February. Most significantly, primary die-off does not occur in the hottest months of

summer.

• Researchers with experience in Florida Bay prior to

the initial primary die-off found the waters to be

clear and the seagrasses mostly free from epiphytes

during 1987–1989. Reduced water clarity and epiphytic growth followed seagrass die-off by several

years.

Factors such as high water temperature, prolonged

hypersalinity, and excessive seagrass biomass (due to

lack of recent disturbances, including hurricanes and

reduced salinities), which lead to increased respiratory

demands, hypoxia, and sulfide toxicity are some of the

physiological stressors thought to have contributed to

Thalassia die-off (Robblee et al., 1991; Carlson et al.,

1994). Observations indicated that the waters of Florida

Bay were generally clear, warm, and hypersaline in

most areas exhibiting die-off. High water-column light

attenuation from suspended sediments or eutrophic

phytoplankton growth was not present at the initiation

of the die-off or for the first several years (Boyer et al.,

1999).

Lesions were frequently observed on blades of

Thalassia in areas affected by die-off, but they were not

universally present during the earliest phases. An undescribed species of marine slime mold in the genus

Labyrinthula was the most common microorganism

isolated from short-shoots affected by die-off (Porter

and Muehlstein, 1989). Physiological stress and a negative carbon balance are associated with infection of

Thalassia by Labyrinthula (Durako and Kuss, 1994).

Based on these early observations and data, the two

conceptual models were formulated (the Zieman et al.

model and the Carlson and Durako model) incorporating distal and proximal elements thought to have

contributed to die-off.These models also included predictions of negative cascades following die-off.

Establishing the relative contribution of (primary)

die-off versus mortality induced by light stress to the

recent losses of Thalassia in western Florida Bay is

problematic. There is a high spatial coincidence between the distribution patterns of seagrass loss,

Labyrinthula abundance (Landsberg et al., 1996;

Blakesley et al., 1998), high sediment-sulfide levels

(Carlson et al., 1994), and turbidity (Phlips et al., 1995;

Stumpf et al., 1999). Increases in Halodule in the bay may

reflect its lower light requirements (Williams and

McRoy, 1982; Dunton and Tomasko, 1994), ability to

rapidly spread into areas where the Thalassia canopy

has been removed (Thayer et al., 1994), or resistance to

disease. There has been little change in Thalassia and

Halodule abundances in basins that are periodically subjected to low salinities. This may reflect the effects of

intermediate disturbance (sensu Connell’s Intermediate Disturbance Hypothesis) in maintaining a mixedspecies sub-climax (See fig. 13 in Zieman, 1982) as well

as offering a low-salinity refugium from disease—

Labyrinthula has never been found in Florida Bay at

salinities below 15 practical salinity units (psu)

(Blakesley et al., 1998).

Summary of Research

Research on seagrass dynamics has focused on the

Figure 7.4 Carlson and Durako model for seagrass die-off.

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100 FWRI Technical Report TR-11

influence of internal conditions in the ecosystem and

external driving factors on plant growth and metabolism. It has also emphasized documentation of the sequence and pattern of seagrass die-off and recovery in

Florida Bay (Table 7.2). The principal findings of this

work are as follows:

• Primary die-off is associated with high-density Thalassia beds. High-density beds result in conditions

that increase stress on Thalassia: high sedimentsulfide levels (greater than 2–4 mmol); self-shading

of shoots; nighttime/early morning anoxia/hypoxia

in meristems; and increased susceptibility to leafto-leaf transmission of Labyrinthula. Recent die-off

in the Barnes Key area occurred where Thalassia

densities and standing crop are among the highest

in Florida Bay.

• Geographic variation in patterns of seagrass loss

suggests multiple causes and a distinction between

primary die-off and secondary mortality.There is a

high spatial coincidence between the distribution of

Thalassia loss, Labyrinthula abundance, high sediment-sulfide levels, and turbidity, but determining

the relative contributions of these factors to die-off

versus secondary mortality has been difficult.

• Recent bay-scale changes in seagrass distribution and

abundance following the region-scale primary seagrass die-off (1987–1991) are largely due to secondary

mortality of Thalassia associated with areas of chronic

turbidity (since 1994) in western Florida Bay. These

changes are most likely due to light limitation as

indicated by stand-thinning in Thalassia beds with

concommitant increases in low-light-adapted species

such as Halodule wrightii and Halophila engelmanii.

There has also been a loss of Thalassia in northern

Little Madeira Bay, near the mouth of the Taylor

River, followed by colonization by Halodule and Ruppia.The cause of this loss is unknown. Primary acute

die-off (rapidly developing dead patches within

dense beds) is known to be currently occurring (initiated in 1999) only in the high-density Thalassia

beds of the Barnes Key area. A chronic die-off is occurring in dense beds in Sunset Cove. However, the

etiologies of these two sites are different.

• The boom-and-bust pattern of Halodule and Ruppia

characteristic of the Joe Bay and Highway Creek

areas of northeastern Florida Bay prior to 1991 has

Table 7.2 Research topics defined by Question 4 (cells in the matrix) and key references to the associated research.

Plant Growth and Seagrass Die-off and 

Research Topic Metabolism Recovery

General Characteristics Robblee et al., 1991; Fourqurean et al., 1992;

Thayer et al., 1994; Hall et al., 1999a,b;

Stumpf et al., 1999; Zieman et al., 1999;

Durako and Hall, 2000; Bacon et al., 2001;

Durako et al., 2002

Temperature and Salinity Chesnes et al., 1999 Blakesley et al., 1998; Boyer et al., 1999;

Fourqurean et al., 2003

Light Attenuation Williams and McRoy, 1982; Prager, 1998; Stumpf et al., 1999;

Dunton and Tomasko, 1994; Fourqurean et al., 2003

Thayer et al.,1994; Frankovich 

and Zieman, 2001; Holmquist

et al., 1989

Nutrients Fourqurean et al., 1992, 2003 Butler et al., 1995; Fourqurean et al., 2003

Anoxia and Sulfide Erskine and Koch, 2000 Barber and Carlson, 1993; 

Carlson et al., 1994; Erskine and Koch, 2000

Disease Durako and Kuss, 1994; Porter and Muehlstein, 1989;

Durako and Kunzelman, 2002 Muehlstein, 1992;

Blakesley et al., 1998, 1999a,b

Dense Grass Beds Durako, 1995; Hall et al., 1999a Durako, 1994, 1995; Durako et al., 2001

Sediment Characteristics Carlson et al., 1994

Climate

Freshwater Inflow Fourqurean et al., 2003

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FWRI Technical Report TR-11 101

stabilized with the occurrence of relatively constant

estuarine/marine salinities and the lack of periodic

intense hypersaline conditions.

• There has been little net change in seagrass distribution and abundance since 1995 in central basins

that are periodically subjected to low salinities or

have shallow sediments and lower overall densities

of Thalassia.These conditions may reduce density-related stresses and allow robust species communities to develop; low salinity may also provide refugia

from disease because Labyrinthula has never been observed in Florida Bay in salinities below 15 psu.

• The variable abundances of seagrass-associated microfossils in cores suggest repeated cycles of seagrass

presence and absence at core locations over the past

200 years.Thus, the recent changes in seagrass abundance may not be unprecedented.

• A statistical, discriminant-function model of seagrass species occurrence has been developed. The

model predicts that an increase in freshwater delivery

to Florida Bay will result in an expansion of Ruppia

maritima and Halodule wrightii distribution into the

northeastern bay with a concomitant loss of Thalassia in this region.

WHY DID SEAGRASS DIE–OFF OCCUR ONLY

IN FLORIDA BAY? 

Unique characteristics of the Florida Bay ecosystem

make it susceptible to conditions that contribute to

primary die-off. Other marine areas with high Thalassia densities and carbonate sediments, such as the

Lakes Region of the lower Keys, did not experience extensive die-off in the late 1980s. Relative to these other

areas, the central basins in Florida Bay are isolated

from tidal influences and are subject to relatively high

continental influence.

Effect of Ecosystem Characteristics

on Growth and Survival

“Physiological stressors such as elevated water temperature, prolonged hypersalinity, excessive seagrass

biomass leading to increased respiratory demands,

hypoxia and sulfide toxicity, and disease are some of the

factors thought to have contributed to Thalassia die-off.

However, the causative mechanisms responsible for initiating the die-off remain incompletely understood (Robblee et al., 1991; Carlson et al., 1994; Durako, 1994; Durako

and Kuss, 1994; Durako et al., 2002).”

—Durako et al., 2002

Changes in plant morphology and metabolism

measured in the field provide indicators of response

of seagrasses to environmental stressors in advance of

die-off. Zieman et al. (1999) reported results of longterm monitoring at sites of pre-existing die-off and at

sites not yet affected by die-off:

“Measurements were made of short-shoot density, standing crop, leaf morphology and productivity of Thalassia

testudinum. Seagrass beds in Florida Bay have continued

to decline in density and biomass since the die-off event

began in 1987. In contrast to the loss of seagrass density

and standing crop, we found that mass-specific productivity increased markedly from 1989 to 1995 at the dieoff sites. The increases in mass-specific productivity of

seagrasses in Florida Bay occurred at the same time

that transmission of light to the bottom was decreasing

dramatically. Despite increasing productivity on a massspecific basis over the period 1989–1995, area productivity

of Thalassia testudinum decreased at three of the five

die-off sites.This can be explained by a change in shoot

density and size.”

Seagrasses in Florida Bay are meristem-dependent and depend on rhizome growth and branching to

maintain or increase their populations. Excess carbon

from photosynthesis is needed for production and

growth of apical meristems to allow lateral growth

and spread of populations. Core samples (15-cm diameter) obtained during the springs of 1998 and 1999

FHAP sampling indicated that the mean branching

frequency was 0.24 during this period, or one rhizome

apical branch produced for every four short-shoots

(Paxson and Durako, 2001). This is very similar to the

branch frequency of 0.28 observed during the early dieoff from 1989 to 1990 (Durako, 1994). The relationship

between rhizome branching in 1998 and shoot density

changes from 1998 to 1999 was significant (P < 0.0001);

however, the r 2 value of 0.12 was low.

Carlson et al. (1994) have reported that alcohol dehydrogenase (ADH) increases in Thalassia exposed to

hypoxic conditions, exacerbating the carbon drain on

below-ground tissues. Apical meristem density and

branch frequency (rhizome apical density normalized

to short-shoot density) may act as “ecoindicators”of the

potential for increase or decrease of seagrass density.

Temperature

The large-scale distribution of marine plants is largely

controlled by temperature.The effects of temperature

on seagrasses have been extensively investigated and

are well known. The range of thermal tolerance in

tropical species is about half that of temperate species;

upper thermal limits are similar for both groups, but

tropical species are less tolerant of low temperatures

(Zieman et al., 1989). Because tropical species are growing near the upper temperature limits, thermal effects

of discharges from electrical power plants can result

in mortality during the warm summer months, although the productivity of these species may have

been enhanced during the winter (Barber and Behrens,

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102 FWRI Technical Report TR-11

1985). Thalassia may show defoliation or reductions in

leaf length when temperatures fall below 15°C or exceed 30°C (Phillips, 1960; Zimmerman and Livingstone, 1976; Durako and Moffler, 1985). Temperatures

below or above these extremes have been observed on

bank tops in Florida Bay. Syringodium seems to be more

tolerant of low temperatures than Thalassia (no defoliation at temperatures just below 15°C); Syringodium

has maximal growth between 23°C and 29°C, whereas

for Thalassia maximal growth occurs between 23°C

and 31°C (Zimmerman and Livingstone, 1976). In shallow bays with widely fluctuating temperatures, Thalassia, Syringodium, and Halodule have narrower leaf

widths, suggesting increased stress (McMillan, 1978).

Ruppia has a wide tolerance to temperatures (7°–35°C)

and has highest growth rates during the cooler spring

months (Phillips, 1960; Pulich, 1985). Water temperatures are not expected to be significantly affected by

altered water management.

Salinity

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 a low

biomass of submerged plants (Montague and Ley,

1993). Mesocosm studies (Chesnes et al., 1999) have produced data on how fluctuating salinities may affect

turtlegrass, shoalgrass, and widgeon grass. Unfortunately, operational difficulties plagued the mesocosm

facility, which slowed progress and resulted in problems with maintaining proper controls.

The role of hypersalinity in seagrass die-off still remains unclear. Salinities during the initial die-off

episodes in 1987–1989 were accompanied by hypersalinities ranging from 45 psu to 70 psu.The Barnes Key

die-off clearly did not correlate with this level of salinity but showed many of the other stresses that accompanied the primary die-off. Although the role of

hypersalinity may vary relative to other stresses, low

salinity provides refugia from Labyrinthula, because infection by that organism does not occur at <15 psu

(Blakesley et al., 1999b).

Light

Light penetration to the leaf blades, which sets the

limits for seagrass photosynthesis, varies as a function

of depth, turbidity, and epiphyte cover.

TURBIDITY

To assess the relationship between water clarity and

seagrass recovery in Florida Bay, Carlson et al. (2001)

have continuously measured subsurface and bottom

photosynthetically active radiation (PAR) at seven stations in Florida Bay since the fall of 1998. In addition

to continuous light data, discrete water samples for

analysis of turbidity, color, chlorophyll, and total suspended solids and plant samples for epiphyte light

attenuation measurements have been collected

monthly. Diffuse attenuation coefficients vary seasonally and between basins within Florida Bay: attenuation is higher in winter than in summer and is

generally higher in the basins that lost large amounts

of seagrass in die-off episodes between 1987 and 1991.

Persistent phytoplankton blooms in the north-central

region of the bay are associated with high (greater

than 3) Kd values, but water clarity at most sites was

higher in 2000 than in 1999.

EPIPHYTES

There are spatial and temporal gradients in species

composition of epiphytes, with coralline red algae

being more dominant in western high-flow areas of the

bay (Frankovich and Fourqurean, 1997). Fleshy epiphytes have restricted distributions, occurring only

near bird islands and near the Keys. The cyanobacterium Lyngbya is very common in the Syringodium filiforme beds in the west (Frankovich and Zieman, 2001).

Epiphyte attenuation is higher in winter and spring

(30%–50%) than in summer and fall (15%–30%), and

values are higher in the eastern region of the bay

(greater than 40%) than in the western region (ca.

20%). Calcium carbonate derived from calcareous algae

and resuspended sediment composes more than half

of the epiphyte load.

Epiphytic light attenuation has also been measured using Mylar strips and a light-attenuation measurement apparatus that has been successfully

employed by investigators in Australia and Chesapeake Bay (Frankovich and Zieman, 2001). The Mylar

strips have been set out within various seagrass meadows across the bay for a sufficient time period to allow

for the accumulation of epiphytic organisms. Distinct

epiphytic communities consisting of benthic diatoms,

coralline red algae (Melobesia membranacea, Hydrolithon

farinosum), and filamentous red and brown algae occur

seasonally in various regions across Florida Bay.These

various epiphyte functional forms, and combinations

thereof, result in differing levels of light attenuation at

the leaf surface relative to the amount of epiphyte

loading. Epiphyte loads at Barnes Key are at lower

than 5% of bay-wide averages (Frankovich and Zieman,

2001). Highest epiphyte loads are in areas of highest

seagrass productivity, so epiphytes are not reducing

production (Frankovich and Zieman, 2001).

Grazers may have an important role in controlling

epiphyte abundance. Snails (50/m2

) and hermit crabs are

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FWRI Technical Report TR-11 103

conspicuous at Rabbit Key Basin (Frankovich and

Fourqurean, 1997). In Florida Bay,Bittium occurs at densities of about 3/m2

,whereas in the Indian River Lagoon,

Bittium occurs at densities of 20/Halodule short-shoot or

at 1000s/m2 .

Macroalgae such as Laurencia are patchy in abundance. It is unknown at present whether its abundance has changed in response to the changes in

seagrass abundance (although these data are in the

FHAP dataset). Jeff Holmquist found that Laurencia

accumulation did not kill underlying Thalassia.

Nutrients

Worldwide, many estuaries have undergone loss of

seagrasses as a result of eutrophication.The mechanism

for nutrient-enrichment-induced seagrass losses is

that increased nutrients lead to increased epiphyte

loads and consequent shading and deterioration of

the seagrasses (Cambridge et al., 1986). Phytoplankton

blooms usually follow rather than cause seagrass dieoffs. As stated above, epiphyte loads on seagrasses are

relatively low in Florida Bay and do not seem to be reducing production.

Sulfide

Carlson et al. (1994) and Erskine and Koch (2000) have

explored the potential links between elevated sediment-sulfide concentrations and seagrass mortality

(a hypothesis long proposed to account for seagrass dieoff). Hydrogen sulfide is a known plant toxin. High sulfide levels in sediments have been observed during and

after die-off episodes (Carlson et al., 1994). Sulfide concentrations greater than 2–4 mmol have been measured in die-off sites, but it is not known if this is a cause

or an effect of die-off. If sulfide concentrations are less

than 2 mmol, no die-off has been observed. However,

photosynthetic rates in Thalassia increase as a function

of increasing sulfide up to 6 mmol, and sulfide levels

up to 10 mmol have failed to produce visual signs of

acute sulfide toxicity, although high sulfide levels have

been shown to result in reduced leaf-elongation rates

(Erskine and Koch, 2000).

A series of field experiments has been undertaken

to help us understand how sulfide concentrations may

influence seagrass growth and abundance and to clear

up uncertainties between the two conflicting sets of results. A submersible pulse-amplitude-modulated fluorometer (Diving-PAM) was used to investigate

photosynthetic yields of Thalassia testudinum leaf material in situ within bucket experiments established by

Carlson. Photosynthetic yields were significantly lower

in buckets with added glucose and acetate (Figure 7.5),

although both treatments should have increased sediment sulfide levels.

Labyrinthula Infection

A theoretical model for die-off as a disease (Blakesley

et al., 1999a) has suggested three different roles that

Labyrinthula might play in Florida Bay under different

environmental conditions. These roles include (1) a

nonpathogenic parasite, (2) an opportunistic secondary

pathogen, and (3) a primary pathogen. Five different

factors were discussed as critical elements in determining the role(s) of Labyrinthula in seagrass health at

a particular site in Florida Bay (Blakesley et al., 1999b).

Salinity controls infection (infection does not occur

at less than 15 psu). Seagrass density determines the

extent to which Labyrinthula infection spreads because

the slime mold transmission is thought to depend on

blade-to-blade contact (Muehlstein, 1992). Pathogenicity of a particular strain of Labyrinthula will determine severity of infection (Muehlstein et al., 1988).

Environmental stressors (abiotic factors), such as low

light or high temperatures, may weaken Thalassia and,

in combination with the infection by pathogenic

Labyrinthula, may cause seagrass die-off. Resistance to

disease due to genetic factors or production of phenolic

compounds may be important in determining the

health of Thalassia in Florida Bay.

Blakesley et al.’s (1999a) model predicted that in

areas with high seagrass density, high salinity, “suboptimal seagrass conditions (environmental stress),”

and presence of pathogenic Labyrinthula, the slime

mold could contribute to either chronic or acute dieoff by acting as an opportunistic secondary pathogen.

Figure 7.5 Photosynthetic quantum yields of Thalassia in Carlson buckets exposed to high glucose (HIG), low glucose (LOG),

molybdate (MO4), acetate (ACE), shaded (SHD), and shaded

with glucose (SHG) relative to controls inside (CON) and outside

(OC) plots.

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104 FWRI Technical Report TR-11

Under the same conditions but without environmental stress, Blakesley et al. (1999b) suggested that

Labyrinthula, acting as a primary pathogen, could still

cause “thinning”or patchy die-off. Labyrinthula occurrence was found to have significant adverse influences

on branching frequency (P = 0.03), shoot density (P =

0.004), and apical density (P = 0.033) (Paxson and Durako, 2001), suggesting that this microorganism may

have an adverse influence on seagrass growth and

carbon balance (sensu Durako and Kuss, 1994).

The occurrence of Labyrinthula has also been shown

to affect photosynthetic characteristics of Thalassia

(Durako and Kuss, 1994). Durako and Kuss (1994)

demonstrated that Labyrinthula infection reduces oxygen production and may increase susceptibility to sulfide.These results indicate that reduced photosynthetic

capacity may be caused by Labyrinthula-induced lesions

or elevated sulfide.This may make Thalassia more susceptible to sulfide toxicity, hypoxia, or disease by imposing a negative carbon drain on belowground tissues.

Photosynthetic yields of Thalassia testudinum leaf

material measured in situ at Sunset Cove and Cross

Bank by using a Diving-PAM were significantly lower

in regions of Thalassia leaves that had visible lesions.

These patterns agree with those previously reported

by Durako and Kuss (1994). However, close-interval

Diving-PAM-fluorescence measurements along an individual leaf with several visible lesions indicated the

reductions in photosynthesis were restricted to the immediate area of the lesion.Quantum yields of lesion-free

leaf regions of short-shoots also declined along transects

from dense, apparently healthy beds to recent die-off

patches at both Barnes Key and Cross Bank. The photosynthetic characteristics of solitary short-shoots within

the die-off patches were significantly lower than those

of shoots along the ecotones and shoots only 1 m inside

the bed. This indicates that photosynthesis may be

reduced even in the absence of visible lesions.The Diving-PAM data also revealed that the severity of stress

imposed by the leaf lesions will be a function of the proportion of total leaf surface that is necrotic,and it remains

to be determined what the lethal threshold is for lesion

coverage.

Sediment Characteristics

Carlson (unpublished data) has observed that highporosity sediments correlate with high hydrogen sulfide levels because of the sediments’low permeability.

Barnes Key surface sediments have very high porosity (lots of water and fine sediments) and high levels

of hydrogen sulfide.

Seagrass die-off has been observed only in biogenic

carbonate sediments, which are usually low in iron.

However, Florida Bay carbonate sediments have relatively high iron levels (Carlson et al., 1994). Spatially,

iron levels are high near the mainland and toward the

west. The role of atmospheric deposition of iron to

Florida Bay remains poorly understood. Addition of

iron to sediments decreases the flux of sulfide to Thalassia (Carlson bucket experiments) and results in a

small but significant increase in growth. Iron distribution may also have a role in controlling phytoplankton blooms in the bay.

Effect of Ecosystem Characteristics

on Community Structure and 

Distribution

“The present distribution and species composition of seagrasses in south Florida are 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. 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 (e.g., Hall et al., 1999; Durako et al., 2001 [2002]). For this reason, accurate data on

the species composition of the seagrass communities

must be collected periodically as a measure of the status of the coastal environment.”

—Fourqurean et al., 2002

Correlation with Patterns of Water Quality

Fourqurean et al. (2003) developed a discriminant function model that uses seagrass species occurrence and

abundance data from 677 locations and water-quality

data from 28 monitoring stations in Florida Bay. This

model predicted that the distribution of benthic habitat types in Florida Bay would likely change if water

quality was affected by changes in water delivery to the

bay. Specifically, an increase in seasonal delivery of

fresh water should cause an expansion of the distribution and abundance of Ruppia maritima and Halodule

wrightii at the expense of Thalassia testudinum along the

northeast region of the bay.

Geographic Variation in Seagrass Die-Off

The production of distribution and abundance maps

of the seagrasses has proven valuable, and these maps

provide a quantitative record of distribution and abundance at both basin and bay scales. The changes between maps provide clear visualization of where and

how much the distribution and abundance changes and

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FWRI Technical Report TR-11 105

can thus point to “hot spots”that may be related to specific forcing events or conditions. The use of seagrass

cover has become an assessment standard in the region (FKNMS, DERM, FHAP). Cover may be a more

sensitive performance measure to short-term changes

than density because density changes require mortality

and recruitment of short-shoots, whereas cover can also

vary in response to changes in the leafiness of shortshoots. Short-shoot density may be a better longer-term

performance measure and is a quantitative measurement. Density is the net effect of mortality, recruitment, and life span (demographics).

The recurrence of an acute die-off in Barnes Key

presented an opportunity to test a portion of the theoretical model by comparing the symptoms and progression of an acute event in the Barnes Key mud bank

area with the symptoms and progression of what we

believed to be chronic die-off in Sunset Cove.The hypothesis was that in Sunset Cove, Labyrinthula acted as

a primary pathogen in an environmentally unstressed

site, whereas in Barnes Key, Labyrinthula more likely

played the role of a secondary pathogen in an environmentally stressed site.

Comparisons of results for the Sunset Cove and

Barnes Key sites revealed that although active die-off

was occurring in both places, the sites were very different. At both sites the pattern of die-off was patchy,

suggesting disease processes rather than a physical

process as the primary cause. Both sites had high salinities (greater than 15 psu) and dense Thalassia beds—

necessary elements for Labyrinthula infection and

transmission. However, the data from the two sites revealed important differences that are summarized in

Table 7.3.

These differences strongly suggest that the mechanisms for the die-offs in Barnes Key and Sunset Cove

are not the same. Blakesley et al. (1999b) proposed that

the acute die-off in Barnes Key resulted from a series

of events beginning with heat stress and an initial infection or disease (not Labyrinthula-induced) that

rapidly kills the infected seagrass.The resultant large

amount of decaying belowground biomass from the

rapidly dying Thalassia roots and rhizomes promotes

microbial activity that in turn elevates the sedimentsulfide levels selectively in those vegetative zones

where the die-off is occurring or has recently occurred.

The high sediment-sulfide levels do not kill seagrass

outright but instead further stress the other seagrass

in the immediate area. Finally, Labyrinthula, acting as

an opportunistic secondary pathogen, infects the already weakened remaining seagrass.

In contrast, the chronic die-off in Sunset Cove appears to be directly caused by Labyrinthula acting as a

primary pathogen. Sediment-sulfide levels may remain relatively low in all the vegetative zones tested

simply because the slow death of Thalassia roots and

rhizomes results in a smaller decaying belowground

biomass. As a result, the sediment-sulfide levels may

remain lower in the vegetative zones than in contrast

to the Barnes Key. Such chronic seagrass die-off is still

ongoing in many parts of Florida Bay, where the Thalassia beds are dense enough for transmission of the

disease and the salinity is high enough for infection to

occur.

Recent high-resolution in situ oxygen measurements show that the short-shoot meristems of Thalassia at the Barnes Key die-off site became anoxic during

the night in November and remained anoxic for up to

five hours (Borum et al., 2001). In contrast, leaf meristems at Rabbit Key Basin did not become hypoxic.The

Barnes Key samples showed a more rapid decline in

oxygen concentration after sunset and slower internal

oxygen concentration increases in the morning than did

samples from Rabbit Key Basin. Thalassia at Barnes

Key is very dense, with over 1,200–1,500 shortshoots/m2

, a very high biomass (SC 300 g/m2), low

turnover rate of grass blades (1.2%/day), and very thick

litter layer. In Rabbit Key Basin, densities are also high

(1,200–1,300 short-shoots/m2), but biomass (SC 109

g/m2

) and turnover rates (1.7%/day) are more moderate (Zieman et al., 1999).The long short-shoot stems at

Barnes Key lead to increases in diffusion distance for

oxygen. Long internodes may be a response to hypoxia or rapid sedimentation.

Table 7.3 Characteristics of seagrass die-off at two locations in Florida Bay.

Barnes Key Sunset Cove

Seagrass loss is rapid. Seagrass loss is slower.

Meristem “rots.” Meristem “healthy.”

Lesions occur after die-off. Lesions occur before die-off.

Water temps are high in summer. Water temps are “normal.”

“High”sediment-sulfide levels (up to “Low”sediment-sulfide levels

7,000 μmol). Levels vary in different zones, but (= 1,600 μmol) in bare, sparse, and 

highest values are found in active die-off zones. dense seagrass zones.

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106 FWRI Technical Report TR-11

Seagrass loss has been recently observed in the

northern portion of Little Madeira Bay from the mouth

of the Taylor River to the south and west (Bacon et al.,

2001), and this area has been recolonized by Halodule

and Ruppia.This new seagrass die-off led to a plan for

a series of observations and experiments to test several

of the hypotheses that might explain seagrass die-off.

Modeling Change in 

Benthic Communities

Statistical modeling was commissioned by the Central

and Southern Florida Comprehensive Review Study

(Restudy) of the U.S. Corps of Engineers consistent with

recommendations from the 1998 Seagrass Modeling

Workshop (Fourqurean et al., 2003). The goal of this

work is to seek relationships between water-quality

variables and seagrass species composition and abundance, which if sufficiently strong, can be used to predict the effects of various alterations in Florida Bay

salinity regimes.The statistical models developed in this

project will be used in conjunction with output from

other models to predict the effects of Restudy scenarios on the benthic habitats of Florida Bay. This statistical model will not address mechanisms or degree of

change in water quality that result from Restudy scenarios; other models (such as the NSM, FATHOM,

and the Florida Bay salinity-transfer-function models

currently employed by the Restudy) must simulate

water-quality changes across Florida Bay that will provide the input to the new models developed in this project. As a consequence, it is anticipated that the benthichabitat-change predictions of the statistical models

will be the most reliable in the regions most closely coupled with water-management practices (i.e., the enclosed, mangrove-lined estuaries on the fringe of

Florida Bay). As the fidelity of the physical water-quality models to the behavior of the system declines, the

reliability of the benthic changes predicted by our

model will also decline. However, because the statistical relationships will be based on data from a larger

number of marine areas and upper estuaries, the basic

relationships between actual water quality (not modeled) and benthic habitats will be robust.

In addition, other seagrass-modeling efforts have

recently been initiated. These include the development of both seagrass unit models for Thalassia and

Halodule and a landscape model that will take output

from the unit models. Smith and colleagues are developing a hierarchical approach to modeling the interaction between plant and physical processes in

Florida Bay that involves two distinct spatial scales—

the demographic unit (ca. 10 m2

) and the landscape unit

(ca. km2). A preliminary landscape model of Thalassia

that explicitly relates patterns of photosynthesis, respiration, and carbon allocation to environmental conditions that include salinity, temperature, PAR, and

nutrient availability has been developed.

Physical processes such as sedimentation, decomposition, and nutrient cycling will be modeled on

a spatial scale of the landscape unit. This approach

provides a hierarchical framework in which the demographic units used to simulate plant processes exist

in the context of the landscape units, which will define

the underlying physical environment.The plant characteristics that are relevant to the feedbacks with the

physical environment (such as primary productivity

and inputs of dead organic matter) are described statistically for the demographic units and used to define

the biological environment for each landscape unit. In

this manner, the heirarchical framework functions as

a dynamic, interactive GIS in which each parameter and

process is described and simulated at the appropriate

temporal and spatial scales.

Current Research

Field Assessment

• FHAP is continuing spring and fall sampling and has

incorporated (since spring 2001) measurements of

photosynthetic characteristics of Thalassia using Diving-PAM-fluorometry at each station in addition to the

Braun-Blanquet and standing-crop measurements.

• FWRI (Carlson) is continuously monitoring benthic

light availability at fixed stations in several basins.

• Zieman is maintaining the long-term productivity

measurements.

Experimental

• SFWMD is funding a new study entitled “High salinity, nutrient and multiple stressor effects on seagrass communities of NE Florida Bay” being

conducted by Koch and Durako. The results of this

study are intended to help determine the parameters for the unit model with data from Florida Bay

seagrasses.

• FWRI (Carlson) bucket experiments have been conducted to examine effects of multiple stressors (sulfide, nitrogen, and light) on seagrasses in situ.

Literature Cited

BACON, J. J., L. N. HEFTY, S. K. KEMP, F. SHAW, K. LIDDELL, and C. AVILA. 2001. Seagrass distribution and

cover abundance in northeast Florida Bay. Pp. 145–147

in 2001 Florida Bay Science Conference. (Abstract)

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BARBER, B. J., and P. J. BEHRENS. 1985. Effects of elevated temperature on seasonal in situ leaf productivity of Thalassia testudinum Banks ex König and

Syringodium filiforme, Kützing. Aquatic Botany 22: 61–69.

BARBER,T. R., and P. R. CARLSON. 1993. Effects of seagrass die-off on benthic fluxes and porewater concentrations of CO2, H2S and CH4 in Florida Bay

sediments. Pp. 530–500 in Biogeochemistry of Global

Change: Radiatively Active Trace Gases (R. S. Oremland, ed.). Chapman and Hall, New York.

BLAKESLEY, B. A., M. O. HALL, and M. W. WHITE.

1999a. Spatial patterns of Thalassia decline and incidence of Labyrinthula infection in Florida Bay, USA. P.

26 inThe American Society of Limnology and Oceanography Conference,“Navigating into the Next Century”

Abstracts, 1–5 February, Santa Fe, New Mexico.

BLAKESLEY, B. A., J. H. LANDSBERG, B. B. ACKERMAN, R. O. REESE, J. R. STYER, C. O. OBORDO, and

S. E. LUCAS–BLACK. 1998. Slime mold, salinity, and statistics: implications from laboratory experimentation

with turtlegrass facilitate interpretation of field results

from Florida Bay. Pp. 164–166 in 1998 Florida Bay Science Conference. (Abstract)

BLAKESLEY, B. A., J. H. LANDSBERG, M. O. HALL, S.

E. LUKAS, B. B. ACKERMAN, M. W. WHITE, J.

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