92 FWRI Technical Report TR-11 

## 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 nu trient 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 domi nant in the hydroscape as they are in south Florida**  **(Fourqurean et al.,**  **2002). Seagrasses are the dominant biological community in Florida Bay, historically cov** **ering more than 90% of the 180,000 ha of subtidal mud banks 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,**

**c hemical, geological, and biological processes in this sys** **tem (Zieman, 1982). Seagrass communities are also im portant to the economy of south Florida because they** **provide food and shelter to numerous fish and inver tebrate species, many of which are commercially im**  **portant 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**  **Ke ys reef tract and the Atlantic Ocean to the south**  **(Schomer and Drew, 1982).The seagrass communities of this region form an important buffer by intercept** **ing the flow of water and reducing nutrient and par ticulate 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 man agers will need to consider actions that might aid in the** **reestablishment of continuous seagrass cover in west ern Florida Bay.This would be an important step in re**  **ducing sediment-resuspension-induced turbidity along this boundary that could reverse the cascading de**  **clines that characterize the present system. Seagrasses act as integrators of**  **net c**  **hanges in**

**w ater-quality variables, which tend to show rapid and**  **wide fluctuations when measured directly.The shallow distribution of seagrasses places them in close prox**  **imity to the land–sea interface, a region experiencing a rapid growth of human populations.This coastal dis**  **tribution also places seagrass communities at the end of the watershed pipe, so the health of these commu**  **nities 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 dis**  **turbances that are associated with changes in water quality along the land–sea interface.To a large extent,** **seagrass abundance determines public perception re garding the “health”of the coastal waters of Florida (Go**  **erte, 1994; Boesch et al.,**  **1993, 1995). Thus, the recent**

**c hanges in the distribution and abundance of sea**  **grasses within Florida Bay have been perceived as a**

**c hange in the health of the bay. For these reasons, sea**  **grasses may be the best indicators of changes in the Florida Bay ecosystem (Fourqurean**  **et al., 1992).** **Although the initial die-off originated in the inte rior basins of the bay (Robblee**  **et al., 1991), the great**  **est 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** 

**w estern 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 north**  **eastern 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

T eam Leaders Michael Durako (University of North Carolina at Wilmington), Joseph Zieman (University of Virginia), Michael Robblee (USGS, PMC)Hunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 93 **community in Florida Bay. Of the more than 14,000 Braun-Blanquet samples (each 0.25 m** **2 ) 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, T** 

**halassia w** **as present at 75.9% and Halodule**  **w as pre**  **sent 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 Ke** **ys National Marine Sanctuary (FKNMS), is domi nated by seagrass habitats. Fourqurean**  **et al., (2002) as**  **sessed seagrass species composition and density at 1,207 sites distributed across 19,402 km**  **2 of nearshore**  **marine and estuarine environments in south Florida. At these sites, a total of 8,434 quadrats (each 0.25 m** **2 total**  **area = 2,108.5 m 2**  **) 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. T**  **halassia testudinum**  **(turtlegrass) was the most commonly en countered 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 mar**  **itima (widgeongrass, 41 sites) and**  **Halophila engelman nii**  **(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://peo**  **ple.uncw.edu/durakom/seagrass/seagrass.htm). This**

**ev ent was first observed by backcountry fishing guides** **who reported the occurrence of “potholes”in the sea grass beds in the bights along the north-central part**  **of Florida Bay. Extensive areas of T**  **halassia began dying**  **rapidly in central and western basins, and by 1990, 4,000 ha were completely lost and 24,000 ha were af**  **fected by the die-off (Robblee et al.,**  **1991). T** **he patterns of changes in seagrass abundance in Florida Bay have recently undergone four phases: 1) pri**  **mary die-off; 2) secondary mortality, with mortality primarily due to light attenuation; 3) seagrass recov** **ery associated with improving water clarity; and 4) re newed 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 oceani cally flushed of all of the extensive south Florida sea**  **grass beds, and it occurred only in the most densely developed beds within the bay. To this day, no occur** **rence of primary seagrass die-off has been found out side of the densest seagrass beds within Florida Bay,**  **and no primary seagrass die-off has occurred in sparse or medium-density beds within the bay.** 

PRIMARY SEAGRASS DIE–OFF **T he 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 T**  **halassia w** **ere high but then de clined 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 die off (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 sus**  **pended 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 T**  **halassia 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** 
**cy anobacteria-dominated microalgal blooms and re**  **suspended 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 tur**  **bidity. Sponge mortality, changes in juvenile lobster population dynamics (Butler**  **et al., 1995), and indica** **tions of cascading effects on plant and animal com munities in adjacent systems (**  **e.g., sea urchin**  **population explosions and unbalanced growth of Sy**  **ringodium filiforme in the waters of the Florida Keys Na**  **tional Marine Sanctuary southwest of Florida Bay; K** **enworthy et al.,**  **1998; Rose et al.,**  **1999) were also ob served. From 1992 to 1995, salinities showed a pro**  **gressive 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 photog raphy became useless in determining seagrass distri**  **bution over much of Florida Bay. Because of the** Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

94 FWRI Technical Report TR-11 T able 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, Ke** **y Basin, Johnson Key Basin, or Rankin Lake. 1987**
**“Potholes”seen in T**  **halassia 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 in** **itiates studies on morphometric changes, Braun-Blanquet abundance, and demography.**  **Carlson begins studies on hypoxia/sulfide-ADH, ethylene, H**  **2 S, conducts bucket experiments.**  **May 1989 Robblee initiates studies of seagrass associated animal responses to die-off in Rankin Lake,**  **J ohnson and Rabbit Key basins. Monitoring of**  **plant community response to T**  **halassia die-off**  **initiated. J** **un 1989 Zieman completes version 1 of his conceptual model of seagrass die-off.**  **Zieman initiates long-term T**  **halassia 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**  Seagrass EcologyHunt and Nuttle Florida Bay Synthesis

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 T**  **halassia 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.** 

**w ashes 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**
**A pr 1993 Zieman**  **et al. look at light-reduction effect on**  **T**

**halassia in Rabbit Key basin and Sunset Cove.**  **Oct 1993 Hefty et al.:**  **DERM initiates seagrass monitoring of northeastern basins.**  **J un 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.**
**V**

**ery 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**  **Ke y basin, suggests a shade-adapted community.** 

**A ug 1997 Stumpf examines satellite imagery to detect** 

**c hanges 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**  **T halassia**  **ecoindicators. Flowering of**  **T halassia**  **and Halodule**  **widespread. May 1998** T able 7.1 **Florida Bay seagrass die-off milestones. (continued)**

Event or Observation Date Significant Publication/Research Initiated Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

96 FWRI Technical Report TR-11 **continuing concern regarding the extent of seagrass**

**c hanges within Florida Bay and the need to monitor the** **effects on seagrass communities of proposed water management alterations for the restoration of the Ever**  **glades-Florida Bay ecosystem, the FHAP was initiated during spring 1995. The turbid conditions in western** **Florida Bay from 1995 to 1997 made measuring and in terpreting seagrass losses and changes in species dis**  **tributions more complicated because of the primary die-off versus secondary effects attributable to light lim**  **itation. 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 in dicated that the chronically turbid regions had exhib**  **ited the most significant losses of T.**  **testudinum.**

STABILIZATION AND RECOVERY **F ollowing 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).**  **T halassia**  **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, con** **trasted with stability or gains in the middle and east ern 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 T**  **halassia is declining, and mixed turtle**  **grass and shoalgrass beds are becoming more common. In the past five years, relative**  **T halassia**  **abundance has dropped from being more than five times that of**  **Halo dule**  **to being less than three times more abundant; in spring 1997,**  **Halodule replaced**  **T halassia**  **as the most abundant seagrass in Johnson Key Basin (Durako**  **et al., 2002). During fall 1996, the small-bodied, low-light**  **adapted seagrass Halophila engelmannii**  **w as observed**  **at one station in Johnson Key Basin. By spring 1998, this species was present at 15 of the 32 stations in this**  **basin. T** **he passage of Hurricane Georges west of the bay in the fall of 1998 uprooted much of the**  **Halophila in**  **J ohnson Key Basin (it was observed only at five stations**  **nine days after the storm); reduced the cover of Tha**  **lassia (especially in areas where it had been sparse); and** **removed much of the litter layer on the bottom. How**

**ev er, by spring 1999,**  **Halophila cover increased in John**  **son 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 T** **halas sia**  **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 T** 

**halassia, crystal-clear water.** 
**Fa ll 1998 Koch: Sulfide phytotoxicity studies on**  **T halassia** 

**. New “classic”die-off observed in Barnes Key Jan 1999 Hall-Blakesley monitor changes in**  **T halassia**  **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 T**  **halassia and**  **Halodule again May 1999**  **observed in core samples from several stations. Hurricane Irene passes west of Florida Bay. Oct 1999**  **Flowering of T**  **halassia observed in field,**  **May 2000 recruiting seedlings observed in cores.**  T able 7.1 **Florida Bay seagrass die-off milestones. (continued)**

Event or Observation Date Significant Publication/Research Initiated Seagrass EcologyHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 97**Georges, suggesting the hurricane resuspended sed iments in Florida Bay. Hurricane Irene also reduced sea**  **grass cover in areas of sparse T**  **halassia and reduced**  **Halophila distribution as determined by before-and**  **after sampling by FHAP. Another recent dramatic change in the ecology of** **seagrasses in Florida Bay is the widespread occur rence of flowering in both**  **T halassia**  **and Halodule.**  **In spring 1999, reproductive short-shoots of**  **T halassia** 

**w ere present at 19 sites in 7 basins across the bay; re**  **productive short-shoots of Halodule**  **w ere present at 24**  **sites in 4 basins in the western part of the bay (Durako et al.,**  **2002). In spring 2000, flowering T**  **halassia was ob**  **served 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 appear**  **ance and spread of Halophila engelmannii**  **are often as sociated 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 T** **halassia from spring 1995 to spring 2001.** Florida Bay Synthesis Hunt and Nuttle

98 FWRI Technical Report TR-11 **productivity, standing crop, and flowering of T**  **halassia and**  **Halodule that seems to be associated with improv**  **ing 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 orig inal die-off sites, a new instance of primary seagrass die**  **off 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 ob**  **served 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 meris**  **tem 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).** 

P

ALEOECOLOGICAL 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 sea** **grass-associated microfossils and by chemical signa tures in the sediments, has shown repeated cycles of**  **presence and absence at the core sites.Thus, the recent**

**c hanges in seagrass cover are apparently not un**  **precedented. However, the paleoecological data come from only a few selected locations, limiting the confi** **dence that can be placed in their general applicabil ity. 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 de**

**v eloped of hypothesized die-off mechanisms and the**  **conditions and processes leading up to the die-off. Initially, the model developed by Zieman**  **et al. w** **as a mostly process-oriented model and placed strong em** **phasis on the historic conditions leading up to the die off. 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. T** **he 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) T**  **halassia ecosystem; an**  **Ini tiation 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 Mainte**  **nance Phase (G–L–G and repeat), in which the process**  **became self-sustaining. In this model, the interactions of the dense**  **T halassia**  **and the environmental stresses formed the primary trigger to the initial die-off**  **episodes. T** **he Carlson and Durako model (Figure 7.4) in cluded over-developed**  **T halassia**  **as a component but focused more on the role of physiological stress, es**  **pecially 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 el ement. 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 mod**  **els matured, seagrass researchers found much common** Figure 7.3 **Zieman et al.**  **model for seagrass die-off.** Seagrass EcologyHunt 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 T**  **halassia 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 sum** 

**mer–fall or fall–early winter. Both the Sunset Cove and Barnes Key die-off events were first observed** **during January–February. Most significantly, pri mary 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 epi phytic growth followed seagrass die-off by several** 
**y ears.** 

**F actors such as high water temperature, prolonged**  **h**

**ypersalinity, 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 T** 

**halassia 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**  **T**

**halassia in areas affected by die-off, but they were not** **universally present during the earliest phases. An un described species of marine slime mold in the genus**  **Labyrinthula w** **as the most common microorganism isolated from short-shoots affected by die-off (Porter** **and Muehlstein, 1989). Physiological stress and a neg ative carbon balance are associated with infection of**  **T**

**halassia 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) incorpo**  **rating distal and proximal elements thought to have contributed to die-off.These models also included pre**  **dictions 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**  **T halassia**  **in western Florida Bay is problematic. There is a high spatial coincidence be**  **tween 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 T**  **halassia canopy**  **has been removed (Thayer et al.,**  **1994), or resistance to disease. There has been little change in**  **T halassia**  **and Halodule**  **abundances in basins that are periodically sub jected to low salinities. This may reflect the effects of**  **intermediate disturbance ( sensu**  **Connell’s Intermedi ate Disturbance Hypothesis) in maintaining a mixed**  **species 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.** Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

100 FWRI Technical Report TR-11 **influence of internal conditions in the ecosystem and external driving factors on plant growth and metabo** **lism. It has also emphasized documentation of the se quence and pattern of seagrass die-off and recovery in**  **Florida Bay (Table 7.2). The principal findings of this**

**w ork are as follows:** 

**• Primary die-off is associated with high-density**  **T ha**  **lassia beds. High-density beds result in conditions**  **that increase stress on Th**  **alassia: high sediment**  **sulfide levels (greater than 2–4 mmol); self-shading of shoots; nighttime/early morning anoxia/hypoxia** **in meristems; and increased susceptibility to leaf to-leaf transmission of**  **Labyrinthula. Recent die-off**  **in the Barnes Key area occurred where T**  **halassia 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 T** 

**halassia loss,**  **Labyrinthula abundance, high sedi**  **ment-sulfide levels, and turbidity, but determining the relative contributions of these factors to die-off**  **v ersus secondary mortality has been difficult.** 

**• Recent bay-scale changes in seagrass distribution and** 

**abundance following the region-scale primary sea grass die-off (1987–1991) are largely due to secondary**  **mortality of T**  **halassia associated with areas of chronic**  **turbidity (since 1994) in western Florida Bay. These**

**c hanges are most likely due to light limitation as**  **indicated by stand-thinning in T**  **halassia beds with**  **concommitant increases in low-light-adapted species such as**  **Halodule wrightii and**  **Halophila engelmanii. T** **here has also been a loss of T**  **halassia in northern**  **Little Madeira Bay, near the mouth of the Taylor River, followed by colonization by**  **Halodule and**  **Rup pia.**  **T he cause of this loss is unknown. Primary acute**  **die-off (rapidly developing dead patches within dense beds) is known to be currently occurring (ini**  **tiated in 1999) only in the high-density T**  **halassia beds of the Barnes Key area. A chronic die-off is oc**  **curring in dense beds in Sunset Cove. However, the etiologies of these two sites are different.**  **• T he boom-and-bust pattern of**  **Halodule and**  **Ruppia**

**ch aracteristic of the Joe Bay and Highway Creek**  **areas of northeastern Florida Bay prior to 1991 has** T able 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;**  **Thay er**  **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**
**Te mperature and Salinity Chesnes**  **et al., 1999**  **Blakesley et al.,**  **1998; Boyer et al.,**  **1999; F** **ourqurean et al.,**  **2003 Light Attenuation**  **Williams and McRoy, 1982; Prager, 1998; Stumpf et al.,**  **1999; Dunton and Tomasko, 1994; Fourqurean**  **et al., 2003**  **Thay er**  **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**

**F reshwater Inflow Fourqurean**  **et al., 2003**  Seagrass EcologyHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 101

**stabilized with the occurrence of relatively constant estuarine/marine salinities and the lack of periodic**  **intense hypersaline conditions. • T** **here has been little net change in seagrass distrib**

**ution and abundance since 1995 in central basins that are periodically subjected to low salinities or**  **have shallow sediments and lower overall densities of**  **T halassia.**  **T hese conditions may reduce density-re** **lated stresses and allow robust species communi ties to develop; low salinity may also provide refugia**  **from disease because Labyrinthula**  **has never been ob served in Florida Bay in salinities below 15 psu.**  **• T he variable abundances of seagrass-associated mi** 

**crofossils in cores suggest repeated cycles of seagrass presence and absence at core locations over the past** **200 years.Thus, the recent changes in seagrass abun dance may not be unprecedented.**  **• A statistical, discriminant-function model of sea** 

**grass 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 T** **halas sia**  **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 T** **halas sia**  **densities and carbonate sediments, such as the Lakes Region of the lower Keys, did not experience ex**  **tensive 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 tem perature, prolonged hypersalinity, excessive seagrass**  **biomass leading to increased respiratory demands, h** 

**ypoxia and sulfide toxicity, and disease are some of the factors thought to have contributed to**  **T halassia**  **die-off. However, the causative mechanisms responsible for ini** **tiating the die-off remain incompletely understood (Rob blee**  **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 long**  **term monitoring at sites of pre-existing die-off and at sites not yet affected by die-off:** 

**“Measurements were made of short-shoot density, stand ing crop, leaf morphology and productivity of**  **T halassia**  **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 pro ductivity increased markedly from 1989 to 1995 at the die**  **off 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 mass**  **specific basis over the period 1989–1995, area productivity of**  **T halassia 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-depen dent 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 di**  **ameter) 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 die**  **off from 1989 to 1990 (Durako, 1994). The relationship between rhizome branching in 1998 and shoot density** 

**c hanges from 1998 to 1999 was significant (** **P < 0.0001);**  **however, the r**  **2 v** **alue of 0.12 was low. Carlson**  **et al. (1994) have reported that alcohol de**  **h**

**ydrogenase (ADH) increases in T**  **halassia exposed to**  **h**

**ypoxic 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.**



### T emperature

**T he 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 grow ing near the upper temperature limits, thermal effects**  **of discharges from electrical power plants can result in mortality during the warm summer months, al**  **though the productivity of these species may have been enhanced during the winter (Barber and Behrens,**  Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

102 FWRI Technical Report TR-11 **1985). T**  **halassia may show defoliation or reductions in** **leaf length when temperatures fall below 15°C or ex ceed 30°C (Phillips, 1960; Zimmerman and Living**  **stone, 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**  **T halassia**  **(no defo liation at temperatures just below 15°C);**  **Syringodium has maximal growth between 23°C and 29°C, whereas**  **for T**  **halassia maximal growth occurs between 23°C** **and 31°C (Zimmerman and Livingstone, 1976). In shal low bays with widely fluctuating temperatures,**  **Tha lassia, 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 tempera**  **tures are not expected to be significantly affected by altered water management.** 


### Salinity

**Salinity plays a very important role in controlling ben thic 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 pro duced data on how fluctuating salinities may affect** **turtlegrass, shoalgrass, and widgeon grass. Unfortu nately, operational difficulties plagued the mesocosm** **facility, which slowed progress and resulted in prob lems with maintaining proper controls.**  **T he role of hypersalinity in seagrass die-off still re**  **mains unclear. Salinities during the initial die-off episodes in 1987–1989 were accompanied by hyper**  **salinities ranging from 45 psu to 70 psu.The Barnes Key die-off clearly did not correlate with this level of salin** **ity but showed many of the other stresses that ac companied the primary die-off. Although the role of**  **h**

**ypersalinity may vary relative to other stresses, low salinity provides refugia from**  **Labyrinthula, because in**  **fection 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 sta tions 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 sus**  **pended solids and plant samples for epiphyte light attenuation measurements have been collected** **monthly. Diffuse attenuation coefficients vary sea sonally and between basins within Florida Bay: atten**  **uation 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. P** 

**ersistent 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 **T here 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 epi phytes have restricted distributions, occurring only** **near bird islands and near the Keys. The cyanobac terium**  **L yngbya**  **is very common in the Syringodium fil**  **iforme 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** 

**v alues 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 mea sured using Mylar strips and a light-attenuation mea**  **surement apparatus that has been successfully employed by investigators in Australia and Chesa**  **peake Bay (Frankovich and Zieman, 2001). The Mylar strips have been set out within various seagrass mead** 

**o ws 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**  **f**

**arinosum ), and filamentous red and brown algae occur**  **seasonally in various regions across Florida Bay.These**

**v arious 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/m 2**  **) and hermit crabs are** Seagrass EcologyHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 103 **conspicuous at Rabbit Key Basin (Frankovich and F** **ourqurean, 1997). In Florida Bay,**  **Bittium occurs at den**  **sities of about 3/m 2** 
**, whereas in the Indian River Lagoon,**  **Bittium occurs at densities of 20/**  **Halodule short-shoot or**  **at 1000s/m 2** 

**. Macroalgae such as**  **Laurencia are patchy in abun** **dance. It is unknown at present whether its abun dance 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**  **T halassia.** 



### Nutrients

**W**

**orldwide, 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 die offs. As stated above, epiphyte loads on seagrasses are** **relatively low in Florida Bay and do not seem to be re ducing production.** 



### Sulfide

**Carlson et al.**  **(1994) and Erskine and Koch (2000) have explored the potential links between elevated sedi**  **ment-sulfide concentrations and seagrass mortality (a hypothesis long proposed to account for seagrass die** **off). Hydrogen sulfide is a known plant toxin. High sul fide levels in sediments have been observed during and**  **after die-off episodes (Carlson et al.,**  **1994). Sulfide con centrations greater than 2–4 mmol have been mea**  **sured 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**  **T halassia**  **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 re** **sults. A submersible pulse-amplitude-modulated flu orometer (Diving-PAM) was used to investigate**  **photosynthetic yields of T**  **halassia testudinum leaf ma**  **terial 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 sed**  **iment 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 deter mining 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). Patho**  **genicity of a particular strain of Labyrinthula**  **will de termine severity of infection (Muehlstein**  **et al., 1988).**  **Environmental stressors (abiotic factors), such as low light or high temperatures, may weaken**  **T halassia**  **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 T**  **halassia in Florida Bay.**  **Blakesley et al.**  **’s (1999a) model predicted that in** **areas with high seagrass density, high salinity, “sub optimal seagrass conditions (environmental stress),”**  **and presence of pathogenic Labyrinthula,**  **the slime mold could contribute to either chronic or acute die**  **off by acting as an opportunistic secondary pathogen.** Figure 7.5 **Photosynthetic quantum yields of T** **halassia in Carl**  **son 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.**  Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

104 FWRI Technical Report TR-11**Under the same conditions but without environmen tal stress, Blakesley**  **et al. (1999b) suggested that**  **Labyrinthula, acting as a primary pathogen, could still**  **cause “thinning”or patchy die-off. Labyrinthula**  **occur rence 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 Du**  **rako, 2001), suggesting that this microorganism may have an adverse influence on seagrass growth and**  **carbon balance ( sensu**  **Durako and Kuss, 1994). T** 

**he occurrence of Labyrinthula**  **has also been shown to affect photosynthetic characteristics of**  **T halassia**  **(Durako and Kuss, 1994). Durako and Kuss (1994) demonstrated that**  **Labyrinthula infection reduces oxy** **gen production and may increase susceptibility to sul fide.These results indicate that reduced photosynthetic**  **capacity may be caused by Labyrinthula**  **-induced lesions or elevated sulfide.This may make**  **T halassia**  **more sus ceptible to sulfide toxicity, hypoxia, or disease by im**  **posing a negative carbon drain on belowground tissues. Photosynthetic yields of**  **T halassia testudinum**  **leaf material measured**  **in situ at Sunset Cove and Cross**  **Bank by using a Di ving-PAM w** **ere signifi cantly lower**  **in regions of T**  **halassia leaves that had visible lesions.**  **T hese patterns agree with those previously reported**  **by Durako and Kuss (1994). However, close-interval**  **Diving-PAM-fl uorescence measurements along an in**  **dividual leaf with several visible lesions indicated the reductions in photosynthesis were restricted to the im**  **mediate 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 pho**  **tosynthetic 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 Div ing-PAM**  **data also revealed that the severity of stress imposed by the leaf lesions will be a function of the pro**  **portion 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 high porosity sediments correlate with high hydrogen sul**  **fide levels because of the sediments’low permeability. Barnes Key surface sediments have very high poros**  **ity (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 rela tively high iron levels (Carlson**  **et al., 1994). Spatially,**  **iron levels are high near the mainland and toward the**

**w est. The role of atmospheric deposition of iron to**  **Florida Bay remains poorly understood. Addition of iron to sediments decreases the flux of sulfide to**  **Tha lassia**  **(Carlson bucket experiments) and results in a small but significant increase in growth. Iron distrib** **ution may also have a role in controlling phytoplank ton blooms in the bay.**  

## Effect of Ecosystem Characteristics on Community Structure and Distribution

**“The present distribution and species composition of sea grasses in south Florida are a result of the interaction of**  **many factors, the most important being water depth,**

**w ater 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**

**c hange in species composition of seagrass beds, not a**  **wholesale loss of seagrass cover ( e.g.,**  **Hall et al.,**  **1999; Du rako**  **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 sta**  **tus of the coastal environment.” —Fourqurean**  **et al., 2002**  

### Correlation with Patterns of Water Quality

**F ourqurean**  **et al. (2003) developed a discriminant func**  **tion 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 habi**  **tat 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 distrib**  **ution and abundance of Ruppia maritima**  **and Halodule**  **wrightii at the expense of**  **T halassia testudinum**  **along the northeast region of the bay.**  

### Geographic Variation in Seagrass Die-Off

**T he production of distribution and abundance maps**  **of the seagrasses has proven valuable, and these maps provide a quantitative record of distribution and abun** **dance at both basin and bay scales. The changes be tween maps provide clear visualization of where and**  **how much the distribution and abundance changes and** Seagrass EcologyHunt and Nuttle Florida Bay Synthesis

FWRI Technical Report TR-11 105**can thus point to “hot spots”that may be related to spe cific forcing events or conditions. The use of seagrass** **cover has become an assessment standard in the re gion (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**

**v ary in response to changes in the leafiness of short**  **shoots. Short-shoot density may be a better longer-term performance measure and is a quantitative measure** **ment. Density is the net effect of mortality, recruit ment, and life span (demographics).**  **T he recurrence of an acute die-off in Barnes Key** **presented an opportunity to test a portion of the the oretical model by comparing the symptoms and pro**  **gression 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 hy pothesis 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 envi ronmentally stressed site.**  **Comparisons of results for the Sunset Cove and Barnes Key sites revealed that although active die-off** 

**w as occurring in both places, the sites were very dif**  **ferent. 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 salin ities (greater than 15 psu) and dense**  **T halassia**  **beds— necessary elements for**  **Labyrinthula infection and** **transmission. However, the data from the two sites re**

**v ealed important differences that are summarized in**  **T**

**able 7.3. T** **hese differences strongly suggest that the mech anisms 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 in fection or disease (not**  **Labyrinthula -induced) that**  **rapidly kills the infected seagrass.The resultant large amount of decaying belowground biomass from the**  **rapidly dying T**  **halassia roots and rhizomes promotes** **microbial activity that in turn elevates the sediment sulfide levels selectively in those vegetative zones**  **where the die-off is occurring or has recently occurred. T** **he 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 al**  **ready weakened remaining seagrass. In contrast, the chronic die-off in Sunset Cove ap**  **pears to be directly caused by Labyrinthula**  **acting as a primary pathogen. Sediment-sulfide levels may re**  **main relatively low in all the vegetative zones tested simply because the slow death of**  **T halassia**  **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**  **Tha lassia**  **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 measure**  **ments show that the short-shoot meristems of T** **halas sia**  **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 meris tems 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.**  **T halassia**  **at Barnes Ke**  **y is very dense, with over 1,200–1,500 short shoots/m**  **2 , a v**  **ery high biomass (SC 300 g/m 2**  **), 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/m**  **2 ), but biomass (SC 109**  **g/m 2**  **) and turnover rates (1.7%/day) are more moder ate (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 hy**  **poxia or rapid sedimentation.** T able 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.**  **W ater 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.** Seagrass EcologyFlorida Bay Synthesis Hunt and Nuttle

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.**  **T his 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**  **W**

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

**w ork is to seek relationships between water-quality** 

**v ariables and seagrass species composition and abun** **dance, which if sufficiently strong, can be used to pre dict 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 scenar ios on the benthic habitats of Florida Bay. This statis**  **tical model will not address mechanisms or degree of**

**c hange in water quality that result from Restudy sce**  **narios; other models (such as the NSM, FATHOM, and the Florida Bay salinity-transfer-function models**  **currently employed by the Restudy) must simulate**

**w ater-quality changes across Florida Bay that will pro** **vide the input to the new models developed in this pro ject. As a consequence, it is anticipated that the benthic**  **habitat-change predictions of the statistical models will be the most reliable in the regions most closely cou**  **pled with water-management practices ( i.e.,**  **the en closed, mangrove-lined estuaries on the fringe of** **Florida Bay). As the fidelity of the physical water-qual ity models to the behavior of the system declines, the**  **reliability of the benthic changes predicted by our model will also decline. However, because the statis**  **tical relationships will be based on data from a larger number of marine areas and upper estuaries, the basic** **relationships between actual water quality (not mod eled) and benthic habitats will be robust.**  **In addition, other seagrass-modeling efforts have recently been initiated. These include the develop**  **ment of both seagrass unit models for T**  **halassia and**  **Halodule and a landscape model that will take output** **from the unit models. Smith and colleagues are de**

**v eloping a hierarchical approach to modeling the in**  **teraction between plant and physical processes in Florida Bay that involves two distinct spatial scales—**  **the demographic unit (ca. 10 m 2**  **) and the landscape unit (ca. km**  **2 ). A preliminary landscape model of**  **T halassia**  **that explicitly relates patterns of photosynthesis, res piration, and carbon allocation to environmental con**  **ditions that include salinity, temperature, PAR, and nutrient availability has been developed.** **Physical processes such as sedimentation, de composition, and nutrient cycling will be modeled on**  **a spatial scale of the landscape unit. This approach provides a hierarchical framework in which the de**  **mographic units used to simulate plant processes exist in the context of the landscape units, which will define** **the underlying physical environment.The plant char acteristics that are relevant to the feedbacks with the**  **physical environment (such as primary productivity and inputs of dead organic matter) are described sta**  **tistically 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**  **T halassia**  **using Div ing-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 salin** 

**ity, nutrient and multiple stressor effects on sea grass communities of NE Florida Bay” being**  **conducted by Koch and Durako. The results of this study are intended to help determine the parame**  **ters for the unit model with data from Florida Bay**
**seagrasses.**

**• FWRI (Carlson) bucket experiments have been con** 

**ducted to examine effects of multiple stressors (sul fide, nitrogen, and light) on seagrasses**  **in situ**

**.** 

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