Marine Biology "2001) 138: 341±354 Ó Springer-Verlag 2001 J. W. Fourqurean á A. Willsie á C. D. Rose L. M. Rutten Spatial and temporal pattern in seagrass community composition and productivity in south Florida Received: 30 January 2000 / Accepted: 24 July 2000 Abstract We document the distribution and abundance the data presented in this paper serve as a benchmark of seagrasses, as well as the intra-annual temporal pat- against which future change in the system can be terns in the abundance of seagrasses and the produc- quanti®ed. tivity of the nearshore dominant seagrass "Thalassia testudinum) in the south Florida region. At least one species of seagrass was present at 80.8% of 874 ran- Introduction domly chosen mapping sites, delimiting 12,800 km2 of seagrass beds in the 17,000-km2 survey area. Halophila Seagrasses have not fared well worldwide in the last decipiens had the greatest range in the study area; it century because of human alteration of the coastal zone. was found to occur over 7,500 km2. The range of In general, human activities have decreased the clarity of T. testudinum was almost as extensive "6,400 km2), fol- the water column, either because of increased turbidity lowed by Syringodium ®liforme "4,400 km2), Halodule or eutrophication; this has led to a concomitant decrease wrightii "3,000 km2)andHalophila engelmanni in seagrasses "see Duarte 1995; Short and Wyllie- "50 km2 ). The seasonal maxima of standing crop was Echeverria 1996 for review). Seagrass beds are often about 32% higher than the yearly mean. The produc- cited as some of the most productive ecosystems on tivity of T. testudinum was both temporally and spatially earth, rivaling cultivated crops in annual net primary variable. Yearly mean areal productivity averaged production "Zieman and Wetzel 1980). Despite the rec- 0.70 g m)2day)1, with a range of 0.05±3.29 g m)2 day)1. ognized importance of seagrasses "Costanza et al. 1997), Speci®c productivity ranged between 3.2 and 34.2 continued human population growth, and the suscepti- mg g)1 day)1, with a mean of 18.3 mg g)1 day)1. An- bility of seagrass communities to anthropogenic distur- nual peaks in speci®c productivity occurred in August, bance, there have been fewdetailed spatially extensive and minima in February. Integrating the standing monitoring programs designed to examine the status and crop for the study area gives an estimate of 1.4 ´ 1011 g trends of the seagrass beds at the regional scale "Duarte T. testudinum and 3.6 ´ 1010 g S. ®liforme, which 1999). Before assessments of trends in such systems can translate to a yearly production of 9.4 ´ 1011 g be accomplished, it is imperative that spatial and intra- T. testudinum leaves and 2.4 ´ 1011 g S. ®liforme leaves. annual patterns in the seagrass beds be understood. We assessed the ecacy of rapid visual surveys for There are fewareas whereseagrasses are as wide- estimating abundance of seagrasses in south Florida by spread and conspicuous as the shallowmarine waters comparing these results to measures of leaf biomass for surrounding the southern tip of the Florida peninsula T. testudinum and S. ®liforme. Our rapid visual surveys "ca. 24.5°N, 80.5°W). Seagrass beds are the most com- proved useful for quantifying seagrass abundance, and mon benthic community type in the region, they cover at least 14,000 km2 "Fourqurean et al. in press). A lack of signi®cant river discharge and vigorous mixing of coastal Communicated by N. Marcus, Tallahassee water bodies with oceanic water results in generally clear waters in south Florida; this water clarity allows su- J. W. Fourqurean "&) á A. Willsie á C. D. Rose á L. M. Rutten cient light to penetrate to the sandy and muddy bottoms Department of Biological Sciences and Southeast Environmental Research Center, of the region to support seagrass growth. Seagrass beds, Florida International University, along with mangrove forests and the only barrier coral Miami, FL 33199, USA reef in the continental United States, provide the habitats e-mail: fourqure@®u.edu that support commercial ®shing and recreational use of Tel.: +1-305-3484084; Fax: +1-305-3484096 the nearshore marine habitat. 342 The litany of recent, perceived environmental prob- protect the physical and biological components on the lems in the nearshore marine environment in south south Florida estuarine and marine ecosystem to ensure Florida is long. Particular concern has been raised over its viability for the use and enjoyment of present and the bleaching of reef corals "Jaap 1985; Williams et al. future generations'' "NOAA 1996). 1987; Fitt et al. 1993), loss of coral cover on the reef In this study, we describe the spatial pattern in the tract "Dustan and Halas 1987; Porter and Meier 1992), present-day distribution of seagrass communities in and increasing occurrence and types of coral diseases south Florida and describe the seasonal patterns in the "Richardson 1998; Richardson et al. 1998). A poorly biomass and productivity of Thalassia testudinum,a understood seagrass dieo event in the late 1980s in dominant seagrass, as a baseline against which future adjacent Florida Bay "Robblee et al. 1991; Thayer et al. change in the ecosystem can be measured. The spatial 1994; Hall et al. 1999) had rami®cations that cascaded scale and temporal resolution of the monitoring network through the ecosystem, causing phytoplankton blooms described herein are without precedent in seagrass eco- and sponge dieos that in turn aected large mobile ®sh systems; only through such large-scale studies can gen- and invertebrates "Butler et al. 1995; Philips and Bad- eralizable and reliable patterns and trends be detected. It ylak 1996; Matheson et al. 1999; Thayer et al. 1999). is possible that relatively local phenomena may be mis- Changing water quality has been implicated, as either a sed in a monitoring program of such scope, but proper cause or an eect, of many of these environmental management of the FKNMS, as well as other seagrass problems "e.g., Lapointe and Clark 1992; Porter et al. ecosystems worldwide, depends on understanding gen- 1999). Direct physical damage to seagrass beds is also eral, regional-scale trends in the ecosystem "Duarte occurring in south Florida, mostly in the form of ``prop 1999). scarring'' caused by inadvertent or negligent operation of boats in shallowseagrass beds "Sargent et al. 1995). Human recognition of the value of the nearshore marine habitats, coupled with perceptions of degraded habitat Materials and methods quality that were partially supported by strong scienti®c We surveyed the seagrass distribution and abundance throughout a data, were largely responsible for the creation of the 17,000-km2 area in south Florida "Fig. 1). The FKNMS is a 9,600- Florida Keys National Marine Sanctuary "FKNMS) in km2 region within this larger area. A strati®ed-random approach, 1990. The goal of the FKNMS is to ``preserve and with distance oshore as the strata, was used to locate 30 perma- Fig. 1 Map of study area showing boundaries of manage- ment areas within the study area and the location of the 30 permanent monitoring sites and the 874 mapping sites 343 nent seagrass monitoring sites within the FKNMS "Fig. 1). Sites mum Braun-Blanquet score "note Di £ Ai). Frequency "F)was were sampled every 3 months from December 1995 through De- calculated as: cember 1998. In addition to these permanent sites, an additional N F i 3 874 mapping sites were randomly selected across the FKNMS and i n the shallowportion of the SouthwestFlorida Shelf to the north of such that 0 £ Fi £ 1. In addition to species-speci®c measures, sea- the FKNMS "Fig. 1); these sites were visited once during the grass species richness S was calculated for each site by summing the summer months of 1996±1998 in order to rapidly estimate spatial number of seagrass species for which D >0. extent and cover of benthic macrophytes in the FKNMS and Net aboveground productivity of Thalassia testudinum was adjacent shallowwatermarine environments. measured on a quarterly basis at each permanent site using a modi- A rapid, visual assessment technique developed early in the 20th ®ed leaf marking technique "Zieman 1974; Zieman et al. 1999). Six century by the plant sociologist Braun-Blanquet "Braun-Blanquet 10 ´ 20-cm quadrats were haphazardly distributed within 10 m of a 1972) was used to assess the abundance of seagrass and macroal- permanent steel rod that marked the site. Within each quadrat, all gae. This method is very quick, requiring only minutes at each short shoots "SS) of the seagrass T. testudinum were marked near the sampling site; yet it is robust and highly repeatable, thereby mini- base of the leaves by driving an 18-gauge hypodermic needle through mizing among-observer dierences, and has recently been applied all of the leaves on a short shoot. Care was taken not to disturb other to seagrass research "Kenworthy et al. 1993; Rose et al. 1999). At plant and animal taxa in the quadrats. The marked SS were allowed each permanent seagrass monitoring site, a 50-m-long transect was to growfor 10±14 days; after whichall above-ground seagrass ma- established at the beginning of the study period by driving steel terial in the quadrats was harvested. The number of SS of each sea- rods into the substratum at both ends of the transect. At each grass species was counted. Plant material was separated by seagrass mapping site, a 50-m transect was set up by extending a meter tape species; and T. testudinum leaves were separated further into newly along the bottom in an up-current direction.