A Review of Certain Aspects of the Life, Death, and Distribution of the Seagrasses of the Southeastern United States 1960-1985

A Review of Certain Aspects of the Life, Death, and Distribution of the Seagrasses of the Southeastern United States 1960-1985 Joseph C. Zieman Department of Environmental Sciences University of Virginia Charlottesville, Virginia 22903 ABSTRACT Seagrass meadows are among the richest and ecologically most important coastal habitats. In the United States,the greatestseagrass resources are along the south and west coastsof Florida, with over 5,500 km' of seagrassin south Florida, and a secondextensive bed coveringover 3,000 kme betweenTampa and Apalachee Bay. Well developed seagrass meadows occur at depths over 10 m in clear waters, but are often limited to less than 2 rn in turbid, polluted estuaries. In these latter areas, suspended particulate rnatter, as well as over- growthsof epiphytic algae,brought about by excessnutrients in the water column,can stress the seagrasses.In more pristine waters, seagrasses maintain high productivity by obtaining nutrients from the sediments via extensive root and rhizome systems, which, coupled with the current-baftling effect of the leaf canopy, protect and stabilize the sediments. In turbid, shallow seagrass systems, much of the food web is based on epiphytic algal grazing, but the dominant trophic pathway in most seagrass systems seems to be via the detrital food web, Seagrass leaves are a relatively rich food source, compared to saltmarsh plants and mangroves, but are grazed directly by few organisms, especially outside of tropical Caribbean waters. In addition to contributing to local food webs, detached seagrass blades are often exported great distances and serve as food sources hundreds of kilometers from their source beds, INTRODUCTION - HISTORICAL PERSPECTIVE Today, seagrassmeadows are recognizedamong Although the time scale is greatly compressed, the richest and most productive of all coastal compared to that of the classical sciences, seagrass ecosystems, providing habitat and food for innumer- ecologyhas progressedthrough recognizable stages able organisms. However, recognitionof the immense in this 25-year period, finally emerging as a mature importanceof these systemsin maintaining the area of study. Initially, nearly all of the literature abundance of commercial and sports fisheries, as was descriptive and qualitative. By 1970, most well as the numerous other benefits that they con- published works were quantitative, and development vey, is a comparatively recent development, In 1960, of conceptual models was just beginning. By 1980, the number of published papers concerning seagrass increasingly robust models of the mechanisms by ecology,or any aspectof seagrassbiology, could be which the systems develop and maintain their pro- counted on the fingers of very few hands. By 1978, ductivity were being proposed and used as guidesto a bibliography compiled by the Seagrass Ecosystem proposed research. Study listed over 1400 titles worldwide Bridges et In addition to advances in scientific equipment al,, 1978!, and, by 1982, a community profile of the and techniques, and increasingly powerful computer south Florida seagrasses contained over 550 refer- facilities, another technological advancement aided ences. the development of seagrass research to a degree 54 FLORIDA MARINE RESEARCH PUBLICATIONS impossible to overestimate: SCUBA. Seagrass re- protection. This seems to be due to the area's search has benefited immeasurably from the in- unique physical conditions, which create one of the creasing availability of safe, reliable, and relatively world's few zero-energy coastlines Murali, 1982; inexpensiveunderwater breathing gear, Researchers Tanner, 1960!. This meadowcovers 3,000 km2 and are no longer constrainedto wading in shallowbeds, is second in size only to the enormous beds in which were often quite stressed due to their shallow Florida Bay and behind the Florida Reef Tract, nature, and, therefore, not representative of most which spread from just south of Key Biscayne to seagrassmeadows. Seagrass meadows are ideal sub- west of Key West, totaling 5,500 km~ in area jects for the use of SCUBA techniques, because Iverson and Bittaker, 1986!. Seagrassesare reduced most occur in depths of less than 10 m, where use or excluded where excess runoff and turbidity create of SCUBA is relatively unrestricted. unfavorable conditions, such as off the Ten Thousand The objectiveof this paper is to review pro- Islands Iverson and Bittaker, 1986! or in Apalachi- gress, over the past 25 years, in our understanding cola Bay Livingston, 1984!, but are found in most of seagrass systems and what makes them so other estuaries. Other major estuarine or lagoonal beneficiaL Topics that will be addressedare distri- seagrass beds are found in Tampa Bay, Charlotte bution, biomassand production,nutrient cycling, Harbor, and the Indian River, decomposition and detrital processing, export, and a brief summary of general trophic structure as it ZONATION relates to the other topics. Although extensive development of seagrass beds is confined to depths of 10-12 m or less, seagrasses have been recorded from as deep as 42 m. DISTRIBUTION The principal factors determining depth distribution are light and perhaps! pressure at depth, and ex- AREAL DISTRIBUTION posure and resultant dessication at the shallow end of the gradient. Nearly all of the seagrass resources in the The seagrasses of Florida are all subtidal, southeastern coastal region of the United States, tolerating little exposure. Exposure to air does occur from South Carolina to the Mississippi River, occur at certain low tides on shallow T. testudinum or in the coastalwaters and estuariesof Florida. By Halodule u rightii Aschers. fiats, and heavy leaf 1960, the distribution of seagrassesin this region mortality results, unless exposure is extremely brief. was relatively well described, but major gapain the Rafts of dead seagrassleaves frequently are carried literature left nearly a quarter of the coastline of from shallow fiats following spring low tides, but, Florida undescribed Phillips, 1960!. Much of this normally, rhizomes are not damaged and the plants unknownarea was addressedby Moore963!, who recover, gave a detailed observational and qualitative des- Numerous studies in Florida, from Phillips cription of the areal distribution of Thalassia 960! and Strawn 961! to Lewis et aL 985!, testudinumBanks ex Konig. McNulty et al. 972! have illustrated patterns of zonation in the state' s estimated the vegetative coverage of nearshore seagrass beds Figure 1!, Localized conditions waters and embayrnentsof inner Florida Bay and create much variability, but patterns of zonation the west coast of Florida, and found seagrass exhibit a definite commonality. The general pattern coverageto total 2,106 km~, tidal marshes,2,615 described below represents a typical gradient ob- km, and mangroves,1,591 km served throughout clear waters in Florida. In turbid Florida possessesone of the largest seagrass water areas, the same pattern could be expected, resources on earth Beds of various sizes are found but the ranges of various species would be attenu- in much of the protected waters from the Indian ated. A vertical zonation gradient that extends 10- River, on the central east coast, to Santa Rosa 15 m in the Keys or the Dry Tortugas, would be Sound, on the northwestcoast. Typically, beds compressed to 2 m or less in highly impacted occur in somewhat protected waters behind reefs, estuaries such as Tampa Bay or northern Biscayne barrier islands,or some other form of protection. Bay, or even in certain basins in Florida Bay that The Big Bend area of the northwest coast of Florida are free of any human impact. is quite unique in having seagrass meadows ex- Halodule terightii and Ruppia maritima L. gener- tending tens of kilometers offshore with no seaward ally are found in the shallowestwater and appear to NUMBER 42 Of all the major species,S. jiliforrne often has the most discontinuousdistribution. It is commonly found intermixed with T. test&inurn in deeper portions of the latter's range, but is virtually never found in waters as shallow as Thalassia or Halodule, because its stiff leaves do not bend sufficiently to conform to the sediment surface and reduce dessication, Syringodiurnleaves are quite brittle, compared to those of other species, and break easily if bent at a sharp angle. However, the plant seems well adapted to both turbulence and rapid water flow. In highly turbulent waters immediately behind the Florida Reef Tract, Synngodiurn is commonly found either in dense monospecificstands or intermixed with Thalassia. Both plants seem to possess sufficientleaf strengthand root holdingcapacity to exist in this rigorous environment. Dense stands of Syrirtgodium are also commonly found in deep channelsthroughout the Florida Keys, where tidal current velocities are very high, and high turbidity reducesthe incident light. Figure l, Zonation of seagrassesha an area of Tampa Bay. From PRODUCTION ECOLOGY Phillips, 1960!, Seagrassesoccur in a very wide range of den- sities, butunder optimum conditions,they can form vast high-density meadows. The literature on this be more tolerant of exposurethan the other species. subjectis extensiveand often bewilderingbecause The relatively high flexibility of their leaves allows values have been reported in a variety of forms. For them to conform to the damp sediment surface consistency, the terms used here conform to those during periods of exposure, thus minunizing leaf of Ziemanand Wetzel 980!: "standingcrop" refers

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