CHAPTER 5 



THE SEAGRASS COMMUNITY - COMPONENTS, STRUCTURE, AND FUNCTION 



Seagrass-associated comriunities are 

 doternined by species conposition and den- 

 sity of seagrass present, as well as abi- 

 otic variables. These communities range 

 from monospecific turtle grass beds in the 

 clear, deep waters behind the reef tract 

 to the shallow, muddy bottoms of upper 

 Florida Bay where varying densities of 

 shoal grass are intermixed with patches of 

 turtle grass. 



Turney and Perkins (1972) divided 

 Florida Bay into four regions based large- 

 ly on temperature, salinity, circulation, 

 and substrate characteristics. Each of 

 these regions proved to have a distinctive 

 molluscan asse;nblage. 



Studies have also shown that great 

 diversity in species number and abundance 

 exists even within communities of similar 

 seagrass composition and density, and 

 within comparatively small geographical 

 regions. Brook (1978) compared the macro- 

 fauna! abundance in five turtle grass com- 

 munities in south Florida, where the blade 

 density was greater than 3,000 blades/m-. 

 Total taxa represented varied from a low 

 of 38 to a high of 80, and average abun- 

 dance of individuals varied from 292 to 

 10,644 individual s/m-'. 



The biota present in the seagrass 

 ecosystem can be classified in a scheme 

 that recognizes the central role of the 

 seagrass canopy in the organization of the 

 system. The principal groups are (1) epi- 

 phytic organisms, (2) epibenthic 

 organisms, (3) infaunal organisms, and (4) 

 the nektonic organisms. 



The term epiphytic organisms is used 

 here the same as that of Harlin (1980) and 

 means any organism growing on a plant and 

 not just a plant living on a plant. Epi- 

 benthic organisms are those organisms that 

 live on the surface of the sediment; in 

 its broadest sense, this includes motile 

 organisms such as large gastropods and sea 

 urchins, as well as sessile forms such as 

 sponges and sea anemones or macroalgae. 

 Infaunal organisms are those organisms 

 that live buried in the sediments. Organ- 

 isms such as penaeid shrimp, however, that 

 lie buried part of the day or night in the 

 sediments, but are actively moving on the 

 sediment surface the rest of the time 

 would not be included as part of the 

 infauna. The infauna would include organ- 

 isms such as the relatively immobile 

 sedentary polychaetes and the relatively 

 mobile irregular urchins, Nektonic organ- 

 isms, the highly mobile organisms living 

 in or above the plant canopy, are largely 

 fishes and squids. 



Kikuchi (1961, 1962, 1966, 1980) 

 originally proposed a functional classi- 

 fication scheme for the utilization of 

 Japanese seagrass beds by fauna that has 

 wide utility. This classification, mod- 

 ified for tropical organisms, would 

 include (1) permanent residents, (2) 

 seasonal residents, (3) temporal migrants, 

 (4) transients, and (5) casual visitors. 

 The third category is added here to 

 include the organisms that daily migrate 

 between seagrass beds and coral reefs. 

 These were not included in the original 

 classification which was based on tem- 

 perate fauna. 



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