are present because of its ability to tol- 

 erate aerobic, unstable sediments and to 

 rapidly extend its rhizone system under 

 these conditions. This is especially evi- 

 dent in back-reef areas. Patriquin (1575) 

 attributes the persistence of nanatee 

 grass in areas around Barbados to recur- 

 rent erosion in areas where the bottom was 

 never stable for a sufficiently long time 

 to allow turtle grass to colonize. Mana- 

 tee grass can have half of its biomass as 

 leaves (Table 4). Thus, while manatee 

 grass is colonizing aerobic disturbed sed- 

 iments, which would be areas of low nutri- 

 ent supply and regeneration, the amount of 

 its root surface available for nutrient 

 uptake would be reduced, and correspond- 

 ingly leaf uptake would become a major 

 source of nutrients. If this is the case, 

 the higher agitation of the water column 

 would be of benefit by reducing the grad- 

 ients at the leaf surface. 



4.4 THE CENTRAL POSITION OF THE SEA- 

 GRASSES TO THE SEAGRASS ECOSYSTEM 



organisms with their widely differing 

 requirements and interactions functioned 

 as a highly intricate web structure that 

 made each individual or each link less 

 necessary to the maintenance of the total 

 system. There was much natural redundance 

 huilt into the system. For certain seg- 

 iients of the community this may be true. 

 The problem is that at climax there is one 

 species for which there is no redundancy : 

 the seagrass. In some cases, if the sea- 

 grass disappears, the entire associated 

 community disappears along with it; there 

 is no other organism that can sustain and 

 support the system. 



This is shown in a sf^all way when 

 minor disturbances occur as was described 

 with the blowouts. As the grass beds in 

 these areas are eroded away, the entire 

 seagrass system disappears, including the 

 top 1 or 2 m of sediment. These features 

 are small and readily repaired, but give 

 an indication of what could happen if 

 there was widespread damage to the sea- 

 grasses. 



Seagrasses are vital to the coastal 

 ecosystem because they form the basis of a 

 three-dimensional, structurally complex 

 habitat. In modern ecology there has been 

 a shift from the autoecological approach 

 of studying individual species independ- 

 ently, to the community or ecosystem ap- 

 proach where the focus is the larger inte- 

 grated entity. With that realization, one 

 could wonder, "Why spend sO much effort on 

 a few species of marine plants, even if 

 they are the most abundant, in a system 

 that has thousands of other species?" The 

 reason is that these plants are critical 

 to most other species of the system, both 

 plant and animal. There are few other 

 systems which are so dominated and con- 

 trolled by a single species as in the case 

 of a climax turtle grass or Zostera mea- 

 dow. H.T. Odum (1974) classified turtle 

 grass beds as "natural tropical ecosystems 

 with high diversity." Taken as a total 

 system, tropical seagrass beds are regions 

 of very high diversity, but this can be 

 misleading. Comparisons between tropical 

 and temperate systems were made at a time 

 when high diversity was equated with high 

 biological stability. The prevailing con- 

 cept was that the multitude of different 



The largest contribution to the di- 

 versity of the system is commonly made by 

 the complex communities that are epiphytic 

 on the seagrass leaves. Hhen defoliation 

 of the seagrasses occurs, most of this 

 community disappears, either by being car- 

 ried out as drifting leaves or becoming 

 part of the litter layer and ultimately 

 the surface sediments. With the leaves 

 gone, the current baffling effect is lost 

 and the sediment surface begins to erode. 

 Algal mats that may form have minimal 

 stabilizing ability; however, the dead 

 rhizomes and mats will continue to bond 

 the sediments, in some cases for several 

 years (Patriquin 1975; Scoffin 1970). 



In south Florida the disappearance of 

 seagrasses would yield a far different 

 seascape. Much of the region v/ould be 

 shifting mud and mud banks, while in many 

 areas the sediments would be eroded to 

 bedrock. Based on the communities found 

 in such areas today, primary production 

 and detrital production would be dramati- 

 cally decreased to the point that the 

 support base for the abundant commercial 

 fisheries and sport fisheries would shrink 

 if not disappear. 



38 



