4.5 STRUCTURAL 

 SEAGRASSES 



AND PROCESS SUCCESSION IN 



As species succession occurs in a 

 shallow marine system, important struc- 

 tural changes occur. Because seagrass 

 systems do not have woody structural com- 

 ponents and only possess relatively simp- 

 listic canopy structure, the main struc- 

 tural features are the leaf area and bio- 

 mass of the leaves as well as the root and 

 rhizome material in the sediment. The 

 most obvious change with community devel- 

 opment is the increase in leaf area. This 

 provides an increase in surface area for 

 the colonization of epiphytic algae and 

 fauna, with the surface area of the climax 

 community being many times that of either 

 the pioneer seagrass, shoal grass, or the 

 initial algal colonizers. In addition to 

 providing a substrate, the increasing leaf 

 area also increases the current baffling 

 and sediment-trapping effects, thus en- 

 hancing internal nitrogen cycling. 



As organisms grow and reproduce in 

 the environment, they bring about changes 

 in their surroundings. In doing so these 

 organisms frequently modify the environ- 

 ment in a way that no longer favors their 

 continual growth. McArthur and Connell 

 (1966) stated that this process "gives us 

 a clue to all of the true replacements of 

 succession: each species alters the envi- 

 ronment in such a way that it can no 

 longer grow so successfully as others". 



In a shallow water successional se- 

 quence leading to turtle grass, the early 



stages are often characterized by a low 

 supply of organic matter in the sediment 

 and open nutrient supply, that is, the 

 community relies on nutrients being 

 brought in from adjacent areas by water 

 movement as opposed to in situ regenera- 

 tion. With the development from rhizophy- 

 tic algae to turtle grass, there is a pro- 

 gressive development in the helowground 

 biomass of the community as well as the 

 portion exposed in the water column. With 

 the progressive increase in leaf area of 

 the plants, the sediment trapping and par- 

 ticle retention increase. This material 

 adds organic matter to further fuel the 

 sedimentary microbial cycles. Although 

 various segments of this successional 

 sequence have been measured by numerous 

 authors, the most complete set of data has 

 recently been compiled by Williams (1981) 

 in St. Croix (Table 8). In St. Croix, 

 where the data were collected, as on many 

 low, small islands with little rainfall, 

 the climax is commonly a mixture of turtle 

 grass and manatee grass. In south Florida, 

 with its higher rainfall and runoff, the 

 climax more commonly is a pure turtle 

 grass stand. In turtle grass beds in 

 south Florida, Capone and Taylor (1977, 

 1980) found that nitrification was highest 

 on the developing periphery of the bods 

 and lower in the centers where particulate 

 trapping and retention were greater. Add- 

 itionally, mature ecosystems, both marine 

 and terrestrial, seem to be based primar- 

 ily on the detrital food web which aids in 

 conserving both carbon and nitrogen, as 

 direct grazing is quantitatively low in 

 these systems. 



39 



