leaves of seagrasses have been shown 

 to fix nitrogen, thus adding to the 

 nutrient pool of the region. In add- 

 ition, seagrasses have been shown to 

 pick up nutrients fron the sediments, 

 transporting then through the plant 

 and releasing the nutrients into the 

 water column through the leaves, thus 

 acting as a nutrient pump fron the 

 sediment. 



4.2 SUCCESSION AND ECOSYSTEM DEVELOPMENT 



In conventional usage, succession 

 refers to the orderly development of a 

 series of communities, or serai stages, 

 which result in a climax stage that is in 

 equilibrium with the prevailing environ- 

 mental conditions. In more contemporary 

 usage, however, succession is more broadly 

 used to mean the succession of species, 

 structure, and functions within an ecosys- 

 tem. Odum (1969) stated the contemporary 

 concept as follows: 



(1) Succession is an orderly process 

 of community development that in- 

 volves changes in species structure 

 and community processes with time; it 

 is reasonable, directional, and 

 therefore predictable. 



(2) Succession results from modifi- 

 cation of the physical environment by 

 the community; that is, succession is 

 community-controlled even though the 

 physical environment determines the 

 pattern and the role of change, and 

 often sets limits as to how far 

 development can go. 



(3) Succession culminates in a sta- 

 bilized ecosystem in which maximum 

 biomass (or high information content) 

 and symbiotic function between organ- 

 isms are maintained per unit of 

 available energy flow. 



Species succession has received by 

 far the most attention as it is most 

 obvious and easily measured. The study of 

 succession of processes or functions is 

 just beginning, hov/ever. It may well prove 

 to be the most important avenue for under- 

 standing ecosystem development. Defining 

 these processes is of much greater impor- 

 tance than mere scientific curiosity. It 



is also the key 

 denuded systems. 



to res tori no damaned or 



4.3 SPECIES SUCCESSION 



Throughout the south Florida rooion, 

 and most of the Gulf of riexico and Carib- 

 bean, the species of plants that partici- 

 pate in the successional sequence of sea- 

 grasses are remarkably few because there 

 are so few marine plants that can colonize 

 unconsolidated sediments. In addition to 

 the seagrasses, one other group, the rhi- 

 zophytic green alqae, has this capability. 

 These algae, however, have only limited 

 rhizoidal development and never affect an 

 area greater than a few centimeters from 

 their base. 



The most common illustration of suc- 

 cession in seagrass systems is the recolo- 

 nization following a "blowout." This loc- 

 alized disturbance occurs in seagrass beds 

 throughout Florida and the Caribbean where 

 there is sufficient current movement in a 

 dominant direction (Figure 9). Usually a 

 disruption, such as a major storm, over- 

 grazing caused by an outbreak of urchins, 

 or a major ripping of the beds caused by 

 dragging a large anchor, is required to 

 initiate the blowout. Once started, the 

 holes are enlarged by the strong water 

 flow which causes erosion on the down cur- 

 rent side. Slowly a crescentic shape a 

 few meters wide to tens of meters wide is 

 formed. A sample cross section in Figure 

 10 shows a mature turtle grass community 

 that has been disrupted and is recovering. 

 The region at the base of the erosion 

 scarp is highly agitated and contains 

 large chunks of consolidated sediment and 

 occasional rhizome fragments. With in- 

 creasing distance from the face of the 

 scarp, turbulence decreases and some mate- 

 rial is deposited. The area has become 

 colonized with rhizophytic algae; Hal imeda 

 and Penicil lus are the most abundant, but 

 Caulerpa , Udotea , Rhipocephalus and 

 Avrainvillea arc also common. These algae 

 provide a certain amount of sediment- 

 binding capability as illustrated in Fig- 

 ure 11, but they do not stabilize the sur- 

 face of the sediments very well (Scoff in 

 1970). A major function of these algae in 

 the early successional stage is the con- 

 tribution of sedimentary particles (Wil- 

 liams 1981), The generalized pattern and 



34 



