seagrasses, the greatest proportion of 

 fresh plant material is not readily used 

 as a food source. For these animals sea- 

 grass organic natter becomes a food source 

 of nutritional value only after undergoing 

 decomposition to particulate organic 

 detritus, vvhich is defined as dead organic 

 natter along with its associated nicro- 

 organisns (Heald 1969). 



The nonavailability of fresh seagrass 

 material to detritus-consuming animals 

 (detritivores) is due to a complex combi- 

 nation of factors. For turtle grass 

 leaves, direct assays of fiber content 

 have yielded values up to 59% of the dry 

 weight (Vicente et al . 1978). Many ani- 

 mals lack the enzymatic capacity to assim- 

 ilate this fibrous material. The fibrous 

 components also make fresh seagrass resis- 

 tant to digestion except by animals (such 

 as parrotfishes and green turtles) with 

 specific morphological or physiological 

 adaptations enabling physical maceration 

 of plant material. Fresh seagrasses also 

 contain phenolic compounds that may deter 

 herbivory by some animals. 



During decomposition of seagrasses, 

 numerous changes occur that result in a 

 food source of greater value to many con- 

 sumers. Bacteria, fungi, and other micro- 

 organisms have the enzymatic capacity to 

 degrade the refractile seagrass organic 

 matter that many animals lack. These 

 microorganisms colonize and degrade the 

 seagrass detritus, converting a portion of 

 it to microbial protoplasm and mineraliz- 

 ing a large fraction. Whereas nitrogen is 

 typically 11 to 4% dry weight of seagrass- 

 es (Table 7), microflora contain 5? to ICl' 

 nitrogen. Microflora incorporate inorganic 

 nitrogen from the surrounding medium — 

 either the sediments or the water column-- 

 into their cells during the decomposition 

 process, enriching the detritus with pro- 

 teins and other soluble nitrogen com- 

 pounds. In addition, other carbon com- 

 pounds of the microflora are much less 

 resistant to digestion than the fibrous 

 components of the seagrass matter. Thus, 

 as decomposition occurs there will be a 

 gradual mineralization of the highly 

 resistant fraction of the seagrass organic 

 matter and corresponding synthesis of 

 microbial biomass that contains a much 

 higher proportion of soluble compounds. 



Microorganisms, because of their di- 

 verse enzymatic capabilities, are a neces- 

 sary trophic intermediary between the sea- 

 grasses and detritivorous animals. Evi- 

 dence (Tenore 1977; Ward and Cummins 1979) 

 suggests that these animals derive the 

 largest portion of their nutritional re- 

 quirements from the microbial coinponent of 

 detritus. Detritivores typically assimi- 

 late the microflora compounds with effi- 

 ciencies of 50"J to almost 100%, whereas 

 plant compound assimilation is less than 

 5% efficient (Yingst 1976; Lopez et al . 

 1977; Cammen 1900). 



During seagrass decomposition, the 

 size of the particulate matter is decreas- 

 ed, making it available as food for a wid- 

 er variety of animals. The reduced parti- 

 cle size increases the surface area avail- 

 able for microbial colonization, thus in- 

 creasing the deconposition rate. The abun- 

 dant and trophically important deposit- 

 feeding fauna of seagrass beds and adja- 

 cent benthic communities, such as poly- 

 chaete worms, amphipods and isopods, oohi- 

 uroids, certain gastropods, and mullet, 

 derive much of their nutrition from fine 

 detrital particles. 



It is important to note that much of 

 the contribution of seagrasses to higher 

 trophic levels through detrital food webs 

 occurs away from the beds. The more 

 decomposed, fine detrital particles (less 

 than 0.5 mm) are easily resuspended and 

 are widely distributed by currents (Fisher 

 et al. lf^79). They contribute to the 

 organic detritus pool in the surrounding 

 waters and sediments where they continue 

 to support an active microbial population 

 and are browsed by deposit feeders. 



Physical Breakdown 



The physical breakdown and particle 

 size reduction of seagrasses are important 

 for several reasons. First, particle size 

 is an important variable in food selection 

 for a wide range of organisms. Filter 

 feeders and deposit feeders (polychaetes, 

 zooplankton, gastropods) are only able to 

 ingest fine particles (less than 0.5 mm 

 diameter). Second, as the seagrass mate- 

 rial is broken up, it has a higher surface 

 area to volume ratio which allows more 

 microbial colonization. This increases 



70 



