Release of Dissolved Organic l^atter 



Seagrasses release substantial 

 amounts of dissolved organic carbon (DOC) 

 during growth and decomposition. The DOC 

 fraction is the most readily used fraction 

 of the seagrass organic matter for micro- 

 organisms and contains much of the soluble 

 carbohydrates and proteins of the plants. 

 It is quickly assimilated by microorgan- 

 isms, and is available to consumers as 

 food in significant quantities only after 

 this conversion to microbial biomass. 

 Thus, the utilization of seagrass DOC is 

 functionally similar to detrital food webs 

 based on the particulate fraction of sea- 

 grass carbon. Both epiphytes and leaves 

 of Zostera are capable of taking up label- 

 led organic compounds (Smith and Penhale 

 1980). 



Experiments designed to quantify the 

 release of DOC from growing seagrasses 

 have yielded a wide range of values. The 

 short-term release of recently synthesized 

 photosynthate from blades of turtle grass 

 was found to be 2% to 10%, using radio- 

 labelled carbon (Wetzel and Penhale 1979; 

 Brylinsky 1977). Losses to the water col- 

 umn from the entire community, including 

 belowground biomass and decomposing por- 

 tions, may be much higher. Kirkman and 

 P.eid (1979) found that 50X of the annual 

 loss of organic carbon from the Posidonia 

 austral is seagrass community was in the 

 form of DOC. 



Release of DOC from detrital leaves 

 may also be substantial. In freshwater 

 macrophytes, leaching and autolysis of DOC 

 load to a rapid 50% loss of weight (Otsuki 

 and Wetzel 1974). In laboratory experi- 

 ments dried turtle grass and manatee grass 

 leaves released 13% and 20%, respectively, 

 of their organic carbon content during 

 leachino under sterile conditions (Robert- 

 son et al . 19C2). 



The carbon released as DOC is ex- 

 tremely labile and is rapidly assimilated 

 by microorganisms (Otsuki and k'etzel 1974; 

 Brylinsky 1977), which leads to its immed- 

 iate availability as food for secondary 

 consumers. In 14-day laboratory incuba- 

 tions, the HOC released by turtle grass 

 and manatee grass leaves supported 10 

 times more mfcrobial bionass per unit 



carbon than did the particulate carbon 

 fraction (Robertson et al. 1982). 



DOC may also become available to con- 

 sumers through incorporation into particu- 

 late aggregates. Microorganisms attached 

 to particles will assimilate DOC from the 

 water column, incorporating it into their 

 cells or secreting it into the extracellu- 

 lar materials associated with the parti- 

 cles (Paerl 1974, 1975). This microbial ly 

 mediated mechanism also makes seagrass DOC 

 available for consumers. 



In most marine systems the DOC pool 

 contains 100 times more carbon than the 

 particulate organic carbon pool (Parsons 

 et al. 1977; references therein). The 

 cycling of DOC and its utilization in de- 

 trital food webs are complex. The highly 

 labile nature of seagrass DOC suggests 

 that it may play a significant role in 

 supporting secondary productivity. 



Role of the Detrital Food Web 



The detrital food web theory repre- 

 sents our best understanding of how the 

 major portion of seagrass organic carbon 

 contributes to secondary productivity. The 

 organic matter of fresh seagrasses is not 

 commonly utilized by many animals because 

 of various factors, including their low 

 concentrations of readily available nitro- 

 gen, high concentrations of fiber, and the 

 presence of inhibitory compounds. The par- 

 ticulate and dissolved fractions of sea- 

 grass carbon seem to become potential food 

 for animals primarily after colonization 

 by microorganisms. During decomposition 

 the chemical nature of the detritus is 

 changed by two processes: loss of plant 

 compounds and synthesis of microbial pro- 

 ducts. 



The decomposer community also has the 

 enzymatic mechanisms and ability to assim- 

 ilate nutrients from the surrounding med- 

 ium, leading to the enrichment of the de- 

 tritus as a food source. As a result, the 

 decomposer community represents a readily 

 usable trophic level between the produc- 

 ers and most animal consumers. In this 

 food web, the consumers derive nutrition 

 largely from the microbial components of 

 the detritus. This decomposer community 

 is influenced by environmental conditions 

 and biological interactions, including the 

 feeding activities of consumers. 



74 



