filainentous fungi to yeasts changed 

 predictably in different wetlands 

 depending on their history. 



Mixed with these decomposers on the 

 soil surface is an active community of 

 autotrophic algae, chiefly diatoms, that 

 enter the food web at the same level as 

 the decomposers and may be an important 

 additional energy source. Most 



investigators, however, are concerned more 

 with the biochemical activity mediated by 

 the microbiota than with species 

 identification. They are satisfied to get 

 some relative index of microbial bioTiass 

 like that afforded by total ATP activity, 

 or to characterize the microbiota by their 

 chemical activity (White et al. 1979). 



The decomposition of underground 

 biomass has been studied very little. No 

 studies are available from the Louisiana 

 delta marshes. The best infonnation on 

 the subject comes from studies in Atlantic 

 coast salt marshes summarized by Valiela 

 et al. (1982), Teal (1983), and Howarth 

 and Hobbie (1982). 



Since the soil environment is anoxic, 

 most of the decomposition must be anaero- 

 bic. The leaching phase of decomposition 

 is the same as aboveground, but subse- 

 quently the disappearance of organic 

 material is slower. Nitrogen stimulates 

 the decomposition rate, indicating that it 

 is limiting belowground as well as in an 

 aerobic environment. One reason is that 

 nitrate may control the metabolic rate by 

 acting as an electron acceptor in the 

 absence of oxygen. Most underground pro- 

 duction, however, is decomposed through 

 the fennentation and sulfate reduction 

 pathways (Howarth and Teal 1979). 



CONSUMERS 



Benthos 



In terms of energy transfer it is 

 assumed that' the microflora act as the 

 intermediary between the organic 



production of the higher plants and the 

 higher trophic levels. At first 



investigators thought that the macroscopic 

 deposit feeders were ingesting 



bacteria-laden detritus; skimming the 

 bacteria from it; and fragmenting. 



packaging, and inoculating the detritus 

 with bacteria in fecal pellets. 



It appears now that bacterial density 

 is too low on most detrital material to 

 provide a sufficient food source for the 

 macro-benthos (Wiebe and Pomeroy 1972). 

 This change in viewpoint is reflected in 

 the trophic diagram of Figure 54. The 

 meiofauna are seen to have a crucial role 

 in energy transfer (1 in Figure 54). They 

 are distinguished from macrofauna 



primarily by size. Both are found in or 

 on the substrate during all or part of 

 their life cycles. Meiobenthos are 

 generally microscopic; macrobenthos are 

 larger and include such taxonomic groups 

 as snails, mussels, and crabs. 



Sikora et al . (1977) found that 

 meiobenthic nematodes account for 70 - 90 

 percent of ttie sediment ATP, indicating 

 that nearly all living biomass in anoxic 

 marsh sedi;nents is meiofaunal, not 

 bacterial. These organisms are thought 

 to be siiial 1 enough to graze the bacteria 

 efficiently and "package" that organic 

 energy supply in bite-sized portions for 

 slightly larger macrobenthic deposit 

 feeders (3 in Figure 54). 



Sikora (1977) showed that the chelae 

 of the grass shrimp (Pal eomonetes spp.) 

 are about the right size to capture 

 nematodes and speculated that grass shrimp 

 are more likely to use this food than 

 detritus. Bell's study (1980) supports 

 this idea. She found that meiobenthic 

 polychaete and copepod densities increased 

 in caged exclosures that reduced macro- 

 faunal predation. Gut analyses seldom 

 turn up nematodes, the dominant meiofaunal 

 taxon, but this is probably because their 

 soft bodies are dissolved rapidly. Macro- 

 benthic deposit feeders are thus ingesting 

 and using as an energy source meiofauna, 

 which in turn have been cropping bacteria. 

 The deposit feeders themselves are prey 

 for the many small fish, shellfish, and 

 birds that use the marsh, marsh creeks, 

 and small marsh ponds (3 and 4, Figure 

 54). Although apparently each step in 

 this energy transfer can be quite 

 efficient - net growth efficiencies up to 

 50 percent for bacteria (Payne 1970), 38 

 percent for nematodes (Marchant and 

 Nicholas 1974) - the trophic pathway from 

 detritus to microbes to meiofauna to 



60 



