plant biomass increases to a maximum of about 

 700 g/m^ in late August and September. Flowering 

 begins at this time and subsequently most of the 

 plants die, reducing the biomass to about 200 g/m^ in 

 January. As a consequence of winter senescence, the 

 amount of dead grass increases rapidly to a peak in 

 late winter. This material is broken down during the 

 following spring and summer, and much of it is swept 

 out of the marsh, as shown by the high rate of disap- 

 pearance of detritus during AprU to July. The timing 

 of the detritus pulse from the marsh corresponds with 

 the arrival of migrating species from offshore such as 

 shrimp that find a ready supply of food (Odum 1967). 

 Thus plants produced in one year became available to 

 consumers the following spring. 



Energy Budget of Wetlands. The organic energy 

 budget of an ecosystem accounts for the amount of 

 organic energy fixed in plants by photosynthesis and 

 the relative energy demand by different consumers. 

 Unfortunately, detailed energy budgets have been 

 constructed for very few ecosystems. In wetlands, 

 only salt marsh budgets are well documented (Teal 

 1962, Day et al. 1973). Nevertheless, quantitative 

 energy budgets for wetland habitats in Louisiana 

 (fig. 4-12) have been estimated and give some indica- 

 tion of tlie relative importance of different wetland 

 processes. 



The following generalizations are drawn from 

 tliese diagrams: 



1 . Direct grazing consumes a small proportion 

 of plant production, ahhough that propor- 

 tion increases from salt to fresh marsh habi- 

 tats as the diversity of grazers increases. 



2. Consumer species of commercial interest 

 probably directly consume much less than 

 1% of net primary production. 



3. Most of the organic matter produced by the 

 system (something over 90%) is processed 

 through a detrital system (part 4.2.4). 



4. A major proportion of primary production 

 is respired to carbon dioxide by benthic and 

 epiphytic organisms, primarily microbial. 



5. Export and deposition of organic materials 

 in marsh sediment are inversely related and 

 together account for 20 to 40% of primary 

 production. Export predominates where 

 flushing action is strong; deposition where it 

 is weak. In the swamp forest habitat a major 

 portion of production is accumulated as 

 wood. 



4.2.4 THE DETRITUS-MICROBIAL COMPLEX 



In wetland habitats little of the total plant pro- 

 duction is consumed by grazers. Instead, plants die 

 and the resulting detritus is modified by microor- 

 ganisms before being consumed by other animals. In 

 effect, the microorganisms are the first consumers in 

 the detrital food web. Microorganisms feed on the 

 lignins and cellulose of the dead plants, converting 

 these compounds into microbial proteins, fats, and 

 sugars (fig. 4-13). 



During the process of enrichment, many small 

 consumers ingest the detrital material, skim off the 

 nutritious microorganisms, and egest the undigestible 

 plant remains. These are recolonized and the process 

 repeated (fig. 4-14; Fenchel 1970). This is true 

 whether the detritus lies on the marsh surface or is 

 carried by flood waters into adjacent water bodies. 



Metabolic Rates. Part of the organic matter in- 

 gested by microorganisms is used to fuel their meta- 

 bolic processes and is lost as heat. Estimates of this 

 heat loss are summarized by Payne (1970), who con- 

 cluded tliat a minimum of 40% of food energy is 

 converted to heat during growth of the microor- 

 ganisms. Gosselink and Kirby (1974) reported that 

 conversion efficiencies of smooth cordgrass detritus 

 to microbial biomass in laboratory cultures were as 

 high as 60%, but in actual marsh conditions it was 

 thought that a more realistic conversion rate was 

 about 25%. The conversion efficiency varies, depend- 

 ing on the degree of aeration of the marsh surface, 

 the flushing regime, the inorganic nitrogen available 

 to the decomposers, and other factors. On the average, 

 however, for every 4g (0.14 oz) of detrital food 

 assimilated by a consumer, some 3 g (0.1 1 oz) are lost 

 as heat. 



Although no data are available for other wetland 

 habitats. Day et al. (1977) found that the total 

 annual metaboUc loss (benthic respiration) in salt 

 marshes in Louisiana was about 600 to 718 g/m^, 

 while Teal (1962) found that the annual loss to 

 microbial respiration was only 730 g/m^ in a Georgia 

 salt marsh. 



Role of Benthic Organisms. Although microor- 

 ganisms are the first to colonize the dead plant tissue, 

 other benthic organisms are probably more important 

 in breaking down this matter to fine particulate 

 detritus. For instance, Fenchel (1970) showed that 

 the degradation rate in marine turtlegrass beds was 

 greatly increased when the leaves were exposed to 

 amphipods. Detritus particles may be further reduced 

 by grinding in digestive tracts of crustaceans (Odum 

 et al. 1972). Crayfish are probably very important in 

 breaking down leaf Utter in the swamp forest habitat. 



Summary. The deep layer of litter on the marsh 

 surface should be recognized as an active biochemical 

 factory (much Uke a cow's rumen), which transforms 

 nutritionally indigestible cellulose to useful protein. 

 The many small consumers Uving in tliis detritus are 

 an important source of protein for animals higher in 

 the food web. Events which impair the abihty of these 

 small consumers to transfonn litter have repercussions 

 throughout the food web. For instance, regular 

 flooding and draining of the marsh stimulates me- 

 taboUsm while continuous flooding results in oxygen 

 depletion and slower decomposition rates (Day et al. 

 unpubhshed). 



165 



