freshwater, terrigenous salt marsh and 

 eelgrass sources. 



The organisms primarily responsible 

 for the initial decomposition of detrital 

 material on tidal flats are a wide variety 

 of microorganisms, mainly fungi and bacte- 

 ria. Fungi are associated with decompos- 

 ing vascular plant material and breakdown 

 cellulose by extending their hyphae into 

 the detrital fragments. Fungi adhering to 

 other particles, such as organic-encrusted 

 mineral grains, are less common in tidal 

 flat sediments (Johnson 1?74). Bacteria 

 are associated with the interstitial water 

 found in sediments as well as the external 

 surface of detrital particles and the con- 

 cave surfaces of mineral grains (Johnson 

 1974). Studies have shown that bacterial 

 standing stock is inversely correlated 

 with particle size in marine sediments 

 (e.g.. Dale 1974). Presumably such a rela- 

 tionship exists because of the increased 

 surface-to-volume ratio of the smaller 

 particles resulting in increased area per 

 unit volume of sedimenc for bacterial 

 colonization and growth. Finer-grained 

 sediments, therefore, have more abundant 

 bacterial populations than coarser-grained 

 sediments. Bacteria are also more abun- 

 dant at the surface of sediments than at 

 depth (Rublee and Dornseif 1978) probably 

 because of the greater amount of detrital 

 material found in near-surface sediment 

 layers (Whitlatch 1981). 



Decomposition rates of detritus are a 

 function of the type and source of the 

 organic substrate, physical and chemical 

 conditions, and the density and type of 

 organism feeding upon the matrix of living 

 and non-living organic material. Detrital 

 material entering tidal flats from terres- 

 trial sources is more resistant to decom- 

 position than much marine-derived detrital 

 material. Terrestrial plants build more 

 structural polymers (e.g., lignins) than 

 marine plants and are much more resistant 

 to bacterial decomposition (MacCubbin and 

 Hodson 1?80). Larger organisms (e.g., 

 invertebrates) feeding upon detrital mate- 

 rial have been shown to accelerate the 

 decomposition process through the reduc- 

 tion of particle size, exposure of grazed 

 surfaces to microbial activity, and 

 selective foraging upon fast-growing 

 microbial cells (Fenchel 197C, 1972; 

 Fenchel and Harrison 1976; Lopez et al. 

 1977). 



The decomposers perform several vital 

 functions in marine coastal habitats. 

 First, microbial decomposition of plant 

 material serves as the primary link be- 

 tween primary and secondary production 

 (Cdum and de la Cruz 1967). Many studies 

 have demonstrated that only small percent- 

 ages of plant material are consumed while 

 plants are living but that after death and 

 physical-biological fragmentation, plant 

 material serves as an energy source for 

 the microbial and fungal populations in 

 the sediment. The resultant microbial 

 activity breaks down detritus and enhances 

 its nutritive value as a food source for 

 many other species of organisms. Second, 

 during the decomposition process, the 

 microbiota convert dead organic material 

 into nutrients that can be utilized by 

 primary producers. Loder and Gilbert 

 (1980), for example, calculated that 7% of 

 the dissolved phosphate entering Great Bay 

 Estuary, New Hampshire, came from the 

 estuarine sediments. Zeitzschel (1980) 

 recently suggested that 30% to 100% of the 

 nutrient requirements of shallow-water 

 phytoplankton growth comes from the sedi- 

 ments. Release of nutrients from the 

 sediment may also be important for tidal 

 flat macroalgal production (B.L. Welsh; 

 University of Connecticut, Avery Point, 

 Groton; February 1981; personal communica- 

 tion). Bacteria can also convert dissolved 

 organic materials from the water column 

 into particulate biomass. While the impor- 

 tance of dissolved organic material in 

 shallow-water marine environments is not 

 fully understood, many types of marine 

 invertebrates can utilize these substances 

 as a food source (Stephens and Schinske 

 1961; Stephens 1975). Tidal flat inverte- 

 brates have well-developed digestive sys- 

 tems for the ingestion of particulate 

 material and it is thought that bacteria 

 can outcompete many of these organisms for 

 dissolved organic material in marine sedi- 

 ments (Fenchel and J0rgensen 1977). Last, 

 the net effect of having bacteria and 

 fungi at the base of the decomposer food 

 web is a stabilization of energy transfer 

 to higher trophic levels within the tidal 

 flat habitat. The availability of food for 

 consumers is not restricted to the growing 

 season of a temperate climate. The energy 

 tied up in the primary detrital fraction 

 is slowly released depending on the rate 

 of microbial degradation to become avail- 

 able to higher trophic levels throughout 

 the year. 



13 



