(leaves, twigs) is in the form of coarse 

 particulate organic matter (CPOM, particle 

 size >1 mm), fine particulate organic 

 matter (FPO^:, particle size <1 mm), or 

 dissolved organic matter (DOM, parti- 

 cles <C.5 microns) (Cummins and Spengler 

 1978). Following autumn leaf fall, leaves 

 are first enriched by bacteria and aquatic 

 hyphomycete fungi which partially digest 

 leaf tissue and build their own cellular 

 protein. Both the quality of POM and the 

 rate of its formation depend on the tree 

 species involved. Elm, ash, and maple 

 leaves disappear faster than oak and beech 

 (Kaushik 1975). Floodplain tree diversity 

 thus insures food over a longer time span. 

 Insect larvae called shredders (some Tri- 

 choptera, Plecoptera, and Diptera) as well 

 as crayfish and amphipods (Thomas 1975) 

 then reduce the leaves to FPOM. In the 

 meantime some scrapers such as snails may 

 "graze" on the attached periphyton (dia- 

 toms) on the CPOH particles. The feces of 

 both groups become FPOM. 



Quantitative estimates of POM are 

 limited. Wallace et al. (1977) found 7.8 

 X 10 particles per liter in the Altam.aha 

 River (GA) in April. Other data suggest 

 that 11,000 kg/day move down the Altamaha 

 (Wharton 1980). Though particulate organic 

 matter is probably the most important 

 source of carbon to the floodplain trophic 

 system, DOM is a far more abundant source 

 (about 257,000 kg/day transport estimated 

 for the Altamiaha River; Wharton and Brin- 

 son 1979a). Labile DOM fractions can be 

 removed by some organisms (Lush and Hynes 

 1973, 1978; Lock and Hynes 1976; Sepers 

 1977); much of the DOM, however, may not 

 be usable by the biotic comn.unity because 

 of their refractory nature. Most of the 

 DOM (up to 30%-40% of their dry weight) 

 leaches out of leaves within a few days 

 after falling into the water. Depending 

 on the pH and the presence of divalent 

 cations such as calcium, some DOM can 

 flocculate and form clumps of FPOM. These 

 abiotic aggregates can then be colonized 

 by bacteria and fungi in quiet backwaters 

 (Kaushik 1975) and consumed by filter 

 feeders. 



Whatever the source, FPOM is used by 

 detrital processors which obtain it from 

 the sediments (some Epheirieroptera and 

 Diptera) or filter it from suspension 

 (some Ephemeroptera, Trichoptera, and 

 Diptera) (Cummins 1973). While some 



caddisflies and other organisms are algal 

 grazers, and blackfly larvae (Simulium) 

 can even trap bacteria, it is the FPOM 

 which fuels the river channel ecosystem. 

 In the permanent waterways (Zone I) an 

 innumerable host of larval blackflies, 

 caddisflies, stoneflies, m,ayflies, midge 

 flies, and adult clams screen these tiny 

 fragments from the water as their energy 

 source. Incredible densities of organisms 

 are supported on snags (20,000/m2), and in 

 sands (40,0G0/m2) of the Satilla River 

 (Benke et al. 1979). Figure 53 indicates 

 the dominant species which live on snags 

 in a blackwater river and in bottom sands 

 and also portrays the dominant organisms 

 which wash down (drift fauna) in both 

 blackwater and alluvial rivers. 



The high productivity of blackwater 

 streams has been documented (Beck 1965; 

 Holder et al. 1970, 1971; Benke et al. 

 1979). The blackwater Okefenokee Swamp 

 has predominantly peat substrates and a 

 summer pH as low as 3.8 (normal pH 4.0), 

 yet has an abundant fauna of amphipods, 

 freshwater shrimp, insect larvae, ancylid 

 snails (F.C. Parrish, Department of Bio- 

 logy, Georgia State University, Atlanta; 

 personal communication), and fish (Wharton 

 1977). Furthermore, the swamp is filled 

 with carnivorous plants such as Utricu- 

 laria , indicative of low nitrogen levels. 

 Some biologists are surprised that such 

 apparently nutrient-poor waters can have 

 such a large and varied fauna. Evidently, 

 since blackwater systems recycle nutrients 

 poorly by conventional means (decomposi- 

 tion and mineralization of plant debris) 

 and have minimal inputs of inorganic 

 nutrients from the watershed, a conserva- 

 tion strategy has evolved. Animals in the 

 food web consuming detritus and/or aggre- 

 gated particles of DOM thus incorporate 

 the large amounts of organic nitrogen 

 present. Organic nitrogen is obtained from 

 animals directly (insectivorous plants) or 

 indirectly (animal excretory products); 

 thus this limited nutrient is recycled and 

 conserved. Other food chains involving 

 grazing of both algae and higher plants 

 may be present, but their importance is 

 undocumented. 



Apart from their toxicity, radionu- 

 clides (5°Sr. 137CS, 60co, 1311, i"OBa, 

 3H) are of interest in tracing the fate of 

 minerals in the floodplain ecosystem 

 (Garten et al. 1975; Pinder and Smith 



98 



