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6 BIOLOGICAL REPORT 90(5) Ecosystem Characteristics and Biological Productivity of Southeastern Coastal Plain Blackwater Rivers Arthur C. Benke by Department of Biology University of Alabama Tuscaloosa, Alabama 35487 Blackwater rivers in the Coastal Plain of the southeastern United States are characterized by broad floodplain swamp forests and the presence of large amounts of woody debris (snags) in the main channel. Since they are located in a relatively flat landscape with low gradient, Coastal Plain streams are not as subject to impoundment as streams and rivers in higher gradients. However, snagging, chan- nelization, and drainage of adjacent wetlands occur frequently in such systems, and all three can cause substantial ecological damage. The Satilla and Ogeechee rivers are blackwater river systems in the southeastern Coastal Plain. They are the two largest river systems in Georgia that remain free from any major impoundments from their headwaters to the sea. Furthermore, there have not been major attempts to clear snags or channelize most of the upper portions of these rivers. However, the lower section of the Satilla was regularly cleared of snags until the early 1950's. In both the Satilla and Ogeechee rivers, snags are the major site of invertebrate biomass, production, and diversity (Benke et al. 1984a, 1984b; Wallace and Benke 1984; Benke and Meyer 1988). River sedi- ments generally have lower production and diver- sity, although they constitute larger areas. Many of the snag animals are filter-feeding aquatic insects (e.g., caddisflies, black flies, midges, mayflies). Other snag-dwellers include invertebrate predators such as dragonflies, stoneflies, and hellgrammites. Further- more, the majority of drifting invertebrates (70-80% of numbers and biomass) originates from the snag habitat in the Satilla River (Benke et al. 1986). Also in the Satilla River, at least 50% of the prey of insec- tivorous fishes (e.g., bluegill, redbreast) originates from snags. Thus, invertebrate production-and hence probably much fish production-is directly related to the abundance of snags. Interactions with the adjacent river swamps (in- creasing river width by as much as 40 times) are extremely important to animal productivity in these Coastal Plain rivers. The major organic foods for snag invertebrates seem to originate from swamps in the form of suspended soil bacteria or fine par- ticulate organic matter (Edwards 1987; Edwards and Meyer 1987; Wallace et al. 1987). During flood periods, invertebrate production increases on the submerged swamp substrate and on wetted tree trunks. Inundation of the floodplain usually occurs for 3-4 months of the year and provides for a great expansion of feeding and spawning opportunities for many fish species. Long-term management of Coastal Plain rivers should emphasize protection of the floodplain for- ests, especially by discouraging snag removal and channelization. In areas that have been largely cleared of snags or channelized, fish and wildlife managers should consider the introduction of woody debris to reestablish productive invertebrate and fish habitat. Information Sources Benke, A. C., R. L. Henry, D. M, Gillespie, and R. J, Hunter. 1985. Importance of the snag habitat for animal production in a southeastern stream. Fisheries 10:8-13. Benke, A. C., R. J, Hunter, and F. K. Parrish. 1986, In- vertebrate drift dynamics in a subtropical blackwater river. J. North Am. Benthol. Soc. 5:173-190. Benke, A. C., and J. L. Meyer. 1988, Structure and func- tion of a blackwater river in southeastern USA. Proc. Int. Assoc. Theor. Appl. Limno!. 23:1209-1218. Benke, A, C" T. C. Van Arsdall, D. M. Gillespie, and F. K. Parrish. 1984a, Invertebrate productivity in a sub- tropical blackwater river: the importance of habitat and life history. Eco!' Monogr. 54:25-63. Benke, A. C., T. C. Van Arsdall, D. M. Gillespie, and F, K. Parrish. 1984b. Invertebrate productivity in a sub- tropical blackwater river. BioScience 34:443-444. Edwards, R. T. 1987. Sestonic bacteria as a food source for filtering invertebrates in two southeastern black- water rivers. Limnol. Oceanogr. 32:221-234.