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10 BIOLOGICAL REPORT 90(5) Aquatic and Terrestrial Linkages: The Role of Floodplains J. Vaun McArthur by Savannah River Ecology Laboratory Drawer E A iken, South Carolina 29801 Linkages between terrestrial and aquatic ecosys- tems occur at several orders of scale. At a landscape scale, stream basin characteristics control longi- tudinal and lateral (floodplain) interactions among aquatic habitats. The amount of organic matter input from the basin is dependent on the shape, recent use history, and nature ofthe floodplain. Lateral move- ment of organic matter has been severely affected in many river systems by levee districts, which have effectively blocked the stream from its valley. The nature and timing of organic matter input into lotic ecosystems will directly affect processing this material. Changes in the species composition of the riparian forest will affect the invertebrate assem- blages in the stream because invertebrate life his- tories have evolved to respond to the resources that have historically been available. Changes in the in- vertebrate community can directly affect the fish assemblages. Removal of snag habitat has been a major cause in the decoupling of terrestrial and aquatic interactions. Debris dams serve as habitat for fish and provide sites for accumulation and subsequent processing of organic matter. The closest link between terrestrial and aquatic ecosystems is bacteria. It has been shown that bacteria can only respond to natural sources of organic matter to which they have been previously exposed. This response is both physiologic and genetic. Populations of bacteria appear to be adapted to the organic matter found in the adjacent riparian and floodplain habitats. By changing the source of organic matter, the aquatic systems become less efficient in processing, and the greater portion of material is lost. As processing efficiency declines, there is less secondary production of in- vertebrates, which will lead to a decrease in fish and other vertebrates that rely on the invertebrate biomass. More research is needed to determine how tight the linkage between terrestrial and aquatic systems is and how changes affect higher trophic levels. Clearly, management of floodplain and riparian forests will affect all trophic levels in stream systems, and it is important that we document these effects so that effective management practices can be developed. Information Sources McArthur, J. V, 1989. Aquatic and terrestrial linkages: floodplain functions. In D. D. Hook and R. Lea,eds. Pro- ceedings of the symposium: the forested wetlands of the southern United States. U,S, For. Serv.Gen. Tech. Rep, SE-50. McArthur, J. V., D, A. Kovacic, and M. H. Smith. 1988. Genetic diversity in natural populations of a soil bacterium across a landscape gradient, Proc. N at. Acad. Sci. 85:9621-9624. Minshall, G. W, 1988, Stream ecosystem theory: a global perspective. J. North Am. Benthol. Soc. 7:263-288. Minshall,G, W., R. C.Petersen, K. W. Cummins,T. L. Bott, J. R. Sedell, C. E. Cushing, and R. L. Vannote. 1983. Interbiome comparison of stream ecosystem dynamics, Ecol. Monogr. 53:1-25. Naiman, R J., H. Decamps, J. Pastor, and C. A. Johnston. 1988. The potential importance of boundaries to fluvial ecosystems,J. North Am. Benthol. Soc. 7:289-306. Webster, J. R., and E. F. Benfield. 1986. Vascular plant breakdown in freshwater ecosystems. Annu. Rev. Ecol. Syst. 17:567-594. ~