MICHAEL H. PALLER, Environmental Sciences Section, Savannah River Technology Center, Westinghouse Savannah River Company, Aiken, South Carolina 29803, USA

The fish assemblage in L Lake, a 400-hectare former reactor cooling reservoir, has been periodically sampled since Steel Creek was impounded in 1985, permitting an evaluation of temporal changes in assemblage structure and the factors that influenced them. The fish assemblage in L Lake was initially composed largely of relatively small stream fishes (e.g., Lepomis marginatus) that were able to successfully colonize L Lake from Steel Creek. This assemblage was quickly replaced by larger species more typical of reservoir ecosystems (e.g., Micropterus salmoides) as they were intentionally or fortuitously introduced into L Lake. More recently, small species and phytophilous species (e.g., Labidesthes sicculus) have again increased in abundance as a result of the proliferation of aquatic vegetation following a macrophyte propagation program. The structure provided by the vegetation beds has increased species diversity by permitting colonization or recolonization by at least five species. Changes in the L Lake fish assemblage appear to be the result of predation and competition among species with the outcome of these processes strongly influenced by changes in the physical and chemical environment.