This proposal seeks to extend ongoing work in reservoirs and small impoundments, pursuing an evaluation of the generality of the trophic cascade hypothesis. This hypothesis is currently being tested in north temperate open-water and inshore, benthic communities. Southeastern reservoirs, the focus of this work, differ dramatically from northern lakes in that an open-water omnivore, gizzard shad, occurs in abundance. Neither controlled by fish predators (owing to impressive fecundity coupled with a narrow window of vulnerability to predators) nor by their zooplankton prey (following the mid-summer zooplankton crash, shad switch to detritus and phytoplankton), gizzard shad regulate community composition via "middle-out" processes, rather than being regulated by top-down or bottom-up forces. These investigators have designed a series of experiments across a range of spatial scales (1 m3 to 100 hectares) to evaluate whether gizzard shad can control zooplankton populations and hence recruitment of bluegill (via exploitative competition for zooplankton) and largemouth bass (by reducing their bluegill prey). Preliminary evidence gathered during the past 5 years strongly supports the perspective that gizzard shad may well drive community composition in Ohio reservoirs, thus serving as a "critical" species that may well operate in a "middle-out" fashion. The experimental approach of this study will evaluate the generality of the cascade hypothesis by assessing its conceptual strength in reservoir food webs comprised of weak biotic linkages.
