FOOD-WEB REGULATION BY A PLANKTIVORE - EXPLORING THE GENERALITY OF THE TROPHIC CASCADE HYPOTHESIS

The trophic cascade hypothesis currently being tested in north tempera te systems may not apply to open-water communities in lower latitude U .S. reservoirs. These reservoir communities differ dramatically from n orthern lakes in that an open-water omnivore, gizzard shad (Dorosoma c epedianum), often occurs in abundance. Neither controlled by fish pred ators (owing to high fecundity and low vulnerability) nor by their zoo plankton prey (following the midsummer zooplankton decline, gizzard sh ad consume detritus and phytoplankton), gizzard shad regulate communit y composition rather than being regulated by top-down or bottom-up for ces. In experiments across a range of spatial scales (enclosures, 1-9 m(2); ponds, 4-5 ha; and reservoirs, 50-100 ha), we evaluated the gene rality of the trophic cascade hypothesis by assessing its conceptual s trength in reservoir food webs. We reviewed the role of gizzard shad i n controlling zooplankton populations and hence recruitment of bluegil l, Lepomis macrochirus (via exploitative competition for zooplankton), and largemouth bass, Micropterus salmoides (by reducing their bluegil l prey). Reservoir fish communities, owing to the presence of gizzard shad, appear to be regulated more by complex weblike interactions amon g species than by the more chainlike interactions characteristic of th e trophic cascade.