Nutrient inputs versus piscivore biomass as the primary driver of reservoir food webs

Abstract

We used an ecosystem-based modeling approach, Ecopath with Ecosim, to explore the relative importance of a top-down biotic management lever (top predator introduction) versus a bottom-up abiotic management lever (alteration of nutrient inputs) in regulating biomass in reservoir food webs. To do so, we modeled three Ohio reservoirs that varied in ecosystem productivity. For each, we simulated five hybrid striped bass (Morone chrysops × Morone saxatilis) (introduced top predator) biomass levels at three nutrient input levels (n = 15 simulations per reservoir). Nutrient inputs influenced the food web more than introduced predators within each reservoir. Further, across all three reservoirs, the impact of stocked hybrid striped bass on the equilibrium biomass of phytoplankton, prey fish (gizzard shad, Dorosoma cepedianum), and native top predators (e.g., largemouth bass, Micropterus salmoides) was <3%, <14%, and <20%, respectively, of the maximum impact of changes in nutrient inputs on these components. Thus, in mesotrophic to hypereutrophic reservoirs that are dominated by omnivorous gizzard shad, manipulating allochthonous inputs of nutrients offers agencies a more powerful means to regulate food web structure than manipulation of top predator biomass.