Regulation of Reservoir Food Webs by External Nutrient Inputs

Ecosystem-based fisheries management is a holistic approach that allows agencies to account for broader food web effects on target fisheries.  Such an approach seems ideal for understanding and potentially forecasting the effects of planned management actions or unplanned perturbations to an ecosystem.  Herein, we used an ecosystem-based modeling approach, Ecopath with Ecosim, to explore the relative importance of a top-down biotic management lever (i.e., introduction of a top predator) versus a bottom-up abiotic management lever (i.e., alteration of nutrient inputs) in regulating food web structure in reservoir ecosystems.  To do so, we modeled three Ohio reservoirs that varied in system productivity and gizzard shad Dorosoma cepedianum abundance, a dominant omnivorous prey fish.  For each reservoir, we simulated three hybrid striped bass Morone chrysops x M. saxatilis (introduced top predator) biomass levels at three different nutrient loading levels (n = 9 simulations per reservoir).  In each reservoir, alternation of nutrients inputs had a larger impact on food web structure than altered top predator biomass, with the biomass of all functional groups in the model increasing with increased nutrient inputs.  These results indicate that, in mesotrophic to hypereutrophic reservoirs dominated by gizzard shad, manipulating allochthonous inputs of nutrients offers agencies a more powerful means to regulate food web structure than manipulation of top predator biomass.