Season and presence of Gizzard Shad influence horizontal spatial distribution of zooplankton in reservoirs of the midwestern United States

We contrasted near- and offshore zooplankton biomass and assemblage structure in 15 reservoirs in the central United States. In particular, we evaluated whether season and the dominant pelagic planktivore, Gizzard Shad (Dorosoma cepedianum), influenced near- and offshore zooplankton dynamics in reservoirs. Independent of Gizzard Shad, nearshore areas had greater zooplankton biomass in all seasons than did offshore areas. This difference was consistent for all cladocerans, copepods, rotifers, and all other zooplankton combined (including Chaoborus, ostracods, Trombidiformes, and mussel veligers). Gizzard Shad presence drove the spatial dynamics of major taxonomic groups, and time of year also influenced the structure of zooplankton assemblages. Additionally, Gizzard Shad presence was correlated with more copepod and rotifer taxa than cladoceran taxa. The relative distribution of these taxa at near- and offshore sites, however, remained consistent whether or not Gizzard Shad were present. Gizzard Shad had the most pronounced effect on zooplankton assemblage structure in the middle of the growing season. Mean annual zooplankton biomass per unit volume was approximately 50% greater in nearshore areas than in offshore areas for reservoirs with and without Gizzard Shad. Our findings, therefore, show that dominant planktivores can alter the assemblage structure and distribution of zooplankton in reservoirs, as they do in natural lakes.