Food Web Feedbacks and Compensation in Trophic Cascades Driven By Zooplankton Size Structure

Fish are an organizing feature in aquatic food webs and capacity to respond to top predators has focused predominantly on environmental factors and food web linkages. Biotically driven compensatory mechanisms in trophic cascades are less well understood. A three by two factorial design was used to evaluate whether the initial size frequency distribution of zooplankton influences food web resilience to changes caused by planktivory, with particular focus on changes in phytoplankton chlorophyll a concentration and zooplankton biomass, size frequency distribution, and production. Specifically, effects of planktivorous bluegill (Lepomis macrochirus) were evaluated in three zooplankton treatments including small zooplankton (Moina micrura), large zooplankton (Daphnia pulex), and a mixture of the two. Bluegill preferentially consumed larger zooplankton and reduced zooplankton biomass in all treatments. Phytoplankton increased among all treatments with fish, but increased at a higher rate and obtained a greater terminal concentration in the large zooplankton only treatment. Weaker effects of planktivory on zooplankton production and phytoplankton in treatments with small and mixed size zooplankton, despite reductions in zooplankton biomass, are likely because reduced size corresponds with a higher ratio of production to biomass.