Zooplankton responses to predation by larval bluegill: an enclosure experiment

SUMMARY 1. Larval fish are gape‐limited predators that forage on prey of specific sizes, and thus may be expected to differentially affect members of a zooplankton community, possibly altering the size‐structure or species composition. 2. I used an enclosure experiment to look at the effect of predation by larval bluegill on the dynamics of two zooplankton communities, one dominated by large‐bodied individuals and the other by small‐bodied individuals. Enclosures containing these zooplankton received a zero, low, medium, or high density of larval bluegill predators. 3. Increasing larval density had a negative effect on zooplankton abundance and abundance declined similarly in the large‐bodied and small‐bodied communities. 4. Zooplankton size‐structure, as estimated by the length of the average zooplankton, increased and then decreased during the experiment, decreasing faster at higher larval fish densities. When zooplankton size‐structure was estimated as the length of the average cladoceran, size‐structure declined in the large‐bodied but not in the small‐bodied community and the greatest decline in size‐structure was seen in the medium and high larval density treatments. 5. Ordination of each community using multidimensional scaling (MDS) indicated that the trajectory of change in species composition differed between the presence and absence of larval fish. In both communities, the degree of response by individual taxa depended on the density of bluegill larvae. This effect on zooplankton abundance, size‐structure and community composition suggests that larval fish may make an important contribution to zooplankton dynamics in many lakes and ponds.