One of the most striking characteristics of aquatic communities is the extreme spatial and temporal heterogeneity in zooplankton abundance. A major consequence of patchiness is that individuals differ in the densities and species they encounter, and therefore are likely to experience different competitive pressures and predation risks. The goal of this research is to determine whether patchiness in zooplankton communities affects the outcome of interactions among commonly co-occurring species under realistic conditions. The research combines field measurements and experimental laboratory manipulations. The field measurements characterize the neighborhoods (the number and composition of the zooplankton and phytoplankton in close proximity) experienced by each species in 2 dates during the growing season. The effect of sampling volume (1, 14 and 50 liters) and diurnal movements on estimates of spatial pattern (patchiness, number of other animals experienced by each individual, species specific tendencies to aggregate) will be tested. The experimental manipulations measure the outcome of 2 types of interactions that are more likely to be affected by close neighbors than by mean population densities: filtering and predation rate inhibition by neighbors. The appropriate neighborhood volume for measuring these 2 types of interactions will be experimentally quantified and compared to the field measurements of spatial pattern at the same scales when possible. This approach links patchiness in zooplankton abundance to the controlled study of species interactions under realistic field conditions. Measuring density dependent interactions among zooplankton is necessary if we are to expand the study of zooplankton interactions to include the striking spatial and temporal distribution patterns observed in nature.
