Linking individual foraging strategies with ecological dynamics: Quantifying zooplankton movements in heterogeneous resource distributions

A major challenge in biological oceanography is to predict consequences of environmental heterogeneity for the abundance, distribution and ecological function of planktonic organisms. In heterogeneous prey distributions such as phytoplankton thin layers, predators experience non-uniform rather than constant and average prey concentrations. Fluctuations in prey concentrations could have ramifications for predictions of trophic and demographic rates and impact the ecological dynamics of marine microbial food-webs. The goal of this study was to examine which behavioral strategies planktonic predators use to locate prey patches.; Empirical and analytical methods were developed to simultaneously quantify individual, three-dimensional movements and population distributions of microscopic organisms over large temporal (seconds to hours) and spatial scales (micrometers to meters). The heterotrophic dinoflagellate Oxyrrhis marina was observed to aggregate rapidly to 5-mm thin layers of phytoplankton prey within a 30-cm water column. Predator population abundance within prey thin layers increased up to 20-fold four hours after layers were introduced. Acquisition of very large (>106) sample sizes allowed statistical characterization of behavioral responses to prey and identification of possible mechanisms of aggregation. Individual predator movements, including increases in turning rates and decreases in vertical velocity, changed significantly in the presence of both prey cells and prey-derived filtrate. Predator population flux was not enhanced by the presence of increasing prey gradients in short-term experiments of 1.5 hours.; Individual-level movement statistics provided parameters for spatially explicit models to predict rates of population re-distribution over ecologically relevant scales. The model predictions suggest that behavioral heterogeneity amongst individuals plays a significant role in shaping population distributions. Improvement of these predictions rests on characterizing behavioral heterogeneity and switches in behavior. Linking individual behaviors with population level effects is a step towards linking ecological causes and effects that may be separated by vast spatial or temporal scales. For instance, estimates of predator growth and grazing rates increased by over one order of magnitude when predators' ability to locate prey patches was taken into account. Overall, these results support the notion that individual-level behaviors are quantitatively significant to ecological dynamics.