A comprehensive evaluation of Daphnia pulex foraging energetics and the influence of spatially heterogeneous food

Abstract

Aquatic organisms are distributed in a heterogeneous, nonrandom manner, and zooplankton are known to seek out and orient themselves within regions of high food concentration. Yet, the energetics associated with this foraging behavior are unknown. We hypothesized that zooplankton foraging behavior increases foraging efficiency measured as food consumption per unit effort. In this study, we measured the energetic costs and benefits for the zooplankton Daphnia pulex foraging in a range of algal concentrations to determine the net energetic benefit of foraging in algal patches. The net energy benefit of foraging increased significantly with increases in algal concentration; this trend was driven by significant increases in prey ingestion with algal concentration, whereas foraging costs did not change. Even considering corrections for changes in assimilation efficiency, foraging in algal patches greatly increases net foraging benefit compared to foraging in low concentrations of algae and thus has the potential for higher reproduction and greater growth in filter-feeding zooplankton.