Meso-scale distributions of lake zooplankton reveal spatially and temporally varying trophic cascades

Abstract

The horizontal distribution of crustacean zooplankton is known to vary spatially in large lakes. Patterns in smaller dimictic lakes and variation through time (hourly and monthly) are less well established. Currents, chlorophyll and predators are all factors that vary through time, potentially affecting zooplankton distribution in stratified waters. Along an intermediate-length transect extending from the littoral area to the pelagic zone, we measured crustacean zooplankton size and horizontal position, as well as chlorophyll, currents and predators. We also assessed zooplankton vertical distribution patterns near each transect end. Horizontally, zooplankton were highly spatially structured in high summer, especially during daytime, when they preferred pelagic regions. A regular diel vertical migration pattern in early summer was replaced by an inverse one, suggesting a strong role for predation. Chlorophyll and zooplankton were inversely related, indicating a strong grazing interaction. The weakest signal was detected for currents. Results indicate that there can be large spatial fluctuations in zooplankton abundance apparently driven by predation. Furthermore, zooplankton distribution correlates inversely with chlorophyll only in the absence of a higher trophic level. We expect that plankton distribution in many small dimictic lakes will similarly be dominated by biotic factors, indicating that spatial variation should be taken into account when trophic interactions are considered. © The Author 2010.