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

The horizontal distribution of crustacean zooplankton is known to vary spatially inlarge lakes. Patterns in smaller dimictic lakes and variation through time (hourlyand monthly) are less well established. Currents, chlorophyll and predators are allfactors that vary through time, potentially affecting zooplankton distribution instratified waters. Along an intermediate-length transect extending from the littoralarea to the pelagic zone, we measured crustacean zooplankton size and horizontalposition, as well as chlorophyll, currents and predators. We also assessed zooplank-ton vertical distribution patterns near each transect end. Horizontally, zooplanktonwere highly spatially structured in high summer, especially during daytime, whenthey preferred pelagic regions. A regular diel vertical migration pattern in earlysummer was replaced by an inverse one, suggesting a strong role for predation.Chlorophyll and zooplankton were inversely related, indicating a strong grazinginteraction. The weakest signal was detected for currents. Results indicate thatthere can be large spatial fluctuations in zooplankton abundance apparentlydriven by predation. Furthermore, zooplankton distribution correlates inverselywith chlorophyll only in the absence of a higher trophic level. We expect thatplankton distribution in many small dimictic lakes will similarly be dominated bybiotic factors, indicating that spatial variation should be taken into account whentrophic interactions are considered.KEYWORDS: spatial heterogeneity; temporal heterogeneity; predators; currents;chlorophyll