Springtime Crustacean Zooplankton Abundance under Cold Water Conditions Is Determined Mainly by Temperature Regardless of Fish Predation

The seasonal succession of plankton communities is initiated as temperature starts to increase in early spring. Here, we examined the abundance of crustacean zooplankton in six large Finnish boreal lakes a few days after the melting of the ice for a period of ten years. We related zooplankton abundance to relevant abiotic and biotic factors, including temperature, wind and predation by fish and assessed the annual variability in zooplankton abundance at three spatial scales (lake, part of a lake, and habitat) and one temporal scale (year). Zooplankton abundance was positively associated with water temperature regardless of fish predation pressure and the spatial scale examined. This finding is significant as mean water temperature in the lakes concerned was very low (5.7 ° C), although the observed temperature range was actually quite wide (2-13 ° C). Predation by newly-hatched coregonids affected zooplankton density only in one lake. The effect of wind in one lake was significant in the littoral but not in the pelagic habitat. Temperature may be the ultimate factor determining zooplankton dynamics, particularly under cold water conditions.