Sampling zooplankton in the pelagic zone of lakes during the day is standard practice in limnology. However, zooplankton commonly exhibit diel vertical and horizontal migration, which may lead to biases in zooplankton demographic estimates collected during the day. We analyzed almost 900 paired night-day samples from the literature and found night crustacean zooplankton density and biomass estimates exceeded day estimates by approximately 255% and 125% on average, respectively, suggesting a potential systematic bias in such estimates and, by extension, rate processes derived from day sampling.

We present the first results of our research on day-night differences of crustacean zooplankton in Lake Glubokoe and Mozhaysk Reservoir. We found increases in pelagic crustacean zooplankton density and biomass at night in both lakes. Changes in quantity and species composition were more prominent and statistically significant in Mozhaysk Reservoir than in Lake Glubokoe. We observed only a slight increase in zooplankton density and biomass and a shift between dominant species distribution in Lake Glubokoe. The changes we observed appear to be caused by horizontal zooplankton migration.