Should we be sampling zooplankton at night?

Limnologists generally sample during the day. Consequently, many estimates of lake ecosystem processes are derived from daytime samples. Zooplankton, which play critical roles in many such processes, are typically sampled during the day in the pelagic zone, although they commonly exhibit diel vertical and horizontal migration. Demographic estimates using day pelagic sampling may be biased depending on the type and extent of migration. We compiled 870 paired night‐day zooplankton samples from the literature and found that crustacean zooplankton had, on average, 255% and 125% greater density and biomass, respectively, at night compared to day. Although limnologists have been aware that night zooplankton estimates are often higher than day estimates, our results provide quantitative estimates of the differences. Such underestimates from daytime sampling are alarming and call into question ecological rate processes derived from sampling zooplankton during the day.