Cooperative Fish and Wildlife Research Units Program: all
Education, Research and Technical Assistance for Managing Our Natural Resources

Diel migrations of zooplankton occur throughout marine and freshwater systems, and can complicate inferences from studies that focus on daytime observations. Sampling only during daytime can mischaracterize zooplankton distributions and abundances if populations perform vertical or horizontal diel migrations. Zooplankton often display reduced capture avoidance at night in addition to occupying areas easier to sample near the surface and away from littoral structure and the benthos. We examined zooplankton during daytime and nighttime new moon periods using discrete depth sampling in oligo-mesotrophic reservoirs in Oregon, USA. These reservoirs have limited littoral structure, but support populations of zooplanktivorous fishes that we expected to drive diel vertical migrations. Contrary to our expectations, most zooplankton taxa were within 2 meters of their daytime distributional peak and did not display significant differences in abundance from day to night sampling. We consider factors that may help predict whether diel migration occurs in a system. Where daytime sampling is sufficient for characterizing zooplankton densities and distributions, costs and risks specific to nighttime sampling may be avoided. Improving our understanding of zooplankton dynamics, particularly in ecosystems with limited diurnal variability, is an important part of understanding lake and reservoir food webs and can optimize the efforts of future studies.