The Aggregation of Fish in the Metalimnion Constrains Zooplankton Diel Vertical Migration

ABSTRACT The role of fish predation in zooplankton diel vertical migration (DVM) is well established, where zooplankton move to deep waters during the day—to hide from visual predators in surface layers—and return to the epilimnion at night (i.e., ‘normal’ DVM), to feed on phytoplankton. A second pattern exhibited by some zooplankters—a ‘reverse’ DVM—occurs in response to invertebrate predators performing normal DVM. However, these patterns may not be evident in temperate lakes colonized only by cold‐stenothermic fish, which avoid the epilimnion during summer because temperatures are too warm. In this case, the surface layers could become a refuge to predation for zooplankton, modifying patterns of zooplankton DVMs. We examined the mechanisms behind DVM patterns of different zooplankton taxa by size classes using high‐frequency in situ imagery and acoustic telemetry to assess the vertical distribution of zooplankton and the cold stenothermic brook charr ( Salvelinus fontinalis ), respectively. Vulnerable taxa such as large cladocerans performed a DVM but within the epilimnion for the whole 24 h cycle during summer, staying away from areas occupied by fish. Less vulnerable zooplankters (i.e., small‐sized individuals or taxa with morphological or behavioural defences) showed a reverse DVM, influenced by temperature and invertebrate predation. In autumn, large cladocerans changed their DVM behaviour in response to the depth change in fish distribution, when the epilimnion cooled and became available to brook charr. Our results highlight the importance of zooplankton DVM plasticity in response to predation risk and inform on an undocumented zooplankton DVM pattern in a stratified temperate lake with deep chlorophyll maximum and cold‐stenothermic predators.