Identifying and quantifying unexpected deep zooplankton diel vertical migration in a large deep lake

Abstract Diel Vertical Migration (DVM), a widespread zooplankton behavior in freshwater and marine systems, affects ecological interactions and biogeochemical cycles. In lakes, DVM has mainly been studied in the upper 50 m of the water column. However, based on acoustic and net sampling data collected in Lake Geneva, Switzerland (~ 309 m depth) during summer 2022, we demonstrate that DVM occurs down to ~ 125 m depth daily throughout the summer season. The daily descents terminated at around zenith when the Relative Rate of light Change (RRC) was the lowest, and the late afternoon ascent started when RRC values exceeded −0.005 s −1 . DVM migration descent/ascent rates were asymmetric with faster mean upward rates () than downward rates (). Migration rates overall increased as summer progressed, corresponding to the intra‐seasonal increase in RRC. Cyclopoid copepods Cyclops prealpinus abundances correlated with the observed deep DVM and their migration responded to exogenous light cues. These new findings, which can also be expected to be relevant for other deep lakes, indicate that Lake Geneva's DVM greatly exceeds maximum migration depths previously reported for a lake. Thus, it is important to study zooplankton DVM dynamics throughout the entire water column in large, deep lakes since it plays an important role in buffering global climate change effects. Furthermore, it is suggested that present zooplankton DVM sampling protocols in large, deep lakes should be revised accordingly.