Effect of temperature on zooplankton vertical migration velocity

Zooplankton diel vertical migration (DVM) is an ecologically important process, affecting nutrient transport and trophic interactions. Available measurements of zooplankton displacement velocity during the DVM in the field are rare; therefore, it is not known which factors are key in driving this velocity. We measured the velocity of the migrating layer at sunset (upward bulk velocity) and sunrise (downwards velocity) in summer 2015 and 2016 in a lake using the backscatter strength (VBS) from an acoustic Doppler current profiler. We collected time series of temperature, relative change in light intensity chlorophyll-a concentration and zooplankton concentration. Our data show that upward velocities increased during the summer and were not enhanced by food, light intensity or by VBS, which is a proxy for zooplankton concentration and size. Upward velocities were strongly correlated with the water temperature in the migrating layer, suggesting that temperature could be a key factor controlling swimming activity. Downward velocities were constant, likely because Daphnia passively sink at sunrise, as suggested by our model of Daphnia sinking rate. Zooplankton migrations mediate trophic interactions and web food structure in pelagic ecosystems. An understanding of the potential environmental determinants of this behaviour is therefore essential to our knowledge of ecosystem functioning.