An abrupt change in zooplankton vertical migratory behavior at the end of the negatively stratified period in a deep lake

Abstract:

During a field campaign in Spring 2019, an Acoustic Doppler Current Meter was deployed to measure currents in the western part of Lake Superior. The ADCP also collected Returned Signal Strength Intensity, which can be used as a proxy for scatterers in the water column. A strong diel vertical migration signal was observed, likely due to migrating Mysids, a common zooplankton in Lake Superior. On 10 June 2019, the DVM signal suddenly weakened considerably, suggesting that the migrators are responding to an environmental cue. This date corresponds exactly to the end of the negatively stratified season at this site, when the lake transitions from negative stratification to neutral stratification. Conductivity-Temperature-Depth profiles of the water column prior to and subsequent to this date show that the distribution of phytoplankton biomass changes with the change in stratification. During negative stratification, phytoplankton are restricted to the convectively mixed surface layers, approximately 80m thick. Subsequently, during neutral stratification, convection mixes phytoplankton over the entire water column. We hypothesize that this sudden availability of food deep in the water column reduces the need for vertical migration on order to feed.