Diel vertical migration under the ice: A preliminary examination of zooplankton communities in a northern temperate lake in winter

Zooplankton exhibit daily movement patterns known as diel vertical migration (DVM).  DVM in stratified lakes is characterized by heterotrophic organisms exploiting the inverse relationship between light intensity and depth to avoid predators and forage for food.  Few DVM investigations have been conducted during periods of ice-cover.  Temperate lakes under ice-cover exhibit limited nutrient availability, inverse temperature stratification, and limited light conditions due to both ice cover and decreased photoperiod.  This suggests that DVM patterns could exhibit seasonal variance.  This study investigated DVM in an ice-covered lake to determine if winter patterns differ from those found in previous summertime studies.  We sampled zooplankton and abiotic variables below the ice in January 2007 at the University of Minnesota’s Lake Itasca field station.  Depth profiles of abiotic factors and the zooplankton community were taken at 07:00, 14:00, and 20:15 hrs from the surface to 4 meters below the ice.  Three classifications of zooplankton were compared: cladocerans, copepods and rotifers.  The overall zooplankton community exhibited statistically significant DVM.  Trends in zooplankton density varied significantly by depth, time of day, and class.  Water temperature and light intensity may influence winter DVM.  Rotifers exhibited the least DVM.  Copepods and cladocerans had more pronounced motility, with greater density closer to the surface in morning and afternoon and deeper in the evening.  The total plankton community was more abundant in shallower depths during the morning and afternoon.  In the evening, plankton were more abundant at lower depths. Our results suggest that winter DVM differs from summer patterns.