Utilization of subsurface food resources for zooplankton reproduction: Implications for diel vertical migration theory

The water columns of lakes and oceans provide a diverse habitat gradient in which light, temperature, food, and predation risk all change with depth. Many planktonic organisms exhibit diel vertical migrations (DVM) in response to daily oscillations in many of these variables. DVM theory often assumes that surface waters are more food-rich than deeper, subsurface layers and proceeds to try to explain why zooplankton migrate out of these beneficial surface layers during the day. Here, we test the assumption that food is best in surface waters by feeding two common crustacean zooplankton with natural epilimnetic and metalimnetic food assemblages from a eutrophic lake and examining their egg production rates. Both Diaptomus and Daphnia showed greater reproductive rates in the metalimnetic water and significant food limitation in the epilimnetic water. Mass-specific ingestion rates were approximately three times higher in the metalimnion than in the epilimnion. In spite of the poorer food in the surface waters, these two crustaceans migrated into the epilimnion at night. These observations are contrary to the assumption that food is best in the surface water, and a review of the literature suggests that food frequently is not best in surface waters. The upward migrations at night are best explained by the warmer temperatures and reduced predation risk in the surface waters at night.