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 gradie nt in which light, temperature, food, and predation risk all change wi th 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 deep er, subsurface layers and proceeds to try to explain why zooplankton m igrate out of these beneficial surface layers during the day. Here, we test the assumption that food is best in surface waters by feeding tw o common crustacean zooplankton with natural epilimnetic and metalimne tic food assemblages from a eutrophic lake and examining their egg pro duction 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 t hree times higher in the metalimnion than in the epilimnion. In spite of the poorer food in the surface waters, these two crustaceans migrat ed into the epilimnion at night. These observations are contrary to th e assumption that food is best in the surface water, and a review of t he literature suggests that food frequently is not best in surface wat ers. The upward migrations at night are best explained by the warmer t emperatures and reduced predation risk in the surface waters at night.