EFFECTS OF AQUATIC INSECT PREDATORS ON ZOOPLANKTON IN FISHLESS PONDS

We removed the surface-orienting aquatic insects from a fishless pond to determine their predation effects on zooplankton behavior and size structure. A second fishless pond served as the unmanipulated referenc e system in this two year study. In the reference pond and the treatme nt pond prior to manipulation, daphnids exhibited pronounced diel vert ical migrations. Following the removal of surface-orienting aquatic in sects from the treatment pond, daphnid migration changed to a reverse migration strategy that was significantly different from that observed in the reference system. Average daphnid body size increased signific antly following predator removal in the treatment system. Our data ind icate that predation by aquatic insect predators, such as notonectids and dytiscids, may affect daphnid migration behavior in fishless syste ms. Vertical migration by daphnids may allow coexistence with surface- orienting insects in ponds that are deep enough to provide a spatial r efuge from these predators.