DIEL VERTICAL AND HORIZONTAL MIGRATION BY ZOOPLANKTON - POPULATION BUDGETS AND THE DIURNAL DEFICIT

Diel horizontal and vertical distributions of zooplankton in Bullhead Pond, Rhode Island, USA were determined from March until June 1988, the period of occurrence of the major species of copepod, Diaptomus sanguineus. As water temperatures rise during this time period, activity of the major planktivore (sunfish) has been shown to increase, causing a drastic elevation in mortality rates of diaptomid copepods. Males did not undergo significant diel vertical migration (DVM) until June, while ovigerous females began to migrate in April. In March no diel differences in distribution patterns of ovigerous females were discernable and these copepods exhibited no directed movement patterns. Beginning in April, however, ovigerous female D. sanguineus were concentrated in the middle, deepest part of the lake during the day, and were found more evenly dispersed and near shore at night. This behavioral pattern should reduce the risk of predation by visually foraging planktivores in the euphotic and littoral zones of lakes. Evidence that horizontal migration behavior of D. sanguineus is due to the presence of fish is indicated by its response to a fish manipulation experiment in a neighboring pond (Little Bullhead Pond). During late April 1988, at noon D. sanguineus adults were concentrated deep in the middle of a quarter containing sunfish and evenly dispersed at night. In a quarter without fish, copepods were randomly dispersed during both day and night. Population budgets indicate that abundances of copepods at midnight were generally greater than at noon (i.e., a diurnal deficit). In both Bullhead and Little Bullhead Pond diurnal deficits were probably in part due to extreme migration by copepods into the sediments, and perhaps due to diel horizontal migrations. Taken together, results from Bullhead and Little Bullhead Pond suggest that this copepod avoids areas of highest predation risk and may be able to change behaviors as activity patterns of predators vary. Responses appear to depend on the life history phenology of the copepod as well as other possible anti-predator adaptations available.