Abstract
The seasonal occurrence, timing, and pattern of diel vertical migration of acoustically estimated zooplankton biomass at the edge of the continental shelf off the eastern United States is described based on the SEEP II (Shelf Edge Exchange Processes in the Southern Middle Atlantic Bight) data set obtained between February 1988 and May 1989. Zooplankton biomass was estimated using a bottom-moored, upward-looking 300
kHz acoustic Doppler current profiler deployed at the 90
m isobath. Vertical distributions of biomass were represented by the median depths of the acoustically estimated biomass which had been normalized relative to the maximum and minimum depths for each day and time standardized such that sunrise and sunset occurred at 0600 and 1800, respectively. Periods of diel vertical migration during the 15 month record were identified by determining daily least-squares fits of hourly vertical velocities to an ideal curve of migrating biomass velocities such that maximum upwards and downwards velocities of zooplankton biomass coincided with sunset and sunrise, respectively. Diel vertical migration occurred for 35, 15, and 20% of the days (
p<0.05) during each of the spring, summer, and fall/winter mooring deployments, respectively. The occurrence of diel vertical migration at the site appeared to be affected primarily by changes in the community composition of the zooplankton which was strongly influenced by the advection of different water types and associated zooplankton communities over the site. Little correlation was observed between the occurrence or magnitude of vertical migration and environmental parameters such as chlorophyll
a concentration and temperature. Daily vertical migrations were timed to the diel light cycle.
