PATTERNS AND OCCURRENCE OF DIEL VERTICAL MIGRATION OF ZOOPLANKTON BIOMASS IN THE MID-ATLANTIC BIGHT DESCRIBED BY AN ACOUSTIC DOPPLER CURRENT PROFILER

The seasonal occurrence, timing, and pattern of diel vertical migratio n of acoustically estimated zooplankton biomass at the edge of the con tinental shelf off the eastern United States is described based on the SEEP II (Shelf Edge Exchange Processes in the Southern Middle Atlanti c Eight) data set obtained between February 1988 and May 1989. Zooplan kton biomass was estimated using a bottom-moored, upward-looking 300 k Hz acoustic Doppler current profiler deployed at the 90 m isobath. Ver tical distributions of biomass were represented by the median depths o f the acoustically estimated biomass which had been normalized relativ e 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 id entified by determining daily least-squares fits of hourly vertical ve locities to an ideal curve of migrating biomass velocities such that m aximum upwards and downwards velocities of zooplankton biomass coincid ed with sunset and sunrise, respectively. Diel vertical migration occu rred for 35, 15, and 20% of the days (p < 0.05) during each of the spr ing, summer, and fall/winter mooring deployments, respectively. The oc currence of diel vertical migration at the site appeared to be affecte d primarily by changes in the community composition of the zooplankton which was strongly influenced by the advection of different water typ es and associated zooplankton communities over the site. Little correl ation was observed between the occurrence or magnitude of vertical mig ration and environmental parameters such as chlorophyll a concentratio n and temperature. Daily vertical migrations were timed to the diel li ght cycle. (C) 1998 Elsevier Science Ltd. All rights reserved.