SEASONAL-VARIATIONS IN A DEEP AGGREGATION OF CALANUS-PACIFICUS IN THESANTA-BARBARA BASIN

A deep aggregation of fifth copepodid (C5) Calanus pacificus in the Sa nta Barbara Basin (SBB) was mapped over 1 1/2 yr with zooplankton net tows, an optical plankton counter, and a moored acoustic Doppler curre nt profiler. High concentrations of diapausing C5 C. pacificus built u p in the deep waters of the SBB during the summer and into the fall. D uring the buildup, the deep aggregation moved up from the bottom as ox ygen became depleted in the basin's deep waters. The deep aggregation apparently builds up due to the basin trapping C5s that migrate below the sill depth from water advected over the basin. C5s are retained wi thin the basin until they swim, or are forced, above the sill depth. P ossible mechanisms responsible for the dispersal of the deep aggregati on are migration to the surface waters with subsequent dispersal by su rface currents, advection over the basin's sill due to the buildup of oxygen-deficient water, or flushing of the basin's deep water.