Seasonal variations in a deep aggregation of Calanus pacificus in the Santa Barbara Basin

Abstract

A deep aggregation of fifth copepodid (C5) Calanus pacificus in the Santa Barbara Basin (SBB) was mapped over 1 1/4 yr with zooplankton net tows, an optical plankton counter, and a moored acoustic Doppler current profiler. High concentrations of diapausing C5 C. pacificus built up in the deep waters of the SBB during the summer and into the fall. During the buildup, the deep aggregation moved up from the bottom as oxygen 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 within the basin until they swim, or are forced, above the sill depth. Possible mechanisms responsible for the dispersal of the deep aggregation are migration to the surface waters with subsequent dispersal by surface currents, advection over the basin's sill due to the buildup of oxygen-deficient water, or flushing of the basin's deep water.