OBSERVATIONS OF A DEEP AGGREGATION OF CALANUS-PACIFICUS IN THE SANTA-BARBARA BASIN

An optical plankton counter/CTD package was used with zooplankton net samples to map the distribution of fifth copepodid (C5) Calanus pacifi cus in the Santa Barbara Basin region during two autumn cruises. Diapa using C5 C. pacificus were aggregated in a layer just above the basin' s oxygen-deficient bottom waters and below its sill depth. The maximal concentration measured was 6,900 ind. m(-3) from a net sample spannin g a depth range double the thickness of the C5 layer Although the C5 c oncentration varied, the layer was found at all stations of sufficient bottom depth within the basin. During November 1994, C5 C. pacificus accounted for 95-97% of all zooplankton caught in net samples from the layer. Relatively low concentrations of deep-dwelling C5 C. pacificus were observed at nearby stations outside the basin. We hypothesize th at C5 C. pacificus descend into the Santa Barbara Basin at diapause, a re trapped, and accumulate in a region of relatively low predator abun dance. The resultant aggregation is estimated to contain a significant fraction of the regional C. pacificus population and thus assumes an important role in its dynamics.