Observations of a deep aggregation of Calanus pacificus in the Santa Barbara Basin

Abstract

Abstract-An optical plankton counter/CTD package was used with zooplankton net samples to map the distribution of fifth copepodid (C5) CuZanus paci$cus in the Santa Barbara Basin region during two autumn cruises. Diapausing C5 C. puci$cus 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 -' from a net sample spanning a depth range double the thickness of the C5 layer. Although the C5 concentration varied, the layer was found at all stations of sufficient bottom depth within the basin. During November 1994, C5 C. pucijcus 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 that C5 C. paci$cus descend into the Santa Barbara Basin at diapause, are trapped, and accumulate in a region of relatively low predator abundance. The resultant aggregation is estimated to contain a significant fraction of the regional C. pucijcus population and thus assumes an important role in its dynamics.Cdanus paciJcus, a marine planktonic copepod, is one of the dominant zooplankton species in the waters off southern California (Fleminger 1964;Star and Mullin 1981). In this region, C. pac@cus is active in the surface 100 m of the water column. However, starting in the late spring, some of the C. paci$cus arrest their development at the fifth copepodid (C5) stage and migrate to deeper waters (300-600 m) where they enter a diapause state (Fleminger 1985). A portion of the population remains dormant through the fall, becoming active again and rejoining the surface population around the first of the year.The Santa Barbara Basin (SBB), located off the coast of southern California, has an eastern sill of 225 m and a western sill extending as deep as 472 m in a narrow region (Fig. 1). Within it, Alldredge et al. (1984) observed a dense aggregation of diapausing C5 C. pacifkus from a submersible during early fall 1982. They estimated C5 concentrations up to 26 X lo6 ind. rnpi in a layer 20 m thick at a depth of 450 m, -100 m above the bottom. Although they had 10 dives over a 1.5&m diameter arc, the spatial extent of the aggregation as well as its concentration relative to nearby locations could not be ascertained. If the C5s within the basin are at concentrations much higher than elsewhere, they could be regionally important to the recruitment of C. paciJicus by serving as a potential seed population (e.g. Smith et al. 1986).To determine if Alldredge et al. (1984) had observed a recurring phenomenon, we sampled zooplankton in the SBB and surrounding waters during 20-22 November 1994 and 7-9 September 1995. Plankton nets were used to obtain the vertical distribution of C5 C. paci$cus while an optical plankton counter (OPC;Herman 1988Herman , 1992) was used to obtain the spatial extent of the deep C5 C. pacijkus aggregation. Net samples were obtained from discrete depth strata with a do...

Comparison of the epipelagic zooplankton samples from a U-Tow and the traditional WP2 net

“…Consequently, there is an urgent need for techniques that allow mesozooplankton to be sampled over extended spatial scales (tens or even hundreds of kilometres) (Marine Zooplankton Colloquium, 1989). To this end, acoustic and optical techniques have been extensively explored in recent years, although both techniques have limited ability to resolve species composition and may also suffer from the presence of too many, or non-living, particles in the water (Mitson et al, 1996;Stanton et al, 1996;Osgood, 1997;Benfield et al, 1998;Brierley et al, 1998a;Brierley et al, 1998b;Liao et al, 1999;Zhang et al, 2000). Systems that actually collect zooplankton samples at high speed and over extended transects would, therefore, be most valuable.…”