Calanus pacificus, a common copepod from the California current, was the subject of a comparative study using two independent means for measuring grazing rate. Pump samples of zooplankton retaining the >333-μm fraction were taken from discrete depth intervals at
12 stations in the Southern California region, sorted for C. pacificus stages ?, ?, and Stage V and IV copepodites. Using data on phytoplankton abundance at each sampling station and depth, grazing rates were computed using published equations based on extensive laboratory studies of
the species. These values were compared to corresponding values of laminarinase activity, a measurement which has been proposed as a relative index of recent grazing activity. Significant differences were shown in both grazing indices between the southerly set of samples from the San Pedro
channel and the northerly set from the Santa Barbara Channel. The latter region, proximate to an active upwelling center, appeared to be a favorable environment for C. pacificus growth because ingestion was calculated to substantially exceed respiratory demands. One station midway between
the regions, where a dinoflagellate bloom occurred, showed depressed grazing and evidence of avoidance of dinoflagellate concentrations. At all other northerly locations, the bulk of C. pacificus biomass was concentrated at depths where chlorophyll concentrations exceeded a calculated
critical level at which respiratory demands are met (0.9 mg·m–3 chlorophyll a). High chlorophyll a concentrations occurred at sampling points with relatively lower grazing pressure. Evidence is presented to support the hypothesis that grazers, such
as C. pacificus, tend to aggregate in regions where primary production rates are highest. One exceptional instance of intense surface shoaling of high densities of Stages V and IV copepodites was observed.
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