SEASONAL CYCLE OF ZOOPLANKTON OFF SOUTHWESTERN BRITISH-COLUMBIA - 1979-89

Average seasonal cycles of zooplankton biomass, species composition, and environmental conditions are estimated from 1979-89 samples collected off the southwest coast of British Columbia. Average total dry weight biomass ranges from a winter minimum < 1.5 g/m2 to a late spring maximum > 8 g/m2. Three subregions within the study area can be distinguished based on bathymetry and current pattern: the inner shelf banks, the shelf break and slope, and a gyre off the mouth of Juan de Fuca Strait. Total zooplankton biomass, species composition, and seasonality differ sharply between subregions, as do temperature, salinity, and nutrient and phytoplankton distributions. From spring through autumn, there are large inputs of upwelled water (and nutrients) to the continental shelf. Phytoplankton biomass is high, especially in mid- to late summer, and is ample to support growth of herbivorous zooplankton. Despite high food availability and relatively low predation pressure, surface layer zooplankton populations decline on the continental shelf from late spring through autumn, while offshore zooplankton populations increase slowly. A high washout rate due to upwelling and subsequent seaward and alongshore transport is the most plausible explanation. For the continental shelf in summer, the advective component of population turnover is probably larger than local predation mortality.