Annual cycle of zooplankton abundance, biomass and production on the northern Gulf of Alaska shelf, October 1997 through October 2000
Zooplankton abundance from March through October on the northern Gulf of Alaska shelf in 1998, 1999 and 2000 was dominated by calanoid copepods; the biomass was dominated by calanoids and cnidarians. Although we sampled during the 1997–1998 El Niño, marked interannual differences in the major copepod taxa were not observed. Zooplankton abundance and species composition were influenced primarily by mean water-column salinity, secondarily by the mean temperature above the thermocline. An annual biomass peak, averaging about 0.5 g wet weight m−3, occurred in May and consisted primarily of the oceanic copepod species Neocalanus cristatus, Neocalanus plumchrus and Neocalanus flemingeri. A second biomass peak, 0.5 g wet weight m−3, was observed in August and consisted mainly of the cnidarian Aequorea spp. Copepod production, estimated from daily growth rates using temperature–body size regressions, peaked in May at about 35 mg C m−2 day−1. Initial calculations suggest an annual copepod production on the order of 6 g C m−2 year−1, probably less than 10% of the annual primary production. The apparent resilience of the zooplankton assemblage on the northern Gulf of Alaska shelf to the 1997–1998 ENSO may have been because of its large geographic separation from the faunal boundary between zooplankton communities in the California Current and North Pacific subarctic gyre.