INTERANNUAL VARIATIONS IN ZOOPLANKTON BIOMASS IN THE GULF-OF-ALASKA, AND COVARIATION WITH CALIFORNIA CURRENT ZOOPLANKTON BIOMASS

Large-scale atmospheric and oceanographic conditions affect the produc tivity of oceanic ecosystems both locally and at some distance from th e forcing mechanism. Recent studies have suggested that both the Subar ctic Domain of the North Pacific Ocean and the California Current have undergone dramatic changes in zooplankton biomass that appear to be i nversely related to each other. Using time series and correlation anal yses, we characterized the historical nature of zooplankton biomass at Ocean Station P (50 degrees, 145 degrees W) and from offshore station s in the CalCOFI region. We found a statistically significant but weak negative relationship between the domains. We investigated whether su ch a relationship arises from different forcing mechanisms or as an op posite response to the same mechanism. We found that the seasonal peak of both data sets occurred in the summer but that the CalCOFI data la gged the Ocean Station P data. A surface-drift simulation model showed that winter trajectories started at Ocean Station P and along 145 deg rees W drifted more into the California Current before the 1976-77 reg ime shift. and more into the Alaska Current after the 1976-77 shift. W e examined physical and biological conditions which may lead to this i nverse relationship between the two ecosystems, and we discuss the imp lications of these results for higher trophic levels.