Changes in oceanic zooplankton populations in the north-east Pacific associated with the possible climatic regime shift of 1998/1999

There is increasing evidence that the winter of 1998–1999 saw a shift in the ecosystem structure of the north-east Pacific. The Continuous Plankton Recorder sampled plankton along a transect from Alaska to California in 1997, the warm period preceding the regime shift, as well as the subsequent cool period of 2000 and 2001. Analyses of mesozooplankton biomass and species composition show significant changes in the plankton communities of the open Gulf of Alaska and the Alaskan shelf, consistent with the changing environmental conditions. Boreal calanoid copepod species showed lower abundances in the warm period while subtropical species showed higher abundances and a more northerly distribution. During the colder period, boreal species had higher abundances and subtropical species were less abundant on the northern part of the transect. Differences were greatest between the years of 1997 and 2001, with 2000 appearing as a ‘transition’ year. The communities in the open Gulf of Alaska appear to be as responsive to climate change as shelf ecosystems, whose responses have been reported elsewhere. The composition changes have implications for higher trophic levels that forage in the open ocean, and for the forcing of the downwelling ecosystem of the Alaskan continental shelf ecosystem by events offshore.