Comparisons of interannual biomass anomalies of zooplankton communities along the continental margins of British Columbia and Oregon

We update and compare multi-year time series of zooplankton community composition anomalies from three continental margin regions covering a 850-km distance along the eastern margin of the Northeast Pacific: the inner continental shelf off Newport Oregon (44.6°N), the continental shelf and slope off southern Vancouver Island, British Columbia (48–49°N), and the continental shelf and slope off northern Vancouver Island (49.5–51.5°N). In all three regions, there have been large, low-frequency fluctuations in biomass of the dominant zooplankton taxa. Particularly strong changes in sign and/or slope of the anomaly time series occurred between 1988–1991 and 1998–1999. Time intervals for both transitions coincided with NE Pacific ‘regime shifts’ identified in other data sets, and also with El Niño–La Niña events. Anomaly time series from southern Vancouver Island and central Oregon were remarkably similar (region-to-region correlations 0.59 and 0.95 for the two best-censused groups of species). Through most of the 1990s, both regions had a strong and cumulative shift to a more ‘southerly’ copepod fauna. This trend reversed sharply in 1999, following the 1997–1999 ENSO event. Since 1999, abundance of most zooplankton taxa has been similar to the 1970s–mid-1980s. Off southern Vancouver Island, empirical statistical fits to environmental indices derived from the middle part of the time series (1985–1997) also perform well both for earlier (1979–1984) and later (1998–2001) time periods. Off northern Vancouver Island the 1998–1999 changes in the zooplankton community were substantially weaker, although identical in timing and direction to the changes observed off southern Vancouver Island and Oregon.