Changes in the zooplankton community of the British Columbia continental margin, 1985-1999, and their covariation with oceanographic conditions

A 15-year zooplankton time series collected off southern Vancouver Island (48-49 degreesN) shows large interannual anomalies of biomass for most major zooplankton species. Variations within groups of ecologically similar species have been more extreme (often 10-fold or greater) than the variation in total biomass (four- to six-fold). For both total biomass and species groups, the zooplankton anomalies develop and persist over time spans of several years and are correlated across a large spatial scale (> 100 km longshore). One dominant mode of recent zooplankton variation was a 1990-1998 cumulative shift to a more "southerly" copepod and chaetognath fauna: order-of-magnitude declines in several species endemic to the Northeast Pacific continental shelf and order-of-magnitude increases of species endemic to the California Current (35-45 degreesN). This trend abruptly reversed in 1999. A second major mode of zooplankton variability consisted of roughly mirror-image fluctuations in the abundance of euphausiids versus subarctic oceanic copepods. Zooplankton anomalies were correlated with year-to-year changes in several physical environmental indices. The patterns of covariance suggest that zooplankton community composition responds strongly to ocean climate fluctuations and in particular to changing current patterns.