Interannual variations in phytoplankton community structure in the northern California Current during the upwelling seasons of 2001-2010

Phytoplankton species were enumerated from 72 samples collected biweekly during the upwelling season (May to August) of 2001-2010 to test for effects of interannual variations in upwelling and decadal basin-scale variability on phytoplankton species composition and community structure. Cluster analysis of phytoplankton community structure identified 7 groups; 1 group was dominated by dinoflagellates while the other groups were dominated by diatoms but with variable ratios of diatom-to-dinoflagellate abundance ranging from 4 to 847. The most abundant diatoms were Thalassiosira spp., Chaetoceros spp., Asterionellopsis glacialis, Cylindrotheca closterium, Leptocylindrus spp., Nitzschia and Pseudo-nitzschia spp., with dominance varying among the 7 groups. Variations in phytoplankton community structure were not related to the strength of upwelling within a given year; rather, differences were related to when a sample was collected within an upwelling/downwelling cycle. Community structure was also analyzed by non-metric multidimensional scaling ordination. The x-axis scores of the ordination, which is an index of community structure, were correlated with the Pacific Decadal Oscillation (PDO) but not with seasonally averaged coastal upwelling strength. Positive values of the index corresponded with positive PDO years (2002-2007), and negative index values with negative PDO years (2001, 2008-2010). Thus changes in the sign of the PDO seem to be more influential in explaining the interannual variations in phytoplankton community structure than seasonally averaged coastal upwelling.