Phytoplankton in the California Current system off southern California: Changes in a changing environment

Patterns of phytoplankton biomass, abundance and composition are examined between 1990 and 2009 over three major scales of environmental variability in the California Current system: seasonal, El Niño-Southern Oscillation (ENSO) and interannual. These patterns are compared with patterns of zooplankton biomass for commonalities and for links that might explain the decrease of zooplankton biomass since the 1970s, and the seemingly paradoxical increase in phytoplankton since 1984.Phytoplankton showed a slow, but significant increase in biomass and a change in species composition. The change appears to be quasi-continuous, with no sign of a more abrupt change around the year 2000. The responses of phytoplankton to ENSO events were weak. Biomass was depressed by El Niño conditions, but an effect of La Niña was not evident. There was no consistent response of species composition to either extreme. The seasonal signal of phytoplankton was strong, evident in biomass, abundance and species composition. The timing and magnitude of the annual maximum changed during the study. Prior to 1998, the annual peak of phytoplankton occurred consistently during the spring months, after which the peak shifted to summer. The shift in annual phytoplankton peak from spring to summer was accompanied by a change in composition, including a 75% decrease in the abundance of Hyalochaete species in the annual peak. The decrease in spring phytoplankton has been offset by an increase in the summer, which suggests a change in the source and timing of the nutrient supply.No evidence was found of an immediate link between phytoplankton abundance or composition and zooplankton biomass. It is postulated that the decline of zooplankton biomass seen since the 1970s is due, not to a decrease in annual primary production, but to shifts in the timing and composition of the annual phytoplankton peak which has destabilized the zooplankton cycle.

► Interannual fluctuations of phytoplankton in the CalCOFI area are described.
► Interannual changes were gradual.
► Annual peaks changed from spring to summer with a change in taxonomic composition.
► The interdecadal decline in zooplankton may be due to phenological changes of phytoplankton.
► Future research into variations of nutrient supplies may strengthen this hypothesis.