Long-term change and stability in the California Current System: lessons from CalCOFI and other long-term data sets

The California Current System (CCS) is a highly variable system, both spatially and temporally, that is strongly affected by low-frequency climatic fluctuations. This paper reviews evidence for long-term (decadal-scale) change in the physics and biology of the CCS over the last 50–100 years, as well as evidence for stability in planktonic community structure and long-term persistence of populations. Increases in water temperature, thermocline depth and stratification in the CCS have been accompanied by changes in populations of kelp, diatoms, foraminifera, radiolarians, intertidal invertebrates, zooplankton, fish and seabirds. However, there is also evidence for stability in assemblages of larval fish, calanoid copepods and radiolarians. Statistical averaging (the portfolio effect) may explain some aspects of stability in assemblages. Advection of planktonic populations may account for rapid recovery of biomass and dominance structure following perturbations such as strong El Niño events. Planktonic populations in the CCS may be adapted to large-scale biotic and abiotic variability, through a combination of advection of populations and life history traits. l lessons may be learned from the California Cooperative Oceanic Fisheries Investigations and other long-term data sets: (1) long time series are needed to understand the dynamics of the ecosystem; (2) life histories are important determinants of species responses to environmental forcing, even in the plankton; and (3) the CCS is simultaneously variable and stable, and these properties are not necessarily in conflict.