UC San Diego

The linkage between physical climate changes and marine ecosystems has been of great interest during recent decades. However, climate-driven primary production changes and their impacts on marine ecosystems over decadal scales are not fully understood. This issue is addressed by analyzing physical-biological changes in the southern California Current (SCC) based on the interdisciplinary datasets of the California Cooperative Fisheries Investigations (CalCOFI) program, Scripps Pier time series, and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) surface chlorophyll. The 55-year CalCOFI dataset reveals that the upper ocean warmed significantly and the density difference across the thermocline increased in the SCC over decadal scales. This surface-intensified warming results in deepening of the isotherms. However the thermocline depth, defined as the depth with the maximum gradient of temperature, has no significant long-term linear trend. As the surface heating changed the strength of stratification, it also changed the slope of the nitrate-temperature relation for the mid-depth waters (roughly 30 m to 200 m). Thus, the quality of upwelled water may have been fundamentally altered after the shift, which may affect primary production and higher trophic levels. Algal blooms at the Scripps Pier in the SCC occur with irregular timing and intensity each year. In the 1990's, the algal blooms occurred earlier in the year and with larger amplitudes compared to those of the 1980's. The annual mean Pier chlorophyll concentration exhibits a clear increasing trend with no concomitant trend evident in the Pier SST from 1983 to 2000. The Pier chlorophyll is highly coherent at 3-7 year periods with nearby offshore in situ surface chlorophyll observations at CalCOFI station 93.27. Upper-ocean nitrate concentration has been measured regularly in CalCOFI since 1984, but its variability in earlier CalCOFI decades is not well known. In order to reconstruct nitrate variability in the earlier decades of CalCOFI since 1950, the leading principal component (PC1) of density seasonal anomalies is used as a quantitative proxy for nitrate since the first principal component (PC1) of upper-ocean density is correlated with the upper-ocean nitrate content. The nitrate proxy PC1 provides a new perspective on the bottom-up forcing of the long-term changes in macrozooplankton observed in CalCOFI. The nitrate proxy decreases from 1950 to the early 1990's, but increases afterwards, which is a tendency that is similar to that exhibited by the surface chlorophyll and displacement volume macrozooplankton biomass time series in CalCOFI, supporting the idea that bottom-up forcing by the physical climate system drives changes in the plankton biomass