Eddies and Filaments Mediate the Transport and Biogeochemical Evolution of Coastal Upwelled Waters in the California Current System

Coastal upwelling in the California Current System, driven by seasonal equatorward along-shore winds, supplies macro- and micro-nutrient rich waters to the surface mixed layer. The transport of upwelled waters offshore is mediated by mesoscale eddies and submesoscale filaments. In this study, we sampled a series of submesoscale and mesoscale features to characterize the chemical and biological evolution of upwelled waters as they are transported from the coastal zone offshore. We combine observations of dissolved inorganic nutrients, trace metals, picophytoplankton abundances and phytoplankton photophysiology (assessed from variable fluorescence and fluorescence lifetime fluorimetry). We find that upwelled nutrients (including trace metals) are differentially drawn down as they are transported away from the coast. This is also reflected in changes in the composition of the (pico)phytoplankton, as well as pronounced shifts in the photophysiological status of the phytoplankton community. We observed high photosynthetic efficiencies (PSE) of phytoplankton in the upwelled region, indicating the absence of nutrient limitation. Away from coast we observed a decrease in phytoplankton PSE related to nutrient limitation. This decrease was accompanied by an increase in the functional cross-section of PSII reflecting a shift in phytoplankton community structure towards smaller species. Overlain on the mesoscale patterns seen in our data, we found the signature of localized upwelling and the entrainment of surrounding waters at the edges of the eddies sampled. This was evident in a shallower nutricline at the eddy edges, with a localized positive effect on phytoplankton PSE. By combining physical, chemical and biophysical data, we show how the evolution of upwelled waters defines the nutrient landscape in the California Current System, and how it, in turn, interacts with the phytoplankton community structure and phytophysiological status in this region.