Multi-Decadal Shoaling of the Euphotic Zone in the Southern Sector of the California Current System

We document a long-term reduction in Secchi depth of 0.06–0.13 m yr⁻¹ in the southern California Current System (CCS) over the period 1969–2007, reflecting a long-term shoaling of the euphotic zone. Calibrated water clarity observations from 1949 to 1954 reinforce the results indicating a progressive shoaling. For the inshore area, 150 km off the coast, Secchi disk has shoaled 8.4 ± 1.2 m since 1949. This change has been accompanied by a nitracline shoaling of 18.2 ± 6.4 m since 1969 and an increase in chlorophyll a since 1978. These changes have occurred despite an increase in density stratification in the CCS. Increased stratification has been linked by others to decreased nutrient fluxes, reduced primary production, increased water transparency, and a deepening of the euphotic zone, contrary to our findings. We explore this apparent paradox with a simple steady-state model of the thickness of the euphotic zone. A sensitivity analysis suggests that the observed shoaling of the euphotic zone is consistent with a doubling in nutrient upwelling, but in this analysis we also consider alternative causes for such shoaling: water-column darkening caused by freshening, increased residence time of the phototrophic biomass in the euphotic zone, and supply of nutrients of anthropogenic origin.