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Multi-decadal shoaling of the euphotic zone in the southern sector of the California Current System

Summary: Multi-decadal shoaling of the euphotic zone in the southern sector of the California
Current System
Dag L. Aksnesa,* and Mark D. Ohmanb
a Department of Biology, University of Bergen, Bergen, Norway
bCalifornia Current Ecosystem Long Term Ecological Research Site, Scripps Institution of Oceanography, La Jolla, California
We document a long-term reduction in Secchi depth of 0.060.13 m yr21 in the southern California Current
System (CCS) over the period 19692007, 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 6 1.2 m since 1949. This change has been accompanied by a
nitracline shoaling of 18.2 6 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.
Increased stratification of the ocean water column is
generally believed to restrict nutrient supply into the


Source: Aksnes, Dag L. - Institutt for biologi, Universitetet i Bergen


Collections: Environmental Sciences and Ecology; Biology and Medicine