Why is the northern end of the California Current System so productive?

Abstract

Although scientists have long believed that, in an eastern boundary current system, alongcoast wind stress is responsible for the upwelling of deep nutrients that initiates phytoplankton blooms, the California Current System (CCS) presents a challenge to this idea. Seasonally averaged chlorophyll concentrations are several times higher along the Washington/southern British Columbia coast than off northern California, where alongcoast wind stress is several times greater. The high chlorophyll concentration is reflected in higher trophic levels, such as zooplankton and fish density in the northern CCS. This article discusses potential reasons for this apparent paradox. The analysis suggests that the northern CCS has several mechanisms that can produce upwelled nutrient concentrations comparable to those in regions with much greater wind stress, including a persistent nutrient supply through the dynamics of the Strait of Juan de Fuca and local upwelling enhancement by submarine canyons. Large-scale upwelling resulting from coastal-trapped waves forced in the areas with stronger wind stress is also likely an important factor, as is iron input by the Columbia River. In addition, in contrast to other parts of the CCS, the high-productivity northern latitudes have numerous physical features that give phytoplankton blooms time to develop fully and to be retained on the shelf, including wide shelves, coastlines without large capes, a large bank, wind intermittency, and density fronts related to the Columbia River.