Physical, chemical and biological structure of a coastal eddy near Cape Mendocino

Abstract

We sampled mesoscale physical, chemical and biological structure in the coastal region of northern California in May, 1987. The circulation was dominated by an equatorward-flowing coastal jet and by a set of mesoscale eddies. High-velocity flow in the core of the coastal jet was composed of low-salinity, low-nutrient water, Cyclonic coastal eddies were observed near Cape Mendocino and Pt. Arena. The Cape Mendocino eddy is examined in detail. This eddy is a common feature which was present both before and after the spring transition, suggesting a relatively weak coupling between the eddy and the local wind field.The coastal circulation strongly affected the chemical and biological structure of the region. The coastal jet and cyclonic eddy increased primary production and phytoplankton biomass by increasing the nutrient supply to the euphotic zone. The change in slope of density surfaces in association with high-velocity flow in the jet and eddy raised isopycnals carrying nutrients to the surface layer. The inshore edge of the coastal jet marked the boundary between rich coastal water and oligotrophic offshore water. Since the jet was not carrying nutrients in the surface layer, advection by this feature was not an important nutrient source for the coastal ocean. The cyclonic flow and secondary circulation associated with the eddy appeared to act as a nutrient pump which provided a continuing input of high nutrient, low chlorophyll water to the center of the eddy. This resulted in low chlorophyll in the eddy center, and high chlorophyll and a large excess in dissolved oxygen at its outer edge. The coupling between physical and biological structure in the study region differs from that described for systems dominated by wind-driven coastal upwelling