Chemical flux, mixed layer entrainment and phytoplankton blooms at upwelling fronts in the California coastal zone

Abstract

Atmospheric and in situ physical processes appear to generate a sustained chemical flux at the frontal boundaries of coastal upwelling systems. In this study off Pt. Sur. California, satellite images and three-dimensionally presented in situ data show the evolution of an upwelling system with a strong chemical front and a large phytoplankton bloom in a surface layer juxtaposed to the frontal boundary. Nutrient uptake from this layer by phytoplankton, ranging from 0.15 to 2.39 μM NO 3 d 1, is indicated by primary production measurements in the bloom. Based on an atmospheric forcing model, wind stress drives the physical entrainment of nutrients from deeper layers into the “mixed layer” adjacent to the front at rates ranging from 0 to 0.76 μM NO 3 d 1. From a dynamic balance of nutrient exchange processes, a lateral cross-front exchange from the upwelling system to the adjacent mixed layer is also indicated. These two mixing processes, sustained by atmospheric forcing may explain the chemostat-like persistence of phytoplankton pigments which appear along the boundaries of upwelling systems in situ and in remotely sensed ocean color images of the California coastal zone.