Do upwelling filaments result in predictable biological distributions in coastal upwelling ecosystems?

Abstract

We used satellite sea surface temperature (SST) images and in situ zooplankton data collected in four summers of field sampling in the northern California Current to address the question: Can the physical variables that indicate advective mesoscale circulation features (e.g. upwelling filaments) be used to infer biological distributions? Transects were selected from research cruises conducted when upwelling filaments were present in our study region and compared to transects occupied when filaments were not present. The hydrography and distribution of several indicator species of copepods were examined along each transect. SST in filaments ranged from 1.5 °C to 3.0 °C lower than the surrounding ocean water. Distributions of coastal and oceanic taxa reflected the SST patterns. Coastal taxa occurred offshore in higher abundances in cold upwelling filaments than in the surrounding warm ocean water. In contrast, species with oceanic and sub-tropical affinities were most abundant where warm surface water was present. Biomass and species abundances varied substantially among cruises and locations and so could not be quantitatively predicted by SST patterns. However, upwelling filaments consistently expanded the across-shelf region in which high zooplankton biomass occurred, ‘spreading’ the biomass from the continental shelf into offshore regions of the upwelling system. We conclude with a discussion of the sampling strategies necessary to capture ecosystem variability caused by these dynamic circulation features.