Sources of phytoplankton to the inner continental shelf in the Santa Barbara Channel inferred from cross-shelf gradients in biological, physical and chemical parameters

Phytoplankton are a major food resource for filter feeding organisms occupying intertidal and subtidal habitats of the inner continental shelf.  In the Santa Barbara Channel (SBC) the continental shelf is only a few kilometers wide such that phytoplankton can be supplied to the inner shelf both through local production and through exchange with offshore waters. Physical, chemical and biological gradients were examined monthly along a 3-km cross-shelf transect in the SBC from January, 2008 through April, 2009.  Chemical and biological distributions followed temporal changes in physical forcing with higher nutrient concentrations and a more intense period of biological production associated with spring upwelling.   Chlorophyll was evenly distributed across the shelf during upwelling, but was present at higher concentrations on the inner shelf under stratified conditions. Similarly, cross-shelf gradients in the distribution of dominant phytoplankton genera were weakest during upwelling when blooms of the prymnesiophyte, Phaeocystis, and the diatoms, Eucampia spp. and Thalassiosira spp. occurred across most of the shelf.  Upon stratification blooms were largely confined to the inner shelf within 0.6 km of the shoreline with an initial bloom of the diatom Leptocylindrus spp. followed by Pseudo-nitzschia spp. and series of dinoflagellate blooms with Protocentrum spp. and Lingulodinium spp. attaining the highest abundances.   Phytoplankton taxonomic similarity decreased with increasing distance separating stations along the transect and was inversely related to stratification intensity.   The observed distribution patterns and these trends in taxonomic similarity imply that for most of the year consumers within rocky intertidal and subtidal ecosystems on the inner shelf are exposed to phytoplankton produced on the continental shelf, and often on the inner shelf.  The contribution of phytoplankton produced beyond the shelf break occurs mainly during spring upwelling.