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

摘要

Phytoplankton are a major food resource for filter-feeding organisms occupying intertidal and subtidal habitats of the inner continental shelf. Phytoplankton in these nearshore waters experience different anthropogenic and natural forcing compared to those offshore such that cross-shelf exchange would allow phytoplankton that were produced offshore to serve as a subsidy to inner shelf consumers when phytoplankton production on the shelf is negatively impacted. In the Santa Barbara Channel (SBC) the continental shelf is only a few kilometers wide facilitating 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 relatively 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 0.75. km of the shoreline with an initial bloom of the diatom. Leptocylindrus spp. followed by. Pseudo-nitzschia spp. and series of dinoflagellate blooms with. Prorocentrum 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 the 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.