Seasonal and Event Scale Forcing of Phytoplankton Abundance and Taxonomic Composition in the Surfzone of an Open-Coast, Rocky Shore

Phytoplankton within the surfzone of rocky shores are an important component of nearshore food webs, yet little research has focused on these phytoplankton assemblages. We investigated the succession of the surfzone phytoplankton assemblage at an open-coast rocky site over two years, compared it to phytoplankton assemblages just beyond the surfzone, and explored the oceanographic mechanisms associated with the patterns observed. Seasonal and event scale oceanographic forcing were strongly linked to patterns of change in the surfzone phytoplankton assemblage, as expected. We were able to categorize three primary bloom types: shelf, centric and Gulf blooms. Shelf blooms occurred in the upwelling season during relaxed wind conditions, were composed of previously documented upwelling system bloom taxa, and were apparently mixed into the surfzone from just beyond it. Highly concentrated centric blooms were dominated by a solitary Thalassiosira sp. and occurred during summer upwelling conditions that co-occurred with peaks in wave energy. We speculate that these centric blooms result when positively buoyant diatoms accumulate in the surfzone by a previously described wave-driven mechanism. Gulf blooms occurred during the relaxation season and were advected into the region with lower salinity water from the Gulf of the Farallones by extended periods of northward flow. While this latter bloom type is specific to the regional context of the particular site we studied, the former two likely apply to other areas. This study contributes to our growing understanding of the distinctive physical and biological characteristics that differentiate the surfzone from other nearshore habitats and underscores the need for more mechanistic investigations of the ecology of nearshore phytoplankton assemblages. These results also have general implications for understanding the structure and dynamics of phytoplankton blooms (including harmful algal blooms) in intertidal, sub-tidal, and other nearshore ecosystems.