Phytoplankton Community Response to Changing Environmental Conditions Across Two Central California Sites

Understanding the role that changing environmental conditions play in altering phytoplankton abundance and community composition, and in turn ecosystem structure and function, will be increasingly important for the sustainable use and management of ocean resources in a changing climate. Characterizing change in nearshore ecosystems requires long-term studies with a broad spatial extent, with most studies sacrificing spatial extent for temporal duration. However, phytoplankton and ecosystem response can vary substantially over small spatial scales due to local oceanographic forcing and anthropogenic influence, making the application of long-term data from one site to another in the same geographic vicinity potentially challenging. In this study, we compare weekly phytoplankton abundance and community composition samples between two sites on the Central California Coast. One site, located in the San Luis Obispo (SLO) Bay upwelling shadow, is a long-term harmful algal blooms (HABs) sampling site with more than a decade of data with well characterized seasonal and interannual variability. The other site (~35 km away) is located at the mouth of Morro Bay, a seasonally low-inflow estuary with multiple aquaculture farms and long-term data from higher trophic levels, where samples were collected at high tide to capture incoming oceanic waters. Comparison of nearly a year of data shows significant correlations in abundance and temperature between sites, highlighting similarities in regional-scale oceanographic processes. Phytoplankton community response, and in particular the relative proportion of diatoms and dinoflagellates, was significantly correlated to temperature at both sites with higher temperatures associated with dinoflagellate-dominated communities and lower temperatures with diatom-dominated communities. Moreover, during large bloom events, while phytoplankton abundance differed by up to an order of magnitude between sites, likely stemming from local-scale processes, the composition reached high levels of similarity. The relationships established here suggest that long-term phytoplankton data from the Cal Poly Pier could potentially be linked with long-term high trophic-level datasets collected in Morro Bay, although a full annual cycle with interannual realizations would provide more certainty on the relationships established