Seasonal and Interannual Variability of Phytoplankton Abundance and Community Composition on the Central Coast of California

Phytoplankton form the base of marine food webs. Therefore, variations in their abundance and composition greatly impact ecosystem structure and function. Within the California Current System (CCS), phytoplankton community structure is tightly coupled to variability in wind-driven coastal upwelling, a process that drives changes to coastal water temperatures and nutrient concentrations. Based on a decade (2008-2018) of weekly phytoplankton measurements, this study provides the first characterization of the seasonal and interannual variability of phytoplankton abundance and composition in San Luis Obispo (SLO) Bay, an understudied region with the CCS. These measurements were collected as part of a harmful algal bloom (HAB) monitoring program and therefore our study focuses on two dominant taxa in the region – diatoms and dinoflagellates. Overall, the seasonality of phytoplankton in SLO Bay mirrored that of the larger CCS; diatoms dominated the community during the spring upwelling season, whereas dinoflagellates dominated the community during the fall relaxation period. We also observed considerable interannual variability among phytoplankton taxa, but of particular note was the absence of a fall dinoflagellate-dominated period from 2009-2014. We hypothesize this regime shift may be linked to a phase shift of the Pacific Decadal Oscillation (PDO) from a negative (cool) to a positive (warm) phase. The impact of these changes is considered in the context of broader biogeochemical cycling, HAB dynamics, and the use of chlorophyll-a as a proxy for changes in phytoplankton abundance.