Physical and Biogeochemical Factors Driving Spatially Heterogeneous Phytoplankton Blooms in Nearshore Waters of Santa Monica Bay, USA

Phytoplankton blooms in nearshore waters are difficult to predict using only biogeochemical factors known to control phytoplankton growth, yet the need to understand these events continues to grow with expanding harmful algal bloom (HAB) events. The present study investigated the spatial and temporal dynamics of phytoplankton blooms and their drivers in King Harbor, a harbor in Santa Monica Bay in the Southern California Bight. High-frequency sensor measurements of environmental variables and biomass and discrete sampling of phytoplankton community composition and nutrients over an annual cycle were obtained. Eleven distinct bloom events, nine of which were numerically dominated by dinoflagellates, were identified over the study period. Results from both regression-based and time series analyses show that these blooms were correlated with increased temperature, changes in nutrient concentrations, and decreased tidally driven mixing, revealing an opportunity for bloom initiation close to neap tides. Predictors of chlorophyll biomass and environmental factors that explained differences in microplankton community structure differed between the two basins of King Harbor, despite their close and connected nature. Biomass and HAB taxa abundances in the harbor were significantly correlated with those in Santa Monica Bay with a 1-week lag in the harbor data, suggesting possible onshore transport of organisms into the harbor. The results of this study quantify the significant influence of tidal cycle as a physical process operating locally and at timescales of hours to days and provide evidence for a high degree of spatial heterogeneity in bloom dynamics in nearshore environments.