Abstract
The California Current System (CCS) is characterized by dynamic coastal upwelling that profoundly influences plankton community structure and diversity and that leads to the emergence of large diatoms nearshore. This study presents a high‐resolution biogeochemical model of the CCS, coupling the Darwin ecosystem model with the Regional Ocean Modeling System (ROMS). The model is configured to capture key plankton patterns in the CCS, including chlorophyll distribution, cross‐shore phytoplankton community composition, and ecosystem size structure. We show that the emergence of large diatoms in nutrient‐rich nearshore waters requires both high nutrient availability and robust grazing pressure, a result that aligns with ecological theory and observational studies. The model also shows size quantization patterns in the plankton biomass spectra, which dynamically adjust to changing environmental conditions. Our results highlight the importance of the interplay between bottom‐up and top‐down controls in shaping the planktonic ecosystem, with a specific emphasis on the factors leading to the emergence of large diatoms in the CCS. These findings provide insights into the mechanisms underlying plankton diversity and community structure in coastal upwelling systems, which we can build upon to improve our understanding of ecosystem responses to environmental changes in the CCS and other highly productive coastal regions.