Diatom community response to high and low iron upwelling plumes in the California current system

• Diatom communities in CCS vary based on chemical composition of the upwelling plume. • Communities were significantly different on either side of California Current jet. • Higher initial dFe upwelling plumes corresponded to higher fluorescence. • Bloom-forming Chaetoceros associated with higher dFe concentration upwelling plumes. Diatoms are important primary producers, accounting for most of the oceanic production and carbon export relative to other photosynthetic eukaryotes. There are important differences among diatom species that enable them to proliferate in different environments. Some have nutrient storage capabilities or lower nutrient quotas suited for oligotrophic oceans. Bloom-forming diatoms are typically found in nutrient-rich coastal waters where they respond quickly to nutrient pulses and can influence biogeochemical cycling in the ocean after blooms. As many physiological differences affect biomass and export, it is important to better understand how physical and chemical parameters influence diatom community composition, especially in dynamic upwelling environments. Examining diatom community responses to upwelling can be challenging, as it is difficult to capture and track upwelling waters from initiation. Here, we present the diatom community composition from two physically and chemically distinct upwelling plumes in the California Current System (CCS), determined using 18S rDNA sequencing, to test the hypothesis that iron (Fe) influences upwelling diatom communities. Diatom community composition, dFe, and dFe:nitrate (NO 3 – ) were all significantly different in the two upwelling plumes. There was higher fluorescence and higher relative abundance of bloom-forming Chaetoceros at the start of the upwelling transect with higher initial dFe. Interestingly, the complex physics of the CCS appeared to influence the dispersion of upwelling-induced diatom communities, with the same Chaetoceros species abundant after passing through a high velocity jet in the lower initial dFe upwelling transect. We conclude with a discussion of possible implications for future predictions of diatom proliferation, transport, and carbon export in the CCS.