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Abstract Coastal upwelling currents such as the California Current System (CCS) comprise some of the most productive biological systems on the planet. Diatoms dominate these upwelling events in part due to their rapid response to nutrient entrainment. In this region, they may also be limited by the micronutrient iron (Fe), an important trace element primarily involved in photosynthesis and nitrogen assimilation. The mechanisms behind how diatoms physiologically acclimate to the different stages of the upwelling conveyor belt cycle remain largely uncharacterized. Here, we explore their physiological and metatranscriptomic response to the upwelling cycle with respect to the Fe limitation mosaic that exists in the CCS. Subsurface, natural plankton assemblages that would potentially seed surface blooms were examined over wide and narrow shelf regions. The initial biomass and physiological state of the phytoplankton community had a large impact on the overall response to simulated upwelling. Following on‐deck incubations under varying Fe physiological states, our results suggest that diatoms quickly dominated the blooms by “frontloading” nitrogen assimilation genes prior to upwelling. However, diatoms subjected to induced Fe limitation exhibited reductions in carbon and nitrogen uptake and decreasing biomass accumulation. Simultaneously, they exhibited a distinct gene expression response which included increased expression of Fe‐starvation induced proteins and decreased expression of nitrogen assimilation and photosynthesis genes. These findings may have significant implications for upwelling events in future oceans, where changes in ocean conditions are projected to amplify the gradient of Fe limitation in coastal upwelling regions. 

We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (kin-app) computed by combining data on: i) Fe content of individual in situ phytoplankton cells; ii) concurrently-determined seawater dFe concentrations; and iii) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002-2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high-affinity Fe uptake system, so data was screened, yielding 560 single cell kin-app values from 31 low-Fe stations. We normalized kin-app to cell surface area (S.A.) to account for cell-size differences. The resulting bioavailability proxy (kin-app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe-siderophore complexes and often approaches that of highly-available inorganic Fe´. Station averaged kin-app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of kin-app/S.A. among stations (ca. 5-fold variation), we computed a grand-averaged dFe availability, which upon normalization to cell carbon (C) yields kin-app/C of 42,200 ± 11,000 L mol C-1 d-1. We utilize kin-app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of kin-app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe-limited Equatorial Pacific.