Understanding Diatoms’ Past and Future Biogeochemical Role in High‐Latitude Seas

Because cold‐water diatoms’ baseline elemental density (BED) is substantially higher than temperate diatoms, previous polar studies may have underestimated diatoms’ contribution to elemental standing stocks, contribution to particulate organic carbon (POC) export and incorrectly modeled their susceptibility to future warming. We apply cold‐water diatom allometry to Arctic field samples and derive diatom growth rates ranging from 0.01–0.68 day−1, versus unrealistically high rates estimated using temperate diatom allometry. Reanalysis of published Southern Ocean data (Antarctic Environment and Southern Ocean Process Study and European Iron Fertilization Experiment) shows that diatom POC was significantly underestimated and diatoms could have accounted for a majority of POC export. However, during some field programs (Kerguelen Plateau), temperate allometry properly accounted for diatom biomass. We also predict that warming sea surface temperature may alter high‐latitude diatom BED, suggesting that even if abundances do not change with warming, the reduced diatom BED will likely lower the trophic‐transfer efficiency and their total carbon flow to consumers. Plain Language Summary High‐latitude marine regions are warming faster than other marine systems. We show that the role of diatoms in these systems, already thought to be important, was systematically underestimated in some studies due to insufficient understanding of cold‐water diatoms’ physiology, specifically their high elemental density. When applying cold‐water‐diatom elemental density relationships to original and previously published high‐latitude field data, diatom growth rates were rapid (up to a doubling per day), and they could account for most of the organic carbon standing stock and its export—original publications inferred contributions