Pacific‐to‐Arctic Oceanic Nitrate Fluxes: First Bering Strait Overwinter Nitrate Time‐Series (2022–2023) Show Winter Replenishment and Suggest Decadal Flux Increase

Abstract The first overwinter (September 2022–July 2023) biogeochemical (nitrate, dissolved oxygen, fluorescence, and turbidity) mooring time‐series from the Bering Strait indicate late summer secondary productivity/ammonification, very minor oxygen consumption in winter, and a spring bloom triggered by sea‐ice retreat. Hitherto unobserved nitrate replenishment (to 10–20 μM) occurs in October/November across the entire strait, with winter variability seemingly linked to across‐channel flow, implying a cross‐strait nitrate gradient in winter. Fall storms before ice‐formation only oxygenate the water column to ∼95% saturation. Data yield the first estimates of year‐round Pacific‐to‐Arctic nitrate flux (10.4 ± 1.6 kmol/s) based on in situ measurements (rather than inferred values); suggest that, due to flow increase, the nitrate flux has most likely increased over the last three decades; and indicate the nitrate‐salinity relationship used by others to predict Bering Strait nitrate breaks down in winter, making in situ measurements essential to constrain the changing Pacific nutrient inflow to the Arctic.