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

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.