Extreme Nitrate Deficits in the Western Arctic Ocean: Origin, Decadal Changes, and Implications for Denitrification on a Polar Marginal Shelf

The western Arctic Ocean is known to be nitrate deficient relative to phosphate but the decadal trend and the processes contributing to the deficit are not clear. To investigate changes in this extreme nitrate deficit of over 10 μmol/kg and its causal mechanisms, nutrient concentrations were examined along a transect spanning the Bering Basin, the Bering–Chukchi Shelf, and the western Arctic Ocean Basin over the last two decades (1994–2018). The results show that the extreme nitrate deficit has extended to greater depths and further north during the past two decades, which coincided with the expansion of Pacific water in the western Arctic Ocean. Subsurface nutrient stocks in the basin areas appear to have increased, but are accompanied by a larger nitrate deficit, which may be due to stronger shelf denitrification. This nitrate loss (∆N) caused by shelf denitrification was estimated to be 7.3 ± 0.1 μmol/kg during the interval 2012–2018, which was ∼10% higher than that in 1994. This suggests an intensification of denitrification on this marginal shelf under climatic and environmental change in the Arctic Ocean. Plain Language Summary Prominent nitrate deficits relative to phosphate of over 10 μmol/kg in the Arctic Ocean are caused by a combination of nitrate loss by denitrification and a pre‐existing nitrate deficit in the inflowing Pacific waters. Over the last two decades (1994–2018), this extreme nitrate deficit extended to greater depths and further north, which coincided with the expansion of Pacific water into the western Arctic Ocean. Meanwhile, subsurface nutrient stocks have increased, however followed by a larger nitrate deficit, which may be due to stronger shelf denitrification in the western Arctic Ocean. Key Points Prominent nitrate deficits of over 10 μmol/kg expanded to greater depths and further north during 1994–2018 Subsurface nutrient stocks have increased in the western Arctic Ocean basin, but with larger nitrate deficit Denitrifying activity on the marginal shelf might be enhanced as a result of climate change