A potential nitrogen sink discovered in the oxygenated Chukchi Shelf waters of the Arctic

Abstract

The western Arctic Shelf has long been considered as an important sink of nitrogen because high primary productivity of the shelf water fuels active denitrification within the sediments, which has been recognized to account for all the nitrogen (N) removal of the Pacific water inflow. However, potentially high denitrifying activity was discovered within the oxygenated Chukchi Shelf water during our summer expedition. Based on 15N-isotope pairing incubations, we estimated denitrification rates ranging from 1.8 ± 0.4 to 75.9 ± 8.7 nmol N2 L−1 h−1. We find that the spatial pattern of denitrifying activity follows well with primary productivity, which supplies plentiful fresh organic matter, and there was a strong correlation between integrated denitrification and integrated primary productivity. Considering the active hydrodynamics over the Chukchi Shelf during summer, resuspension of benthic sediment coupled with particle-associated bacteria induces an active denitrification process in the oxic water column. We further extrapolate to the whole Chukchi Shelf and estimate an N removal flux from this cold Arctic shelf water to be 12.2 Tg-N year−1, which compensates for the difference between sediment cores incubation (~ 3 Tg-N year−1) and geochemical estimation based on N deficit relative to phosphorous (~ 16 Tg-N year−1). We infer that dynamic sediment resuspension combined with high biological productivity stimulates intensive denitrification in the water column, potentially creating a nitrogen sink over the shallow Arctic shelves that have previously been unrecognized.

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

“…The shelf area maintains high biological productivity (Grebmeier et al., 2006, 2015) and effective particle removal (Chen et al., 2012) throughout the year, which is believed to fuel denitrification. Inorganic nitrogen removal by water column denitrification has also been found in the bottom waters of the productive Chukchi shelf, which might be attributed to high sediment resuspension and particle‐associated bacteria activity (Zeng et al., 2017).…”

“…Extreme nitrate deficit in the subsurface of the western Arctic Ocean has a different causal mechanism, with minimal local or in situ water column denitrification (Reeve et al., 2019). Here, nitrate deficit coincides with the lateral transport of a water mass that is continuously influenced by denitrification process on the shelf sea floor (Brown et al., 2015; Granger et al., 2018; Zeng et al., 2017).…”

Distribution and Production of N2O in the Subtropical Western North Pacific Ocean During the Spring of 2020

“…Similarly, in the relatively oxygen-rich Pacific Ocean outside the OMZ, potential denitrification in the microreducing environment created by marine snow in the water column has also been suggested despite aerobic conditions (Yamagishi et al, 2005;Kim et al, 2013). In addition, the coupling of many surface-supplied particles and denitrifiers may contribute to denitrification in the oxygenated Chukchi Shelf waters (Zeng et al, 2017). In addition to potential denitrification within the water column, denitrification occurs in permeable sediments under high oxygen concentrations (even above 100 mmol L -1 ) (Marchant et al, 2017).…”

Nitrogen Fixation in a Changing Arctic Ocean: An Overlooked Source of Nitrogen?

“…In fact, the often prevalent N limitation of primary production in the AO (e.g., Codispoti et al, 2013;Tremblay et al, 2015;Mills et al, 2018) is predicted to intensify in some areas due to, e.g., increased stratification (Vancoppenolle et al, 2013;Slagstad et al, 2015). However, large regionsparticularly in the eastern AO -are undersampled, and the many mechanisms regulating input and availability of N across the AO are intensively debated: e.g., turbulent nitrate fluxes (Randelhoff et al, 2020), advection of Pacific and Atlantic water (Lewis et al, 2020), glacial melt (Hopwood et al, 2020), riverine discharge (Terhaar et al, 2019), denitrification processes (Zeng et al, 2017), atmospheric deposition (Kühnel et al, 2013), shelf-break eddies (Watanabe et al, 2014), and photoammonification (Xie et al, 2012). Hence, accurate determination of sources and sinks of new N is a critical prerequisite for predictions of future net primary production and sequestration of carbon in the AO.…”