Increased DIN Storage and ΔDIC/ΔDIN Ratio in the Subsurface Water of the Canada Basin, 1990‒2015.

The subsurface layer of the Canada Basin contains large reservoirs of dissolved inorganic nitrogen and carbon (DIN and DIC, respectively). Under rapid change of Arctic climate system, these reservoirs may not only serve as potential material bases for primary production, but also for carbon source and sink. However, the long‐term trends in reservoirs and the interaction between reservoirs and the upper ocean are not fully understood. This study used two data sets to evaluate the long‐term trends in the DIN and DIC reservoirs in the subsurface layer of the Canada Basin during 1990–2015. The results indicate an increase of ∼35% in the subsurface DIN reservoir over 25 years, whereas the DIN concentration remained fairly constant. The discrepancy between the DIN reservoir and DIN concentration was primarily owing to the expansion of the subsurface layer, which increased the DIN stock but maintained a relatively stable DIN concentration. Additionally, the ΔDIC/ΔDIN ratio in the subsurface layer increased by ∼33% over 25 years, primarily due to denitrification on the pathway of Pacific inflow, such as Chukchi shelf. In the Canada Basin, the enhanced mixing between the subsurface and surface layers may promote primary production and render the subsurface layer a carbon source. Consequently, when studying the biogeochemical cycles in the changing Arctic Ocean, the long‐term interactions between the subsurface and surface layers in the Canada Basin should be further considered. Plain Language Summary: Nutrients in the subsurface layer of the Canada Basin are potential material bases for primary production. Rapid climate change and enhanced mixing between the subsurface and surface layers impact the biogeochemical cycles in the Canada Basin. Two data sets were collected to determine the dissolved inorganic nitrogen and carbon (DIN and DIC, respectively) concentrations, DIN reservoir, and ΔDIC/ΔDIN ratio in the subsurface layer from 1990 to 2015. The results revealed that the subsurface DIN concentration remained constant, whereas the DIN and DIC reservoirs and ΔDIC/ΔDIN ratio dramatically increased. The increased Pacific inflow and expansion of subsurface layer may have led to an increase in the subsurface DIN and DIC stocks in the Canada Basin. Simultaneously, denitrification on the Chukchi shelf may have slowed the increase in the DIN reservoir, leading to an increase in the ΔDIC/ΔDIN ratio. Increased regional events such as upwellings, eddies, and storms can enhance the mixing of the upper layers (subsurface and surface layers) and thus may promote primary production. However, the increased ΔDIC/ΔDIN ratio implies that the mixing of the upper layers also supplies DIC upwardly, which might negatively impact the air‐sea CO2 flux in the Canada Basin and reduce carbon uptake from the atmosphere. Key Points: The subsurface dissolved inorganic nitrogen (DIN) reservoir in the Canada Basin increased by ∼35% during 1990–2015The subsurface DIN concentration in the Canada Basin remained fairly constant during 1990–2015The ratio of subsurface delta dissolved inorganic carbon (DIC) to delta DIN (ΔDIC/ΔDIN) increased by ∼33% during 1990–2015 [ABSTRACT FROM AUTHOR]