Nitrate assimilation and regeneration in the Barents Sea: insights from nitrogen isotopes

• Main Authors: Tuerena, Robyn E., Hopkins, Joanne, Ganeshram, Raja S., Norman, Louisa, Vega, Camille, Jeffreys, Rachel, Mahaffey, Claire
• Format: Text
• Language: English
• Published: 2020
• Subjects: Arctic; Arctic Ocean; Barents Sea; Arctic Basin; Phytoplankton; Sea ice
• Online Access: https://doi.org/10.5194/bg-2020-293; https://bg.copernicus.org/preprints/bg-2020-293/

While the entire Arctic Ocean is warming rapidly, the Barents Sea in particular is experiencing significant warming and sea ice retreat. An increase in ocean heat transport from the Atlantic is causing the Barents Sea to be transformed from a cold, salinity stratified system into a warmer, less-stratified Atlantic-dominated climate regime. Productivity in the Barents Sea shelf is fuelled by waters of Atlantic origin (AW), which are ultimately exported to the Arctic basin. The consequences of this current regime shift on the nutrient characteristics of the Barents Sea are poorly defined. Here we use the stable isotopic ratios of nitrate (δ 15 N-NO3, δ 18 O-NO3), to determine the uptake and modification of AW nutrients in the Barents Sea. In summer months, phytoplankton consume nitrate, surface waters become nitrate depleted, and particulate nitrogen (δ 15 N-PN) reflects the AW nitrate source. The ammonification of organic matter in shallow sediments resupplies N to the water column through the season. Low δ 18 O-NO3 in the northern Barents Sea reveals that the nitrate in lower temperature Arctic Waters is > 80 % regenerated through seasonal nitrification. During on shelf nutrient uptake and regeneration, there is no significant change to δ 15 N-NO3 or N * , suggesting benthic denitrification does not impart an isotopic imprint on pelagic nitrate. Our results demonstrate that the Barents Sea is distinct from other Arctic shelves, where coupled partial nitrification-denitrification enriches δ 15 N-NO3 and decreases N * . Our results suggest that any current or future changes to productivity on the Barents Sea shelf are unlikely to alter the magnitude or isotopic signature of nutrient supply exported to the central Arctic basin. However, we suggest that the AW nutrient source ultimately determines Barents Sea productivity and changes to this supply may alter Barents Sea primary production and subsequent nutrient supply to the central Arctic Ocean.