Abstract
Abstract. 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 (δ15N-NO3, δ18O-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 (δ15N-PN) reflects the AW nitrate source. The ammonification of organic matter in shallow sediments resupplies N to the water column and replenishes the nitrate inventory for the following season. Low δ18O-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 δ15N-NO3 or N*, suggesting that 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 benthic denitrification enriches δ15N-NO3 and decreases N*. As nutrients are efficiently recycled in the Barents Sea and there is no significant loss of N through benthic denitrification, changes to Barents Sea productivity are unlikely to alter N availability on shelf or the magnitude of N advected to the central Arctic Basin. However, we suggest that the AW nutrient source ultimately determines Barents Sea productivity and that changes to AW delivery have the potential to alter Barents Sea primary production and subsequent nutrient supply to the central Arctic Ocean.
Highlights
The Arctic Ocean is warming (Huang et al, 2017), experiencing sea ice loss (Notz and Stroeve, 2016) and freshening (Coupel et al, 2015) as a direct response to climate change
The Barents Sea Opening (BSO) between Norway and Svalbard was dominated by saline (S > 34.8) Atlantic inflow, notably in Bear Island Trough (BIT; Figs. 1a, 2b)
We show that nitrogen availability in the Barents Sea is supported through Atlantic Water (AW) supply and the efficient replenishment of nutrients through seasonal cycling processes
Summary
The Arctic Ocean is warming (Huang et al, 2017), experiencing sea ice loss (Notz and Stroeve, 2016) and freshening (Coupel et al, 2015) as a direct response to climate change. It is an enclosed basin filled with waters from the Atlantic and Pacific oceans, which provide varying concentrations of nutrients (Torres-Valdes et al, 2013).
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