Nitrate isotope investigations reveal future impacts of climate change on nitrogen inputs and cycling in Arctic fjords: Kongsfjorden and Rijpfjorden (Svalbard)

[摘要] Ongoing climate change in the Arctic has caused tidewaterglaciers to retreat while increasing the discharge of freshwater andterrestrial material into fjords. This can affect both nutrient inputs andcycling within the fjord systems. In particular, tidewater glaciers and thepresence of associated subglacial meltwater plumes can have a large impacton fjord circulation and biogeochemistry. In this study, we assess theinfluence of tidewater glaciers on nitrogen inputs and cycling in two fjordsin Svalbard during the summer using stable isotopic analyses of dissolvednitrate ( δ 15 N and δ 18 O) in combination withnutrient and hydrographic data. Kongsfjorden receives inputs from tidewaterglaciers, whereas Rijpfjorden mainly receives surface inputs fromland-terminating glaciers. Results showed that both fjords are enriched innutrients from terrestrial inputs. Nutrient ratios indicate excess Si and Prelative to N. In both fjords, terrestrial nitrate from snowpack and glaciermelting are identified as the dominant sources based on high δ 18 O-NO 3 - and low δ 15 N-NO 3 - ofdissolved nitrate. In Kongsfjorden, mixed-layer nitrate is completelyconsumed within the fjord system, which we attribute to vigorous circulationat the glacial front influenced by the subglacial plume and longer residencetime in the fjord. This is in contrast to Rijpfjorden where nutrients areonly partially consumed perhaps due to surface river discharge and lightlimitation. In Kongsfjorden, we estimate terrestrial and marine Ncontributions to the nitrate pool from nitrogen isotopic values ( δ 15 N-NO 3 - ), and this suggests that nearly half the nitrate inthe subglacial plume (50  ±  3 %) and the water column (44  ±  3 %) originates from terrestrial sources. We show that terrestrial Ncontributes significantly to the regenerated N pool (63 %–88 %) within thisfjord suggesting its importance in sustaining productivity here. Given thisimportance of terrestrial nutrient sources within the fjords, increase inthese inputs due to climate change can enhance the fjord nutrient inventory,productivity and nutrient export offshore. Specifically, increasingAtlantification and warmer Atlantic Water will encourage tidewater glacierretreat and in turn increase surface discharge. In fjords akin toRijpfjorden this is expected to foster more light limitation and lessdynamic circulation, ultimately aiding the export of nutrients offshorecontributing to coastal productivity. Climate change scenarios postulated forfjords such as Kongsfjorden include more terrestrial N-fuelled productivityand N cycling within the fjord, less vigorous circulation due to the retreatof tidewater glaciers, and the expansion of oxygen-depleted deep watersisolated by the sill.

[学科分类] 大气科学