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

Abstract. Ongoing climate change in the Arctic has caused tidewater
glaciers to retreat while increasing the discharge of freshwater and
terrestrial material into fjords. This can affect both nutrient inputs and
cycling within the fjord systems. In particular, tidewater glaciers and the
presence of associated subglacial meltwater plumes can have a large impact
on fjord circulation and biogeochemistry. In this study, we assess the
influence of tidewater glaciers on nitrogen inputs and cycling in two fjords
in Svalbard during the summer using stable isotopic analyses of dissolved
nitrate (δ15N and δ18O) in combination with
nutrient and hydrographic data. Kongsfjorden receives inputs from tidewater
glaciers, whereas Rijpfjorden mainly receives surface inputs from
land-terminating glaciers. Results showed that both fjords are enriched in
nutrients from terrestrial inputs. Nutrient ratios indicate excess Si and P
relative to N. In both fjords, terrestrial nitrate from snowpack and glacier
melting are identified as the dominant sources based on high δ18O-NO3- and low δ15N-NO3- of
dissolved nitrate. In Kongsfjorden, mixed-layer nitrate is completely
consumed within the fjord system, which we attribute to vigorous circulation
at the glacial front influenced by the subglacial plume and longer residence
time in the fjord. This is in contrast to Rijpfjorden where nutrients are
only partially consumed perhaps due to surface river discharge and light
limitation. In Kongsfjorden, we estimate terrestrial and marine N
contributions to the nitrate pool from nitrogen isotopic values (δ15N-NO3-), and this suggests that nearly half the nitrate in
the subglacial plume (50 ± 3 %) and the water column (44 ± 3 %) originates from terrestrial sources. We show that terrestrial N
contributes significantly to the regenerated N pool (63 %–88 %) within this
fjord suggesting its importance in sustaining productivity here. Given this
importance of terrestrial nutrient sources within the fjords, increase in
these inputs due to climate change can enhance the fjord nutrient inventory,
productivity and nutrient export offshore. Specifically, increasing
Atlantification and warmer Atlantic Water will encourage tidewater glacier
retreat and in turn increase surface discharge. In fjords akin to
Rijpfjorden this is expected to foster more light limitation and less
dynamic circulation, ultimately aiding the export of nutrients offshore
contributing to coastal productivity. Climate change scenarios postulated for
fjords such as Kongsfjorden include more terrestrial N-fuelled productivity
and N cycling within the fjord, less vigorous circulation due to the retreat
of tidewater glaciers, and the expansion of oxygen-depleted deep waters
isolated by the sill.
