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

Abstract

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.

Silicon Isotopes Highlight the Role of Glaciated Fjords in Modifying Coastal Waters

“…Samples from the Chla-max (10-20 m deep) fall above the 1:1 ratio line and therefore NO 3 is not assumed to be the limiting nutrient for diatoms. There is excess PO 4 with respect to NO 3 throughout the water column (Figure S3 in Supporting Information S1), which is also seen in other fjord systems (Santos-Garcia et al, 2022), due to the rapid uptake of NO 3 by primary producers and the low NO 3 concentrations contributed by glacial meltwaters.…”

Multifactorial effects of warming, low irradiance, and low salinity on Arctic kelps

“…In fact, nitrate concentration in water was higher in T2 (1.68 ± 0.8 µM/L) treatment than the control (0.87 ± 0.9 µM/L, data not shown) during the duration of the 315 experiment. Arctic coastal waters are known to be nitrate-limited (Santos-Garcia et al, 2022). The influx of fresh and potentially more nitrate-rich waters may have induced an increase in the N metabolism of S. latissima which was nitrogen limited.…”

Tidewater glaciers as “climate refugia” for zooplankton-dependent food web in Kongsfjorden, Svalbard

“…Our stable isotope analysis revealed that the glacial bay houses a separate and wider isotopic niche than communities outside the sill to the inner fjord basin. As the isotopic niche is defined by d 15 N and d 13 C, this does imply different primary carbon and nitrogen sources for this zooplankton community (Santos-Garcıá et al, 2023). It is unknown to us at present if the community of zooplankton and fish represents a different trophic niche as isotopic niche is not necessarily correlated to the trophic niche (Jackson et al, 2011).…”

“…These water masses mix on the shelf and are advected into the fjord as AW (T>3.0 °C, S>34.65) and Transformed Atlantic Water (TAW: T 1.0-3.0 °C , S >34.65; Svendsen et al, 2002). During the summer, AW and TAW typically dominate the hydrography of Kongsfjorden (Hop et al, 2006;Assmy et al, 2023;Santos-Garcıá et al, 2023), which is the main reason for referring to it as sub-Arctic when compared to the Arctic fjords in northern Svalbard (Hop et al, 2002;Santos-Garcıá et al, 2023).…”

“…The water temperature in the inner fjord basin is typically lower, or more Arctic, than further down fjord and offshore (Santos-Garcıá et al, 2023). It is suggested that such cold and saline waters are remnants of winter-cooled waters resulting from heat loss to the atmosphere and contact with glacial fronts (Torsvik et al, 2019).…”