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

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 15 this current regime shift on the nutrient characteristics of the Barents Sea are poorly defined. Here we use the stable isotopic ratios of nitrate (d15N-NO3, d18O-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 (d15N-PN) reflects the AW nitrate source. The ammonification of organic matter in shallow sediments resupplies N to the water column through the season. Low d18O-NO3 in the northern Barents Sea reveals that the nitrate in lower temperature Arctic Waters is 20 >80% regenerated through seasonal nitrification. During on shelf nutrient uptake and regeneration, there is no significant change to d15N-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 d15N-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, 25 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. https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction 30
The Arctic Ocean is warming (Huang et al., 2017) and 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). In turn, these nutrient supply pathways influence the distribution and extent of primary production throughout the Arctic Ocean (Lewis et al., 2020).
Approximately 50% of the Arctic Ocean is made up of productive shelves that support large fisheries and diverse habitats 35 (Dalpadado et al., 2014;Friedland and Todd, 2012). As the Arctic continues to warm and more sea ice is lost, phytoplankton growth will become less limited by light availability. Instead, nutrient availability, principally nitrate (Codispoti et al., 2013), may become the primary control on phytoplankton growth (Arrigo and van Dijken, 2015;Lewis et al., 2020). A greater understanding of how nitrate is supplied to the Arctic inflow shelves and on shelf nutrient dynamics is therefore imperative in order to predict changes to Arctic primary production and food web dynamics. 40 Atlantic Water (AW) is supplied to the Arctic via Fram Strait and the Barents Sea Opening and fills most of the deep basins of the Arctic. It supplies nutrients to the Eurasian shelves with nitrate and phosphate concentrations close to Redfield (15-16N:1P), and low concentrations of silicate, which can limit the extent of diatom growth (Hatun et al., 2017). AW is a mixture of nutrient-rich North Atlantic subpolar and nutrient-poor subtropical origin water advected into the Norwegian Sea. Over the 45 last two decades there has been a 7% and 20% decrease in nitrate and silicate concentrations respectively in the Barents Sea (Rey, 2012). This has been driven by shallower winter mixing in the subpolar gyre, coupled with weakening and westward retraction of the gyre which has increased the proportion of subtropical origin water entering the Norwegian Sea (Rey, 2012;Hatun et al., 2017).

50
As warm and saline AW inflow water is transported across the Barents Sea, it is modified by atmospheric cooling and is mixed with cold, fresh Arctic origin water (ArW) and the Norwegian Coastal Current (NCC). ArW found across the northern Barents Sea comprises fresh Arctic river runoff, sea ice melt and precipitation, and contains the remnants of the winter mixed layer (Rudels et al., 1996). It isolates the sea surface and ice cover from warm AW below (Lind et al., 2016) and during the summer is capped by a well-mixed surface layer of fresh melt water (Polar Surface Water). Sea ice import from the Nansen Basin and 55 Kara Sea is the most important source of freshwater in the northern Barents Sea (Lind et al., 2016;Ellingsen et al., 2009). The transition between AW and ArW is marked by the Polar Front, which can be identified from the sea surface temperature gradient (Barton et al., 2018;Oziel et al., 2016). https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License.
Intense cooling of AW across the Barents Sea, reinforced by brine rejection due to ice formation creates dense Barents Sea 60 Water (BSW) that cascades into the deeper troughs of the central and eastern Barents Sea (Arthun et al., 2011;Oziel et al., 2016). BSW eventually leaves the shelf, mainly through St. Anna Trough (Smedsrud et al., 2013), where it is entrained into Arctic Intermediate Water and spreads further into the Arctic basin (Schauer et al., 1997).
The Barents Sea is experiencing a rapid decline in winter and summer sea ice cover (Onarheim and Arthun, 2017;Arthun et 65 al., 2012), full-depth warming driven by both increased ocean heat transport from the Atlantic and amplified atmospheric warming over the Arctic (Arthun et al., 2012;Onarheim et al., 2015;Serreze et al., 2009), alongside increases in salinity (Lind et al., 2018;Barton et al., 2018). The area occupied by AW is increasing and the southern expression of the Polar Front is moving north (Oziel et al., 2016;Oziel et al., 2020). In the northern Barents Sea, a reduction in sea-ice import and therefore a loss of freshwater is weakening stratification and enhancing vertical mixing (Lind et al., 2018). The northern Barents Sea is 70 therefore transitioning from a cold, salinity stratified shelf into a warmer, less stratified Atlantic dominated climate regime (Lind et al., 2018), a process described as 'Atlantification'.
On the other side of the Arctic, the Pacific Ocean supplies high concentrations of nutrients onto the Chukchi and East Siberian shelves, fuelling productivity and nutrient uptake (Granger et al., 2011). Increases in volume transport through the Bering 75 Strait in recent years (Woodgate, 2018) have increased Pacific nutrient supply to the Arctic basin. These waters are relatively deplete in nitrate (in comparison to phosphate), and combined with sedimentary denitrification on the shallow shelves (Fripiat et al., 2018;Granger et al., 2018), promote nitrogen limitation in the western Arctic Ocean (Mills et al., 2018).
Although many studies have found the western Arctic Ocean to be strongly N limited (Mills et al., 2018;Granger et al., 80 2018;Brown et al., 2015), we know less about the extent of N limitation and occurrence of sedimentary denitrification in the eastern Arctic Ocean. Nitrate isotope measurements can give integrated estimates of nitrogen cycling processes, yet there is currently no data on the North Atlantic inputs which provide nutrients to the Arctic basin via the Barents Sea. The respectively) provide complementary information about nitrate uptake by phytoplankton and regeneration processes (Sigman et al., 2009b) and can be used to determine the relevant N cycling processes. 85 Nitrate consumption through algal uptake fractionates both N and O in a 1:1 ratio (Granger et al., 2004) with an isotope effect close to ~5‰ (Sigman et al., 2009b). Nitrogen loss through denitrification in the water column leads to enrichment in N and O isotopes in the residual nitrate pool with a fractionation of 25-30‰ (Sigman et al., 2009a). In sediments, denitrification does not usually impart a signature on nitrate isotopes as N is efficiently lost to N2 before fractionation is expressed (Sigman et al., 90 2003). However, multiple studies from the western Arctic and Bering Sea show that benthic denitrification can impart a signature on the overlying water column (e= 0-5‰) when high d 15 N-NH4 is released, a process termed coupled partial nitrification-denitrification or CPND (Brown et al., 2015). https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License.
Over most of the ocean fixed nitrogen is efficiently recycled in surface waters and the regeneration of nitrate in the water 95 column retains a d 15 N signature from the N source (Sigman et al., 2000). The d 15 N signature imparted on nitrate can therefore be used to identify nutrient sources from partial utilization of nutrients (Rafter et al., 2012), different water masses (Sigman et al., 2000;Tuerena et al., 2015), new N inputs (Knapp et al., 2008;Marconi et al., 2017), atmospheric inputs (Altieri et al., 2016) and rivers (Thibodeau et al., 2017).

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In contrast to d 15 N-NO3, where N atoms are internally recycled during nitrification, oxygen atoms are sourced from ambient O2 and seawater, in general providing a nitrification signature of d 18 O-H2O plus 1.1‰ (Buchwald et al., 2012;Sigman et al., 2009b). The contrasting sources of N and O atoms and thus their distinct isotopic signatures allow the relative importance of preformed and regenerated nitrate to be investigated (Rafter et al., 2013). d 18 O-NO3 has been used to quantify the extent of regeneration on the Bering Sea shelf (Granger et al., 2011) and as evidence for the significance of nitrate regeneration in 105 sustaining nutrient stocks on Arctic Shelves (Fripiat et al., 2018;Granger et al., 2018).
When nitrate is not fully consumed in surface waters, d 15 N-NO3 and d 18 O-NO3 can indicate the extent of seasonal nitrate uptake by phytoplankton (DiFiore et al., 2009). The d 15 N of surface water nitrate increases as nitrate is progressively utilized by phytoplankton, through the preferential consumption of 14 N (Sigman et al., 1999). Together with other N cycling processes 110 this can be described by Rayleigh fractionation systematics (Mariotti et al., 1981). Nitrate utilisation by phytoplankton in an environment where there is no resupply of nutrients, i.e. a stratified upper ocean in summer, follows Rayleigh fractionation systematics for a closed system, with d 15 N-NO3 falling on a fractionation trend for its isotopic effect (ε) (Granger et al., 2004).
In combination with dissolved nutrients, the d 15 N of particulate nitrogen (d 15 N-PN) can track the extent of biological utilisation, contrasting nutrient sources, and the significance of new versus regenerated nutrients (Altabet and Francois, 1994). 115 In this study we report the first stable isotope measurements of dissolved and particulate N in the Barents Sea and use them to understand the relative sources of nutrients fuelling contemporary Barents Sea productivity. We use stable isotope tracers to investigate how N cycling processes vary across the Barents Sea, in contrast to other Arctic shelves, and reflect upon the susceptibility of the ecosystem to climate change. 120

Materials and Methods
Samples were collected in the Barents Sea as part of the ARISE project (NERC Changing Arctic Ocean programme). Shipboard measurements were taken from the RRS James Clark Ross during July-August 2017 (JR16006). A 2,200 km transect was completed, comprising 59 full depth CTD casts, starting from the northern tip of Norway and ending at the shelf edge north-https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License. east of Svalbard (Figure 1a). The transect crossed the Barents Sea Opening (BSO), between Norway and the southern tip of 125 Svalbard. Then, from Hopen Trench it continued north towards Kong Karls Land (along 30°E), and to the shelf edge and Nansen Basin.
Standard CTD measurements and water sampling were performed using a stainless steel rosette equipped with a full sensor array and twenty-four 20-litre OTE bottles. Conductivity, temperature, and pressure were measured using a CTD system 130 (Seabird 911+). Derived salinity was calibrated on-board with discrete samples using an Autosal 8400B salinometer (Guildline) (Dumont et al., 2019) and an SBE43 oxygen sensor was calibrated against oxygen samples analysed using the Winkler method. A Biospherical QCP Cosine PAR sensor measured down-welling photosynthetically available radiation (PAR; 400-700 nm). We define the base of the euphotic zone to be the depth where PAR decreased to 1% of its surface value.
The mean depth of the euphotic zone was 34.3 ±11.9 m. 135 Dissolved inorganic nutrient concentrations were determined using a Bran and Luebbe QuAAtro 5-channel auto analyser (SEAL Analytical) and AACE operating platform (V 6.1) following standard colorimetric methods with a CRM precision of 0.3%, 0.8% and 1.9% for nitrate+nitrite, phosphate and nitrite, respectively. Nitrate isotope samples were collected and filtered inline from the CTD using an Acropak and were frozen at -20°C until analysis. Δ 15 N-PN samples were collected by gently 140 vacuum filtering through combusted GF/F filters (450 °C, 4hr, Whatman, 48 mm or 25 mm, nominal pore size 0.7 µm) until a good colour was obtained on the filter (8 to 12 L for the 48 mm diameter filters and 2 to 5 L for the 25 mm diameter filters).
The filters were dried at 60 °C to remove all moisture and were stored folded and wrapped in combusted aluminium foil until return to the home laboratory where they were placed in a -80 °C freezer until analysis. Of the 59 CTD casts in 2017, 23 were sampled for nitrate isotopes. 145 To quantify the distinct nutrient concentrations, nutrient ratios and isotopic values of Atlantic Water (AW), Barents Sea Water (BSW) and Arctic Water (ArW), we define the water mass type of each sample using the water mass properties in Oziel et al. (2016). These are summarised in Table 1. Seawater was filtered through combusted GF/F filters for δ 15 N-PN analysis and aliquots of the filtrate were placed into acid cleaned HDPE bottles and stored at -20 °C for the analysis of δ 15 N-NO3 and inorganic nutrients.

160
The isotopic composition of nitrate+nitrite (d 15 N-NO3 and d 18 O-NO3) was determined by the denitrifier method (Sigman et al., 2001;Casciotti et al., 2002) and following GEOTRACES protocols (Schlitzer et al., 2018). Samples were corrected using international reference standards N3 and USGS-34 ) and expressed in delta notation (d 15  lower than nitrate (Casciotti et al., 2007), we correct our d 18 O-NO3 data following Kemeny et al., (2016). Our isotopic measurements are compared to studies where the nitrite in a sample has been removed using sulphamic acid (Granger and Sigman, 2009), to account for this, where nitrite was >2.5% of nitrate+nitrite, samples were re-run with sulphamic acid removal. For samples where nitrite was <2.5% of nitrate+nitrite, d 15 N-NO3+NO2 samples were corrected assuming a d 15 N-170 NO2 of -24‰ (Kemeny et al., 2016). d 15 N-PN was determined by EA-IRMS using a Costech Instruments Elemental Analyser coupled to Thermo Scientific Delta V Advantage mass spectrometer fitted with Conflo IV gas handling system. The instrumentation was operated using ISODAT 3.0 isotope ratio MS software. Prior to analysis the filters were wrapped in tin foil cones (OEA Laboratories) and pelletised. 175
Dense Barents Sea Water (BSW) was observed near the seabed in Hopen Trench (Figure 2a, HT). Above it lay cooled Atlantic origin water and a thermally stratified surface layer. North of the narrow sill joining the Spitzbergen and Great Banks ( Figure  185 2a, SB-GB), the approximate location of the Polar Front, colder (< 0 °C) and fresher (S < 37.7) ArW occupied depths below 50 m (Figure 2b). This was capped with an even fresher (S < 34) layer of sub-zero temperature melt water. This Polar Surface Layer extended southwards from the Nansen Basin, becoming progressively thinner. Below 100 m depth, over the shelf break https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License. and continental slope of the Nansen Basin, high salinity (cooled) Atlantic origin water within the Boundary Current that had entered the Arctic via the Fram Strait was observed. 190 In the AW, nitrate concentrations were relatively homogenous below the mixed layer (11.8 ±1.8 µM) but low or below the limits of detection in the euphotic layer (Figure 2c). NH4 + concentrations were highest close to the seafloor over the Spitsbergen Bank (Figure 2d, Figure 4).  1 ±1.1 µM). BSW d 18 O-NO3 was 2.1 ±0.5‰, lower than AW but higher than ArW, reflecting a mix between these two water masses.

Origin of Atlantic Water supplied to the Barents Sea
The nutrient concentration within the AW that is advected into the Barents Sea is controlled by the relative contribution of 210 nutrient-rich North Atlantic subpolar water and nutrient-poor subtropical waters that reach the Norwegian Sea together with the biological and physical transformations en-route. (Hatun et al., 2017;Rey, 2012;Johnson et al., 2013). Pre-bloom nutrient concentrations set the upper limit on seasonal primary productivity. Therefore, the origin of AW is important. Here we consider the contribution of subtropical and subpolar water to the AW sampled in the Barents Sea based on known nitrate isotope end members. We discuss the processes that the source waters are likely to have undergone and consider historical and 215 future long-term trends in d 15 N-NO3.

235
In the North Atlantic, low d 15 N-NO3 is associated with high salinity of the subtropical gyre (Knapp et al., 2008). The salinity of the AW supplied to the Barents Sea has increased in recent years (Barton et al., 2018;Oziel et al., 2016). We suggest that continued increases in salinity and the associated decrease in nitrate supply (Rey, 2012), has the potential to decrease d 15 N-NO3 of Arctic nitrate supply albeit to a small degree. Based upon the salinity-d 15

Nitrate utilisation and limitation in the Barents Sea
In July 2017, nitrate was depleted in the euphotic zone, coinciding with an increase in both d 15 N-NO3 and d 18 O-NO3. The seasonal uptake of nitrate by phytoplankton fractionates d 15 N-NO3 and d 18 O-NO3 with an isotope effect (e), close to 5‰ 245 (Sigman et al., 2009b). This relationship can determine the relative importance of algal uptake versus other processes such as dilution and regeneration (DiFiore et al., 2006;Rafter et al., 2012). Here we find that in the Arctic, e is often muted in surface waters through dilution with nitrate-deplete freshwater.
The southern Barents Sea remains ice-free all year round and away from the Norwegian Coastal Current, the near surface 250 salinity remains high. During the spring and summer months a warm surface mixed layer is established which triggers https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License. phytoplankton growth. As nitrate decreases, both d 15 N-NO3 and d 18 O-NO3 increase and algal uptake of nitrate is the dominant N cycling process occurring in the euphotic zone and is fuelled by new production (nitrate). We estimate a d 15 N-NO3 uptake fractionation of 4.7-4.9‰ (Figure 5a and c), with isotopic data following a trend for Rayleigh fractionation, or a closed system (Mariotti et al., 1981). This finding is anticipated since strong stratification isolated the euphotic zone from deeper waters 255 during the time of sampling.
In the northern Barents Sea, d 15 N-NO3 and d 18 O-NO3 increase as nitrate decreases in the euphotic zone. These waters are cooler and fresher and are likely to have undergone at least one seasonal cycle on the Barents Sea shelf, where there is evidence for nutrient regeneration (Section 4.3). We find a muted uptake fractionation in this region of 1.8‰ which is likely due to 260 dilution of the nitrate concentration by fresh, nutrient depleted surface water (Figure 5a and c).
The stable isotopic signal recorded in the Arctic marine food web is primarily dependent upon the particulate organic material produced by phytoplankton, representing the base of the food web, whose 15 N is controlled by the dissolved nutrient source.
With knowledge of the mechanism behind isotopic fractionation during nitrate uptake, and if nitrate uptake is the primary N 265 cycling process occurring in the euphotic zone, then d 15 N-PN may be predicted.
The JR16006 cruise was conducted during summer when the southern Barents Sea was thermally-stratified. Further north, seaice melt had established a fresh surface mixed layer resulting in salinity-driven stratification. Throughout the Barents Sea, particulate organic matter load was highest in the euphotic zone (average of 31.7 ±14.7 µg L -1 ), and decreased to 9.5 ±3.4 µg 270 L -1 below 70 m. We found that d 15 N-PN largely followed nitrate concentration, falling close to the trend for the integrated product of N uptake (Figure 6a). In areas where there was still nitrate available to phytoplankton, d 15 N-PN was lower representing the preferential consumption of the lighter isotope.
where u = NO3observed /NO3initial, d 15 Ninitial = 5.1‰, e=5‰, and NO3initial = 11.8 µM. We find the spatial trends are captured in the modelled data with the highest modelled d 15 N-PN where the concentrations are the lowest and vice versa (Figure 6c).
Deviations from the trend, representing a lower isotopic effect, are in lower temperature samples from ArW (Figure 6c). At 280 these locations the upper euphotic zone is salinity stratified and polar surface water dilutes the nitrate concentration. If the ArW samples are corrected to the lower isotope effect of 1.8‰ and nitrate concentration (10 µM), as predicted from our d 15 N-NO3 data, we find a Pearson's correlation, r=0.87, df=33, p=0 (Figure 6c). https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License.
These results support the finding that nitrate from the Atlantic is the primary source of nutrients to phytoplankton in surface 285 waters and that the organic matter in the euphotic zone is principally autotrophic. When there is still nitrate readily available in surface waters the phytoplankton preferentially take up 14 N and a lower d 15 N is expressed in particulate N. As there is full utilisation of nutrients over the growing season, we suggest that the integrated source of organic matter to the sediments and food web is ~5‰ throughout the Barents Sea. November and March were relatively constant at around 5‰. In June d 15 N-PN decreased as the lighter isotope is preferentially consumed by phytoplankton (Figure 7f). The relatively constant value of 5‰ for the rest of the annual cycle reflected the AW source value of ~5‰, suggesting that there is limited new production occurring over the winter months and that d 15 N-PN represents the integrated product of nitrate uptake from the previous growing season.

Nitrification
As inflowing AW cools and freshens across the Barents Sea, d 18 O-NO3 decreases from its AW source value of 2.8±0.3‰ to 1.6±0.3‰ (Figure 3). This decline represents N recycling and nitrification. A range in nitrified nitrate values of -1.5 to 1.3‰ have been reported from nitrifier cocultures (Buchwald et al., 2012). Previous field and modelling studies have used a nitrifying d 18 O value of 1.1‰ plus d 18 OH2O (Granger et al., 2013;Sigman et al., 2009b). As nitrate is regenerated, newly nitrified nitrate 310 tracks the d 18 O of seawater, which, in the Barents Sea is ~0.2‰ (Schlitzer et al., 2018), therefore as the proportion of regenerated nitrate increases, d 18 O will decrease towards ~1.3‰.
The recycling of nitrate in-situ is a common feature on Arctic shelves, evidenced using nitrate isotopes on the West Siberian Shelf (Fripiat et al., 2018) and the Canadian Shelf (Granger et al., 2018). In these regions, d 18 O-NO3 tracks d 18 O-H2O showing the importance of N recycling in sustaining the N-limited primary production the following season. As we have characterised the Atlantic source d 18 O-NO3, we can track the extent of nitrification across the Barents Sea to give an estimate of the proportion of regenerated nitrate on the Barents Sea shelf (Granger et al., 2013). north of which the proportion of regenerated nitrate increases from <10% in the south, to >80% in ArW. The nutrient concentration of ArW is coupled to winter mixing, driven by atmospheric cooling and brine release during sea ice formation.
The ArW experiences nutrient regeneration and nitrification over winter which works to decrease d 18 O-NO3. As the shelf waters on the Barents Sea cool and freshen, the nitrate inventory is also replenished from the nitrification of ammonium which is supplied to the water column from sediments. The resupply and mixing of nutrients from the sediments is an important 330 component in replenishing the N inventory.

Nitrogen resupply from sediments
Organic matter produced in surface waters will ultimately be regenerated in the water column or sink to the seafloor. The release of NH4 + from relatively shallow Arctic sediments has been noted in previous work, where the organic rich shelf 335 sediments provide a source of NH4 + to the water column (Brown et al., 2015). Studies from the Chukchi Sea suggest there are annually varying rates of nitrification, with much higher rates in winter (Christman et al., 2011). This suggests that there is a build-up of NH4 + in summer and NH4 + concentrations decrease into winter as nitrification rates exceed NH4 + release. We found NH4 + was enhanced over the Spitzbergen Bank and in the Hopen Trough, with the highest concentrations close to the sediment rather than the euphotic zone, indicating that the sediments are releasing NH4 + to the water column. 340 NH4 + is generated in sediments by the ammonification of organic material and can be released by diffusive and non-diffusive fluxes (Granger et al., 2011). Previous studies have suggested that the NH4 + produced during ammonification should be similar to the organic matter source, but that there is a large isotopic effect (~14‰) associated with the nitrification of NH4 + to NO2 - (Casciotti et al., 2003). Over the course of the season, once all NH4 + that has been released from the sediments and oxidised, 345 d 15 N-NO3 should reflect the N source (in this study: 5.1 ±0.1‰). There were a few samples with low (<4.8 ‰) d 15 N-NO3 on the flanks of the Spitzbergen Bank, near the seabed at the head of Hopen Trench and over the SB-GB sill, which may be associated with partial N recycling processes and the retention of 15 N in NH4 + (Casciotti et al., 2003). However, nitrate d 15 N https://doi.org/10.5194/bg-2020-293 Preprint. Discussion started: 9 September 2020 c Author(s) 2020. CC BY 4.0 License. below the nitricline was relatively homogenous across our sampled transect, reflecting the AW source value of 5.1 ±0.1‰ (Table 1). 350 In the western Arctic, Bering Sea and East Siberian Sea the release of NH4 + from sediments leads to a decrease in d 18 O-NO3 and an enrichment in d 15 N-NO3 over the timescales of water mass transit (Fripiat et al., 2018;Granger et al., 2018). In these regions, remineralisation is greater than nitrification, therefore NH4 + diffuses out of the sediments which is higher in 15 N, as low d 15 N nitrified nitrate is lost to benthic denitrification in sediments (Granger et al., 2011). This process enriches NH4 + in 355 15 N, a signature which is subsequently imparted on the overlying water column when NH4 + is released from the sediments and oxidized by nitrifiers (Brown et al., 2015). These trends are combined with concomitant decreases in N*, demonstrating the prevalence of benthic denitrification on Arctic shelves, where d 15 N increases from ~6.5‰ at the Bering Strait (Brown et al., 2015;Lehmann et al., 2007) to 8‰ on the Canadian and Siberian Shelves (Fripiat et al., 2018;Granger et al., 2018).

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If coupled partial nitrification-denitrification was occurring in the Barents Sea sediments, there should be an observed increase in d 15 N-NO3 with the decrease in d 18 O-NO3 through NH4 + release and nitrification. This is not evident in our dataset. (Table   1, Figure 4). Instead, we found no clear increase in d 15 N-NO3 or decrease in N* from the AW entering the shelf, to ArW and BSW further north and east (Table 1). This finding suggests that either the process of NH4 + release from the sediments is insignificant to the water column nitrate inventory, or that in contrast to the Canadian and Siberian Shelves, the layer of low 365 oxygen (and thus denitrification), is separated from the layer of ammonification and NH4 + release from sediments. The high NH4 + , which exceeds 25% of the dissolved inorganic N inventory at the base of some profiles, suggests that NH4 + was accumulating in the water column at the time of our study.
In the Barents Sea, the shallow banks and slopes (e.g. Spitzbergen Bank) experience strong tidal and frontal currents which 370 induce significant mixing (Sundfjord et al., 2007), and in shallower water winter convection is able to overturn the whole water column, processes that are able to remobilise and increase the oxygenation of surficial sediments. In the shallow regions we would therefore predict a deeper depth of denitrification within sediments and the faster release of NH4 + to the water column, largely by advective rather than diffusive fluxes.

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The contrasting findings between this study in the Barents Sea and other Arctic shelves may result from a number of factors.
The Pacific inflow supplies the much shallower Chukchi and Beaufort shelves (< 60m), with higher concentrations of macronutrients (Granger et al., 2013). In contrast, the Atlantic inflow to the Barents Sea provides lower concentrations of macronutrients to a deeper shelf (> 100m), therefore the organic load to sediments and thus benthic denitrification is expected to be lower (Chang and Devol, 2009). 380 We show that nitrogen availability in the Barents Sea is supported through AW supply and the efficient replenishment of nutrients through seasonal cycling processes. By the end of the growing season, all nitrate is consumed in surface waters and the d 15 N of PON reflects the AW source. The N inventory is dependent on the NH4 + release from sediments and nitrification. 385 In contrast to other Arctic shelf regions, we find no evidence for benthic denitrification communicating with the water column (and no loss of N relative to P Previous work has suggested that increasing NPP on Arctic shelves would increase organic matter supply to sediments and thus increase sedimentary denitrification rates (Arrigo and van Dijken, 2015). As N is the primary limiting nutrient to Arctic phytoplankton (Mills et al., 2018), this would have downstream consequences to NPP in the central Arctic basin. Our findings suggest that increased organic matter supply to sediments through increasing NPP is unlikely to exacerbate N limitation in the 395 Barents Sea. We conclude that changes to productivity across the Barents Sea are unlikely to have a significant impact on the nutrient inventory in the Barents Sea and thus the magnitude of nutrients transported to the Arctic waters, which is more dependent on variability in the Atlantic Water inflow.