The Arctic is undergoing unprecedented environmental change. Rapid warming, decline in sea ice extent, increase in riverine input, ocean acidification and changes in primary productivity are creating a crucible for multiple concurrent environmental stressors, with unknown consequences for the entire arctic ecosystem. Here, we synthesized 30 years of data on the stable carbon isotope (δ13C) signatures in dissolved inorganic carbon (δ13C‐DIC; 1977–2014), marine and riverine particulate organic carbon (δ13C‐POC; 1986–2013) and tissues of marine mammals in the Arctic. δ13C values in consumers can change as a result of environmentally driven variation in the δ13C values at the base of the food web or alteration in the trophic structure, thus providing a method to assess the sensitivity of food webs to environmental change. Our synthesis reveals a spatially heterogeneous and temporally evolving δ13C baseline, with spatial gradients in the δ13C‐POC values between arctic shelves and arctic basins likely driven by differences in productivity and riverine and coastal influence. We report a decline in δ13C‐DIC values (−0.011‰ per year) in the Arctic, reflecting increasing anthropogenic carbon dioxide (CO2) in the Arctic Ocean (i.e. Suess effect), which is larger than predicted. The larger decline in δ13C‐POC values and δ13C in arctic marine mammals reflects the anthropogenic CO2 signal as well as the influence of a changing arctic environment. Combining the influence of changing sea ice conditions and isotopic fractionation by phytoplankton, we explain the decadal decline in δ13C‐POC values in the Arctic Ocean and partially explain the δ13C values in marine mammals with consideration of time‐varying integration of δ13C values. The response of the arctic ecosystem to ongoing environmental change is stronger than we would predict theoretically, which has tremendous implications for the study of food webs in the rapidly changing Arctic Ocean.

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.

Arctic primary production has increased by >50% in the last two decades, fueled by increased light and nutrient availability, but whether or not this trend continues will depend on a sustained nutrient supply to phytoplankton (Arrigo & van Dijken, 2015;Lewis et al., 2020). Currently, nitrogen (N) is the main limiting nutrient to phytoplankton in the Arctic Ocean (Krisch et al., 2020;Mills et al., 2018). As warming continues, the degree of N limitation may change as the water mass properties supplied from the Atlantic and Pacific are altered (Hatún et al., 2017;Woodgate, 2018). Stable isotope measurements of nitrate and organic nitrogen can be used to provide insights into past and present N limitation to phytoplankton (Francois Abstract The hydrography of the Arctic Seas is being altered by ongoing climate change, with knockon effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the east, and southward flowing cold and fresh Polar Surface Water in the west. Here, we assess how physical processes control nutrient dynamics in the Fram Strait using nitrogen isotope data collected during 2016 and 2018. In Atlantic Waters, a weakly stratified water column and a shallow nitracline reduce nitrogen limitation. To the west, in Polar Surface Water, nitrogen limitation is greater because stronger stratification inhibits nutrient resupply from deeper water and lateral nitrate supply from central Arctic waters is low. A historical hindcast simulation of ocean biogeochemistry from 1970 to 2019 corroborates these findings and highlights a strong link between nitrate supply to Atlantic Waters and the depth of winter mixing, which shoaled during the simulation in response to a local reduction in sea-ice formation. Overall, we find that while the eastern Fram Strait currently experiences seasonal nutrient replenishment and high primary production, the loss of winter sea ice and continued atmospheric warming has the potential to inhibit deep winter mixing and limit primary production in the future. Plain Language SummaryThe Fram Strait is the main gateway of the Arctic Ocean. In the east, warm, salty waters from the Atlantic flow north into the Arctic basin, and in the west, cold, fresh waters flow south from the central Arctic into the North Atlantic. We examined how changes to the availability of nutrients (which are essential for algae to grow) may limit algae growth in the Fram Strait, both as a result of changes to their source and also how easily the upper ocean mixes nutrients from depth. In the eastern Fram Strait, there is a high availability of nitrate, one of the main nutrients to support algae growth, and winter mixing sustains nutrient supply and biological production in recent decades. However, in the western Fram Strait, the outflowing surface waters do not easily mix with deeper waters and are depleted in nitrate, and nut...

Knowledge of species trophic position (TP) is an essential component of ecosystem management. Determining TP from stable nitrogen isotopes (δ15N) in predators requires understanding how these tracers vary across environments and how they relate to predator isotope composition. We used two seal species as a model for determining TP across large spatial scales in the Arctic. δ15N in seawater nitrate (δ15NNO3) and seal muscle amino acids (δ15NAA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ15NNO3 and δ15NAA. Our results show that the spatial variation in δ15NAA in seals reflects the δ15NNO3 end members in Pacific vs. Atlantic waters. This study provides a reference for best practice on accurate comparison of TP in predators and as such, provides a framework to assess the impact of environmental and human‐induced changes on ecosystems at pan‐Arctic scales.

Environmental contaminants and parasites are ubiquitous stressors that can affect animal physiology and derive from similar dietary sources (co-exposure). To unravel their interactions in wildlife, it is thus essential to quantify their concurring drivers. Here, the relationship between blood contaminant residues (11 trace elements and 17 perfluoroalkyl substances) and nonlethally quantified gastrointestinal parasite loads was tested while accounting for intrinsic (sex, age, and mass) and extrinsic factors (trophic ecology inferred from stable isotope analyses and biologging) in European shags Phalacrocorax aristotelis . Shags had high mercury (range 0.65–3.21 μg g –1 wet weight, ww) and extremely high perfluorooctanoic acid (PFOA) and perfluorononanoic acid (PFNA) residues (3.46–53 and 4.48–44 ng g –1 ww, respectively). Males had higher concentrations of arsenic, mercury, PFOA, and PFNA than females, while the opposite was true for selenium, perfluorododecanoic acid (PFDoA), and perfluooctane sulfonic acid (PFOS). Individual parasite loads ( Contracaecum rudolphii ) were higher in males than in females. Females targeted pelagic-feeding prey, while males relied on both pelagic- and benthic-feeding organisms. Parasite loads were not related to trophic ecology in either sex, suggesting no substantial dietary co-exposure with contaminants. In females, parasite loads increased strongly with decreasing selenium:mercury molar ratios. Females may be more susceptible to the interactive effects of contaminants and parasites on physiology, with potential fitness consequences.