Author Contribution Statement: C.D.L.V., C.M., S.S., J.H., A.T., and R.M.J. designed the study. D.J.Y., S.H.F., G.B.S., M.B., E.S.N., T.H., and R.E.T. collected the samples. C.D.L.V. and R.E.T. analyzed the samples. C.D.L.V. performed the data and statistical analyses and wrote the first draft of the manuscript. All authors contributed to editing and refining the manuscript.

Data Availability Statement: Data are available via the UK polar data center (PDC): de la Vega, C. (2020). ARISE project - Work package 2: Stable nitrogen isotopes of nitrate in sea water, and of bulk tissue and amino-acids of muscle of harp and ringed seals from the Arctic and sub-Arctic (Version 1.0) [Data set]. UK Polar Data Centre, Natural Environment Research Council, UK Research & Innovation. https://doi.org/10.5285/66AAF3C8-FA58-41DE-8EF1-480A2E125408.

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Abstract

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 (δ¹⁵N_NO3) and seal muscle amino acids (δ¹⁵N_AA) were determined to independently characterize the base of the food web and the TP of harp and ringed seals, demonstrating a direct link between δ¹⁵N_NO3 and δ¹⁵N_AA. Our results show that the spatial variation in δ¹⁵N_AA in seals reflects the δ¹⁵N_NO3 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.

Scientific Significance Statement

Decadal trends in the trophic position (TP) of marine predators, ascertained through tracers such as stable nitrogen isotopes, have been used to infer the impact of environmental change on ecosystems. Understanding how the environment is altering stable nitrogen isotopes at the base of the food web is key to interpreting these tracers in predators. Here, we demonstrate that the stable nitrogen isotope signatures in harp and ringed seals across the Arctic are directly controlled by the stable nitrogen isotope signature of the water masses they forage in. This has important implications for accurately estimating the TP of predators, as water mass circulation in the Arctic Ocean has been altered during the last decades as a result of climate change.

Marine ecosystems are being modified as a result of multiple stressors, including global environmental change, fish exploitation, pollution, and habitat degradation (IPCC 2019). Determining the resilience of marine ecosystems to perturbations is essential for sustainable management in a changing environment (Silberberger et al. 2018). Food webs interconnect a diverse range of species and body sizes, over large spatial scales and across a variety of different habitats. Food web structure is inherently linked to ecosystem function and resilience (Yen et al. 2016). Trophic position (TP) of top and near-top predators is a fundamental property of ecological communities. It has the general function of reflecting changes in ecosystems overall, and can be used to assess food web structure, food chain length, and functional roles of predators (Post 2002).

Stable nitrogen isotopes are commonly used as a chemical tracer to reconstruct food webs and estimate TP of predators. The ratio between heavy (¹⁵N) and light (¹⁴N) isotopes of bulk tissue (δ¹⁵N_bulk) increases by ~ 3‰ at each trophic level, providing a continuous measure of TP (Post 2002). However, δ¹⁵N_bulk is influenced by δ¹⁵N at the base of the food web, or “baseline.” Variation in δ¹⁵N_bulk in predators can therefore reflect changes in either (1) TP (Fig. 1a) or (2) δ¹⁵N at the baseline (Fig. 1b).

Details are in the caption following the image — **Fig. 1**
Open in figure viewer PowerPoint

Examples of variation in the δ¹⁵N_bulk observed in a predator caused by (a) change in the food chain length or in the predator diet and (b) variation in the δ¹⁵N at the baseline (e.g., nitrate), and predictions of (c) relative δ¹⁵N at the baseline (e.g., nitrate) in Atlantic (Atl.) and Pacific (Pac.) influenced Arctic regions and (d) relative baseline-corrected δ¹⁵N (cor-δ¹⁵N) in harp (HS) and ringed (RS) seals in the Arctic.

Compound-specific stable nitrogen isotopes of amino acids (δ¹⁵N_AA) is a powerful approach that disentangles baseline and trophic level effects from the analysis of consumer tissue alone. The δ¹⁵N of “source” amino acids experiences negligible fractionation during trophic transfer and conservatively traces the δ¹⁵N baseline, whereas significant fractionation of “trophic” amino acids results in ¹⁵N enrichment between each trophic transfer (McMahon and McCarthy 2016). The uncertainty regarding trophic fractionation factors between “source” and “trophic” amino acids across taxa in entire food webs prevents accurate estimation of an organism's absolute TP (Nielsen et al. 2015). However, this approach allows robust estimation of relative TP (TP_rel) and is particularly insightful when comparing TP_rel of mobile predators, which integrate the biochemical characteristics of their foraging habitats over large spatial scales with potentially different baselines.

Here, we used two key marine predators, the ringed (Pusa hispida) and harp (Pagophilus groenlandicus) seal, as model species for determining TP_rel across large spatial scales and environmental gradients in the Arctic and sub-Arctic. The Arctic Ocean is experiencing unprecedented rates of environmental change compared to the rest of our planet (IPCC 2019). Changes in sea ice extent and thickness, and hydrographic structure have altered the timing and magnitude of primary production (Arrigo and van Dijken 2015). The warming ocean is leading to changes in zooplankton (Dalpadado et al. 2016) and fish (Fossheim et al. 2015) communities. Collectively, these food web alterations are affecting the phenology, behavior, and distribution of top predators in the Arctic (IPCC 2019). Understanding food web structure in the Arctic and sub-Arctic is vital for the development of policies to manage and conserve these unique polar ecosystems.

Phytoplankton underpins marine food webs and their δ¹⁵N mainly reflects the δ¹⁵N of seawater nitrate (δ¹⁵N_NO3), an essential nutrient (Mariotti et al. 1981). Nitrate is supplied to the Arctic Ocean by Atlantic water entering through the Barents Sea and on the eastern side of Fram Strait, and by Pacific water crossing the Bering Strait (Fig. 2) (Torres-Valdés et al. 2013). Pacific water δ¹⁵N_NO3 is enriched in ¹⁵N by ~ 3‰ compared to Atlantic water δ¹⁵N_NO3, as a result of the biological processing within the Pacific and Atlantic oceans (Somes et al. 2010). Pacific and Atlantic waters are further modified by the physical and biogeochemical changes that occur within the Arctic basin, before exiting via the Canadian Archipelago and on the Western side of Fram Strait (Fig. 2; Torres-Valdés et al. 2013). Gradients in δ¹⁵N_NO3 across the Arctic and sub-Arctic therefore reflect water mass supply, mixing processes and in situ nitrogen cycling. To reliably detect pan-Arctic trends in seal TP_rel, it is essential to account for spatial variation in the δ¹⁵N at the baseline.

In this study, we used both δ¹⁵N_bulk and δ¹⁵N_AA to determine the TP_rel of harp and ringed seals. Ringed and harp seals are abundant near-top trophic level generalists that have a wide distribution. Generally, their diet consists of a large variety of pelagic invertebrates and forage fish (Wathne et al. 2000; Folkow et al. 2004; Nordøy et al. 2008; Lindstrøm et al. 2013; Ogloff et al. 2019). Given these characteristics, ringed and harp seals are suitable model species with which to quantify spatial variation in food web structure. We specifically focus on δ¹⁵N of the source amino acid phenylalanine (δ¹⁵N_phe), which reflects variations in the baseline, and δ¹⁵N of three main trophic amino acids (glutamic acid, aspartic acid, and leucine), allowing accurate estimation of TP_rel McMahon and McCarthy 2016). In addition, we compared δ¹⁵N_phe in seals and δ¹⁵N_NO3 from the seal foraging areas. We predict that: (1) δ¹⁵N_bulk in Arctic seals varies across the Arctic, (2) spatial variation in δ¹⁵N_bulk is driven by variation in δ¹⁵N_phe, (3) spatial variation in δ¹⁵N_phe is driven by spatial variation in δ¹⁵N_NO3 reflecting water mass characteristics (Fig. 1c), and (4) harp and ringed seals are at similar TP (Fig. 1d), which does not vary across the Arctic.

Materials and methods

Seal sampling

A total of 210 muscle samples were obtained from the longissimus dorsi of adult (older than 6 year old) harp and ringed seals at six sites across the Arctic and sub-Arctic (Southern Barents Sea, Northern Barents Sea, Greenland Sea, Labrador shelf, Baffin Island, and Canadian Archipelago; Fig. 2, Table 1). Harp seals from the Barents Sea were sampled by the Norwegian Institute of Marine Research as part of Norwegian commercial sealing. Permission for the scientific catch of harp seals from the Greenland Sea in 2018 and 2019 was obtained from the Ministry of Foreign Affairs of Denmark (Utenrigsministeriet) in verbal notes (JTHAV sagsnr 2017-4885 and JTHAV sagsnr. 2019-9877) and from the Norwegian Directorate of Fisheries (letter ref. 18/1124 and 18/14793). Ringed and harp seals from the Canadian Archipelago, Baffin Island and Labrador Shelf were collected by trained, licensed hunters following the humane hunting requirements, as part of the Inuit subsistence and commercial harvests. All samples were immediately frozen and stored at −20°C.

Table 1. Seal sampling sites and regions, seal species, total number of seal samples (N), number of seal samples selected for δ¹⁵N analyses on amino acids (n), seal sampling years, seal sampling months, Arctic regions reflected in seal muscle tissue, mean δ¹⁵N_NO3 ± SD (sample number) in the region(s) integrated by seal muscle tissue (see Fig. 3); seal and nitrate sampling regions are labeled on Fig. 2.

Sampling sites	Sampling regions	Species	N*	n*	Sampled years	Sampling months	Region(s) integrated in muscle	δ¹⁵N_NO3 ± SD (n)
Cape Kanin	Southern Barents Sea	Harp	17	17	2018	April, May	Southern and northern Barents Sea	5.1 ± 0.1‰ (n = 11)
North Svalbard	Northern Barents Sea	Harp	4	4	2016	September	Southern and northern Barents Sea	5.1 ± 0.1‰ (n = 11)
Jan Mayen	Greenland Sea	Harp	17	17	2018, 2019	March	Greenland Sea and northern Barents Sea	5.1 ± 0.2‰ (n = 4)
Labrador shelf	Labrador shelf	Harp	59	14	2017, 2018	January, February	Baffin Island area and Labrador shelf	5.6 ± 0.3‰ (n = 6)^†
Pangnirtung	Baffin Island	Harp	8	8	2015, 2016	July, September	Baffin Island area and Labrador shelf	5.6 ± 0.3‰ (n = 6)^†
Pangnirtung	Baffin Island	Ringed	4	4	2015, 2016	September, October	Baffin Bay	6.2 ± 0.2‰ (n = 3)^†
Resolute	Canadian archipelago	Ringed	10	10	2015, 2016	June, July, August	Canadian archipelago	6.8 ± 0.2‰ (n = 1)^†

^* Total number of females (F), males (M) and unknown sex (U), and number of females (f), males (m) and unknown sex (u) selected for δ¹⁵N analyses on amino acids; Harp seals F/M/U (f/m/u): 71/33/1 (45/14/1); ringed seals F/M/U (f/m/u): 8/5/1 (8/5/1).
^† Data from Lehmann et al. (2019).

Seal sampling design

Muscle tissue, which integrates the δ¹⁵N of the diet over 4–5 months (Vander Zanden et al. 2015), reflected seal foraging over different seasons depending on the sampling month (Table 1; Fig. 3).

The harp seal populations of Greenland and southern Barents Sea (Fig. 2) partially overlap in the northern Barents Sea during the summer and autumn (Folkow et al. 2004; Nordøy et al. 2008). In late November/early December, harp seals migrate back toward their breeding and molting areas in the Greenland Sea and the southern Barents Sea/White Sea. Muscle samples collected in March from harp seals from the Greenland Sea, reflected the diet integrated from late autumn to late winter (Fig. 3), whereas muscle samples collected in spring from the southern Barents Sea (Table 1) reflected the diet integrated from winter to spring (Fig. 3). These seals were foraging within the sampling regions during both periods (Table 1, Fig. 3). Muscle tissue of harp seals from the northern Barents Sea reflected a combination of diets consumed in the Greenland Sea, southern and northern Barents Sea (Table 1, Fig. 3).

Harp seals from Newfoundland spend summer and autumn in Arctic waters (Baffin Island and Davis Strait) and migrate south to the Labrador shelf in early winter (Lacoste and Stenson 2000). Harp seal samples from the Labrador shelf, collected in winter, and from Baffin Island, collected in summer, reflected both a combination of the diet from the Baffin Island area and the Labrador shelf (Table 1; Fig. 3).

Ringed seal samples from Baffin Island, collected in autumn, and from the Canadian Archipelago, collected in summer, reflected the diet during seasons when seals were foraging within the sampling regions (Yurkowski et al. 2016) (Table 1; Fig. 3).

Stable nitrogen isotopes analyses in seals

Stable isotope (δ¹⁵N_bulk and δ¹⁵N_AA) analysis of seal muscle tissue was carried out at the Liverpool Isotopes for Environmental Research laboratory, University of Liverpool and results are reported in standard δ-notation (‰) relative to atmospheric N₂ (Hobson and Welch 1992; Hobson et al. 1997; Germain et al. 2013). Details of sample preparation, instrument configuration, and reproducibility are detailed in Supporting Information S1. All samples were analyzed for δ¹⁵N_bulk and a subset were selected for δ¹⁵N_AA (Table 1; de la Vega 2020).

TP estimation

We used the δ¹⁵N of phenylalanine (δ¹⁵N_Phe) to track the δ¹⁵N of the baseline and the δ¹⁵N of three amino acids (δ¹⁵N_trophic; glutamic acid, aspartic acid, and leucine) to estimate TP. The uncertainty regarding trophic fractionation factors between “source” and “trophic” amino acids across taxa in entire food webs prevents accurate estimation of an organism's absolute TP (Nielsen et al. 2015). To compare the relative TP (TP_rel) across sampling sites, we subtracted the δ¹⁵N_Phe values from δ¹⁵N_trophic providing baseline-corrected δ¹⁵N_trophic values (cor-δ¹⁵N_trophic; Supporting Information S2). To prevent variation in the absolute values from overwhelming trends in relationships among AA, cor-δ¹⁵N_trophic values were scaled using z-transformation. We applied principal component analysis (PCA) on the scaled cor-δ¹⁵N_trophic values (Supporting Information S3) and used the scores of the PCA axis 1 as a proxy for TP_rel of harp and ringed seals.

Nitrate

Seawater for nitrate analysis from the European Arctic (Fig. 2) was collected as part of the NERC Changing Arctic Ocean program, from the RRS James Clark Ross in July–August 2017 (JR16006) and May–June 2018 (JR17005). Seawater was collected using a 24-position stainless steel rosette equipped with a SBE911plus CTD and 20-liter OTE bottles. δ¹⁵N_NO3 (Table 1; de la Vega 2020) were determined at the University of Edinburgh, UK, using the denitrifier method (Sigman et al. 2001) and following Geotraces protocols (Schlitzer et al. 2018). Samples were corrected using international reference standards N3 and USGS34 and analyzed in duplicate with a reproducibility < 0.2‰. δ¹⁵N_NO3 data from the North American Arctic (Table 1) were compiled from Lehmann et al. (2019). Mean values were calculated from samples below the mixed layer (mean sampling depth = 202 ± 107 m) and were representing the nitrate isotope end member in a given region prior to biological utilization.

Statistical analyses

Statistical analyses were performed in R v. 3.5.1 (R Core Team 2018), mainly following Zuur et al. (2009a) and Zuur et al. (2009b).

The effect of species on δ¹⁵N_bulk, δ¹⁵N_Phe, and TP_rel (scores of the PCA axis 1) was tested through linear models, with model fit being checked by residual analyses with visual inspection of quantile-quantile plots, and residuals and standardized residuals vs. fitted values plots.

As samples for both species were only available at one site, separate models were fitted for harp and ringed seals. Multifactorial linear models were used to investigate the influence of site, individual body length, and sex on δ¹⁵N_bulk, δ¹⁵N_Phe, and TP_rel (scores of the PCA axis 1) for harp and ringed seals separately. Explanatory variables were not significantly collinear (variance inflation factors [VIFs] < 3). Model selection was based on Akaïke information criterion scaled for small sample sizes (AICc). We compared a list of biologically meaningful candidate models, with the maximal model being: δ¹⁵N = site + length + sex. Model specification was validated via residual analyses of maximal model. For each specific model, we calculated the AICc, the difference between AICc of the specific model and the best model (ΔAICc), and the AICc weight (normalized weight of evidence in favor of the specific model, relative to the whole set of candidates). Variables included in the best model (lowest AICc) were considered to best explain variation in δ¹⁵N_bulk, δ¹⁵N_Phe, and TP_rel. For harp seals that were sampled at more than two sites, we applied ANOVAs followed by Tukey pairwise comparison tests on δ¹⁵N_bulk, δ¹⁵N_Phe, and TP_rel to test the effect of the most accurate explanatory factors derived from the model selection (Supporting Information S4). Significance was considered when the 95% confidence interval of the slopes did not cross zero. p values (α = 0.005; Benjamin et al. 2018), R², F-statistics, and df are reported for each model (Supporting Information S4).

The δ¹⁵N_NO3 values in seawater were averaged within the seals foraging areas (Table 1). The relationship between δ¹⁵N_bulk, δ¹⁵N_Phe and TP_rel in seal tissues and the averaged δ¹⁵N_NO3 were investigated using linear models and Pearson correlations.

Results

Spatial variation of δ¹⁵N_bulk in seals

δ¹⁵N_bulk in harp seals ranged from 13.2 ± 0.7‰ (Greenland Sea) to 14.4 ± 0.9‰ (Baffin Island). δ¹⁵N_bulk in ringed seals ranged from 16.3 ± 0.1‰ (Baffin Island) to 17.4 ± 0.4‰ (Canadian Archipelago, Fig. 4a). The best models for δ¹⁵N_bulk included “site” for both seal species (Tables 2, 4 in Supporting Information S4). In these models, δ¹⁵N_bulk varied significantly between sites in both harp (linear model, p < 0.005, R² = 41.2%, n = 105; Tables 2, 3 in Supporting Information S4) and ringed seals (linear model, p < 0.005, R² = 67.7%, n = 14; Table 5 in Supporting Information S4). δ¹⁵N_bulk in harp seals from the Greenland Sea was depleted in ¹⁵N compared to harp seals from the Southern Barents and Labrador Shelf (Tukey tests following ANOVA: p < 0.005; Fig. 4a, Table 3 in Supporting Information S4). The δ¹⁵N_bulk in ringed seals from the Baffin Island was depleted in ¹⁵N compared to the Canadian Archipelago (Fig. 4a, Table 5 in Supporting Information S4). δ¹⁵N_bulk of ringed seals was enriched in ¹⁵N compared to harp seals (linear model: p < 0.005, R² = 41.8%, n = 119; Table 1 in Supporting Information S4).

Spatial variation in the baseline

δ¹⁵N_NO3 of seawater was enriched in ¹⁵N by ~ 2‰ in the Pacific influenced Canadian Archipelago water (6.8‰), compared to the Barents Sea (5.1 ± 0.2‰) and Labrador Shelf (5.0 ± 0.3‰; Table 1). δ¹⁵N_Phe, representing the δ¹⁵N of the baseline in seal tissues ranged from 6.2 ± 0.9‰ (Greenland Sea) to 9.8 ± 0.7‰ (Baffin Island) in harp seals, and from 11.2 ± 0.2‰ (Baffin Island) to 12.1 ± 0.6‰ (Canadian Archipelago) in ringed seals (Fig. 4b,c). The best models for δ¹⁵N_Phe included “site” for both seal species (Tables 2, 4 in Supporting Information S4). In these models, δ¹⁵N_Phe varied significantly between sites in both harp (linear model, p < 0.005, R² = 56.4%, n = 60; Table 3 in Supporting Information S4) and ringed seals (linear model: p = 0.020, R² = 34.5%, n = 14; Table 5 in Supporting Information S4). δ¹⁵N_Phe in harp seals from the Greenland Sea and Northern Barents Sea were depleted in ¹⁵N compared to harp seals from the Labrador Shelf and Baffin Island (Tukey tests: p < 0.005; Table 3 in Supporting Information S4). δ¹⁵N_Phe in ringed seals from the Baffin Island was depleted in ¹⁵N compared to ringed seals from the Canadian Archipelago (Fig. 4b,c). δ¹⁵N_Phe of ringed seals was enriched in ¹⁵N compared to harp seals (linear model: p < 0.005, R² = 56.4%, n = 74; Table 1 in Supporting Information S4). δ¹⁵N_Phe were positively correlated with δ¹⁵N_bulk (Linear model: p < 0.005, R² = 68.9%; Pearson correlation: 85%, n = 74; Fig. 4b; Table 6 in Supporting Information S4) and with δ¹⁵N_NO3 from the seals foraging areas (linear model: p < 0.005, R² = 88.4%; Pearson correlation: 93%, n = 7; Fig. 4c; Table 6 in Supporting Information S4).

Seal TP

δ¹⁵N_bulk was ~ 3‰ higher in ringed seals than in harp seals, which would indicate that ringed seals occupy a higher TP than harp seals (Post 2002). However, δ¹⁵N_bulk was poorly and negatively correlated with TP_rel (linear model: p < 0.005, R² = 23.4%; Pearson correlation: −49%, n = 74; Fig. 4e; Table 6 in Supporting Information S4).

Cor-δ¹⁵N_trophic were enriched in ¹⁵N by ~ 4‰ in harp seals compared to ringed seals (linear model: p < 0.005, R² = 39.9%, n = 74; Supporting Information S2 and Table 1 in Supporting Information S4) indicating that harp seals are in fact approximately one TP higher than ringed seals (McMahon and McCarthy 2016). These trends are supported by the higher δ¹⁵N_bulk, but lower TP_rel of ringed seals compared to harp seals, specifically at sampling site 5 (Figs. 2, 4a,d), the only site where we were able to compare directly between species. TP_rel (as given by PCA axis 1 of Cor-δ¹⁵N_trophic) did not vary with site for any of the seals species, as the best model for TP_rel in harp seals only included length, and none of the models for TP_rel in ringed seals was better than the null model (Tables 2, 4 in Supporting Information S4).

Discussion

When using the δ¹⁵N_AA approach and correcting for variations in the baseline using δ¹⁵N_Phe, our results show that: (1) within each seal species, the TP_rel does not vary across the Arctic, confirming our prediction and (2) ringed seals are at a lower TP than harp seals, contradicting our prediction. If the traditional interpretation of δ¹⁵N_bulk in predators was applied here, we would conclude that: (1) the TP of seals varies between Arctic regions, as suggested by the spatial variation of δ¹⁵N_bulk between sampling sites and (2) ringed seals are one TP higher than harp seals, as evidenced by their ~ 3‰ enrichment in ¹⁵N (Post 2002). These findings highlight the power of using δ¹⁵N_AA when examining spatial variation in TP of predators and demonstrate the need to account for variation in the δ¹⁵N of the baseline to avoid misinterpretation of δ¹⁵N_bulk in consumers.

Harp seals are generally larger than ringed seals (Ogloff et al. 2019). While harp and ringed seals feed on broadly similar prey species, stomach content analysis has shown that ringed seals have a greater reliance on smaller fish and invertebrates in the upper water column compared to harp seals, which rely to a greater extent on larger fish at deeper depths, probably related to differences in body size and habitat preferences (Wathne et al. 2000; Ogloff et al. 2019). This is in agreement with the lower TP_rel of the smaller ringed seals compared to the larger harp seals.

Variation in δ¹⁵N_bulk in Arctic seals was largely driven by variation of δ¹⁵N of the baseline, as evidenced by the strong positive correlation between δ¹⁵N_Phe and δ¹⁵N_bulk in seal tissue and supported by the weak and negative correlation between TP_rel and δ¹⁵N_bulk. In turn, despite the small sample size (n = 7), the strong positive correlation between δ¹⁵N_Phe and δ¹⁵N_NO3 confirmed that spatial patterns in δ¹⁵N_Phe were driven by the δ¹⁵N of water masses associated with the seal foraging areas. The offset observed between δ¹⁵N_NO3 in water masses and δ¹⁵N_Phe in seal tissues demonstrates that there is some fractionation of phenylalanine from the base of the food web to the upper trophic levels. This has previously been reported as ~ 1.5‰ between each trophic step (Bradley et al. 2014; McMahon and McCarthy 2016), which agrees with observations in this study (1.1 ± 0.5‰, assuming the seals to be at trophic level 3).

For the first time, we demonstrate a direct link between δ¹⁵N_NO3, δ¹⁵N_Phe, and δ¹⁵N_bulk in predators, using observations of all three properties. Crucially, the ¹⁵N-enrichment of δ¹⁵N_Phe in seals from the Canadian archipelago and Baffin Island reflects the influence of the δ¹⁵N_NO3 of the Pacific derived water exiting the Arctic via the Canadian Archipelago (Lehmann et al. 2019), which is ¹⁵N enriched by ~ 2‰ compared to the Atlantic water inflow (Somes et al. 2010). Our results show that δ¹⁵N_Phe in seals can be used as tracers of spatial variation of environmental gradients across the Arctic. Any future changes in Arctic circulation, such as an increase of Pacific inflow through the Bering Strait (Woodgate 2018) or a weakening of the North Atlantic subpolar gyre (Hátún et al. 2017), will influence the δ¹⁵N baseline of the Arctic Ocean, and in turn the δ¹⁵N in Arctic seals, convoluting the detection of temporal trends in food web structure without baseline correction.

Changes in species composition of Arctic communities have already been observed as a result of environmental change. The northward shift of warmer water zooplankton (Dalpadado et al. 2016) and fish communities (Fossheim et al. 2015) has led to an increased abundance of boreal species at the expense of Arctic species, a process commonly referred to as “borealization.” This has implications for Arctic food web structure (Kortsch et al. 2015; Yurkowski et al. 2018) and more specifically, prey availability to mobile predators including ringed and harp seals. With continued climate warming and environmental change, the impact on Arctic and sub-Arctic ecosystems will intensify, potentially having divergent effects on harp and ringed seals due to differences in dietary plasticity as a result of differing life-history strategies (Ogloff et al. 2019). Regional and local rates of borealization could also lead to a different ecosystem response between Canadian and European Arctic (Moore et al. 2019). Decadal assessment of δ¹⁵N_AA values in Arctic seals is urgently required to assess past and future impact of environmental and human-induced changes on seal TP over pan-Arctic scales. Our study provides a reference for best practice on accurate comparison of TP_rel across large spatial and temporal scales, not only in the Arctic and sub-Arctic but also in other marine and terrestrial environments.

Conflict of Interest

None declared.

Supporting Information

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Acknowledgments

This work resulted from the ARISE project (NE/P006035/1, NE/P006310/1, and NE/P006000/2), part of the Changing Arctic Ocean programme, funded by the UKRI Natural Environment Research Council (NERC).

Citing Literature

Volume6, Issue1

February 2021

Pages 24-32

Arctic seals as tracers of environmental and ecological change