Besides the expected inter‐specific variations largely confirming known feeding patterns, we also found considerable intra‐specific variability in the FA profiles of the investigated under‐ice fauna species. All amphipod species and Calanus spp. from the Amundsen Basin regime had higher proportions of the FA 16:1 n −7 compared to the samples from the Nansen Basin regime. Additionally, all amphipods from the AB regime showed lower proportions of all other algal FAs than those sampled in the NB regime. The FA 16:1 n −7 was largely limited to I‐POM samples in our dataset. Hence, the observed variability between the two environmental regimes was probably driven by variability in ice algal communities rather than phytoplankton, assuming lipid turnover rates in these herbivores were fast compared to changes in algal composition (Graeve et al. 2005 ). An impact of the variability of sea ice communities on the FA composition is corroborated by pronounced differences in the community composition of protists in sea ice between the two environmental regimes (K. Hardge et al. unpubl.), as well as by differing drift pathways of sea ice between the NB and the AB in 2012 (David et al. 2015 ).

The carnivorous pteropod C. limacina is assumed to feed exclusively on Limacina helicina (Conover and Lalli 1974 ; Phleger et al. 2001 ). In our study, the FA composition of C. limacina was characterized by the lowest proportion of the diatom‐specific FA 16:1 n −7 and the highest proportion of the dinoflagellate‐specific FA 22:6 n −3, possibly reflecting a pelagic‐based diet of diatoms and dinoflagellates in L. helicina . The pteropod L. helicina was first described as a pure herbivore, but more recent studies reported an omnivorous diet consisting of small copepods and juvenile L. helicina (Gilmer 1974 ; Gilmer and Harbison 1991 ; Falk‐Petersen et al. 2001 ). The low levels of the Calanus ‐specific FAs found in our study in C. limacina , however, indicated that Calanus copepods were not important in the L. helicina ‐based pathway of the food web during the weeks before our sampling.

Carnivorous amphipods, such as E. holmii and T. libellula , constitute important links between lipid‐rich herbivores and top predators (Noyon et al. 2011 ). These two amphipod species also showed high levels of the diatom‐specific FAs 16:1 n −7 and 20:5 n −3. In T. libellula , a higher proportion of the dinoflagellate‐specific FA 22:6 n −3 indicated a greater importance of dinoflagellate‐derived carbon than in E. holmii . Both species, but particularly T. libellula , displayed elevated levels of the Calanus ‐specific marker FAs. Our findings are consistent with other feeding studies, which identified T. libellula as a part of the Calanus ‐based food web (Scott et al. 1999 ; Dalpadado et al. 2008 ; Kraft et al. 2013 ).

Calanus copepods are able to synthesize the long‐chain FAs 20:1 n −9 and 22:1 n −11 in large amounts de novo. These FAs can also be used as trophic indicators for a copepod‐related diet in higher consumers (Sargent et al. 1977 ; Wold et al. 2011 ). Accordingly, high values of the FA 20:1 n −9 indicated a partly Calanus ‐based diet in the omnivorous amphipod O. glacialis . G. wilkitzkii has been reported to also feed extensively on copepods, primarily during adult stages (Scott et al. 2001 ). However, we found only small amounts of the Calanus ‐specific FAs 20:1 n −9 and 22:1 n −11 in this amphipod, indicating that Calanus may not have been important in their diets before the sampling.

The FA composition of the amphipod A. glacialis indicated a diet dominated by diatom‐derived carbon, evident by high proportions of the diatom‐specific FAs 16:1 n −7 and 20:5 n −3, accompanied by low levels of the dinoflagellate‐specific FA 22:6 n −3. A diatom‐dominated diet is in agreement with several studies showing that A. glacialis primarily feeds on the under‐ice flora and phytodetritus (Bradstreet and Cross 1982 ; Scott et al. 1999 ; Tamelander et al. 2006a ). Together with O. glacialis and G. wilkitzkii , A. glacialis is known to live permanently associated with the Arctic sea ice (Poltermann 2001 ; Gradinger and Bluhm 2004 ). Thus, it is not surprising that O. glacialis and G. wilkitzkii contained high levels of the diatom markers 16:1 n −7 and 20:5 n −3, with considerably lower levels of the dinoflagellate‐specific FA 22:6 n −3.

Both Calanus spp. are known to be key Arctic grazers, utilizing both ice algae‐ and pelagic phytoplankton‐derived carbon (Søreide et al. 2010 ; Durbin and Casas 2013 ). There was little difference in the FA profiles between C. glacialis and C. hyperboreus , indicating that the primary carbon sources were similar for both Calanus species. As frequently shown, the FA composition of Arctic Calanus spp. was characterized by high amounts of the diatom‐specific FAs 16:1 n −7 and 20:5 n −3 (Graeve et al. 1994b ; Wang et al. 2015 ). Furthermore, our results showed that both copepod species contained high amounts of the dinoflagellate‐specific marker FA 22:6 n −3, which together suggests sources of carbon from both diatoms and dinoflagellates.

The FA profiles of the under‐ice fauna species revealed variable associations with diatom‐and dinoflagellate‐related marker FAs. Although it may be possible for herbivorous invertebrates to synthesize 20:5 n −3 and 22:6 n −3 from 18:3 n −3 (Moreno et al. 1979 ), FA 18:3 n −3 was only found in trace amounts (< 1%) in the species from this study. This indicates that biosynthesis of 20:5 n −3 and 22:6 n −3 likely did not occur, and these FAs were derived through the trophic chain from algal sources.

In our study, the FA profiles of the I‐POM samples suggested a diatom‐dominated ice algal community. The small amounts of the dinoflagellate‐specific FAs 18:4 n −3 and 22:6 n −3 in the I‐POM samples indicated that a small part of the sea ice flora consisted of dinoflagellates, which was in agreement with the results of molecular analyses of the primary community structures (K. Hardge et al. unpubl.). Based on the marker FA proportions, the phytoplankton community consisted of a mixture of both diatoms and flagellates. The dominance of dinoflagellates in the water column and a substantially higher proportion of diatoms in the sea ice community compared to the pelagic community during our sampling were also confirmed by genome sequencing (K. Hardge et al. unpubl.). The lower levels of the diatom‐specific FA 20:5 n −3 accompanied with the distinctly higher levels of the diatom‐FA 16:1 n −7 in the I‐POM samples compared to the P‐POM samples could indicate a different diatom‐community in sea ice compared to the water column. Supporting our assumption, previous studies found a dominance of pennate diatoms in sea ice vs. a dominance of centric diatoms in the water column (Gosselin et al. 1997 ; Arrigo et al. 2010 ).

Importance of ice algae‐produced carbon to the Arctic under‐ice community

In most investigated species, the α Ice estimates based on BSIA were higher than those based on the single FAs. Unlike CSIA of FAs, which is limited to molecules assumed to be unchanged by metabolic processes, the interpretation of BSIA results can be more complicated. Besides the lipid components, proteins and carbohydrates are also subject to various mass‐dependent metabolic processes, influencing the carbon stable isotope signal of a species. Compared to proteins and carbohydrates, lipids are more depleted in the heavy carbon stable isotope (Deniro and Epstein 1977; Søreide et al. 2006). To correct for a potential bias in the BSIA results introduced by variability in lipid content, both a priori lipid removal and post‐analytical corrections, e.g., with the normalization algorithm proposed by McConnaughey and McRoy (1979), have been used in previous studies. Several studies showed, however, that the extraction can cause fractionations in δ15N (Pinnegar and Polunin 1999; Sweeting et al. 2006; Post et al. 2007). On the other hand, there are studies indicating that normalization models do not account for different lipid levels in different species in an appropriate way (Sweeting et al. 2006; Post et al. 2007). Therefore, we based our calculations on the non‐corrected data. It remains difficult to conclude to which degree and in which species BSIA‐based estimates of α Ice were influenced by lipid content, taxon‐specific, habitat‐related, and/or trophic level‐related effects on metabolically active compounds. Yet, both BSIA and CSIA‐derived α Ice estimates yielded a consistent hierarchical order of the investigated species, ranging from a highly sea ice algae‐related trophic dependency in A. glacialis to a considerably lower trophic dependency on sea ice algae in C. limacina within the food web.

Based on the CSIA results, the isotopic values of carbon in the FAs 20:5n−3 and 22:6n−3 were used to investigate the proportional contributions of sea ice algae‐produced carbon α Ice vs. phytoplankton‐produced carbon to the body tissue of abundant under‐ice fauna species. Budge et al. (2008) traced the carbon flux in an Alaskan coastal ecosystem using the stable isotope values of carbon in the FA 20:5n−3, which they assumed to represent a realistic estimate of the ice algae contribution relative to all other types of phytoplankton. Wang et al. (2015) also suggested that the use of only FA 20:5n−3 could be most accurate if diatoms dominated the POM composition. Due to the mixed taxonomic composition of the primary communities in our dataset, we additionally calculated α Ice using the FA 22:6n−3 in combination with 20:5n−3 to account for the contribution of the dinoflagellate‐dominated pelagic communities in our samples.

To estimate the relative contribution of carbon sources to higher trophic levels, Budge et al. (2008) made several assumptions and simplifications that we also included in our study. We assumed that the major sources of FA 20:5n−3 were either ice‐related diatoms or pelagic diatoms, and isotopic fractionation and routing processes were negligible. Furthermore, we assumed that our measured carbon stable isotope ratios actually reflected the ratio at the base of the food web. This means that the algae‐derived lipid composition during the time of sampling was representative of the time when they were ingested. Consumers at lower trophic levels show a quick lipid turnover rate ranging between hours and days (Graeve et al. 2005), indicating that this was indeed the case for the more herbivorous species.

The highest α Ice estimates were found when only the diatom‐specific FA 20:5n−3 was used (model a). The dinoflagellate‐specific FA 22:6n−3 showed generally lower δ13C values compared to I‐POM in all under‐ice fauna species. Thus, α Ice estimates were considerably lower in some species when 20:5n‐3 was used in combination with 22:6n−3 (model b). This indicates that the sole use of the diatom‐specific FA 20:5n−3 underestimates the contribution of dinoflagellate‐produced carbon when the proportion of diatoms vs. dinoflagellates varies between sea ice and water column, causing a potential bias towards ice algae‐produced carbon.

As expected, the sympagic amphipods showed a high trophic dependency on the ice algal production. Surprisingly, many species classified as rather pelagic also showed a considerable input of ice algae‐produced carbon, further emphasizing the importance of ice algae for the entire food web. In Calanus spp., the estimated relative contribution of ice algae‐derived carbon based on the BSIA and the CSIA profiles indicated a mix of pelagic and ice‐associated carbon sources. Our results were comparable to a recent study in the Bering Sea, suggesting that the mean proportion of 20:5n−3, which originated from ice algae, was between 39% and 57% in Calanus spp., depending on the ice conditions (Wang et al. 2015). The reported mean α Ice values for the combination of 20:5n−3 and 22:6n−3 were somewhat higher than our values, ranging between 31% and 63% (Wang et al. 2015). The ice algae‐dependency of Calanus spp., however, seems to have a high variability, depending on region, season, and environmental properties. For example, Søreide et al. (2006) found a higher ice algae contribution for both Calanus copepods in autumn compared to spring, based on bulk stable isotope values.

Among the amphipods, A. glacialis showed the highest dependency on ice algal‐produced carbon. O. glacialis, G. wilkitzkii, and E. holmii showed also high α Ice values for both BSIA and CSIA, indicating a generally high trophic dependency on the ice algae production for all investigated sympagic amphipods, which is consistent with previous studies (Søreide et al. 2006; Tamelander et al. 2006a). In contrast, Budge et al. (2008) estimated the mean ice algae carbon contribution in Apherusa sp. near Barrow, Alaska, based on FA 20:5n−3, to be distinctly lower (61%) than our results. The mean proportional contributions of ice algae‐produced carbon in Onisimus sp. and Gammarus sp., estimated by Budge et al. (2008), were also clearly lower than our findings (Onisimus sp.: 36%, Gammarus sp.: 46%). These differences could be explained by a combination of regional, seasonal, or inter‐annual variability. In a shelf system, pelagic production may be higher due to higher nutrient and light availability, and amphipods have better access to recycled pelagic POM. In the ice‐covered high Arctic deep‐sea, however, ice algae represent a highly important carbon source for species, such as A. glacialis or Onisimus spp., and pelagic production is low (Fernández‐Méndez 2014).

Based on FA 20:5n−3, Wang et al. (2015) reported that T. libellula consumed substantial amounts of ice algae‐produced FAs with a proportional contribution between 47% and 63% in the Bering Sea, with variations according to ice conditions. These values correspond well to the results of our BSIA analysis and our model a, which is based on the same FA. Our results from model b, however, indicate that the true dependency of this species on sea ice‐produced carbon was probably lower when the proportional consumption of dinoflagellate‐produced FAs is considered. In fact, a previous study, based on bulk stable isotope compositions, also indicated that T. libellula primarily depends on pelagic carbon sources (Søreide et al. 2006).

In the pteropod C. limacina, we found the lowest trophic dependency on ice algae‐produced carbon compared to all other species, irrespective of the method and the mixing model used. A low trophic dependency (<20%) on ice algae‐produced carbon based on BSIA values was also found by e.g., Søreide et al. (2006). However, the subsequent loss of shelter from predators might be more pronounced in certain species than the dependency on sea ice in terms of food supply.

Altogether, a CSIA‐based approach including the effect of multiple potential carbon producing taxa at the base of the food web (such as our model b) appears to be the most conservative approach to estimate the contribution of sea ice algae in food web studies.