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Linking production, transfer and subsequent bioavailability of nutritionally significant matter from phytoplankton to higher trophic levels is a fundamental aspect in understanding marine food webs. The plant–animal interface is of interest because of the highly variable transfer between producers and consumers, and the myriad of factors that influence it. Essential fatty acids (EFAs) are dietary nutrients that are necessary for normal function in all consumers, yet it remains unclear how efficiently they are transferred through marine food webs. We introduced a 13C-labelled carbon source to the cryptophyte Rhodomonas salina to quantify primary production of two omega-3 long-chain polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). We investigated transfer and assimilation efficiencies of these EFAs from phytoplankton to the calanoid copepod Calanus finmarchicus in an 8-day feeding experiment. We found low production of both EFAs in R. salina. Assimilation efficiencies of both EFAs ranged from 5 to 15% throughout the experiment, remaining slightly higher on average for DHA. This was mirrored in more efficient trophic transfer of DHA (up to 28%, compared to 13% for EPA). These results add to previously scarce experiments empirically quantifying the assimilation and transfer efficiency of EFAs in a basic marine planktonic food chain.

Copepods are an important link in marine food webs, transferring crucial nutrients such as essential fatty acids (EFA) from primary producers to higher consumers. Yet data on lipid dynamics in marine ecosystems associated with small lipid-poor copepods are scarce. In this study, we used the coastal calanoid copepod Eurytemora herdmani, which is an opportunistic generalist feeder, to assess EFA-specific gross growth efficiency (GGEEFA), defined as the fraction of ingested EFA retained in copepod tissue. We conducted two eight-day feeding experiments with 13C-labeled phytoplankton to quantify incorporation and GGEEFA of eicosapentaenoic acid (20:5n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) in stage CV E. herdmani copepodites consuming either the EPA-replete diatom Thalassiosira weissflogii or the DHA-replete mixotrophic dinoflagellate Heterocapsa triquetra. EFA incorporation was quantified using gas chromatography. We recorded rapid shifts (within 48 h) in copepod tissue fatty acid (FA) composition following diet change, but not directly mirroring dietary algal FA. We also found high GGEEFA, with maxima of 46% and 61% for EPA and DHA, respectively, in copepods ingesting T. weissflogii and 39% and 87%, respectively, in copepods ingesting H. triquetra. GGEDHA remained higher than GGEEPA throughout the experiments regardless of dietary algae. GGEEFA decreased overall when copepods matured and produced nauplii, indicating EFA allocation for reproductive purposes. GGEDHA values in non-reproducing E. herdmani copepodites were high, suggesting that under predicted shifts in copepod community size structure, trophic transfer by small copepods could help compensate for DHA reductions resulting from lower abundances of large, lipid-rich species.

Baleen whales accumulate fat reserves during the summer to sustain reproduction while fasting in the winter. The southern right whale Eubalaena australis population that calves off Península Valdés, Argentina, experienced high calf mortality events from 2003 to 2013 and poor nutritional states of mothers could be a contributing cause. Previous studies found that the population’s reproductive success is influenced by prey availability. Mothers unable to build sufficient fat reserves or feeding on prey with different nutritional value may fail to meet the demands of lactation. Milk is the only source of nutrients and energy for calves at Valdés, so their fatty acids (FAs) and stable isotopes should reflect their mother’s diet and feeding-ground locations. Here, we compared FA profiles and C and N stable isotopes of dead calves with those of living calves to evaluate the potential impact of maternal nutrition on calf survival. We found no differences in the FA composition of blubber in dead and living calves, indicating similar maternal diets. Likewise, the isotopic values of living and dead calves imply that their mothers had similar foraging ranges. However, FA composition was greatly affected by calf length, indicating effects of calf age and duration of nursing. These findings suggest that mothers of dead calves did not feed on different diets or feeding grounds compared to mothers of living calves. Future research should further assess the overall health and body condition of the Valdés southern right whale calves. Fil: Marón, Carina Flavia. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Diversidad Biológica y Ecológica; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Instituto de Conservación de Ballenas; Argentina Fil: Budge, Suzanne M.. Dalhousie University Halifax; Canadá Fil: Ward, Robert E.. Utah State University; Estados Unidos Fil: Valenzuela, Luciano Oscar. Universidad Nacional del Centro de la Provincia de Buenos Aires. Facultad de Ciencias Sociales. Departamento de Arqueología. Laboratorio de Ecología Evolutiva Humana (Sede Quequén); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tandil; Argentina Fil: Di Martino, Matías. Programa de Monitoreo Sanitario Ballena Franca Austral; Argentina Fil: Ricciardi, Marcos. Instituto de Conservación de Ballenas; Argentina Fil: Sironi, Mariano. Instituto de Conservación de Ballenas; Argentina. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Diversidad Biológica y Ecológica; Argentina. Programa de Monitoreo Sanitario Ballena Franca Austral; Argentina Fil: Uhart, Marcela. Programa de Monitoreo Sanitario Ballena Franca Austral; Argentina. University of California; Estados Unidos Fil: Seger, Jon. University Of Utah. Department Of Biology; Estados Unidos Fil: Rowntree, Victoria J.. University Of Utah. Department Of Biology; Estados Unidos. Instituto de Conservación de Ballenas; Argentina. Programa de Monitoreo Sanitario Ballena Franca Austral; Argentina. Whale Conservation Institute/Ocean Alliance; Estados Unidos

Fatty acids are commonly used as biomarkers for making inferences about trophic relationships in aquatic and soil food webs. However, researchers are often unaware of the physiological constraints within organisms on the trophic transfer and modification of dietary biomarkers in consumers. Fatty acids are bioactive molecules, which have diverse structures and functions that both complicate and enhance their value as trophic tracers. For instance, consumers may synthesize confounding non-dietary sourced markers from precursor molecules, and environmental conditions also affect fatty acid composition. There is a vital need for more research on the uptake and transfer of trophic biomarkers in individual organisms in order to advance the field and make meaningful use of these tools at the scale of populations or ecosystems. This special issue is focused on controlled feeding experiments on a diverse taxonomic breadth of model consumers from freshwater, marine and soil ecosystems with a goal of creating a more integrated understanding of the connection between consumer physiology and trophic ecology. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids'.

Knowledge of predator diets, including how diets might change through time or differ among predators, provides essential insights into their ecology. Diet estimation therefore remains an active area of research within quantitative ecology. Quantitative fatty acid signature analysis (QFASA) is an increasingly common method of diet estimation. QFASA is based on a data library of prey signatures, which are vectors of proportions summarizing the fatty acid composition of lipids, and diet is estimated as the mixture of prey signatures that most closely approximates a predator’s signature. Diets are typically estimated using proportions from a subset of all fatty acids that are known to be solely or largely influenced by diet. Given the subset of fatty acids selected, the current practice is to scale their proportions to sum to 1.0. However, scaling signature proportions has the potential to distort the structural relationships within a prey library and between predators and prey. To investigate that possibility, we compared the practice of scaling proportions with two alternatives and found that the traditional scaling can meaningfully bias diet estimators under some conditions. Two aspects of the prey types that contributed to a predator’s diet influenced the magnitude of the bias: the degree to which the sums of unscaled proportions differed among prey types and the identifiability of prey types within the prey library. We caution investigators against the routine scaling of signature proportions in QFASA.

Quantitative fatty acid signature analysis has become an important method of diet estimation in ecology, especially marine ecology. Controlled feeding trials to validate the method and estimate the calibration coefficients necessary to account for differential metabolism of individual fatty acids have been conducted with several species from diverse taxa. However, research into potential refinements of the estimation method has been limited. We compared the performance of the original method of estimating diet composition with that of five variants based on different combinations of distance measures and calibration-coefficient transformations between prey and predator fatty acid signature spaces. Fatty acid signatures of pseudopredators were constructed using known diet mixtures of two prey data sets previously used to estimate the diets of polar bears Ursus maritimus and gray seals Halichoerus grypus, and their diets were then estimated using all six variants. In addition, previously published diets of Chukchi Sea polar bears were re-estimated using all six methods. Our findings reveal that the selection of an estimation method can meaningfully influence estimates of diet composition. Among the pseudopredator results, which allowed evaluation of bias and precision, differences in estimator performance were rarely large, and no one estimator was universally preferred, although estimators based on the Aitchison distance measure tended to have modestly superior properties compared to estimators based on the Kullback-Leibler distance measure. However, greater differences were observed among estimated polar bear diets, most likely due to differential estimator sensitivity to assumption violations. Our results, particularly the polar bear example, suggest that additional research into estimator performance and model diagnostics is warranted.

We analyzed the fatty acid (FA) composition and carbon stable isotope ratios of indi- vidual FAs (δ 13 CFA) of 3 zooplankton species (Themisto libellula, Calanus marshallae/glacialis, and Thysanoessa raschii) sampled from the Bering Sea during winter maximum ice extent, spring ice melt, and summer ice-free conditions in 2009 and 2010. Our goal was to assess diets of these eco- logically important species and estimate the proportional contribution of pelagic and sympagic carbon sources to their diets. FA profiles showed little variation in diet within species between ice conditions or years but revealed differences in diet among species. FA biomarkers confirmed that T. libellula was predominately carnivorous and that C. marshallae/glacialis and T. raschii were primarily herbivorous. Estimates from 4 stable isotope mixing models using combinations of δ 13 CFA values of diatom FA markers (16:1n-7, 20:5n-3), and a flagellate FA marker (22:6n-3) showed that substantial, albeit highly variable, proportions of these FAs originated from organic matter originating from sea ice algae (T. libellula 36 to 72%, C. marshallae/glacialis 27 to 63%, and T. raschii 39 to 71%). Our results suggest that ice algae may be an important food source for zooplankton when water column phytoplankton are not available during critical periods in their life history. Predicted increases in water column phytoplankton production in the Bering Sea may help offset the expected reduction in ice algal production and any detrimental effects that this might have on consumers such as zooplankton.

Knowledge of animal diets provides essential insights into their life history and ecology, although diet estimation is challenging and remains an active area of research. Quantitative fatty acid signature analysis (QFASA) has become a popular method of estimating diet composition, especially for marine species. A primary assumption of QFASA is that constants called calibration coefficients, which account for the differential metabolism of individual fatty acids, are known. In practice, however, calibration coefficients are not known, but rather have been estimated in feeding trials with captive animals of a limited number of model species. The impossibility of verifying the accuracy of feeding trial derived calibration coefficients to estimate the diets of wild animals is a foundational problem with QFASA that has generated considerable criticism. We present a new model that allows simultaneous estimation of diet composition and calibration coefficients based only on fatty acid signature samples from wild predators and potential prey. Our model performed almost flawlessly in four tests with constructed examples, estimating both diet proportions and calibration coefficients with essentially no error. We also applied the model to data from Chukchi Sea polar bears, obtaining diet estimates that were more diverse than estimates conditioned on feeding trial calibration coefficients. Our model avoids bias in diet estimates caused by conditioning on inaccurate calibration coefficients, invalidates the primary criticism of QFASA, eliminates the need to conduct feeding trials solely for diet estimation, and consequently expands the utility of fatty acid data to investigate aspects of ecology linked to animal diets.

Juvenile polar cod (Boreogadus saida) are often found in close association with sea ice and represent an important trophic link in the Arctic food web. However, the proportional contribution of sea ice algal production via the sympagic food web to the diet of polar cod is unknown. To estimate the proportional contribution of fatty acids (FAs) from sea ice-derived particulate organic matter (i-POM) to the diet of juvenile polar cod, we used FA profiling and compound-specific stable carbon isotope analysis of individual FAs from juvenile polar cod collected from three regions in the Beaufort Sea. The δ13C values of the FAs 14:0, 16:4n-1, 18:0, 20:5n-3 and 22:6n-3 in the polar cod were found to most strongly resemble pelagic POM rather than i-POM. Results from isotope-mixing models using diatom FA markers indicated that the proportional contribution of FAs from i-POM to juvenile polar cod was ≤2 %, which suggests that juvenile polar cod had not sourced their FAs from i-POM. Thus, changes in sea ice coverage due to environmental change may not affect juvenile polar cod in regard to nutrients such as FAs but may still affect their populations by reducing critical shelter from predators.

Phocids routinely fast for extended periods. During these fasts, energetic requirements are met primarily through the catabolism of blubber lipid. To assess whether fatty acid (FA) composition changes during the postweaning fast in northern elephant seals, blubber biopsies were acquired longitudinally from 43 pups at 2.3 ± 1.5 and 55.2 ± 3.7 days postweaning in 1999 and 2000. At weaning, short-chain monounsaturated FA (SC-MUFA, ≤18 carbons) dominated the blubber while saturated FA (SFA) were found in the next highest proportion. The major FA (all ≥1 % by mass) comprised approximately 91 % of total blubber FA. In both years, 18:1n-9 and 16:0 were the most prevalent FA. Major FA mobilized during the fast consisted of polyunsaturated FA (PUFA), SFA, and SC-MUFA. Long-chain MUFA (>18 carbons) tended to be conserved. The fractional mobilization value of 20:5n-3 was the highest, resulting in significant reductions of this PUFA. Although concentrations of some blubber FA changed significantly during the postweaning fast, the general FA signature of blubber was similar at weaning and near the end of the fast. Changes in some FA differed across years. For example, the concentration of 20:4n-6, a minor PUFA, was significantly reduced in 1999 but not in 2000. FA mobilization patterns in northern elephant seal pups are somewhat similar to those reported previously for other fasting phocids and terrestrial mammals, though there are some notable differences. Differences in FA mobilization patterns across mammalian species may be related to differences in diets, geographical distribution, environmental factors, physiological adaptations, and life history stage.

Climate-mediated changes in the phenology of Arctic sea ice and primary production may alter benthic food webs that sustain populations of Pacific walruses (Odobenus rosmarus divergens) and bearded seals (Erignathus barbatus). Interspecific resource competition could place an additional strain on ice-associated marine mammals already facing loss of sea ice habitat. Using fatty acid (FA) profiles, FA trophic markers, and FA stable carbon isotope analyses, we found that walruses and bearded seals partitioned food resources in 2009-2011. Interspecific differences in FA profiles were largely driven by variation in non-methylene FAs, which are markers of benthic invertebrate prey taxa, indicating varying consumption of specific benthic prey. We used Bayesian multi-source FA stable isotope mixing models to estimate the proportional contributions of particulate organic matter (POM) from sympagic (ice algal), pelagic, and benthic sources to these apex predators. Proportional contributions of FAs to walruses and bearded seals from benthic POM sources were high [44 (17-67)% and 62 (38-83)%, respectively] relative to other sources of POM. Walruses also obtained considerable contributions of FAs from pelagic POM sources [51 (32-73)%]. Comparison of δ13C values of algal FAs from walruses and bearded seals to those from benthic prey from different feeding groups from the Chukchi and Bering seas revealed that different trophic pathways sustained walruses and bearded seals. Our findings suggest that (1) resource partitioning may mitigate interspecific competition, and (2) climate change impacts on Arctic food webs may elicit species-specific responses in these high trophic level consumers.

Carnivora includes three independent evolutionary transitions to the marine environment: pinnipeds (seals, sea lions, and walruses), sea otters, and polar bears. Among these, only the pinnipeds have retained two forms of insulation, an external fur layer and an internal blubber layer for keeping warm in water. In this study we investigated key factors associated with the transition to the use of blubber, by comparing blubber characteristics among the pinnipeds. Characteristics included gross morphology (blubber thickness), fat composition (fatty acid profiles, percentage lipid, and water), and thermal conductivity. Sea lions, phocids, and walrus, which have lower fur densities than fur seals, have thicker blubber layers than fur seals (P

This study was performed to evaluate the stability of the fatty acid (FA) composition of blubber from Baltic grey seals Halichoerus grypus archived at −25°C in the Swedish Environmental Specimen Bank (SESB). Possible alterations of FA compositions with storage time and temperature were evaluated. Blubber from 5 seals was kept at +2°C and at −20°C and sampled for up to 6 mo and 33 mo, respectively. Furthermore, FA profile stability was evaluated in blubber stored in the SESB for 4 and 6 yr. This was done by re-sampling and re-analyzing blubber samples from SESB. In both parts of the study, a principal component analysis was performed on 48 FAs comprising 98 to 99% of the total FAs in blubber of Baltic grey seals. We found no indication of a critically altered FA composition, neither in blubber stored at +2°C for 6 mo nor in blubber stored at −20°C for 33 mo. The re-analysis of samples stored in SESB for ~4 to 6 yr only differed for a few FAs in some samples. The most remarkable differences were a slight increase in some long-chain unsaturated FAs (18:2n-6, 20:3n-3, 20:4n-3, 22:4n-3). As an increase in long-chain polyunsaturated FAs due to storage is highly unlikely, we believe that this is probably more due to the sampling technique than to a critically altered FA composition due to storage. Our results suggest that archived seal blubber may be safely used in retrospective analyses of diet using an FA signature analysis technique.

Fat bodies in the heads of toothed whales, which serve to transmit and receive sound, represent extraordinary examples of physiological specialization in adipose tissues among mammals, yet we know surprisingly little about their biochemical composition. We describe the spatial distributions and development of unusual endogenous lipids (branched-chain ["iso"] molecules and wax esters) in the mandibular fat bodies of bottlenose dolphins (Tursiops truncatus) using an ontogenetic series (fetus to adult; n = 10). Although concentrations of iso-acids, iso-alcohols and waxes were lower in younger dolphins than in adults, the same relative spatial arrangement was present in all age classes, implying a set "pattern" of acoustic lipid distribution that is established very early in life. In all age classes, a small region of blubber overlying the lateral region contained unusually high concentrations of iso-acids, exhibiting a tenfold increase over "normal" adjacent blubber. Being chemically more similar to the acoustic fat bodies, this region may serve as an entry point for sound into the head. Developmental accumulations of some iso-acids and iso-alcohols occurred more rapidly than others, implying that not only are the spatial distributions of branched-chain molecules under extremely fine-scale control, but the regulatory mechanisms controlling acoustic lipid synthesis are also highly complex.

Four species of pagophilic phocid seals in the Western Arctic—bearded (Erignathus barbatus), ringed (Phoca hispida), ribbon (Phoca fasciata), and spotted (Phoca largha)—are particularly vulnerable to arctic warming trends. Documenting diet composition over time is one way in which the flexibility of a species, in the face of broad ecosystem changes, can be measured. Because the fatty acid (FA) composition of depot lipids has long been known to reflect diet, we analysed the FA composition of blubber samples collected from bearded (n = 30), ringed (n = 15), ribbon (n = 32) and spotted seals (n = 24). All animals were taken near Little Diomede Island in May and June 2003, providing a unique opportunity to study interspecies differences with minimal effects of spatial and temporal variation. Bearded seal FA composition was significantly different from all other seals (P < 0.001 in all cases), ringed seal FA composition was significantly different from that of both ribbon and spotted seals (P < 0.001), but ribbon and spotted seal FA compositions could not be distinguished from each other. Overall, the blubber FA compositions of ribbon, ringed and spotted seals implied diets dominated by pelagic prey. Inferences we made about current diets of bearded and ringed seals suggest that certain prey important to them in this area in the 1970s remain important today, despite notable changes in the ecosystem. We believe that blubber FA analysis will be a useful tool in assessing the response of ice-associated pinnipeds and their ecosystem to changes associated with arctic warming.

Global warming and the loss of sea ice threaten to alter patterns of productivity in arctic marine ecosystems because of a likely decline in primary productivity by sea ice algae. Estimates of the contribution of ice algae to total primary production range widely, from just 3 to >50%, and the importance of ice algae to higher trophic levels remains unknown. To help answer this question, we investigated a novel approach to food web studies by combining the two established methods of stable isotope analysis and fatty acid (FA) analysis--we determined the C isotopic composition of individual diatom FA and traced these biomarkers in consumers. Samples were collected near Barrow, Alaska and included ice algae, pelagic phytoplankton, zooplankton, fish, seabirds, pinnipeds and cetaceans. Ice algae and pelagic phytoplankton had distinctive overall FA signatures and clear differences in delta(13)C for two specific diatom FA biomarkers: 16:4n-1 (-24.0+/-2.4 and -30.7+/-0.8 per thousand, respectively) and 20:5n-3 (-18.3+/-2.0 and -26.9+/-0.7 per thousand, respectively). Nearly all delta(13)C values of these two FA in consumers fell between the two stable isotopic end members. A mass balance equation indicated that FA material derived from ice algae, compared to pelagic diatoms, averaged 71% (44-107%) in consumers based on delta(13)C values of 16:4n-1, but only 24% (0-61%) based on 20:5n-3. Our estimates derived from 16:4n-1, which is produced only by diatoms, probably best represented the contribution of ice algae relative to pelagic diatoms. However, many types of algae produce 20:5n-3, so the lower value derived from it likely represented a more realistic estimate of the proportion of ice algae material relative to all other types of phytoplankton. These preliminary results demonstrate the potential value of compound-specific isotope analysis of marine lipids to trace C flow through marine food webs and provide a foundation for future work.

Fatty acids (FA) have a diversity of structures that are transferred with little modification through food webs, making them valuable in assessing diets of animals that cannot be directly observed feeding. Before using FA to estimate diets, it is necessary to evaluate variation in FA signatures within and among individuals of a given species. To begin assessing diets and foraging of western Arctic bowhead whales ( Balaena mysticetus) , we examined the FA in blubber of 64 bowheads taken in the spring and fall subsistence hunts in 1997–2002 at Barrow and Kaktovik, Alaska. We found no significant differences in FA characteristics of inner blubber layers taken from either duplicate samples on the dorsal surface, or between dorsal and ventral sites. Significant differences were found in the FA composition between inner and outer layers of blubber at the same body site. We also found age, season and year to have significant effects on FA composition; however, gender was not found to be significant. While the importance of the Beaufort Sea as a feeding ground of bowhead whales remains uncertain, our results indicate that adults and sub-adults foraged to some extent on different prey and that both age classes consumed copepods there in summer at sufficient levels to significantly alter their blubber FA profiles. Both of these findings correspond with dietary conclusions reached from the analysis of stomach contents. Furthermore, we found compelling evidence that yearly variation in bowhead FA reflect changes in FA compositions of phytoplankton at the base of the food web, probably in response to climate variation. Variability in phytoplankton-derived FA in blubber was correlated significantly with yearly mean values of the Pacific Decadal Oscillation. FA in bowhead whale blubber, therefore, might be used to monitor effects of climate change on lower trophic levels and production processes in the western Arctic.

Dramatic multiyear fluctuations in water temperature and seasonal sea ice extent and duration across the Bering-Chukchi continental shelf have occurred in this century, raising a pressing ecological question: Do such environmental changes alter marine production processes linking primary producers to upper trophic-level predators? We examined this question by comparing the blubber fatty acid (FA) composition and stable carbon isotope ratios of individual FA (δ¹³CFA) of adult ringed seals (Pusa hispida), bearded seals (Erignathus barbatus), spotted seals (Phoca largha), and ribbon seals (Histriophoca fasciata), collectively known as "ice seals," sampled during an anomalously warm, low sea ice period in 2002-2005 in the Bering Sea and a subsequent cold, high sea ice period in 2007-2010. δ¹³C(FA) values, used to estimate the contribution to seals of carbon derived from sea ice algae (sympagic production) relative to that derived from water column phytoplankton (pelagic production), indicated that during the cold period, sympagic production accounted for 62-80% of the FA in the blubber of bearded seals, 51-62% in spotted seals, and 21-60% in ringed seals. Moreover, the δ¹³CFA values of bearded seals indicated a greater incorporation of sympagic FAs during the cold period than the warm period. This result provides the first empirical evidence of an ecosystem-scale effect of a putative change in sympagic production in the Western Arctic. The FA composition of ice seals showed clear evidence of resource partitioning among ringed, bearded, and spotted seals, and little niche separation between spotted and ribbon seals, which is consistent with previous studies. Despite interannual variability, the FA composition of ringed and bearded seals showed little evidence of differences in diet between the warm and cold periods. The findings that sympagic production contributes significantly to food webs supporting ice seals, and that the contribution apparently is less in warm years with low sea ice, raise an important concern: Will the projected warming and continuing loss of seasonal sea ice in the Arctic, and the associated decline of organic matter input from sympagic production, be compensated for by pelagic production to satisfy both pelagic and benthic carbon and energy needs?

Fatty acids (FA) represent a large group of molecules that comprise the majority of lipids found in all organisms. Their great diversity, biochemical restrictions and, in some cases, unique origin among plants and animals has fostered a number of areas of research, ranging from assessment of animal nutrition and metabolism, to investigating trophic interactions and ecosystem structure. Over the past three decades, we have observed the use of FA develop from a potential tool for delineating food webs (Ackman and Eaton 1966) to a powerful technique for quantitative assessment of predator diets (Iverson et al. 2004). Studies that have compared the FA found in predator fat stores with those found in their prey have allowed both qualitative (e.g., Horgan and Barrett 1985; Smith et al. 1996; Raclot et al. 1998; Dahl et al. 2000, 2003; Falk-Petersen et al. 2004) and quantitative or semi-quantitative analyses of diet (Kirsch et al. 2000; Iverson et al. 2001b, 2004; Iverson and Springer 2002) and

The fat and fatty acid compositions of 28 species of fish and invertebrates (n = 954) from the Scotian Shelf, Georges Bank, and the Gulf of St. Lawrence were determined. Discriminant analysis of the 16 most numerous species (n [Formula: see text] 18 each), using 17 major fatty acids, classified species with greater than 98% accuracy and grouped species into three general clusters (gadids, flatfish, and planktivores) with similar fatty acid compositions, and likely, similar diets. A number of species exhibited changes in fatty acid signatures with increasing size (multivariate analysis of variance), which corresponded with known dietary shifts reported from stomach contents analyses. Location effects were also observed among the three major geographical regions and were probably due to broad-scale variations in prey assemblages and phytoplankton composition in the northwestern Atlantic. Despite these effects, within-species variation was still substantially less than among-species variation. Thus, fatty acid signatures can be used to distinguish and characterize fish and invertebrate species in a given ecosystem, as well as to study finer-scale trophic interactions of these species. These data also have applications at higher trophic levels and will serve as a prey database for studying the diets of other fish and marine mammal predators using fatty acid signatures.

Fatty acid biomarkers were used to investigate the biogeochemistry of a former blue mussel (Mytilus edulis) aquaculture site in a shallow, sheltered cove in northeastern Newfoundland. High levels of polyunsaturated fatty acids (PUFA) in net-tow and sediment trap samples indicated a substantial phytoplankton source of organic matter, and fluctuations in specific fatty acid biomarkers reflected the changing abundances of diatoms and dinoflagellates. In comparison, sediments contained very low levels of PUFA (

Polar bears (Ursus maritimus) have experienced substantial changes in the seasonal availability of sea ice habitat in parts of their range, including the Beaufort, Chukchi, and Bering Seas. In this study, we compared the body size, condition, and recruitment of polar bears captured in the Chukchi and Bering Seas (CS) between two periods (1986–1994 and 2008–2011) when declines in sea ice habitat occurred. In addition, we compared metrics for the CS population 2008–2011 with those of the adjacent southern Beaufort Sea (SB) population where loss in sea ice habitat has been associated with declines in body condition, size, recruitment, and survival. We evaluated how variation in body condition and recruitment were related to feeding ecology. Comparing habitat conditions between populations, there were twice as many reduced ice days over continental shelf waters per year during 2008–2011 in the SB than in the CS. CS polar bears were larger and in better condition, and appeared to have higher reproduction than SB bears. Although SB and CS bears had similar diets, twice as many bears were fasting in spring in the SB than in the CS. Between 1986–1994 and 2008–2011, body size, condition, and recruitment indices in the CS were not reduced despite a 44-day increase in the number of reduced ice days. Bears in the CS exhibited large body size, good body condition, and high indices of recruitment compared to most other populations measured to date. Higher biological productivity and prey availability in the CS relative to the SB, and a shorter recent history of reduced sea ice habitat, may explain the maintenance of condition and recruitment of CS bears. Geographic differences in the response of polar bears to climate change are relevant to range-wide forecasts for this and other ice-dependent species.

Pelagic primary production in Arctic seas has traditionally been viewed as biologically insignificant until after the ice breakup. There is growing evidence however, that under-ice blooms of pelagic phytoplankton may be a recurrent occurrence. During the springs of 2011 and 2012, we found substantial numbers (201-5713 cells m-3) of the large centric diatom (diameter >250 µm) Coscinodiscus centralis under the sea ice in the Canadian Arctic Archipelago near Resolute Bay, Nunavut. The highest numbers of these pelagic diatoms were observed in Barrow Strait. Spatial patterns of fatty acid profiles and stable isotopes indicated two source populations for C. centralis: a western origin with low light conditions and high nutrients, and a northern origin with lower nutrient levels and higher irradiances. Fatty acid analysis revealed that pelagic diatoms had significantly higher levels of polyunsaturated fatty acids (mean ± SD: 50.3 ± 8.9%) compared to ice-associated producers (30.6 ± 10.3%) in our study area. In particular, C. centralis had significantly greater proportions of the long chain omega-3 fatty acid, eicosapentaenoic acid (EPA), than ice algae (24.4 ± 5.1% versus 13.7 ± 5.1%, respectively). Thus, C. centralis represented a significantly higher quality food source for local herbivores than ice algae, although feeding experiments did not show clear evidence of copepod grazing on C. centralis. Our results suggest that C. centralis are able to initiate growth under pack ice in this area and provide further evidence that biological productivity in ice-covered seas may be substantially higher than previously recognized.