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Abstract. Climate change is having profound impacts on Arctic ecosystems with important implicationsfor coastal productivity and food web dynamics. We investigated seasonal variations in resource use of 16 invertebrate taxa in lagoon ecosystems along the Alaska Beaufort Sea coast using a combination of fatty acid (FA) biomarkers, bulk stable carbon isotope measurements of whole animals, and compound-specific stable carbon isotope measurements of total lipid extracts and individual FAs. Invertebrates were collected during full-ice cover (April), ice breakup (June), and open water (August) periods. Amphipods (Onisimus glacialis) had higher proportions of 18:2n-6 and 18:3n-3 FAs in April than in the other months. These elevated markers were accompanied by relatively low bulk and 18:2n-6 δ 13 C values, indicating proportionally higher contributions from terrestrial/freshwater sources in April. A wider range of invertebrates examined during June and August showed increases in algae-specific markers and higher proportions of essential FAs (e.g., 22:6n-3 [docosahexaenoic acid] and 20:5n-3 [eicosapentaenoic acid]) later in the summer. There were also marked differences in FA characteristics among invertebrates that highlighted differential feeding modes. For example, proportions of bacterial FAs were generally higher in deposit-feeding invertebrates than in suspension feeders. These results highlight the current role of diverse carbon sources to Arctic coastal food webs, which may change with future warming.

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