Arctic river watersheds are important components of the global climate system and show an amplified response to climate change. Here, we characterize origins and transformations of dissolved organic matter (DOM) in five major Arctic rivers (Kolyma, Lena, Yenisei, Ob, Mackenzie) over 3 years with seasonal sampling periods using measurements of carbohydrates, amino acids, bacterial biomarkers (D-amino acids), and plant protein biomarkers (hydroxyproline). A strong seasonal cycle of bioavailable DOM export was observed that correlated with discharge, vegetation, river morphology and water residence time. The chemical composition of bioavailable DOM was different among rivers reflecting unique characteristics of Arctic river watersheds. Trends in specific bacterial biomarkers were synchronous to changes in bacterial community compositions demonstrating that bacterial communities responded to the seasonal shifts in organic matter quality and chemical composition. Extensive heterotrophic processing of plant and soil-derived DOM resulted in major inputs of bacterial detritus, and bacterial organic matter accounted for 21–42% of DOC in all watersheds. Dissolved organic nitrogen sources were dominated by bacterially-derived nitrogen and important contributions of soluble plant protein during the Spring freshet. Overall, our results demonstrated the importance of watershed characteristics and bacterial metabolism in regulating DOM composition, reactivity and carbon fluxes in Arctic river watersheds.

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A suite of lipid biomarkers plus compound-specific carbon isotopes of major sterols were determined in muscle tissues across increasing sizes of northern Neptune whelks Neptunea heros, developing eggs and potential diets to link trophic patterns, metabolism and carbon sources on the Chukchi Sea shelf. Analysis of primary prey included the northern clam Astarte borealis, water column particulate organic matter (POM) and surface sediments near the collection sites. Sterols specific to major algal groups along with algal-derived polyunsaturated fatty acids (C20:5n-3, C20:4n-3, C22:6n-3) in whelk muscle tissue reflected the importance of algal primary production to benthic consumers and its direct incorporation. Compound-specific carbon isotope values of cholesterol and Δ5,7-sterols present in foot muscle of N. heros also suggest a transition from scavenging/detrital feeding in smaller juveniles to predatory consumption by larger adults. Sexually mature adult female N. heros (shell lengths >10 cm) contained lower sterol concentrations compared to smaller whelks, which may reflect reduced consumption rates or lipid translocation during the annual reproductive cycle. Analysis of N. heros eggs at 3 developmental stages ranging from internal egg masses to mature egg cases showed significant transfer of algal-derived sterols and fatty acids which were conserved during egg maturation. The suite of individual lipid signatures and δ13C sterol values observed in N. heros suggest that the northern Neptune whelk relies on both direct lipid incorporation as well as metabolic modification of algal-derived food sources for sterol requirements that transition as animals mature and become predatory consumers.

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