Arctic deltas as biogeochemical hotspots affecting the delivery of nutrients and dissolved organic matter to the Arctic Ocean

The Mackenzie River and Delta were sampled during hydrologically-defined seasons in four consecutive years to assess 1) the importance of sampling during the rising limb of the flood hydrograph (rising freshet) for accurately characterizing constituent fluxes and quality, and 2) how floodplain processes affect discharge to the Arctic Ocean. Including rising freshet samples had a modest effect on annual sediment and nutrient flux estimates for the Mackenzie River (-9 to +26% difference). Nutrient quality was very different during the rising freshet, however, with relatively high concentrations of carbon-rich dissolved organic matter (DOM), phosphorus-rich particles, and nitrogen-rich inorganic nutrients. Mackenzie River DOM quality was relatively fresher, more terrigenous, and younger (radiocarbon values suggesting ages < 15 years) during the rising freshet, indicating a high proportion of recently-fixed vascular plant material. The Mackenzie was also a net absorber of carbon dioxide during the rising freshet (-112 to -258 mg-C m-2 d-1), switching to net emission after peak flood. Open water (freshet through summer) fluxes of dissolved organic carbon (1.4 Tg) and lignin (7.1 Gg) in the Mackenzie River were greater than previously reported total annual fluxes, likely due to the inclusion of rising freshet data herein. Optical parameters, and statistical relations between fluorescence components derived from Parallel Factor Analysis (PARAFAC; six in delta channels, five in delta lakes) and chemical biomarkers (e.g. lignin phenols), suggest substantial modification of DOM in delta lakes and on the floodplain during downstream transport. When incubated (14 days) under solar conditions similar to those on the floodplain, Mackenzie River DOM (isolated during peak flood) experienced photochemical changes on par with those observed in delta lakes over the entire open water period. Photodegraded DOM significantly reduced abundances but fueled per-cell growth in bacterial populations from delta habitats, indicating rapid shifts in community composition. Gradients in chemical biomarkers were related to the delta-wide gradient of lake hydrological connectivity. These results emphasize the importance of the rising freshet in accurately characterizing Mackenzie River DOM quality and carbon fluxes, and the need to sample downstream sites in lake-rich circumpolar deltas to constrain flux estimates and characterize discharge to the Arctic Ocean.