Interactions between rivers and coastal margins in the Alaskan Arctic

Abstract

Arctic coastal margins serve as the interface between Arctic terrestrial and oceanic carbon reservoirs. Characterized by extreme seasonality and annual variability, Alaskan Arctic coastal margins only receive riverine input of terrestrial organic material from late spring through late summer. This work examines the delivery, transformation, and distribution of organic material within Stefansson Sound, an Alaskan river-fed estuary, at the beginning and end of this period. We found that during the spring freshet, the landfast icescape over Stefansson Sound is composed of five ice and water features that are optically unique in terms of their surface reflectance spectra. One of these features is representative of freshwater flooding on ice which was found to persist in Stefansson Sound until late July. This floodwater is characterized by decreased reflectance in the blue portion of the visible light spectrum which indicates strong absorption by organic material contained within the freshet waters. Alaskan Arctic rivers drain watersheds composed of mixed proportions of tundra and mountains. During the freshet, organic material leached from the thawed soil surface horizons in the tundra is more available for microbial consumption over short periods than organic material originating in the mountains. This labile material is estimated to be higher molecular weight and more aromatic according to absorbance and fluorescence optical proxies. Fluorescence analysis revealed that tundra water contained more humic material and less amino acids when compared to water originating in the Brooks Range. This result challenges traditional assumptions that high molecular weight material in Arctic fluvial networks is refractory. In late summer, distributions of organic material in the surface waters of Stefansson Sound are driven by meteorological conditions such as precipitation over the watersheds and wind direction and speed in the estuary. Increased riverine discharge results in higher concentrations of organic material in the surface waters while faster wind speeds create deeper, well-mixed riverine plumes. Strong linear relationships between salinity and organic material indicate conservative mixing of freshwater in Stefansson sound, while new relationships between optical properties of water and organic material provide avenues forward for monitoring future climate driven changes regarding the biogeochemistry of this delicate ecosystem.