Seasonal aspects of the biogeochemistry of the Chena River near Fairbanks, Alaska: how the hydrological flow regime drives the system

by

Tom Douglas (CRREL Alaska)
Laodoing Guo andYihua Cai (University of Southern Mississippi)
Joel Blum and Katie Keller (University of Michigan)

Abstract:
Northern watersheds experience a wide range of flow regimes from winter base flow to intense spring melt runoff events to summer wet and dry periods. We collected surface water from the Chena River biweekly during 2005-2006 and the waters were analyzed to investigate the relationship between flow and biogeochemistry. Major element concentrations and strontium isotopes were measured to elucidate geochemical signatures in the watershed. The results suggest that base flow yields a geochemically distinct signature dominated by carbonate weathering. We also measured carbon and nitrogen species including organic carbon (DOC), dissolved inorganic carbon (DIC), and particulate organic matter (POC). Over 50% of the carbon we measured occurs in the form of DIC, while ~40% is DOC and <10% resides within the POC pool. A strong linear relationship exists between DOC and discharge excluding the spring freshet, indicating the hydrological control of river DOC storage and export. The DOC concentrations during spring runoff are much higher than those expected from the linear relationship between DOC and discharge implying a more efficient leaching of DOC from surface soils and overlying plant litter during spring melt. Spring melt runoff also exhibits heavier PO13C and P15N, low specific POC concentrations, and low C/N ratios. POC concentrations are elevated in late August, coincident with elevated chlorophyll-a concentration, suggesting contributions from aquatic biological activities during later summer/early fall. The strong seasonal control on river water biogeochemistry we measured suggests that monitoring campaigns focused on northern watersheds must sample enough to constrain the varied flow regimes and their associated biochemical sourcings.

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