Seasonal dynamics of organic carbon cycling in the Fraser River, Canada

Wednesday, 17 December 2014
Britta Voss, Woods Hole Oceanographic Institution, Woods Hole, MA, United States, Timothy I Eglinton, ETH Swiss Federal Institute of Technology Zurich, Zurich, Switzerland, Bernhard Peucker-Ehrenbrink, WHOI, Marine Chemistry, Woods Hole, MA, United States, Robert G Spencer, Woods Hole Research Center, Falmouth, MA, United States, Valier Galy, WHOI-MCG, Woods Hole, MA, United States and Zhaohui Aleck Wang, WHOI-Marine Chem & Geochem, Woods Hole, MA, United States
The Fraser River basin in southwestern Canada is a large (250,000 km2), relatively undisturbed temperate river basin with a wide range of topographic, geologic, and climatic features. Extensive sampling of dissolved and particulate organic and inorganic geochemical properties in recent years have created a detailed framework for understanding the cycling of inorganic weathering products, sediments, and organic matter. Organic carbon fluxes in the Fraser River are influenced by spatial heterogeneity in source contributions and seasonal variability in hydrology. Fluxes of both dissolved and particulate organic carbon (DOC, POC) to the Strait of Georgia are dominated by the annual spring freshet. During the very early portion of the freshet, concentrations of both DOC and suspended sediments increase rapidly. While the organic carbon content of sediments decreases, increased sediment concentration compensates to cause a rise in POC concentration. The compositions of DOC and POC also change during the spring freshet. Optical properties of DOC shift from less aromatic, lower molecular weight pre-freshet DOC towards more aromatic, higher molecular weight DOC during the freshet, reflecting a shift from deep to shallow runoff flowpaths. DOC composition gradually returns to pre-freshet conditions throughout the summer, then undergoes a second shift towards freshet-like composition during the fall, likely due to basin-wide rain events. Concentrations of dissolved inorganic carbon (DIC) also vary between seasons and in different tributaries. Using the DIC and major element composition of a headwater tributary strongly influenced by carbonate weathering, we estimate that the majority of DIC in the Fraser River is derived from sources other than carbonate weathering. A significant correlation between non-carbonate-derived DIC and DOC concentration suggests that DOC respiration is the primary source of non-carbonate-derived DIC, and is responsible for >80% of the total DIC flux of the Fraser River throughout the year. The combined flux of DOC (2.7x1010 mol C yr-1) and POC (~1.1x1010 mol C yr-1) represents ~1% of annual net primary productivity in the Fraser basin, which is a relatively high proportion for large rivers globally, particularly given its relatively low suspended sediment yield.