Fluvial carbon export and CO2 efflux in representative nested headwater catchments of the eastern La Plata River Basin

摘要

This study involved a baseline evaluation of fluvial carbon export and degas rates in three nested rural catchments (1 to 80 km(2)) in Taboao, a representative experimental catchment of the Upper Uruguay River Basin. Analyses of the carbon content in stream waters and the catchment carbon yield were based on 4-year monthly in situ data and statistical modeling using the United States Geological Survey load estimator model. We also estimated pCO(2) and degas fluxes using carbonate equilibrium and gas-exchange formulas. Our results indicated that the water was consistently pCO2 saturated (similar to 90% of the cases) and that the steep terrain favors high gas evasion rates. The mean calculated fluvial export was 5.4 tC.km(-2).year(-1) with inorganic carbon dominating (dissolved inorganic carbon: dissolved organic carbon ratio >4), and degas rates (similar to 40 tC.km(-2).year(-1)) were nearly sevenfold higher than the downstream export. The homogeneous land use in this nested catchment system results in similar water-quality characteristics, and therefore, export rates are expected to be closely related to the rainfall-runoff relationships at each scale. Although the sampling campaigns did not fully reproduce storm-event conditions and related effects such as flushing or dilution of in-stream carbon, our results indicated a potential link between dissolved inorganic carbon and slower hydrological pathways related to subsurface water storage and movement.