Extreme gradients in CO₂ losses downstream of karstic springs

Rivers are significant sources of CO₂ to the atmosphere, and karstic watersheds are particularly important in this respect due to their large availability of inorganic carbon. This study examines characteristics of dissolved inorganic carbon (DIC) and excess partial pressures of CO₂ (epCO₂) in the source springs and headwaters of four watersheds in a Central European karstic region, via dissolved inorganic carbon concentration and stable carbon isotope measurements. Our results show the most ¹³C-depleted δ¹³C_DIC values at the source springs, which become rapidly enriched downstream due to CO₂ degassing. Concurrently, epCO₂ values, while consistently in excess of atmospheric concentrations at the spring sources, show decreases of up to 92% within only 50 m downstream distance. In conjunction with the large observed flux estimates of up to 88 g C m^-2 day^-1, these findings suggest that karstic springs are major CO₂ sources to the atmosphere. Because headwater streams constitute the bulk of the surface area of most watersheds, they may provide a disproportionately large contribution to CO₂ effluxes in carbonate-dominated basins, in which source springs play a particularly important role.