Riverine carbon dioxide evasion along a high-relief watercourse derived from seasonal dynamics of the water-atmosphere gas exchange.

The high-relief catchment of the Tavignanu River (Corsica Island, France) with an elevation range from sea level to 2622 m above sea level was investigated for its riverine carbon budget and stable carbon isotopes. Major riverine dissolved inorganic carbon (DIC or TCO) sources depended on seasons and sub-catchment lithology. In winter δC values of -2 to -7‰ (VPDB) indicated influences of atmospheric CO. δC values decreased gradually to values between -9 and -12‰ in July, which indicates elevated soil CO contribution. An observed downstream increase in the total amount of carbon species correlated with inputs from carbonate bearing tributaries and evaporation effects in summer. Main channel partial pressure of CO (pCO) was seasonally highly variable in the upper silicate catchment and the lower coastal plain, where summer values exceed up to six times atmospheric levels. During winter, the central section of the Tavignanu River was found to be undersaturated with respect to atmospheric CO levels. The median values for main channel pCO were below atmospheric levels in winter and spring and above in summer and autumn. The annual carbon flux across the air-water boundary (F) along the Tavignanu River was calculated with (0.77 ± 0.24) Gg C yr, which is about seven times higher than the riverine TCO transport to the sea of about 0.11 Gg C yr. While large sections of the river experienced year-round atmospheric CO uptake or equilibrium, the river as a whole was a small but continuous net source of carbon to the atmosphere. This underlines the important, but so far not well-constrained, contributions of smaller streams and rivers to the terrestrial carbon flux and the need of incorporating them into future global carbon cycle models.