Altitudinal Gradients and Forest Response:
Climate, Hydrology and Isotope Variability
of a Mediterranean Ecosystem

Abstract:

The western Mediterranean is one of the most prominent climate change hotspots in Europe and also worldwide. A closely related scientific question to such climatic changes is the contribution of rivers, streams and connected groundwater to the amount of carbon dioxide released to the atmosphere from inland waters. The role of rivers and associated groundwater within the global carbon cycle as a major source of CO2 has long been neglected and even more so headwater streams. So far, only few studies have taken into account carbon stable isotopes as a tracer for sources and sinks of carbon. The high-relief Tavignanu River (Corsica, France) with a total length of 89 km and an altitude difference of 1750 m, from a mountainous source lake to the river mouth to the Mediterranean Sea, was investigated for potential processes that influence the riverine carbon budget. These processes include water mixing (especially with groundwater), weathering, changes in lithology, and seasonal variations. Riverine dissolved inorganic carbon (DIC) concentrations and its stable isotope ratios (?13CDIC) showed seasonal variations. In February, ?13CDIC values of –2 to –7 ‰ (VPDB) indicated influences of atmospheric CO2. During springtime, stable isotope values started to gradually decrease to lower values in July that ranged from –9 to –12 ‰, that indicate an increased contribution from soil- and grondwater-derived CO2 to the riverine DIC pool. The median pCO2 values averaged over the entire Tavignanu River course were 160 µatm for February, 340 µatm for May, and 540 µatm for July thus showing the river as a CO2 sink in winter and only a slight source in summer.