SOURCES AND FATE OF DIC IN SWEDISH STREAMS

Abstract:

DIC export by streams and rivers is a major component of the global C cycle. However, many questions remain about the source and fate of aquatic DIC and CO₂. Stable carbon isotope δ¹³C can provide information about the source and evolution of DIC and CO₂ along hydrological networks. But the interpretation of δ¹³C values must be made with caution, since several biogeochemical processes affect the isotopic signal. In this study, we developed a systematic approach resolving these influences when interpreting large-scale patterns in δ¹³C-DIC and δ¹³C-CO₂ values with regard to the source and fate of C in low order streams. We analyzed δ¹³C-DIC values in streams from four different regions of Sweden. Taken together they span large gradients in climate, geomorphology and lithology. The source of the DIC pool was predominantly biogenic in three of the regions (δ¹³C-DIC_source = -17.4‰), but not the northernmost, where a clear geogenic input could be identified (δ¹³C-DIC_source =-8.2 ‰). Our results suggest that soil respired CO₂ is the main source of stream CO₂ (δ¹³C-CO2_source=-22.9‰) in all four regions, yet aquatic processes can also be a contributing component of the DIC pool in streams, with corresponding influence on the δ¹³C values. Once CO₂ was in the stream, degassing was the primary control on its fate. However, there were indications that aquatic biological processes added CO₂, (by DOC degradation) in the southernmost region, and that CO₂ was removed (by photosynthesis) in the most central region. Correctly interpreted, the carbon stable isotope data can serve as a powerful tool for identifying the source and fate of stream DIC.