Factors affecting diurnal dissolved inorganic carbon and its isotopic composition in a small stream on a volcanic island in South Korea

Water samples were collected hourly at a small stream on Jeju Island, South Korea in both wet and dry seasons to identify the factors affecting dissolved inorganic carbon (DIC) and its isotopic composition (δ13CDIC). In the wet season, dissolved oxygen (DO) displayed a diurnal pattern with lower values in samples collected between sunset and sunrise compared to samples at other times. Likewise, a diurnal pattern was observed for pH, DIC and δ13CDIC. An overall negative correlation between DIC and DO indicated that photosynthesis and respiration in the stream are the main factors controlling DIC concentration and behavior. Notably, samples at night displayed two trends in plots of DO and DIC, with different δ13CDIC values. This was due to CO2 outgassing from the stream to the ambient atmosphere. Partial pressure of CO2 (pCO2), calcite saturation index (SIcalcite), and equilibrium isotope fractionation between HCO3− and CO2 gas were related to the δ13CDIC value, verifying that photosynthesis and respiration determined DIC behavior. In addition to microbial activity, CO2 outgassing contributed to DIC in the stream after respiration and before photosynthesis. In comparison, δ13CDIC values in the dry season were similar to those in the wet season but showed no clear relationship with pCO2 and SIcalcite. Unlike the wet season, this indicated that in the dry season, photosynthesis and respiration are not the dominant processes and that CO2 exchange between the atmosphere and water is more important in determining DIC.