Carbon isotopes in dissolved inorganic carbon as tracers of carbon sources in karst waters of the Plitvice Lakes, Croatia

Carbon isotope composition of dissolved inorganic carbon (DIC) was studied at springs, lakes and tributaries of the Plitvice Lakes. The Plitvice Lakes are a unique and complex karst lakes system consisting of 16 flow-through lakes connected by waterfalls and streams and characterized by intense calcium carbonate precipitation in the form of tufa and lake sediments. Two main springs have different ¹⁴C concentrations due to different geology and their variations were controlled by flow rates. Further downstream, at lakes to the Korana River that outflows from the lakes, the ¹⁴C activity and δ¹³C consistently increased down the flow. Carbon isotope composition from 2010 to 2015 was compared with the values measured 30 years earlier. The variation in δ¹³C_DIC was accounted for seasonal change, while ¹⁴C activity of surface water DIC decreased 7–12% during the 30 years. Using a semi-empiric model, it is calculated that the downstream increase is controlled by the exchange of DIC and atmospheric CO₂ carbon and by introduction of decomposed biogenic carbon from the top soil organic matter of the lakes surrounding area in almost equal proportions.