Spatial and seasonal variations in the stable C isotope composition of dissolved inorganic carbon and in physico-chemical water parameters in the Plitvice Lakes system

Plitvice Lakes waters were collected at 14 sampling points, including springs, tributaries and lakes, for the period 2002-2007. The results of the physical and chemical conditions of calcite precipitation as well as the delta C-13 values of dissolved inorganic carbon (DIC) were used to study the processes influencing calcite precipitation. Significant differences between spring, lake and stream waters as well as changes in the downstream direction were observed. The correlation between delta C-13(DIC) values and physico-chemical conditions for calcite precipitation showed that calcite precipitates in lake waters which are oversaturated with respect to CaCO3 (I-sat values 4-10) and with delta C-13(DIC) values between -11.5 and -8.5 parts per thousand. In spring waters, the delta C-13(DIC) values were more negative, from -14 to -12 parts per thousand, and Isat values of 1-2 indicated that equilibrium conditions for calcite precipitation were not attained. The downstream increase in delta C-13(DIC) correlated with the increase in the delta C-13 values of calcite in the lake sediments, suggesting that the freshwater calcite was mainly of autochthonous origin and precipitated within the water column in isotopic equilibrium with DIC.