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 δ¹³C 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 δ¹³C_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 δ¹³C_DIC values between?11.5 and?8.5 ‰. In spring waters, the δ¹³C_DIC values were more negative, from?14 to?12 ‰, and I _sat values of 1–2 indicated that equilibrium conditions for calcite precipitation were not attained. The downstream increase in δ¹³C_DIC correlated with the increase in the δ¹³C 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.