engleski

Delta 13C values of dissolved inorganic carbon in the Plitvice Lakes

Plitvice Lakes, situated in the Dinaric karst, Central Croatia, consist of 16 lakes of different sizes and positioned at different altitudes, connected by waterfalls and streams. The precipitation of calcite from water in the form of tufa and lake sediments is very intensive. The results of delta13C values of dissolved inorganic carbon (DIC) in the Plitvice Lakes water collected at 17 sampling points (springs, tributaries and lakes) in the period 2004-2007, were used for the study of carbon isotope exchange processes between the atmospheric CO2 and the DIC, the equilibrium/non-equilibrium conditions of calcite precipitation (comparison with delta13C of calcite), as well as for the study of the origin of carbon in the lake sediments (autochthon/allochthon). The influence of temperature and amount of precipitation on the delta13CDIC values as well as the influence of some other processes, e.g. primary productivity on the delta13CDIC were also studied. The delta13CDIC values showed a steady increase in the downstream direction, rising from the springs (mean delta13CDIC -12.3‰ ) to the lakes (up to -8.3‰ ). The increase is due to carbon exchange between the DIC and the atmospheric CO2, as well as to photosynthesis process in the lake waters. The more negative delta13CDIC values in springs and tributaries show a higher influence of the CO2 of biogenic origin on the DIC composition. The downstream increase of delta13CDIC correlates with the increase of the delta13C values of calcite in the lake sediments in the same direction, showing that calcite is mainly of autochthonous origin precipitated under equilibrium conditions. Seasonal measurements of delta13CDIC show variations of delta13CDIC in the lake waters influenced by temperature variation, and more stable delta13CDIC values in the spring and tributary waters. The delta13CDIC values of the Plitvice Lakes water measured for the period 2004 -2007 were compared with those for the period 1984 – 2002, and correlated with mean annual air temperature and annual precipitation amount. The most positive delta13CDIC values for majority sampling points were observed in the year 1984 and 2004. These years were characterized by higher air temperature, lower amount of precipitation and lower water outflow. Higher air temperature caused higher water temperature and increase of water evaporation, as well as the acceleration of the process of photosynthesis/primary productivity. The final effect was the increase of delta13CDIC values. The rise of the delta13CDIC values was also reflected in the increase of the delta13C values of the surface lake carbonate, particularly in small lakes. The correlation between the delta13CDIC values and the concentration of the dissolved organic carbon (DOC) in the Plitvice Lakes water showed that in this eco-system photosynthesis is the dominant process in the lakes, with enhanced process of decay of organic material in the spring and tributary waters.

delta13C; dissolved inorganic carbon; Plitvice Lakes; tufa; lake sediments; carbon isotope exchange processes; origin of carbon

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