The isotope composition of precipitation has been monitored in monthly precipitation at Zagreb, Croatia, since 1976. Here, we present a statistical analysis of available long-term isotope data (3H activity concentration, δ2H, δ18O, and deuterium excess) and compare them to basic meteorological data. The aim was to see whether isotope composition reflected observed climate changes in Zagreb: a significant increase in the annual air temperature and larger variations in the precipitation amount. Annual mean δ18O and δ2H values showed an increase of 0.017‰ and 0.14‰ per year, respectively, with larger differences in monthly mean values in the first half of the year than in the second half. Mean annual d-excess remained constant over the whole long-term period, with a tendency for monthly mean d-excess values to decrease in the first half of the year and increase in the second half due to the influence of air masses originating from the eastern Mediterranean. Changes in the stable isotope composition of precipitation thus resembled changes in the temperature, the circulation pattern of air masses, and the precipitation regime. A local meteoric water line was obtained using different regression methods, which did not result in significant differences between nonweighted and precipitation-weighted slope and intercept values. Deviations from the Global Meteoric Water Line GMWL (lower slopes and intercepts) were observed in two recent periods and could be explained by changes in climate parameters. The temperature gradient of δ18O was 0.33‰/°C. The tritium activity concentrations in precipitation showed slight decreases during the last two decades, and the mean A in the most recent period, 2012–2018, was 7.6 ± 0.8 Tritium Units (TU).
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 δ13CDIC 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.
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