engleski

Chemical and Isotopic Measurement of Recent Sediment at the Plitvice Lakes

Recent lake sediment with its mineral, organic and isotope composition and trace element content, records environmental changes that occurred in the lake ecosystem and its watershed. It can also indicate whether these changes were natural or anthropogenic. In this study, lake sediment cores in the Plitvice Lakes area were collected to investigate how the recent environmental changes of the lakes are reflected in the sediment geochemistry and carbonate isotopes. The Plitvice Lakes, situated in the Dinaric karst in central Croatia, consist of 16 lakes of different sizes, connected by streams and waterfalls. The lakes receive water from 2 main springs and 2 tributaries. The lakes are characterized by intense calcium carbonate precipitation from the water, forming tufa barriers and fine-grained lake sediments mainly composed of authigenic calcite. The area is sparsely populated and is protected as a national park. Consequently, human activities in the lake watersheds are relatively limited. However, numerous tourists visit this area every year. To study the environmental conditions in the Plitvice Lakes and possible anthropogenic influence on the system the following investigations were performed: - Recent sediments, five short cores (top 40 - 45 cm of sediment) from four lakes of the Plitvice Lakes system were measured for trace elements and isotopes 210Pb, 137Cs, a14C, d13C, d18O to study the influence of environmental changes on the sediment system in small and large lakes. Lake sediments consisted mainly of autochthonous carbonates with higher sedimentation rate in small lakes. Chemical analyses of recent sediments (last ~150 yr) in 2 big and 2 small lakes showed differences between lakes with decreasing concentrations of trace elements in downstream direction, and higher concentrations close to tributaries. All lakes showed an increase in 14C in the carbonate sediments above the first occurrence of 137Cs which was interpreted as a damped and decades-delayed consequence of the bomb-induced increase in 14C in atmospheric CO2. For the small lakes, the increase of δ13C in the last 2 decades is probably due to an increase in primary productivity, which enhanced biologically induced calcite precipitation. Downstream increase in the 14C and δ13C in the carbonate fraction of the sediment corresponds to an increase of 14C and δ13C of dissolved inorganic carbon (DIC) in the water as a result of CO2 degassing and carbon isotope exchange between atmospheric CO2 and DIC and photosynthetic activity - Recent sediments in Lake Kozjak were used to study the impact of the material delivered to the lake sediment via Rječica Brook. Distribution of organic matter, C/N values, d13Corg and d13Ccarb in sediment profiles showed strong influence of terrestrial organic and carbonate fraction run-off via Rječica Brook which decreased rapidly in the direction of the open lake. The sedimentation rate as well as the ratio of allogenic and authigenic fraction in sediments was estimated. The increase of the allochtonous fraction in the lake sediment, e.g. organic material from the catchment area enhances the growth of macrophytes and therefore the eutrophication process in this part of the lake.

Sediments; Plitvice Lakes; Isotopes

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