ABSTRACT. Samples of sediment cores from two lakes in the karst area of northwest Yugoslavia were analyzed. Both Lakes Kozjak and Prosce are in the Plitvice National Park, Central Croatia. 14C dating, sedlimentologic, seismic, and isotopic studies, and distribution of diatoms are presented.14C dating of lake marl revealed a uniforn sedimentation rate in Lake Prosce as opposed to Lake Kozjak. Both lake sediments belong to the Holocene period. 14C dating of lake sediment is in agreement with seismic profiles, sedimentologic analysis, and diatom frequency measurements both in an undisturbed as well as in a disturbed lake sediment. INTROI)UCTIONA comprehensive study of aquatic chemistry, hydrobiology, and hydrogeology of the Plitvice Lakes included coring of lake sediment and a subsequent systematic analysis of cores. Samples of cores taken every 5 to 50cm (depending on the type of analysis) were analyzed for pollen, diatom, and ostracode content, followed by sedimentologic, trace element, x-ray diffraction, stable isotope, and 14C analyses. The 14C, stable isotope, and diatom population analyses are presented here, along with a brief description of seismic and sedimentologic work.Lake Kozjak core was retrieved from 24m water depth. Even though a surprisingly thick lake sediment prevented the extraction of all core material overlaying bedrock, the first 12m of sediment gave very useful and, to a certain extent, unexpected data. Radiocarbon ages of an acoustically wellstratified top section indicated a fairly constant sedimentation rate of 0.8mm/a for the last 1800 years. Lake Prosce core was retrieved from 17.2m water depth and reached the clayey residual layer overlaying bedrock. Radiocarbon dating of lake sediment revealed a uniform sedimentation rate(1.4mm/a) as opposed to Lake Kozjak sediments. Both lake sediments belong to the Holocene.The x-ray diffraction analyses showed that the sediments consist of calcite with traces of dolomite, indicating that the sediments were formed by decomposition of dissolved calcium bicarbonate without any significant contribution from detrital limestone or dolomite. This was substantiated by b' 3C data, ranging between -8.5%o and -8.8%o, which is typical of sediments of biogenic origin. Geologic SettingAccording to Polsak (1979) the Plitvice Lakes are surrounded by Triassic, Jurassic, and Cretaceous beds. The whole area is gently folded and intersected by numerous faults. Two most prominent faults run in a north-
ABSTRACT. 613C and 14C activity measurements were made on terrestrial, marsh and aquatic plants growing in their natural habitat of the Plitvice Lakes in northwest Yugoslavia. &3C values were ca -47%o for aquatic mosses, which indicate that the carbon source was dissolved inorganic carbon (DIC) from alkaline karst waters, following a C3 pathway, and ca -25%o for marsh plants, indicating the carbon source was atmospheric CO2.
ABSTRACT. Anthropogenic activities that introduce an excess of nutrients and other pollutants into rivers and lakes are causing significant changes in their aquatic environment. Excessive nutrients greatly accelerate eutrophication, and lake marl formed during eutrophication differs from that formed in oligotrophic water.We analyzed recent sediment cores from Prone and Kozjak lakes located in Plitvice National Park, central Croatia.Analyses consisted of 14C activity of calcareous lake marl, the ratio of stable isotopes (b13C, 5180), organic compounds in the sediment and the distribution of diatoms. Previous 14C activity measurements helped to determine the sedimentation rate and thus the time period of increased input of nutrients into lakes. We determined the increased 14C activity in lake sediments caused by nuclear bomb effect in recent depth profiles. We attributed the sudden increase in diatom species, Cyclotella operculata unipuctata andAchnanthes clevei rostrata, in the uppermost 5-cm layer, to eutrophication of the lake water.We performed a molecular characterization of hydrocarbons isolated from the sediments, and applied computer-assisted high-resolution gas chromatography/mass spectrometry to estimate contributions of biogenic, fossil and pyrolytic hydrocarbons.
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