engleski

Naturaly and anthropogenicaly induced environmental changes reconstructed from recent lake sediment in the Plitvice Lakes

Plitvice Lakes are complex natural phenomenon that comprises interaction among geological structure of karst area, water, vegetation and atmosphere. Interconnection of physico-chemical and biological processes results in calcium carbonate precipitation from water, and formation of tufa barriers and lake sediments, i.e. lakes itself. Lakes are very sensitive to environmental changes, either of natural origin, like climate change, or caused by anthropogenic influence. Processes of both origin could result in enhancement of primary productivity and eutrophication of the lakes. Lake sediments with their geochemical and isotopic composition preserve information of past environmental changes and thus they can be used for assessment of possible anthropogenic influence. In this paper we present investigation of sediments from four lakes: the uppermost big Lake Prošće, small Lake Gradinsko big Lake Kozjak (location K1), and the lowermost small Lake Kaluđerovac. We studied the influence of environmental changes on the Plitvice Lakes system and differences between small and big lakes in response to these changes. Cores from these sites were sampled in the middle of the lakes, and additionally, the core K2 was sampled near the confluence of Rječica Brook to Lake Kozjak. Sediment cores were collected by scuba divers in November 2003 with plastic corers (50 cm length, 10 cm diameter). The frozen sediment cores (~45 cm length) were cut into 1 2 cm thick layers and the following analyses were performed: mineralogical, CHN analyses of organic matter (OM), trace element and isotopic analyses. The 210Pb dating method showed that all sediments represent the period of 100 – 150 years. Sedimentation rates for big lakes Prošće and Kozjak are 1.3 and 0.8 kg m-2y-1, respectively, and for smaller lakes, Gradinsko and Kaluđerovac, are 3.4 and 2.6 kg m-2y-1, respectively (1). Mineralogical analyses showed that lake sediments consist mainly of calcite with lower calcite content in big lakes (70 – 85%) and higher in smaller lakes (90 – 99%). Fractions of dolomite and quartz were minor in big lakes and almost negligible in smaller lakes. The OM content in sediment cores decreased from uppermost Lake Prošće (10%) to lowermost Lake Kaluđerovac (2%). The K2 core had the lowest calcite content (below 60%) and highest organic matter content (up to 36%) because of high influence of Rječica Stream. Distribution of calcite and OM content from bottom to the top of all sediment cores were nearly uniform with slightly decreasing trend in calcite content and increasing trend in OM content. Atomic C/N ratios obtained from CHN analyses of OM were between 10 and 16 thus showing mainly autochtonous origin of OM. The decreasing trend of C/N ratio from the bottom to the top of the cores is a consequence of OM degradation in oxygenated sediments and partly indicates lower input of allochtonous OM. Analyses of trace elements (Al, Cd, Cr, Cu, B, Ba, Mn, Ni, Sr, Zn, and P) showed slight variations in core profiles and concentrations which indicated natural origin of most analysed elements. The exception is Cd with relatively high concentration in sediment from Lake Prošće and Kozjak K2 and its origin could be terrestrial transported by streams of Matica and Rječica. Lead concentration increases from the bottom to the top in cores from lakes Prošće and Kozjak K1. Cluster analysis indicated anthropogenic origin of lead which could be explained as a consequence of traffic which run along the lake shore in the sixties and seventies of last century. Due to construction of new main road further away, concentration of lead decreases in the top 5 cm of both cores. Isotopic measurements (a14C, δ 13C and δ 18O) of carbonate fraction of sediment showed that calcite is mainly autochtonous, precipitated from lake water dissolved inorganic carbon (DIC) in nearly isotopic equilibrium. Increase of a14Ccarb was observed downstream from uppermost Lake Prošće (mean value 65.5 pMC), to the lovermost Lake Kaluđerovac (mean value 80.6 pMC) as a consequence of carbon exchange between DIC and atmospheric CO2. Similar trend is noticed in δ   Ccarb values, mean value increases downstream from -8.8‰ to -7.8‰ . In upper layers (10 cm) of small lakes δ   C values have sudden increase indicating nonequilibrium conditions of calcite precipitation. Peaks of a14C are visible in four cores from the middle of the lakes and they represent the reflection of global atmospheric contamination with bomb 14C due to thermonuclear weapon test during sixties of the last century. Increase of a14C of atmospheric CO2 was 100%, and in sediment cores observed increase of a14Ccarb was only 5 – 7% in two big lakes, and ~10% in two smaller lakes. Peaks were noticed at top 3 – 5 cm in big lakes and at ~10 cm layers in small lakes. Delayed and damped 14C signal in the lake sediments is consequence of geochemical carbon cycle. The results of isotopic measurements in OM were: a14Corg between 83 pMC and 88 pMC, and δ 13Corg between -34‰ and -30‰ , and δ 15Ntot values between 0.5 and 1.8‰ . a14Corg values are typical for organic matter produced in hard-water lakes (hard-water effect). Isotopic composition of OM (a14C, δ 13C) and δ 15Ntot values are characteristic for mainly autochtonous origin of OM in all lake sediments, except for the K2 core where isotopic composition (a14Corg ~102 pMC, δ 13Corg ~ -29‰ and δ 15Ntot ~ -1‰ ) indicates mainly allochtonous origin of OM. Stable isotopes in both carbonate and organic fractions indicated slight increase of primary productivity in the last 20 30 years. According to isotopic results in all sediment cores, including core K2, main nutrient contribution comes from streams runoff. Thus, observed increase in primary productivity which is more pronounced in small lakes, could be explained by the atmospheric temperature increase, i.e. climate change. Global anthropogenic influence was also seen in a14C activity of sediments.

lake sediment; environmental changes; Plitvice Lakes

nije evidentirano