Changes in accumulation of organic matter and stable carbon and nitrogen isotopes in sediments of two Slovenian mountain lakes (Lake Ledvica and Lake Planina), induced by eutrophication changes

We measured accumulation rates of organic carbon (OC) and total nitrogen (TN) and stable carbon ((delta)13Corg) and nitrogen isotopes ((delta)15N) in the sediments of two mountain lakes (Lake Ledvica and Lake Planina, northwest Slovenia). Marked variations of these parameters were observed in both sediments. OC accumulation rates ranged from 4 to 23 g m^-2 yr^-1 in Lake Ledvica, whereas in Lake Planina they were one order of magnitude higher and increased substantially in the upper sediments. (delta)13Corg and (delta)15N decreased upward in Lake Ledvica, from -25.5% to -28.4%, and from +2.1% to -3.4%, respectively. In contrast, substantial variations in the (delta)13Corg profile, ranging from -30.9% to -37.4%, but a quite uniform (delta)15N profile of approximately +1.8%, was observed in Lake Planina. Elemental and isotopic changes of bulk sedimentary organic matter in the lakes were related to changes in the past trophic state of the lakes and their watersheds, inferred by natural development of the lake ecosystems, anthropogenic activities, as well as earthquakes and forest fires. Observed changes and differences in the (delta)13Corg and (delta) 15N records could be attributed to global changes in isotopic composition of atmospheric CO2 and nitrate in atmospheric deposition, as well as to differences in organic matter, dissolved inorganic carbon, and dissolved inorganic nitrogen sources at various altitudes. Although Lakes Ledvica and Planina are remote mountain lakes, the sediment records showed severe eutrophication in Lake Planina, whereas in Lake Ledvica eutrophication is still moderate, thus reflecting a high sensitivity of these ecosystems to any external inputs.