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 (dCorg) and nitrogen isotopes (d15N) 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 m22 yr21 in Lake Ledvica, whereas in Lake Planina they were one order of magnitude higher and increased substantially in the upper sediments. dCorg and d15N decreased upward in Lake Ledvica, from 225.5‰ to 228.4‰, and from 12.1‰ to 23.4‰, respectively. In contrast, substantial variations in the dCorg profile, ranging from 230.9‰ to 237.4‰, but a quite uniform d15N profile of approximately 11.8‰, 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 dCorg and d15N 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. The organic matter accumulated in lake sediments constitutes an important fraction that provides crucial information in studies of the lacustrine paleoenvironment, the history of climate change, and the effects of man on local and regional ecosystems. It is introduced to lakes by multiple pathways (Meyers and Ishiwatari 1995; Herczeg et al. 2001). Terrigenous (allochthonous) organic matter, originating mostly from the catchment area via tributaries, consists mainly of degraded remains of leaves and grass and soil organic matter, as well as material that falls directly into the lake from the riparian zones. Additional contributions from more distant sources are delivered by precipitation and wind. On the other hand, biota (animals, plants, and bacteria) within the water column and the sediments contribute aquatic (autochthonous) organic matter. The different types of biota populating a lake and its watershed produce organic matter having a distinctive biochemical composition. Changes in the community structure of these biota create variations in the amounts and types of organic matter deposited at different times in the history of a lake (Meyers and Ishiwatari 1995). 1 Corresponding author ([email protected]).