Effects of land use and fish stocking on a mountain lake - evidence from the sediment

Background and purpose: Mountain and high-mountain lakes all over the Alps were originally without fish, in medieval times a lot of lakes were stocked with fish, but not in Slovenia. Lake Jezero Na Planini Pri Jezeru (1430 m a.s.i.; NW Slovenia) was stocked in 1951. Other human activities, particularly forges and alpine diary farming, were known there since the 14(th) century. From 1950 onward tourism has became an important human activity around the lake too. Today the lake is highly eutrophic. To reveal the main milestones during eutrophication process of the lake analyses of the sediment were done. Material and Methods: A sediment corer was employed to get a 40 cm long sediment core from the deepest point of the lake. The core was immediately sliced into 0.5- 1 cm thin slices for further analyses. Dating (Pb-210 method) and analyses of physical properties of the sediment (dry weight, loss-on-ignition) as well as analyses of diatom and Cladocera remains were performed to reconstruct events in the lake's catchment and lake itself Historical data were used to comment results obtained from the sediment analyses. Results: Forging, associated with clear-cutting influenced the lake since the very beginning but the most destructive influence had introduction of fish in 1951. A change in zooplankton community started at that time and was enhanced after hotel reconstruction in mid 80s. The result is highly eutrophic lake with permanent depletion of oxygen below 5 m and low Secchi-disc depth. Prevalence of small-bodied zooplankton Crustacea, with Eubosmina as the most abundant species is characteristic for the present situation. Diatom species, Stephanodiscus parvus, an indicator of eutrophication, is very common, too. Conclusion:Anthropogenic influence of the lake started almost live centuries ago, but was moderate till 1950. Homeostasis of the lake was strong enough to neutralize most of the negative effects of human activities and the result was low eutrophication, fish introduction in mid 1950s and increase in tourism and activities associated with it have definitely destroyed lake homeostasis and started intensive eutrofication.