Achensee and Lk. Teletskoye - 5000 km apart: similarities and differences of two mountain lakes

Lakes account for 3% of the Earth's surface and they constitute important habitats for manifold biota [1]. In the 19th century at Walden pond (Massachusetts, USA) the research field of limnology developed, which is nowadays dealing with physical, chemical and biotic aspects in all kind of freshwaters [2,3]. While in the past pollution was a main driver in lake ecosystems [4], nowadays rising water temperatures negatively affect Alpine lakes throughout the world [5]. Thus, we selected two mountainous lakes in the Alps as well in the Altai Mountains - two distant, but ecologically similar regions - to compare their characteristics, research history, pressures as well as management strategies.Lake Achensee is the largest lake (6,8 km²) of the Austrian federal state Tyrol at an altitude of 928.78 m. It`s length is 8.6 km and the maximal depth amounts to 133.02 m. Investigations of the lake started in the beginning of the 20th century, focusing on zooplankton [6] and bathymetry. Since 1927 this natural lake is used as a reservoir for hydropower production, with water level fluctuations of up to 5 m. In the 1970ties intensified tourism, resulted in an eutrophication of the lake and blooms of Planktothrix rubescens. Since the construction of a sewer around the lake, the situation improved, and it is again considered as an oligotrophic lake since the end of the 20th century. Commercial fisheries were stopped in 2000, due to reduced stocks (reduction of nutrients) as well as the occurrence of Triaenophorus crassus [7]. Nowadays only recreational fishing takes place. In Europe lakes > 50 hectares are assessed regularly under the WFD, including the biological quality elements phytoplankton, macrophytes and fish – revealing a good ecological potential of the lake.The 78 km long Lake Teletskoye (Altyn-Kol) is the largest (223 km²) and deepest (up to 325 m) lake of the Russian Altai at an altitude of 434 m. It has been known since the 17th century, but the first large-scale studies, including bathymetric measurements and hydrobiological collections, started only in 1901 [9]. This cold oligotrophic lake is characterized by very low fish productivity. Attempts to organize industrial fishing were made in the 1930ties, but the fishery was considered impractical. At the same time, Lake Teletskoye is one of the largest tourist and recreational centers in Russia, and in conditions of outbound tourism restrictions, the tourist flow increases annually. The state environmental monitoring of the lake includes hydrological and hydrochemical measurements only. Since 1987, a scientific floating station of the Russian Academy of Sciences is operating on the lake, whose tasks include analyzing long-term changes in the composition and structure of aquatic communities [e.g. 10, 11]. The available data indicate the high ecological status of Lake Teletskoye.Our synthesis highlights, that long-term data is crucial in order to understand changes related to human activities as well as climate change. We exemplify dynamics and catchment interactions of mountain lakes, using two lakes and discuss similarities and differences.