Piburger See is a small mountain lake in the Eastern Alps (Austria) and was part of the OECD study on eutrophication (OECD, 1980). This lake experienced cultural eutrophication during several decades in the 20th century. In the 1950s and 1960s, recreational activities increased with a concurrent rise in tourism, and increasing amounts of fertilisers were used on nearby fields (Pechlaner, 1968). This resulted in enhanced primary production and rising hypolimnetic oxygen depletion in the lake (Pechlaner, 1979). Lake restoration started in 1970 by exporting anoxic and nutrient-rich hypolimnetic waters with an Olszewski tube (Pechlaner, 1971, 1979). External nutrient loading was reduced by altering fertiliser application and by diverting sewage from a public bath (Psenner, Pechlaner & Rott, 1984). After the installation of the Olszewski tube in June 1970, phytoplankton biomass increased, accompanied by an increase in chlorophyll "a" and total phosphorus (TP) (Rott, 1983; Tolotti & Thies, 2002). The period from 1970 until 1988 was characterised by a marked increase in the filamentous cyanobacterium Oscillatoria limosa C.A. Agardh, which became the dominant algal taxon in the lake and contributed up to 40% to the mean annual phytoplankton biovolume (Rott, 1976; Wolf, 1991). The response of phytoplankton biovolume to lake restoration was delayed by two decades and re-oligotrophication of Piburger See began during the late-1980s (Pechlaner, 1979; Rott, 1983; Pipp & Rott, 1995). At present, Piburger See is oligomesotrophic (Tolotti & Thies, 2002). Since the early 2000s, chlorophyll "a" and phytoplankton biovolume values suggest a reversing trend in lake trophic status with a rising contribution of diatoms including blooms of Asterionella formosa (Tolotti et al., 2005; Tolotti net al. 2012, Thies et al. 2012). Monitoring of lake water quality, control of deep water siphoning discharge and operation of gauges are current activities at this site. A rowing boat for lake water sampling is available. A smaller part of the lake has the status of a research area. During summer, tourism is an important factor at this site (public bath, rowing boats, angling, swimming, walking).