Neusiedlersee, the lake with the largest surface area in Austria (320 km2), is situated on the Austrian-Hungarian border with its southernmost part and its outflow in Hungary. Thick stands of Phragmites communis occupy the littoral zone of the lake which account for nearly 170 km2 swamp area (largest marsh area of Central Europe). The lake is characterized by its shallowness and high turbidity, caused by the erosion of silty bottom material. The lake water has a mean conductivity (S) of 1,800 (under ice cover it can increase up to 3,000) with an average ionic content of: Na+:Mg2+:Ca2+:K+ = 51:39:7:3 and HCO3-:SO42-:Cl- = 24:10:18.

The lake occupies a tectonic depression which came into existence only some 13,000 years B. P. at the end of the Pleistocene. The lake has been extremely astatic throughout its history with more than one hundred times of desiccation and high water with the lake area almost twice its present surface. Since the last desiccation in 1868, extensive pure growth of P. communis have expanded mainly along its western and southern shores constituting nearly 99% of the emergent macrophytes vegetation.

At present the lake is threatened with eutrophication problems caused mainly by tourism and agriculture. During the past two decades nutrient levels in the lake increased steadily resulting in successive algal blooms. Since 1976 the bloom species was Microcystis pulverea. Other man-made impacts include the artificial regulation of lake water level, hunting and poaching, introduction of exotic species and reclamation of reed areas for agriculture and tourism.

Neusiedlersee was the site of an IBP project during 1967-1972 and the succeeding IHD and MAB programs. In 1978 the Austrian part and in 1979 the Hungarian part of the lake was declared as a biosphere reserve, thus establishing the first international biosphere reserve. An up-to-date account of the limnology of the lake is given by Loffler (1979)(Q, 1).