Lake Neusiedl wetland (Austria/Hungary) in a changing environment: anthropogenic versus anthropogenic effects

Lake Neusiedl and adjacent lakes at the Austrian/Hungarian boundary represent a unique wetland, situated at the geodynamical and geomorphological boundary between the Alps, Carpathians and the Pannonian Basin, and therefore represents an important transition zone concerning terrain, climate, vegetation and fauna. The more than 700 km² large area is one of the flattest regions of Austria with less than 17 m relief variation. Today, Lake Neusiedl measures some 320 km² with less than 1.8 m depth and a tectonic origin is widely accepted. East of the lake, about 30 shallow lakes still exist, of which the largest measures less than 2 km length and less than 1 m depth. At present, the water level of Lake Neusiedl shows the lowest values since 1965 and all the shallow lakes were dry during the summer 2022. We use high-resolution topographical, sedimentological, geomorphological as well as historical maps and historical charters to investigate the formation end evolution of Lake Neusiedl and adjacent lakes. Our data show that the present-day conditions and processes of Lake Neusiedl strongly diverge from conditions in the past. The earliest preserved record of modification of the lake´s hydrological conditions is from 1568, followed by increasing drainage efforts and the building of a dam road in the southeast, finished in 1780, which subsequently cut off the lake from its most important tributaries from the south. This palaeohydrological reconstruction of Lake Neusiedl and Hanság also implies an episodic lower salt content of the water compared to modern values, especially during flood periods. Therefore, it is not useful to compare the hydrological situation of Lake Neusiedl before 1780 (or even 1568) and after. The documented variation of the water level of Lake Neusiedl between desiccation and highest flood level is around 4.2 m. These variations affected enormous areas in this low-relief region, with a huge impact on the landscape, fauna, vegetation, human settlement patterns, land use, communication routes and even possible occurrence of malaria – which should be considered in regional archaeological, historical and biological interpretations. The numerous shallow lakes and presently dry basins, detected in the high-resolution airborne laser scanning data in the Seewinkel originally formed as thermokarst lakes during permafrost degradation after the end of the Last Glacial Maximum. In total, more than 370 enclosed depressions are visible. The depressions of eastern Austria represent one of the first Latest Pleistocene thermokarst lakes documented in central Europe. Although man-made climate change has a clear impact on the water balance of the lakes, they reinforce past hydrological modifications.