Hidrológiai Közlöny, 2016 (96. évfolyam)

2016 / 3. szám - HISTORICAL SNAPSHOT - Borics Gábor - Ács Éva - Boda Pál - Boros Emil - Erős Tibor - Grigorszky István - Kiss Keve Tihamér - Lengyel Szabolcs - Reskóné Nagy Mária - Somogyi Boglárka - Vörös Lajos: Water bodies in Hungary - an overview of their management and present state

G. Borics et al: Water bodies in Hungary - an overview of their management and present state

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Lake Fertő

Lake Fertő/Neusiedlersee is a wind-exposed, extremely shallow steppe lake (~ 1.3 m), straddling the Austrian-Hungarian border at 166.5 m above Adriatic Sea level (Löffler 1979, Dokulil and Herzig 2009). The lake basin covers a total area of 315 km2, of which 76% (240 km2) are on the Austrian and 24% (75 km2) are on the Hungarian territory. The lake has an elongated shape with a maximum length of 36 km and a maximum width of 12 km. Two major inflows are River Wulka and Rákos stream. Originally, the lake has no natural outflow, but at the end of the 19th century an artificial channel (Hanság channel) was constructed. Nowadays, the lake water level is regulated by a sluice on Hungarian territory near Fertőújlak, and bilateral issues are dealt with by the Austro- Hungarian water commission which was established in 1956 (Loiskandl et al. 2012). In the 18th century the lake had dried out four times and the last vanishing was from 1864 to 1870 (Loiskandl et al. 2012).

More than half (ca. 55%, i.e. 171 km2) of the surface area is covered by emergent vegetation, mainly reed

(Phragmites australis (Cav.) Trin. ex Steud). In Hungary, reed comprises a significantly larger area along the shoreline (up to a width of >5 km), than in Austria (up to 2 km). Degradation of the reed stands has been documented from the early 1980s (Dinka et al. 2010) '.This problem is more aggravated in the Hungarian part: while in Austria only 10% of the reed stands are degraded, in Hungary this proportion is 30% (Márkus et al. 2008, Wolfram et al. 2014). As a result, there are numerous reedless brown- water ponds (inner lakes) of variable size within the reed belt, which is intersected with artificial canals connecting the inner ponds with the open water areas (Fig. 7-8). Electric conductivity is about 2,200 pScnV1, and the pH varies around 9. The dominant salt is sodium bicarbonate (NaHC03) and thus, Lake Fertő classified as a soda lake (Wolfram et al. 2014). As a result of wind-induced sediment resuspension, the open water of the lake is characterized by high inorganic turbidity and usually low Sec- chi-disk transparency (Löffler 1979). Within the reed cover, water is not turbid due to less exposure to wind and brown in colour due to humic substances (Löffler 1979, Wolfram et al. 2014).

Figure 7-8. The open water and the reed belt of Lake Fertő/Neusiedlersee

The lake underwent strong eutrophication during the 1970s and 1980s, when phosphorus concentrations have shown a pronounced rise due to increase in tourism and economic development in the catchment as well as an enhanced consumption of fertilisers in the intensified agriculture (Dokulil and Herzig 2009, Wolfram et al. 2014). In the open water, annual mean concentration of total phosphorus rose from about 40 pgL 1 to more than 160 pgL~' at the peak of the eutrophication period (Wolfram et al. 2014). Due, however, to extreme light limitation of algae, enhanced nutrient availability did not induce a significant increase in phytoplankton biomass, and algal blooms were mainly restricted to sheltered areas. Various management measures were taken to reduce the nutrient input into the lake. These have significantly reduced anthropogenic nutrient loads since the beginning of the 1980s. Total P loads fell from about 80 t in the beginning of the 1980s (only Austrian part of the catchment area) to less than 20 t around 2000 (total catchment area). As a consequence, phosphorus concentrations have also decreased since the 1980s (Wolfram et al. 2014).

During the last century, the fish community has experienced significant changes, mainly due to fisheries and introduction of exotic species (topmouth gudgeon, pump- kinseed). Stocking with eel was clearly one of the most

serious impacts on the autochthonous fish community from the middle of the 20th century (Wolfram et al. 2014). It resulted in local extinction of small-sized fish, like mudminnow and weatherfish. Since the beginning of the 21st century, when stocking of glass eel was stopped, the density of eel has significantly decreased. Recently, species, which had become locally extinct, were reintroduced in the Hungarian part of the lake, e.g. the weatherfish (Wolfram et al. 2014).Toáay, there are less than 15 professional fishermen in Austria, while about 40-50 of them worked in the whole lake at the end of the 19th century. Angling and fishing on the Hungarian side has never been as important as in Austria, due to the large extension of the reed covered area.

Because of the environmental diversity the Lake Fertő region provides habitats for many wildlife species especially birds and an important wetland in Central Europe. For conservation and wise use of the wetland and its resources the “Neusiedler See-Seewinkel” is designated to the Ramsar Convention on Wetlands in 1982. Aside from that, for nature and natural resources conservation a national park “Neusiedler See-Seewinkel” was founded in 1993 and co-managed with the Hungarian national park “Fertő-Hanság”, founded in 1991 (Loiskandl et al. 2012).