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
60 Hungarian Journal of Hydrology Vol. 96. No. 3. 2016. stream), flow periodicity, fluctuation of water level, relatively low water depth, excessive water abstraction and lack of buffer zones. Although there is now an urgent need to pay more attention to the conservation-oriented management of small streams, it is in conflict with the objectives of the agriculture. Lake Balaton Lake Balaton - the largest lake of Central Europe - lies almost exactly in the centre of Transdanubia, the ancient Roman province of Pannónia. The mean lake level is 104.8 m above the Adriatic Sea level. At mean lake level the surface area is 596 knr, the mean depth is 3.25m, so that Lake Balaton is a shallow lake having a large surface area. This lake has an elongated shape with a length of 78 km and an average width of 7.6 km. Along the longitudinal axis the depth decreases gradually to the SW-end. The northern shore decorated by a range of picturesque hills (extinct volcanoes in the Tapolca basin) (Fig. 5) while its southern shore is a fertile flatland. Figure 5. Lake Balaton with extinct volcanoes in the background The southern shore is a sandy beach, the bottom drops steeply here, along the northern shore, the bottom of the lake deepens sharply. The catchment area covers 5775 km2. The lake has 51 inflows but the only outflow is the Sió Canal at the south-eastern end of the lake. One-half of the inflow iinto the lake is the River Zala, which flows into the smallest westernmost basin. The Zala River drains an area of 2622 km". Limestone and dolomitic rocks predominate in the catchment area, consequently the waters discharged the lake predominated by calcium-, magnesium- and bicarbonate ions. The concentration of total dissolved solids is quite high, about 500 mg/L, and the typical pH of the water is 8.4. Almost one-half (110 km) of the shoreline covered by a dense reed stands. The total area of the reed cover is 15 km2 (Herodek et al. 1988). The water temperature is normally above 20 °C from the end of May to early September, and this period is considered suitable for bathing and aquatic sports. Lake Balaton is one of the greatest natural assets and main touristic destination in Hungary due to its sweet water, mild climate, and picturesque landscape. The eutrophication of Lake Balaton has become a serious problem in the eighties of the last century. Summer blooms of nitrogen-fixing filamentous cyanobacteria became frequent phenomenon in the western part of the lake, as a consequence of the large phosphorus load of River Zala. The large scale eutrophication control measures (sewage water diversion, phosphorus removal at the sewage treatment plants, construction of pollution control reservoirs on the catchment area) resulted in a significant decrease of the external phosphorus load and phytoplankton biomass of the lake (Clement et al. 2005). During the 1980s the summer maximum of the chlorophyll a concentration usually exceeded the 200 pg/L, nowadays it is far below the 50 pg/L in the most productive western basin, which means that the whole lake is suitable for bathing (the WHO limit is 75 pg/L in case of cyanobacteria dominance) (Fig. 6). kD kD kO k£> kO kO kO k£> kD kD kO kD kD kD kD kD kD D kD kD kD kD kO kD kO Figure 6. Long-term changes of the phytoplankton biomass (chlorophyll a concentration) in the most productive western basin of Lake Balaton
