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
62 Hungarian Journal of Hydrology Vol. 96. No. 3. 2016. Lake Velencei Lake Velencei, having unique value from geological, nature conservational and ecological viewpoint, is the second largest soda lake in Hungary (area: 24 km2, mean depth: 1.6 m; Electrical conductivity: -3000 pS cm ') and an important touristic and recreational centre ( Reskóné 1999). In the past the lake was characterised by drastic water level fluctuations; occasionally dried up, or since it has no natural outlets, flooded its surroundings. To lower these extremities the first interventions were implemented in 1880 when the extended wetlands on the south-western part of the lake were drained and a natural outlet was established, by which an effective water level regulation could be achieved. From the 1930s due to the development of holiday resorts around the lake the anthropogenic effects became more intense, accelerating the process of natural eutrophication that was already in advanced stage independently of anthropogenic load. The reed-bed proliferated more and more both along the shore and in the inner parts of lake-basin and the mud that accumulated in the midst of roots of the reed arrested the movement of the floating marshes that were characteristic formations of the western part of the lake. (Gorzó 1990). By the beginning of 1960s, the 59% area of the lake was already covered by reed, while in the open water areas the mass proliferation of algae resulted in water blooms; which frightened tourism and water uses. To overcome these Figure 9-10. The Western (laeft) and Eastern (right) parts Soda pans Soda dominated waters are characterized by large amounts of sodium hydro-carbonate/carbonate and usually hypersaline with high alkalinity (pH>9), which property clearly distinguishes them from other inland saline waters. Soda lakes/pans can be found in each continent of the World, but their distribution is confined to specific geographic regions. The Carpathian Basin is the western border of the soda lakes/pans distribution (Austria, Hungary and Serbia) in Eurasia, and they can be found only in few regions of Asia. The intermittent (astatic) soda pans are very shallow (water depth >lm) and small (open water 1-200 ha), which were formed on various geological substrates by specific climatic, geologic and hydro- logic environment in the Carpathian Basin at the end of Pleistocene and the beginning of Holocene, which were influenced by human impact in the last two centuries. The intermittent soda pans are situated in the groundwater undesirable processes a comprehensive rehabilitation of the lake (reed-cutting, dredging of the basin and shore regulation) started in the 60ties. In the course of dredging, 9 million nT of mud was removed from the basin. This mud was used for banking up one part of the shore and creating two small artificial islands. Parallel with this a shoreline regulation (1962-1985) was also done. In the catchment area of the lake two reservoirs were built, which can supply the lake with water in dry periods. Due to these measures the water level of the lake can be regulated safely. At present, Lake Velencei can be divided into two, clearly separable parts. The reedy-marshy Bird Nature Reserve of the western basin (Fig. 9) covers approximately one-third of the lake. The other, eastern part turned into a large, open water area (Fig. 10) after the reconstruction, and now it is an important recreation area. Despite the measures taken during the last decades several problem still have to be faced, i.e. cyanobacterial blooms, or occasionally occurring imbalance between the wetland and open water areas of the lake, which creates undesirable processes concerning the lake’s chemical and biological processes (Reskóné and Törökné 2000, Reskóné et al. 2001, Acs 2007). These highlight the need for deeper understanding and protection of the delicate biological system of the lake. of the Lake Velencei with wetlands and open water areas discharge areas of a closed hydrographic (endorheic) basin, in which groundwater inflow exceeds the surface- related watershed inflow and precipitation. The most important ecological criteria of the intermittent natural soda pans are listed below (Boros et al. 2014): • Shallow astatic open water bodies with bare pan bed (emerged and submerged macrophytes are sparse or absent); • Annual average of salinity exceeds 1 g/L; • The presence of characteristic species of flora and fauna. The most frequent type of soda pans is the basic alkaline type (Na-HCCfi), the second and third subtypes are the chloride and sulphate, beside the hydro- carbonate/carbonate dominance. Magnesium sometimes arises as a secondary dominant cation beside sodium.
