Lake Balaton

Ecological characterization

Lake Balaton is the largest shallow, freshwater lake in Central Europe. It is located in West-Hungary, with the area of 596 km2, and with the mean depth of 3.25 m. The area of the whole catchment is 5765 km2 (Figure 18).

The formation of the Balaton basin has been started approximately 15.000 years ago, when a depression happened and several smaller, separated sub-basins have been filled up with water (Figure 19). The lake has been heavily modified during centuries and today the lake is subdivided into 4 basins: Keszthely, Szigliget, Szemes and Siófok.

- Figure 18. Lake Balton - Figure 19. Formation of the pre-Balaton basin-set (based on Tullner and Cserny 2003)

Recharge of Lake Balaton comes from 20 permanent and 31 temporal inflows and from numerous springs in the lakebed. The northern watershed covers 820 km2 where mostly fast flowing creeks can be found while in the south shallow ditches collect waters from the surrounding areas. Of recharge waters, the most important is River Zala whose bed was artificially modified in the 1800s. In the 19th century a sluice was built at Siófok (Figure 20), on the south of the lake, which connected Lake Balaton to the Danube River. Via the sluice, the water level of the lake was controlled and lowered, resulting in the partial drying up of the Kis-Balaton wetland which functioned as a natural filter zone, retaining nutrients carried by River Zala.

- Figure 20. The Sió floodgate in Siófok (Source: www.siocsatorna.hu)

The disappearance of this natural filter zone, together whit the increasing emission of agricultural runoffs, industrial wastewater (food industry) and inefficient sewage treatment supported eutrophication processes in Lake Balaton by the mid 60’s, especially in the westernmost Keszthely basin. Critical water qualities during summers prevailed until 1994 and were characterized by unpredictable blooms of Cylindrospermopsis raciborskii, an invasive cyanobacterium of tropical origin.

A large scale restoration program aimed to restore the original ecological status of the lake was initiated in the early 1980s and later on it became the largest scale wetland conservation/restoration/management project worldwide.

Today discharges deriving from facilities working in settlements are gathered in the public sewer after necessary pretreatment, and are treated together whit municipal wastewater (Figure 21). Municipal waste management services are available in almost 100 % of the settlements, only a few small populated settlements are not involved in the public waste disposal network. The number of the landfill sites dropped drastically after 2008; 52 of the total 124 were closed.

- Figure 21. Public and industrial wastewater discharges round Lake Balaton.

The restoration project also included the construction of the Kis-Balaton Water Protection System. Nowadays the reconstructed Kis-Balaton area together with the marshlands (traditionally called “berek” in Hungarian) of the southern shoreline are under the protection of Ramsar convention. The most important habitat of the Lake itself is the reed belt, which provide feeding and spawning habitats for fishes and also important as filtering zone.

However, even though the ecological status of the open lake is now good during most of the year since 1995, the shoreline zones remained heavily modified (dominated by concrete banks), what largely prevents further improvement of water quality.

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