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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Salinity varies widely in sub- (0.5-3 g/L) and hypersaline (>50 g/L) ranges, while the pH varies in 8-10 range. Beside the high alkalinity, the high inorganic turbidity (Secchi depth 0.5-30 cm), polyhumic concentration of dissolved humic substances, hypertrophic conditions of the soda pans represent a unique aquatic ecosystem in Hungary (Boros et al. 2013, Boros et al. 2014) (Fig. 11).

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Figure 11. Soda pan in the Damibe-Tisza Interfluve

Due to their lower salinity, biodiversity of soda waters is generally higher than other continental saline waters with characteristic flora and fauna, benthic and planktonic communities reflect a strong structuring role of salinity, turbidity and trophic state, and the intermittent standing water incapable of supporting resident fish populations. The characteristic soda pan habitats are listed in Annex 1 of the EU Habitat Directive (92/43/EGK) and are thus considered to be of high priority (Natura 2000 Network). Coupled with the characteristic physical and chemical conditions the waterbirds have an important role in the regulation of trophic relationships of the soda pans (Boros et al. 2008a, Boros et al. 2008b, Vörös et al. 2008), and several of them have been designated as Wetlands of International Importance by the Ramsar Convention on Wetlands and as Special Protection Areas for birds. Because of the significant (85%) loss of these habitats during the last 60 years in the Carpathian Basin, they are “ex lege” protected in Hungary. Due to to unstable water balance and small size these pans are particularly threatened by the climate change combined with human activities.

Oxbow lakes

The oxbows are characteristic parts of the landscape in the Great Hungarian Plane. Although some of them are formed naturally, most oxbows were created in the second half of the 19th century, during the comprehensive regulation of rivers in the Tisza valley. More than 100 meander loops were cut off, resulting in hundreds of oxbows with a total length of 589 km and with an area of about 3000 ha. Oxbows of the floodplain are quickly filled with sediment, but those ones which are separated from the watercourses by flood protection embankments can keep their open water character for centuries. (Fig. 12)

Now the oxbows in Hungary occupy the complete range of successional stages, from the relatively deep open water lakes to the shallow marshlands (Krasznai et al. 2010) and provide habitats for many rare and protected plants and animals. However, oxbows are very sensitive ecosystems. Most of them have left their direct contact with the living river, and thus, total exchange of water in the lake basin with fresh water is not possible. The other important hydromorphological characteristic of these lakes is the small surface area to shoreline ratio which means, that oxbows are extensively exposed to anthropogenic disturbances.

Figure 12. Oxbow lake at Tiszadob village

Ecological state of these water bodies is strongly determined by the type of land use in the immediate catchment. Oxbows surrounded by arable lands are the most threatened because of agricultural diffuse pollution from fertilisers. The enhanced nutrient input accelerates the filling succession of the lake basin, and drives the system towards an unstable hypertrophic state, which is characterised by dense marshy vegetation and extreme values in water quality. The other threat to the oxbows is the intensification of sport fishing, which coincides with the drastic artificial modification of the fish fauna, increased pollution caused by the feeding of fish, and ultimately, with enhanced algal blooms (Borics et al. 2013). Several oxbows were dredged and restored in the recent years, and now because of their unique flora and fauna many of them are under nature protection. However the multiple uses of these waters results in conflicts between stakeholders (local residents, fishermen, conservationists, agriculturists), therefore restoration and conservation of these valuable aquatic systems will require the development of comprehensive ecologically-based management strategies in the near future.

Wetlands

Great Plain wetlands are remnants of the former extensive floodplains of river Tisza and its tributaries. Before river regulation (mid- 19th century), inundation by floods was part of the natural dynamics of the vast Tisza floodplain. Floods represented periodically occurring disturbances that led to the formation of a mosaic of habitat types ranging from constant marshes through periodically flooded marshes to irregularly flooded marshes and meadows (Aradi and Lengyel 2003). Descriptions from the 18th and 19th centuries attest that floodplain wetlands were used for fishing, egg collecting, plant collecting, reed harvesting and livestock grazing. Regular floods and the variety of land use ensured that the mosaic habitat complexes of wetlands hosted highly diverse communities of plants and animals on which people depended.