The main characteristics of phytoplankton populations in the Hungarian section of the River Tisza in connection with the WFD

Even though the hydrological and hydromorphological characteristics of the River Tisza could qualify it as a standard for international water quality assessment systems, it is among the less known streams due to the lack of systematic middle-and long-term examination programmes. The natural self-purification abilities of the river are often put to test by pollution conveyed by its tributaries. If we want to maintain, or even improve, the present ecological conditions of the Tisza, we have to take its whole drainage basin as an integrated ecological system, with all the advantages and disadvantages this means. According to algological and physico-chemical variables, the main channel of the River Tisza can be divided into three parts in Hungary. The first part, the Tiszabecs section, receives special attention, as this is the only “ upstream-type” large river in Hungary with all featurespertaining to such streams. The following section of the mean channel extends to Ároktő at the mean water stages, and to Tiszadob at low water stages. This is the most problematic section in terms of water quality. As a result of the slowed-down flow of the river, “ nutrients” arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out of the Hungarian streams, the River Szamos and the Lónyai-channel have the most significant effect on the plankton composition of the Tisza, as both convey considerable amounts of pollutants into the Tisza. The tributaries have such a strong influence on the main channel – especially at low water periods – that they can entirely transform its structure of organisms. Owing to this, it is practically impossible to define a planktonic population pertaining to this section. Therefore, it is essential to properly detect the pollution load on the Tisza coming from its tributaries. The quite balanced lower section reaches from Ároktő at mean water periods and from Tiszadob at low water periods to Tiszasziget. The section below the Kisköre-reservoir to the entry of the River Hármas-Körös is characterised by a fairly rich plankton population. However, this abundance falls back between the entry of the Hármas-Körös and the collection point at Tiszasziget.