Preliminary evaluation of the quality of water in the Mierzęcka Struga River by a biological method

River quality assessment upon the basis of aquatic macrofauna incidence is already applied in European Union countries. The EU Water Framework Directive (Directive 2000/60/EC) dated October 23, 2000, lays the member countries under an obligation to carry out the biomonitoring of rivers. River classification applied in Poland since 1991 (Regulation of 1991) until 2004 (Regulation of 2004) is based mainly upon physical-chemical data and bacteriological indicators (Coli titre), as well as upon the measurements of saprobic index of seston. Upon the basis of the measured values, the quality of water was assessed according to three classes of quality. In which particular class the tested water should be included is often decided by an indicator least important from the point of view of its usefulness for given purposes. In the European Union countries biotic indices are commonly used for the assessment of the quality of rivers. They are based mainly upon the analysis of the taxonomic composition of macroinvertebrates. Different geographical rangelands of macroinvertebrate species as well as biotypological diversification among watercourses do not allow the use of one universal biotic index. It is necessary to adjust the indices to the local conditions of particular countries. Fulfilling the river quality assessment criteria consistent with the European Union requirements necessitates the undertaking of research that will allow the appropriate biotic indices to be evaluated. The River Mierzęcka Struga is not exposed to industrial contamination, moreover, in its basin there are 39 lakes including a few large ones that are sinks which neutralize pollutions from cultivated fields. This provides an opportunity to test a particular biotic index for small watercourses in the northwest of Poland. First, we tried to assess the quality of the waters of the Mierzęcka Struga using the Brown and Schroeder (Häffner 1993) method.
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