BIOMONITORING OF THE WATER-QUALITY IN THE RIVER WARCHE (BELGIUM) - IMPACT OF TRIBUTARIES AND SEWAGE EFFLUENT

Between March 1994 and March 1995, sampling campaigns were carried out each fortnight in order to determine the physical, chemical and biolo gical quality of the water in the river Warche. Macroinvertebrate and rotifer biomonitoring (based on the global and Belgian biotic index an d on the Sladecek index respectively) were used as bioindicators. Thes e biotic indices were related to the main physical and chemical parame ters monitored between the source and the town of Malmedy (i.e., a riv er length of 40 km). Important seasonal and longitudinal changes of ph ysical, chemical and biological water quality occur. The deterioration of the water quality in the river Warche is obvious in two sectors of the river. Strong organic pollution and eutrophication were observed between the source and the sampling station located downstream of Bull ingen. Indeed, along this sector, tributaries, sewage and dairy efflue nt flow into the Warche and induce significant increase of nitrate, ni trite, ammonium and phosphate concentrations. As a consequence, biotic indices decrease significantly downstream of Bullingen and the most a bundant rotifer species are clearly eutrophic indicators. Another impo rtant decrease of water quality was observed downstream of an importan t paper mill effluent which induces a significant increase of temperat ure and of nitrite and sulphate concentrations. The presence of two la kes along the Warche course induces downstream either a restoration of the water quality during a mixing period of the lake or an increase o f ammonium and phosphate concentrations after water stratification as a result of a temperature gradient in the lake.