THE EFFECT OF HYDROLOGICAL DROUGHT ON CHEMICAL QUALITY OF WATER AND DISSOLVED ORGANIC CARBON CONCENTRATIONS IN LOWLAND RIVERS

The effects of drought on river water quality and dissolved organic carbon (DOC) fluctuations were studied in 22 lowland rivers in north-east Poland with different size (from 3.3 km to 77.2 km), and discharge (mostly in range 1-8 m(3) s(-1)). The extensive agriculture, housing and plantation forestry (afforestation from 7% to 97%) dominate in the landuse in catchments. All investigated rivers were characterized by similar hydrological regime and relatively high natural level of organic matter in waters (10 mg DOC dm(-3)). The autumn 2000 drought in Poland was one of the most severe in many years when the monthly sum of precipitation in October made only 6% of average long-term norm for the region. Significant differences in concentrations of investigated chemical parameters were noted between results from wet (1996-1998) and dry (2000) autumn periods. Concentrations of ions derived from non-point sources increased in many locations, which was confirmed by increases in Ca, and HCO3 concentrations. Average dissolved organic carbon concentrations, and water colour were two times higher in the wetter autumn than in dry period. The hydrological drought in autumn 2000 caused significant decrease in the range of DOC concentrations (about 30%). The largest difference in DOC concentrations between the wet autumn period and extremely dry one were recorded in the rivers flowing through dense forests. Our results indicate that dissolved organic carbon in most natural catchments decreased as a result of the lack of dilution. Some more natural north-east Polish rivers gradually lost contact with their catchments, and ecotone zones did not play an important role in supplying organic matter to rivers during droughts. Most of river and catchment morphological parameters became less important in shaping the amounts of DOC in rivers. The current study indicates that small and medium rivers were not resistant in hydrochemical terms to flow decreases what has environmental consequences for transport of nutrients, organic matter and finally on sediments, flora and fauna habitats.