IMPACT OF HYDROLOGY ON THE CHEMISTRY AND PHYTOPLANKTON DEVELOPMENT INFLOODPLAIN LAKES ALONG THE LOWER RHINE AND MEUSE

The impact of hydrology (floods, seepage) on the chemistry of water an d sediment in floodplain lakes was studied by a multivariate analysis (PCA) of physico-chemical parameters in 100 lakes within the floodplai ns in the lower reaches of the rivers Rhine and Meuse. In addition, se asonal fluctuations in water chemistry and chlorophyll-a development i n the main channel of the Lower Rhine and five floodplain lakes along a flooding gradient were monitored. The species composition of the sum mer phytoplankton in these lakes was studied as well. At present very high levels of chloride. sodium, sulphate. phosphate and nitrate are f ound in the main channels of the rivers Rhine and Meuse, resulting fro m industrial. agricultural and domestic sewage. Together with the actu al concentrations of major ions and nutrients in the main channel, the annual flood duration determines the physico-chemistry of the floodpl ain lakes. The river water influences the water chemistry of these lak es not only via inundations, but also via seepage. A comparison of rec ent and historical chemical data shows an increase over the years in t he levels of chloride both in the main channel of the Lower Rhine and in seepage lakes along this river. Levels of alkalinity in floodplain lakes showed an inverse relationship with annual flood duration, becau se sulphur retention and alkalinization occurred in seepage waters and rarely-flooded lakes. The input of large quantities of nutrients (N, P) from the main channel has resulted, especially in frequently floode d lakes, in an increase in algal biomass and a shift in phytoplankton composition from a diatom dominated community towards a community domi nated by chlorophytes and cyanobacteria.