Abstract
Human pressure is now severe on most of the rivers worldwide. The long term fluxes of dissolved geogenic and biogenic matter are changing dramatically, causing notable changes in aquatic bioactivity. Typical patterns of anthropogenic pressure that influence eutrophication, salinization and chemical contamination are discussed. The heavily influenced rivers Rhine and upper Danube will be used as examples, also considering their geological settings. In the past decade sewage treatment reduced nitrate and orthophosphate loads in both basins. This influenced bioactivity in the rivers, causing less silica depletion due to diatom blooms in the Rhine. Therefore a notable increase in minima concentrations of dissolved silica can be observed. In the upper Danube, however, an increase in orthophosphate concentration since 2003 is noticeable; breaking the former decreasing trend, despite treatment efforts. The hydrochemistry of major ions in both basins is strongly influenced by the ratios of carbonate, siliciclastic sediment and igneous or metamorphic rock outcrops. In addition Mesozoic evaporites and salt mining were responsible for extremely high levels of Cl, Na and SO4 in the Rhine, peaking in the 70s and 80s at concentrations of 350, 180 and 140 mg/l, respectively. Water basin management efforts cut former high levels to less than a half. Heavy metals and persistent organic pollutant concentrations are declining in the Rhine as well. A combination of climate change and anthropogenic water inputs resulted in an increase of water temperature of the Rhine by 3.5 °C during the past 50 years. In the upper Danube such a trend in water temperature can not be observed.
