Decreasing salinity and initial reactions of the phytoplankton in the rivers Werra and Weser
The salinity in the Werra and Weser, principally due to wastewater discharge from potassium industries, has been reduced to about one-third between 1990 and 1995. Corresponding to that decrease, there have been changes in the species community, phytoplankton dynamics and spatial intensity of primary production. Halotolerant centric diatoms formerly dominated the biomass and cell numbers, but they now only appear at times of lower discharge in summer. Concentrations of up to 500 µ chlorophyll-a have been measured, as well as cell numbers of 500 x 10 6 cells per litre. The zone of maximum primary production moves upstream in the course of the year, and significant oxygen deficits occur downstream in the impounded part of the Weser.