FIXATION OF DISSOLVED SILICATE AND SEDIMENTATION OF BIOGENIC SILICATE IN THE LOWER RIVER RHINE DURING DIATOM BLOOMS

Concentrations of dissolved silicate and particulate biogenic silicate were measured in three branches of the lower river Rhine in The Netherlands in order to analyse the role of this element in the eutrophication of the river basin. Particulate silicate followed the seasonal development of the phytoplankton, which was dominated by diatoms. The concentration of dissolved silicate fell during blooms (< 0.1 mg.l-1 ), but the amounts of biogenic silicate measured (∼ 1 mg.l-1) were insufficient to explain the seasonal decrease in the dissolved fraction; this indicates retention of silicate upstream. Some particulate biogenic silicate in river water settled in man-made sedimentation areas in the Rhine delta. The observations suggest that changes in silicate fixation in the Rhine may have contributed to the incidence of non-diatom phytoplankton blooms in receiving waters. © 1990 Kluwer Academic Publishers.