Impact of eutrophication on the silicate cycle of man-made basins in the Rhine delta

The impact of eutrophication on the biogeochemical cycle of silicate in the Rhine delta was analysed by 1) comparing the seasonal variation in river water, stagnant fresh water and coastal seawater and 2) observations in well-controlled experimental reservois subjected to different regimes of phosphate precipitation. The high input of dissolved sliciate from Rhine water was rapidly depleted in receiving waater systems through vigorous phytoplankton (datom) growth. In reservoirsca. 50% of the silicate input was retained over a seven years' observation period. Regeneration of silicate immobilized by diatoms was accelerated by very dense blooms of phytoplankton in reservoirs and in Lake IJssel that increased the pH value over 9. The prevailling lowratios of Si/N and Si/P in Rhine water entall the risk of massive blooms of flagellates or cyanobacteria in receiving waters. The 50% reduction in nitrogen and phosphate emission agreed upon in the Rhine Action Plan may reduce butnot eliminate this risk.