As a result of changed land use and concomitantly increased drainage, in combination with increased use of water for industrial, agricultural and other human purposes, there is an increasing shortage of “natural” water in many areas of The Netherlands in summer. To compensate for this shortage, increasing amounts of hard water from the Rivers Rhine and Meuse are diverted to these lowlands. It is commonly assumed that the disappearance of Stratiotes aloides L. in peaty lowlands is caused by a decline of the water quality, owing to high concentrations of nutrients in the incoming water. However, field observations in “de Weerribben”, a large mire complex in The Netherlands, indicated that the decline of the water quality in this area is caused by an altered ion composition of the water. The incoming Rhine-type water from Lake IJsselmeer is very alkaline, rich in HCO 3 − and SO 4 2−. These anions appear to be responsible for a change in the decomposition processes. Consequently, the peat becomes very fine and soft, and because of increased bioturbation the water becomes turbid. As a result of increased reduction processes in the soil there is an increased mobility of phosphorus, leading to internal eutrophication and greatly increased levels of reduced compounds such as S 2−, CH 4 and NH 4 + in the soft, upper layer of the bottom. Transplantation experiments with Stratiotes aloides plants showed that the plants started to decay in winter, after migration to the soft, upper sapropel layer. Culture experiments proved that the S 2− and NH 4 + levels in this layer are lethal to Stratiotes plants.