Mechanisms involved in the decline of Stratiotes aloides L. in The Netherlands:: sulphate as a key variable

In the last century, Stratiotes aloides L. (Water Soldier) has declined considerably in the Netherlands. The species has disappeared almost completely from the littoral zones of the shallow peaty lakes it once occupied, and at present it is still abundant in ditches in nature reserves and in some agricultural areas. Originally Stratiotes aloides is a species from moderately nutrient rich and moderately alkaline waters. In large parts of the Netherlands, hydrological conditions have changed considerably owing to intensified human activities. In most peaty lowlands, water quality has been influenced not only by direct eutrophication but also by the inlet of water from the rivers Rhine and Meuse, which is characterized by a high alkalinity and relatively high sulphate levels. Additionally seepage of groundwater has strongly declined, resulting in decreased iron inputs in these aquatic systems. Increased sulphate reduction rates result in the immobilisation of iron as iron sulphide and in the generation of alkalinity. As a result phosphate is liberated from iron phosphate complexes and the decomposition of organic matter is accelerated (internal eutrophication). Next, dissolved iron levels decrease strongly while toxic sulphide accumulates in sediment pore water. As a result, iron deficiency, sulphide toxicity, ammonium toxicity and increased competition by nonrooting species have al been observed to affect Stratiotes aloides vegetation. We conclude that sulphate is the master variable in the decline of S. aloides in the Netherlands.