Internal eutrophication, iron limitation and sulphide accumulation due to the inlet of river Rhine water in peaty shallow waters in the Netherlands

Abstract

Due to agricultural activities and increased water extraction, ground water levels have generally decreased in large parts of the peaty lowlands in the Netherlands. As a result (iron containing) seepage has decreased in many regions while alkaline river Rhine water, rich in sulphates and poor in iron, has been used to compensate for the shortage of water. This has resulted in increased alkalinity and organic sediment breakdown. Increased sulphate reduction leads to iron sulphide precipitation and internal alkalinity generation. As a result of these processes phosphate and ammonium levels in sediment pore water have increased strongly. Release of these nutrients to the water layer has resulted in internal eutrophication of the peatland ecosystems. Further, iron levels have decreased strongly due to decreased seepage and iron sulphide precipitation. As a result sulphide accumulates in sediment pore water and reaches toxic levels. Furthermore, decreased iron levels appear to be correlated with increased phosphate levels in sediment pore water. The strong decline of aquatic macrophytes in many minerotrophic peatlands in non agricultural regions in the Netherlands can probably be attributed to internal eutrophication, internal intoxication and iron limitation, resulting from changed hydrological situations i.e. decreased seepage and inlet of river Rhine type water.