Internal eutrophication of aquatic ecosystems: mechanisms and possible remedies

Summary Eutrophication is an important problem in aquatic environments in Europe. In the Netherlands, many aquatic macrophytes have strongly declined while, at present their former habitats are characterized by non-rooting species such as Lemna species, Spirodela polyrhiza (L.) Schleiden and Azollla filiculoides Lamark. The experiments reveal that processes that cause eutrophication without the external input of nutrients (i.e. internal eutrophication) play an important role in the eutrophication of semi-aquatic habitats. The alkaline nature and high sulphate content of the river Rhine water which is allowed into large parts of the peaty lowlands of the Netherlands is suggested to be responsible for this internal eutrophication. Internal eutrophication appears to be caused by increased mineralisation of organic matter in semi aquatic and aquatic habitats due to the alkalinisation of sediments. Alkalinisation is caused by the production of bicarbonate owing to the reduction of sulphate and the alkaline nature of the inlet water. Next, free iron levels in the sediment have decreased due to increased iron sulphide precipitation and decreased iron inputs by seepage. Finally, sulphide and phosphorus levels have increased after the exhaustion of mobile iron in the sediment. The results also show that the mobilisation of phosphorus from the sediment is prevented by the addition of iron to iron depleted-sediments. Some possible remedies that can prevent or diminish internal eutrophication are discussed.