INTERNAL EUTROPHICATION OF AQUATIC ECOSYSTEMS - MECHANISMS AND POSSIBLE REMEDIES

Eutrophication is an important problem in aquatic environments in Euro pe. In the Netherlands, many aquatic macrophytes have strongly decline d while, at present their former habitats are characterized by non-roo ting species such as Lemna species, Spirodela polyrhiza (L.) Schleiden and Azollla filiculoides Lamark. The experiments reveal that processe s that cause eutrophication without the external input oi nutrients (i .e. internal eutrophication) play an important role in the eutrophicat ion of semi-aquatic habitats. The alkaline nature and high sulphate co ntent of the river Rhine water which is allowed into large parts of th e peaty lowlands of the Netherlands is suggested to be responsible for this internal eutrophication. Internal eutrophication appears to be c aused by increased mineralisation of organic matter in semi aquatic ac id aquatic habitats due to the alkalinisation of sediments. Alkalinisa tion is caused by the production of bicarbonate owing to the reduction of sulphate and the alkaline nature of the inlet water. Next, free ir on levels in the sediment have decreased due to increased iron sulphid e precipitation and decreased iron inputs by seepage. Finally, sulphid e and phosphorus levels have increased after the exhaustion of mobile iron in the sediment. The results also show that the mobilisation of p hosphorus from the sediment is prevented by the addition of iron to ir on depleted-sediments. Some possible remedies that can prevent or dimi nish internal eutrophication are discussed.