Recovery from eutrophication: experiences of reduced phosphorus input to the four largest lakes of Sweden.
In-lake concentration changes of phosphorus (P) and nitrogen (N) in lakes Vättern, Vänern, Mälaren, and Hjälmaren in response to diminished input has been examined from the mid-1960s onwards. In the former two deep and oligotrophic lakes with slow water renewal, drastic reductions in P-input from the middle of 1970s caused just minor reductions in P-concentration over a very long time. At the same time accumulation occurred in the water mass of inorganic N and possible reasons are discussed. In the latter two mesotrophic to hypertrophic lakes, two shallow basins in L. Hjälmaren showed slow recovery due to release of P from sediments. The same basins and two basins in L. Mälaren have suffered from N-deficiency, particularly during the pre-phosphorus reduction years, and nitrogen fixation was indicated. In two L. Mälaren basins recovery of in-lake P concentrations was better than expected in comparison to the so-called IMSA-model for lake recovery from nutrient pollution. In the other five lakes/basins chlorophyll concentrations after 20 years were similar compared to those modeled.
Wilander
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Department of Environmental Assessment, Swedish University of Agricultural Sciences, Box 7050, SE-750 07, Uppsala, Sweden. Anders.Wilander@ma.slu.se
Persson
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D018533:Biomass; D004784:Environmental Monitoring / Q000379:methods; D005068:Eutrophication; D005618:Fresh Water / Q000737:chemistry*; D009584:Nitrogen / Q000032:analysis; D010758:Phosphorus / Q000032:analysis*; D010839:Phytoplankton; D013548:Sweden; D013995:Time; D014874:Water Pollutants, Chemical / Q000032:analysis*; D014876:Water Pollution / Q000517:prevention & control*
D014874:Water Pollutants, Chemical; D010758:Phosphorus; D009584:Nitrogen