Lake Jabel restoration project: Phosphorus status and possibilities and limitations of diversion of its nutrient-rich main inflow

External and internal phosphorus loading was studied in 1996 in the anthropogenically eutrophicated Lake Jabel, which is located in Mecklenburg (north‐eastern Germany). It has a dimictic 23‐m‐deep northern basin, a 19‐m‐deep middle basin and a 6‐m‐deep polymictic southern basin. The main tributary, Grabowhöfer Grenzgraben, which enters via a leaching field from a waste‐water treatment plant, has been identified as the major nutrient discharge (80% of total phosphorus loading) into the northern basin. The sediment is the second largest source of phosphorus for Lake Jabel, contributing 25.7%, 27.4% and 2.4% of the overall phosphorus load to the northern, middle and southern basins, respectively. Although the favoured diversion of the Grabowhöfer Grenzgraben would contribute to a reduction of 1.22 t P and 20.94 t NO₃^‐–‐N per year, it is mainly rejected for the following reasons. If the extreme monthly nitrate supply, between 4.84 and 189.6 mg N m^‐2 day^–1 ceased, the release of redox‐sensitive phosphorus in the northern basin would increase by a factor of 0.54, which is with 41.1% total phosphorus, twice as high as that in the middle basin. In addition, the nitrate supply can substantially lower the seasonal precipitation of stable iron sulphides, which would otherwise lower the concentration of redox‐sensitive phosphorus, but cannot change the phosphorus retention in the long term. Because of internal phosphorus sources, the diversion of the Grabowhöfer Grenzgraben would result in no substantial changes in the level of trophy.