Cultural eutrophication has considerably impaired lake ecosystems worldwide (Hutchinson 1973, Schindler 1977, Sas 1989, Wetzel 1990). In northern temperate lakes, total phosphorus (TP) is regarded as the key factor of eutrophication (Schindler 1977). To improve water quality, attempts have been made during the last two decades to reduce the external nutrient loading of lakes by improving waste-water treatment, by increasing catchment retention capacity and by reducing the phosphorus content of fertilizers and detergents (Phillips et al. 1999, Van der Moelen Portielje 1999). However, many lakes have subsequently suffered from high internal loading, delaying recovery (Ryding 1981, Marsden 1989, Sas 1989). While studies of the response of nutrient and phytoplankton to nutrient loading reduction are plentiful (Sas 1989), little emphasis has been placed on the higher trophic levels and changes in seasonality. For the last 15 years, eight shallow Danish lakes have been monitored intensively following nutrient loading reduction without the undertaking of additional fish manipulation to precipitate recovery (Jeppesen et al. 2002 and 2005a, Sondergaard et al. 2002 and 2005). In the present paper we highlight the major results of these studies and include more recent monitoring data from the lakes. We further refer briefly to a recent comparative study of 35 north tem-perate lakes (Jeppesen et al. 2005a).
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