The Loosdrecht lakes are a system of shallow, interconnected, peat lakes in the centre of The Netherlands. The main environmental functions of the Loosdrecht lakes are nature and recreation. From the point of view of the Dutch policy, a Specific Environmental Quality ('Bijzondere Milieukwaliteit') should be set for these lakes.
The most serious environmental problem of the area is eutrophication.
The Loosdrecht lakes have, by increasing external phosphorus loading, changed, from clear lakes with few macrophytes, followed by a period of abundant characean growth, to turbid lakes dominated by cyanobacteria and detrital matter. Eutrophication was counteracted by use of sewerage systems and dephosporization of the supply water. The resultant decrease in external phosphorus loading did not result in a decrease of turbidity by suspended particles.
The eutrophication of the lake ecosystems was described as a series of phases. One of those phases, the status around 1940, has been used as an ecological reference system.
By means of a graphical presentation technique, the so-called 'AMOEBE-approach', the state of the environment of the Loosdrecht lakes has been visualized. Thirty-two ecological parameters, including both biotic and abiotic factors, have been selected and quantified. Concrete target values for these parameters have been derived from historical reports and from Lake Western Loenderveen, located close to the Loosdrecht lakes, but less eutrophic.
The general conclusion is that the state of the environment of the Loosdrecht lakes is far from what is required with respect to a Specific Environmental Quality, as many of the selected parameters, like water transparency, total phosphorus, mineral nitrogen, cyanobacteria, bream, pike, macrophytes, birds and otter, deviate by over an order of magnitude from their desired levels. '500 *o 50 *f 5 *f 20 *f Remarks *1 Sum of the ions NO;, NO; and NH; *2 Sum of ions SO:-, HCO;, Cl-, Mg", Na+ and Kf *3 Number of individuals in Southern Vecht lakes area (Crutzberg ef al., 1969) *4 Number of individuals hibernating in the Loosdrecht lakes ares (Leentvaar & M(3rzer Bruijns, 1962) *5 Number of breeding pairs in Soutem Kievits Area (Morel, 1989