The national monitoring network of benchmark lakes consists of ten lakes situated in different Polish lakelands. They do not receive wastewater from point sources of pollution and their catchment areas are predominantly forested. Changes in their water quality should be natural or almost natural due to very low anthropogenic pressure. In the majority of the benchmark lakes the values of eutrophication parameters during the 1999-2004 period did not exceed 0.060 mg dm-3 P for total phosphorus or 10 μg dm-3 for chlorophyll