Changes of water quality in benchmark lakes in the Polish national monitoring network in the 1999-2004 period 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 a, and Secchi disc readings generally exceeded 2.5 m. Trend analyses in the majority of cases did not reveal significant changes in eutrophication parameters over time.