Monitoring of Lake Mjøsa, S Norway. Annual report for 2015

Summary

The report presents data from the monitoring project on Lake Mjøsa, its tributaries and the outlet river Vorma, emphasizing results from 2015 and long-term trends in water quality and environmental status for the period 1972-2015. As a result of comprehensive pollution abatement measures, the water quality of Lake Mjøsa has improved markedly since the 1970ies and the 1980ies. The mean phytoplankton biomass has been reduced by 65 %, and the mean concentration of chlorophyll-a has been reduced by 50 % since the 1970ies. However, even in some later years, the phytoplankton biomass has been rather high especially during diatom “blooms” in late August and early September. Based on mean algal biomass and composition of the phytoplankton in 2015, the ecological status was classified as good at all four sampling stations.