Monitoring of Lake Mjøsa, S Norway. Annual report for 2014

Summary

The report presents data from the monitoring project on Lake Mjøsa, its tributaries and the outlet river Vorma, emphasizing results from 2014 and long-term trends in water quality and environmental status for the period 1972-2014. 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 45 % 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 2014, the ecological status was classified as good at all four sampling stations. This indicates an improvement of the environmental conditions compared to the situation in 2013, when both the composition of the phytoplankton and the higher algal biomass indicated more nutrient rich conditions. At the main station, tot-P mean value for the algal growth season declined from 7-11 µg P/l in the 1970ies to 3-5 µg P/l in years 2002-2008. In years 2009-2013 the tot-P mean values were slightly higher, varying in the range 4-7 µg P/l. This increase was probably caused by relatively large P-inputs from the catchment during periods with high run off, especially during the late spring floods in 2011, 2013 and 2014. In 2014 the tot-P concentration was within the environmental target for Lake Mjøsa. Mean values for tot-P concentrations in the six main inlet rivers indicated high ecological status for the rivers Gudbrandsdalslågen, Hunnselva and Flagstadelva, good status for Lena and Svartelva, and moderate status for Gausa. The tot-P concentration in river Gausa has increased considerably since 2011, probably caused by erosion in association with construction works with the purpose to prevent flood damage. Based on investigations of benthic communities of algae macro invertebrates the ecological status was classified as good at the sampling stations situated in the lower parts of the rivers Vismunda and Stokkelva in 2014.