Sammendrag
The report presents data from the operational monitoring of Norway’s largest lake Mjøsa, its tributaries and the outlet river Vorma, emphasizing results on ecological status from 2016 based on phytoplankton, nutrients and secchi depth and long-term trends in temperature, riverine phosphorus discharges, water quality and environmental status for the period 1972-2016. 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 more than half of what is was during 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 2016, the ecological status was classified as good at all four sampling stations. However, at the sampling station Furnesfjorden, the ecological station was close to moderate. 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 the years 2002-2008. In the years 2009-2016 the tot-P mean values were slightly higher, varying within 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 2016 the tot-P concentration was within the range of good ecological status, and did not exceed the local environmental target for Lake Mjøsa. Median values for tot-P concentrations in the six main inlet rivers were relatively low in 2016 and indicated high ecological status for the rivers Gudbrandsdalslågen, Gausa, Hunnselva, Lena and Flagstadelva and good status for the rivers Lena and Svartelva. Based on investigations of benthic algae and macro invertebrates the ecological status was classified as moderate at the sampling stations situated in the lower parts of the rivers Mesna and Brumunda in 2016.
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