Monitoring of Lake Mjøsa, S Norway. Annual report for 2013

Summary

The report presents data from the monitoring project on Lake Mjøsa, its tributaries and the outlet river Vorma, emphasizing results from 2013 and long-term trends in water quality and environmental status for the period 1972-2013. As a result of comprehensive pollution abatement measures, the water quality of Lake Mjøsa has improved markedly since the 1970ies and the 1980ies. The amount of algae, expressed as mean chlorophyll-a and mean phytoplankton biomass, has been reduced by 50 % and 60 % respectively since the 1970ies. However, in 2013, the phytoplankton biomass was rather high during a diatom “bloom” in late August and early September, indicating mesotrophic conditions. At that time diatoms comprised approximately 90 % of the phytoplankton biomass, with Tabellaria fenestrata as dominant species and Fragilaria crotonensis and Asterionella formosa as subdominant species. Based on mean algal biomass and composition of the phytoplankton in 2013, the ecological status was classified as moderate at all sampling stations except for station Brøttum, situated in the northern part of the lake. This means that environmental condition of Lake Mjøsa concerning eutrophication was not acceptable in 2013. Using data from the 3-year period 2011-2013 improved the results to good ecological status for all four sampling stations, but close to moderate status for the sampling station Furnesfjorden. Tot-P mean value for the algal growth season declined from 8-12 μg P/l in the 1970ies to 4-5 μg P/l in years 2002-2008. In years 2009-2013 the tot-P mean values varied in the range 5-8 μg P/l. This increase was probably caused by relative large P-inputs from the catchment during periods with high run off in later years, especially during the late spring floods in 2011 and 2013. Median values for tot-P concentrations in the six main inlet rivers indicated high or good ecological status for the 3-year period 2011-2013. However, in several rivers like Gausa, Flagstadelva and Svartelva volume weighted mean values for tot-P seem to have increased during later years. Based on investigations of the communities of benthic algae and benthic invertebrates the ecological status was classified as good at the sampling stations situated in the lower parts of the rivers Gudbrandsdalslågen and Svartelva in 2013.