Operational monitoring of Lake Mjøsa. Annual report for 2019.

Summary

Objective and content The objective of the operational monitoring of Norway’s largest lake Mjøsa and its tributaries is to document the current ecological status and long-term trends concerning impacts of nutrients on biomass and species composition of phytoplankton, zooplankton and relevant physico-chemical variables. The monitoring includes investigations at four stations in the lake, and in the 12 largest tributaries (according to a cyclical programme), as well as in the outlet river Vorma. Ecological status of the lake The overall classification in 2019 indicates moderate ecological status in Furnesfjorden and good ecological status for the other monitoring stations. However, the Kise station was near the good/moderate boundary and the main station Skreia was in the lower part of good ecological status class. In addition, the average value of the normalized EQR values for the last three years (2017-2019) has decreased from the previous three-year period (2014-2016) at all the stations except Brøttum. The massive bloom of cyanobacteria at the beaches around most of Mjøsa in the latter part of July 2019 is also a sign of a possible deterioration of the ecological status. The reason for the reduced ecological status is not clear but may be due to a combination of short-term phosphorus loads from local rivers in connection with heavy rainfall episodes during summer, as well as warmer waters. This may indicate a need for mitigation measures to reduce the phosphorus loads from local tributaries. (...)