Monitoring of Lake Vestvannet and Lake Borredalsdammen in Østfold County, SE Norway, 2017.

Summary

NIVA and FREVAR conducted a monitoring survey of the water quality in Lake Vestvannet and Lake Borredalsdammen in Fredrikstad in 2017, with focus on planktonic algae and cyanobacteria. The findings are compared to data from previous years. Norwegian Food Safety Authorities guidance together with NIVAs proposition for suitability criteria for drinking water (Solheim et al. 2008) is taken into consideration in the discussions of the results. In addition, the most recent guidance (Veileder 02:2013 – rev. 2015) for ecological classification of waters is used as a tool in this report. The content of the nutrients nitrogen and phosphorus was slightly reduced in 2017 compared to 2016. The algal biomass and chlorophyll quantity have decreased in Vestvannet in 2017 compared to previous years, while the same parameters have increased in Borredalsdammen. There are no observed significant changes in any parameters measured in short term or long term, nor in algal composition. Most of the algal community consists of species that are common in lakes in Østfold county, and these are not toxin producers. However, a small amount of microcystin was measured in Borredalsdammen in August 2017, but the level measured was well below the limits for both drinking water supplies and bathing water. The biomass of cyanobacteria is generally low in both lakes. Lake Vestvannet is classified to high ecological status by the Water Framework Directive guidance, and both lakes are considered to be well suitable for drinking water with regards to microcystin. In a total assessment of the lakes suitability as drinking water, both are considered poorer.