Monitoring of Lake Vestvannet and Lake Borredalsdammen in Østfold County, SE Norway, 2014

Summary

Norwegian Institute for Water Research (NIVA) conducted in 2014 a monitoring survey of the water quality in Lake Vestvannet and Lake Borredalsdammen in Fredrikstad, with focus on planktonic algae and cyanobacteria. The findings are compared to data from previous years. NIVAs new 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 Direktoratsgruppa, Vanndirektivet 2013) for ecological classification of waters is used as a tool in this report. The content of the nutrients nitrogen and phosphorus in Lake Vestvannet and Lake Borredalsdammen are somewhat reduced in 2014 compared to previous years, as is the algal biomass. Chlorophyll-a was also reduced in Lake Vestvannet, while it increased in Borredalsdammen. There are no critical changes observed for any of the parameters included in this survey, nor for the algal community. Most of the community is made up by species that are common in the lakes of Østfold County, and they are not toxin producers. There was, however, a small amount of microcystin measured in Lake Vestvannet in October, probably caused by Planktothrix, which, although in small amounts, was the dominating cyanobacterium at the time. The biomass of cyanobacteria is generally low in both lakes, and the microcystin level measured was well below the limits for both drinking water supplies and bathing water. As before, Lake Vestvannet and Lake Borredalsdammen appear as weakly mesotrophic humic lakes, with moderate contents of nutrients and algal biomass. Lake Vestvannet is classified as very good ecological status by the Water Framework Directive guidance, and both lakes are considered to be well suitable for drinking water abstraction with regards to microcystin. However, both are considered poorer with regards to phosphorous contents and chlorophyll levels.