Sammendrag
NIVA and FREVAR conducted a monitoring survey of the water quality in Lake Vestvannet in Sarpsborg and Lake Borredalsdammen in Fredrikstad in 2020, focusing on planktonic algae and cyanobacteria. The findings are compared to data from previous years. The Norwegian Food Safety Authorities’ guidelines for drinking water and NIVA’s proposed environmental objectives and class limits for physico-chemical parameters in lakes and rivers (Solheim et al. 2008) are taken into account in the evaluation of the results. In addition, the Norwegian guidance for ecological classification of waters (Veileder 02:2018, Direktoratsgruppa, Vanndirektivet 2018) is used to classify the ecological status of Vestvannet and Borredalsdammen.
Compared to 2019 the total phosphorous concentrations have increased in both lakes in 2020. The total nitrogen concentration has decreased in Lake Vestvannet but has increased in Borredalsdammen in relation to 2019. Lake Vestvannet was characterized by a lower average algal biomass and a higher average chlorophyll a concentration than in 2019. In Borredalsdammen phytoplankton biomass and chlorophyll a concentration increased compared to 2019. Most of the algal community consisted of species that are common in lowland lakes. Small amounts of the cyanobacterial genus Planktothrix and the toxin microcystin were detected in low concentrations in Borredalsdammen in October 2020. However, the cyanobacterial biomass was generally low in both lakes.
Lake Vestvannet is classified to “good” ecological status and Borredalsdammen to “moderate” ecological status by the Water Framework Directive guidelines, and both lakes are considered suitable for drinking water with regards to microcystin. In a total assessment of the lakes’ suitability as drinking water, both are considered “not suitable”, due to increased colour in Vestvannet and high total phosphorous concentrations and increased colour in Borredalsdammen. However, as FREVAR performs extensive treatment of the water, there is still potential for delivering good quality drinking water.
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