Sammendrag
The report presents results from monitoring of water quality, biota and ecological status of Lake Næra and tributaries in 2016. Long time changes of the lakes environmental condition have also been assessed.
Based on mean algal biomass, the composition of phytoplankton and physic-chemical parameters in 2016, the ecological status of Lake Næra was classified as good at all three sampling stations. However, at the sampling station Næra S the ecological status was close to moderate, and at the sampling station Næra N both the phytoplankton biomass and the phytoplankton trophic index (PTI) indicated moderate status.
Based on investigations of benthic algae and macro-invertebrates the ecological status was classified as moderate at four out of five sampling stations in inlet brooks, and as bad at one of the sampling stations.
Both the concentrations of total phosphorus and the phytoplankton biomass seems to have increased in Næra in later years compared to 10-20 years ago. In 2015, an algal bloom (dominated by the Chrysophyceae Uroglenopsis americana) and observations of dead fish in the lake were reported.
Lake Næra is probably in an unstable state, which means that relatively small increases in nutrient inputs may cause algal blooms and significantly deteriorating water quality. We therefore recommend measures to reduce nutrient inputs to the lake. To be able to evaluate the response of nutrient reduction efforts in the future, it is also important to continue the monitoring of the lake.
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