Effects of water level regulations on the ecological status of Lake Helgeren, a drinking water reservoir.

Summary

In 2013, a review was made of the existing hydromorphological impacts in the 51 lakes that are included in Oslo's water supply, as well as a theoretical assessment of whether the interventions have sufficient impact on ecological status to trigger measures, according to the EU Water Framework Directive (WFD; Berge 2013). The report concludes that there may be a need for measures in Lake Helgeren, as this is one of the lakes in this area that are regulated most actively. In normal years, the water level varies from one to three meters. In order to verify the theoretical assessments that were made in 2013, and generally to assess whether the drinking water regulation impacts the biology and the ecological status according to the WFD, the Agency for Water and Sewage Works in Oslo wanted to carry out a biological assessment in Lake Helgeren, and a classification of ecological status. In 2018, NIVA surveyed water plants (aquatic macrophytes), benthic invertebrates and fish in Lake Helgeren. Water plants were surveyed once by standard methodology, and we calculated the water level index WIc to assess effects of the water level fluctuations. We sampled benthic invertebrates in the upper littoral zone and along depth transects both in spring and autumn. We also put out crayfish traps to verify the presence of noble crayfish (Astacus astacus). Fish was examined with standard gillnet fishing in the fall, and minnow traps. An assessment of the spawning streams and the recruitment conditions for trout was also conducted. In addition to assessing the effects of the water level regulations, we calculated the acidification indices for water plants and benthic invertebrates. The surveys show that Lake Helgeren is in moderate ecological status. This is based on the water level index for aquatic plants, which indicated moderate status. There are no indices that directly quantify the effects of regulation on benthic invertebrates or fish in Norwegian lake basins, however, based on an expert assessment, we believe that neither the benthic invertebrate fauna nor the fish community will achieve the objective of good ecological status according to the WFD. The benthic invertebrate fauna was relatively species poor and has strikingly low densities. The low diversity can probably be attributed to water level fluctuations, as is shown earlier in the literature from both Norway and abroad. Lake Helgeren is not affected by acidification, as the acidification indices for aquatic plants and benthic invertebrates showed good and very good ecological status, respectively. The fish community is influenced by the common minnow, which in lake Helgeren probably is a foreign species, and whitefish have also been introduced, through the tunnel from Lake Trehjørningen. The perch populations in Lake Helgeren is good, with individuals is all size classes, but it is reasonable to believe that the production of trout has been harmed by the water level regulations - again as found in other sympatric fish communities. Still, data from several years of test fishing will be important to obtain more reliable data on the fish community. Based on WIc and expert evaluations of benthic invertebrates and fish, we conclude that the ecological status in lake Helgeren is below the environmental objective, and that the reason is physical changes related to the water level regulation.