Monitoring of the lakes in the Lake Eikeren watercourse 1974-2015

Summary

The monitoring is performed as part of the control programs for the water supply from Lake Eikeren by Eikeren Vannverk IKS (Eikeren Water Works). The water quality of Lake Eikeren complied with the good water quality class in the Norwegian classification system for deep lakes after the European Water Framework Directive. The water chemistry, phytoplankton concentration and species composition, as well as concentrations of E. coli were favorable with respect to using the water for drinking water production. However, during the whole monitoring period from the mid-seventies until now, there has been a small, but statistic significant, increase in the chlorophyll a of the lake Eikeren. The lakes upstream the Lake Eikeren, are classified from mesotrophic to eutrophic in character due to discharges of sanitary wastewater and agriculture runoff. Most of the pollution enters the watercourse in the Lake Hillestadvannet area. During the growth seasons heavy blooms of cyanobacteria (blue-green algae) from the genera Aphanizomenon, Dolichospermum (Anabaena) and Microcystis occurred in Lake Hillestadvannet which spread all the way downstream to Lake Eikeren. In Lake Eikeren the cyanobacteria were diluted and did not survive over time due to lack of nutrient supplies. The supply streams for the lakes Haugestadvannet and Hillestadvannet were polluted. Upstream of Lake Hillestadvannet the watercourse consisted of mesotrophic lakes. The authorities should locate and improve the causes of pollution. The monitoring programs should be continued and also include the data from water intake in the report.