Compensatory nutrient enrichment in an oligotrophicated mountain reservoir – effects and fate of added nutrients

Abstract.

Most Scandinavian rivers are impounded for power production. High altitude reservoirs retain water during summer and fall for power production during winter and spring. The oligotrophic lakes Mjölkvattnet and Burvattnet were impounded in 1942. Annual water level fluctuations caused by regulation have resulted in a loss of littoral habitat, and Arctic char (Salvelinus alpinus) and brown trout (Salmo trutta) populations have declined. To assess compensatory nutrient enrichment as a remedy against declining fish populations, we added dissolved phosphate and nitrate to Mjölkvattnet in June and July in 2002 and 2003, and used the upstream lake Burvattnet as a reference system. Nutrient addition doubled water column total phosphorus concentration, from 3 to 6 μg P/L and increased nitrogen concentration by about 20 μg/L. Half of the added phosphorus settled out as organic matter, and about one third was lost downstream. Phytoplankton production and biomass increased, but species composition remained principally unchanged. Rotifers and cladocerans responded rapidly, as did the condition of fish. After two years of nutrient addition, five year old Arctic char had doubled in weight and increased significantly in length, reaching pre-impoundment conditions.