Jarenvannet, Gran, Hadeland, in the summer of 1969. In just a couple of years, Jarenvatnet...

Jarenvannet, Gran, Hadeland, in the summer of 1969. In just a couple of years, Jarenvatnet has changed from a nice fishing lake to a porridge of green plants that it is difficult to calm down a boat. The water is attacked by water plague (Elodea Canadensis), a plant consisting of long, fragile stems that spread quickly via outline from the stems. Bjørn Rørslett (pictured) studies ecology. He has followed the water plague's invasion of the Jarenvannet and says the water is particularly rich in lime and nutrients that give the plant good conditions. The water is supplied with several tonnes of phosphates through sewage annually in addition to runoff from agriculture. When the plant rotes on the bottom it uses the oxygen in the water and the fish stock is in danger. Without water plague, Jarenvannet would have been threatened by algae growth that thrives under the same conditions. Photo: Aage Storløkken / Aktuell / NTB.