Ecological Changes in Lake Risten, Sweden, caused by Anthropogenic Influences - A Theoretical and Practical Study-

Sammanfattning: The ecological status in lake Risten, Sweden, is important in many ways. The lake is with its size a huge resource for recreation all around the year, since it provides good opportunities for activities like swimming, fishing and camping. Lake Risten is the first big lake in the river system downstream the deposits of the former copper mine in Bersbo, which has been affecting its downstream recipients over many years, although a very successful restoration was accomplished during 1987-1989. The fish from the lake is to a high extent used for consumption, and therefore comprehensive investigations has been necessary to check if the concentrations of metals in the fish are low enough not to make out a risk for human health when consuming it. It has been discovered, from a literature study, that the nutrient status of a lake is crucial for the uptake of metals in fish. Another problem in lake Risten, which has been more recently noticed by the local people, is an early stage of eutrophication. In the whole lake, but especially close to local nutrient sources from agriculture, an increase in under water vegetation and bottom sediment is occurring, which in turn is resulting in a change of habitats affecting the fauna of the lake. One species, which can be seriously affected by such a change, is the crayfish, and the fact is that the crayfish population has decreased over the last years. Whether the eutrophication is the main reason for the decrease or not cannot be stated without further investigations. This thesis is based on a combination of analysis of a thorough literature study and practical work, in form of water sampling for measuring of nutrient values in the inlet- and outlet streams of the lake and measurements of physical parameters of those streams. The report presents the situation in which the lake is found today and its history, but more importantly it presents how its ecological status could be improved in the future by application of suggested solutions. The results from the measurements can not give a totally accurate answer, but it can hopefully give an idea of the situation and tell if further investigations are needed or not.