Browsing by Author "Varjus, Tomi"

Enäjärvi is a shallow, longish lake in Southern Finland. The factors that led to its hypereutrophic state are a water level adjustment during the last century and a 25 years lasting point source nutrient load from sewage waters of Nummela. The restoration aimed at improving the status of the lake started in 1993. The improvement actions have included, inter alia, the oxidation of the hypolimnion, food chain restoration by removing roaches and the reduction of scatter load by building sedimentation basins, wetlands and border strips to the lake's catchment area. The lake is still highly eutrophic. Its surface sediment has been studied in two different years before. The first one was made in 1991, two years before the start of the restoration projects and the second one in 1999. In 1991 it was found out that the surface sediment was completely dark sulphide-colored gyttja that had poor nutrient retaining capacity. In the 1999 study the sediment had turned back to oxidiced, healthy sediment that worked as a sink for nutrients. The purpose of this thesis is to repeat those two studies. Together they will account of phosphorus retention monitoring in Enäjärvi for more than 20 years. The sampling was carried out in early spring 2013 from the same sample sites as in the earlier studies. The samples were taken from three different depths from 30 sample points. They were analyzed for water content, loss on ignition and total phosphorus. In addition the sediment surface layer was also analyzed for phosphorus fractions. The results were compared to previous findings with diagrams, percent change analysis, Sperman's rank correlation coefficients and regression line analysis. The water quality data collected by Economic Development, Transport and the Environment centre was also used to support the findings. The results of this study show that the restoration activities carried out on the lake have reduced the scattered loading to the lake and also bioturbation based leaching of phosphorus. The good quality of the surface sediment and the high content of redox-sensitive phosphorus fraction that increases with depth point out that the oxygen levels have remained high enough to bind phosphorus to metal oxides. Though at some sample points sediment surface had iron sulphide precipitates that point to depletion in oxygen level at the hypolimnion to some extent.