Sisäinen ravinnekuormitus Vesijärven Enonselällä

Internal loading of nutrients (the return of sediment nutrients into the water column) is one of the most important factors regulating the productivity of aquatic ecosystems. During the previous decades, external nutrient loading from industry and municipalities has been reduced, but in thousands of lakes internal loading prevents the ecosystems recovering from eutrophication. An old and very widely accepted paradigm in limnology is that internal phosphorus (P) loading is prevented or strongly diminished by aerobic conditions at the sediment-water interface. The oxygen-internal loading paradigm has had a very strong impact on the research and management of aquatic ecosystems. Artificial aeration has been one of the most intensively used methods in the restoration of eutrophicated lakes. During the recent years, the overwhelmingly important role of low oxygen concentrations in controlling the internal P loading has been questioned. It seems that in numerous cases high pH and sediment resuspension caused by wind may be the main regulators of internal loading. It is possible that due to the climate change, conditions in aquatic ecosystems (thermal stratification, wind effects) have changed so much that results from the 80-year-old experiments forming the cornerstone of the oxygen-internal loading paradigm are not valid anymore. However, conclusions on this aspect can not be made based on the present knowledge, because ecosystem-scale studies on aerobic vs. anaerobic P release are lacking. Therefore, in this study, the importance of aerobic vs. anaerobic phosphorus release in the Enonselkä basin of Lake Vesijärvi is studied. The lake also gives an exceptionally good opportunity to study the effects of artificial aeration, since a very large aeration project was started in the Enonselkä basin in 2010. The level of internal loading and the importance for aerobic and anaerobic bottoms for internal loading are studied by mass balance calculations and resuspension-based methods. The effects of aeration on internal nutrient loading, temperature, and stratification characteristics of the lake are also intensively studied. The project will be continued in 2011.