Background Lake Fure (Furusøen) is one of the largest (940 ha) and deepest (37 m) lakes in Denmark. It is naturally eutrophic and at the beginning of the 20th century, was one of the cleanest lakes in Northern Europe and one of the richest in species. In 1911, 33 species of submarine plants were registered in the lake, including 18 species of phanerogams and 10 species of stonewort. However, the lake is located close to the suburbs of Copenhagen and has consequently suffered from decades of heavy loading with nutrients, (particularly phosphorus), from waste and sewage water. These high nutrient levels embedded themselves in the sediment at the bottom of the lake, depriving it of oxygen and causing a massive build-up of phytoplankton, algae, and so-called 'trash' fish. The predatory fish can not see their prey in the unclear water which means that the number of nonpredatory fish becomes significantly higher, especially roach and beam. Consequently, the submerged vegetation for which the lake was once famous had all but disappeared. Moreover, this situation had an impact not only on the flora and fauna but also on the many recreational interests of the lake. By 2002 the external sources of pollution had been reduced to very low levels thanks to the installation of wastewater treatment plants and corresponded to the situation at the turn of the 20th century. However, the internal loading of phosphorus within the lake was still high. It would require a significant one-off investment to bring the situation back to how it should be, so that the submerged vegetation and other natural features could re-establish themselves. Objectives The overall objective of the project was to ensure the total restoration of the biological system of Lake Fure (habitat type 3150 which includes species of Potamageton and Chara), especially the submerged vegetation for which the lake was once famous. This was to be achived by two key actions: 1) To reduce the standing biomass of trash fish by 80%, from 162 tonnes to 31 tonnes. Several techniques were to be used, including dragnets operated from boats and pound nets. It was expected that after 3-5 years the large predatory fish (particularly Perca fluviatilis) would be able to control the remaining trash fish to acceptable levels. 2) To re-introduce pure oxygen into the bottom of the lake to 'clean out' the accumulated phosphorus pools in the sediment. These techniques had been tried with success elsewhere in Denmark. By the end of the project the worst of the effects should have been addressed but the actions would need to be maintained, albeit at lower levels, for up to ten years afterwards. Results The Lake Fure project is a good example of what can be done to improve the conservation status of a very euthropic lake. However, although the conditions of the lake have improved during the project, further work is needed both with oxygenation and monitoring, (especially of fish stock) in order to clarify if additional bio-manipulation actions are needed. Key results of the project are as follows: - 212 tons of fish have been caught during the project period. These were distributed as follows: 77 tons of roach, 121 tons of bream and 14 tons of other species. Around 140 tons have been fished by pound net and 72 tons by seine. However, the biomass of coarse fish in Lake Fure and the adjacent waters was markedly higher than estimated. The results from the fish stock investigations showed a clear change in the fish fauna towards a higher percentage both in abundance and biomass of piscivorous fish, primarily perch (Perca fluviatilis) in the main basin as well as in Store Kalv. However, the improved proportion of piscivorous fish is not yet considered adequate to maintain a natural regulation of the coarse fish fauna, especially not in Store Kalv. Therefore, although from an environmental point of view, the composition of the fish fauna has improved, the response of the piscivorous fish fauna to biomanipulation will not be known until a few years time. - Oxygen concentration of 4 mg/liter in the hypolimnion (bottom layer of water in a thermally-stratified lake) was achieved in 2004. These improved conditions at the bottom of the lake have resulted in the spread of several oxygen-demanding mosquito species and mussels in the deeper parts. In 2006, the mosquito, (Chironomus anthracinus), a species characteristic for deep, clean stratified lakes was well distributed on the bottom of Lake Fure. In 2005, the relict crayfish (Mysis relicta) was found in the deepest parts of Lake Fure for the first time in many years. - Already in the first year of oxygen supply to the hypolimnion, the phosphorus release from the sediment of the lake was reduced. This decrease continued for the next 3 years. However, there was no effect on the phosphorus concentration in the upper part of the water (epilimnion). In 2006, the phosphorus concentration was at the same level as the years before. The target phosphorus concentration of 0.04 mg/l was therefore not achieved. A reduction in the phosphorus concentration to lower levels in the summer period is essential to minimize the algae production. - In the first years of the restoration project the conditions of the submerged vegetation improved. The number of species as well as their depth limit increased. During a hot summer of 2006 this positive trend was broken due to a big bloom of blue-green algae early in the growth period. The algae displayed a carpet-like surface on the water surface, and thereby limited the penetration of light to the bottom. The consequence was that the submerged vegetation did not grow on depths deeper than 2 meter in 2006. The target (8 meter growth depth) for submerged vegetation was not reached. This growth of algae also had a negative effect on the transparency of the water in 2006 and the objective (secchi disk transparency of 4 meters) was not reached.