Summary 1.We analysed the development of submerged macrophytes in Lake Fure, Denmark, experiencing a 30-fold increase of phosphorus input from year 1900 to 1970 and a subsequent decline to twice the 1900 level in 2005. Nutrient enrichment stimulated phytoplankton growth and restricted macrophyte distribution by reducing water transparency from a summer mean of 5-6 m in the early 1900s to a minimum of 1.6 m at the peak of eutrophication, followed by recovery to a recent maximum of 4.1 m. 2. Macrophyte occurrence and abundance changed in accordance with altered environmental conditions and species' life-history traits. Small angiosperms, mosses and characeans disappeared in the 1970s to 1980s, along with all vegetation in deeper waters (5 -8 m), and have only partly recovered recently. Tall angiosperms became dominant while small species vanished. All 10 characeans originally present disappeared at the peak of eutrophication, but four reappeared. Mesotrophic macroalgae were replaced by hypertrophic species whose dominance has persisted. 3. Species richness decreased from 37 to 13 species at the peak of eutrophication, before returning to 25 species during the recent recovery. Species richness increased with transparency because deeper growth generates more niches. 4. Reduction of species distribution and richness has been reversible following nutrient reduction of the long eutrophied lake, whereas species composition and abundance have not. The historical legacy of community composition is strong, as reflected by closer correlations to time than to measures of nitrogen and phosphorus availability and water transparency. 5. Synthesis . Although phosphorus input may decline further, reassembly of the original macrophyte community will face difficulties. Oligotrophic freshwater species have become rare throughout Denmark, reducing the probability of recolonization. Species reaching Lake Fure may fail to establish because sediments have become richer in nutrients and organic matter and less consolidated, while shading and competition have increased from emergent reeds, tall submerged angiosperms and fast-growing macroalgae.

Stratified eutrophic lakes often suffer from hypolimnetic oxygen depletion during summer. This may lead to low redox conditions and accumulation of phosphate and ammonia in the hypolimnion. Hypolimnetic oxygenation has been used as a lake management strategy to improve the water quality in five eutrophic dimictic Danish lakes where oxygenation was conducted for 4-20 years. In one lake, the hypolimnetic oxygen concentration clearly improved by oxygenation, whereas the other four lakes still exhibited low mean summer levels (\2.2 mg O 2 l -1 ). Oxygenation generally increased the hypolimnetic water temperature by 0.5-2°C, but in one lake it increased by 4-6°C. In all lakes, oxygenation significantly reduced the hypolimnetic concentrations of phosphorus and ammonia during stratification. The accumulation of phosphorus and ammonia typically decreased by 40-88%. In two lakes oxygenation was stopped for 1-2 years and here hypolimnion concentrations of both phosphorus and ammonia increased again. Surface water quality only improved in one lake, but was likely also influenced by simultaneously occurring changes in external nutrient loading. Overall, it is concluded that hypolimnetic oxygenation reduces the hypolimnetic accumulation of phosphorus and ammonia and may prevent anoxia in the deeper parts of the lake. However, long-term oxygenation is required and it is uncertain whether the overall lake water quality can be improved by oxygenation. Reduction of the external nutrient loading is still essential to improve lake water quality.