100 years of vegetation decline and recovery in Lake Fure, Denmark

Summary 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. 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. 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. 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. 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.