Macrophyte decline in Danish lakes and streams over the past 100 years

Summary 1 Freshwater habitats in cultivated and densely populated lowland regions of Europe have experienced profound changes during the last 100 years. We take advantage of the long interest in aquatic plants in Denmark to compare the submerged flora in lakes and streams in 1896 and 1996. 2 Most of the lakes which contained a diverse submerged vegetation 100 years ago now have the high phytoplankton biomasses and summer transparencies below 2.0 m characteristic of eutrophication. The majority of 17 lakes included in both old and recent studies have lost all or most of their submerged species. Species richness for those lakes that were vegetated did not, however, differ significantly between old and recent studies. 3 Species richness declined markedly in the 13 streams included in both studies. Over all sites, there was also a significant decline of species richness per locality. Potamogeton species declined from 16 to 9, despite an 8‐fold increase in the number of sites surveyed. 4 Similar compositions and rank‐abundances of Potamogeton species in lakes and streams studied 100 years ago reflect suitable growth conditions and mutual exchange of propagules. Today, low habitat diversity and frequent disturbance in streams and low recruitment from lakes favours only robust, fast‐growing species capable of regrowth following weed cutting and dredging. 5 A positive interspecific relationship observed in the contemporary stream vegetation between mean local abundance and number of occupied sites was probably promoted by redistribution of plants as a result of disturbance and efficient dispersal in the interconnected stream network. 6 The freshwater macrophyte flora in north‐west Europe presently includes a high proportion of rare species which are threatened by extinction. Both species typical for oligotrophic conditions (e.g. P. filiformis and P. polygonifolius ) and another group of large, slow‐growing species (e.g. P. alpinus, P. lucens, P. praelongus and P. zosterifolius ), were once common but are now infrequent, while other transient species have remained rare (e.g. P. acutifolius, P. colouratus, P. densus and P. rutilus ). The presence of many species that barely survive in small and distant populations will make re‐assembly of submerged aquatic communities difficult.