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

1 Freshwater habitats in cultivated and densely populated lowland regions o f Europe have experienced profound changes during the last 100 years. We ta ke advantage of the long interest in aquatic plants in Denmark to compare t he submerged flora in lakes and streams in 1896 and 1996. 2 Most of the lakes which contained a diverse submerged vegetation 100 year s ago now have the high phytoplankton biomasses and summer transparencies b elow 2.0 m characteristic of eutrophication. The majority of 17 lakes inclu ded 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, how ever, differ significantly between old and recent studies. 3 Species richness declined markedly in the 13 streams included in both stu dies. Over all sites, there was also a significant decline of species richn ess per locality. Potamogeton species declined from 16 to 9, despite an 8-f old 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 mu tual exchange of propagules. Today, low habitat diversity and frequent dist urbance 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 p robably 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 s pecies typical for oligotrophic conditions (e.g. P. filiformis and P. polyg onifolius) and another group of large, slow-growing species (e.g. P. alpinu s, 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.