Changes in aquatic macrophytes since 1933 in an urban lake, lidesjarvi, SW Finland

Encroaching urbanisation changed Lidesjarvi from being a lake largely in a rural area surrounded by cultivated fields, pastures and meadows at the beginning of the 20th century to its present-day urban land setting. The lake has been damaged by pollution, road construction, dumping of toxic waste, introduced species and building of new settlements. To assess the impact of these temporal changes in the aquatic plant biodiversity of Iidesjarvi this study analysed macrophyte survey data collected during the course of the 20th century. The study indicated that despite of the environmental changes the total number of aquatic macrophytes remained relatively stable, but the species composition has changed considerably reflecting the changes in land use, water quality and the invasion of alien species. Some previously dominant species have disappeared with eutrophication affecting particularly elodeids and ceratophyllids. Some species have returned to the lake after a long absence. Similar lakes in the region form a dynamic system where the exchange of species from one to another may be possible. The semi-isolated lagoons may also help to re-establish aquatic macrophyte species in the main basin of lidesjarvi, if the water quality improves in the future.