Long-term changes in the flora and vegetation of lake Mikołajskie (Poland) as a result of its eutrophication

Abstract

Changes in littoral flora as well as aquatic and swamp vegetation were analysed with increasing eutrophication of the mesotrophic Lake Mikołajskie. Over 30 years the habitat conditions of the lake deteriorated and the phytolittoral was reduced from a zone 6 metres wide to one of only 2 metres. In addition, the number of submerged macrophyte species decreased by 50% and the frequency of most of the remaining species declined severalfold. No new species were encountered. Species retreating from the lake littoral included all Chara species, Patamogeton obtusifolius, P. natans and Hydrocharis morsus-ranae. A significant lowering of the phytosociological diversity and species richness of aquatic and swamp communities was observed. By 1994, six of the 12 associations identified in 1964 and representing the submerged and floating-leaved vegetation (e.g. Nitellopsidetum obtusae, Charetum asperae and Potamogetonetum compressi) were no longer present. In turn, 6 swamp communities from among the original 14 identified in the lake were lacking (e.g. Typhetum angustifoliae, Sagittario-Sparganietum emersi and Eleocharitetum palustris). At the same time, two new aquatic and swamp communities appeared (Ranunculetum circinati, Myriophylletum spicati, Caricetum acutiformis and Caricetum distichae). In contrast there was an increase in the species richness of reedswamp communities due to an influx of marshland species. While the 1990s witnessed a distinct decrease in concentrations of nutrients in Lake Mikolajskie, the consequent increase in water transparency was not associated with an increase in the area of submerged macrophytes, or the species richness of aquatic vegetation.