Changes in plant species diversity of aquatic ecosystems in the agricultural landscape in West Poland in the last 30 years

The aim of this study was to diagnose changes in species composition and their dynamics caused by human impact in aquatic and marsh vegetation in the contemporary agricultural landscape. The most serious threat was an increase in the nitrogen content of surface waters and groundwater which had resulted from the intensive management of the surrounding areas. Floristic data were collected in a lake, peaty pools and a drainage ditch within agricultural areas in the Wielkopolska region (West Poland) in 2006–2007, and compared with data from 1976–1980. The species richness had significantly increased and the shares of sociological and geographical–historical groups had changed clearly. This was reflected in an increase in the synanthropization index from 32.6 to 48.5. All the changes resulted from human impact, which had caused the acceleration of eutrophication in the water bodies. The natural plant succession, especially in the reed beds and sedge communities, was also accelerated due to human pressure. The vascular plant species richness in aquatic and marsh habitats increased by 40, because 15 species had disappeared and 55 new had appeared. However, six moss and four stonewort species had disappeared and only three new moss species were found. The increase in species richness did not raise the natural value of the flora, because the new species group is composed mainly of synanthropic species: native (apophytes) and alien, whereas all the lost species are endangered and rare native species (spontaneophytes), which occur only in natural or seminatural conditions. On the local scale, as species richness increases, plant communities lose their natural value. As an effect of synanthropization the stenotopic species disappear, while the widespread species spread even more. The increase in species richness is a consequence of appearance of species with a wide ecological scale that are not typical for aquatic ecosystems in patches of emergent vegetation. Finally, the species diversity of the whole landscape declines.