Is the flora of oxbow lakes different from that of fishponds? A comparison of two types of water reservoirs in the Noteć River valley and Bydgoszcz Canal valley (NW Poland)

Abstract

The study was conducted in the growing seasons of 2011–2013 and involved 10 oxbow lakes located in the vicinity of Chobielin and 10 fishponds near Ślesin (north-western Poland). The oxbow lakes were characterized by a greater floristic diversity (212 species) compared with the ponds (131 species). Their flora has a more natural character, which is manifested in a higher pro- portion of native species, including non-synanthropic spontaneophytes (45.3%) as well as some legally protected species. In the ar- tificial reservoirs, non-native species represented 12.2% of the total number of species, in natural reservoirs – only 8.5%. The main differences in the spectrum of life forms were manifested in a higher relative abundance of phanerophytes in the vegetation of the oxbow lakes (10.8%), and a higher abundance of hydrophytes along with helophytes (17.6%), therophytes (18.3%), and geophytes (16.3%) in the ponds. The flora of the oxbow lakes was represented by species belonging to 20 phytosociological classes, while the flora of the ponds – 14 classes. Molinio-Arrhenatheretea and Phragmitetea australis accounted for the highest relative species abun- dance in both types of water bodies. Oxbow lakes are characterized by a much greater proportion of species from Querco-Fagetea, almost exclusively non-synanthropic spontaneophytes. Management practices associated with the maintenance of the ponds translate into a higher proportion of ruderal species, by nearly 6%. As a result, taxa from Artemisietea vulgaris were more numerous in the ponds. Edificators from Lemnetea minoris and Potametea were also abundant there. The differences in the analysed flora, resulting primarily from different origins of the reservoirs and their surroundings (mainly forests in the case of the oxbow lakes and meadows around the ponds), was confirmed by statistical analysis and synanthropization indices. Despite their different origin and management methods, both types of reservoirs are valuable refuges for native flora and are characterized by high and similar values of floristic diversity indices. While the oxbow lakes are protected from degradation as protected habitats, the existence and condition of vegeta- tion in fishponds are entirely dependent on the economic regime of fish farms.