Habitat conditions of the Salvinia natans phytocoenoses in the Vistula and Odra river valleys in Poland

The Floating Fern, Salvinia natans, is a species of fern associated with stagnant water, primarily oxbow lakes, but also ponds clay pits, low-flow drainage canals and ditches. In these ecosystems, it most often forms phytocoenoses of Salvinio natantis-Spirodeletum polyrhizae. Due to anthropogenic changes in the river valleys where this species and its communities usually occur, they are under threat of extinction in many parts of Europe. The floristic composition and structure of the phytocoenoses dominated by S. natans in Poland and other parts of Europe is well-recognized. However, there is not much information about their ecology, especially the physical and chemical properties of water in which they develop. Therefore, the main purpose of the paper was to determine the range of habitat conditions necessary for phytocoenoses dominated by S. natans, in its common areas of occurrence in Poland (which are in the Vistula and Odra River valleys), and to specify the conditions necessary for their protection. In addition, an attempt was made to determine the degree of habitat distinction of Salvinio natantis-Spirodeletum polyrhizae phytocoenoses in relation to the most common phytocoenoses of Lemno-Spirodeletum polyrhizae in the same areas. 53 stands of Salvinio natantis-Spirodeletum polyrhizae were tested for phytosociological and habitat characteristics, and the results compared to data collected from 48 stands of Lemno-Spirodeletum polyrhizae. 12 physical and chemical water properties were tested for: water depth, pH, colour, phosphate concentration, ammonium nitrogen, nitrates, calcium, magnesium, sodium; potassium, total iron, and soluble silica. The analyses showed the difference between Lemno-Spirodeletum polyrhizae and Salvinio natantis-Spirodeletum polyrhizae in range of floristic composition. Salvinio natantis-Spirodeletum polyrhizae phytocoenoses had a greater share of aquatic species (Potamogetonetea class) and rush plants from the Phragmito-Magnocaricetea class. The phytocoenoses of Salvinio natantis-Spirodeletum polyrhizae more often inhabited deeper, more alkaline waters that were richer in soluble silica and total iron, and poorer in calcium and potassium. In addition to methods for the protection of S. natans and its phytocoenoses a permanent hydrochemical monitoring of the richest S. natans sites in both river valleys should be carried out. It is particularly important to maintain proper water depth (> 0.5 to +/-1.5 m), alkaline pH and the concentrations of other properties that differentiate the water habitats of Salvinio natantis-Spirodeletum polyrhizae from Lemno-Spirodeletum poyrhizae.