Habitat differentiation of the Myriophyllum altemiflorum and Littorella uniflora phytocoenoses in Poland
The present study showed the ecological differentiation of phytocoenoses dominated either by <em>Myriophyllum alterniflorum</em> or by <em>Littorella uniflora</em>, which in Poland are classified within the Myriophylletum alterniflori or Myriophyllo-Littorelletum association. The properties which best differentiated the waters of the above types of phytocoenoses were calcium and factors associated with the carbonate complex (pH, electrolytic conductivity, total and carbonate hardness), and pH in the case of substrates. The most distinct differences in the habitats were found between the phytocoenoses dominated by <em>L. uniflora </em>from the Pomeranian <em>Lobelia </em>lakes and those dominated by <em>M. alterniflorum </em>from the Łęczna-Włodawa Lake District, in which other species from the Littorelletea uniflorae class were absent. The <em>L. uniflora </em>phytocoenoses are associated with soft waters poor in Ca<sup>2+</sup> and with the lowest values of pH, electrolytic conductivity, dissolved organic matter (COD-KMnO<sub>4</sub>) and dissolved SiO<sub>2</sub>. The substrates they inhabit are mineral and more acidic. By contrast, the values of the above-mentioned properties are considerably higher in the case of the phytocoenoses from the Łęczna-Włodawa Lake District. The habitats of the <em>M. alterniflorum </em>phytocoenoses from the Pomeranian <em>Lobelia </em>lakes occupy an intermediate position and are more similar to those of <em>M. alterniflorum </em>from the Łęczna-Włodawa Lake District. The results obtained in this study suggest that the phytocoenoses of <em>L. uniflora </em>and <em>M. alterniflorum </em>should not be included in the same association. Further studies are, therefore, necessary to resolve this problem. The comparative analysis of the present ecological findings and data obtained from other regions of Europe show that in Poland the phytocoenoses dominated either by <em>L. uniflora </em>or by <em>M. alterniflorum</em>, in which the contribution of Littorelletea uniflorae species is appreciable, are clearly associated with soft waters and their habitats are representative of the Littorelletea uniflorae class. The massive development of the phytocoenoses with both <em>L. uniflora </em>and <em>M. alterniflorum </em>in the <em>Lobelia </em>lakes is not always indicative of the increase in water hardness and euthrophication of waters typical of the communities of the Littorelletea uniflorae class. The <em>M. alterniflorum </em>dominated phytocoenoses without other Littorelletea species could be good indicators of the above processes taking place in such water ecosystems.