Diversity of macrophytes in riverine aquatic habitats:comparing active river channel and its cut-offs

The study area was a small lowland river valley (the L^g river) located in the south-east ofPoland. The object of investigation was the macrophytes of 10 river lakes with corresponding activeriver channel stretches of the same length as the cut-offs. The aim was to check the difference inspecies diversity between cut-off and active river channels. The second aim was to test the followinghypothesis: vegetation of river lake has been shaped under the influence of contiguous river stretchwhich has left a measurable mark in species abundance and composition. To test this, we checkedwhether a cut-off channel s flora is more similar to flora of the contiguous river stretch, than to floraof a farther river stretch. During the course of the study it was found that the average species richnesswas approximately two times higher in the cut-off channels than in the river stretches. The numberof the species exclusive for the river lakes was nine times higher in comparison with the river sexclusives (not found in the cut-offs). The Shannon diversity index definitely spoke in favor of theriver lakes. These results clearly show the significance of river lakes in maintaining biodiversity ofaquatic ecosystems in a small river valley. We did not confirm our hypothesis concerning floristicrelation between an active channel and its cut-offs. The floristic similarity between a given cut-offchannel and the contiguous active river channel stretch is not stronger than the similarity betweenthis lake and more distant river stretches. The cause of such a state of affairs may be high naturaldynamics ofinvestigated habitats and anthropogenic transformation ofthe river valley.