Characteristics of the pelagic phytoplankton in shallow oxbows

Numerous oxbows of different sizes, depths and ages accompany alluvial rivers in the Carpathian basin. These water bodies provide various habitats for macroscopic and microscopic assemblages. Phytoplankton of 13 oxbows in the Tisza valley was studied between 2005 and 2008. In this article, we focussed on the following questions (i) do the oxbows have unique microflora? (ii) does the macrophyte coverage have a large effect on the composition and biomass of the algal assemblages? and (iii) does the higher plants dominated state result in clear-water conditions in the oxbows? The studied oxbows were in different stages of the ageing process. We classified the sampled oxbows according to their macrophyte vegetation into five types. A total of 646 species of algae were recorded in the oxbows over the study period. The microflora was dominated by cyanophytes, chlorococcalean green algae, euglenophytes and diatom species. Phytoplankton species were allocated into 32 coda. For the determination of typical algal assemblages, we used Kohonen's Self Organizing Map (SOM) statistical analysis combined with K-means clustering, which has resulted five different types of phytoplankton associations. These types were dominated by coda Y, L (O) , W1, W (S) and J. Chlorophyll a data of the sparsely and densely vegetated oxbows did not differ, since a wide range of values characterised both types of the lakes. In this study, the composition of the microflora is similar to that of other eutrophic lakes, but some rarely occurring taxa such as Peridinium gatunense Nygaard and Peridiniopsis elpatiewskyi (Ostenf.) Bourelly might play a key role in the phytoplankton succession of the oxbows. The effect of the macrophyte coverage was decisive in the composition of the algal assemblages, and our investigations indicated that, in the case of the oxbows, the macrophyte-dominated state does not necessarily result in a clear-water state.