Ábrahám Levente (szerk.): Válogatott tanulmányok VI. - Natura Somogyiensis 19. (Kaposvár, 2010)

BALÁZS A. & ERDŐS L.: Botanical survey of Southern Transdanubian oxbow lakes and the changes of the last 10 years

BALÁZS A. & ERDŐS L.: OXBOW LAKES 47 2008. Ceratophyllum demersum was dominant in 2002, whereas it has disappeared by 2008. Lemna trisulca and Spirodela polyrrhiza have also disappeared. Lemna minor, Hidrocharis morsus-ranae, Salvinia natans, Utricularia vulgaris and Trapa natans became extremely rare. Nuphar lutea is the only water-weed species which was frequent both earlier and in 2008. Fortunately, Typha angustifolia and the protected species Thelypteris palustris were frequent on the eastern shore. We found only some individuals of Solidago gigantea in this oxbow lake. Új-tó: The greatest changes have happened at this oxbow lake. We could not find any water-weed species during our studies. 10 years ago, Nymphaea alba was frequent and also Trapa natans was found here (DÉNES and ORTMANN-NÉ AJKAI 1999). Phragmites australis was the only dominant species in 2009, surrounding the shores with a dense stripe, the other species reached only low RPM%. Cladium mariscus was found in this oxbow lake in more sections. Due to the dense reed-bed around the lake, invasive and weed species (e. g. Solidago gigantea and Amorpha fruticosa) were found only in small numbers in Új-tó. Conclusions We found 6 protected species in the six oxbow lakes. Nymphaea alba was found in all oxbow lakes earlier but now it is present only in three oxbow lakes, reaching a greater abundance only in one of them. On the other hand, the quantity of Nymphoides peltata has risen or remained stable in four oxbow lakes. Urtica kioviensis was found only in two oxbow lakes in small numbers. From a nature conservation point of view, Mailáthpusztai-tó proved to be the most valuable, since number of protected species was found to be high whereas number of invasive and weed species is low. Trapa natans, Nuphar lutea, Ranunculus trichophyllus, Myriophyllum spicatum and M. verticillatum have not been reported from this oxbow lake previously (cf. DÉNES and ORTMANN-NÉ AJKAI 1999, CZIROK et al. 2000, 2002, 2003, 2004). At the other end of the scale, Új-tó can be found, where no protected species occur, neither are there any water-weed species. Although the number of invasive species is very low at this oxbow lake, Új-tó is undoubtedly in the worst condition of the six oxbow lakes studied. Compared to the earlier years, not only the quantity of protected species has changed but also the quantity of the water-weeds has decreased and many plants have disappeared. This is caused by the settlement of the adventive herbivorous fish species Ctenopharyngodon idella and the chemicals used for the eradication of the water-weeds. In addition, in the case of Uj-tó, the water-level may not be appropriate, since several water-weeds can not tolerate deep waters (on some parts, Új-tó is as deep as 4 m) (cf. BORHIDI and SÁNTA 1999). The grass carp (Ctenopharyngodon idella) does not make great demands upon its habitat, it tolerates the low solute 0

2 concentration well, it tolerates both shallow and deep waters with bushy water-weeds or without them, so grass caip could find appropriate habitats in these oxbow lakes. It feeds most intensively during the hot summer months, when it is already two years old. The quantity of the grass carp is decreasing, because of the law no. LIII. of 1996 on nature conservation, which banned the settlement of all adventive fish species in the natural waters (FÜSTÖS 2003). In the case of three oxbow lakes, the indicator species of degradation spread because protection zone was either lacking or too narrow. In contrast, where there was a wider forest or reed protection buffer zone along the shores, degradation-tolerating species