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

Natura Somogyiensis 19 41-50 Kaposvár, 2011 Botanical survey of Southern Transdanubian oxbow lakes and the changes of the last 10 years 'ADRIENN BALÁZS & 2LÁSZLÓ ERDŐS 'H-8800 Nagykanizsa, Zemplén Győző utca l/A 8. em. 52., Hungary, e-mail: aedrienn@gmail.com 2University of Szeged, Department of Ecology H-6726 Szeged, Közép fasor 52., Hungary, e-mai 1: Erdős. Laszlo@bio.u-szeged.hu BALÁZS

, A. & ERDŐS, L.: Botanical survey of Southern Transdanubian oxbow lakes and the changes of the last 10 years. Abstract: We studied six oxbow lakes along the River Drava with the Kohler-method. Oxbow lakes were chosen, in which occurences of Nvmphaea alba were mentioned earlier. In comparison to the former studies, the quantity of Nymphaea alba has decreased dramatically in most oxbow lakes. Moreover, several other water-weeds have disappeared. Probably, this problem is caused by the settlement of Ctenopharyngodon idella, the grubbing-up of water-weeds, the pollution from the nearby cultivated fields and the fact that nature conservational aspects are not considered in the management of the lakes. The invasive weed species have mostly spread at Bresztik and Zalátai Ó-Dráva while at the other oxbow lakes, their quantity has been found generally low. Among the invasive species, Solidago gigantea seems to be the most dangerous. Keywords: Bresztik, Vájási-tó, Zalátai Ó-Dráva, Mailáthpusztai-tó, Hótedra, Kovácshidai tavak, River Drava, Nymphaea alba, Kohler-method. Introduction The Drava plain was onee rich in natural values. However, due to the river canalizations, the landscape suffered severe changes (

ORTMANN-NÉ AJKAI 2004). By now only some small pieces of the forests, wet fields and oxbow lakes have remained. Unfortunately, after the establishment of Danube-Drava National Park, only a few areas became protected, although many areas can be found in Drava plain which are valuable from a nature conservation point of view (

DÉNES et al. 1998). The Drava plain has been studied during the last few decades. Floristical data on the region are given by KLUJBER et al. (1963), VÖRÖSS (1964, 1965) and ORTMANN-NÉ AJKAI and DÉNES (1997). Vegetation of the oxbow lakes was studied by KLUJBER et al. (1963), VÖRÖSS (1965), KOVÁCS and KÁRPÁTI (1973, 1974), ORTMANN-NÉ AJKAI (1997a, 1997b) and DÉNES and ORTMANN-NÉ AJKAI (1999, 2006). We made a survey of six oxbow lakes of River Drava (Bresztik, Vájási-tó, Zalátai Ó-Dráva, the northern lake of Mailáthpusztai-tó, Hótedra, Új-tó in Kovácshida) with the Kohler-method. These oxbow lakes were last examined in 1999 and in 2006. So our aim was to point out the changes during the last years and to compare our records to the former examinations. We laid special emphasis on the degree of dispersion of the pro-