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 45 iscus, occuring only in Új-tó, reaches a high quantity on some sections in the middle of the left shore. Moreover, we found considerable differences between the individual sections in the case of disturbance-tolerants and invasive species. Invasive species and other plants indicating degradation can be found mainly on the left shore in the case of Bresztik, Vájási-tó and Zalátai Ó-Dráva. For example, Solidago gigantea is much more abundant on the left shore of Zalátai Ó-Dráva (Fig. 1). The explanation of this phenomenon is that the roads and cultivated fields are very close to the left shores of the oxbow lakes mentioned above, and a protective forest buffer zone is lacking. In contrast, the Új-tó is surrounded by a stripe of Phragmites australis, supporting a protective belt from every direction. Thus we found no considerable differences between the two shores regarding the weed and invasive species. In the case of Mailáthpusztai-tó, a narrow protective forest buffer zone lies along the left shore while on the right shore there is a much wider forestzone. As this oxbow lake contains hardly any weed and invasive species, the narrow forestzone seems to be enough to prevent the penetration of these species. Despite the fact that there is only a narrow buffer zone around Hótedra, we found hardly any weed and invasive species in this lake. A lot of other species can be found mainly on the right shore. Changes of the last 10 years We compared our data with the results of DÉNES and ORTMANN-NÉ AJKAI (1999) and CZIROK et al. (2000, 2002, 2003, 2004) to reveal possible changes. The results are as follows: Bresztik: This oxbow lake had rich water-weed vegetation 10 years ago. Unfortunately, the vegetation has suffered a considerable impoverishment: we could find only 5 waterweed species in 2009. The stands of Typha latifolia and Thelypteris palustris have disappeared from the southern shore and the quantity of Nuphar lutea has decreased much due to the dredging in 2002. On the other hand, Nuphar lutea has recovered fastly since it was already very frequent when our study was carried out (Table 3). Nymphoides peltata turned up in this oxbow lake in 2004 and it became dominant by 2009. Nymphaea alba was not present in this oxbow lake between 2002 and 2004 and we did not find it in 2009, either (Table 3). The decrease of the quantity of Nymphaea alba is due to the dredgings and Ctenopharyngodon idella, this herbivorous fish species. During our survey, Solidago gigantea proved to be the most dangerous invasive species and it was wide-spread along both shores of Bresztik (Table 3). Vájási-tó: Fortunately, the number of water-weed species has not decreased during the years analysed. Nuphar lutea has already been dominant since 1999. The quantity of Nymphoides peltata was fluctuating and it was frequent in 2003 and 2009. The number of water-weed species is low, but these few species reach high RPM% values. Utricularia vulgaris was frequent on the right shore in 2004 and it was wide-spread in the whole oxbow lake by 2009. However, Nymphaea alba could be found only in a few sections and in low quantity. The reason of this could be the dominance of Nuphar lutea in this oxbow lake (cf. BORHIDI and SÁNTA 1999) and the introduction of Ctenopharyngodon idella. Also, this fish may be responsible for the disappearance of other water-weed species. The invasive species reached only low RPM%. Zalátai Ó-Dráva: Unfortunately, we found that both species number and abundance of most water-weed species have decreased, for example we could not find Ceratophyllum demersum, Utricularia minor, Trapa natans, Myriophyllum spicatum, Ranunculus circinatus and Potamogeton crispus, all of which were present previously. Nymphaea alba was present only in low numbers in the south-eastern part of the oxbow lake, which is