B. Papp szerk.: Studia Botanica Hungarica 32. 2001 (Budapest, 2001)

Buczkó, Krisztina; Rajczy, Miklós: Changes of attached diatoms in a dead arm of the Danube between 1992-1999 at Ásványráró (Szigetköz section)

monitoring work in the Szigetköz more than 700 algal samples were collected and analysed. The detailed data and results of monitoring were submitted to the Hungarian Ministry for Environment and Regional Policy in Annual Reports. Meanwhile, some of our previous observations appeared in publications (ACS and BUCZKO 1996, BUCZKO 1999, 2001, BUCZKO and ÁCS 1992, 1994,1996, BUCZKO etal. 1997). In this paper we present the results of the study of 38 samples, restricted to one dead arm of the Danube in the branch system near Ásványráró (Figs 1-2). Our first collecting trip was led here in 1992. Between 1995 and 1999 we collected samples from different substrata regularly, twice a year. Beginning from 1996, we also made observations on the algal mats. In the summer of 1999 during our regular biomonitoring collecting trip in the Szigetköz we recognised, that two of our standard sampling locations have markedly changed. Astonishingly, almost the whole water surface of the dead arm was covered by duckweed (Lemna minor and Spirodela polyrrhizd). Only a few square metres of the water surface at the entrance of the dead arm were free of duckweed; Lemna minor and Spirodela polyrrhiza have almost totally overrun the rest of the surface. In the autumn we also went to this dead arm to collect samples. At that time we found larger free water surface, but 80-90% of the dead arm surface was still covered by duckweed. There was a sharp border between the free and the covered surface. Although we regularly collect periphyton samples from this place from 1992, we never observed such an overrun by duckweed here. Otherwise, duckweed invasion is not a rare phenomenon in Hungary (FELFÖLDY 1990). With the studies we aimed at learning how the shading of duckweed influences the quantitative and qualitative parameters of periphyton. Győr Fig. 1. Location of the Szigetköz in Hungary and that of the Ásványráró branch system within it (shown by a box).