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)
that in 1996 we began to collect them. Unfortunately only 1 sample was collected in 1996, 3 in 1998 and 2 in 1999 in that arm. Altogether 57 diatom taxa were recorded in these 6 samples. Among them 4 (namely Amphora lybica, Navicula lenzii, Surirella ovalis, Navicula halophila) were found only in mats. Achnanthes minutissima was the dominant species as in the periphyton, its contribution in the total number of algae was more than 40%, in the autumn of 1998 it exceeded 80% (Fig. 11). The second most abundant species was Amphora veneta in mats. It is interesting, because this taxon is usually absent or has a very limited presence in periphyton samples. However, it looks characteristic in the mats. One of the main problems we faced in our biomonitoring studies, that we have no sufficient data for the comparison of the conditions before and after the diversion of the Danube. Before the dramatic changes caused by the diversion only sporadic collections were made in the region. We managed to find only one paper (HALÁSZ 1937) to compare our data on algal mats. Halász worked in a side arm of the Danube at Soroksár, and investigated the diatoms on Cladophora. She mentioned 11 taxa as abundant or very abundant. SZEMES (1967) affirmed that these taxa were dominant there for decades. (The only exception was Gomphonema augur, which was very abundant according to Halász, while Szemes mentioned it as a rare taxon.) Diatoma vulgare and Rhoicosphaenia abbreviata were abundant and constant also in our algal mat samples, i.e. only 2 of Halász' s 11 taxa. 100% 80% 6CP/o 40% 20% 0% • others M Nitzschi a recta M Nitzschia small U Nitzschia angustata H Navicula veneta S Navicula saprophila Él Fragilaria construens m Fragilaria capucina M Amphora veneta U Achnanthes minutissima Fig. 11. Relative abundance of the predominant taxa in the algal mats.
