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)

Hierarchical cluster analysis (Czekanowski measure, UPGMA fusion technique), and principal component analysis (standardised PCA) were carried out with the raw data. For the computations, the program package SYN-TAX was used (PODANI 1993). RESULTS AND DISCUSSION Altogether 38 samples were collected from this dead arm in the Ásványráró branch system between 1992 and 1999. Following the microscopical analysis 36 samples were suitable for statistical analysis (2 from 38 samples contained only a few cells of algae). The later omitted two samples were the 2p92626 and a2n98812. Only a few valves (26) occurred in sample a2p92626, collected in 1992. We can only assume that most of the algae belonged to order Centrales. It is well-known, that this group has been characteristic of the phytoplankton of the Danube for decades, throughout the year (KISS 1999). In 1992 they were very common also in the periphyton of the Szigetköz (BUCZKO 1999). Sample a2n98812 was collected in the summer of 1998. It contained only a few Nitzschia valves which was insufficient for further study. About the taxa During the analysing of the remaining 36 samples altogether 123 diatom taxa were identified (Tables 2-4). The combined form of these three tables contains the complete list of diatom taxa found in that dead arm of the Danube. Only 6 taxa were found in all of the 6 investigated years: Achnanthes minutissima, Cocconeis placentula, Cymhella affinis, Gomphonema parvulum, G. truncatum and Nitzschia dissipata. Among them three (A. minutissima, C. placentula and N. dissipata) are mass forming species. Below we provide a brief summary on the periphyton samples collected from different substrata (including coatings of reed). Later we give a separate analysis of the periphyton samples collected only from reed. Then the enumeration of the diatoms found in algal mats follows. Periphyton samples A total of 119 diatom taxa were recorded in the 30 samples. Table 2 contains the occurrences of taxa in the years between 1992 and 1999. Achnanthes minutissima occurred in all of the samples, and the following taxa were also very abundant (with constancy 5): Cymbella affinis, Navicula veneta,