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)

Studia bot. hung. 32, pp. 39-61, 2001 CHANGES OF ATTACHED DIATOMS IN A DEAD ARM OF THE DANUBE BETWEEN 1992-1999 AT ÁSVÁNYRÁRÓ (SZIGETKÖZ SECTION) K. BUCZKO and M. RAJCZY Department of Botany, Hungarian Natural History Museum H-1476 Budapest, Pf. 222, Hungary; E-mail: buczko@bot.nhmus.hu Periphytic diatoms and diatoms living on algal mats have been studied in a dead arm of the Szigetköz section of the Danube for 9 years. Altogether 123 taxa were recorded. In the last year of the study period notable changes were observed in the flora and in the relative abundance of the dominant taxa. The changes may be attributed to the invasion (and shading) of duckweed (Lemna minor and Spirodela polyrrhiza). In 1999, the coatings were characterised by Achnanthes hungarica, Cocconeis placentula and Navicula cf. saprophila. For years long, Achnanthes minutissima was the dominant species. This means that with the appearance of duckweed the attached flora, especially the dominance of the diatom species, has changed sharply. Key words: biomonitoring, diatoms, Danube, Szigetköz INTRODUCTION Researchers of the Department of Botany, Hungarian Natural History Museum began to study the cryptogams of the Szigetköz in 1991. Our investigation on the attached algae has been part of the Hungarian biomonitoring research in the Szigetköz, coordinated by the Szigetköz Committee of the Hungarian Academy of Sciences. The tasks of this committee are to co-ordinate and to synthesise the results of ecological research in the Szigetköz region after the diversion of the Danube. In October of 1992 the main arm of the Danube was diverted into a new, artificial bed at Dunacsúny (Cunovo) as a part of the construction of the Gabcikovo Water Project. "The constructions at Bős (Gabcikovo) involved water management interventions that have brought about significant changes on the Hungarian side, particularly in the Szigetköz region which had formerly been interwoven by a whole system of river branches. These interventions included the so-called variant C of the barrage system construction and the diversion of the Danube, both arbitrarily carried out by the Slovak party, as well as the building of the temporary river-bed sills to make up for at least part of the ensuing catastrophic water loss." (LÁNG et al. 1997). Our task in the Department of Botany has been to follow the changes of attached algae (esp. diatoms) living on different substrata. During the 9 years of our Studia Botanica Hungarica 32, 2001 Hungarian Natural History Museum, Budapest