Investigation of epilithic algae on the River Danube from Germany to Hungary and the effect of a very dry year on the algae of the River Danube
Acs, Eva; Szabo, K.; Kiss, A. K.; Toth, B.; Zaray, Gy.; Kiss, K. T.
Epilithic algae of the River Danube were investigated alongside the river from the source streams Breg and Brigach up to the end of the Hungarian stretch. Beside the data acquisition about the species composition of epilithic algae, diatom-based water quality analysis was performed with the aid of the widely used European software OMNIDIA. Additional samplings were carried out with monthly frequency at Göd (river-km 1669) in order to investigate the impact of a dry period on the water quality. We found that the diatoms Achnanthidium minutissimum and Amphora pediculus, furthermore the Chlorophyta species Protoderma viride gained an overall strong dominance on most parts of the Danube. However, species composition was very different in the source streams Breg and Brigach and on a short upper stretch of the Danube. Thus, two well-defined parts could be distinguished on the basis of epilithic algal composition: the source area and the lower stretch including the total length of the Austrian, Slovakian and Hungarian stretches. We concluded that the differences between the upper stretch and the middle stretch of the Danube have practically vanished which is the result of the large-scale dam constructions on the river. Two invasive diatom species were found in the Danube: Didymosphenia geminata and Reimeria uniseriata. Using OMNIDIA program we found that the indices IPS, IBD and CEE gave reliable water quality assessment in the River Danube. According to these, the water quality of the Danube is good or excellent around the sources and, through gradual deterioration, it becomes moderate or partly bad in Slovakia and Hungary. In August 2003, unusually low phytoplankton abundances were found at Göd, which did not correspond with the low water discharge. At the same time epilithic diatoms showed unexpectedly vigorous multiplication and produced large masses of gelatinous matrix. The diatom-based water quality turned out to be poor (polluted). The peculiar phenomenon could only partly be explained; however, it draws the attention to the necessity of water quality analysis based on several (monthly) sampling per year.