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)

A mass occurrence of Navicula cf. saprophila was observed in the autumn of 1999 (Fig. 10). The coexistence of Navicula cf. saprophila and Achnanthes hungarica is remarkable. They both were absent in the coatings until 1997. They appeared at the same time, in the autumn of 1998, and became abundant in 1999. We must emphasise, that Navicula cf. saprophila was a characteristic and mass forming species in the autumn only, whereas Achnanthes hungarica could be found in summer samples as well. The change of the relative abundance of Cocconeis placentula (Fig. 6) was similar to that of Achnanthes hungarica; its abundance increased in 1999. This increase was probably the consequence of the duckweed shading. The average relative abundance of Amphora pediculus was 2.73% before the appearance of duckweed, and 4.65% in 1999 (Fig. 7). Amphora pediculus is characteristic in the autumn samples; it could be a shadow-adapted species. Figure 8 shows the relative abundance of Navicula veneta. This species looks more abundant in the autumn samples, but there is no notable difference between before and after the invasion of duckweeds. Algal mats In the beginning of the biomonitoring we paid no special attention to algal mats. During the collecting trips the mass of filamentous algae was so eye-catching 60% 50% 40% 30% 20% 10% 0% Fig. 10. Changes of relative abundance of Navicula cf. saprophila between 1992 and 1999. The samples collected in 1999 are black.