L. Lőkös szerk.: Studia Botanica Hungarica 25. 1994 (Budapest, 1994)

Ács, Éva, Buczkó, Krisztina; Lakatos, Gyula: Changes in the mosaic-like water surfaces of the Lake Velence as reflected by reed periphyton studies

Table 2. Distribution of taxa 1989 1990 Cyanophyta 14 20 Euglenophyta 7 3 Chrysophyta Xanthophyceae 1 1 Bacillariophyceae 84 64 Cryptophyta 2 Chlorophyta 39 33 Sum 145 123 the reservoir went dry. The estimated amount of missing water was about 12-14 X 106 m

3 in 1993, which was one third of the total water stock of the lake. Due to the shortage of water, the algal cover of the basement in the Német-tisztás got degraded, broken, and floated to the surface causing anaerobic conditions underneath (GORZÓ mscr.). The dendrograms of the 1978 and 1988 surveys show fairly distinct groups for the samples collected from planktonically eutrophised water districts (Fürdető, Öreg-tisztás, Kárászos) (Figs 2-3). These samples, however, cannot be distinguished from the rest on the 1989 and 1990 graphs (Figs 4-5). The coating of the planktonically eutrophised waters can be characterized by a low taxon number (LAKATOS and BARTHA 1989). In 1978, the taxon number of the sampling sites Fürdető and Öreg-tisztás were smaller than at the other three sampling years (Fig. 7). Though the sampling site Kárászos belonged to the brown water district, too, its individual number of planktonic algae was smaller than at the other two sampling points (LAKATOS and BARTHA 1989). In the last ten years, there was a general sinking observable in the trophic level of the NE parts of the lake. The artificial reed field planted at Kápolnásnyék in 1986-87 reserved 25-30% of nitrogen and 90-100% of phosphorus (GORZÓ mscr.). Probably this fact can explain the rise in the taxon number of the coating observable since 1988. Salinisation as a consequence of the sinking water level may be the cause of the increasing relative abundance of the siliceous alga Chaetoceros muelleri (Fig. 8) since 1988. This species is well known for its resistance to higher salinity. In 1978 samples taken from different water quality regions of the lake formed several distinct clusters at 50% level of SICC (Similarity Index Community Coefficient) (CUNG et al 1991) (Fig. 2). By 1988 all the samples joined at higher levels of similarity than 50% (Fig. 3). Since 1989 we have observed a gradual separation of the algal coating of the Német-tisztás, while all the