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

quality existing in those days. Besides these an additional sampling point was selected at the 'Német-tisztás', which is located within the protected area (Fig. 1). The elaboration method of the samples has already been published in details (ÁCS et al. 1991). Cluster analysis based on Czekanowski-index (CZEKANOWSKI 1909), was performed with the WPGM fusion algorithm using the software package SYNTAX III (PODANI 1988). Sampling points and their abbreviations on the Figures, correspond with the water quality districts of 1978, are as follows: a) regions of dark brown water: Vendel-tisztás (Ve), Nagytó-Rigya (Na), Gallér (Ga), Vaskapu (V), Hollós (Ho), Német-tisztás (Né) b) regions of grey water: Nagy-tisztás (Nt), Felső-tó (Fe), Nádas-tó (N), Belső-tisztás (B) c) regions of algal brown water: Öreg-tisztás (Ö), Kárászos (K) d) regions of green water: Fürdető (Fü) e) regions of water with transitional character: Hosszú-tisztás (Ht), Lángitisztás (L), Kayak-canoe grounds (Kk) RESULTS AND DISCUSSION From the coatings in 1989 and 1990, 119 and 110 algal taxa were identified respectively (Table 1). Their distribution according to phyla is summarised in Table 2. There were remarkable changes in the reed coating of Német-tisztás (Né). The sample taken from this site in 1988 did not appear as separate from the others on the dendrogram of the cluster analysis (Fig. 3). In the subsequent year this sample joined to the rest at 0.42 similarity rate (Fig. 4), while in 1990 it formed a considerably separate group together with the sample of the neighbouring Vendel-tisztás (Ve) (Fig. 5). Changes in the taxonomic composition are demonstrated on Fig. 6. It is remarkable that the taxon number of blue algae has increased, together with their individual number. While in 1988 and 1989 the three predominant algal organisms were diatoms (Achnanthes minutissima = ACHMIN, Cymbella lacustris = CYMLAC, Fragilaria capucina var. vaucheriae = FRACVV, and A. minutissima, F. construens = FRACON, Rhoicosphenia abbreviata = RHOABB respectively), in 1990, two filamentous blue-green algal taxa were found in high quantities (Oscillatoria chlorina = OSCCHL, Pseudanabaena tenuis PSETEN), though the most abundant was a diatom (Cocconeis placentula = COCPLA) again. The two blue-green algae can be described as inhabitants of base sediments, i.e. rotting silt (FELFÖLDY 1972b). Due to the extremely dry years since 1989, an increasing deficiency of water appeared in the lake. In 1990