L. Forró szerk.: Miscellanea Zoologica Hungarica 7. 1992 (Budapest, 1992)

Gulyás, P.; Forró, L.: Composition and abundance of microcrustacean fauna in the Upper Reservoir (Hídvégi-tó) of the Kis-Balaton

In 1985-87, great planktonic crustacean communities were only formed in the middle part of the lake and in the cassette. In contrast, from 1988 number of individuals exhibited no decreases in the southern parts either, obviously owing to the drastic diminishing of the stands of aquatic macrophytes. On the basis of planktonic crustacean biomass, the water of this lake was eutrophic or even hypertrophic during most of the year. Number of individuals did not decrease even during Aphanisomenon and Microcystis blooms lasting for several months. In samples collected with funnel traps, species compositions of communities also exhibited spatial and temporal changes. Kelso & Ney (1985) have observed in a Virgininian reservoir, that until July species typical of open water dominated, and later species living among aquatic plants attained preponderance. No such changes were observed in this lake during this two-year study, and on each occasion Acanthocyclops robustus i limnetica, the most conspicuous member of the plankton, dominated. Collections with funnel traps were also made in three different basins of Lake Balaton (Forró 1992). These collections differed in taxonomical composition. Among crustaceans, species of Oslracoda and Amphipoda also occurred in greater individual number. The fact that the epibenthic crustacean fauna- in the Hídvégi-tó, which is constructed in the vicinity of Lake Balaton, is different from that of found in Lake Balaton could be explained; besides other basic differences between them; by the age differences of the two lakes. The few years which have passed since the construction of this lake were not sufficient to produce corresponding communities. This statement is also supported by the fact that the abundances found by means of funnel traps in this first stage of the Kis Balaton Protective System were very small. The values obtained were much lower than those for Lake Balaton (Forró 1992), and also fall behind those for European and North American waters (Whiteside et. al. 1978, Whiteside & Lindegaard 1982, Kelso & Ney 1985). It was observed by Kelso & Ney (1985) in connection with the seasonal changes of abundance that after a maximum in July it continually decreased until November, which they explained first of all by prédation. In contrast, maximum abundance was observed by Whiteside et. al. (1978) in early summer and in autumn. In the latter case, however, the autumn peak for species living on mud surface was less distinct compared to those living among aquatic plants. In the Hídvégi-tó, the individual number of animals collected with funnel traps from mud surface diminished both from July through October 1987 and during summer 1988. These changes are difficult to explain, and it is likely that here prédation or degradation of water quality have also played an important role. The results suggest that the trophic level of the lake is much higher as compared with the River Zala and the Bay of Keszthely. Therefore the deterioriation of water quality in Lake Balaton will not be slowed down or stopped, but even accelerated because of water input from the Hídvégi-tó.