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

Metaphytic species were the most frequent there (e.g. Alonella excisa, Ceriodaphnia quadrangula, Disparalona rostrata, Macrothrix hirsuticornis, Pleuroxus aduncus, Eucyclops serrulatus, Macrocyclops albidus, Thermocyclops crassus). In the water flowing out from the cassette and in the central part of the lake Acanthocyclops robustus f. limnetica and Mesocyclops leuckarti formed great populations during summer. In the following year great stands of aquatic macrophytes were not produced in the middle part of the lake. The majority of Zooplankton communities were made up on each occasion and in every place by rotifers. Species number of crustaceans, the diversity of their communities markedly decreased even in the southern regions of the lake. As a result of the great flood which started in early August and lasted for about three weeks, the water quality regions distinguished previously completely disappeared. Only the cassette (sampling point 8) formed an exception to that, in which the abundance of Zooplankton remained unchanged because of the longer retention time. In 1988, planktonic crustacean communities exhibited essential changes in species composition. The number of species continued to decrease. At the same time, however, number of individuals became many times greater. In most cases and in most places, the dominance of species typical of eutrophic standing waters and fishponds could be found: Bosmina longirostris, Chydorus sphaericus, Diaphanosoma brachyurum, Moina micrura, Acanthocyclops robustus f. limnetica, Mesocyclops leuckarti, Thermocyclops crassus, Thermocyclops oithonoides). Moina micrura occurred only sporadically in the previous years, but from this year it has become a characteristic cladoceran species of the Hídvégi-tó. It is also worth mentioning here that Eudiaptomus gracilis has been frequently found in this lake, and often formed great populations in it. The mosaic nature of the lake observed earlier could not be detected in this year. It is interesting that during the bloom of the blue-green alga Microcystis aeruginosa during the drought in July and August, the density of planktonic crustacean communities did not diminish. In 1989, collectings were repeated in the whole lake. These collectings had the special aim to determine water quality. In that period mass productions of the blue-green algae Microcystis, Aphanisomenon occurred in the whole reservoir. On the basis of species compositions of planktonic crustacen communities, the lake proved to be almost homogenous at that time. The earlier water quality regions ceased to exist (Fig. 4). The number of true euplanktonic species (Bosmina longirostris, Daphnia longispina, Daphnia hyalina, Diaphanosoma brachyurum, Leptodora kindtii, Eudiaptomus gracilis, Cyclops vicinus, Mesocyclops leuckarti, Thermocyclops crassus, Thermocyclops oithonoides) has practically not changed compared to previous years. At the same time, however, several species were recovered from the plankton samples, which are inhabitants of the littoral of shallow waters and the waters among the vegetation (Disparalona rostrata, Iliocryptus sordidus, Leydigia leydigi, Oxyurella tenuicaudis, Eucyclops serrulatus, Macrocyclops albidus). The greatest changes took place in the southern part of the lake. In the preceding years the number of individuals decreased markedly, species numbers increased and species not found elsewhere were frequently recorded. In this year of examination, there was no decrease in abundance, and the dominant species of communities were identical with those found in the vicinity of the brook "Szabari patak" and the mill stream of Rada.