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

Objectives The aim of the qualitative and quantitative studies on cladocerans and copepods inhabiting the open water and mud surface in this lake was to find out- which species of these animal groups occur there, - how the assemblages of these animals change in space and time, - whether it is possible to distinguish different water quality regions in the lake on the basis of the occurrence of species, - how cladoceran and copepod abundance and biomass change in the various water quality regions of the lake. To obtain answer to the aforementioned questions, these two groups were systematically studied, simultaneously with the chemical and phytoplankton investigations, at 11 points of the lake from 1985 through 1989. The species occurring at these points as well as their number of individuals and biomass were determined. During another series of studies qualitative samples were collected from the lake at about 30-40 points on two occasions in 1985, and on one occasion in 1989. Crustacen biomass and its changes during these five years were evaluated on the basis of presences and absences of species. Material and methods For the analysis of crustacean plankton water samples were collected from the surface water layer. From each sample 20-50 liter was filtered through a plankton net of 70 um mesh. The filtrates were conserved in situ with formaldehyde to a final concentration of 4%. Quantitative analyses were performed in the counting chambers of 5 ml of an Utermöhl inverse microscope. From the mud surface, the animals were collected with funnel traps. The method is based on the fact the vertically migrating crustaceans actively get into the collecting bottle. This method can be very effectively used for collecting quantitatively various animal groups living on mud surface (mainly Cladocera and Copepoda) (Whiteside & Williams 1975. Whiteside & Lindegaard 1980). Each trap consisted of three glass funnels of 10 cm diameter, which were connected with a bottle by means of a plexi plate. The traps were set up at three points along the longitudinal axis of the lake, using three traps at each point. The traps were placed out in the afternoon and removed next morning. Thus, collectings were performed for at least 18 hours with this device. The sampling sites are indicated in Fig. 1. Results Planktonic communities Qualitative studies Following the filling of the Hídvégi-ló, enormous mass productions of two cladocerans, Daphnia magna and Daphnia longispina could be observed during summer 1985, which lasted until the middle of July. Later, these species were not so numerous. Besides the aforementioned two species, Bosmina longirostris, Chydorus sphaericus, Acanthocyclops robustus, Eucyclops serrulatus, Mesocyclops leuckarti and Thermocyclops crassus were also frequent in the plankton samples. Simultaneously with Daphnia mass production, water bloom caused by the blue-green alga Aphanisomenon flos-aquae also occurred, and in addition almost impenetrable stands of aquatic macrophytes covered a great part of the lake, among which the water was clear down to the bottom. It is also an interesting observation, that during that time species number, abundance as well as biomass of Rotatoria were extremely small.