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

Species compositions of planktonic crustacean communities in the cassette differed from those in the external reservoir space. In the latter populations of Acanthocyclops robustus t limnetica and Moina micrura exhibited great densities in early spring and during summer. On the basis of the analyses on crustaceans performed in 1988-89 the first stage of the Kis-Balaton Protective System can be divided into the following water quality regions: - the inlet section of the River Zala and the stream "Bárándi víz", - the external reservoir space, - the cassette, - that section of the River Zala, which collects the waters flowing from the external reservoir space and the cassette (Fig. 4). Quantitative studies The results of quantitative studies are given in individuum/dm

3 unit, and those of biomass studies in wet weight mg/m

3 dimension. In the water of the River Zala entering the Hídvégi-tó, both the number of individuals and biomass of cladocerans and copepods were small. During the fiveyear studies their densities were 1-28 i/dm

3 and their biomass varied between 1.2 and 74.7 mg/m

3

. Following the filling of the lake, values for number of individuals and biomass of Daphnia magna and Daphnia longispina were 24-327 i/dm

3 resp. 488275265 mg/m

3 in the central part of it (sampling places 4-6) and on one occasion in its southern part (sampling place 7) during summer 1986. Such great Zooplankton mass productions did not occur in the following years. In 1986-88 the greatest planktonic crustacean populations were generally found in the central parts of the lake (sampling places 4 and 5). These communities were almost without exception made up by the individuals of Acanthocyclops robustus £ limnetica and Moina micrura. The latter species appeared in greater quantity in 1988. During July and August of the same year, its mass production was registered in the environment of sampling place 4 (88 i/dm

3

. biomass 13689.6 mg/m

3

). On some occasions the individuals of Acanthocyclops were also numerous (max. 239 i/dm

3

, biomass = 2041.2 mg/m

3

). In the cassette, number of individuals and biomass of species were generally identical with those found in the central part of the lake. In 1988, the yearly average number of individuals pro dm

3 of Cladocera and Copepoda were computed for the various parts of the lake. The values obtained varied considerably. In the inflowing River Zala, in the vicinity of sampling places 2 and 3 this value was 21, in the middle part of the lake (sampling places 4, 5, 8) 133, in the southern part of the lake (sampling places 6, 7) 35, and at the outlet 92. The value for the whole storage space averaged 84. The values well illustrate the essential structural changes of planktonic crustacean communities in the lake. In 1989, the planktonic crustacean communities attained great dimension not only in the middle part of the lake, but in the southern part, as well (sampling places 6, 7). The differences established earlier between the various parts of the lake ceased to exist.