Investigations on planktonic crustacea in Lake Balaton VI.: Quantitative changes in the Eudiaptomus gracilis population at variorus regions of Lake Balaton

In the periods of 1965-67 and 1972-73 the authors investigated the variation of Eudiaptomus gracilis populations on the basis of samples taken monthly at water areas of different trophic level of Lake Balaton. In the course of the investigation the following could be stated: 1. The development of the population of the species differs in space and in seasons. At the south-western end of the lake (section M) where the water quality is hypertrophic a spring peak of 36-40 per cent frequency was found, while in the other basin (sections A-E) summer peaks dominated (25—40 p er cent). In th e middle of the lake mostly spring-autumn peaks were obseved (20-28 per cent). 2. Adapting the formula of Marczewski-Steinhaus the patterns of the highest similarity were found at the mesotrophic areas (sections G-A), while those of the less similarity at sections K-G. 3. In the population the relative quantity of forms of large body size (females and egg-bearing females) gradually decreases from section A (mesotrophy) to section M (hypertrophy). Regarding the mean values of the period 1966-67 and 1972, it is seen that the total number of the species is the highest at the south-west end of the lake (11 individuals/litre) gradually decreasing to the north-east basin (7.2 individuals/litre). 4. The egg-number/egg-sac was found to be the highest in the hypertrophic south-west basin (14.3) decreasing to 6.2 at the other end of the lake