Population Dynamics,Biomass and Biomass Production of Eudiaptomus gracilis (G. O. SARS) in Two Water Areas of Differing Trophic State of Lake Balaton (Hungary)

Population dynamics and production of Eudiaptomus gracilis (G. O. Sars) were investigated in two basins of differing water quality of Lake Balaton from February through December, 1977. One of the basins (Keszthely) is considered to be hypertrophic. the other (Tihany) moderately eutrophic. Plankton samples were collected at weekly intervals. The fecundity was 9.61 ± 3.39 at Tihany, and 16.53 ± 3.60 at Keszthely. The difference between the two basins was significant (P<0.1%). At Tihany the average number of eggs per m³ was lower by 36% relative to Keszthety. The number of nauplii differed only a little between the two basins with the exception of April, when at Tihany the amount of individuals was nearly two times as great as at Keszthely. In the case of younger copepodites (Stages I … III) the difference in individual numbers between the two areas of water was marked from February to June, while in other periods it proved to be small. At Tihany, the number of young copepodites was greater by 8%, on the average, than at Keszthely. The amount of larger copepodites (IV … V) relative to the total number of younger individuals of the population was by 63% resp. 77% smaller at Tihany and Keszthely. In the whole period of these studies the number of adults was less by 44% at Keszthely than at Tihany. Their amount relative to that of nauplii was smaller by 6% at Tihany and by 45% at Keszthely. At Tihany, the population biomass was at a constant high level during winter and spring (40 to 50 mg/m³), in summer values being lower (15 … 35 mg/m³). In the Keszthely-Basin biomass showed more extreme changes, reaching a peak as high as 40 … 43 mg/m³ in April and November, but hardly exceeding 6 mg during July–August. During the period of investigation, total biomass in the Tihany-Basin was 342 mg/m³, at Keszthely 204 mg/m³. Net biomass production-two methods for estimation having been used – was higher in the Tihany-Basin (17.25 and 33.83 KJ/m²), while in the Keszthely-Basin it was considerably lower (13.06 and 9.50 KJ/m²).