LIFE-CYCLE OF CYCLOPS-VICINUS IN RELATION TO FOOD AVAILABILITY, PREDATION, DIAPAUSE AND TEMPERATURE

The seasonal cycle of Cyclops vicinus was studied during a 5 year period in the shallow and hypertrophic Lake Sobygard. The annual number of generations varied between three and six and sometimes included midsummer generations. Naupliar and copepodite density was extremely high, the maximum recorded being 1313 and 745 l-1 respectively. The seasonal fluctuation in the abundance of C.vicinus was related to the concentration of edible phytoplankton (EDP): when diatoms and cryptomonads were abundant, nauplii and copepodites were also abundant, whereas when greens such as Scenedesmus spp. were dominant, no cohorts were observed. The maximum density of copepodites in the separate cohorts and the amount of EDP, expressed as carbon content, were found to be significantly correlated. The development of the second cohort also seemed to depend on the occurrence of a peak in rotifer density. High number of eggs per female generally coincided with high biovolume of EDP, suggesting that algae constituted a major part of the diet of adult C. vicinus. The duration of the naupliar stage was inversely related to temperature and is one of the shortest times that have been recorded in the literature. The short duration is probably attributable to the abundance of food. Fish composition and the calculated sex ratio of C. vicinus indicate low predation pressure from planktivorous fish. The high density of C. vicinus during the summer, when the whole population would be expected to have entered diapause, is probably attributable to the concurrence of low predation and the abundance of food.