Differential success of cyclopoid copepods in the pelagic zone of eutrophic lakes

Numerous investigations have shown that some species of cyclopoid copepods invaded the pelagic zone of lakes during the eutrophication, i.e. that eutrophication favours cyclopoids. Cyclops vicinus, for example, is very common in the pelagic zone of eutrophic lakes; Acanthocyclops robustus and Diacyclops bicuspidatus, however, are less successful. A review of population characteristics showed that C. vicinus displays extremely flexible life cycles, that many A. robustus populations contain an extremely low percentage of females relative to males and that in some D. bicuspidatus populations only juveniles invaded the pelagic zone whereas most adults remained benthic. Enclosure experiments revealed that the low percentage of females in A. robustus populations is caused by fish predation. Comparisons of body sizes and clutch sizes suggest that the combination of large egg sacs and low to intermediate female sizes makes A. robustus and probably D. bicuspidatus particularly vulnerable to fish predation so that their success in the pelagic zone of eutrophic lakes is reduced when fish predators are abundant. The present results on C. vicinus suggest that many eutrophic lakes provide conditions that enable this species to overcome the clear water period and which permit development of juveniles and reproduction of adults almost the year round.