Calanoid-cyclopoid interactions: evidence from an 11-year field study in a eutrophic lake

1. The interactions between calanoid and cyclopoid copepods were examined in a 2. A diminishing ratio of calanoids to cyclopoids was observed. The responses, however, were species specific. While two cyclopoid species responded with increased populations (Cyclops vicinus, Thermocyclops oithonoides), one species exhibited marked declines (C. kolensis). Other species extended the duration of their pelagic phase (Mesocyclops leuckarti, Diacyclops bicuspidatus, T. oithonoides, C. vicinus), leading to higher population overlaps. Eudiaptomus graciloides and E. gracilis, which used to be present throughout the year, were more frequently absent. 3. These changes coincided with a series of mild winters during which prey availability (algae and rotifers) was high. 4. Enhanced prey availability, in conjunction with a flexible diapausing strategy, are hypothesized to underly the increased success of C, vicinus. Predatory losses due to early occurrence and enhanced abundances of C. vicinus, rather than competition for the same food resources, are hypothesized to be responsible for the declines of C. kolensis and Eudiaptomus. 5. Summer species tended to develop higher abundances early in the season, which led to enhanced seasonal means during spring. These changes cannot be explained by the interaction between calanoids and cyclopoids alone, but were probably a result of changes in fish predation.