Experimental studies on the factors limiting Tropocyclops prasinus (Fischer) 1860 in an oligotrophic lake

Abstract

In Placid Lake Cyclops bicuspidatus thomasi was the abundant cyclopoid copepod and Tropocyclops prasinus was rare. Feeding experiments showed that T. prasinus was omnivorous in the later instars but this species was neither cannibalistic nor did it prey on other microcrustaceans. Consequently, late instar and adult diets of T. prasinus and C. b. thomasi did not overlap. Seasonal abundance peaks revealed a temporal segregation of the naupliar instars of these two cyclopoid copepods. Competition between T. prasinus and C. b. thomasi was therefore unlikely. Laboratory experiments showed that the only other potentially important invertebrate predator found in Placid Lake, Chaoborus flavicans, ate more C. b. thomasi than T. prasinus but in the absence of Chaoborus in beaker environments, C. b. thomasi always outnumbered T. prasinus after 50 or 100 days. Large scale enclosure experiments revealed that lake densities of C. b. thomasi could limit the abundance of T. prasinus in Placid Lake. Survivorship curves indicated that there was high T. prasinus naupliar mortality even in the absence of C. b. thomasi but when C. b. thomasi was included in the enclosure plankton, mortality of the nauplius 4 to copepodid I instars was greatly increased.