Spatial segregation between rotifers and cladocerans mediated by Chaoborus

Abstract

In a field experiment I examined the effect of Chaoborus spp, on the vertical distribution of three rotifer species, Kellicottia longispina, Keratella cochlearis and Polyarthra sp. and on the spatial overlap of these rotifer species with three Daphnia species (D. pulicaria, D. rosea and D. retrocurva). In the presence of Chaoborus, total rotifer abundance increased, while total cladoceran abundance decreased. Patterns of migratory behavior varied among rotifer species. Kellicottia longispina and Polyarthra sp. showed vertical migration, while K. Cochlearis did not. Kellicottia longispina mean depth was deeper during the day than during the night. The presence of Chaoborus had no significant effect on its vertical distribution. Polyarthra mean depth was significantly shallower during the day than during the night, but a marginally significant interaction suggests that day-night differences occurred only in the absence of Chaoborus. No vertical migration was observed in any Daphnia species in the absence of Chaoborus. D. pulicaria mean depth was significantly shallower in the presence of Chaoborus, and a marginally significant Chaoborus x time interaction suggests that D. pulicaria migrate upward during the night. The spatial overlap of K. longispina with each Daphnia species was not affected by Chaoborus. Keratella cochlearis was spatially segregated from D. pulicaria in the absence of Chaoborus, but the spatial overlap between these two species significantly increased in the Chaoborus treatment. Spatial segregation occurred between Polyarthra and D. pulicaria in absence of Chaoborus, however a significant Chaoborus x time interaction indicated that the spatial segregation occurred only during the day. These results suggest that Chaoborus could have complex indirect effects on rotifer-Daphnia interactions. Rotifer populations could be released from competition due to Chaoborus predation on Daphnia. Chaoborus presence, however, could intensify rotifer-Daphnia competitive interactions by increasing their spatial overlap.