Spatial segregation between rotifers and cladocerans mediated by Chaoborus

In a field experiment I examined the effect of Chaoborus spp, on the vertic al 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. retrocurv a). 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 ve rtical migration, while K. cochlearis did not. Kellicottia longispina mean depth was deeper during the day than during the night. The presence of Chao borus had no significant effect on its vertical distribution. Polyarthra me an depth was significantly shallower during the day than during the night, but a marginally significant interaction suggests that day-night difference s occurred only in the absence of Chaoborus. No vertical migration was obse rved in any Daphnia species in the absence of Chaoborus. D. pulicaria mean depth was significantly shallower in the presence of Chaoborus, and a margi nally significant Chaoborusxtime interaction suggests that D. pulicaria mig rate upward during the night. The spatial overlap of K. longispina with eac h Daphnia species was not affected by Chaoborus. Keratella cochlearis was s patially segregated from D. pulicaria in the absence of Chaoborus, but the spatial overlap between these two species significantly increased in the Ch aoborus treatment. Spatial segregation occurred between Polyarthra and D. p ulicaria in absence of Chaoborus, however a significant Chaoborusxtime inte raction 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, how ever, could intensify rotifer-Daphnia competitive interactions by increasin g their spatial overlap.