Long-term changes in Lake Ontario rotifer abundance and composition: A response to Cercopagis predation?

Rotifers and the predatory Cercopagis, Bythotrephes, Leptodora, and Diacyclops were collected from 1984 to 2013 in Lake Ontario. In contrast to a previous study, total rotifer abundance decreased. Of the six common genera, Polyarthra and Keratella abundances significantly decreased; Conochilus, Kellicottia, Synchaeta, and Asplanchna abundances did not. Relative abundances of Conochilus and Keratella remained the same, but relative abundance of Kellicottia increased from 12.6% to 24.7% of the population after 1999. Bythotrephes predation on rotifers was minimal, as this species was not observed in quantity during the summer months when rotifers were present. Although Leptodora and Diacyclops predation cannot be ruled out, populations of these genera were higher prior to the Cercopagis invasion and did not appear to be causal in the observed decline in rotifers. The changes in rotifer populations observed did coincide with the introduction of the exotic, predaceous cladoceran Cercopagis. Selective feeding on rotifers by Cercopagis was likely, as relative abundance of colonial and loricated large-spine species increased and non-loricated forms such as Polyarthra decreased in abundance and relative abundance. However, with the long-term decrease in chlorophyll and phosphorus, which were reduced to ~ 10 from ~ 25 μg P/L in the late 1990s, a definitive causal factor cannot be determined. The decrease in abundance of total rotifers was likely a response to decreased chlorophyll levels in Lake Ontario, while the change in relative abundance and composition of loricated and colonial forms suggests that a top-down selective response may be occurring.