Rotifer Seasonal Succession and Copepod Predation in Lake Michigan

Abstract

Phytoplankton and zooplankton population dynamics in southeastern Lake Michigan provide indirect evidence that selective Cyclops predation and food resource levels are important determinants of rotifer species succession. The spring rotifer community, dominated by the soft-bodied Synchaeta, declined in abundance by late spring and was succeeded by a summer assemblage dominated by spiny ( Kellicottia, Keratella) and colonial ( Conochilus) rotifers, and then by an autumn assemblage of spiny rotifers and the soft-bodied, evasive-swimming Polyarthra vulgaris. In contrast to Synchaeta, these latter forms are resistant to predation by Cyclops bicuspidatus thomasi, the major invertebrate predator in the study area. The density of Synchaeta was negatively correlated with the population density of late instar C. bicuspidatus thomasi and positively correlated with standing stocks of flagellates and small, non-flagellate (<30 μm on one dimension) phytoplankton. These results suggest that Cyclops predation and food resource levels were important factors affecting the abundance of this genus. Conservative estimates of predation mortality of Synchaeta by Cyclops can account for most of the estimated population death rates during spring and early summer when Synchaeta abundances declined markedly. The population dynamics of rotifers resistant to Cyclops predation, especially Keratella cochlearis and Kellicottia longispina, were positively correlated with Cyclops populations, suggesting that abundances of these populations were not predation-limited. A high correlation between egg ratios of these resistant species and phytoplankton standing stocks was found. However, no significant correlation was observed between estimated death rates of resistant species and Cyclops abundance, suggesting that population dynamics of these predation-resistant species were more affected by food resources than by cyclopoid predation.