Predatory Impact of Exotic Cladoceran Cercopagis pengoi on the Zooplankton of Lake Ontario

Abstract

Despite growing knowledge of the ecology of Cercopagis in North America, little information exists on its effects on the composition and abundance of zooplankton and phytoplankton. I assessed the impact of Cercopagis on the lower food-web of Lake Ontario by analyzing the historical and seasonal abundance of the crustacean zooplankton, by conducting laboratory feeding experiments, and by estimating consumption demand of Cercopagis based on bioenergetic considerations. A comparison of average pre-and post-invasion abundances of Daphnia retrocurva, Bosmina longirostris and Diacyclops thomasi suggests that Cercopagis is having a major effect on zooplankton abundance in the lake. In the laboratory Cercopagis fed on a number of zooplankton including the rotifer Asplanchna priodonta and the cladocerans, Daphnia retrocurva, Bosmina longirostris, Ceriodaphnia lacustris, Scapheloberis kingi, and Leptodora kindtii. Between 1999 and 2001 decreases in the abundance of common members of the zooplankton community coincided with an increase in the abundance of Cercopagis. Daphnia retrocurva populations declined despite high birth rates in all three years, indicating that food limitation was not responsible for the patterns. Chlorophyll-a concentration generally increased concomitant with declines in cladoceran zooplankton abundance. Consumption demand of average mid-summer populations of Cercopagis, estimated from the bioenergetic model of confamilial Bythotrephes, were sufficient to decimate much of the crustacean community in a few days. Predatory effects exerted by Cercopagis on the Lake Ontario zooplankton community have decreased steadily since the species was established in the lake.