Impact of Bythotrephes longimanus on zooplankton assemblages of Harp Lake,Canada: an assessment based on predator consumption and prey production

1. Following the 1993 invasion of Harp Lake by Bythotrephes longimanus a nu mber of small-bodied zooplankton declined dramatically compared to pre-inva sion densities, and some larger species increased. 2. To test whether these changes were caused by Bythotrephes, we measured i ts consumption of zooplankton in 1995 and compared this to production by th e prey species with which it overlapped spatially and temporally. 3. On a seasonal basis Bythotrephes consumption was 199 mg dry mass (DM) m( -2) 25% of zooplankton production which was 783 mg DW m(-2). However, for s ome species, such as the small copepod Tropocyclops extensus, consumption g reatly exceeded production-this was one of the species noted to decline in Harp Lake following the appearance of Bythotrephes. By contrast, for the la rger cladocerans Daphnia galeata mendotae and Holopedium gibberum, consumpt ion was much less than production-these species were observed to increase f ollowing the invasion. 4. Our results thus support the hypothesis that zooplankton changes in Harp Lake were caused by Bythotrephes. We speculate that lower quality prey rem aining in Harp Lake may lead to reduced densities of Bythotrephes, or impos e strong selective forces that lead to new adaptations by this predator.