Impact of predation by Mysis relicta on zooplankton in Flathead Lake, Montana, USA

We documented major changes in the zooplankton community of Flathead Lake f ollowing the appearance of Mysis relicta. The three common cladocerans foun d in the lake decreased in abundance, most notably Daphnia longiremis which virtually disappeared from the lake. Copepods were also affected by M.reli cta, especially Diacyclops thomasi which decreased in abundance by an order of magnitude. The only macrozooplankton species which seemed to benefit fr om the presence of M.relicta was Leptodiaptomus ashlandi. The varied respon ses of dominant zooplankton species appeared to be the result of a combinat ion of factors. Laboratory feeding studies indicate that M.relicta preferre d Cladocera over Copepoda, with the following order of feeding preference: Bosmina longirostris > D, longiremis > Daphnia thorata > L. ashlandi > D. t homasi. The two zooplankton species which declined most following the appea rance of M.relicta showed the greatest degree of habitat overlap with M.rel icta. Daphnia longiremis and D. thomasi, together with M. relicta, are cold stenotherms and were concentrated in the hypolimnion during the summer. Fi nally, another trait shared by D. longiremis and D. thomasi was the absence of a diapause stage, which made them vulnerable to predation by M.relicta throughout the year.