Long-term effects of a trophic cascade in a large lake ecosystem

Ellis, Bonnie; Stanford, Jack; Goodman, Daniel; Stafford, Craig; Gustafson, Daniel; Beauchamp, David; Chess, Dale; Craft, James; Deleray, Mark; Hansen, Barry

Brief Description: Follow up paper to 1991 report by Craig Spencer (Ref #336210) describing lake ecosystem dynamics in Flathead Lake, Montana Full Description: Introductions or invasions of nonnative organisms can mediatemajor changes in the trophic structure of aquatic ecosystems. Herewe document multitrophic level impacts in a spatially extensivesystem that played out over more than a century. Positive interactionsamong exotic vertebrate and invertebrate predatorscaused a substantial and abrupt shift in community compositionresulting in a trophic cascade that extended to primary producersand to a nonaquatic species, the bald eagle. The opossum shrimp,Mysis diluviana, invaded Flathead Lake, Montana, the largestfreshwater lake in the western United States. Lake trout had beenintroduced 80 y prior but remained at low densities until nonnativeMysis became established. The bottom-dwelling mysids eliminateda recruitment bottleneck for lake trout by providing a deepwater source of food where little was available previously. Laketrout subsequently flourished on mysids and this voracious piscivorenow dominates the lake fishery; formerly abundant kokaneewere extirpated, and native bull and westslope cutthroat trout areimperiled. Predation by Mysis shifted zooplankton and phytoplanktoncommunity size structure. Bayesian change point analysisof primary productivity (27-y time series) showed a significantstep increase of 55 mg C m−2 d−1 (i.e., 21% rise) concurrent withthe mysid invasion, but little trend before or after despite increasingnutrient loading. Mysis facilitated predation by lake trout andindirectly caused the collapse of kokanee, redirecting energy flowthrough the ecosystem that would otherwise have been availableto other top predators (bald eagles).