Timing and pattern of population establishment by the invasive zooplankton, Bythotrephes longimanus, in a Minnesota lake using sediment records
Range expansions by non-native species have become commonplace. One noteworthy example is the predatory zooplankton, Bythotrephes longimanus (Crustacea: Cladocera) which has colonized >100 North American lakes since the early 1980s when it was first detected on the continent. Although research on B. longimanus has accelerated since its North American invasion, most studies are short term and began post invasion. This is symptomatic of research on invasive species in general because usually an invasion event is unplanned and the invader is not detected until it has established a sizable population. Consequently, dates of introduction, rates and patterns of early population growth, and prior knowledge of food-web configurations are rarely known. To overcome this, we studied dated (Lead 210 and Cesium 137), sectioned (0.5 cm slices) sediment cores from 4 sites (3 deep, 1 shallow) in Island Lake Reservoir (Minnesota, USA). Sediment layers from 1970 to 2009 were searched for B. longimanus and daphnids whose densities were reconstructed from tail spine fragments and ephippia, respectively. Growth trajectories of B. longimanus densities were fit iteratively with nonlinear models. Changes in daphnid assemblage composition and density were assessed in coordination with growth of B. longimanus.
Results indicate that B. longimanus first appeared in the lake in 1982, 8 years prior to its first detection in the water column, making it one of the earliest documented invasions in North America. This raises management concern because it demonstrates that an ecosystem may serve as a source to other lakes for years before the threat is recognized. Logistic models fit to 3-year running averages of sediment fossil densities produced R2 values > 0.9 and showed that the population required at least a decade to achieve an annual carrying capacity at the deep sites. This indicates a predictable, but gradual buildup of the population including several initial years of low densities that may be the result of Allee effects (since B. longimanus must reproduce sexually in the autumn) or vulnerabilities of the obligate overwintering egg stage (reduced longevity, reduced environmental tolerance) that delay egg bank accumulation. Post-invasion, Daphnia mendotae became proportionally more abundant than other daphnids in one of the two basins, but neither Daphnia pulex nor Daphnia retrocurva demonstrated the types of significant declines commonly reported from other lakes post-invasion.