Freshwater food webs have been dramatically altered by the introduction of nonindigenous species. For instance, shortly after its introduction into the Laurentian Great Lakes in the 1980s, the non-native predatory zooplankter, Bythotrephes longimanus, is believed to have significantly altered the community structure of mesozooplankton. However, ongoing effects of B. longimanus on zooplankton community structure and dynamics are not well understood and can be difficult to disentangle from other factors (e.g., seasonality, density dependence, temperature, and food resources). To address this gap, we used generalized additive models to examine the effects of B. longimanus on estimated population growth rates for common species in Lake Michigan, generated from long-term, offshore time series data (1994-2012). The approach used allowed us to assess predator effects from field observational data while accounting for potential confounding effects of seasonality and other covariates.
Results/Conclusions
Results of the generalized additive model analysis suggest that zooplankton species differ in the occurrence and magnitude of B. longimanus effects, with observed differences consistent with expectations based on experimental measurements of B. longimanus effects. For example, at maximum B. longimanus abundance, cladocerans experienced substantial reductions in estimated population growth rate (estimated reductions of 20% and 24% in Daphnia and Bosmina growth rates, respectively), compared to no effect on calanoid copepods. Such effects likely alter the production and composition of zooplankton, with potential consequences for important planktivorous fish species. Our results thus provide valuable, field-based evidence for ongoing impacts of this invader in Lake Michigan, with implications for ecosystem-scale processes.