Simulation of zebra mussels (Dreissena polymorpha) invasion and evaluation of impacts on Mille Lacs Lake, Minnesota: An ecosystem model

In less than a decade after being first noticed in 2005, Zebra mussels (Dreissena polymorpha) became fully established in Mille Lacs Lake, Minnesota, USA. To explore the ecosystem-wide impact of this invasion in the premier walleye (Sander vitreus) lake, an ecosystem model with 51 functional groups was built using Ecopath and Ecosim (EwE) modelling suite. The model which represents the 1985 ecosystem condition of the lake was tuned to observed time series of fish abundance and fisheries catch data from 1985 to 2006. Zebra mussels were setup with a high initial biomass, and an adequate fishing pressure was applied on it with an aim to neutralize the effect on ecosystem caused by the inclusion of the mussels. At the onset of 2005 (the first year the mussels were observed in the lake), the fishing pressure was released with different trajectories so that we could mimic the non-nutritional challenges the species could have faced during its irruption in the lake. The fitted model was simulated to the year 2036 (30 years). To enhance the credibility of the model prediction, we compared the prediction with the available field data from 2007 to 2012: the model successfully forecasted most of the changes seen in the lake after the period of fitted-data. The simulation results indicated system-wide collapse of major predators including walleye due to the bottom-up trophic control as zebra mussels efficiently filter out the phytoplankton from the system. The result also indicated that the population of zebra mussels in the lake stabilized after attaining the maximum density within few years of the invasion. Furthermore, the model predicted a significant boost in smallmouth bass (Micropterus dolomieui) population when the mussels were incorporated in the diet of crayfish; remarkably, the predatory pressure did not cause a large impact on zebra mussels biomass. Our capability to predict the response of Mille Lacs Lake to zebra mussels invasion would largely depend on the dynamics of plankton groups, the response of juveniles of higher trophic fish species like walleye to the changing dynamics of plankton groups, and the response of yellow perch (Perca flavescens) population—a major prey in the system.