Understanding the risk–release relationship (the relationship between density of organisms released and associated risk of establishment of a population) of aquatic invasive species is important for setting policy standards to protect natural water bodies from species spread through human-mediated vectors, in particular ballast discharge. To test the viability of an experimental and analytical approach to investigate this relationship, we conducted a mesocosm-based experiment using a test organism, Melosira varians (a freshwater phytoplanktonic diatom native to the Great Lakes). Varying densities of the test organism were added to 19-l mesocosms of water from the Duluth-Superior Harbor at Superior, Wisconsin, in three consecutive trials over 4 months. Each mesocosm was sampled weekly for 4 weeks, and the size of the M. varians population and phytoplankton community was measured via assessments of cell densities. Population responses varied by initial M. varians density. Based on a logistic model, the inoculation density necessary for establishment of M. varians was approximately 12 cells/ml. These findings suggest mesocosm experiments coupled with logistic modeling have the potential to characterize risk–release relationships. Additional investigations using similar methods should be undertaken with a variety of test organisms and environmental conditions to further vet this method and extend understanding of risk–release relationships.
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Acknowledgements The authors thank Richard Green for providing valuable feedback during the analysis and writing phases of the project, Travis Mangan for onsite support and assistance in setup and troubleshooting, and Matt Julius (St. Cloud State University) for generating and providing the Melosira varians culture. The proposal ‘‘Characterizing the Risk-Release Relationship for Aquatic Invasive Species in the Great Lakes’’ was funded in 2011 by the Great Lakes Protection Fund to make this research possible (Great Lakes Protection Fund 2011). The authors also thank the Northeast-Midwest Institute for providing the venue for the experiment and the Water Resources Science Program for support throughout the degree process. Hugh MacIsaac, Sidinei M. Thomaz, Katya Kovalenko, and six anonymous reviewers provided valuable comments on this manuscript.
- Invasive species