Evaluation of a method for ballast water risk–release assessment using a protist surrogate

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.