Evaluation of a method that uses one cubic meter mesocosms to elucidate a relationship between inoculation density and establishment probability for the nonindigenous, invasive zooplankter, Bythotrephes longimanus

The introduction and spread of aquatic invasive species by ships is a global concern, and development and implementation of effective prevention measures are an urgent priority. Two questions plague science and policy. First, what level of reduction in live organism discharges from ships yields a given desired level of reduction in species establishment probability? Second, how can this question be empirically tested in a practicable and replicable manner? We evaulated the extent to which experiments using 1 m³ mesocosms could help elucidate a relationship between inoculation density and reproductive output (a proxy for establishment probability) of a surrogate invader, Bythotrephes longimanus (Crustacea: Cladocera). The inoculation densities were 1, 5, 10, 20, or 40 individuals m⁻³. All individuals in each inoculum were added as a single event at the start of each of four, two-week-long experiments. B. longimanus were inoculated into ambient water pumped from the Duluth-Superior Harbor at the start of each experiment between June and August, 2015. The mesocosm volume and range of inoculation densities has relevancy to the International Maritime Organization’s Ballast Water Management Convention D2 standard which allows a density of < 10 viable organisms m⁻³ in discharged ballast water for organisms > 50 μm minimum dimension. Reproductive output was detected among 78 of 80 mesocosms after 2 weeks. Among 13 abiotic and biotic variables, inoculation density was overwhelmingly the best predictor, and water temperature was the second best predictor, of B. longimanus reproductive output. Net reproductive value of the inoculants was ≥ 1 among 35 of 80 mesocosms after 2 weeks. There was evidence for density-dependent reduction in population growth rate in the higher inoculation density mesocosms. We examined various parts of our method for robustness. Translation of the results into meaningful estimates of establishment probability in the field remains a vivid challenge. We introduce the idea of using annual hatching rates of natural banks of dormant eggs in lake sediments as a method to characterize relationships between propagule pressure and establishment probability at the scale of an ecosystem for seasonally transient species such as B. longimanus.