Ballast water management systems protect the Great Lakes from secondary spread of non-indigenous species

Approximately 65% of established non-indigenous species (NIS) identified in the Great Lakes-Saint Lawrence River basin (GLSLR) since 1959 were introduced by ballast water discharges from transoceanic vessels. While the rate of new detections has sharply declined, NIS already present may spread within the system-including upstream-through secondary invasions by domestic ballast water transferred mainly by 'laker' vessels. Canada has mandated that all vessels loading or unloading in waters under Canadian jurisdiction in the GLSLR will need to use ballast water management systems (BWMS) by 2030. Here we used simulations informed by empirical data to investigate the expected efficacy of BWMS in reducing zooplankton and phytoplankton introductions on a per-trip basis, and the corresponding probabilities of survival and establishment related to ballast water discharges within the GLSLR. We investigated three ballast water scenarios: no treatment, full treatment, and treatment by a partially-functioning BWMS (owing to malfunctions or challenging water quality). Fully-functioning BWMS reduced community pressure by > 99% and corresponding establishment risk of NIS by 38% and 66% relative to untreated ballast discharges for zooplankton and phytoplankton, respectively. Partial treatment (modelled as a 95% reduction in organism concentrations) resulted in 10-20% reduction in per-trip probability of NIS establishment; results indicate that trips with BWMS inoperability caused by highly turbid uptake conditions may be less risky than trips with BWMS inoperability due to plankton blooms. The implementation of BWMS is expected to reduce risk of secondary spread within the GLSLR system by ballast water, even if the BWMS are subject to periodic malfunction.