Abstract
Ships' ballast water is a major vector for aquatic non-native species, particularly in the Great Lakes and St. Lawrence River (GLSLR) region. Effective September 8, 2024, international ships must comply with an international convention limiting live organisms in discharged ballast water, typically by installing onboard ballast water management systems (BWMS). This study assessed BWMS effectiveness in the GLSLR using a paired uptake-discharge sample design (before and after treatment), to evaluate live organism abundance in the two regulated size classes, and plankton diversity, using microscopy and high-throughput sequencing. Samples from 10 ships (11 paired events) collected in 2019 and 2022 revealed uptake abundances of living organisms up to 107,600 organisms/m(3) for the large (≥ 50 μm) and 169 organisms/mL for the small (≥ 10 to < 50 µm) size classes, respectively. While all discharge samples met the small size class limit, four samples exceeded the large size class limit, with BWMS reducing abundances by at least 98%. High uptake abundance and BWMS filter mesh size were key predictors of organism abundance in discharge samples, highlighting potential challenges in meeting the discharge standard in eutrophic waters. BWMS also reduced plankton diversity in discharge samples compared to uptake samples, with molecular methods showing greater sensitivity than microscopy but potentially detecting DNA from both live and dead organisms. The findings underline the need for further research to address BWMS performance in different water qualities and to improve technology. Complementary use of microscopy and molecular methods offers a comprehensive approach to evaluating plankton diversity and BWMS effectiveness.