Presence of zooplankton, eggs, and resting stages in ballast water samples from the Laurentian Great Lakes

To minimize the introduction and spread of aquatic nuisance species, the United States Coast Guard (USCG) and the International Maritime Organization (IMO) regulate the allowed microbe, protist, and zooplankton densities present in ballast water discharged from commercial ships. However, the density of eggs and resting stages in ballast discharge are not routinely assessed when determining the effectiveness of ballast water treatment systems. While the density of eggs and resting stages found in residual sediment has been examined by various researchers, the density in ballast water uptake and discharge samples was previously unexamined. Untreated uptake and discharge ballast water samples from 2017, 2022, and 2023, collected on ships within the Laurentian Great Lakes from August through January, were analyzed for eggs and resting stages. The samples contained a total density of 325–140,859 eggs and resting stages per cubic meter. These results demonstrate that uncounted eggs and resting stages are present in ballast water, contributing to the risk of spreading aquatic nuisance species within the Great Lakes.