Diapausing zooplankton eggs remain viable despite exposure to open-ocean ballast water exchange: evidence from in situ exposure experiments

To reduce the transfer of nonindigenous species, regulations require transoceanic ships to exchange ballast with ocean water before discharging into the Great Lakes. Although ballast water exchange (BWE) is effective for live freshwater animals, laboratory experiments provide mixed results with regards to its impact on diapausing zooplankton eggs. We conducted an in Situ test of the effectiveness of BWE for treating diapausing eggs in ballast sediments. Incubation chambers containing ballast sediment were placed in ballast tanks of cargo vessels transiting from North America to Europe. Each vessel had paired ballast tanks, one of which remained filled with Great Lakes water (control), while the second was exchanged with mid-ocean water. Laboratory viability tests were then conducted to compare viability of eggs recovered from sediments placed in both treatments, as well as identical sediments that remained at the laboratory in cold storage. No significant differences in egg viability were detected between treatments, but more species hatched from sediment that remained in cold storage. Results indicate that physical conditions in ballast tanks may affect egg viability, but saltwater exposure does not eliminate the risk of species introductions via diapausing eggs. Strategies that minimize sediment accumulation in ballast tanks can reduce the risk of species introductions via diapausing eggs.