Bacterial dynamics in ballast water during trans-oceanic voyages of bulk carriers: environmental controls

Bacterial abundance, population dynamics, and related environmental parameters were determined in ballast water during 2 trans-Pacific voyages from Japan to the west coast of Canada and 2 trans-Atlantic voyages from the Netherlands to the east coast of Canada. Water samples were collected from ballast tanks that either underwent mid-ocean exchange (MOE) or were unexchanged (control). Bacterial abundances in the MOE and control tanks were not significantly different at the end of the voyage. Moreover, abundances were not lower immediately after compared with immediately before MOE, suggesting that MOE alone is not effective in reducing ballast water bacterial abundances. Results of this study suggest that the duration of the voyage plays a critical role in determining the potential bacterial propagule pressure. Although the bacterial growth dynamics were complex, we found robust relationships and a highly significant predictive relationship between bacterial abundance and both temperature and dissolved oxygen concentrations of the ballast water (p < 0.001). This robust relationship allows the prediction of potential bacterial propagule pressure from environmental characteristics within the ballast tanks.