ViewpointBiological controls on bacterial populations in ballast water during ocean transit

- We determined bacterial growth rates and grazing mortality in ballast water.
- When the grazer component was removed, bacterial abundances significantly increased.
- Grazer-mediated mortality for ballast water from ports was greater than MOE water.
- Mortality decreased over time for the source port ballast water.
- Bacterial populations in transit are controlled by microzooplankton grazing.

Bacteria (and viruses) numerically dominate ballast water communities, but what controls their population dynamics during transit is largely unexplored. Here, bacterial abundance, net and intrinsic growth rates, and grazing mortality were determined during a trans-Atlantic voyage. The effects of grazing pressure by microzooplankton on heterotrophic bacteria during transit were determined for source port, mid-ocean exchange (MOE), and six-day-old source port ballast water. When the grazer component was removed, bacterial abundances significantly increased. Additionally, we determined that the grazer-mediated mortality for ballast water originating from ports was greater than MOE water and that mortality decreased over time for the source port ballast water. This study shows that bacterial populations in transit are controlled by microzooplankton grazing. If these findings are representative of ballast water environments, they suggest that if the grazing component is selectively removed by various treatment methods, bacterial populations may increase; this could have environmental and human health consequences.