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.