Mid-ocean exchange of container vessel ballast water. 2: Effects of vessel type in the transport of diatoms and dinoflagellates from Manzanillo, Mexico, to Hong Kong, China

Plankton samples were collected from 4 container ships which took on ballast water in Manzanillo, Mexico, and discharged it 21 d later in Hong Kong, China. As expected, the lack of light during transport in ballast tanks was inimical to the survival of many autotrophic (phytoplankton) species. After 21 d at sea, few of the dinoflagellate and diatom species taken on in Manzanillo Harbour were alive in the ballast water delivered to Hong Kong. In addition, 5 ships from Manzanillo which reballasted with open ocean water were sampled. To assess the effectiveness of mid-ocean exchange, the mean number of diatoms and dinoflagellates in the coastal ballast water (838 cells l–1) was compared with the number in the open ocean ballast water (436 cells l–1) delivered to Hong Kong. Open ocean exchange of ballast water (reballasting) was 48% effective in reducing diatom and dinoflagellate abundance. When we compared the Manzanillo study with our previous study of ships from Oakland, California, we concluded that the older container ships such as those coming from Manzanillo were not as effective in getting rid of diatom and dinoflagellate species as the newer container ships. This is probably because the reballasting design of the older ships is not as efficient in removing the water and sediments located near the bottom of the ballast tanks. This bottom water is associated with a large number of resting cysts and cells.