Mid-ocean exchange of container vessel ballast water. 1: Seasonal factors affecting the transport of harmful diatoms and dinoflagellates

Our study of 34 ships (20 direct from Oakland, California, USA, and 14 more after open ocean exchange) is the first year-long seasonal study to assess the effectiveness of open ocean exchange of ballast waters. The highest nu mber of harmful species occurred in April and February when water temperatu res in Hong Kong (China) and Oakland were low. The periods with the greates t species richness of harmful species did not correspond to periods with th e highest abundance of harmful species. The latter occurred in early Septem ber and mid-August, when Skeletonema costatum (14 000 cells l(-1)) dominate d the plankton in the ballast water from Oakland Harbor. From April 1996 to April 1997, ballast water samples were collected from 34 Orient Overseas C ontainer Lines Ltd skips which took 16 d to travel from Oakland to Hong Kon g. Of the 34 ships, 14 exchanged their Oakland Harbor ballast water for ope n ocean water (referred to here as reballasting). Open ocean was defined as waters with a depth >2000 m. Once reballasting was completed, the contents of ballast tank no. 1 were not discharged until the ship reached Hong Kong . Six harmful diatoms and 9 harmful dinoflagellate species were observed in the ballast water from Oakland Harbor. These included the diatom Chaetocer os concavicornis and the PSP (paralytic shellfish poisoning) toxin producin g dinoflagellate Alexandrium catenella. Of the 15 harmful species found in the Oakland Harbor ballast water, 8 (53 %) were also found in the ballast w ater of ships that had exchanged coastal for open ocean ballast water. The abundance of harmful species in open ocean ballast water was 87 % lower tha n those in the ballast water from Oakland Harbor. The reason that mid-ocean exchange failed to eliminate all harmful diatoms and dinoflagellates was p robably because the ballast tank was never completely emptied before it was reballasted with mid-ocean water.