Morphological and molecular identity of diatom cells retrieved from ship ballast tanks destined for Vancouver, Canada

Nine diatom phylotypes were recovered from trans-Pacific crossing ship ballast water tanks (25 day duration Trans-Pacific Voyage 2) and from associated ports (ballast water source in Osaka, Japan and destination in Vancouver, Canada). Diatoms were characterised morphologically (SEM) and genetically (ITS and rbeL DNA fragments) using one intact cell or a clonal chain of cells as a DNA source. Concurrently we monitored the tank physico-chemical environment (temperature, chl a, oxygen, and mineral nutrient concentrations) daily to provide an environmental context for the diatoms that persisted throughout the crossing. Most diatoms perished in ballast tank waters (probably from hypoxia, darkness and pathogens) but a few persisted several days into the voyage and could have been a part of de-ballasted water inocula in the destination port of Vancouver had the voyage been shorter and ballast tank environment more hospitable. New diatoms were taken up onboard with oceanic water during mid-ocean ballast water exchange (including species tolerant of coastal environments, such as Chaetoceros peruvianus) and some of these also persisted for several clays. Taxa recovered from ballast tanks include diatoms considered non-native to other coastal Canadian waters (e.g.. Atlantic coast, such as Coscinodiscus wailesii, Odontella sinensis and Thalassiosira punctigera) and potentially toxigenic species (Pseudo-nitzschia pungens var. aveirensis). Our results illustrate limitations of the benefits of using mid-ocean exchange as a sole means of ballast water management.