We examined the risk of introduction associated with potentially toxic or otherwise harmful algae (HA) or nonindigenous species (NIS) of dinoflagellates in ballast water from 63 commercial ships visiting ports of eastern Canada in 2007–2009. Ship categories included transoceanics
undergoing ballast water exchange (BWE) and coastal ships with or without BWE. Of 159 species of dinoflagellates observed in Lugol-preserved samples, 15 were potential HA (six Dinophysis spp.) and 46 were NIS (including three HA). We found at least one species of HA in 81% of all ships
examined, and maximum cell concentrations reached nearly 4000 cells·L–1. Coastal nonexchanged tankers carried the greatest cell concentrations of HA. NIS dinoflagellates were found in 56% of ships, significantly more in ships with BWE. There was no evidence that ships
with BWE contained significantly fewer taxa or lower concentrations of HA dinoflagellates, indicating that BWE is not efficient in controlling the introduction of these organisms. In fact, BWE promoted the transport of NIS dinoflagellates, possibly because of the wide distribution of several
of these species. Coastal ship traffic is a significant introduction pathway for HA (ships with and without BWE) and NIS (ships with BWE) dinoflagellates in eastern Canada.
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