High colonization and propagule pressure by ship ballast as a vector for the diatom genus Pseudo-nitzschia

Most harmful diatoms belong to the marine, planktonic genus Pseudo-nitzschia and are responsible for amnesic shellfish poisoning through the production of domoic acid. Fifteen Pseudo-nitzschia species, nine of them toxigenic (approximately 60% of the species found in our samples) were recovered from 185 ship ballast tanks (water and sediment) destined for Canadian ports. Our results demonstrate that the three Canadian coastal regions receive considerable total propagule pressure (1.2 × 10, 2.6 × 10, 1.5 × 10 cells from water ballast; 4.5 × 10, 3.7 × 10, 5.3 × 10 cells from sediments), and colonization pressure (15, 11 and 3 species) from these diatoms for Atlantic, Pacific and Great Lakes ports, respectively. Neither ballast water exchange method (intercoastal unexchanged, intercoastal exchanged, transoceanic exchanged), or ship port-of-destination seemed to affect sample groupings recovered by hierarchical clustering and multidimensional scaling. Only weak separations of samples by coastal region, propagule pressure/colonization pressure, and the number of days in the ballast since mid ocean water exchange were recovered. The Canadian Atlantic coast is under particularly high individual propagule pressure from P. turgidula, and the Canadian Pacific from P. seriata. Both species are toxigenic and not yet reported from either region. Alien to the Atlantic Ocean, highly toxigenic P. australis and P. turgidula have been recently found in Scottish waters, but not yet in Atlantic Canada, with the former relatively common in our ballast water samples. A greater number of species may be dispersed by ballast waters than by sediments because lightly silicified and narrow-valve species were absent in our sediment samples. However, these hardy survivors in the ballast sediments may be better adapted to tolerate suboptimal growth conditions when introduced to non-native regions and/or environments. Pseudo-nitzschia delicatissima, found in Great Lakes ship ballast waters, is known for its tolerance of brackish waters and may be one of the potentially toxigenic coastal species well suited for establishment in the Great Lakes, as has been the case for some other coastal marine