LOW SINKING RATES OF PSEUDO-NITZSCHIA: A COMPETITIVE FEATURE CONTRIBUTING TO THE DEVELOPMENT AND MAINTENANCE OF TOXIC BLOOMS

Benjamin Beall¹, and Charles G. Trick^1,2, William P. Cochlan³, Vera Trainer⁴, and Mark L. Wells⁵

¹Department of Biology, University of Western Ontario, London, ONT, N6A 5B7
²Schulich School of Medicine and Dentistry, University of Western Ontario, London, ONT, N6A 5B7, Canada
³Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, 94920, USA
⁴NOAA Northwest Fisheries Science Center, Seattle, 98112 USA
⁵School of Marine Sciences, University of Maine, Orono, ME 04469, USA

The toxigenic diatom genus Pseudo-nitzschia, responsible for amnesic shellfish poisoning, is a significant harmful algal bloom taxon. Efforts to predict the blooms and subsequent fate of Pseudo-nitzschia cells require understanding the factors that contribute to the growth and mortality of the diatom. Balancing cell growth and nutrient uptake rates against grazing-determined loss rates is the foundation for many models of bloom maintenance. However, sedimentation is a major loss mechanism for diatoms often not considered in describing bloom development. In addition to their unique cell morphology, Pseudo-nitzschia spp. often display behaviours (e.g. optional chain formation) that have the potential to minimize cell loss through sedimentation, and thereby extending the duration and magnitude of toxigenic diatom bloom events. The relative sinking rates of natural communities containing Pseudo-nitzschia were studied along the coasts of Washington State, U.S.A., and British Columbia, Canada, as part of the ECOHAB-PNW project. Sinking rates of Pseudo-nitzschia were substantially lower than those of the entire phytoplankton community during the early development and midpoint of toxic bloom formation. Measurable sinking rates of Pseudo-nitzschia were observed only late in the bloom. Even so, the maximum observed sinking rate of Pseudo-nitzschia was less than measured for the phytoplankton community as a whole. This change in Pseudo-nitzschia sinking rates corresponded with a sharp reversal of intracellular vs. extracellular domoic acid concentrations, where high concentrations of dissolved DA were released by the cells. The increase in Pseudo-nitzschia sinking rates also was accompanied by a higher potential formation of aggregates, measured in a Couette device, that was likely mediated by increased concentrations of exocellular polysaccharides. These results suggest that the low observed sinking rate of Pseudo-nitzschia during bloom development could contribute to the competitive success of this taxon, and sedimentation may only be significant after periods of substantial aggregation and subsequent sinking of the diatom flocs.