| The purpose of this study was to examine the sequence of changes in the gross morphology and toxicity of the raphidophyte alga Heterosigma akashiwo in the context of blooms of this species in the Strait of Georgia, British Columbia. Field work focused on this alga's 1989 fish-killing bloom.;Information was collected on water column structure, macronutrient concentrations, and the distribution of algae and fish mortalities. Fish mortalities were coincident with the first sighting of waters discoloured with Heterosigma akashiwo. The alga appeared to be transported to the areas by currents. Termination of the bloom was associated with a weakening of the water column stratification and the reappearance of non-limiting concentrations of nitrogen in the surface waters.;Work in culture demonstrated that there was a sequence of reproducible changes in cell size and shape that improved uptake when nutrients were at the concentrations seen in the Strait at the start of the toxic bloom. These changes involved reduction in cell volume by a factor of between 2- and 4-fold and cells changing from rounded, almost football-shaped cells (oblate spheroids), to plate-like (prolate spheroids). Depending on the nutrient history of the population, changes in cell shape could continue beyond the plate-like form with significant portions of the algal population developing surface protuberances when adequate nutrients and energy were available. The elaboration of surface processes could further improve uptake dynamics.;As cells reach the end of their life cycle in culture, they revert to the oblate spheroid form and, if no new nutrients are added, will form resting cysts. However, cyst formation can also be triggered earlier in the life cycle by the addition of nutrients.;In addition to changes in shape, Heterosigma akashiwo cells produce a toxic agent. Production of those toxins begins and declines immediately prior to decline in population numbers. Similar to the differentiation of cell shape, the level of toxicity expressed by the cells appears to be correlated with the nutrient history of the population.;Heterosigma akashiwo would appear to have adaptations which enhance its ability to compete and survive in the nutrient-limited waters of late summer. Lack of vertical migration and a protracted period of a stable, stratified water column with depleted nutrients appear to be critical to the genesis of toxicity in wild populations. (Abstract shortened by UMI.). |
