Behavioural patterns and growth strategies of red tide organisms of the southern Benguela

Abstract

Red tides are a common feature of the southern Benguela upwelling system and are usually dominated by migratory flagellates and the ciliate Mesodinium rubrum. Seasonal blooms of dinoflagellates occur in response to seasonal upwelling and typically succeed diatom blooms. High biomass, multispecies red tides result from concentration by various physical forces and are characteristically found in warm, stratified, nutrient-depleted water overlying cold, nutrient-rich bottom water. The influence of turbulent mixing, light and the availability of nutrients on the migratory behaviour of red tide species was studied by means of both mesocosm and field studies. The mesocosm experiments were conducted in a 3m laboratory column in which a red tide community, collected from the field, was introduced above nutrient-rich bottom water. All the dominant species exhibited directed vertical migration, with ascent and descent starting before sunrise and before sunset respectively. Observations support the hypothesis that red tide organisms can sustain high concentrations in nitrogen depleted surface waters by growing at the expense of nitrate taken up during nocturnal descent. Vertical niche separation of different red tide species was evident both during the night and the day. Observations support the hypothesis that species are capable of coexisting within a red tide bloom. Division rates were determined from the frequency of paired nuclei and cells. C. furca recorded the highest growth rate (u = 0.24). The relatively low growth rates emphasise the importance of physical processes, as opposed to biological processes, in the formation of red tides within upwelling systems.