The effect of changes in light availability caused by mixing on the growth of Anabaena circinalis (Nostocales, Cyanobacteria) and Aulacoseira sp (Centrales, Bacillariophyceae)

Experiments were undertaken to study the effects of vertical distribution controlled by mixing on the population growth of Australian isolates of the toxic cyanobacterium Anabaena circinalis and the diatom Aulacoseira sp. Transparent tubular microcosms were identically illuminated in all experiments, with a surface maximum irradiance of 225 mumol photons m(-2) s(-1) attenuating to a minimum of 18 mumol photons m(-2) s(-1) at the base of the microcosm at 1 in depth. In separate experiments, microcosms containing A. circinalis or Aulacoseira sp. were mixed at 10 min (MIXED treatment) or 48 h (CALM treatment) intervals to simulate well-mixed and calm environments respectively. The vertical distribution or cells was predicted using a model that utilized growth-irradiance relationships and sinking and flotation rates measured during the experiments. For A. circinalis, there was no significant difference in population growth rate between MIXED and CALM treatments (MIXED, mu = 0.47 d(-1): CALM, mu = 0.45 d(-1)). The predicted vertical distribution of A. circinalis was skewed toward the surface in the CALM treatment, indicating that an increased portion of the population received greater irradiance. However, both predicted and actual population growth were no greater in the CALM treatment, owing to the low flotation rate of the cells. Any light-harvesting advantage gained through buoyancy was insufficient to produce a significant difference in growth. Aulacoseira sp. had significantly different population growth rates in different treatments (MIXED, mu = 0.39 d(-1) CALM, 4 = 0.28 d(-1)). Predicted differences in the vertical distribution of Aulacoseira sp. indicate that a large proportion of the cellular population received reduced light as a consequence of sinking in the CALM treatment. These results indicate that population growth in Aulacoseira sp. is highly sensitive to reduced light caused by sinking from the well-lit surface waters. Our microcosm experiments suggest that in the natural environment, reduced growth of Aulacoseira sp., in calm conditions in thermally stratified and/or low, flow conditions would put it at a competitive disadvantage relative to A. circinalis.