Effect of cell flotation on growth of Anabaena circinalis under diurnally stratified conditions

We tested the hypothesis that the growth rate of Anabaena circinalis, under diurnally stratified conditions, would increase as flotation velocity increased owing to higher light availability. An in situ experiment compared the growth of diurnally stratified populations of A. circinalis with flotation velocities of 0.5 and 1.0 m h−1, with neutrally buoyant populations that were exposed to either mixed or persistently stratified conditions. The experiment was conducted in the turbid lower Murray River in South Australia (vertical attenuation coefficient = 4.52 ± 0.36 m−1). To represent the mixing patterns, A. circinalis was contained in diffusion chambers that were moved to different positions in the water column throughout the day. Diurnal populations with flotation velocities of 1.0 and 0.5 m h−1 grew at 0.23 ± 0.01 and 0.15 ± 0.01 day−1, respectively. Mixed populations grew at 0.19 ± 0.01 day−1, whereas persistently stratified populations grew at 0.43 ± 0.01 day−1. Results were used to extend a model that predicts growth of A. circinalis when exposed to the different mixing patterns. The model showed that blooms are unlikely to be formed when the period of diurnal stratification is 1 week, flotation velocity is important and a bloom may form depending on values assigned to the growth period and maximum mixed depth (Zm).