Vertical Disentrainment of Anabaena circinalis in the Turbid, Freshwater Darling River, Australia: Quantifying Potential Benefits from Buoyancy

The migration patterns of phytoplankton through time and depth were measure in the Darling River at Revallyn, New South Wales, Australia during a bloom of Anabaena circinalis. Anabaena circinalis was able to disentrain and maintain position witin surface waters during the early morning, coinciding with the diel period of least wind speeds and a state of no detectable thermocline (0.1°C detection limit). Anabaena circinalis concentrations were up to 10 times higher in the surface waters than in the bottom waters during the morning sampling periods. Afternoon and midnight sampling periods revealed either a small amount of surface accumulation or none. All other phytoplankton were found to have a relatively even distribution throughout the water column at all time periods measured (except Aulacoseira on one occasion). These vertical distribution data were used to determine the potential benefit buoyant A. circinalis could gain over an evenly distributed population using a quantitative estimate of primary productivity. The buoyant population was found to have a daily integral of photosynthetic O2 production of 3.63 mol m –2, five times greater than that for the evenly distributed population. Losses due to respiration were greater for the evenly distributed population (29.5%) than the buoyant population (4.8%), probably due to the amount of time cells spent outside the euphotic zone. It is suggested that buoyancy may offer considerable advantage to A.circinalis in gaining dominance in turbid freshwater rivers. Further, buoyancy may offer some advantage even without strong thermal gradients.