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 d in the Darling River at Trevallyn, New South Wales, Australia during a bl oom of Anabaena circinalis. Anabaena circinalis was able to disentrain and maintain position within surface waters during the early morning, coincidin g with the diel period of least wind speeds and a state of no detectable th ermocline (0.1 degreesC detection limit). Anabaena circinalis concentration s were up to 10 times higher in the surface waters than in the bottom water s during the morning sampling periods Afternoon and midnight sampling perio ds revealed either a small amount of surface accumulation or none. All othe r phytoplankton were found to have a relatively even distribution throughou t the mater column at all time periods measured (except Aulacoseira on one occasion). These vertical distribution data were used to determine the pote ntial benefit buoyant A. circinalis could gain over an evenly distributed p opulation using a quantitative estimate of primary productivity. The buoyan t population was found to have a daily integral of photosynthetic O-2 produ ction of 3.63 mol m(-2), five times greater than that for the evenly distri buted population. Losses due to respiration were greater for the evenly dis tributed population (29.5%) than the buoyant population (4.8%), probably du e to the amount of time cells spent outside the euphotic zone. It is sugges ted that buoyancy may offer considerable advantage to A. circinalis in gain ing dominance in turbid freshwater rivers. Further, buoyancy may offer some advantage even without strong thermal gradients.