Transitions between Aulacoseira and Anabaena dominance in a turbid river Weir pool

The transitions between the diatoms Aulacoseira spp. (Melosira) and the cya nobacteria Anabaena spp, as dominant phytoplankton species in a turbid-rive r weir pool are shown to depend directly on the establishment or destructio n of persistent thermal stratification. A transition from high to low flow through the pool resulted in the establishment of persistent thermal strati fication, causing Aulacoseira to sink out of the euphotic zone at a speed o f 0.95 m d(-1). Concurrently, the slightly buoyant Anabaena grew within the euphotic zone with a specific growth rate of 0.37 d(-1), climaxing after a pproximately 14 d at a population of 20,000-30,000 cells ml(-1), at which p oint its biomass may have been limited by the availability of phosphorus. T he stratification thus caused the phytoplankton population to separate into two distinct layers, with Anabaena occupying the illuminated surface layer and Aulacoseira found only in the lower layer below the euphotic depth. Un der stratified conditions, the ratio of the surface layer depth to euphotic depth, z(s1):z(eu) was approximately 1, whereas for a mixed water column t hat ratio was >3. Access to light appeared to be the main factor determinin g the dominant phytoplankton species.