The Effects of Surface Mixers on Stratification, Dissolved Oxygen, and Cyanobacteria in a Shallow Eutrophic Reservoir

Top-down surface mixers are increasingly used in drinking water reservoirs to prevent the development of stratification, control cyanobacteria, and limit sediment release soluble manganese. A targeted field investigation enabled discrimination artificial mixing by from wind convection a shallow (6.6 m), eutrophic reservoir. were effective at reducing vertical temperature dissolved oxygen gradients over 20 m radius, within which turbulent kinetic energy (TKE) input exceeded maximum TKE contribution convection. Meteorological conditions appeared have stronger influence beyond 60 radius mixers. Near-bed velocities measured using an Acoustic Doppler Velocimeter (ADV) ∼ 30 north significantly lower when not operating; operating, ADV signal amplitude showed localised resuspension. Cyanobacteria cell counts high throughout reservoir but low-light adapted Planktothrix sp. highest near mixers, indicating mixer operation may improve growing for Planktothrix. While destratification goal was accomplished locally, limited range left >90% subject diurnal anoxia, potential internal loading inorganic nutrients metals, restricting effectiveness as in-reservoir management technique raw quality systems.