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