Fate of Artificially Injected Oxygen in the Hypolimnion of a Two-Basin Lake: Amisk Lake, revisited

Abstract

Bubble-plume diffusers are increasingly used to add dissolved oxygen (DO) to the hypolimnion of lakes and reservoirs. Bubble plumes are successful at replenishing hypolimnetic DO, but they also introduce mixing energy that induces subtle changes in the thermal structure of the reservoir, driving changes in plume behavior. To account for this complex plume-reservoir interaction, a double bubble-plume model is coupled with a three-dimensional hydrodynamic model. The coupled model is used to reassess a field-scale analysis of the bubble-plume diffuser in two-basin Amisk Lake, aiming at evaluating the relative role of bubble-induced circulation and internal-seiching in driving inter-basin transport under stratified conditions. A large-scale plume-induced circulation was previously thought to be the main driver of inter-basin oxygen transport. This interpretation was based on the attribution of the time-averaged circulation in the channel due to plume operation. However, the intrinsic complexity of the hydraulic system and the sparseness of the field data introduced large uncertainties in the previous analysis. Here, we demonstrate that the time-averaged circulation is primarily the result of wind-driven internal seiches. Oxygen exchange is shown to be controlled by the interaction between internal seichedriven horizontal transport along the channel, and, the rate at which added oxygen reaches the layers above the sill, which is mainly controlled by plume-induced circulation. Internal-seiche driven transport through basin constrictions will vary depending on the magnitude of the wind forcing, depth of the thermocline and the channel geometry. These results highlight the importance of understanding water movement prior to introducing restoration actions in lakes.