INTERACTION BETWEEN OXYGEN-TRANSFER MECHANISMS IN LAKE MODELS

Dissolved oxygen (DO) distributions in stratified lakes can be simulated by a one-dimensional (vertical), deterministic, unsteady (time-variant) diffusion equation with surface gas transfer as a boundary condition and photosynthetic oxygen production as an internal source. Both surface gas transfer and vertical diffusion depend on turbulence near the water surface. The interaction between these two mechanisms, and photosynthetic productivity of dissolved oxygen in lake models is therefore studied analytically and numerically. The analytical solution of the DO transport equation in a semiinfinite medium is developed. Predicted DO concentrations at depth z are shown to depend on three dimensionless parameters, and the strength of net photosynthetic oxygen production is an important parameter in this analysis. The increases of DO concentrations due to photosynthesis are modulated by surface aeration and vertical diffusion in the lake. A numerical solution of the dissolved oxygen transport equation is also obtained and used to illustrate simulated DO profiles over an entire open water season in two Minnesota lakes. The analytical solution is used to explain certain features of the numerical results.