Gas exchange in San Francisco Bay

Abstract

Gas exchange across the air-water interface is one of the most important processes controlling the concentrations of dissolved gases in estuarine systems. A brief review of principles and equations to predict gas exchange indicates that both current shear and wind shear are possible sources of turbulence for controlling gas exchange rates in estuaries. Rates of exchange determined by constructing a mass balance for radon-222 indicate that wind shear is dominant in San Francisco Bay. Because many estuaries have wind shear and current speeds comparable to this system, this conclusion may be true for other systems as well. A compilation of gas exchange rates measured in San Francisco Bay with those for other wind-dominated systems updates previous compilations and yields an equation for predicting gas exchange: K _l = 34.6 R _v (D_m20)^1/2 (U₁₀)^1.5 where R_ν is the ratio of the kinematic viscosity of pure water at 20° C to the kinematic viscosity of water at the measured temperature and salinity, D_m20 is the molecular diffusivity of the gas of interest at 20°C in cm² s⁻¹, U₁₀ is the wind speed at 10 meters above the surface in m s⁻¹, and K_L is the liquid phase gas transfer coefficient in m d⁻¹. This relationship fits the available field data within 20% for wind speeds between 3 and 12 m s⁻¹. It is used to show that the residence time of dissolved oxygen in San Francisco Bay should range from 2 days during windy summer periods to as much as 15 days during calm winter periods. Because these times are short compared to time constants for other processes controlling oxygen distribution in this system, dissolved oxygen concentrations in San Francisco Bay are usually near atmospheric equilibrium. Other systems, such as Chesapeake Bay, may differ. There, despite ample air-water gas exchange rates, some bottom waters become anoxic during summer months due to slow vertical mixing.