Aquatic Biogeochemical Eddy Covariance Fluxes in the Presence of Waves

Abstract

The eddy covariance (EC) technique is a powerful tool for measuring atmospheric exchange rates that was recently adapted by biogeochemists to measure aquatic oxygen fluxes. A review of aquatic biogeochemical EC literature revealed that the majority of studies were conducted in shallow waters where waves were likely present, and that waves biased sensor and turbulence measurements. This review identified that larger measurement heights shifted turbulence to lower frequencies, producing a spectral gap between turbulence and wave frequencies. However, some studies sampled too close to the boundary to allow for a spectral turbulence-wave gap, and a change in how EC measurements are conducted and analyzed is needed to remove wave-bias. EC fluxes have only been derived from the time-averaged product of vertical velocity and oxygen, often resulting in wave-bias. Presented is a new analysis framework for removing wave-bias by accumulation of cross-power spectral densities below wave frequencies. This analysis framework also includes new measurement guidelines based on wave period, currents, and measurement heights. This framework is applied to sand, seagrass, and reef environments where traditional EC analysis resulted in wave-bias of 7.0% ± 9.2% error in biogeochemical (oxygen and H+) fluxes, while more variable and higher error was evident in momentum fluxes (10.5% ± 21.0% error). It is anticipated that this framework will lead to significant changes in how EC measurements are conducted and evaluated, and help overcome the major limitations caused by wave-sensitive and slow-response sensors, potentially expanding new chemical tracer applications and more widespread use of the EC technique.