Inland lakes play important roles in water and greenhouse gas cycling in the environment. This study aims to test the performance of a flux-gradient system for simultaneous measurement of the fluxes of water vapor, CO₂, and CH₄ at a lake–air interface. The concentration gradients over the water surface were measured with an analyzer based on the wavelength-scanned cavity ring-down spectroscopy technology, and the eddy diffusivity was measured with a sonic anemometer. Results of a zero-gradient test indicate a flux measurement precision of 4.8 W m^–2 for water vapor, 0.010 mg m^–2 s^–1 for CO₂, and 0.029 μg m^–2 s^–1 for CH₄. During the 620 day measurement period, 97%, 69%, and 67% of H₂O, CO₂, and CH₄ hourly fluxes were higher in magnitude than the measurement precision, which confirms that the flux-gradient system had adequate precision for the measurement of the lake–air exchanges. This study illustrates four strengths of the flux-gradient method: (1) the ability to simultaneously measure the flux of H₂O, CO₂, and CH₄; (2) negligibly small density corrections; (3) the ability to resolve small CH₄ gradient and flux; and (4) continuous and noninvasive operation. The annual mean CH₄ flux (1.8 g CH₄ m^–2 year^–1) at this hypereutrophic lake was close to the median value for inland lakes in the world (1.6 g CH₄ m^–2 year^–1). The system has adequate precision for CH₄ flux for broad applications but requires further improvement to resolve small CO₂ flux in many lakes.

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