Evaluation of the eddy covariance method for the assesment of coastal CO2 flux

The air-water CO₂ flux in coastal regions should be an important factor for the mitigation initiative　of the climate change. However, the measurement method of coastal air-water CO₂ flux has not been established due to the large temporal and spatial fluctuations of the aquatic conditions. For example, the bulk formula method, which requires the measurement of water pCO­₂, is difficult to evaluate coastal air-water CO₂ flux comprehensively because the continuous and wide-range measurement of water pCO₂ takes a lot of work and time. This problem is also applied to other existing methods (e.g., the floating chamber method). On the other hand, the eddy covariance method, which has been applied to the air-land scalar flux, is suitable to the coastal measurement due to the large footprint and the ease of the long-time measurement. However, the application of the eddy covariance method in water areas has proved to be challenging. Previous measurement has indicated to be one or two order of magnitude larger than other methods such as the bulk formula method or the floating chamber method. Although some studies analyzed the reason of the difference and proposed the correction method, there is still controversy about the explanation of the measurement bias. Here, we aim to evaluate the eddy covariance method as the coastal CO₂ flux measurement method using the continuous measurement data at a seagrass meadow (Furen lagoon, Japan). The multi-point measurement by the bulk formula method was also performed in order to examine the effect caused by the coastal heterogeneous condition. Although the average and the deviation of the eddy covariance measurement was different from the estimate of the bulk formula method, some time series variations of the eddy covariance method were consistent with the bulk formula measurements. In this study, we focused to the fluctuation of the air pCO₂  as a parameter for flitering the CO₂ flux bias, and considered the assessment procedure of the eddy covariance method as coastal air-water CO₂ flux measurement.