Blue Carbon is the carbon taken up by marine organisms and stored in the ocean. Shallow coastal ecosystems store approximately 80% of the total carbon stored in marine sediments. On the other hand, the fugacity of carbon dioxide (fCO2) in shallow coastal waters fluctuates due to the temporal variations in environmental conditions and biological activities. Shallow coastal waters, thus, can be both sources and sinks of CO2. However, because fCO2 has not been sufficiently measured in shallow coastal waters, the comprehensive analyses and prediction methods have not been established. Hence, the objectives of this study were to statistically extract important environmental factors that regulate the fCO2 in shallow coastal waters and to estimate the current fCO2 dynamics.
Field observations were conducted from 2010 to 2015 to obtain data on the fCO2 and environmental factors in various ecosystems. Generalized linear models were constructed to estimate the fCO2 in seawater using the dataset. Path analyses were applied to the entire data set to infer the causal relationship between the fCO2 values and the environmental factors. We found that the air–water CO2 fluxes were mainly affected by (1) wind speed, (2) allochthonous inputs, and (3) biological processes. In particular, the fCO2 was strongly related to the indices of biological production (ΔDIC) and calcification (ΔTA). These results suggest that respiration, decomposition, and calcification mainly increase the fCO2, and photosynthesis mainly decreases the fCO2.
The statistical models developed in this study can be used to estimate the fCO2 in shallow coastal waters where the effects of physical and biological processes can be quantified by setting appropriate boundary conditions between land and sea. Our findings help planning the conservation, restoration, and creation of shallow coastal ecosystems for enhancing their function of CO2 uptake.

Key Words: blue carbon, CO2 uptake, seagrass meadows, tidal flats, coral reefs