In situ measurements of seawater pCO₂ using sightseeing boat

Abstract

In this paper, we report the results of measuring pCO₂ (partial pressure of carbon dioxide) in seawater around Tomari wharf, Naha Port using the sightseeing boats “Marine Star” and “Harmony” owned by Marine Tourism Development Co., Ltd.. The measuring device used was a portable NDIR-based carbon dioxide measuring device, which used non-dispersive infrared absorption (NDIR) to measure pCO₂ in seawater and the atmosphere. During observations conducted on February 15, 2022, confirmed that seawater pCO₂ was approximately 400 ppm at the Tomari wharf, but it dropped to 328 ppm directly above the soft coral colony area in the Naha port. This suggests that soft corals absorb CO₂ from seawater. However, spatiotemporal fluctuations in seawater pCO₂ are thought to be influenced by many environmental factors, including day and night photosynthesis and respiration processes, solar radiation, freshwater inflow, tidal advection and diffusion, and the stratified structure of water masses. Future research will require continuous observation and data analysis to clarify the relationship between these factors and seawater pCO₂, and to more accurately calculate the amount of CO₂ absorbed by soft corals. In order to accurately assess the carbon reduction potential of blue carbon ecosystems, it is important to conduct highly accurate pCO₂ measurements on a broad temporal and spatial scale, which forms the basis for estimating CO₂ absorption between the atmosphere and the ocean.