Air-Sea CO2 exchange and dissolved inorganic carbon distribution in an inner part of Osaka Bay, Japan

The carbon captured by coastal oceans and ecosystems called “blue carbon” is attracting attention to reduce greenhouse gas. Recently, field surveys on air-sea CO2 exchange have been conducted in various coastal seas to reveal the blue carbon function. In this study, we investigated the vertical distribution of the dissolved inorganic carbon (DIC) system in the inner part of Osaka Bay to reveal the air-sea CO2 exchange characteristics in port and harbor area. Field surveys were conducted on a vertical seawall in Naruo-hama of Osaka Bay, Japan, during 12 months from November 2014 to October 2015 and during summer from August to September 2016. The vertical distributions of DIC and pH were measured on site, and the CO2 partial pressure in water (pCO2w) was calculated based on carbon acid equilibrium. Water temperature, salinity and DO were also measured to examine the relationships the vertical distribution of DIC system and water mass structure. Furthermore, the air-sea CO2 flux (FCO2) was estimated to examine the characteristics of the air-sea CO2 exchange. DIC ranged from 787 to 2,111 μmol kg−1, with seasonal and vertical change. DIC in the bottom layer increased during high water temperature period due to CO2 production by respiration and decomposition, and DIC accumulated along with formation density stratification. The correlation between DIC and pCO2w in surface layer was smaller than that in bottom layer because surface pCO2w fluctuated due to inflow of river water or seawater mixing. This study area acts as CO2 sinks under calm conditions (FCO2 = -3.4 mmol-C m−2 d−1), however the site acts CO2 sources (1) when density stratification was relaxed (FCO2 = 2.1 mmol-C m−2 d−1); (2) when water was mixed by disturbance such as typhoon (FCO2 = 1.3 mmol-C m−2 d−1); and (3) when it rained (FCO2 = 15.5 mmol-C m−2 d−1).

Endo T., Shimano J., Harada N., Sakai D. & Fujiwara R., 2019. Air-Sea CO2 exchange and dissolved inorganic carbon distribution in an inner part of Osaka Bay, Japan. International Conference on Asian and Pacific Coasts APAC 2019: 1073-1079. Article (subscription required).

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