5:15 PM - 7:15 PM
[AOS13-P01] Estimating long-term variation of pCO2 in the Seto Inland Sea
Keywords:carbon dioxide, CO2 fluxes, partial pressure of CO2 in seawater, random forest
Studies on carbon dioxide (CO2) in coastal waters are increasing, especially related to problems such as ocean acidification and blue carbon. As much as the open ocean, the amount of sea-air CO2 exchange in coastal regions has been estimated, however that is still much uncertainty. Thus, it is necessary to estimate in each coastal area. The environment in the Seto Inland Sea has changed significantly in the last half century, shifting from eutrophic to oligotrophic. In this study, we estimated the long-term variation of pCO2 (partial pressure of CO2 in seawater) in the Seto Inland Sea.
We collected data during research cruises on Fukaemaru of the Graduate School of Maritime Science, Kobe University. These cruises were conducted winter (March) and Summer (July, August, September) from 1994 to 2010. We measured pCO2, T (Seawater temperature, ℃), S (Salinity), DO (Dissolved Oxygen, mg/L), and pH from intake water 3m depth from sea surface. Additionally, we measured pCO2 air (partial pressure of CO2 in atmosphere) and U (wind speed) from samples taken at a height of 10m from the surface. We built estimation models by random forest, which estimate pCO2 from T, S, pH and DO, with about 5 to 14% of the estimation error. We estimated pCO2 by inputting data from the Seto Inland Sea Comprehensive Water Quality Survey and Regional Comprehensive Water Quality Survey, with winter (January to March), spring (April to June), summer (July to September) and autumn (October to December). Using estimated pCO2, averaging year and season in each area, we analysed its long-term variation. We divided the region into 6 areas, Hiuchi Nada, Bisan Seto, Harima Nada, western Osaka Bay, eastern Osaka Bay and Kii channel.
Figure 1 shows the long-term variation of pCO2 for each area. pCO2 had increased in Hiuchi Nada, Bisan Seto, western Osaka Bay, and Kii Channel. In summer, pCO2 had increased in all areas except eastern Osaka Bay, with the highest rate of increase. As the CO2 solubility decreases with increasing water temperature, the pCO2 tends to increase. Water temperature showed an increasing trend in all areas, with an annual increase of 0.02-0.05 ℃ y-1. Using the coefficient of the relationship between pCO2 and seawater temperature (Yamashita et al., 1993), we estimated the contributions of seawater temperature and other factors. In Hiuchi Nada and Kii Channel, the contribution of seawater temperature was larger, suggesting its significant impact. Especially in summer, its influence was relatively small in Bisan Seto, Harima Nada, and western Osaka Bay. For other factors, we considered such as primary production and COD in bottom layer.
Chl.a, which was used as an indicator of primary production, dereased in all regions except Hiuchi Nada and Harima Nada, with the steepest decrease in Osaka Bay. In western and eastern part was approximately four times, and ten times greater than that in Bisan Seto respectively. This suggests that the declining primary production led to reduced CO2 consumption, contributing to rising pCO2, especially in western Osaka Bay.
COD in bottom layer increased in all areas except eastern Osaka Bay. From spring to summer, Hiuchi Nada and Harima Nada tend to be stratified in summer, and hypoxic water masses are generated by organic matter decomposition in their bottom layers, suggesting the presence of high CO2 water masses (Fujita et al., 2024; Taguchi and Fujiwara, 2010). Due to the increase of COD in bottom layer, CO2 in there was considered to have increase. Having transported that water masses to the surface layer of Bisan Seto and both Nada areas through the bottom layer of Bisan Seto, pCO2 in there increased.
Calculations of CO2 flux showed a significant downward trend in all areas. Hiuchi Nada, Harima Nada and Kii Channel, which were CO2 sources in the 1980s, became CO2 sinks after 2000s. This suggests that the Seto Inland Sea has gradually shifted toward absorbing more CO2 over time.
We collected data during research cruises on Fukaemaru of the Graduate School of Maritime Science, Kobe University. These cruises were conducted winter (March) and Summer (July, August, September) from 1994 to 2010. We measured pCO2, T (Seawater temperature, ℃), S (Salinity), DO (Dissolved Oxygen, mg/L), and pH from intake water 3m depth from sea surface. Additionally, we measured pCO2 air (partial pressure of CO2 in atmosphere) and U (wind speed) from samples taken at a height of 10m from the surface. We built estimation models by random forest, which estimate pCO2 from T, S, pH and DO, with about 5 to 14% of the estimation error. We estimated pCO2 by inputting data from the Seto Inland Sea Comprehensive Water Quality Survey and Regional Comprehensive Water Quality Survey, with winter (January to March), spring (April to June), summer (July to September) and autumn (October to December). Using estimated pCO2, averaging year and season in each area, we analysed its long-term variation. We divided the region into 6 areas, Hiuchi Nada, Bisan Seto, Harima Nada, western Osaka Bay, eastern Osaka Bay and Kii channel.
Figure 1 shows the long-term variation of pCO2 for each area. pCO2 had increased in Hiuchi Nada, Bisan Seto, western Osaka Bay, and Kii Channel. In summer, pCO2 had increased in all areas except eastern Osaka Bay, with the highest rate of increase. As the CO2 solubility decreases with increasing water temperature, the pCO2 tends to increase. Water temperature showed an increasing trend in all areas, with an annual increase of 0.02-0.05 ℃ y-1. Using the coefficient of the relationship between pCO2 and seawater temperature (Yamashita et al., 1993), we estimated the contributions of seawater temperature and other factors. In Hiuchi Nada and Kii Channel, the contribution of seawater temperature was larger, suggesting its significant impact. Especially in summer, its influence was relatively small in Bisan Seto, Harima Nada, and western Osaka Bay. For other factors, we considered such as primary production and COD in bottom layer.
Chl.a, which was used as an indicator of primary production, dereased in all regions except Hiuchi Nada and Harima Nada, with the steepest decrease in Osaka Bay. In western and eastern part was approximately four times, and ten times greater than that in Bisan Seto respectively. This suggests that the declining primary production led to reduced CO2 consumption, contributing to rising pCO2, especially in western Osaka Bay.
COD in bottom layer increased in all areas except eastern Osaka Bay. From spring to summer, Hiuchi Nada and Harima Nada tend to be stratified in summer, and hypoxic water masses are generated by organic matter decomposition in their bottom layers, suggesting the presence of high CO2 water masses (Fujita et al., 2024; Taguchi and Fujiwara, 2010). Due to the increase of COD in bottom layer, CO2 in there was considered to have increase. Having transported that water masses to the surface layer of Bisan Seto and both Nada areas through the bottom layer of Bisan Seto, pCO2 in there increased.
Calculations of CO2 flux showed a significant downward trend in all areas. Hiuchi Nada, Harima Nada and Kii Channel, which were CO2 sources in the 1980s, became CO2 sinks after 2000s. This suggests that the Seto Inland Sea has gradually shifted toward absorbing more CO2 over time.