2:45 PM - 3:00 PM
[ACG41-11] Development of Numerical Model for Dynamics of Partial Pressure of CO2 in Seawater Considering Effects of Stratification in the Yatsushiro Sea
Keywords:partial pressure of CO2 in seawater, air-sea CO2 flux, ecological model, blue carbon
About 55 % of the carbon fixed by living organisms on the Earth is that fixed by marine organisms, so-called "blue carbon". In particular, coastal areas with vegetation such as seagrass are considered to be important in reducing atmospheric CO2.
On the other hand, estimates of the amount of CO2 absorbed by coastal areas are currently uncertain. In general, the partial pressure of CO2 in seawater (pCO2) is used to calculate the amount of CO2 absorbed from the atmosphere to seawater. However, pCO2 in the coastal zone fluctuates greatly in space and time affected by flow, biological activities, etc. Therefore, in order to improve the accuracy of estimating the amount of CO2 absorbed in coastal areas, it is necessary to accumulate data on pCO2 based on various conditions such as flow and ecosystem (Macreadie et al., 2019).
It has also been reported by Kone et al. (2008) and others that pCO2 fluctuates with the development of stratification. However, there are few examples of field studies and model development focusing on the relationship between stratification and pCO2.
In this study, we conducted the following to obtain basic knowledge on CO2 dynamics in coastal areas. First, we conducted a field survey on pCO2 under three stratification conditions. Then, based on the survey results, a numerical model that can reproduce the pCO2 dynamics under each stratification condition was developed. Furthermore, using the model, we made an attempted to elucidate the spatio-temporal variability of CO2 absorption in coastal areas. The Yatsushiro Sea, the target area of this study, is characterized as a temperate area where both seagrasses and corals live.
The survey sites were selected near the mouth of the Kuma River, where strong stratification could develop. The survey dates were August 26, 2018 (weak stratification period), December 7, 2018 (mixed period), and August 2, 2019 (strong stratification period). Measured items were vertical distribution of water temperature, salinity, etc. from water quality measurement, and the dissolved inorganic carbon concentration (DIC) and total alkalinity (TA) from water samples. pCO2 was calculated from water temperature, salinity, DIC, and TA from the chemical equilibrium relationship of the carbon system.
A numerical model of pCO2 was developed using Delft3D, a general-purpose numerical model for coastal flow. The model consists of a flow model and an ecological model. The horizontal resolution is about 250 m, and the vertical resolution is 17 layers in the σ- coordinate system. The components of the ecological model were phytoplankton, nutrients, particulate organic matter, dissolved oxygen DO, DIC and TA. The main processes are photosynthesis, respiration, and death of phytoplankton, sedimentation and mineralization of organic matter, nitrification, and re-aeration.
The vertical fluctuation of pCO2 with stratification development was revealed from the field survey. Based on the observations, a numerical model of pCO2 dynamics was developed. The model was able to reproduce the vertical distribution of pCO2 under each stratification conditions. The model results confirmed that the CO2 sinks around the mouth of the river fluctuated significantly in space and time with the river discharge. In addition, the following two effects of saline stratification on CO2 absorption in coastal areas were confirmed; (1) After a flood, the saline stratification develops and river water concentrates in the surface layer, which accelerates the release of CO2 from seawater. (2) As mixing progresses and the saline stratification weakens, a large layer of phytoplankton appears on the surface, and CO2 absorption into the seawater is promoted by photosynthesis.
In the future, it is necessary to investigate the effects of seagrass and coral inhabitation, through field observations and to incorporate these effects into the model. These studies are expected to contribute to the understanding of blue carbon dynamics in coastal areas.
On the other hand, estimates of the amount of CO2 absorbed by coastal areas are currently uncertain. In general, the partial pressure of CO2 in seawater (pCO2) is used to calculate the amount of CO2 absorbed from the atmosphere to seawater. However, pCO2 in the coastal zone fluctuates greatly in space and time affected by flow, biological activities, etc. Therefore, in order to improve the accuracy of estimating the amount of CO2 absorbed in coastal areas, it is necessary to accumulate data on pCO2 based on various conditions such as flow and ecosystem (Macreadie et al., 2019).
It has also been reported by Kone et al. (2008) and others that pCO2 fluctuates with the development of stratification. However, there are few examples of field studies and model development focusing on the relationship between stratification and pCO2.
In this study, we conducted the following to obtain basic knowledge on CO2 dynamics in coastal areas. First, we conducted a field survey on pCO2 under three stratification conditions. Then, based on the survey results, a numerical model that can reproduce the pCO2 dynamics under each stratification condition was developed. Furthermore, using the model, we made an attempted to elucidate the spatio-temporal variability of CO2 absorption in coastal areas. The Yatsushiro Sea, the target area of this study, is characterized as a temperate area where both seagrasses and corals live.
The survey sites were selected near the mouth of the Kuma River, where strong stratification could develop. The survey dates were August 26, 2018 (weak stratification period), December 7, 2018 (mixed period), and August 2, 2019 (strong stratification period). Measured items were vertical distribution of water temperature, salinity, etc. from water quality measurement, and the dissolved inorganic carbon concentration (DIC) and total alkalinity (TA) from water samples. pCO2 was calculated from water temperature, salinity, DIC, and TA from the chemical equilibrium relationship of the carbon system.
A numerical model of pCO2 was developed using Delft3D, a general-purpose numerical model for coastal flow. The model consists of a flow model and an ecological model. The horizontal resolution is about 250 m, and the vertical resolution is 17 layers in the σ- coordinate system. The components of the ecological model were phytoplankton, nutrients, particulate organic matter, dissolved oxygen DO, DIC and TA. The main processes are photosynthesis, respiration, and death of phytoplankton, sedimentation and mineralization of organic matter, nitrification, and re-aeration.
The vertical fluctuation of pCO2 with stratification development was revealed from the field survey. Based on the observations, a numerical model of pCO2 dynamics was developed. The model was able to reproduce the vertical distribution of pCO2 under each stratification conditions. The model results confirmed that the CO2 sinks around the mouth of the river fluctuated significantly in space and time with the river discharge. In addition, the following two effects of saline stratification on CO2 absorption in coastal areas were confirmed; (1) After a flood, the saline stratification develops and river water concentrates in the surface layer, which accelerates the release of CO2 from seawater. (2) As mixing progresses and the saline stratification weakens, a large layer of phytoplankton appears on the surface, and CO2 absorption into the seawater is promoted by photosynthesis.
In the future, it is necessary to investigate the effects of seagrass and coral inhabitation, through field observations and to incorporate these effects into the model. These studies are expected to contribute to the understanding of blue carbon dynamics in coastal areas.