11:00 AM - 11:15 AM
[ACG39-07] Estimation of the origin of organic matter that contributes to the coastal ocean acidification and hypoxia in Shizugawa Bay, Miyagi Prefecture,
Keywords:Ocean acidification, Hypoxia, Coastal area, Stable carbon isotope, Oyster farming
In the oceans, global warming, ocean acidification, and hypoxia caused by increased atmospheric carbon dioxide (CO2) concentrations are now considered as problems (IPCC, 2021). In coastal areas, there is also concern about local acidification and hypoxia caused by the nutrients and decomposition of organic matter from land and overuse of coastal areas (Cai et al., 2011; Yamamoto-Kawai et al., 2021). In this study, to understand the current status and to elucidate the causes of ocean acidification and hypoxia round Japan coast, here we conducted a survey in Shizugawa Bay, Miyagi Prefecture, Japan. The bay is known as a high productive ocean, where oysters, seaweed Undaria pinnatifida known as "Wakame", and chum salmon are cultivated.
Two points (Togura A and B) were established at the oyster farming site and seawater temperature, salinity, pH, and dissolved oxygen (DO) sensors were installed at the surface and bottom for continuous monitoring in 2018. In addition, water temperature, salinity, pH, DO, and Chl-a concentrations were measured vertically at 14 sites with a direct-reading multiple sensor five times during different seasons from November 2021 to December 2022. In July 2023, an additional 9 sites were observed. Surface and bottom water samples were collected from each site during summers of 2022 and 2023 for total alkalinity (TA), dissolved inorganic carbon (DIC), nutrients, 18O, δ13CDIC, particulate organic carbon concentration (POC), and δ13CPOC analyses. In addition, water samples were taken from six major rivers flowing into the bay in 2023. The origin of organic matter at each site was estimated using a mixed model with open ocean and river water as end-members to evaluate the causes of acidification and hypoxia.
As a results, a strong correlation between DO and pH was observed in the bottom of Togura B after 2020, and hypoxic seawater with DO below 2.0 mg/L and acidic seawater with pH below 7.7 were observed in the summers of 2021 and 2022. Hypoxic and acidic seawater were also observed in the bottom of Togura A, especially in the summer of 2022. Strong temperature and salinity stratification was also observed, suggesting that the DO and pH of seawater decreased due to oxygen consumption and CO2 emission by respiration and decomposition of organic matter in the bottom layer due to vertical stratification. Additionally, DO and pH tended to decrease especially in the southern part of the bay. The hydrodynamic model suggests that Tsubaki Island in the southern part of the bay may be reducing the current velocity, which may trap the water and causes accumulation of organic matter at the bottom. The δ13CPOC values was also found to differ between the east and west parts of the bay. From the mixing model the δ13C of organic matter decomposed in western and eastern water was estimated to be -27.1‰ and -20.7‰, respectively. Since the δ13C of rice and wheat, which are used as feed for the chum salmon cultivated near the western station, is approximately -27.0 to -26.0‰, it can be suggested that the organic matter derived from salmon farm is one of the causes of the hypoxia and acidification in the bottom of this area. On the other hand, δ13C of the pacific oyster feces collected in Shizugawa Bay was -20.8‰, suggesting that the organic matter from oyster farm may have influenced the formation of hypoxic water mass in the east part of the bay.
The present results indicated that acidified and hypoxic water expected at the end of this century can be already observed in summer, particularly at the bottom of southern area of Shizugawa Bay. The organic matter derived from the aquaculture of Pacific oyster and chum salmon may contribute to the oxygen consumption and CO2 increases during summer stratification. The water field within the bay was also suggested to influence the place of acidified and hypoxic water masses formation within the bay.
Two points (Togura A and B) were established at the oyster farming site and seawater temperature, salinity, pH, and dissolved oxygen (DO) sensors were installed at the surface and bottom for continuous monitoring in 2018. In addition, water temperature, salinity, pH, DO, and Chl-a concentrations were measured vertically at 14 sites with a direct-reading multiple sensor five times during different seasons from November 2021 to December 2022. In July 2023, an additional 9 sites were observed. Surface and bottom water samples were collected from each site during summers of 2022 and 2023 for total alkalinity (TA), dissolved inorganic carbon (DIC), nutrients, 18O, δ13CDIC, particulate organic carbon concentration (POC), and δ13CPOC analyses. In addition, water samples were taken from six major rivers flowing into the bay in 2023. The origin of organic matter at each site was estimated using a mixed model with open ocean and river water as end-members to evaluate the causes of acidification and hypoxia.
As a results, a strong correlation between DO and pH was observed in the bottom of Togura B after 2020, and hypoxic seawater with DO below 2.0 mg/L and acidic seawater with pH below 7.7 were observed in the summers of 2021 and 2022. Hypoxic and acidic seawater were also observed in the bottom of Togura A, especially in the summer of 2022. Strong temperature and salinity stratification was also observed, suggesting that the DO and pH of seawater decreased due to oxygen consumption and CO2 emission by respiration and decomposition of organic matter in the bottom layer due to vertical stratification. Additionally, DO and pH tended to decrease especially in the southern part of the bay. The hydrodynamic model suggests that Tsubaki Island in the southern part of the bay may be reducing the current velocity, which may trap the water and causes accumulation of organic matter at the bottom. The δ13CPOC values was also found to differ between the east and west parts of the bay. From the mixing model the δ13C of organic matter decomposed in western and eastern water was estimated to be -27.1‰ and -20.7‰, respectively. Since the δ13C of rice and wheat, which are used as feed for the chum salmon cultivated near the western station, is approximately -27.0 to -26.0‰, it can be suggested that the organic matter derived from salmon farm is one of the causes of the hypoxia and acidification in the bottom of this area. On the other hand, δ13C of the pacific oyster feces collected in Shizugawa Bay was -20.8‰, suggesting that the organic matter from oyster farm may have influenced the formation of hypoxic water mass in the east part of the bay.
The present results indicated that acidified and hypoxic water expected at the end of this century can be already observed in summer, particularly at the bottom of southern area of Shizugawa Bay. The organic matter derived from the aquaculture of Pacific oyster and chum salmon may contribute to the oxygen consumption and CO2 increases during summer stratification. The water field within the bay was also suggested to influence the place of acidified and hypoxic water masses formation within the bay.