Mechanism-based deduction of subsurface aragonite saturation state in a semi-enclosed and seasonally stratified coastal sea

The North Yellow Sea is a semi-enclosed shallow-water marginal sea of the western North Pacific. Below the seasonal thermocline, subsurface aragonite saturation state (Omega(arag)) values of 1.0-1.5 have frequently been observed in summer and autumn, indicating that marine calcifying organisms in the area are under threat. In this study, we developed mechanism-based models relating summertime and autumnal subsurface Omega(arag) to seawater temperature, practical salinity (S-P), dissolved oxygen and atmospheric CO2 concentration, taking into consideration the combined effects of atmospheric CO2 intrusion, metabolic CO2 increase, and region-specific water mixing modes. For the central offshore areas of the North Yellow Sea, we found that cold water mass Omega(arag) = 0.00885 x [(61.745 x SP + 320 - 17/138 x AOU) (DICWinter + 106/138 x AOU)] + 0.46, where DICWinter is the air-equilibrated bottom-water dissolved inorganic carbon concentration at the beginning of the year, and AOU is the apparent oxygen utilization. In the southern area that was likely influenced by the adjacent Bohai Sea water, we found that nearshore subsurface Omega(arag) = 0.00939 x [(73.245 x SP 17/138 x AOU) (DIC(Winte)r + 106/138 x AOU)] + 0.53. Subsurface Omega(arag) values reconstructed from the two formulae are in general agreement with Omega(arag) values calculated from observed dissolved inorganic carbon and total alkalinity values, with a standard deviation of 0.15 (n = 333). This is a successful attempt to reconstruct coastal subsurface Omega(arag) values from common hydrochemical data using mechanism-based models, and the approach may have applicability in similar coastal systems.

Automated summary

In this study, mechanism-based models were developed to establish relationships between summertime and autumnal subsurface Omega(arag) in the North Yellow Sea and seawater temperature, practical salinity, dissolved oxygen, and atmospheric CO2 concentration. The reconstructed subsurface Omega(arag) values from the models were in good agreement with values calculated from observed hydrochemical data, indicating potential applicability in similar coastal systems. This successful attempt provides insights into the threat posed to marine calcifying organisms in the area and the importance of understanding the impacts of environmental factors on oceanic carbonate chemistry.