Red Tide Events and Seasonal Variations in the Partial Pressure of CO2 and Related Parameters in Shellfish-Farming Bays, Southeastern Coast of Korea

Mixed results have been reported on the evaluation of the coastal carbon cycle and its contribution to the global carbon cycle, mainly due to the shortage of observational data and the considerable spatiotemporal variability arising from complex biogeochemical factors. In this study, the partial pressure of carbon dioxide ( p CO 2 ) and related environmental factors were measured in the Jinhae–Geoje–Tongyeong bay region of the southeastern Korean Peninsula in February 2014, August 2014, April 2015, and October 2015. The mean p CO 2 of surface seawater ranged from 215 to 471 μatm and exhibited a high correlation with the surface seawater temperature when data for August were excluded ( R 2 = 0.69), indicating that the seasonal variation in CO 2 could be largely attributed to the variation in seawater temperature. However, a severe red tide event occurred in August 2014, when the lowest p CO 2 value was observed despite a relatively high seawater temperature. It is considered that the active biological production of phytoplankton related to red tides counteracted the summer increase in p CO 2 . Based on the correlation between p CO 2 and temperature, the estimated decrease in p CO 2 caused by non-thermal factors was approximately 200 μatm. During the entire study period, the air–sea CO 2 flux ranged from −14.2 to 3.7 mmol m –2 d –1 , indicating that the study area served as an overall sink for atmospheric CO 2 , and only functioned as a weak source during October. The mean annual CO 2 flux estimated from the correlation with temperature was −5.1 mmol m –2 d –1 . However, because this estimate did not include reductions caused by sporadic events of biological production, such as red tides and phytoplankton blooms, the actual uptake flux is considered to be higher. The mean saturation state (Ω Ar ) value of carbonate aragonite was 2.61 for surface water and 2.04 for bottom water. However, the mean Ω Ar of bottom water was