An initial study on ocean acidification in Southern waters of Vietnam

This process is the result of large quantities of CO2, produced by vehicles and industrial and agricultural activities. The burning of fossil fuels and destruction of forests by humans have been mentioned as the main reasons to increase the partial pressure of CO2 in the atmosphere. The level of CO2 in the atmosphere increased about 40%, from 280 ppm (pre-industrial period) to 384 ppm (2007). In addition, it is estimated that about 29% (170 ± 20 GtC) of atmospheric CO2 emissions were absorbed by the oceans from 1750–2013. The seawater carbonate system is governed by a series of chemical reactions: CO2 (gas) ↔ CO2 (aqua) + H2O ↔ H2CO3 ↔ H+ + HCO3- ↔ 2H+ + CO32-. Once dissolved in water, gaseous CO2 reacts with water to form carbonic acid H2CO3, which can dissociate by losing hydrogen ions, to form bicarbonate (HCO3-) and carbonate (CO32-) ions. Adding CO2 in seawater will increase aqueous CO2, bicarbonate, and hydrogen ion concentrations, and reduce pH value and carbonate ion concentrations. The calcification plays an important role in the coral reef construction, as well as the growth of other calcareous organisms. The calcification depends on the aragonite saturation state (Ωar), defined as the ion product of calcium and carbonate ion concentrations: Ω = [Ca2+] [CO32-]/Ksp, where Ksp is the solubility coefficient of aragonite. When Ω > 1, shell and skeleton formation occurs, and dissolution occurs when Ω < 1. The dissolved CO2 concentrations increase to decrease pH value and aragonite saturation state, it can be expressed by the equation: CO2 + CaCO3 + H2O ↔ 2HCO3- + Ca2+. Ocean acidification can reduce the calcification rate of coral reefs, as well as increase bioerosion and dissolution of CaCO3 structures, impact the growth of calcareous organisms. Some studies analyzed the cores of the Great Barrier reef and showed that the calcification rate decreased by 21% in the period 1988–2003. In addition, the study of Gazeau et al., (2007) suggested that the calcification rate of Mytilus edulis and the Pacific oyster Crassostrea gigas decreased by 25% and 10%, respectively, with the CO2 level ⁓ 740 ppmv. Samples were collected at 20 stations from Khanh Hoa to Ca Mau, all stations are shown in figure 1. Water samples were collected at 2 layers, the upper layer was 5 m below the surface, the lower layer was at 20, 30, 40 or 60 m depending on the depth of stations. Totally, 40 samples were collected and analyzed. The pH of seawater was measured by a high accuracy method, using the indicator dye m-cresol purple; TA determination used open-cell titration method; bicarbonate (HCO3-), carbonate (CO32-) ions, dissolved CO2 concentrations, aragonite saturation values (Ω) were calculated by the use of CO2sys software; temperature and salinity values were in situ measured by Seabird CTD (SBE 19+ V2). The data showed that in transection 2, seawater temperature, total alkalinity (TA) and carbonate ion concentrations tended to decrease from the coast to offshore stations whereas salinity, HCO3- and CO2 concentrations illustrated an opposite trend. In this transection, the highest values of pCO2 and CO2 concentration and the lowest value of CO32- concentration were recorded at station 25. Besides, Ωara values were higher at the lower layer, at the offshore stations. In transection 3, values of temperature, salinity, pH ranged slightly. Distributions of HCO3-, and CO32- concentrations were similar, increased at the middle stations (35, 36, 37), and decreased at the offshore stations (38, 39). The trend of CO2 concentration and pCO2 had the same patterns. Those parameters had high levels at the onshore areas, decreased at the offshore areas. Most of CO2 concentrations were greater than 10 µmol/kgSW, the smallest value (8.6 µmol/kgSW) was found at station 39. The Ωar values were always higher than 3, and increased at the offshore areas, the highest value was found at station 39. The mean values of pH, total alkalinity (TA), and CO32- concentrations were 8.04 (7.92–8