Answer:
If the pH of seawater decreases while pCO2 remains the same, the equilibrium between CO2, HCO3-, and CO32- will shift to compensate for the change in pH. Specifically, as pH decreases, the concentration of H+ ions increases, which will drive the reaction towards the consumption of H+ ions by HCO3- and CO32- to form carbonic acid (H2CO3):
CO2 + H2O <-> H2CO3 <-> H+ + HCO3- <-> 2H+ + CO32-
This will result in an increase in the concentrations of both HCO3- and CO32- in seawater. However, since the equilibrium constant (K) for the reaction HCO3- <-> H+ + CO32- is relatively small, the concentration of CO32- will change more than HCO3- in response to a change in pH. This is because any increase in H+ ions in the seawater will preferentially react with HCO3- to form H2CO3, which then drives the reaction to consume more CO32- to maintain the equilibrium.
Therefore, the concentration of CO32- will show a greater change than HCO3- in response to a decrease in pH, even though the concentration of both ions will increase.