This is because when carbon dioxide dissolves in the ocean, it forms an acid (H2CO3), thus a decrease in the pH of the ocean waters.
The reaction takes place as shown;
CO2(g) + H2O(l) = H2CO3(aq)
Carbon iv oxide reacts with water to form carbonic acid which is a weak acid, therefore increasing the amount of CO2 dissolved in ocean water results to an increase of more carbonic acid formed which results to an increase in the acidity of the waters (low pH).
