What is ocean acidification?

Ocean acidification is caused by the chemical reaction of CO2 dissolving into seawater, where it immediately dissociates into carbonic acid and then into bicarbonate and hydrogen ions which decreases the pH value as shown below.
CO2 + H2O <=> H2CO3 (carbonic acid) <=> HCO3^- (bicarbonate ion) + H⁺
The dwindling level of carbonate ions means that the saturation state of calcium carbonate in the ocean also decreases as shown below.
HCO3^- (bicarbonate ion) <=> CO3^2- (carbonate) + H⁺
The calcification rate of some species such as echinoderms becomes longer due to the decreasing availability of essential ions which can leave species exposed to predators, and can also negatively impact the fertilisation of species, ultimately limiting population growth