Lower pH value (more acidic) affects the amount of calcium carbonate in the water of coral reefs. This has consequences on the coral skeletons, because they are made of calcium carbonate. It can be a huge problem (since the coral reefs are resource of food and tourism), it is going to be harder to them to calcify or they could even dissolve.
So, decreased pH could decrease biodiversity of the coral reefs if 'ocean acidification' continues due to absorption of atmospheric carbon dioxide.