Hot dose it happen
Ocean acidification is actually caused by the increase of carbon dioxide level in the atmosphere. After entering into ocean CO2 rapidly goes through a series of chemical reactions which increase the acidity of the surface seawater ( lowering pH ). The ocean has already removed about 30% of anthropogenic CO2 over the last 250 years, decreasing pH at a rate not seen for around 60 million years.
Source : Hot, Sour & breathless-ocean under stress
Ocean Acidification reduced : calcification, growth and the reproduction rates in many species ( it change the ecosystem ). It change also to the carbon and nitrogen composition of organic material. To summarise, during this period there is less life ( reproduction and growth ), it's more difficult for fish and other animals of the sea to feed theirselves ( less food chain ). Change the biodiversity.
Source : The Ocean Stress Guide