What is ocean acidification??

When carbon dioxide (CO₂) from different sources like industries,different human activities is absorbed by seawater, chemical reactions occur that reduce seawater pH, carbonate ion concentration, and saturation states of biologically important calcium carbonate minerals. These chemical reactions are termed "ocean acidification" or "OA" for short. Calcium carbonate minerals are the building blocks for the skeletons and shells of many marine organisms. In areas where most life now congregates in the ocean, the seawater is supersaturated with respect to calcium carbonate minerals. This means there are abundant building blocks for calcifying organisms to build their skeletons and shells. However, continued ocean acidification is causing many parts of the ocean to become under saturated with these minerals, which is likely to affect the ability of some organisms to produce and maintain their shells.

How does it affect Ecosystem ?

Any organism in ocean ecosystem is source of food of other any organism in same ecosystem.Any change in population of any organism in ecosystem whether it is producers of consumer directly affects the food web of that ecosystem. Eg :Coral reefs host an abundant and diverse array of marine life. Corals themselves are living organisms on which the whole ecosystem depends. Coral reefs host an abundant and diverse array of marine life. Corals themselves are living organisms on which the whole ecosystem depends.ecosystem services provided by coral reefs are fish to eat, protection of coastal areas from storms, and tourism activities like snorkeling and scuba diving.Human economies rely on ecosystem services provided by healthy oceans and coasts, but ocean and coastal acidification threaten these services.

This can represented by figure below.

https://www.epa.gov/sites/production/files/2016-09/shellfish-fig-1-hires_0.png