Shorter than a tweet, but an excellent review of ocean acidification from the National Academy.
Bio types can follow on to the next chapter.
Of course, this arose in one of those Dunning-Krugar things about acidification of the oceans, like how can you talk about acidification when then pH of the oceans is ~8. Eli has had some words about that in the past (see also the comments by Kenneth Johnson and Bernard J).
The root of teh probelm dates back to Arrhenius (yes, him again) who was the first to define the chemical basis of acids and bases. Since the Earth is a water world and in 1884 organic chemistry was the wild west and the properties of ions in solutions newfangled stuff that would win him a Nobel Prize in 1903, Svante A defined an acid as a source of H+ ions and a base as a source of OH- ions. The problem is that there are other ions which are the sources of both via the hydrolysis of water. For example, if you dump a bunch of carbonate (CO3)2- ions into water you get OH- via
there is no OH(aq)- generated in this reaction but the carbonic acid is neutralized by the CO3(aq)2- which is the base. In the reverse reaction one of the hydrogen carbonates is an acid (proton doner) and the other a base (proton acceptor). FWIW water and HCO3(aq) - can both catch and toss protons, so they are called amphoteric.
From this point of view alkalinity is defined as the capacity to neutralize acid, or if you will to catch protons.
On the water world, this makes sense because the lakes, streams and oceans are filled with ions of weak acids like carbonic acid, which can hydrolyze water. That is why alkalinity and especially the alkalinity of the oceans is defined as the concentrations (indicated by  and see the link to the NAS pub above)