|MadSci Network: Chemistry|
There are many other things in the ocean. Bicarbonate is one species that highly contributes to the pH and alkalinity. Carbonate and sulfate also contribute. The chemistries are well explained in detail at the second reference.
Wilson RW, Millero FJ, Taylor JR, Walsh PJ, Christensen V, Jennings S, Grosell M. (2009)
Contribution of fish to the marine inorganic carbon cycle. Science. 16;323(5912):359-62.
Note that I used "pH and alkalinity". They are not the same and you should look up the definitions (there are two main types of alkalinity). Even a solution at pH=6 has one form of alkalinity.
Try the links in the MadSci Library for more information on Chemistry.