How does acid rain change the conditions in small lakes, streams, oceans, rivers?

When carbon dioxide dissolves into seawater, a portion of it creates carbonic acid, #"H"_2"CO"_3#, which in turn ionizes to form hydrogen ions and bicarbonate. The bicarbonate ions further ionize to some extent into carbonate and hydrogen ions.

The natural pH of the seas is approximately 8.1, which means it is somewhat alkaline (not acidic). The reactions that take place due to more carbon dioxide in the atmosphere (summarized above) release hydrogen ions, which make the water less alkaline.

Compared to preindustrial times, the seas have already experienced a drop in pH of about 0.1 and it is highly likely that marine pH will fall by another 0.3 by the end of this century.

If that happens, the seas will be more acidic than they have been for hundreds of millions of years. So, this will cause end of some creatures and environmental conditions.

For instance, carbonate is needed to build the calcium carbonate in shells and other body parts of marine creatures such as snails, starfish, clams, urchins and others. Read more here.

Some of the most important organisms likely to be affected by a lack of carbonate due to atmospheric #"CO"_2# buildup are certain types of plankton and small marine snails that are a major source of food for fish species and marine mammals. Coral reefs may also be impacted, since they are built from secretions of calcium carbonate by small sea anemone-like animals and coralline algae (the reefs).

Similar to the seas, rivers, lakes, and streams will suffer from increasing #"CO"_2# in the atmosphere both physically and ecologically.

References: Masters, G.M. and Ela, W. P. (2008). Introduction to Environmental Engineering and Science (3rd edition). Pearson International Education, Upper Saddle River, NJ, USA.

To learn more about ocean acidification, see this link.