DISSOLVED CO2
The carbon dioxide dissolved in water has even more effect than the oxygen. Oxygen remains as an O2 molecule, whether it's in its gas phase or in solution, but when CO2 is dissolved in water, a small proportion of it reacts chemically with H2O to form carbonic acid, H2CO3. (There's no mystery about that: just add up the six atoms.) In water carbonic acid dissociates rapidly to form a H+ ion and HCO2 (bicarbonate), so it affects the carbonate equilibrium, and pH values change as a result. Dissolved CO2 lowers the average pH of rainwater to 5.7, even where "acid rain" caused by pollution isn't a factor. The gentle acidity of rainwater is a major source for the weathering of minerals, which the carbonic acid leaches from rocks and which eventually find their way to the ocean.
KARST PROCESSES