CO2 in Nature

[TheSeanMeister at webtv_net (Sean Meister)]

With all the on going talk about CO2 supplementations in the home
aquarium, I keep wondering how plants in lakes, rivers, streams, ponds
etc. get their CO2.

I assume that aquatic plants wouldn't get as much CO2 in the "wild" as
in the aquarium. 

 I also assume that most (or some) CO2 must come from groundwater.  So
could plants in a aquarium get enough CO2 from daily water changes for
normal growth found in nature? 

 I have a ten gallon planted tank without CO2 addition and I notice O2
bubbles on leaves after I change about 1/3 of the water.  Any thoughts?

Sean Meister