In Ben's sticky on lowering TA (http://www.poolforum.com/pf2/showthread.php?t=191), he says that the 4 carbonate-ish (my term) players in the mix are :
bicarbonates (-HC03)
carbonates (=CO3)
carbonic acid (H2CO3)
carbon dioxide (CO2)

at high pH, the carbonate alkalinity is "in the form of carbonates: there is NO bicarbonate and NO carbonic acid"

and that at low pH, the carbonate alkalinity is "carbonic acid + carbon dioxide, with NO carbonates and NO bicarbonates"

Now, I will be the first to admit that I am not highly chemistry-oriented, so I don't always expect to grasp all of it. To visualize this in my own little empty head, I made a graph, of sorts...




It isn't graphed to any real numbers, it is just supposed to indicate that CO3 is high at high pH, H2CO3 is high at low pH, and I am guessing that the CO2 may increase as the pH drops, but I don't know that. Like I said, it is supposed to be an indicator/visual aid only.

What I am having difficulty with, is this: How do the bicarbonates fit in? They don't seem to be there in either high or low pH cases. Can someone tell me the relationship with it?