Oh I think I see now (since you added that last sentence). It seemed like you meant that you were dosing nitrate for the denitrifying bacteria. So you are saying that the denitrifying bacteria are reducing all the nitrate, leaving none for the macroalgae. So you add an excess of NO3 to allow the macroalgae to grow, and thus consume more phosphate. That makes more sense now. But can you still clarify how carbon dosing ties into this, and that point about CO2, carbon sources, and bacteria?