Does more CO2 improve phytoplankton growth in culture?

We noticed that the Pseudo-nitzschia delicatissima (strain LaPn-4) cultures were growing better than expected. We speculated that the culture’s growth suddenly improved because it was getting more CO2 than before. We reached this conclusion because:
1.The culture grew better as flasks were changed more frequently
2.The culture grew better as the flasks we kept them in increased in volume
3.The culture was in flasks and had to be poured to take fluorometer readings and to do dilutions
To investigate this, we used three 100mL cultures in 250mL Erlenmeyer flasks.
F-28 was aerated using a pasteur pipette attached with rubber tubing and airline hose to an aquarium pump. The volumes needed for measurements and dilutions were removed and added using a sterile pipette. F-27 was not aerated. In order to take measurements and dilute the appropriate volume was pipetted in and out of the flask. F-26 was not aerated either. In order to obtain culture for measurements and dilutions the appropriate volume was poured into and out of the flask. No pipettes were used. All flasks were shaken to suspend cells evenly before fluorescence measurements were taken.
Later, upon comparing the length of the cells in the same culture of the previous year, we noticed that the cells were significantly longer this year. This told us that the increased growth, health, and chain length was due to the culture undergoing sexual reproduction.
Chain length for F-28 was numerous at 4-8 cells per chain while both the pipetted and poured culture was most numerous at 1-2 cells per chain. Cell length was about 57 m in length and was uniform over the three flasks.
I used an epifluorescent microscope to count and measure the cells. Guidelines that I used:
1.Count at least 100 chains
2.Dividing cells count as one cell in the chain