Fertilizing the algae will not only not effect the mount of CO2 absorbed by the seawater from the air, but will reduce the amount of CO2 in the water.
Actually, reducing the CO2 in seawater would change the CO2 concentration of the atmosphere. Removing CO2 from the ocean changes the equilibrium state between CO2 in the air and CO2 in the water. This causes more CO2 to be dissolved in the sea from the atmosphere through gas exchange, which is the whole point of Fe fertilization schemes.
Whether or not the algal blooms would successfully sequester the carbon on the ocean floor is another matter entirely.