When doing carbon footprint calculation, how to estimate the percentage of carbon in the dissolved CO2 in water by photosynthetic activity of plants?

I am working in a project involving the environment foot printing in Taiwan. In this project one of the main focus is the carbon consumption in agriculture. As plants require CO2 for photosynthesis it is important to know the role of water in this complex interaction (between plant uptake of water and carbon) and how to incorporate water’s role through dissolved CO2 in water in the carbon foot print calculation.