A solution of the inorganic carbon mass balance equation and its relation to algal growth rates

The mass balance equation describing the dissolved inorganic carbon content of river water is solved for varying conditions of net community productivity, alkalinity, temperature, atmospheric exchange, and accrual of groundwater carbon to give CO2 concentration throughout the day. Without accrual of groundwater the CO2 concentration must lie in the range where it will influence growth rate of some plant species, but groundwater input prevents this situation in most rivers. In lakes, groundwater input and exchange with the aphotic zone may prevent the influence of CO2 on algal growth rates. In waters with little groundwater input or little vertical mixing the CO2 concentration may influence relative growth rates of species and thus the community composition but probably not the total plant biomass produced.