because in areas of low water flow the exposure to co2 would be less.

Around every leaf of foliage on a plant there exists a thin barrier where most of the gas absorbtion takes place, if your lacking water flow across this area, then the plant is able to take up far less carbon than a plant that is in the main flow, as the CO2 saturation in that layer is less frequently replenished.

Im not saying this is definatly the answer, just that it could be