The effect of hydrodynamics on the mass transfer of dissolved inorganic carbon to the freshwater macrophyte Vallisneria americana

The combined effects of water velocity ( U ) and dissolved inorganic carbon (DIC) concentration on photosynthesis rates of Vallisneria americana were investigated. The net photosynthesis rate or O 2 flux ( J obs ) from leaves increased with U from 0.20 ± 0.01 (mean ± standard error) µmol m ‐2 s ‐1 at U = 0 m s ‐1 (i.e., in stagnant water) to 2.1 ± 0.07 µmol m ‐2 s ‐1 at U = 0.066 m s ‐1 . The velocity where J obs was saturated (U sat ) was inversely proportional to the DIC concentration ([DIC]) and decreased monotonically from 0.04 ± 0.01 m s ‐1 at 0.46 mol m ‐3 to 0.006 ± 0.004 m s ‐1 at 4.8 mol m ‐3 . If the net photosynthesis rate and DIC uptake are equal, HCO 3 ‐ uptake rates contributed ≫90% of DIC uptake at all [DIC] at U = 0.005 m s ‐1 and contributed less at higher velocities. The proportion of HCO 3 ‐ uptake to DIC uptake decreased linearly with increasing [DIC]. The measured local Sherwood numbers ( Sh x ) and the parameter a (2.24 ± 1.32) for O 2 of the equation, Sh x = a Re b x Sc 0.33 were higher than predicted for a laminar flat plate boundary layer, indicating that physicochemical activity, such as photosynthesis, influenced Sh x . The thickness of the measured concentration boundary layer (Δ CBL ) and the diffusive sublayer (Δ DSL ) were 63% and 70% smaller, respectively, than theoretical values based on hydrodynamic theory. Theoretical hydrodynamic predictions of mass transfer need to account for biological reactions.