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

Nishihara, Gregory N., Josef D. Ackerman

Limnol. Oceanogr., 51(6), 2006, 2734-2745 | DOI: 10.4319/lo.2006.51.6.2734

ABSTRACT: 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.