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

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 O2 flux (Jobs) 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 Jobs was saturated (Usat) 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, HCO3‐ 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 HCO3‐ uptake to DIC uptake decreased linearly with increasing [DIC]. The measured local Sherwood numbers (Shx) and the parameter a (2.24 ± 1.32) for O2 of the equation, Shx = a Rebx Sc0.33 were higher than predicted for a laminar flat plate boundary layer, indicating that physicochemical activity, such as photosynthesis, influenced Shx. The thickness of the measured concentration boundary layer (ΔCBL) and the diffusive sublayer (ΔDSL) 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.