Diffusive boundary layers do not limit the photosynthesis of the aquatic macrophyte vallisneria americana at moderate flows and saturating light levels

Abstract

Hydrodynamic models of mass transport assume that diffusive processes next to the surface limit transport and that there are no biological and chemical processes that control the supply and demand of the scalar. The validity of these assumptions was examined by measuring the momentum boundary layer (via particle image velocimetry) and the concentration boundary layer (via O 2 microsensors) over the leaves of Vallisneria americana. The O 2 flux (Jobs) was highest at x 5 2 cm downstream from the leading edge of the leaf and was 1.8 to 1.4 times higher than Jobs measured at the trailing edge of the leaf at 0.5 cm s21 and 6.6 cm s21 mean velocity (U), respectively. The maximum Jobs was 0.44 6 0.07 (mean 6 SE) vs. 0.50 6 0.09 mmol m22 s21 at 0.5 vs. 6.6 cm s21. Interestingly, the surface O2 potential (Δ[O2] 5 [O2]surface 2 [O 2]bulk) was also unimodal at the low velocity (D[O 2]max 5 36 6 5 mmol m23 at x 5 3 cm) but was uniform at the higher velocity (Δ[O2]= 9 + 0.7 mmol m -3). An analysis of the time scale of nutrient diffusion (τD) vs. nutrient uptake (τup) through the measured diffusion boundary layer revealed that uptake was always the slower process (i.e., τD < τupi τD and τup increased with x and decreased with U). Under moderate water velocities and saturating irradiance, uptake rates rather than diffusive transport processes appear to control mass transfer rates regardless of the location on the leaf and the water velocity.