Diffusive boundary layers do not limit the photosynthesis of the aquatic macrophyte, Vallisneria americana, at moderate flows and saturating light levels
Limnol. Oceanogr., 54(6), 2009, 1874-1882 | DOI: 10.4319/lo.2009.54.6.1874
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 O2 microsensors) over the leaves of Vallisneria americana. The O2 flux (Jobs) was highest at x = 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 s-1 and 6.6 cm s-1 mean velocity (U), respectively. The maximum Jobs was 0.44 ± 0.07 (mean ± SE) vs. 0.50 ± 0.09 µmol m-2 s-1 at 0.5 vs. 6.6 cm s-1. Interestingly, the surface O2 potential (D[O2] = [O2]surface - [O2]bulk) was also unimodal at the low velocity (D[O2]max = 36 ± 5 mmol m-3 at x = 3 cm) but was uniform at the higher velocity (D[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 < τup; 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.