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

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 (J(obs)) was highest at x = 2 cm downstream from the leading edge of the leaf and was 1.8 to 1.4 times higher than J(obs) 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 J(obs) was 0.44 +/- 0.07 (mean +/- SE) vs. 0.50 +/- 0.09 mmol m(-2) s(-1) at 0.5 vs. 6.6 cm s(-1). Interestingly, the surface O-2 potential (Delta[O-2] = [O-2](surface) - [O-2](bulk)) was also unimodal at the low velocity (Delta[O-2](max) = 36 +/- 5 mmol m(-3) at x = 3 cm) but was uniform at the higher velocity (Delta[O-2] = 9 +/- 0.7 mmol m(-3)). An analysis of the time scale of nutrient diffusion (tau(D)) vs. nutrient uptake (tau(up)) through the measured diffusion boundary layer revealed that uptake was always the slower process (i.e., tau(D) < tau(up); tau(D) and tau(up) 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.