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 (J obs ) 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 J obs measured at the trailing edge of the leaf at 0.5 cm s 21 and 6.6 cm s 21 mean velocity (U), respectively. The maximum J obs was 0.44 6 0.07 (mean 6 SE) vs. 0.50 6 0.09 mmol m 22 s 21 at 0.5 vs. ). An analysis of the time scale of nutrient diffusion (t D ) vs. nutrient uptake (t up ) through the measured diffusion boundary layer revealed that uptake was always the slower process (i.e., t D , t up ; t D and t 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.

Evaluating oxygen fluxes using microprofiles from both sides of the sediment‐water interface

“…Sediment O 2 consumption can alternatively be the rate-limiting step to mass transport in physiologically active systems experiencing moderate flow (Nishihara and Ackerman 2009). We observed R O2v to decrease with depth, suggesting that R O2v may have been dominated by mineralization of newly deposited organic matter at the sediment surface (Santschi et al 1990;Zhang et al 1999).…”

“…Whereas the effective δ DBL approach has been found to overestimate δ DBL (Nishihara and Ackerman 2007;O'Connor et al 2009), direct method δ DBL was typically slightly lower (11% on average) than mean δ DBL (Fig. 3).…”

“…Our results are in agreement with those obtained by Jørgensen (2001), who found effective δ DBL to be similar to δ DBL estimated as a function of hydrodynamics using alabaster plate measurements. Although the direct method was relatively easy to apply to our O 2 profiles with comparable results, Nishihara and Ackerman (2007) have suggested a similar but alternate approach using a hyperbolic tangent (rather than a straight line) to define ∂C/∂z, which may be optimal for profiles exhibiting nonlinearity in the DBL. u * method: While the other four methods are based purely on O 2 microsensor data, the u * method is unique in that it incorporates water-column hydrodynamics (as characterized by u * ) into the assessment of δ DBL .…”

“…Although numerous approaches are used for assessing J O2 , no single method has been shown to be optimal, and further evaluation of how the various methods compare is needed (Nishihara and Ackerman 2007;Glud 2008). Furthermore, because sediment O 2 uptake can be affected by processes on both sides of the SWI, evaluating J O2 from a one-sided perspective could result in different and biased interpretations of J O2 and R O2 .…”

“…A wealth of literature exists on the various methods used for estimating J O2 (Jørgensen and Revsbech 1985;Nishihara and Ackerman 2007;Glud 2008). J O2 is typically evaluated from either a water-or a sediment-side perspective (O'Connor et al 2009).…”