On the determination of mass transfer in a concentration boundary layer

Abstract

The mass transfer of scalar quantities (e.g., O2 and nutrients) in aquatic environments is an important and complex process involving diffusion and advection. In a flowing environment, concentration boundary layers (CBL) occur above the surfaces of organisms when they are a sink or source of scalars. In this study, we used an O2 microsensor to profile the O2 concentrations in the CBL above photosynthesizing freshwater macrophyte (Vallisneria americana) leaves that were oriented parallel to the flow in a recirculating flow chamber at 0.5 and 3.3 cm s−1. Measured O2 profiles were nonlinear indicating the effect of higher order processes near the surface. O2 flux (Jobs) was estimated from these profiles by two nonlinear techniques, hyperbolic tangent and logarithmic models, and the commonly applied linear model. An integrated measurement of O2 flux (Jint) for each leaf was also measured independently in a stirred chamber. Whereas Jobs determined from the hyperbolic tangent (0.42 ± 0.04 [mean SE] µmol m−2 s−1) and linear (0.31 ± 0.04 µmol m−2 s−1) models overestimated and underestimated Jint (0.37 ± 0.05 µmol m−2 s−1), respectively, and were not velocity dependant, the hyperbolic tangent model provided the best fit (r2 = 0.88) compared with the linear model (r2 = 0.77). In addition, the slope of the regression against Jint (1.08 ± 0.06) was closest to 1.00 (i.e., a “perfect” fit). The logarithmic model varied with velocity and overestimated Jobs (0.98 ± 0.22 µmol m−2 s−1 at 0.005 m s−1 and 0.90 ± 0.18 µmol m−2 s−1 at 0.033 m s−1). These results were confirmed in an analysis of 21 published O2 concentration profiles measured next to sediments, microbial biofilms, planktonic algae, and epilithic algae. We would, therefore, recommend the hyperbolic tangent model to estimate mass transfer in a CBL.