Mass-transfer-limited nitrogen and phosphorus uptake by stream periphyton: A conceptual model and experimental evidence

We conducted flow-tank experiments to examine nitrate (NO3) and dissolved reactive phosphorus (DRP) uptake by stream periphyton over a wide range of near-bed velocities and turbulence levels. Our aims were to characterize the hydraulic conditions under which nutrient uptake is controlled by mass transfer through diffusive boundary layers (DBLs) or by membrane kinetics and to identify factors that affect mass-transfer rates. Nutrient uptake was mass-transfer limited under all hydraulic conditions used; there was no indication of kinetic control. We developed a conceptual model describing mass-transfer-limited uptake in terms of periphyton canopy height relative to DBL thickness. Three uptake regimes comprise the model: (1) when the canopy is submerged within the DBL covering the substratum, uptake is controlled by the thickness of this DBL; (2) when canopy height is greater than but comparable with the substratum DBL thickness, uptake is controlled jointly by the substratum DBL and by individual DBLs surrounding the periphyton elements that protrude above the substratum DBL; and (3) when the substratum DBL is very thin and most of the canopy protrudes above it, uptake is controlled by the DBLs surrounding periphyton elements. Patterns of NO, and DRP uptake in the experiments were consistent with the third regime. Periphyton extended well above the substratum DBL even at the slowest near-bed velocities (similar to1 cm s(-1)). We suggest that, in oligotrophic streams, the second and third uptake regimes prevail, DBLs around periphyton elements pose greater diffusive resistance to nutrient transport than the substratum DBL, and nutrient uptake is generally mass-transfer controlled and rarely kinetically controlled.