Boundary‐layer and internal diffusion effects on phosphorus fluxes in lake periphyton1

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Abstract
Phosphate uptake kinetics of periphyton were evaluated by addition of carrier‐free radiophosphate under controlled laminar flow conditions. In most cases, excluding only poorly colonized substrata, tracer fluxes were found to be limited by boundary‐layer mass transfer. Tracer flux could be described as a power function of flow velocity and as a negative power function of distance from the leading edge of the communities, as expected from mass transfer calculations. Observed deviations from theoretical functions were interpreted as effects of community patchiness and of internal water flow in the periphyton matrix. Phosphate turnover by periphyton in suspension was within the range of values for lake plankton, when related to biomass. Kinetic calculations based on these turnover measurements indicated that internal recycling of phosphate and recycling from the boundary layer, rather than external uptake, accounted for most phosphate turnover within intact periphyton films. This situation is likely typical under oligotrophic conditions. Boundary‐layer thickness was within the range of published estimates from littoral systems. The results indicate that chemical conditions within periphyton communities differ considerably from the immediate surroundings and that boundary‐layer development is a key factor affecting the metabolism of periphyton communities.