Anomalous mass transfer of phosphate on coral reef flats

To estimate a maximal rate of phosphate (P) uptake into coral reef‐flat communities, we introduce analogies between fluid flow in pipes and water flow over coral reef flats. On the basis of results from engineering studies of mass transfer in pipes, the best estimate of the Stanton number (St m , a dimensionless number giving the ratio of uptake rate to the rate of advection of the substance past the uptake surface) for P uptake into a sample reef flat is 5 × 10 ‒5 . Field observations yield a value of at least 1.7 × 10 ‒3 , a factor of 40 higher. This very large enhancement is due in part to wave surge but also could be due to the difference between the nature of the biotic surfaces and that of the engineering surfaces. The engineering data have been obtained for simple geometries with low ratios of wetted area to projected surface area. It is likely that much of the enhancement arises from the large ratio of wetted to projected area for the coral reef‐flat community and the fractal nature of this surface.