Phosphate uptake by a coral reef flat : a measurement of the Stanton number

The mass-transfer-limited equation for nutrient uptake is m = St×Ub×Cb, where St is the Stanton number, which is a physical descriptor of the bottom; Ub is water velocity, and Cb the bulk water nutrient concentration. We measured St in the field by using phosphate uptake across a 300m wide mixed coral-algal reef flat in Reunion Island. Phosphate concentrations and current velocities were measured at six stations evenly distributed along a cross-shore transect. We used peristaltic pumps to get integrated water samples for 4-6 hr over the rising tide, to smooth the variability created by complex current trajectories. The gradient (k) of the natural logarithm of phosphate concentration, versus distance along the transect, was (43±6)*10-4 m -1. Water velocities were measured in the middle of the transect, using an acoustic Doppler current profiler. Depth-averaged cross-reef horizontal velocity (Ux) was 0.042±0.011 m.s-1. To account for oscillatory flows, we measured Urms (0.102±0.030 m.s -1), using dissolution of plaster forms at the six stations. k was multiplied by the mean water depth, and Ux/Urms, giving the reef flat Stanton number. St was (19±4)*10-4, which is consistent with published values for coral communities in flumes, given the low water velocities and high 3-dimensional relief of the reef. This corroborates the idea that phosphate uptake by reef flats operates very near a mass-transfer limit.