Water velocity dependence of phosphate uptake on a coral-dominated fringing reef flat, La Reunion Island, Indian Ocean

Phosphate uptake (P-uptake) into coral reef communities has been hypothesized to be mass-transfer limited. One method of demonstrating mass-transfer limitation of P-uptake is to show dependence of P-uptake on water velocity. Water velocity across reef flats varies with tides and swell; thus, we measured P-uptake over the entire reef flat on eight different days, representing a range in water velocities. P-uptake was calculated from changes in P concentration of the water column. Changes in P concentration were measured by water sampling at six sites along a 300-m cross-reef transect while simultaneously measuring water velocity. To smooth the variability in phosphate concentrations, peristaltic pumps were used to get time-integrated water samples for 4-6 h at each site. Water velocities were measured in the middle of the transect using an acoustic Doppler current profiler and were averaged to match the time-integrated water sampling. Depth-averaged cross-reef water velocities were 0.031 +/- A 0.013 m s(-1) (mean +/- A SD), while the root-mean-square water velocities, accounting for oscillatory flow, averaged 3.3 times higher, 0.101 +/- A 0.021 m s(-1) (mean +/- A SD). Phosphate decreased along all transects. The first-order rate constant for P-uptake (S) was 8.5 +/- A 2.4 m d(-1) (mean +/- A SD) and increased linearly with root-mean-square water velocity. The Stanton number derived from oscillatory flow, the ratio of the first-order rate constant for P-uptake to the root-mean-square water velocity (S/U (rms)), was (9.4 +/- A 1.2) x 10(-4) (mean +/- A SD). P-uptake ranged from 0.2 to 1.1 mmol P m(-2) d(-1), demonstrating that P-uptake is variable on short time scales and is directly related to P concentration and water velocity.