EFFECTS OF WATER VELOCITY ON PHOSPHATE-UPTAKE IN CORAL REEF-FLAT COMMUNITIES

To determine a 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 a reef-flat community without the effects of wave surge, we placed communities of reef organisms in the bottom of a flume, 9.3 m long and 0.35 m wide. Water was pumped over the experimental communities at 13 different surface flow velocities, ranging between 2.3 and 58 cm s-1. Rate of phosphate (P) uptake at each velocity was measured by raising the ambient P concentration to approximately 2-mu-M and measuring the rate of decrease in P concentration. St(m) for P uptake in the flume was significantly greater than estimates of St(m) derived from engineering studies of mass and heat transfer. This result confirms the finding of anomalously high mass transfer based on field data. Rates of P uptake increased as water velocity increased. The log of the first-order rate coefficient for P uptake was directly proportional to the log of velocity with a slope of approximately 0.76. These results indicate that the rate of P uptake into reef benthos is controlled by diffusive boundary layers near the surfaces of organisms and that the surface geometry is such that improved transfer in comparison with engineering surfaces is obtained.