Ammonium uptake by coral reefs: Effects of water velocity and surface roughness on mass transfer

To test the hypothesis that ammonium uptake by coral reefs is mass-transfer limited and to determine the relative effects of water velocity and community roughness on uptake, we measured the rates of ammonium uptake by reefs at velocities ranging from 3.3 to 37.5 cm s(-1). Two assemblages of coral rubble (relatively rough and relatively smooth) and two assemblages of live coral (Porites compressa and Pocillopora damicornis) were used. Rate constants for uptake ranged from 16 to 144 x 10(-6) m s(-1) and were positively correlated to water velocity. We estimated roughness of each assemblage as mean height, relief, and by measuring the friction coefficient of each assemblage at water velocities ranging from 3.3 to 58.0 cm s(-1). Friction coefficients ranged from 0.033 to 0.264. Stanton numbers (a dimensionless ratio of uptake rate to advection past the uptake surface) calculated from experimental data were within 80% of those calculated from empirically derived engineering equations describing heat transfer to rough surfaces. Results support the hypothesis that ammonium uptake is mass-transfer limited and are consistent with mass transfer relationships for nonbiotic surfaces.