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 sI. Two assemblages of coral rubble (relatively rough and relatively smooth) and two assemblages of live coral (Porites compressa and Pocilloporu dumicornis) were used. Rate constants for uptake ranged from 16 to 144X 10 .6 m s-l 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 sI. 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 arc consistent with mass transfer relationships for nonbiotic surfaces. Coral reefs are highly productive systems that occur in low-nutrient water. How this high productivity is maintained has received considerable attention. Much of this research has focused on recycling of nutrients between autotrophs and heterotrophs (e.g. Odum and Odum 1955; Pomeroy 1970; Pomeroy et al. 1974); however, recent evidence suggests that enough nutrients are delivered to the reefs from the water column to account for productivity levels if the flux of nutrients to the reefs is considered (Atkinson 1992). The proposed hypothesis is that nutrient uptake by coral reefs is controlled by diffusion of the nutrients through a depleted boundary layer adjacent to the surfaces (i.e. uptake is “masstransfer limited”. Atkinson 1992; Bilger and Atkinson 1992; Atkinson and Bilger 1992). If this is the case, reef productivity is likely tightly coupled to oceanographic processes such as wave-generated turbulence and flow velocity. Both the shape and small-scale roughness of a surface affect the characteristics of the diffusive boundary layer and thus rates of mass transfer to the surface (e.g. Kays and Crawford 1993). This relationship between characteristics of the reaction surface and rates of mass transfer has been studied extensively for nonbiotic surfaces by engineers (see Kays and Crawford 1993). Biologists have adapted these engineering relationships to study the effects of morphology and water flow on productivity of corals at the scale of individual colonies (Patterson et al. 1991; Lesser et al. 1994) and polyps (Patterson 1992) as well as transport of dissolved nutriI Present address: Dauphin Island Sea Lab, POB 369-370, Dauphin Island, Alabama 36528.