Measurement and prediction of mass transfer to experimental coral reef communities

Abstract

The uptake of nutrients (N and P) into coral reef communities is proposed to be limited by diffusion through concentration‐depleted boundary layers between the water and organisms, or what is termed “mass transfer limitation.” The mass transfer rate is a physical limit to the rate of nutrient uptake. Maximum uptake rates by highly rough biological surfaces have not yet been evaluated. Engineering correlations indicate that increased surface roughness should increase mass transfer, although it has been difficult to quantify roughness of living corals. In this paper, the effects of highly rough coral surfaces on mass transfer were investigated by using dissolution of gypsum (plaster‐of‐paris) from flat smooth surfaces and coral skeletons. The gypsum dissolution rates were measured as an increase in concentration of calcium ions in freshwater recirculating over experimental surfaces. Stanton numbers (Stm, a dimensionless number giving the ratio of uptake rate per unit area to the rate of advection of the substance past the uptake surface) of experimental smooth surfaces ranged from 2.6 to 3.5 × 10−5 and were the same as values in the engineering literature for smooth surfaces. Stm for coral‐shaped surfaces ranged from 70 × 10−5 at 0.03 m s−1 to 17 × 10−5 at velocities up to 0.50 m s−1 and were in general 9 ± 1 times that of smooth surfaces. The measured Stm for each coral‐shaped surface was the same as the predicted Stm (±10%) calculated from measured friction and roughness using a correlation of heat transfer. Stm for ammonia uptake on living coral reef communities show the same relationship between mass transfer, friction, and roughness as the coral‐shaped gypsum surfaces. The transport rates of nutrients to reef surfaces are controlled by large‐scale roughness, typically associated with coral heads, and not small‐scale roughness elements on the organisms, nor biological alterations of diffusive boundary layers. Nutrient uptake and possibly other metabolic exchange rates arc governed by concentration, water velocity, and friction dissipated over the reef, denoting that coral reef community metabolism is physically forced.