Effects of nutrient loading on mass‐transfer rates to a coral‐reef community

Abstract

The apparent first‐order rate constants for NH 4 + (N) and PO 4 3− (P) uptake into a coral‐reef community ( S N and S P ) were measured 61 times in a 9‐m experimental flume over 43 d. Water velocities (U b ) varied from 4 to 39 cm s −1 to determine the apparent mass‐transfer exponent—the slope between log S N or log S P and log U b . S N varied from 26 to 147 × 10 −6 m s −1 , while S P varied from 4.8 to 60. Apparent mass‐transfer exponents varied from 0.3 to 0.7. Both S N and S P and their apparent mass‐transfer exponents were inversely correlated with nutrient loading. We consider these effects to be caused by decreases in the rate constants for the real surface reactivity, S rN and S r P , following abnormally high loading of nutrients. As a result of nutrient loading, concentrations of nutrients at the surface of the benthos rise and reduce the rate of diffusion through the boundary layer. A method is presented for calculating mass‐transfer coefficients and Stanton numbers for mass transfer from such data. Results for these physically determined quantities are consistent with earlier measurements and theoretical predictions. To avoid such nutrient‐satiation effects, it appears to be important to avoid exceeding the nutrient‐loading levels of the natural habitat.