Stimson, J., and S.T. Larned. 2000. Nitrogen efflux from the sediments of a subtropical bay and the potential contribution of macroalgal nutrient requirements. J. Exp. Marine Biology & Ecol. 252:159-180. NHEERL-COR 2359J
The concentration of dissolved inorganic nitrogen (DIN) in the porewaters of shallow-water tropical marine sediments can be as high as 50 to 100 µM, at sediment depths of shallow as 20 cm. These concentrations are at least 2 orders of magnitude greater than the DIN concentration in the overlying water. High porewater concentrations, and the resulting concentration gradient, result in substantial efflux of DIN from the sediments to the water column. This sediment-derived DIN may be an important nutrient source for benthic algae. In Kaneohe Bay, Hawaii, a mean ammonium efflux rate of 490 µmol m-2 d -1 and a mean nitrate + nitrite efflux rate of 123 µmol m-2 d-1 were measured on reef slopes in the habitat occupied by benthic algae. It has been demonstrated that this nutrient source is essential for the growth of at least one abundant alga, Dictyosphaeria cavemosa, and possibly others. The DIN concentrations in Kaneohe Bay sediment porewaters, and the rates of DIN efflux from those sediments, are greater than porewater concentrations and efflux rates reported for other, more pristine tropical sites. The rate of sedimentation of particulate nitrogen is similar to rates reported from other tropical lagoons, and about twice as high as the efflux rate of total dissolved nitrogen. These results support the view that nutrient efflux from the benthos, and the subsequent slow diffusion and advection from beneath the algal thalli, are in part responsible for the persistence of D. cavemosa on the coral reefs of Kaneohe Bay, given the present low nutrient concentrations in the water column. It is possible that efflux of DIN from sediments may be responsible for sustained benthic algal productivity in similar habitats on other tropical reefs.