Effects of diffusion and resuspension on nutrient release from submerged sediments

Nutrient release from a coarse-sand (CS) and a fine-sand (FS) estuarine sediment into the overlying water column was studied under static (diffusion-controlled) and dynamic (resuspension) conditions. Resuspension increased the amounts of NH₄⁺, NO₃^- and P released from the sediment cores compared to those released by diffusion alone due to direct release of these nutrients from the readily available pore-water and ion exchange fractions. The average effective NH₄⁺, NO₃^- and P diffusion coefficients (D_e) were found to be similar for both sediments, and did not vary significantly between static and dynamic conditions. Values of D_e were typically of the order of 10^-5 to 10^-6 cm² s^-1. The average daily NH₄⁺, NO₃^- and P diffusive flux density from CS and FS into the overlying water column of the study area was estimated to be 24.6, 0.02 and 0.39 mg m^-2 d^-1, and 66.9, 0.017 and 0.002 mg m^-2 d^-1, respectively. These fluxes suggest that nutrient release from the sediments via diffusion and small-scale resuspension events may represent only a minor source of NH₄⁺, NO₃^- and P to the water column of the study area.