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 (De) were found to be similar for both sediments, and did not vary significantly between static and dynamic conditions. Values of De 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