DIFFUSIVE FLUXES ACROSS SEDIMENT–WATER INTERFACE IN THE SETO INLAND SEA, JAPAN

Abstract: Spatial investigations of pore water nutrient concentrations and their theoretical fluxes were carried out in six different sediment types within a coastal environment (Shido Bay, the Seto Inland Sea, Japan). Nutrient concentrations in pore waters reached maximum values of 711 µM-NH4+, 20.1 µM-PO43- and 195 µM-Si(OH)4 at depth in cores. Diffusive fluxes of NH4+, PO43- and Si(OH)4 were estimated using a modification of a Fick's first law from concentration gradients. Fluxes were positive at all stations, ranging from 0.29 to 4.15 mmol m-2 d-1 for NH4+, 0.01 to 0.08 mmol m-2 d-1 for PO43- and 0.11 to 0.72 mmol m-2 d-1 for Si(OH)4. These fluxes from the bottom to the overlying water are related to the type of sediment and its biogeochemical compositions. Spatial variations of NH4+ and PO43- fluxes were found to be linearly correlated with the organic matter content of surface sediments. Also, NH4+ and PO43- fluxes increased with the C:N ratio of sedimentary organic matter. Differently, the highest Si(OH)4 fluxes were related with the relative high pheo-pigments and biogenic silica (Bio-Si) contents in the surface sediment, which are affected by fecal matter from the oyster culture. As a result, organic matter availability was found to be an important factor in regulating spatial variability of NH4+ and PO43- fluxes, whereas significant contents of sedimentary pheo-pigments and Bio-Si characterise the sites where Si(OH)4 is available.

Keywords: Nutrients, Porewater, Diffusive fluxes, Sediment-water interface, the Seto Inland Sea.