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.