The influence of environmental variability on silicate exchange rates between sediment and water in a shallow-water coastal ecosystem, the Seto Inland Sea, Japan

Silicate regeneration was determined in a shallow-water coastal ecosystem (Shido Bay, the Seto Inland Sea, Japan) during 1999–2000. The present study was carried out directly by core incubation and by determining gradients of dissolved silicate (DSi) in the pore water. Incubated fluxes ranged from 25.5 to 132.6 mg Si m−2 d−1, and were 1.6–21.6 times greater than diffusive fluxes (5.4–43.3 mg Si m−2 d−1). The disparity between fluxes measured by core incubation and modeling pore water indicated that other physical, chemical or biological processes, in addition to diffusion of DSi from below, contribute to DSi fluxes measured during the incubation of undisturbed cores. A regression analysis revealed that water temperature and salinity explained 24% and 23%, respectively, of season to season variability in incubated fluxes. Microphytobenthos was responsible for 37% of the variability in measured rate of DSi fluxes, with greatly reducing DSi release rates due to their own DSi demand. Moreover, the inverse relationship between the DSi fluxes and biogenic silica (Bio-Si) concentrations in the surface sediment, suggested that about 41% of the variability in the DSi fluxes were explained by Bio-Si concentrations in the surface sediment. As a result, Shido Bay showed silicate regeneration of incubated cores to be a consequence of Bio-Si dissolution depending on microphytobenthos, temperature and salinity, while diffusive fluxes appeared to be limited by DSi in the pore water. An annual average of DSi flux (68.7±32.9 mg Si m−2 d−1) from the sediments to the water column corresponds to 38% of DSi, required for primary production by phytoplankton in Shido Bay.