Effect of bacterial activity on nutrient regeneration and release from bottom sediments

The effect of bacterial activity of surface sediment layer on nutrient regeneration and release from bottom sediment should be an important factor in estimating nutrient fluxes. In this study, core incubation experiments were conducted to investigate the bacterial activity in a shallow coastal area, Shido Bay, the Seto Inland Sea, from May to September 2016. High nutrient upward fluxes were observed in a high temperature period. Sodium azide (NaN3) was added to the overlying water of the core incubation experiment system to inhibit bacterial activity. The results of adding NaN3 showed that the bacteria could not only regenerate nutrients but also assimilate nutrients in the bottom sediments. Moreover, bacteria in the superficial layer, the top 2–3 mm layer of surface sediment (SL), markedly assimilated NH4 and PO4, although they did not assimilate Si(OH)4. Judging from these results, we concluded that the upward nutrient fluxes were fundamentally regulated by temperature, which affected the physicochemical processes and bacterial activity, because our previous study showed that the nutrient fluxes were scarcely affected by the activities of microphytobenthos, which were active only during winter when sufficient light penetrated to the seafloor.