Yasushi Hosokawa

Dissolved inorganic nitrogen cycling in Banzu intertidal sand-flat, Japan

The sediment-water exchange of nitrogen, nitrification, denitrification and sedimentary oxygen production were simultaneously measured in the Banzu intertidal sand-flat, Tokyo Bay, Japan. The exchange flux across the sediment-water interface showed a sink of dissolved inorganic nitrogen (DIN) in the light. However, in the dark, the intertidal sediment acted as a source. The diffusive flux obtained by the ammonium profile explained only 22% of the source flux directly measured in the dark, suggesting that bioturbation or excretion by macrofauna greatly contributed to the exchange flux. The total microphytobenthic uptake of DIN estimated from O2 productivity and the Redfield ratio was 573.4 ± 64.4 μmol N m2 h-1, 31% (175.9 ± 64.4 μmol N m-2 h-1) of which was assumed to be derived from the overlying water. The release rate of DIN from the sediment to the water column (1.1 mmol N m-2 day-1) was balanced with the removal rate of DIN from the water column by benthic microalgae on a diurnal basis. This result suggests that DIN was recycled within the sediment, and the microalgae on the sediment surface played a significant role in suppressing the release of mineralized DIN from the sediment. The measured denitrification rate using an acetylene inhibition technique was 99.6 ± 23.5 μmol N m-2 h-1. Since the direct supply of nitrate or nitrite from the water column only accounted for 27% of the total denitrification at the highest estimate, nitrogenous oxide in the sediment pool was the major contributor to sedimentary denitrification.

Flow and particulate nutrient removal by wetland with emergent macrophyte

Coastal wetland with emergent reed has the important function of settling particles in the flow. We undertook some experiments in order to ascertain the hydraulic characteristics of shallow surface flow in reed fields. The decline of water surface and the flow resistance coefficient were measured for low velocity in an experimental channel. Particle tracers were injected into the flow and the particle removal rate was calculated from the change of the particle concentration along the flow. The one-dimensional model with water level correction fits well to the data. The wetland system with reed will be a powerful means of eliminating particulate nutrient load in coastal areas.

Water quality management and waterfront development of a eutrophic bay

ABSTRACT Excess nutrient loads from urban areas give rise to many problems with the water quality in shallow littoral zones. New waterfront development should be accompanied by a system of water quality improvement. A perspective of counter-measures, other than sewage treatment systems, is proposed and some case studies are given. Utilization of natural energies and coastal ecological activities is essential for economic improvement. Numerical water quality predictions are made with the assigned removal efficiency of the systems. Construction costs of treatment systems are roughly estimated for each study area.