Optical, Biological, and Chemical Properties of Aoshio, Hypoxic Milky Blue-Green Water, Observed at the Head of Tokyo Bay

Abstract

Aoshio-hypoxic water which can be distinguished from surrounding coastal water by its milky blue-green color-is often observed at the head of Tokyo Bay when coastal upwelling of sulfidic bottom water occurs during summer and early autumn. Optical properties and the biological-chemical composition of suspended matter in the aoshio water were investigated using nearby “normal” coastal water as a reference.
Suspended matter in the aoshio water contributed to the large beam-attenuation coefficient and also to the strong upward radiance with maximal intensity around 550 nm as a result of the scattering process. No significant difference in bacterial density was observed between samples of aoshio water and reference coastal water. The low chlorophylls concentration in aoshio water samples indicates that biological pigments may not play a dominant role in the color of the aoshio events.
The suspended particles in aoshio water samples had much higher contents of sulfur and manganese and a lower silicon content than those in reference coastal water samples. The colloidal elemental sulfur and manganese-rich particles identified in the aoshio water appear to be oxidation products of dissolved sulfide and manganese (II) in the anoxic water.
On the basis of these findings, it may be concluded that there is a high probability that elemental sulfur and manganese-rich particles are responsible for the color and turbidity in aoshio.