Vertical distribution of heavy metals and its seasonal variations in Beppu Bay

From July to November, the thermocline which has strong temperature gradient (0.7C m^–1) is formed in the bottom water of Beppu Bay, and it prevents the downward mixing of surface water. This has caused the bottom water of the basin to become depleted in oxygen, and in November the bottom water below about 60 m depth becomes anoxic. Accordingly manganese and iron are reduced and more soluble under the anoxic condition, those concentrations are high relative to surface water, and the maximums are 1,240g l^–1 and 80g l^–1. Under the anoxic condition, the flux of dissolved manganese from the sediment is about 10g cm^–2 day^–1.