Temporal variation of particulate organic carbon flux at the mouth of Tokyo Bay

A sediment trap experiment was conducted at a depth of 750 m at the mouth of Tokyo Bay to clarify the quantity and transport process of particles from the bay to the open ocean. The high total mass flux (8.7 ± 4.5 g m(–2) d(–1)) suggests that the particles not only originate in the surface layer right above the trap, but are also focused in Uraga Channel and discharged into the bay mouth. The organic carbon and nitrogen isotope ratios (δ(13)C(org), δ(15)N) of the trapped particles were like those of the surface sediment in the bay, that is, a mixture of particles in rivers and suspended particles in the surface layer of the bay. Compared with the results of the experiment conducted in 1995–2002, the average total mass flux was reduced by 70% and organic carbon content was reduced by 50%. The δ(13)C(org) values of trapped particles were also lower than those observed in the previous experiment, indicating a lower contribution from surface-suspended particles with high δ(13)C(org) values in the bay. These results could partly reflect a decrease of the concentration of the suspended particulate carbon in the bay by half over 20 years. Another factor contributing to the decrease of the flux at the bay mouth would be that the intrusion of Kuroshio coastal water into the bay, which pushes particles out to the bay mouth, has not occurred in recent years.