RESUMEN
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 (δ13Corg, δ15N) 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 δ13Corg values of trapped particles were also lower than those observed in the previous experiment, indicating a lower contribution from surface-suspended particles with high δ13Corg 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.
RESUMEN
In Mikawa Bay, where hypoxia occurs in the bottom layer during summer, six shipboard observations were conducted from the mouth to the head of the bay from May to August 2014 to investigate the spatiotemporal variation in the bottom layer oxygen consumption rate (OCR). The OCR was determined from the dark incubation of sample waters using an optical oxygen sensor, which showed a range of 5.7-38.3 mmol m-3 days-1. A high OCR was observed at the inner-most station, where higher concentrations of nutrients and chlorophyll a (Chl a) than at the other stations were found, and bottom hypoxic water appeared during the observation period after late June. These OCRs can deplete the oxygen dissolved in water within a week. The OCR showed a highly significant positive correlation with particulate organic carbon concentrations in the bottom water. Considering the relatively low carbon-to-nitrogen mole ratio (~ 6.4-7.6) and high carbon isotope ratio (between approximately - 20.2 and - 18.8) of particulate organic matter at the stations, the supply of fresh organic matter produced in the bay as opposed to the land may have affected the OCR by acting as a substrate for microbial aerobic respiration. High temporal resolution data from two automated observation buoys near the bay mouth and the inner area captured increases in Chl a at both sites in response to typhoon events, along with the subsequent appearance of bottom hypoxic water at the inner site and its expansion at the mouth. This supports our hypothesis that enhanced organic matter production due to nutrient input to the surface layer through vertical mixing would increase the bottom OCR, resulting in hypoxia. The apparent oxygen decline in the bottom layer from the buoy data was consistent with incubation-based OCRs during the observation period. Therefore, it is essential to model the OCR in numerical simulations of hypoxia, to which the variability characteristics that we revealed made significant contributions.