10:15 AM - 10:30 AM
[ACG39-05] The vertical distribution and the property of decomposition of colored dissolved organic matter(CDOM) in Osaka Bay
Keywords:coastal sea, CDOM, refractory organic matter, labile organic matter
The dynamics of terrestrial materials in coastal waters is not simple, and there are many issues in quantifying the process of their transport through coastal waters to offshore areas. We aim to clarify the pathway of terrestrial materials to the offshore area using colored dissolved organic matter, which mainly consists of humic substances, as a tracer and to obtain the balance of terrestrial materials in the coastal sea area. In this study, we investigate seasonal changes in the vertical distribution of colored dissolved organic matter in Osaka Bay and conduct a 100-day organic matter decomposition experiment to clarify the decomposition characteristics of colored dissolved organic matter.
In May, August, and November 2023, water samples were taken from the surface layer, depths of 5 m, 10 m, 20 m, and 1 m above the bottom at a total of 12 sites in Osaka Bay, and the surface layer of the Yodo River and Yamato River flowing into Osaka Bay. At each site, a CTD (JFE Advantech ASTD) was lowered to measure water temperature, salinity, chlorophyll, and dissolved oxygen concentration, and a CDOM logger (PME Cyclops7) was put in the water sampled from each layer. Two sets of each sample were obtained, one set was filtered on board or within 4 hours after landing, and one set was put in an incubator setting at the water temperature at the time of water sampling and filtered after 100 days. Dissolved organic carbon (DOC) and the optical absorption coefficient of dissolved organic matter at 443 nm (ay(443)) were calculated for the samples obtained (CDOM).
CDOM in river water sampled in May and August decreased by 20-30% after 100 days, suggesting the presence of labile colored dissolved organic matter. On the other hand, there was no difference in CDOM in the sea area in May between samples filtered on board and those filtered 100 days later at each site and in each layer, suggesting that colored dissolved organic matter in Osaka Bay is almost refractory. In August, CDOM after 100 days decreased by 20-30% in the inner part of the bay and by 40-50% from the center to the mouth of the bay. This suggests that there are relatively large amounts of easily degradable colored dissolved organic matter (CDOM) in the sea area.
In May, August, and November 2023, water samples were taken from the surface layer, depths of 5 m, 10 m, 20 m, and 1 m above the bottom at a total of 12 sites in Osaka Bay, and the surface layer of the Yodo River and Yamato River flowing into Osaka Bay. At each site, a CTD (JFE Advantech ASTD) was lowered to measure water temperature, salinity, chlorophyll, and dissolved oxygen concentration, and a CDOM logger (PME Cyclops7) was put in the water sampled from each layer. Two sets of each sample were obtained, one set was filtered on board or within 4 hours after landing, and one set was put in an incubator setting at the water temperature at the time of water sampling and filtered after 100 days. Dissolved organic carbon (DOC) and the optical absorption coefficient of dissolved organic matter at 443 nm (ay(443)) were calculated for the samples obtained (CDOM).
CDOM in river water sampled in May and August decreased by 20-30% after 100 days, suggesting the presence of labile colored dissolved organic matter. On the other hand, there was no difference in CDOM in the sea area in May between samples filtered on board and those filtered 100 days later at each site and in each layer, suggesting that colored dissolved organic matter in Osaka Bay is almost refractory. In August, CDOM after 100 days decreased by 20-30% in the inner part of the bay and by 40-50% from the center to the mouth of the bay. This suggests that there are relatively large amounts of easily degradable colored dissolved organic matter (CDOM) in the sea area.