Dynamics of Dissolved Organic Matter in Hiroshima Bay

In order investigate the dynamics of dissolved organic carbon (DOC) in Hiroshima Bay, water samples from 7 stations in the bay, its three inflowing rivers and two wastewater treatment plants (WWP) were taken from Aug. 1997 to Aug. 1998. The filtrated organic matter was fractionated into hydrophobic (Hob-DOC) and hydrophilic (Hil-DOC) components using XAD-8 resin. Biodegradation tests by a 100-day incubation and mixing experiments of those samples for evaluating the sedimentation of DOC were also conducted.The concentrations of DOC and Hil-DOC in the bay showed clear seasonal changes (high in summer and low in winter), suggesting aquagenic production of Hil-DOC. Those of river waters had similar seasonal changes, but not so clear. The percentages of Hob-DOC were 31-35% for the bay waters, 34-46% for the river waters, and 31-35% for the effuent from the WWP waters, whereas those of labile DOC were 9-25%, 18-51%, and 17-30%, respectively. The DOC concentrations had positive correlation with chlorophyll a (increments of DOC to Chlorophyll a : 0.01-0.04mg · l-1/μg · l-1), and negative correlation with salinity. The mixing experiment suggested negligible deposition of DOC at the estuaries of inflowing rivers. In conclusion, Hob-DOC went through the estuaries and the bay with negligible change in the concentration in both summer and winter. In contrast, both labile and refractory components of Hil-DOC were produced mainly in the inner part of the bay during summer, and a large part of the former component was subjected to biodegradation during the transport to the outer part.