Elucidating the Hidden Nonconservative Behavior of DOM in Large River-Dominated Estuarine and Coastal Environments

Understanding the mixing behavior and biogeochemical cycling of heterogeneous dissolved organic matter (DOM) at the land-ocean interface has been a major challenge, especially in large river-dominated estuarine and coastal environments with multiple and complex end-members of riverwater and seawater. Significant correlations and thus an apparent conservative behavior are generally observed for the bulk DOM when plotting concentration-salinity relationships, but the relatively small deviations from the apparent conservative behavior are difficult to be quantified. In July 2016, the flood in the Changjiang River basin caused the Changjiang Diluted Water extended widely to the western East China Sea, making it a natural laboratory to examine the river-sea mixing behavior of riverine DOM and other optical properties. Here a new biogeochemical approach was established to elucidate their hidden nonconservative behavior in the study area. Comparisons in the bulk dissolved organic carbon and the three chromophoric DOM properties among 13 pairs of same-salinity patches (with similar salinities but quite different distances from the river mouth) revealed that the East China Sea shelf was a significant sink for all the three chromophoric DOM properties. The Changjiang Diluted Water surface branches transporting southward tended to serve as a sink for the bulk dissolved organic carbon while the ones transporting northward were less a sink or even a source. The different removal extents of various biogeochemical constituents can also be calculated from the same-salinity patch comparisons. Our results obtained by the new approach provide detailed magnitudes of sink or source for different DOM properties and their in situ removal/production extents, giving new insights into biogeochemical processes in large river-dominated estuarine and coastal environments.