Constraining the composition and biochemical activity of organic carbon in a large eutrophic estuary using size-fractionated analysis

The presence of refractory dissolved organic carbon (RDOC) can result in the misestimation of organic pollution, and documentation regarding the characteristics of organic carbon (OC) and its relationship with pollution is limited. This study employed physical separation, biological incubation, and chemical analysis to examine the size-fractionated composition and bioavailability of OC in the Yangtze River Estuary, one of the most polluted estuarine areas in China. Results revealed that OC chemical features were highly diverse, with RDOC constituting approximately 65.8% ± 9.2% of dissolved organic carbon (DOC). During incubation, less than 10% of CHO molecules (molecules composed solely of carbon, hydrogen and oxygen atoms) identified by ultra-high resolution mass spectrometry were degraded. A significant positive linear relationship between OC and RDOC in size-fractionated OC indicated greater recalcitrance in smaller size fractions. The OC present in the >0.45 μm fraction was notably important for labile OC, including the particulate fraction of OC, which is relevant to chemical oxygen demand (COD) assessments. Excluding RDOC allows for a more accurate estimation of the contribution of labile OC to COD, as represented by the equation: COD_Labile = 0.47 × COD_Bulk - 0.03. Approximately 0.44 ± 0.10 Gt of refractory OC, including 0.31 ± 0.07 Gt of RDOC, is transported annually into the ocean via rivers. This linear relationship of COD reveals an overestimation in current assessments of organic pollution and a neglect of RDOC's role in carbon preservation, thereby necessitating a revision of the COD evaluation practices in estuaries. This study highlights the differentiated impacts of refractory and labile OC on the quantification of OC pollution in a large eutrophic estuary.