Current assessments of organic pollution primarily emphasize the quantity of organic carbon (OC), often neglecting its biochemical activity in estuaries. The presence of refractory dissolved organic carbon (RDOC) can misrepresent organic pollution, and documentation on the characteristics of OC and its association with pollution is limited. This study utilized physical separation, biological incubation, and chemical analysis to examine the size-fractionated composition and bioavailability of OC in the Yangtze River Estuary, a most polluted estuary area in China. Results indicated that OC chemical features were notably diverse, with RDOC comprising approximately 65% of dissolved organic carbon (DOC). A linear relationship between DOC and RDOC in size-fractionated OC indicated greater recalcitrance in smaller size fractions. The OC present in the >0.45 μm fraction is significant for labile OC, relevant to chemical oxygen demand (COD) assessments. Excluding RDOC allows for estimating labile OC's contribution to COD using the equation COD_Labile = 0.47COD_Bulk - 0.03. Annually, approximately 0.30 Gt of refractory OC, including 0.13 Gt of RDOC, is transported into the ocean via rivers. This first linear relationship of COD reveals an overestimation of organic pollution and a neglection of RDOC as carbon preservation, necessitating a revision of the COD evaluation in estuaries.
 

refractory dissolved organic carbon, microbial availability, size-fractionated, organic pollution, major estuary