High‐molecular‐weight dissolved organic matter (HMW‐DOM, > 1,000 Daltons) is actively involved in the global biogeochemical cycling of many elements, but its carbon sources and detailed formation pathways are still not well understood. In this study, we measured bulk stable carbon and nitrogen isotopic ratios, lipid composition, and compound‐specific carbon isotopic ratios of HMW‐DOM samples collected from four U.S. estuaries...