The distributions of dissolved organic carbon (DOC) and the natural carbon isotope ration of DOC (DO13C) in estuaries reflect the predominant sources and sinks of organic matter from both allochthonous and autochthonous origins. The traditional view is that DOC in land-margin ecosystems reflects mainly the mixing of land-derived and oceanic DOC. However, this view is not consistent with the bulk of our data from a survey of DOC and DO13C distributions in estuaries on the East and Gulf coasts of the USA. While it is accurate that the DOC in estuaries includes material derived from land and from the ocean, the distribution of DOC and DO13C in several estuaries reflect additional DOC inputs form estuarine phytoplankton and tidal marshes. Even when DOC concentrations were distributed conservatively, the isotopic composition of the DOC revealed the existence of a dynamic cycle of DOC input and removal in some systems.