Coastal margins, where rivers serve as the dominant control on productivity and delivery of dissolved and particulate materials, have been understudied. The potential importance of certain river-dominated margins (RiOMars), such as those of the Mississippi River plume, to the global carbon budget is garnering increased attention because of their disproportionate role in transporting terrigenous materials to the ocean <Dagg et al., 2004; McKee et al., 2004>.