Downcore porewater dissolved organic matter composition was analyzed by FT-ICR MS at two contrasting sites in the Chesapeake Bay. One site is in the oligohaline upper Bay (Chesapeake Bay Water Quality Monitoring Program site CB2.2) approximately 30 km south of the Susquehanna River. The turbidity maximum frequently occurs in this area. The other site is in the mesohaline mid Bay (Chesapeake Bay Water Quality Monitoring Program site CB4.3c) approximately 95 km south of the Susquehanna River, where bottom experience serve summer hypoxia from May - September.