Biodegradation of riverine dissolved organic carbon in five estuaries of the southeastern United States

The biological turnover of riverine dissolved organic carbon (DOC) discharg ed into five southeastern United States estuaries was examined in long-term respiration bioassays. Measures of bacterial oxygen consumption indicated surprisingly large differences in the inherent biodegradability of DOC amon g the five estuaries, despite their close geographic proximity. Differences of up to 13-fold in biodegradation rates were also found temporally within a single estuary. For most of the southeastern United States estuaries, me asured rates of riverine DOC biodegradation were low relative to rates repo rted for other freshwater and marine environments. This was particularly tr ue for the coastal plain ("blackwater") rivers that contribute about 35% of the riverine DOC exported to coastal marine environments in this region; e xtrapolation of biodegradation rates to the adjacent continental shelf pred ict biodegradation of a maximum of 11% of exported blackwater DOC within th e estuary-shelf system (with transit times of up to 140 d). DOC from Piedmo nt rivers was more biologically labile, with maximum losses of 30% predicte d within the estuary and adjacent shelf. Short exposures to natural sunligh t increased the lability of the riverine DOC and enhanced biodegradation ra tes by over 3-fold in some cases, although significant inter-estuary differ ences in susceptibility of riverine DOC to photolysis were also evident.