The biological turnover of riverine dissolved organic carbon (DOC)
discharged into five southeastern U.S. estuaries was examined in long-term
respiration bioassays.  Measures of bacterial oxygen consumption indicated
surprisingly large differences in the inherent biodegradability of DOC among
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 U.S. estuaries, measured rates
of riverine DOC biodegradation were low relative to rates reported for other
freshwater and marine environments.  This was particularly true for the
coastal plain ("blackwater") rivers that contribute about 35% of the
riverine DOC exported to coastal marine environments in this region;
extrapolation of biodegradation rates to the adjacent continental shelf
predict biodegradation of a maximum of 11% of exported blackwater DOC 
within the estuary/shelf system (with transit times of up to 140 d). DOC from
Piedmont rivers was more biologically labile, with maximum losses of 30%
predicted within the estuary and adjacent shelf.  Short exposures to natural
sunlight increased the lability of the riverine DOC and enhanced
biodegradation rates by over 3-fold in some cases, although significant
inter-estuary differences in susceptibility of riverine DOC to photolysis
were also evident.