Dissolved organic nitrogen (DON) and carbon (DOC) often dominate the dissolved nitrogen and organic carbon fluxes from rivers, yet they are not considered to affect coastal water quality because of their assumed refractory nature. The objective of this study was to quantify DON and DOC bioavailability to bacteria in 9 rivers on the east coast of the United States during a 6 d dark bioassay experiment. Water was collected from the freshwater portion of a forest stream in New Jersey (Forest 17a), and from the Bass (New Jersey), Delaware (New Jersey), Hudson (New York), Altamaha (Georgia), Savannah (Georgia), Pocomoke (Maryland), Choptank (Maryland), and Peconic (New York) Rivers during base-flow conditions. DON concentrations ranged from 1 to 35 pM and comprised 8 to 94 % of the total dissolved nitrogen (TDN) in these rivers. Bioassay results indicate that 23% (±4) of the DON (2 ± 1 μM) was bioavailable in all the rivers except the Bass and Pocomoke, where no DON consumption was measured. Of the TDN consumed by bacteria, DON comprised 43 % (±6), demonstrating that DON is an important nitrogen source for bacteria. In contrast, only 4% (±1) of DOC (12 ± 3 pM), was bioavailable in the 9 rivers. Percent-wise, 8 times more DON was consumed relative to DOC in 6 of the rivers, demonstrating that DON cycles faster than DOC. Overall, our study demonstrates that DON is an important part of the TDN pool that needs to be incorporated into coastal nitrogen loading budgets because it is bioavailable on the order of days.
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