Occurrence of Dissolved Organic Nitrogen (DON) in Low-relief Streams on the Eastern Shore of Virginia, USA

The agricultural use of nitrogenous fertilizer in watersheds along the Atlantic Coast, USA, has fueled concerns and investigations into the upland-derived nitrate (NO₃^­-) discharging to coastal waters.  Past studies of low-relief, gaining streams in small watersheds on the Eastern Shore of Virginia, USA, have quantified the NO₃^-­-N flux to seaside lagoons and the Atlantic Ocean.  The contribution of dissolved organic nitrogen (DON) to the total nitrogen loading to coastal waters had not previously been evaluated.  This study quantified concentrations of DON, NO₃^­-, and total dissolved nitrogen (TDN) under baseflow conditions in 15 streams varying in watershed size and cropland use on the Eastern Shore of Virginia across a one-year period.  Mean concentrations of DON in streams ranged from 0.328 to 2.14 mg N L^-1 and represented 12 to 70% of the TDN pool.  In 14 of the 15 streams, NO₃^- was the principal form of nitrogen ranging in mean concentrations from 0.094 to 6.06 mg N L^­-1.  Instream DON concentrations were independent of NO₃^- concentrations, watershed area, and cropland use.  Unlike NO₃^-, DON varied seasonally with highest DON concentrations observed in spring.  DON ranged from 6 to 41% of the TDN in shallow groundwater with concentrations from 0.776 to 2.12 mg N L^-1_.  These concentrations were lower than the respective concentrations determined in overlying surface-water samples (0.001 to 0.773 mg N L^-1) collected concurrently.  In a laboratory experiment, DON of 1.02 mg N L^-1 was eluted in the effluent from an intact streambed sediment core using artificial groundwater influent containing NO₃^- only and represented nearly 60% of the TDN in the core effluent.  The results of this study establish DON as an important and dynamic constituent of the TDN pool in freshwater streams discharging from the Eastern Shore of Virginia, USA, to the coastal waters of the Atlantic Ocean.