Occurrence, Spatiotemporal Variability, and Sources of Dissolved Organic Nitrogen (DON) in Low-relief Streams on the Eastern Shore of Virginia

Intensive agriculture and widespread fertilizer use on the Eastern Shore of Virginia has fueled concerns and investigations into the upland-derived nitrate (NO3-­) in low-relief streams draining into the shallow, seaside coastal lagoons. However, a thorough understanding of the dynamics of dissolved organic nitrogen (DON) in freshwater sources that may also contribute to nitrogen loading to the lagoons is lacking. 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 to examine spatial and seasonal patterns. 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 maximal DON concentrations observed in spring. DON ranged from 6 to 41% of the TDN in groundwater with concentrations from 0.776 to 2.12 mg N L-1. These concentrations were lower than the respective concentrations determined in surface-water samples (0 to 0.773 mg N L-1) collected concurrently. In a laboratory experiment, DON was eluted in the effluent (1.02 mg N L-1) from an intact sediment core using artificial groundwater influent containing NO3- only and represented nearly 60% of the TDN in the core effluent sample. The data from this study establish DON as an important and dynamic constituent of the TDN pool in freshwater systems on the Eastern Shore of Virginia.