Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

The Catskill Mountains of southeastern New York State have among the highest rates of atmospheric nitrogen deposition in the United States. Some streams draining Catskill catchments have shown dramatic increases in nitrate concentrations while others have maintained low nitrate concentrations. Streams in which exchange occurs between surface and subsurface (i.e. hyporheic) waters are thought to be conducive to nitrate removal via microbial assimilation and/or denitrification. Hyporheic exchange was documented in the Neversink River in the southern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate (NO3 ) losses along hyporheic flowpaths were negligible. In this study, Neversink River water was amended with natural, bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series of experimental mesocosms that simulated hyporheic flowpaths. DOC and N dynamics were examined before and throughout a three week BDOC amendment. In addition, bacterial production, dissolved oxygen demand, denitrification, and six extracellular enzyme activities were measured to arrive at a mechanistic understanding of potential DOC and NO3 − removal along hyporheic flowpaths. There were marked declines in DOC and complete removal of nitrate in the BDOC amended mesocosms. Independent approaches were used to partition NO3 − loss into two fractions: denitrification and assimilation. Microbial assimilation appears to be the predominant process explaining N loss. These results suggest that variability in BDOC may contribute to temporal differences in NO3 − export from streams in the Catskill Mountains.