Retention of NO$_3^ -$ in an upland streamenvironment: A mass balance approach

Abstract

Models of the effects of atmosphericN deposition in forested watersheds have notadequately accounted for the effects of aquatic andnear-stream processes on the concentrations and loadsof NO$_3^ -$ in surface waters. This studycompared the relative effects of aquatic andnear-stream processes with those from the terrestrialecosystem on the retention and transport ofNO$_3^ -$ in two contrasting stream reaches ofthe Neversink River, a forested watershed in theCatskill Mountains of New York that receives among thehighest load of atmospheric N deposition in thenortheastern United States. Stream water samples werecollected every two hours and ground-water andtributary samples were collected daily at base flowconditions during four 48-hour periods from April toOctober 1992, and NO$_3^ -$ mass balances werecalculated for each site. Results indicated diurnalvariations in stream NO$_3^ -$ concentrations inboth reaches during all four sampling periods; this isconsistent with uptake of NO$_3^ -$ byphotoautotrophs during daylight hours. Mass-balanceresults revealed significant stream reach losses ofNO$_3^ -$ at both sites during all samplingperiods. The diurnal variations in NO$_3^ -$concentrations and the retention of NO$_3^ -$relative to terrestrial contributions to the streamreaches were greater downstream than upstream becausephysical factors such as the head gradients ofinflowing ground water and the organic matter contentof sediment are more favorable to uptake anddenitrification downstream. The mass retention ofNO$_3^ -$ increased as the mean 48-hr streamdischarge increased at each site, indicating that theresponsible processes are dependent onNO$_3^ -$ supply. Low stream temperatures duringthe April sampling period, however, probably reducedthe rate of retention processes, resulting in smallerlosses of NO$_3^ -$ than predicted from streamdischarge alone. Water samples collected from thestream, the hyporheic zone, and the alluvial groundwater at sites in both reaches indicated that the neteffect of hyporheic processes on downstreamNO$_3^ -$ transport ranged from conservativemixing to complete removal by denitrification. Therelative effects of biological uptake anddenitrification as retention mechanisms could not bequantified, but the results indicate that bothprocesses are significant. These results generallyconfirm that aquatic and near-stream processes causesignificant losses of NO$_3^ -$ in the NeversinkRiver, and that the losses by these processes atdownstream locations can exceed the NO$_3^ -$contributions to the stream from the terrestrialenvironment during summer and fall base-flowconditions. Failure to consider these aquatic andnear-stream processes in models of watershed responseto atmospheric N deposition could result inunderestimates of the amount of NO$_3^ -$leaching from forested ecosystems and to an inabilityto unequivocally relate geographic differences inNO$_3^ -$ concentrations of stream waters tocorresponding differences in terrestrial processes.