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Determining the role of hydrologic variability on DIN export from the Catskill Mountains
Citation:
OELSNER, G. P. Determining the role of hydrologic variability on DIN export from the Catskill Mountains. Presented at 2009 Gordon Catchment Science Meeting, Andover, NH, July 12 - 19, 2009.
Impact/Purpose:
The Catskill region of New York State receives some of the highest rates of atmospheric nitrogen deposition in the northeastern US (approximately 6.2 kg ha-1 yr-1), and headwater streams have elevated nitrate concentrations (mean of stream surveys = 0.27 – 0.35 mg N l-1).
Description:
The Catskill region of New York State receives some of the highest rates of atmospheric nitrogen deposition in the northeastern US (approximately 6.2 kg ha-1 yr-1), and headwater streams have elevated nitrate concentrations (mean of stream surveys = 0.27 – 0.35 mg N l-1). Although wet deposition rates of nitrogen have not significantly changed since the mid-1980’s, monitoring data from four streams show decreasing annual nitrate concentrations between 1990 and 2000 followed by increasing concentrations through 2007. To determine whether the increasing nitrate concentrations result in increasing nitrate export, we calculated the average seasonal and annual dissolved inorganic nitrogen (DIN) loads for each of the four streams using mean monthly concentration and discharge data. Annual export of DIN was 60-100% higher in all four streams during 2003-2006 than in previous years. Furthermore, DIN export from each watershed represented 59-98% (average = 84%) of the total wet nitrogen deposition to the watersheds during 2003-2006 which is higher proportion than during 1991-2002 (25-75%, average = 45%). Nitrate concentrations generally increase with discharge in these four watersheds and discharge was 28-51% higher between 2003-2007 than 1990-2002. To determine whether the increases in DIN export loads were due to increases in discharge or a change in biogeochemical cycling within the watershed, we compared the relationships between nitrate concentrations and log discharge values at each stream during the two time periods (1991-2002 and 2003-2006). The relationship between nitrate concentration and discharge was significantly different in these two time periods during the growing season (May – September). However, the relationship between discharge and nitrate concentration was not significantly different between time periods for either the dormant season (October – February) or during the spring snowmelt period (March and April). Therefore, changes in nitrate export during the dormant season and spring snowmelt are likely controlled by hydrology, whereas changes in nitrate export during the summer growing season are likely controlled by both hydrology and biogeochemical cycling within the watershed. Ongoing data analyses seek to quantify the relative roles of hydrology and watershed biogeochemistry in controlling nitrate export.