Abstract
Abstract. Streamwater nitrate dynamics in the River Hafren, Plynlimon, mid-Wales were investigated over decadal to sub-daily timescales using a range of statistical techniques. Long-term data were derived from weekly grab samples (1984–2010) and high-frequency data from 7-hourly samples (2007–2009) both measured at two sites: a headwater stream draining moorland and a downstream site below plantation forest. This study is one of the first to analyse upland streamwater nitrate dynamics across such a wide range of timescales and report on the principal mechanisms identified. The data analysis provided no clear evidence that the long-term decline in streamwater nitrate concentrations was related to a decline in atmospheric deposition alone, because nitrogen deposition first increased and then decreased during the study period. Increased streamwater temperature and denitrification may also have contributed to the decline in stream nitrate concentrations, the former through increased N uptake rates and the latter resultant from increased dissolved organic carbon concentrations. Strong seasonal cycles, with concentration minimums in the summer, were driven by seasonal flow minimums and seasonal biological activity enhancing nitrate uptake. Complex diurnal dynamics were observed, with seasonal changes in phase and amplitude of the cycling, and the diurnal dynamics were variable along the river. At the moorland site, a regular daily cycle, with minimum concentrations in the early afternoon, corresponding with peak air temperatures, indicated the importance of instream biological processing. At the downstream site, the diurnal dynamics were a composite signal, resultant from advection, dispersion and nitrate processing in the soils of the lower catchment. The diurnal streamwater nitrate dynamics were also affected by drought conditions. Enhanced diurnal cycling in Spring 2007 was attributed to increased nitrate availability in the post-drought period as well as low flow rates and high temperatures over this period. The combination of high-frequency short-term measurements and long-term monitoring provides a powerful tool for increasing understanding of the controls of element fluxes and concentrations in surface waters.
Highlights
Nitrogen (N) is a key nutrient in river systems worldwide, but an over-enrichment of N can lead to problems of eutrophication, acidification and reduced biodiversity (e.g. Dise et al, 2009; Galloway et al, 2004; Jarvie et al, 1998; Neal and Jarvie 2005; Smith et al, 1999; Stevens et al, 2006; Sutton et al, 2011; UK-NEA 2011; Wright et al, 2001)
Within the Hafren catchment, NO3 was the dominant form of streamwater N, accounting for approximately 52 % of the total dissolved N (TDN); with NO2 accounting for less than 1 %, NH4 for approximately 10 % and dissolved organic N (DON) approximately 37 %
DON measurements were limited in both the long-term and high-frequency data sets and high-frequency variability was visible in the observations, this appeared dominated by flow event responses with no identifiable, regular, cyclical pattern
Summary
Nitrogen (N) is a key nutrient in river systems worldwide, but an over-enrichment of N can lead to problems of eutrophication, acidification and reduced biodiversity (e.g. Dise et al, 2009; Galloway et al, 2004; Jarvie et al, 1998; Neal and Jarvie 2005; Smith et al, 1999; Stevens et al, 2006; Sutton et al, 2011; UK-NEA 2011; Wright et al, 2001). Nitrate (NO3) is generally the dominant N fraction, with dissolved organic N (DON) a critical component in certain systems, on a seasonal basis (Chapman et al, 2001; Reynolds and Edwards, 1995; Sutton et al, 2011). Halliday et al.: Upland streamwater nitrate dynamics, Plynlimon mid-Wales
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