Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales

Halliday, S.J.; Skeffington, R.A.; Wade, A.J.; Neal, C.; Reynolds, B.; Norris, D.; Kirchner, J.W.. 2013 Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales. Biogeosciences, 10. 8013-8038. 10.5194/bg-10-8013-2013

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Abstract/Summary

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 only; 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, corre sponding 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 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.

Item Type:	Publication - Article
Digital Object Identifier (DOI):	10.5194/bg-10-8013-2013
Programmes:	CEH Topics & Objectives 2009 - 2012 > Water > WA Topic 3 - Science for Water Management > WA - 3.3 - Better represent hydrological and biogeochemical processes in Earth System Models
UKCEH and CEH Sections/Science Areas:	UKCEH Fellows Emmett
ISSN:	1726-4170
Additional Information. Not used in RCUK Gateway to Research.:	Open Access paper - Official URL link provides full text
NORA Subject Terms:	Hydrology
Related URLs:	Discussion paper
Date made live:	03 Dec 2014 11:11 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/508964

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