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Open AccessArticle

Sources and Mechanisms of Low-Flow River Phosphorus Elevations: A Repeated Synoptic Survey Approach

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Agricultural Catchments Programme, Teagasc, Johnstown Castle, Co., Wexford Y35 Y521, Ireland
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Environmental Research Centre, Teagasc, Johnstown Castle, Co., Wexford Y35 Y521, Ireland
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Geography and Environmental Science, School of Social Sciences, University of Dundee, Dundee DD1 4HN, UK
*
Author to whom correspondence should be addressed.
Water 2019, 11(7), 1497; https://doi.org/10.3390/w11071497
Received: 19 June 2019 / Revised: 5 July 2019 / Accepted: 11 July 2019 / Published: 18 July 2019
High-resolution water quality monitoring indicates recurring elevation of stream phosphorus concentrations during low-flow periods. These increased concentrations may exceed Water Framework Directive (WFD) environmental quality standards during ecologically sensitive periods. The objective of this research was to identify source, mobilization, and pathway factors controlling in-stream total reactive phosphorus (TRP) concentrations during low-flow periods. Synoptic surveys were conducted in three agricultural catchments during spring, summer, and autumn. Up to 50 water samples were obtained across each watercourse per sampling round. Samples were analysed for TRP and total phosphorus (TP), along with supplementary parameters (temperature, conductivity, dissolved oxygen, and oxidation reduction potential). Bed sediment was analysed at a subset of locations for Mehlich P, Al, Ca, and Fe. The greatest percentages of water sampling points exceeding WFD threshold of 0.035 mg L−1 TRP occurred during summer (57%, 11%, and 71% for well-drained, well-drained arable, and poorly drained grassland catchments, respectively). These percentages declined during autumn but did not return to spring concentrations, as winter flushing had not yet occurred. Different controls were elucidated for each catchment: diffuse transport through groundwater and lack of dilution in the well-drained grassland, in-stream mobilization in the well-drained arable, and a combination of point sources and cumulative loading in the poorly drained grassland. Diversity in controlling factors necessitates investigative protocols beyond low-spatial and temporal resolution water sampling and must incorporate both repeated survey and complementary understanding of sediment chemistry and anthropogenic phosphorus sources. Despite similarities in elevation of P at low-flow, catchments will require custom solutions depending on their typology, and both legislative deadlines and target baselines standards must acknowledge these inherent differences. View Full-Text
Keywords: phosphorus; low-flow; synoptic survey; mobilization; point source; sediment phosphorus; low-flow; synoptic survey; mobilization; point source; sediment
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Vero, S.E.; Daly, K.; McDonald, N.T.; Leach, S.; Sherriff, S.C.; Mellander, P.-E. Sources and Mechanisms of Low-Flow River Phosphorus Elevations: A Repeated Synoptic Survey Approach. Water 2019, 11, 1497.

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