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

Sara E Vero,Per-Erik Mellander,Simon Leach,Noeleen T Mcdonald,Sophie C Sherriff,Karen Daly

doi:10.3390/w11071497

Sara E Vero, Per-Erik Mellander + Show 4 more

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https://doi.org/10.3390/w11071497

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Abstract

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.

Highlights

The qualitative status of European waterbodies under the Water Framework Directive (WFD) [1] is determined relative to fixed quantitative thresholds of chemical concentration and physical/ecological status
Monitoring at the outlets of agricultural catchments has revealed that concentrations of nutrients, phosphorus (P), in stream water become elevated during low-flow conditions and may exceed legislated thresholds [2,3]
High temporal resolution monitoring provides insights into the seasonal dynamics of water quality [8], as this is typically done at a single point in the river, its outlet, it reflects the sum of all nutrient sources, hydrological pathways, and biogeochemical processes throughout its entire catchment [9,10]

Summary

Introduction

The qualitative status of European waterbodies under the Water Framework Directive (WFD) [1] is determined relative to fixed quantitative thresholds of chemical concentration and physical/ecological status. Monitoring at the outlets of agricultural catchments has revealed that concentrations of nutrients, phosphorus (P), in stream water become elevated during low-flow conditions and may exceed legislated thresholds [2,3]. Such elevated concentrations, while not necessarily reflective of the mean quality of the stream over the entire course of the year, may occur at critical times affecting. Water quality parameters have been measured on a ten-minute interval at each catchment outlet since 2009/2010 using bankside analyzers (Hach-Lange Sigmatax and Phosphax).

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