Abstract
This study provides a first national-scale assessment of the nutrient status of British headwater streams within the wider river network, by joint analysis of the national Countryside Survey Headwater Stream and Harmonised River Monitoring Scheme datasets. We apply a novel Nutrient Limitation Assessment methodology to explore the extent to which nutrients may potentially limit primary production in headwater streams and rivers, by coupling ternary assessment of nitrogen (N), phosphorus (P), and carbon (C) depletion, with N:P stoichiometry, and threshold P and N concentrations. P limitation was more commonly seen in the rivers, with greater prevalence of N limitation in the headwater streams. High levels of potential P and N co-limitation were found in the headwater streams, especially the Upland-Low-Alkalinity streams. This suggests that managing both P and N inputs may be needed to minimise risks of degradation of these sensitive headwater stream environments. Although localised nutrient impairment of headwater streams can occur, there were markedly lower rates of P and N impairment of headwater streams relative to downstream rivers at the national scale. Nutrient source contributions, relative to hydrological dilution, increased with catchment scale, corresponding with increases in the extent of agricultural and urban land-use. The estimated nutrient reductions needed to achieve compliance with Water Framework Directive standards, and to reach limiting concentrations, were greatest for the Lowland-High-Alkalinity rivers and streams. Preliminary assessments suggest that reducing P concentrations in the Lowland-High-Alkalinity headwater streams, and N concentrations in the Upland-Low-Alkalinity rivers, might offer greater overall benefits for water-quality remediation at the national scale, relative to the magnitude of nutrient reductions required. This approach could help inform the prioritisation of nutrient remediation, as part of a directional approach to water quality management based on closing the gaps between current and target nutrient concentrations.
Highlights
IntroductionIt is increasingly recognised that headwater streams are critical for freshwater biodiversity and play a key role in ecosystem service delivery (Biggs et al, 2017; Kelly-Quinn et al, 2017), and that they contribute cumulatively and often disproportionately to downstream water quality and to the wider functional integrity of downstream aquatic ecosystems (Alexander et al, 2007; Nadeau and Rains, 2007; Neal et al, 2012; Triska et al, 2007)
Higher reactive phosphorus (RP) and total oxidised nitrogen (TON) concentrations were observed in the major rivers compared with the headwater streams (Fig. SI1): median and mean headwater stream RP concentrations were 0.01 mg-P L−1 and 0.124 (0.591) mg-P L−1 respectively, compared with 0.06 and 0.205 (0.390) mg-P L−1 for the rivers
This study shows that there were markedly higher rates of P and N impairment of rivers relative to headwater streams at the national scale
Summary
It is increasingly recognised that headwater streams are critical for freshwater biodiversity and play a key role in ecosystem service delivery (Biggs et al, 2017; Kelly-Quinn et al, 2017), and that they contribute cumulatively and often disproportionately to downstream water quality and to the wider functional integrity of downstream aquatic ecosystems (Alexander et al, 2007; Nadeau and Rains, 2007; Neal et al, 2012; Triska et al, 2007) Their large contributing drainage areas relative to water-body size and low dilution capacity mean that headwater streams can be highly vulnerable to water–quality impairment (Dupas et al, 2015; Jarvie et al, 2010; Jarvie et al, 2008b; Withers et al, 2011). Compared with larger rivers, there is a relative paucity of large-scale strategic monitoring of headwater streams by national agencies, and they are largely excluded from water quality management planning, as there is no statutory obligation for monitoring headwater streams (Biggs et al, 2017; Lassaletta et al, 2010)
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