Abstract
Abstract. This paper introduces new insights into the hydrochemical functioning of lowland river systems using field-based spectrophotometric and electrode technologies. The streamwater concentrations of nitrogen species and phosphorus fractions were measured at hourly intervals on a continuous basis at two contrasting sites on tributaries of the River Thames – one draining a rural catchment, the River Enborne, and one draining a more urban system, The Cut. The measurements complement those from an existing network of multi-parameter water quality sondes maintained across the Thames catchment and weekly monitoring based on grab samples. The results of the sub-daily monitoring show that streamwater phosphorus concentrations display highly complex dynamics under storm conditions dependent on the antecedent catchment wetness, and that diurnal phosphorus and nitrogen cycles occur under low flow conditions. The diurnal patterns highlight the dominance of sewage inputs in controlling the streamwater phosphorus and nitrogen concentrations at low flows, even at a distance of 7 km from the nearest sewage treatment works in the rural River Enborne. The time of sample collection is important when judging water quality against ecological thresholds or standards. An exhaustion of the supply of phosphorus from diffuse and multiple septic tank sources during storm events was evident and load estimation was not improved by sub-daily monitoring beyond that achieved by daily sampling because of the eventual reduction in the phosphorus mass entering the stream during events. The results highlight the utility of sub-daily water quality measurements and the discussion considers the practicalities and challenges of in situ, sub-daily monitoring.
Highlights
The chemical and ecological status of surface waters is an increasing concern worldwide as demand for water and food grows with population increases
River system water quality is affected by changes in the controlling factors of land use and management, atmospheric deposition, water usage and climate; much research has been devoted to the development of integrated catchment management strategies to protect and improve the freshwater resource and ecological status whilst the landscape is used for homes, industry, farming, forestry and recreation (Beman et al, 2005; Boyer et al, 2006)
In addition to insights into the water quality dynamics of these sites and ecosystem function in The Cut in terms of a preliminary analysis of productivity, photosynthesis and respiration, this paper considers the challenges associated with running analytical instrumentation in the field and details the methodologies
Summary
The chemical and ecological status of surface waters is an increasing concern worldwide as demand for water and food grows with population increases. The spatial and temporal variations in streamwater chemistry are highly complex and the links between these changes and the controlling factors are subject to intense study, yet the interactions between the physical and chemical environment of rivers and their ecological status are poorly understood (Hilton et al, 2006). More work is required to improve understanding of the hydrochemical changes at sub-daily intervals to elucidate the links between the physical and chemical environment of a river system and its ecological structure, function and biodiversity Work is needed to determine if chemical thresholds have meaning when assessing ecological status to implement the European Union Water Framework Directive (WFD) and Habitats Directive
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