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Tryptophan-like fluorescence (TLF) is used to indicate anthropogenic inputs of dissolved organic matter (DOM), typically from wastewater, in rivers. We hypothesised that other sources of DOM, such as groundwater and planktonic microbial biomass can also be important drivers of riverine TLF dynamics. We sampled 19 contrasting sites of the River Thames, UK, and its tributaries. Multivariate mixed linear models were developed for each site using 15 months of weekly water quality observations and with predictor variables selected according to the statistical significance of their linear relationship with TLF following a stepwise procedure. The variables considered for inclusion in the models were potassium (wastewater indicator), nitrate (groundwater indicator), chlorophyll-a (phytoplankton biomass), and Total bacterial Cells Counts (TCC) by flow cytometry. The wastewater indicator was included in the model of TLF at 89 % of sites. Groundwater was included in 53 % of models, particularly those with higher baseflow indices (0.50-0.86). At these sites, groundwater acted as a negative control on TLF, diluting other potential sources. Additionally, TCC was included positively in the models of six (32 %) sites. The models on the Thames itself using TCC were more rural sites with lower sewage inputs. Phytoplankton biomass (Chlorophyll-a) was only used in two (11 %) site models, despite the seasonal phytoplankton blooms. It is also notable that, the wastewater indicator did not always have the strongest evidence for inclusion in the models. For example, there was stronger evidence for the inclusion of groundwater and TCC than wastewater in 32 % and 5 % of catchments, respectively. Our study underscores the complex interplay of wastewater, groundwater, and planktonic microbes, driving riverine TLF dynamics, with their influence determined by site characteristics., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024. Published by Elsevier B.V.)

Catchment based solutions are being sought to mitigate water quality pressures and achieve multiple benefits but their success depends on a sound understanding of catchment functioning. Novel approaches to monitoring and data analysis are urgently needed. In this paper we explore the potential of river water fluorescence at the catchment scale in understanding nutrient concentrations, sources and pathways. Data were collected from across the River Thames basin from January 2012 to March 2015. Analysing emission excitation matrices (EEMs) using both PARAFAC and optimal area averaging produced consistent results for humic-like component 1 and tryptophan-like component 4 in the absence of a subset of samples that exhibited an unusual peak; illustrating the importance of inspecting the entire EEM before using peak averaging methods. Strong relationships between fluorescence components and dissolved organic carbon (DOC), soluble reactive phosphorus (SRP), and ammonium clearly demonstrated its potential, in this study basin, as a field based surrogate for nutrients. Analysing relationships between fluorescence, catchment characteristics and boron from across the basin enabled new insights into the provenance of nutrients. These include evidence for diffuse sources of DOC from near surface hydrological pathways (i.e. soil horizons); point source inputs of nutrients from sewage effluent discharges; and diffuse contributions of nutrients from agriculture and/or sewage (e.g. septic tanks). The information gained by broad scale catchment wide monitoring of fluorescence could support catchment managers in (a) prioritising subcatchments for nutrient mitigation; (b) providing information on relative nutrient source contributions; and (c) providing evidence of the effectiveness of investment in pollution mitigation measures. The collection of high resolution fluorescence data at the catchment scale and, in particular, over shorter event timescales would complement broad scale assessments by enhancing our hydro-biogeochemical process understanding., (Copyright © 2018. Published by Elsevier B.V.)

Recent river studies have observed rapid phytoplankton dynamics, driven by diurnal cycling and short-term responses to storm events, highlighting the need to adopt new high-frequency characterisation methods to understand these complex ecological systems. This study utilised two such analytical methods; pigment analysis by high performance liquid chromatography (HPLC) and cell counting by flow cytometry (FCM), alongside traditional chlorophyll spectrophotometry and light microscopy screening, to characterise the major phytoplankton bloom of 2015 in the River Thames, UK. All analytical techniques observed a rapid increase in chlorophyll a concentration and cell abundances from March to early June, caused primarily by a diatom bloom. Light microscopy identified a shift from pennate to centric diatoms during this period. The initial diatom bloom coincided with increased HPLC peridinin concentrations, indicating the presence of dinoflagellates which were likely to be consuming the diatom population. The diatom bloom declined rapidly in early June, coinciding with a storm event. There were low chlorophyll a concentrations (by both HPLC and spectrophotometric methods) throughout July and August, implying low biomass and phytoplankton activity. However, FCM revealed high abundances of pico-chlorophytes and cyanobacteria through July and August, showing that phytoplankton communities remain active and abundant throughout the summer period. In combination, these techniques are able to simultaneously characterise a wider range of phytoplankton groups, with greater certainty, and provide improved understanding of phytoplankton functioning (e.g. production of UV inhibiting pigments by cyanobacteria in response to high light levels) and ecological status (through examination of pigment degradation products). Combined HPLC and FCM analyses offer rapid and cost-effective characterisation of phytoplankton communities at appropriate timescales. This will allow a more-targeted use of light microscopy to capture phytoplankton peaks or to investigate periods of rapid community succession. This will lead to greater system understanding of phytoplankton succession in response to biogeochemical drivers., (Crown Copyright © 2017. Published by Elsevier B.V. All rights reserved.)

River phytoplankton blooms can pose a serious risk to water quality and the structure and function of aquatic ecosystems. Developing a greater understanding of the physical and chemical controls on the timing, magnitude and duration of blooms is essential for the effective management of phytoplankton development. Five years of weekly water quality monitoring data along the River Thames, southern England were combined with hourly chlorophyll concentration (a proxy for phytoplankton biomass), flow, temperature and daily sunlight data from the mid-Thames. Weekly chlorophyll data was of insufficient temporal resolution to identify the causes of short term variations in phytoplankton biomass. However, hourly chlorophyll data enabled identification of thresholds in water temperature (between 9 and 19°C) and flow (<30m(3)s(-1)) that explained the development of phytoplankton populations. Analysis showed that periods of high phytoplankton biomass and growth rate only occurred when these flow and temperature conditions were within these thresholds, and coincided with periods of long sunshine duration, indicating multiple stressor controls. Nutrient concentrations appeared to have no impact on the timing or magnitude of phytoplankton bloom development, but severe depletion of dissolved phosphorus and silicon during periods of high phytoplankton biomass may have contributed to some bloom collapses through nutrient limitation. This study indicates that for nutrient enriched rivers such as the Thames, manipulating residence time (through removing impoundments) and light/temperature (by increasing riparian tree shading) may offer more realistic solutions than reducing phosphorus concentrations for controlling excessive phytoplankton biomass., (Crown Copyright © 2016. Published by Elsevier B.V. All rights reserved.)

Eighteen months of 7-hourly analyses of rainfall and stream water chemistry are presented, spanning a wide range of chemical determinands and building on over 20 years of weekly records for the moorland headwaters of the river Severn. The high-frequency time series data show that hydrochemical responses to major hydrological and biological drivers of short-term variability in rainfall and rivers are not captured by conventional low-frequency monitoring programmes. A wealth of flow related, flow independent, diurnal, seasonal and annual fluctuations indicate a cacophony of interactions within the catchment and stream. The complexity of the chemical dynamics is visually obvious, although there appears to be no clear way of translating this complexity into a simple algorithm. The work provides a proof of concept for the complex structure of catchment functioning revealed by extensive high-frequency measurements coupled with high analytical sensitivity and reproducibility. It provides new insights into hydrogeochemical functioning and a novel resource for catchment modelling., (Copyright © 2011 Elsevier B.V. All rights reserved.)

Soluble reactive phosphorus (SRP) concentrations in the River Thames, south east England, have significantly decreased from an annual maximum of 2100 μg l(-1) in 1997 to 344 in 2010, primarily due to the introduction of phosphorus (P) removal at sewage treatment works within the catchment. However, despite this improvement in water quality, phytoplankton biomass in the River Thames has greatly increased in recent years, with peak chlorophyll concentrations increasing from 87 μg l(-1) in the period 1997 to 2002, to 328 μg l(-1) in 2009. A series of within-river flume mesocosm experiments were performed to determine the effect of changing nutrient concentrations and light levels on periphyton biomass accrual. Nutrient enrichment experiments showed that phosphorus, nitrogen and silicon were not limiting or co-limiting periphyton growth in the Thames at the time of the experiment (August-September 2010). Decreasing ambient SRP concentration from 225 μg l(-1) to 173 μg l(-1) had no effect on periphyton biomass accrual rate or diatom assemblage. Phosphorus limitation became apparent at 83 μg SRP l(-1), at which point a 25% reduction in periphyton biomass was observed. Diatom assemblage significantly changed when the SRP concentration was reduced to 30 μg l(-1). Such stringent phosphorus targets are costly and difficult to achieve for the River Thames, due to the high population density and intensive agriculture within the Thames basin. Reducing light levels by shading reduced the periphyton accrual rate by 50%. Providing shading along the River Thames by planting riparian tree cover could be an effective measure to reduce the risk of excessive algal growth. If the ecology of the Thames is to reach the WFD's "good ecological status", then both SRP concentration reductions (probably to below 100 μg l(-1)) and increased shading will be required., (Crown Copyright © 2011. Published by Elsevier B.V. All rights reserved.)

Chlorophyll-a and nutrient concentrations were monitored at weekly intervals across 21 river sites throughout the River Thames basin, southern England, between 2009 and 2011. Despite a 90% decrease in soluble reactive phosphorus (SRP) concentration of the lower River Thames since the 1990s, very large phytoplankton blooms still occur. Chlorophyll concentrations were highest in the mid and lower River Thames and the larger tributaries. Lowest chlorophyll concentrations were observed in the smaller tributaries, despite some having very high phosphorus concentrations of over 300 μg l(-1). There was a strong positive correlation between river length and mean chlorophyll concentration (R(2)=0.82), and rivers connected to canals had ca. six times greater chlorophyll concentration than 'natural' rivers with similar phosphorus concentrations, indicating the importance that residence time has on determining phytoplankton biomass. Phosphorus concentration did have some influence, with phosphorus-enriched rivers having much larger phytoplankton blooms than nutrient-poor rivers of a similar length. Water quality improvements may now be capping chlorophyll peaks in the Rivers Thames and Kennet, due to SRP depletion during the spring/early summer phytoplankton bloom period. Dissolved reactive silicon was also depleted to potentially-limiting concentrations for diatom growth in the River Thames during these phytoplankton blooms, but nitrate remained in excess for all rivers throughout the study period. Other potential mitigation measures, such as increasing riparian shading and reducing residence times by removing impoundments may be needed, alongside phosphorus mitigation, to reduce the magnitude of phytoplankton blooms in the future., (Crown Copyright © 2012. Published by Elsevier B.V. All rights reserved.)

The water quality of the River Frome, Dorset, southern England, was monitored at weekly intervals from 1965 until 2009. Determinands included phosphorus, nitrogen, silicon, potassium, calcium, sodium, magnesium, pH, alkalinity and temperature. Nitrate-N concentrations increased from an annual average of 2.4 mg l⁻¹ in the mid to late 1960s to 6.0 mg l⁻¹ in 2008-2009, but the rate of increase was beginning to slow. Annual soluble reactive phosphorus (SRP) concentrations increased from 101 μg l⁻¹ in the mid 1960s to a maximum of 190 μg l⁻¹ in 1989. In 2002, there was a step reduction in SRP concentration (average=88 μg l⁻¹ in 2002-2005), with further improvement in 2007-2009 (average=49 μg l⁻¹), due to the introduction of phosphorus stripping at sewage treatment works. Phosphorus and nitrate concentrations showed clear annual cycles, related to the timing of inputs from the catchment, and within-stream bioaccumulation and release. Annual depressions in silicon concentration each spring (due to diatom proliferation) reached a maximum between 1980 and 1991, (the period of maximum SRP concentration) indicating that algal biomass had increased within the river. The timing of these silicon depressions was closely related to temperature. Excess carbon dioxide partial pressures (EpCO₂) of 60 times atmospheric CO₂ were also observed through the winter periods from 1980 to 1992, when phosphorus concentration was greatest, indicating very high respiration rates due to microbial decomposition of this enhanced biomass. Declining phosphorus concentrations since 2002 reduced productivity and algal biomass in the summer, and EpCO₂ through the winter, indicating that sewage treatment improvements had improved riverine ecology. Algal blooms were limited by phosphorus, rather than silicon concentration. The value of long-term water quality data sets is discussed. The data from this monitoring programme are made freely available to the wider science community through the CEH data portal (http://gateway.ceh.ac.uk/)., (Crown Copyright © 2011. Published by Elsevier B.V. All rights reserved.)

The changing patterns of riverine boron concentration are examined for the Thames catchment in southern/southeastern England using data from 1997 to 2007. Boron concentrations are related to an independent marker for sewage effluent, sodium. The results show that boron concentrations in the main river channels have declined with time especially under baseflow conditions when sewage effluent dilution potential is at its lowest. While boron concentrations have reduced, especially under low-flow conditions, this does not fully translate to a corresponding reduction in boron flux and it seems that the "within-catchment" supplies of boron to the river are contaminated by urban sources. The estimated boron reduction in the effluent input to the river based on the changes in river chemistry is typically around 60% and this figure matches with an initial survey of more limited data for the industrial north of England. Data for effluent concentrations at eight sewage treatment works within the Kennet also indicate substantial reductions in boron concentrations: 80% reduction occurred between 2001 and 2008. For the more contaminated rivers there are issues of localised rather than catchment-wide sources and uncertainties over the extent and nature of water/boron stores. Atmospheric sources average around 32 to 61% for the cleaner and 4 to 14% for the more polluted parts. The substantial decreases in the boron concentrations correspond extremely well with the timing and extent of European wide trends for reductions in the industrial and domestic usage of boron-bearing compounds. It clearly indicates that such reductions have translated into lower average and peak concentrations of boron in the river although the full extent of these reductions has probably not yet occurred due to localised stores that are still to deplete.

Phosphorus concentrations in the upper River Thames Basin (southeastern England) are described and linked to sewage effluent sources. Weekly surveys between 1997 and 2007 of the Thames and two of its major tributaries, the Thame and the Kennet indicated that phosphorus was mainly in soluble reactive (SRP) form. Baseflow concentrations in the Thames reduced from 1584microg/l in 1998 to 376microg/l in 2006 and from 2655 to 715microg/l for the Thame. Flow response, flux and endmember mixing analysis indicated that these declines resulted from SRP reductions in sewage treatment works (STW) effluent following phosphorus stripping for the major STWs in the region. This was confirmed by comparing our analysis with direct measurements of SRP in the effluents based on Environment Agency data. A within-river loss under baseflow of approximately 64% (range 56-78%) of the SRP-effluent input was estimated for the Thames, with a near balance for the Thame. SRP concentrations in the Kennet were an order of magnitude lower than the Thames/Thame: non-point sources dominated and were important for all the rivers at high flows. It was concluded that removal of SRP from effluents would be insufficient SRP in the Thames and Thame to meet annual average environmental targets of 50 to 120microg/l. The paper flags the value of combining hydrological/chemical tracing and concentration/flux approaches to data interrogation and the bonus of having actual measurements of the effluent. It highlights the need for fuller assessment of water storage/sediment/biota interactions for phosphorus and for caution in using boron as a long-term tracer for effluent inputs, its concentrations having declined markedly in response to reduced usage in washing powders: the value of using sodium as a tracer for examining SRP changes is shown.

Chlorophyll-a concentration variations are described for two major river basins in England, the Humber and the Thames and related to catchment characteristics and nutrient concentrations across a range of rural, agricultural and urban/industrial settings. For all the rivers there are strong seasonal variations, with concentrations peaking in the spring and summer time when biological activity is at its highest. However, there are large variations in the magnitude of the seasonal effects across the rivers. For the spring-summer low-flow periods, average concentrations of chlorophyll-a correlate with soluble reactive phosphorus (SRP). Chlorophyll-a is also correlated with particulate nitrogen (PN), organic carbon (POC) and suspended sediments. However, the strongest relationships are with catchment area and flow, where two straight line relationships are observed. The results indicate the importance of residence times for determining planktonic growth within the rivers. This is also indicated by the lack of chlorophyll-a response to lowering of SRP concentrations in several of the rivers in the area due to phosphorus stripping of effluents at major sewage treatment works. A key control on chlorophyll-a concentration may be the input of canal and reservoir waters during the growing period: this too relates to issues of residence times. However, there may well be a complex series of factors influencing residence time across the catchments due to features such as inhomogeneous flow within the catchments, a fractal distribution of stream channels that leads to a distribution of residence times and differences in planktonic inoculation sources. Industrial pollution on the Aire and Calder seems to have affected the relationship of chlorophyll-a with PN and POC. The results are discussed in relation to the Water Framework Directive.

The spatial and temporal patterns of in-stream nitrate concentrations for the upper Thames and selected tributaries are described in relation to point and diffuse sources for these rural catchments. The rivers associated with catchments dominated by permeable (Cretaceous Chalk) bedrock show a smaller range in nitrate concentrations than those associated with clay and mixed sedimentary bedrock of lower permeability. The differences reflect the contrasting nature of water storage within the catchments and the influence of point and diffuse sources of nitrate. Nitrate concentrations often increase in a gradual way as a function of flow for the rivers draining the permeable catchments, although there is usually a minor dip in nitrate concentrations at low to intermediate flow due to (1) within-river uptake of nitrate during the spring and the summer when biological activity is particularly high and (2) a seasonal fall in the water table and a change in preferential flow-pathway in the Chalk. There is also a decrease in the average nitrate concentration downstream for the Kennet where average concentrations decrease from around 35 to 25 mg NO(3) l(-1). For the lower permeability catchments, when point source inputs are not of major significance, nitrate concentrations in the rivers increase strongly with increasing flow and level off and in some cases then decline at higher flows. When point source inputs are important, the initial increase in nitrate concentrations do not always occur and there can even be an initial dilution, since the dilution of point sources of nitrate will be lowest under low-flow conditions. For the only two tributaries of the Thames which we have monitored for over 5 years (the Pang and the Kennet), nitrate concentrations have increased over time. For the main stem of the Thames, which was also monitored for over 5 years, there is no clear increase over time. As the Pang and the Kennet river water is mainly supplied from the Chalk, the increasing nitrate concentrations over time clearly reflect increasing nitrate concentrations within the groundwater. It primarily reflects long-term trends for agricultural fertilizer inputs and significant aquifer storage and long water residence times. The results are discussed in terms of hydrogeochemical processes and the Water Framework Directive and are compared with data from other eastern UK rivers. The importance of diffuse sources of nitrate contamination is highlighted. On a flow weighted basis, the average diffuse component of nitrate is around 95% for the Thames Basin rivers draining Chalk and for the corresponding rivers draining less permeable strata, there is a more significant but not major point source component (at least in terms of flux); the average diffuse component is 79% in this case. These data fit well with earlier assessments of agricultural sources to UK surface waters. Under baseflow conditions the diffuse sources remain dominant for the Chalk fed Thames Basin rivers, but point sources can be dominant for the low permeability cases. On a proportionate basis, the Thames Basin rivers are similar to the rural rivers of the Tweed and Humber Basins in terms of percentage diffuse components although the lower intensity agriculture occurring for the rivers monitored means that the average nitrate concentrations are lower for the rural rivers of central and northern England and the borders with Scotland: the Humber and Tweed.

The water quality of the River Cherwell and a tributary of it, the Ray, are described in terms of point and diffuse sources of pollution, for this rural area of the upper Thames Basin. Point sources of pollution dominate at the critical ecological low flow periods of high biological activity. Although the surface geology is predominantly clay, base flow is partly supplied from springs in underlying carbonate-bearing strata, which influences the water quality particularly with regards to calcium and alkalinity. The hydrogeochemistry of the river is outlined and the overall importance of urban point sources even in what would normally be considered to be rural catchments is stressed in relation to the European Unions Water Framework Directive. Issues of phosphorus stripping at sewage treatment works are also considered: such stripping on the Cherwell has reduced phosphorus concentrations by about a factor of two, but this is insufficient for the needs of the Water Framework Directive.

The water quality of the River Thame, a tributary of the River Thames in the Thames basin, is described in relation to point and diffuse contaminant inputs and runoff from permeable and impermeable bedrock geology with their own characteristic water quality. The data is examined to see if the market town of Aylesbury in the upper part of the catchment influences water quality. Previous studies highlighted the influence of Aylesbury sewage treatment works (STW) on soluble reactive phosphorus (SRP) concentrations in the river before and after phosphorus (P) stripping at the STW. Variations in water quality along the river are described and the study indicates that, apart from SRP, water quality determinants seem to be relatively unaffected by Aylesbury. The Thame water quality is compared with other catchment typologies and it is very similar to that of the main stem of the Thames even though the Thames is mainly Chalk groundwater fed. Differences in water quality largely link to the amount of STW effluent within the rivers and to the endmember compositions of the groundwater and near surface water sources.

Variations in phosphorus (P) concentrations in an agriculturally impacted river draining a Chalk aquifer and an associated canal in the west of the Thames Basin, southern England are examined and linked to agricultural and sewage sources and within river/canal process controls. The study area comprises the River Dun, the adjacent River Kennet and the Kennet and Avon (K&A) Canal. Large seasonal variations are observed for soluble reactive phosphorus (SRP) and dissolved silicon (Si) with low concentrations in the spring and summer times when biological activity is high. The K&A Canal shows the largest SRP and Si concentration declines. This reflects high biological activity coupled with higher temperatures and higher water residence times. The extent of SRP removal is examined in relation to organic (uptake/release with phytoplankton growth/decay) and, to a lesser extent, inorganic (SRP coprecipitation with calcite) mechanisms. Boron (B) is used as a tracer of sewage sources. Agricultural inputs of both dissolved and particulate P (PP) can be important particularly under conditions where the catchment is wet and near surface/overland flow is important: sewage treatment works effluent and septic tank discharges to groundwater also probably provide a major component of the SRP occurring within the water column. The canal, and to a lesser extent the river, acts as sink for P in sewage effluent sources due to the high biological activity especially during the spring and summer. The aquifer probably acts as a major sink for agricultural and septic tank inputs of P.

An extensive study of acidic and acid sensitive moorland and forested catchments in mid-Wales is used to show the water quality functioning with respect to nitrate and ammonium. For this, long-term records of rainfall, cloud water, throughfall, stemflow and stream water (up to 18 years of weekly data) are combined with shorter duration information on stream water associated with small tributary sources and drainage ditches, ground water from a network of exploratory boreholes and paired control and felled catchments. The ratio of nitrate to ammonium is about one in rainfall, cloud water, throughfall and stemflow but the concentrations are much lower in rainfall (approximately 25 microM l(-1)) than in cloud water (approximately 300 microM l(-1)) while throughfall and stemflow are intermediate (approximately 80 microM l(-1)). Within the streams draining moorland and forested areas, nitrate concentrations are close to the mean value in rainfall while ammonium concentrations are often over an order of magnitude lower in the stream than in rainfall and are typically only about a fifth that of nitrate. With felling, stream water nitrate concentrations increase for podzolic soils but show a variable response for gley soils. For the streams draining forested podzols, the concentrations of nitrate can be up to an order of magnitude higher for the first few years after felling compared to than pre-fell values but in later years, concentrations decline to pre-fell and even lower levels. Felling for the podzolic soils barely leads to any changes in ammonium concentration. For the gley soils, felling results in an order of magnitude increase in nitrate and ammonium for a small drainage ditch, but the pulse barely reaches the main stream channel. Rather, within-catchment and within-stream processes not only take up the nitrate and ammonium fluxes generated, but in the case of nitrate, concentrations with- and post-felling are lower than pre-felling concentrations. Groundwater concentrations of nitrate for the moorland and forested catchments are slightly lower than for the streams while for ammonium the reverse is the case: ammonium concentrations in groundwater are about a tenth those of nitrate. With felling, groundwater nitrate concentrations show sporadic increases. For two boreholes, these increases occur during wet periods when groundwater levels are at their shallowest; for one other borehole, there is a gradual and sustained increase over several years. The results are explained in relation to the dominant hydrogeochemical processes operative.

Soluble reactive phosphorus (SRP) data are presented for rainfall, cloud water, soil waters, stream waters and groundwaters at the Plynlimon catchments in mid Wales to examine the hydrochemical functioning of inorganic phosphorus for an acidic and acid sensitive area characteristic of much of the UK uplands. In general, stream water concentrations are low compared to lowland areas. Average concentrations of SRP in rainfall and cloud water (0.3 and 0.9 microM l(-1), respectively) are higher than in stream water with wider ranges in concentration (0-19.3 and 0-20.9 microM l(-1), respectively). Throughfall and stemflow is enriched in SRP compared to rain and cloud water by a factor of approximately twofold and sixfold, respectively: the average concentrations and ranges are 0.73 and 0-6.61 microM l(-1) for throughfall and 2.12 and 0-18.61 microM l(-1) for stemflow. Soil water SRP concentrations measured in the surface layers of representative areas of podzol and gley soils, are further enriched with respect to inputs. Average concentrations and ranges for the L/F and Oh horizons in the podzols are 3.1 microM l(-1) (range: 0.03-17.2 microM l(-1)) and 0.75 microM l(-1) (range: 0.03-2.64 microM l(-1)), respectively. Correspondingly, the average values and ranges for the L/F and Oh horizons in the gley are 2 microM l(-1) (range: 0.03-16.65 microM l(-1)) and 0.4 microM l(-1) (range: 0.03-8.61 microM l(-1)). SRP concentrations in stream and ground water are lower than in atmospheric inputs and surface soil waters and show marked spatial variability. This variability is linked to three catchment features. (1) For streams draining podzolic soils, most of the SRP is retained by the catchment. For this situation, stream and ground waters have average concentrations of approximately 0.05 microM l(-1) with a range of 0-1.47 microM l(-1). There is no clear stream or groundwater SRP response to felling despite a large release of SRP from felling debris (brash) and the forest floor (L/F horizon) with average post-felling concentrations of 11.02 microM l(-1) (0.40-155.0 microM l(-1)) and 23.60 microM l(-1) (0.26-172.23 microM l(-1)), respectively. (2) For forested catchments with gley soils, stream water SRP concentrations are more variable with, in one case, much higher concentrations than for the podzol counterparts (range in average 0.05-0.46 microM l(-1)). (3) For the streams draining gley soils, felling results in a mixed SRP response. At the local scale (ditch drainage), there is a marked enrichment in SRP concentration (average concentrations increase from 0.05 to 1.31 microM l(-1), with a peak concentration of 4.0 microM l(-1)). This response is consistent with the observed mobilisation of SRP from brash and forest floor material (post-felling mean concentrations of 9.39 and 11.94 microM l(-1), respectively). However, stream water concentrations are an order of magnitude lower than observed in the soil waters implying considerable immobilisation of SRP between the soils and the stream. At the larger catchment scale, no discernable enrichment in SRP is observed following felling. The results are related to input-output budgets and the findings interpreted in terms of the dominant hydrogeochemical processes operative and environmental management issues.

The relationship between calcium carbonate saturation and phosphorus concentrations for seven sites on the upper reaches of the River Kennet are examined. The findings are related to issues of groundwater supplies and the introduction of phosphorus treatment of effluent from the Marlborough sewage treatment works (STW) at part of the way along the study reach. Being supplied from a Cretaceous Chalk aquifer, the Kennet is mainly of a calcium-bicarbonate type and has a relatively constant composition of many major water quality determinands. Typically, the waters average a pH of approximately eight (range approx. 7.5-8.5) during the day with the lowest values occurring at the upstream site. Dissolved carbon dioxide varies from approximately 5 to 35 times atmospheric pressure during the late morning with the highest values occurring at the upstream site. However, in-stream biological activity gives rise to marked diurnal fluctuations in pH and dissolved carbon dioxide concentrations and during the summer months, by mid to late afternoon, pH is at its maximum and dissolved carbon dioxide is at its lowest: this is shown by continuous measurements at one of the river sites. Alkalinity and calcium concentrations remain relatively constant at approximately 4,700 microEq/l (range 3,500-6,000 microEq/l) and 120 mg/l (range 85-150 mg/l), respectively, and the waters are oversaturated with respect to calcium carbonate (calcite) typically by a factor of six (range 2-25). Along the reach, soluble reactive phosphate (SRP) increases from the first to the second site with the introduction of sewage supplies from the Marlborough STW, and then declines further downstream as sewage dilution and uptake by the river bed/aquatic plants increases. The differences in concentration decrease after phosphorus removal from Marlborough STW. Despite this change, there is no clear indication of any calcite solubility control except perhaps at times of extreme baseflow during the growing season when within-stream photosynthesis is maximal and within-stream residence times are longer. A comparison of river and groundwater data shows that the groundwaters have similar alkalinities and calcium concentrations. However, the groundwaters have (a) higher carbon dioxide saturations (a factor of 2-5 times the value for the river), (b) lower pHs (0.5-1.5 units), (c) lower SRP concentrations (a quarter or less of the river values) and (d) waters near calcite saturation (unlike the surface waters which are oversaturated). The findings indicate a river system dominated by the input carbon dioxide laden groundwaters in approximate equilibrium with calcite attenuated by within-channel biological and physical processes. Within the river: (a) the waters degas carbon dioxide increasing the pH, producing oversaturated conditions; and (b) oscillating pH-dissolved carbon dioxide levels occur between day and night due to changing balances between photosynthesis and respiration. It seems that lowering the phosphorus levels have not resulted in calcite precipitation within the water column and that no significant within-stream self-cleansing mechanisms are occurring that might be predicted from theory: other components in the water such as dissolved organic carbon may inhibit calcite nucleation. However, the low SRP levels in the groundwater coupled with calcite saturation, may well indicate that phosphorous concentrations within the groundwater are regulated by such processes: the number of calcite nucleating sites are orders of magnitude higher and the calcite inhibitors may be less prevalent.

This paper examines the growth and uptake of phosphorus into algal biofilms in the River Kennet, a lowland chalk (Cretaceous-age) stream in southern England. Algal biofilms were grown on artificial plastic substrates (templates) placed (i) on the riverbed and (ii) within the mid-water column. Experiments were set up to examine differences in growth rates of newly colonising biofilms compared with biofilms left to accumulate for periods of up to 6 months. Rates of algal biofilm production were measured by the chlorophyll a concentration that had accumulated per cm2 over the number of days that the biofilm template had been immersed in the river water. An algal biofilm bloom occurred in early spring, prior to peak suspended chlorophyll a concentrations within the water column. Biofilm samples collected in February and March had the highest chlorophyll a and total phosphorus concentrations. The biofilm bloom corresponded with increased solar radiation and declining river flow conditions. Periodic increases in soluble reactive phosphorus concentrations in the overlying river water did not correspond with any significant increase in biofilm production. These results suggest that light, rather than phosphorus is a key factor for biofilm growth in the River Kennet. Higher rates of chlorophyll a development in mid-water column biofilms may be linked to greater light exposure; however, maximum total-P concentrations were similar for both bed and water column biofilms. Newly colonising biofilms exhibited higher chlorophyll a and total-P concentrations than biofilms left to accumulate over longer terms, suggesting that fresh substrate availability promotes high rates of biofilm growth. Both 'condensed and organic' P (stored in biomass) and 'inorganic' (mineral) P fractions within the biofilms were present in varying proportions, although the early spring biofilm bloom resulted in maximum proportions and absolute concentrations of 'condensed and organic' P. Calcite was the only crystalline mineral detected within the biofilms. Ratios of Ca:inorganic P are largely consistent with the presence of CaCO3-P co-precipitates, although one very low value suggested that there may also be additional sources of inorganic P, possibly P adsorbed to clays or organics within the biofilm. However, poor linkages between CaCO3 and inorganic P concentrations suggest that, although the inorganic P fraction within the biofilm may be derived largely from CaCO3-P co-precipitation, the subsequent processes controlling overall CaCO3 and inorganic P concentrations in the biofilm are complex.

The water quality of seven sites on the upper reaches of the River Kennet round the market town of Marlborough is described and related to the introduction of phosphorus treatment of effluent from Marlborough sewage treatment works (STW). The River Kennet is mainly groundwater-fed from a Cretaceous chalk aquifer and hence the river water is calcium- and bicarbonate-bearing and has a relatively constant composition of many major water quality determinants. In-stream biological activity gives rise to marked diurnal fluctuations in pH (of approx. 0.8 units). Dissolved carbon dioxide and dissolved oxygen also show marked diurnal fluctuations. Dissolved carbon dioxide varies from approximately 10 to 70 times atmospheric pressure, indicating net release of carbon dioxide and the dominance of heterotrophic (respiratory) processes over autotrophic processes (photosynthesis). Much of the excess carbon dioxide is probably associated with carbon dioxide laden groundwater inputs and the relatively short within-stream residence times ensures only limited degassing to the atmosphere. Diurnal fluctuations in dissolved oxygen vary from approximately 20% to 200% saturation. For both dissolved carbon dioxide and dissolved oxygen, the amplitude of fluctuations is much lower during the winter period, when biological activity is at its lowest. The concentrations of soluble reactive phosphorus (SRP), total phosphorus (TP) and boron increase markedly just downstream of the sewage works as a result of this point source input. These concentrations slowly decline further downstream as additional groundwater inputs dilute the effluent further. The introduction of chemical treatment of sewage effluent for phosphorus reduction at Marlborough STW resulted in a marked decrease in within-river SRP and TP concentrations to levels approximately the same as those upstream of the STW. A comparison of SRP and boron concentrations reveals a reduction in in-stream SRP concentrations by approximately 75% following effluent treatment. In terms of within-river processes controlling in-stream phosphorus concentrations, previous studies have indicated that one potentially important mechanism within calcium bicarbonate bearing rivers may be related to co-precipitation of phosphorus with calcium carbonate (calcite). The present study shows that the waters are oversaturated with respect to calcium carbonate, that no equilibrium conditions exist and that phosphorus removal has led to undetectable changes in calcium carbonate oversaturation. Hence, it is concluded that the primary changes in phosphorus levels within the river is directly associated with changing point source contributions from the STW and physical dilution within the river. However (1) the results relate to only the first year of study and subsequent differences may become apparent and (2) reactions between the water column and plant and bottom sediment interfaces may be important in regulating phosphorus fluxes within the river. The results presented in this paper mark a pilot phase of a longer-term initiative and this paper provides a background setting. The paper discusses the longer-term objectives and important gaps in knowledge of the system that requires further address.

This paper examines spatial and temporal variability in freshwater inputs of trace elements and major ions to the tidal reaches of the River Tweed. The relationships between concentrations of major ions and trace elements (in dissolved and acid-available particulate forms) and flow are examined, and a simple two-component endmember mixing analysis performed to link river water chemistry to catchment sources, and to identify whether within-river processes modify concentrations to produce non-conservative behaviour. The results indicate that most dissolved major ions and trace elements behave conservatively in the lower reaches of the River Tweed, the variability in concentrations being dominated by hydrology and the existence of different high-flow and low-flow endmember runoff chemistries. This suggests that the variability in concentration of most dissolved trace elements and major ions in the lower Tweed can be modelled using simple mixing relationships. However, the relationships between pH, Ca, Mg and Gran alkalinity show pronounced non-conservative behaviour, indicating the importance of understanding within-river process for modelling these determinants. The non-conservative behaviour identified is related to biological controls and the resultant diurnal variations in pH which promote daytime removal of Ca from solution by precipitation of calcium carbonate (and corresponding removal of Mg by co-precipitation) during extreme low-flow conditions and algal blooms during the summer.

The water quality of a tributary of the Thames, the Pang, draining a rural part of the Thames basin is described and related to the influences of inputs from farming activity and groundwater from the underlying chalk aquifer as modified by biological within-stream processes. The groundwater inputs ensures that the waters are calcium and bicarbonate bearing and have relatively uniform concentrations. Agricultural inputs result in enhanced levels of nutrients, nitrate and soluble reactive phosphorus (SRP) in particular. For nitrate, the concentrations are higher during the winter months due to increased surface runoff. In contrast, SRP shows a more erratic behaviour with higher concentrations occurring during the early storm hydrograph rise following summer baseflow recession. Within the stream, biological activity results in: (a) marked diurnal fluctuations in the dissolved levels of carbon dioxide and oxygen; (b) a strong seasonal pattern in chlorophyll a levels; and (c) dissolved silica concentration reductions during the early spring period. Carbon dioxide levels are particularly high in the groundwater (typically approximately 60 times the atmospheric value) as observed at a spring discharge. However, within the stream, considerable degassing occurs although values remain an order of magnitude above atmospheric pressure. The findings are discussed in the context of the water quality functioning of agriculturally and sewage impacted southern eastern UK rivers. For example, the work shows that unlike for riverine systems with point sewage discharges or limited groundwater storage, there is a very poor link between chemical concentrations and flow even for components such as SRP and boron which are often connected to sewage discharges.

Water quality information is presented for the River Thames 34 km downstream of the market town of Oxford in Oxfordshire to provide an overview of the hydrochemical functioning of a major agriculturally impacted river entering the North Sea. The data, which cover the period from the spring of 1997 to the spring of 1999, relate to three types of data. These types are: (1) weekly spot sampling for determination of major, minor and trace elements, pH, alkalinity and herbicides; (2) tri-weekly spot sampling for pH, alkalinity and dissolved silicon; and (3) continuous measurements of pH and dissolved oxygen. Calcium and bicarbonate provide, respectively, the dominant cation and anion in solution and their compositions remain relatively constant through time, irrespective of flow levels. In contrast, many determinands show seasonally related fluctuations. Concentrations for most of the major anions, sodium, potassium as well as soluble reactive phosphorus and several soluble trace elements such as boron, antimony, arsenic and molybdenum decrease as flow increases. A reverse pattern is observed for nitrate, some herbicides and trace elements associated with particulate phases: concentrations increase with increasing flow. These patterns reflect the influence of: (1) a calcium carbonate rich groundwater system which provides the main stream flow component; (2) dilution of point source pollutant inputs associated with sewage and possibly light industry at high flows for several major, nutrient and trace elements; (3) enhanced nitrate and herbicide runoff from agricultural land at high flows; (4) enhanced microparticulate trace metal levels associated with increased suspended sediment loads at high flows; and (5) biological processes which affect pH, dissolved silicon, dissolved carbon dioxide and dissolved oxygen levels. An examination of soluble reactive phosphorus (SRP) and boron relationships reveals a reduction in concentrations for SRP associated possibly with phosphorus removal from a major sewage treatment works on an upstream tributary of the Thames, the Thame.

Assessment of phosphate species for bicarbonate-bearing lowland rivers of the Thames valley, England and acidic Welsh upland streams (the Plynlimon area, mid-Wales) reveal problems of silica interference when employing standard automated 'phosphomolybdic acid' colorimetric methodologies. The interference occurs under a combination of two conditions: (1) when the reagents and samples are heated to speed up the rate of formation of phosphomolybdic acid; and (2) when the strength of a sulfuric acid reagent is too low. While this paper alerts researchers to potential analytical problems with silica interference within phosphomolybdic acid methodologies to determine soluble reactive phosphorus (SRP) concentrations, it cannot detail whether or not there is a general problem. This lack of specificity occurs because there are a variety of phosphomolybdic acid methodologies available which vary in their reagent recipes and it is not clear which methods will or will not suffer from silica interference. Changing the sulfuric acid reagent strength by a factor of two overcame the problem, in this case, with regards to the determination of soluble reactive phosphorus. However, even here, the method may not be applicable to very high alkalinity waters owing to their potential for reducing the acidity of the analyte. With regard to total dissolved phosphorus (TDP) and total phosphorus (TP) measurements, the importance of undertaking the analysis under ambient conditions is clearly shown. There are many small variants on standard phosphomolybdate methods for determining SRP, TDP and TP in use and it is not always clear which methods will or will not show silica interference for particular water types. It is therefore recommended that individual laboratories check their methodologies for silica interference using phosphate-free solutions with similar silica and alkalinity ranges for the waters being assayed.