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Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK
Science of the Total Environment 336 (2005) 225 – 241 www.elsevier.com/locate/scitotenv
Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK M.J. Bowes *, D.V. Leach, W.A. House Centre for Ecology and Hydrology, Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD, UK Received 24 October 2003; received in revised form 19 May 2004; accepted 22 May 2004
Abstract Chalk streams provide unique, environmentally important habitats, but are particularly susceptible to human activities, such as water abstraction, fish farming and intensive agricultural activity on their fertile flood-meadows, resulting in increased nutrient concentrations. Weekly phosphorus, nitrate, dissolved silicon, chloride and flow measurements were made at nine sites along a 32 km stretch of the River Frome and its tributaries, over a 15 month period. The stretch was divided into two sections (termed the middle and lower reach) and mass balances were calculated for each determinand by totalling the inputs from upstream, tributaries, sewage treatment works and an estimate of groundwater input, and subtracting this from the load exported from each reach. Phosphorus and nitrate were retained within the river channel during the summer months, due to bioaccumulation into river biota and adsorption of phosphorus to bed sediments. During the autumn to spring periods, there was a net export, attributed to increased diffuse inputs from the catchment during storms, decomposition of channel biomass and remobilisation of phosphorus from the bed sediment. This seasonality of retention and remobilisation was higher in the lower reach than the middle reach, which was attributed to downstream changes in land use and fine sediment availability. Silicon showed much less seasonality, but did have periods of rapid retention in spring, due to diatom uptake within the river channel, and a subsequent release from the bed sediments during storm events. Chloride did not produce a seasonal pattern, indicating that the observed phosphorus and nitrate seasonality was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors. D 2004 Elsevier B.V. All rights reserved. Keywords: Chalk stream; Mass balance; Nutrient dynamics; Phosphorus; Nitrate; Silicon
1. Introduction Chalk streams are one of the most productive and environmentally important riverine habitats, due to their unique physical characteristics. They are pre-
* Corresponding author. Tel.: +44-1305-213500; fax: +441305-213600. E-mail address: [email protected] (M.J. Bowes). 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.05.026
dominantly groundwater fed, resulting in a relatively low fluctuation in flow and water temperature (Berrie, 1992), and low suspended sediment concentration (Heywood and Walling, 2003). This stable flow, temperature, and resulting clear waters support large populations of fish (Mann, 1971) and invertebrates (Wright, 1992). The sources of these chalk streams tend to seasonally migrate up and down their valleys with fluctuating water-table level, which produces a unique assemblage of invertebrates which are able to
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withstand the summer drought conditions in these ‘winterbourne’ headwaters (Berrie and Wright, 1984). The high nutrient status and stable flow regimes of chalk streams provide excellent conditions for the growth of aquatic plants (Berrie, 1992). These rivers tend to have distinctive macrophyte assemblages, dominated by Ranunculus penicillatus in the middle channel and cresses along the margins. The very factors that make chalk streams unique and environmentally important also make them particularly susceptible to human impact. Their high productivity and the high nutrient quality of the water have meant that many have been impacted by fish farming and watercress production (Casey, 1981; Casey and Smith, 1994). Intensification of agriculture on the fertile flood meadows has increased the diffuse input of phosphorus and nitrate (Casey et al., 1993), which can lead to eutrophication. Drinking water is often abstracted from chalk aquifers, thereby reducing river discharge, resulting in increases in the concentration of pollutants and silt accumulation within gravel beds, which can destroy salmon and trout spawning grounds and reduce invertebrate biodiversity (Wood and Armitage, 1999; Milan et al., 2000). Chalk streams in the United Kingdom are found in an area stretching from Dorset in southern England, through the North Downs and Norfolk, to as far north as Humberside. Perhaps one of the most intensely studied of these rivers is the River Frome, Dorset, which has had its flora, fauna and water chemistry routinely monitored since 1965 (Casey and Newton, 1973; Casey et al., 1993; Welton et al., 1999). The objective of this study was to determine the spatial and temporal pattern of nutrient retention and export along the River Frome. Nutrient mass balances (the mass of a nutrient exported from a river section minus the total mass of nutrient entering that river section from surface waters) have been used in previous nutrient dynamics studies (Hill, 1982; Brunet and Astin, 1998, 2000; House and Warwick, 1998b; Hetling et al., 1999; Jain, 2000; Bowes and House, 2001). However, in chalk rivers, there will also be a significant input of nutrients from groundwater, and so a traditional mass-balance approach, based on surface water inputs, would not be suitable. This study aimed to include estimates of this groundwater supply of nutrients into the mass-balance calculation, thereby
extending this technique to be applicable to chalkriver environments. In addition, this paper aimed to present mass-balance data that was more comprehensive than most previous studies, by studying the dynamics of the three major plant nutrients (phosphorus, nitrate and silicon) simultaneously, alongside a conservative ion (chloride), at a relatively short (weekly) time interval for a 15 month period.
2. Study area The River Frome catchment, Dorset, UK, covers an area of 414 km2 (Casey and Newton, 1973), extending from Evershot (National Grid Reference ST 047576) on the Dorset – Somerset border, to Poole Harbour (Fig. 1). The dominant rock type for the study area is chalk, which outcrops over 46% of the River Frome catchment (Paolillo, 1969). In addition, there are areas of cretaceous greensand (River Hooke catchment), with fluvial sands and gravels in the lower reaches, downstream of Dorchester (Casey et al., 1993). The land use within the catchment is primarily agricultural, mainly grassland and cereals (Casey et al., 1993). The town of Dorchester is the only significant urban area in the study reach. The sewage treatment works (STW) serving the town has a population equivalent of 27,000 and discharges its treated effluent directly into the River Frome, ca. 30 m downstream of the Louds Mill gauging station, Dorchester (sampling site 5). The locations of other STW with population equivalents greater than 1000 are shown in Fig. 1.
3. Materials and methods 3.1. Fieldwork Water samples (1 l) were collected at weekly intervals from nine sampling sites along the River Frome catchment, between Frampton (site 1) and East Stoke (site 9) (Fig. 1), from June 1999 to October 2000. The samples were collected in acid-washed plastic sample bottles. After rinsing three times with river water, the bottles were immersed at least 30 cm below the water surface (when the depth allowed), capped to exclude air, and then returned to the laboratory for analysis.
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Fig. 1. The River Frome catchment, showing location of sampling sites.
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Water depths and flow velocities (at 60% of the total depth, using a SENSA RC-2 ultrasonic flow meter) were measured at the time of sampling, at the midpoint across each sampling site. In addition, comprehensive depth and flow velocity measurements were acquired at 0.5 m intervals across each sampling site, up to nine times throughout the sampling period (under a range of flow conditions and seasons), to allow estimates of river discharge to be made from the weekly spot measurements. Additional river discharge data were obtained from the Environment Agency (UK) gauging stations located at Louds Mill (sampling site 5), East Stoke (sampling site 9) and Sydling St. Nicholas (Fig. 1). The daily amount of precipitation was monitored using a rain gauge sited at East Stoke. The main river sampling sites were located at three points along the River Frome, at Frampton, Dorchester and East Stoke. This allowed the river to be divided into a ca. 12 km middle reach (between Frampton and Dorchester) and a ca. 22 km lower reach (between Dorchester and East Stoke). These river section boundaries are shown in Fig. 1. Due to the River Frome’s high sinuosity/branched course in its lower reaches, two branches of the river needed to be sampled at Dorchester [site 4 (Stinsford Road Bridge) and site 5 (Louds Mill)] to enable the total discharge and load of the river at that point to be calculated. The other weekly sampling sites were located on the five tributaries that enter the River Frome within the study reach. A number of samples from the STW outfalls at Bradford Peverell, Dorchester and Wool were also taken during the study. Information on the areas, river lengths and landuse for the catchment were obtained from the Centre for Ecology and Hydrology (CEH) digitised 1:50,000 scale Intelligent River Network and the 1990 CEH Land Cover Map of Great Britain (Fuller et al., 1994), using GIS software (ARCVIEW), and was based on 25 land cover types at 25 m grid resolution. 3.2. Chemical analysis A 100 ml aliquot of each sample was filtered through a 0.45 Am cellulose nitrate membrane, within 6 h of sampling. The unfiltered and filtered samples were analysed for total phosphorus (TP) and total
dissolved phosphorus (TDP) respectively, by digesting with acidified potassium persulphate in an autoclave at 121 jC for 40 min, then reacting with acid ammonium molybdate reagent to produce a molybdenum –phosphorus complex. This intensely coloured compound was then quantified spectrophotometrically at 880 nm (Eisenreich et al., 1975). Each batch of samples were analysed alongside a quality control (QC) standard, produced by dissolving analyticalgrade potassium dihydrogen orthophosphate (dried to constant weight at 105 jC) in deionised water. The concentrations of soluble molybdate-reactive phosphorus (SRP), nitrate and dissolved reactive silicon were determined using Flow Injection Analysis (Foss-Tecator FIAStar model 5012). Each sample was analysed in triplicate. SRP was determined spectrophotometrically by a method modified from Murphy and Riley (1962) (Tecator, 1983a). Sample filtering and SRP analysis was completed within 8 h of sampling, to minimise errors associated with sample instability (Haygarth et al., 1995; House and Warwick, 1998a). The combined nitrate/nitrite concentration of filtered samples was determined by reducing the nitrate present to nitrite by passing the sample through a cadmium column, and then reacting with sulphanilamide and N-(1-naphthyl)-ethylenediamine dihydrochloride. The resultant azo-dye was then measured spectrophotometrically at 540 nm (Tecator, 1983b). Silicon concentration was determined by reacting the filtered sample with ammonium molybdate and then oxalic acid, to produce molybdosilic acid, which was then reduced using tin(II) chloride to produce a blue complex which was analysed at 720 nm (Tecator, 1998). Quality control standards were analysed alongside each batch of flow injection analysis. Phosphorus and nitrate QC standards were produced from dried analytical-grade potassium dihydrogen orthophosphate and sodium nitrate respectively. The silicon QC standard was produced by diluting a 1000 mg l-1 standard solution (Spectrosol silicon standard solution; BDH laboratory supplies, Poole, UK). Chloride concentrations were determined by ion chromatography (Dionex, model DX100) using an IonPac AS14 analytical column (4 250 mm) and an ASRS-Ultra 4 mm suppressor with sodium carbonate/bicarbonate eluent. A chloride quality control standard (produced using dried, analytical-grade so-
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dium chloride, dissolved in deionised water) was analysed with each batch of samples. 3.3. River discharge estimates The water depths and flow velocities (measured at 60% of the total depth) of each ungauged sampling site were measured at 0.5 m intervals across the river, at ca. eight times throughout the study period, under a range of different flow conditions and seasons. These comprehensive measurements were used to calculate river discharge by multiplying the cross-sectional area of each 0.5 m section by the mean water velocity of that section, and then summing the discharge from each section to get the total river discharge. The total measured discharge from these comprehensive measurements was then regressed against the discharge measured at the central 1 m of the river. These correlations between total discharge and midpoint discharge were then used to estimate the total discharge at the time of sampling, using the weekly spot measurements of depth and water velocity taken at the midpoint of the river. 3.4. Theory The concentration of each sample and the corresponding river discharge estimate were used to calculate instantaneous loads for each determinand, as follows: Li ¼ Ci Qi
ð1Þ
where Li, Ci and Qi are the instantaneous load, concentration and river water discharge at the ith time period. For each study reach, the load of a particular determinand entering the section was subtracted from the load measured at the downstream site, in order to produce a mass balance for that determinand for that particular sampling day. Mj;Downstream ¼ Mj;Upstream þ RMj;Tributaries þ RMj;STW þ Mj;Residual
ð2Þ
Mj,Upstream represents the load at time j of the River Frome site at the upstream end of the section,
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Mj,Downstream is the load exported from the section, Mj,Tributaries is the load from the tributaries entering the reach, and Mj,STW is the estimated input from sewage treatment point sources entering directly into the River Frome within the section. In the case of the middle study reach, Mj,Downstream is the measured load at Dorchester (i.e., the combined load at sites 4 and 5). Mj,Upstream is the River Frome load entering the section at Frampton (site 1). Mj,Tributaries is the combined load entering the middle reach from the two tributaries, Sydling Water (site 2) and the River Cerne (site 3). Mj,STW is the estimated input from the Bradford Peverell STW (Fig. 1). Mj,Residual is the mass-balance residual at time j, and is composed of four components:
Diffuse nutrient inputs and small unmonitored point sources (i.e., field drains and septic tanks) within the study reach. The internal riverine processes that are occurring within the river section. Errors in sampling, chemical analysis and river flow estimates. Errors associated with river transit time. These have been minimised in this study by eliminating data gathered during sharp, intense storm events. In the absence of errors in the load estimates, if the inputs to a section equal the outputs, then the Mj,Residual will be zero. However, if the mass of determinand being exported from a section is less than is entering from upstream, then the Mj,Residual will be negative, indicating nutrient storage within the channel. Conversely, a positive Mj,Residual will indicate a net-export of determinand from the reach, and will consist of diffuse inputs and any remobilisation of stored nutrients from the river channel. This approach has been used successfully in previous studies in catchments with small diffuse inputs (House and Warwick, 1998b; Bowes and House, 2001). However, because chalk rivers are predominantly groundwater-fed, they receive a constant and significant supply of water and associated diffuse nutrients. In order to use a mass-balance approach for chalk streams, this source of subsurface, diffuse input needs to be estimated and included as an extra component in Eq. (2). The diffuse
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groundwater contribution in this study was estimated by first quantifying the amount of water being generated within each study reach. This was done by calculating the water mass-balance residual, using
Eq. (2), for each sampling day. This quantifies the difference between the amount of surface water entering and flowing out of the study reach. It would be expected that the surface water entering the study
Fig. 2. Relationships between spot measurements and total measured discharge.
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reach would always be less than the discharge at the downstream extent of the study reach, and this extra water, the water residual, Wj,residual, at time j, would consist of
successfully in a previous study (May et al., 2001). These groundwater estimates were then used to refine the mass-balance equation for chalk rivers [Eq. (2)]. Mj;Downstream ¼ Mj;Upstream þ RMj;Tributaries
Wj;residual ¼ Wj;groundwater þ Wj;runoff þ Wj;seepage þ Wj;interception þ Wj;errors
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ð3Þ
þ RMj;STW þ Mj;groundwater þ Mj;Residual
where Wj,groundwater, Wj,runoff, Wj,seepage and Wj,interception are the discharges of groundwater, runoff, soil through-flow/seepage and direct rainfall interception, respectively, entering the section. During dry periods, the water residual will approach the groundwater input (plus errors in river discharge estimates), as runoff and seepage are likely to be minor, and interception will be zero. The quantity of water derived from groundwater was estimated using River Frome hydrographs from the Dorchester (Louds Mill) and East Stoke gauging stations. It was assumed that on ‘dry’ sampling days, the water mass-balance residual would consist entirely of groundwater. These ‘dry’ data points were used to estimate the percentage contribution of the groundwater to the total River Frome flow throughout the year. This approach has been used
ð4Þ
where Mj,groundwater is the mass of determinand entering the study reach via the groundwater. This was calculated by multiplying the estimated volume of groundwater by the concentration of that determinand in the groundwater, obtained from chemical analysis of borehole samples (Fig. 1, site 10) from a previous study (Casey and Neal, 1986). This borehole was sited at Waterston Manor in the adjacent River Piddle catchment, and was only 5 km north east of the River Frome at Dorchester. It is assumed in this study that the groundwater entering the River Frome will have similar concentrations of nutrients. Mj,Residual in Eq. (4) now consists of within-stream processing, surface diffuse inputs and unmonitored point sources, and errors associated with flow gauging and chemical analysis.
Fig. 3. Hydrograph from the River Frome at Louds Mill, Dorchester (site 5) during the study period, showing sampling dates (n), ‘dry’ sampling dates with no rain for 4 days previously (5) and sampling dates that were later removed from the data set, due to coinciding with sharp storm events (D). The dashed line indicates the quantity of the total river flow estimated to be due to groundwater input.
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produce total discharge estimates for all sampling sites. The hydrograph of the River Frome at Louds Mill, Dorchester, including sampling points, is shown in Fig. 3. The unfilled symbols (5) represent sampling days where no rainfall was recorded at East Stoke for at least 4 days previously. During the summer months, it was assumed that on these dry sampling days, all of the river flow was due to groundwater input. The discharges on these dry sampling points were connected to produce estimates of the groundwater contribution, as shown by the dashed line in Fig. 3. During periods of high rainfall, the contribution of groundwater to the total river flow was more difficult to assess, but was again estimated by connecting the hydrograph discharges measured on dry days before and after the rainfall events (dashed line in Fig. 3). Samples taken during
4. Results and discussion 4.1. River discharge and load estimates Regressions of the measured discharge in the centre of the river channel and the total discharge (Fig. 2) produced robust relationships for all sites, except site 2, Sydling Water at Grimstone. Problems with the flow gauging at the Sydling Water site were probably caused by widely varying river flow patterns throughout the year, as a result of excessive plant growth at the sampling site. The discharge at this site was estimated by regressing the measured discharges and the recorded discharge from the Sydling St. Nicholas gauging station, 5 km upstream (Fig. 1). The linear regressions produced in Fig. 2 were applied to the spot measurements of depth and velocity taken at the time of sampling to
Table 1 Summary of nutrient concentrations and discharges at each sampling site Site name (National Grid Reference)
Site number
River Frome, Frampton (SY 623949)
1
Sydling Water (Sy 640945)
2
River Cerne (SY 678936)
3
River Frome, Stinsford Road Bridge (SY 703909)
4
Frome, Louds Mill (SY 707903)
5
South Winterborne (SY 724896)
6
Tadnoll Brook (SY 811882)
7
River Win (SY 832869)
8
River Frome, East Stroke (SY 866867)
9
Load (mol h-1)
Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum
TP
TDP
SRP
NO3
Si
Cl
34.5 504 7.7 4.5 44 0.6 8.9 92 1.4 9.6 45 1.2 56.7 494 12.7 1.4 10 0.05 13.0 74 1.6 3.5 113 0.02 131 487 44
20.0 144 6.7 2.5 10 0.6 5.7 27 1.2 6.3 21 1.2 37.7 231 12.1 1.1 9 0.01 7.3 34 0.6 1.0 31 0.01 90 192 37
17.8 107 6.3 2.3 10 0.4 5.4 24 1.2 5.9 21 1.2 33.7 160 11.9 1.0 8 0.01 6.7 31 0.6 0.7 22 0.01 84 191 35
1747 6031 772 537 1857 154 651 2585 73 786 2418 119 3998 12817 1209 533 2108 39 721 1620 115 126 935 1.4 7577 26371 2313
752 3106 212 184 597 61 208 789 27 275 826 32 1425 4363 360 101 435 10 214 538 41 38 267 2.3 2500 8520 760
2789 15630 789 634 2033 190 831 2854 113 1080 3184 191 5672 17052 1694 582 2297 42 1353 3642 258 329 3045 17 11995 37953 4174
Discharge (m3 s-1) 1.45 7.45 0.46 0.38 1.12 0.14 0.47 1.57 0.07 0.56 1.56 0.11 2.97 9.97 0.98 0.23 0.85 0.02 0.46 1.14 0.09 0.10 0.93 0.01 5.13 15.60 1.87
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three particularly sharp storm events (represented by the symbol D in Fig. 3) were removed from the data set, as samples taken at these times would be at different points in the hydrograph, causing inconsistent results. For instance, if the upper part of the study-reach was sampled during a sharp storm event, and the site at the downstream extent of the reach was sampled before this flood-wave reached it, the mass-balance residuals would be large and negative. Alternatively, if the upper reach was sampled after the storm event had passed, and the lower reach was sampled at the height of the storm event, then the mass balance residuals would be large and positive. It was therefore deemed necessary to remove these three particular data points to avoid these transit-time errors. A summary of the flow and loads for each sampling site is given in Table 1. The mean concentration
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of all nutrients in the River Frome increased downstream. Approximately 55% to 80% of the phosphorus load was in dissolved form at all sampling sites (except the River Win), and 90% to 95% of this dissolved phosphorus was SRP. The River Win had much less phosphorus load in dissolved form (31%), and less dissolved phosphorus as SRP (64%). This is probably due to the absence of STW input to the River Win. 4.2. Mass balances 4.2.1. Water The water mass-balance residuals for the two study reaches, derived from Eq. (2), are shown in Fig. 4(a). The gaps in the data are due to sampling not taking place during those periods, and also data points coinciding with three particularly sharp storm events
Fig. 4. Water mass-balance residuals, shown as (a) cubic meters per second and (b) mass balance (including estimated groundwater input), normalised to the total discharge from each study section.
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(Fig. 3) being removed. Mass-balance residuals were positive throughout the study period, showing that additional water was always entering the River Frome from groundwater and unmonitored surface inputs (such as drainage ditches, runoff and soil seepage). The proportion of this water residual accounted for by groundwater input was estimated using Fig. 3. This estimated volume of groundwater was then included as an input in the mass-balance calculation [Eq. (4)]. The resulting discharge residual was divided by the total discharge at the downstream site, in order to normalise the data [Fig. 4(b)], and shows the proportion of the total discharge derived from diffuse inputs and unmonitored point sources (i.e., drainage ditches). Fig. 4(b) highlights that diffuse inputs can comprise over 30% of the total flow during periods of sustained heavy rainfall.
4.2.2. Phosphorus The TP mass-balance residuals [Eq. (4)] showed a clear seasonal trend, particularly in the lower reach, with a net-retention in the summer months and export during the rest of the year [Fig. 5(a) and (b)]. The large negative TP residuals (over 80% of the total load for the lower reach) during the summer period showed that the majority of phosphorus entering the reach was being retained within the river channel, due to processes such as sequestering of dissolved phosphorus from the water column by bed sediments (House and Denison, 2002), deposition of particulate-bound phosphorus (PP) during periods of low-flow (Owens et al., 2001), and bioaccumulation of phosphorus within aquatic plants and epilithic biofilms during the growing season (Woodruff et al., 1999; House et al., 2001a). The net export of phosphorus from each reach
Fig. 5. Total phosphorus (TP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total TP load exported from the section.
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during the autumn to spring period was a result of remobilisation of bed-sediment PP, release of phosphorus-rich sediment pore-water (Casey and Farr, 1982), loss of P from decaying biota and the input of diffuse phosphorus from the catchment. Similar seasonal patterns have been observed previously (Dorioz et al., 1998; Bowes and House, 2001; Bowes et al., 2003). The TDP and SRP mass-balance residuals are shown in Figs. 6 and 7, respectively. They also show retention of dissolved phosphorus during low-flow periods and a net-export in winter. The patterns and rates of TDP and SRP retention and export are very similar, indicating that transformation from SRP to soluble unreactive forms within this section of the catchment is negligible. The middle reach was a net-source of all forms of phosphorus, with an average net export of 9.4 mol h-1 TP over the course of the study period (equivalent to
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0.78 mol h-1 TP per km of river), indicating that diffuse phosphorus inputs are significant between Frampton and Dorchester throughout the year. The lower reach was a net-sink for phosphorus, retaining on average 14.1 mol h-1 TP over the study period (equivalent to 0.64 mol h-1 TP per km of river), which may be a result of the large-scale overbank flooding that regularly occurred in the flood meadows of the lower section, resulting in transfer of phosphorus from the channel to the floodplain. The average rate of TP, TDP and SRP retention in the lower reach, between July –September 1999 and 2000, was relatively uniform at ca. 30 – 40 mol h-1 Figs. 5(a), 6(a) and 7(a)]. Almost all of the phosphorus sequestered from the water column was in soluble reactive form. Surface diffuse inputs of phosphorus during these dry periods are negligible, and so the rate of retention is almost entirely within-channel storage
Fig. 6. Total dissolved phosphorus (TDP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total TDP load exported from the section.
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Fig. 7. Soluble reactive phosphorus (SRP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total SRP load exported from the section.
as a result of adsorption to bed-sediments and bioaccumulation. The lower study reach had an average phosphorus retention rate of ca. 83 Amol m-2 h-1 throughout these summer months, assuming an estimated channel-bed area of 418,000 m2. Similar retention rates (57 Amol m-2 h-1) have been observed in the River Swale, northern England, during summer lowflow conditions (Bowes and House, 2001). The middle reach also showed a marked seasonal pattern in TP residuals [Fig. 5(b)], with rates of export in the winter of 1999 and release in the summer of 2000 that were almost identical to the rates observed in the lower reach. However, there was very little seasonality in the TDP and SRP residuals in the middle reach [Figs. 6(b) and 7(b)]. These results demonstrate that the retention of ca. 40% of the phosphorus entering the middle reach during the summer of 2000 was predominantly in particulate-bound form.
Over the course of the study, there was a net retention of phosphorus from the lower reach and approximately equal inputs and outputs from the middle reach. However, as the mass-balance residual is a product of both within-stream processing and diffuse nutrient sources, there should be a net loss from the system (as extra nutrients are being added to the catchment in the form of fertiliser). During the wet periods between autumn and spring, there will be large, episodic inputs of diffuse phosphorus and rapid remobilisation of sediment-bound within-channel phosphorus (Brunet and Astin, 1998; Bowes and House, 2001). These rapid exports of phosphorus will be underrepresented by our weekly sampling regime, especially as we have had to remove data associated with sudden, sharp, storm events, due to transit-time errors. If more frequent sampling was adopted during high-flow events, then the phosphorus residuals
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would be expected to be large and positive during these winter storms. 4.2.3. Nitrate Net exports of up to 6 kmol h-1 nitrate were observed in both study reaches during periods of heavy rainfall in January 2000 [Fig. 8(a)], mainly as a result of surface diffuse inputs (runoff and soil seepage) to the river. These diffuse inputs would be exacerbated by the lack of vegetation cover in the winter months in many land-use categories. There is a clear seasonal pattern in nitrate residuals for both study reaches. The middle reach showed a net export for most of the year, with the highest mass-balance residuals in the winter and spring. The lower reach only showed a net export during periods of heavy rainfall, and a net retention at other times. The rate of nitrogen storage was greatest during periods of summer low-flow, where ca. 1 kmol h-1 nitrate was being
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stored within the channel, which was equivalent to 2.4 mmol m-2 h-1. Very similar nitrate retention rates (2.0 mmol m-2 h-1) were measured in the River Frome at East Stoke under low-flow conditions in 1992 (House et al., 2001b). Studies on other rivers have estimated retention rates ranging from 0.12 mmol m-2 h-1 (Hill, 1981) to 4.2 mmol m-2 h-1 (Owens et al., 1972). This negative nitrate residual was probably a result of bioaccumulation by aquatic plants and epilithic algae during the growing season, a lack of diffuse inputs from the catchment during dry periods, and could also indicate that microbial denitrification was occurring in the anaerobic bed sediments (Pattinson et al., 1998). 4.2.4. Dissolved silicon Silicon mass-balance residuals showed very little seasonal pattern (Fig. 9), compared with nitrate and phosphorus. However, there were short periods of
Fig. 8. Nitrate mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total nitrate load exported from the section.
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Fig. 9. Dissolved silicon mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total dissolved silicon load exported from the section.
major retention of dissolved silicon in the spring/early summer (of up to 500 mol h-1) in both reaches, equivalent to ca. 20 –30% of the expected load. Retention rates may have been even greater during March and April 2000, but samples were not taken, or had to be rejected due to coinciding with sharp storm events. There were also short periods of silicon storage between July and September. This pattern of major silicon storage in the spring, followed by smaller and episodic silicon-stripping in the summer has been observed previously in the River Frome (Casey et al., 1981; Marker et al., 1984; House et al., 2001b) and other UK rivers (Edwards, 1974). The removal and storage of dissolved silicon from the water column is a result of assimilation by diatoms (Edwards, 1974; Woodruff et al., 1999; House et al., 2001b), and, to a smaller extent, chemisorption to minerals in the bed
sediments (Casey and Neal, 1986). The silicon retention rate during the initial spring diatom bloom was 1.24 and 0.88 mmol m-2 h-1 for the lower and middle reaches, respectively, which was comparable to the rates observed in a previous study of the River Frome at East Stoke (between 1.9 and 2.6 mmol m-2 h-1; House et al., 2001b) and a study of an adjacent chalk stream catchment (between 1.5 and 4.5 mmol m-2 h-1 in the Bere Stream, Dorset, UK) (Casey et al., 1981). Major positive mass-balance residuals occurred in each study-reach in September and October 1999, and January 2000 [Fig. 9(a) and (b)], coinciding with periods of heavy rainfall/high river discharge (Fig. 3), which was equivalent to approximately a 20– 30% increase in silicon load. This export of silicon during storm events is most likely caused by the disturbance of the upper bed sediments by the increasing flow
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Fig. 10. Chloride mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total chloride load exported from the section.
velocity, resulting in the release of silicon-rich porewaters (produced by the dissolution of diatom frustules since the end of the growing season) (Woodruff et al., 1999; House et al., 2000). This additional silicon may also be derived from diffuse inputs from the floodplain, directly entering the River Frome. Similar patterns of silicon release during storms have been observed in previous studies (Casey and Farr, 1982; Bowes and House, 2001; House et al., 2001b). 4.2.5. Chloride Chloride residuals were small, relative to those of phosphorus and nitrate, and did not show a seasonal pattern [Fig. 10(b)]. However, there was a large (30 – 40%) generation of chloride during periods of high rainfall in December 1999, and May 2000, possibly due to atmospheric inputs and diffuse inputs from the floodplain, directly into the River Frome. The application of rock salt to roads during winter may also
contribute to this increased chloride load during the winter of 1999. During the rest of the monitoring period, the chloride residuals are relatively stable, varying within a range of ca. 20%. This contrasts strongly with the variation observed in the TP [Fig. 5(b)] and nitrate [Fig. 8(b)] residuals in the lower study reach, which varied by 100% and 50%, respectively. These seasonal trends in nitrogen and phosphorus must therefore be a result of internal riverine processing and diffuse nutrient inputs from the catchment, rather than an artefact of errors associated with discharge measurement, transit time or sampling.
5. Conclusions This use of mass-balance residuals has provided a valuable insight into the annual phosphorus, nitrate and silicon dynamics of the River Frome. All nutrients
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showed a net retention within the channel during the dry, summer months, as a result of bioaccumulation and, in the case of phosphorus, adsorption to bed sediments. During the wetter autumn to spring periods, there was a net generation of nutrients within each study reach, as increased runoff transported nutrients from the floodplain into the river channel. The high river discharges during these wet periods also remobilised phosphorus and silicon stored in the bed sediment, and this release of nutrients back into the water column also contributed to a positive massbalance residual. The chloride residuals did not show a seasonal pattern, indicating that the seasonality of the phosphorus and nitrate dynamics was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors. The seasonal pattern in phosphorus and nitrate residuals was more pronounced in the lower reach, compared with the middle study reach. There are a number of possible explanations for this. First, it may be a result of changes in land-use down the catchment. The lower reach has a higher proportion of tilled land (4%, as opposed to 1% in the middle reach). This will have low diffuse nutrient losses during crop production in the summer, but high losses after harvesting. The lowland also has 10% less grassland and seminatural meadow (which will have more consistent nutrient losses throughout the year). Therefore, the diffuse nutrient losses from the lower reach would be expected to have a higher seasonality. Changes in the rates of within-channel nutrient processing downstream will also contribute to the increased seasonality in the lower reach. Higher biomass and an increasing proportion of fine sediment in the lower reach (due to attrition of the sediment load as it is transported downstream) will result in enhanced nutrient storage in summer and subsequent loss during the autumn – spring storms. Finally, the middle reach has a higher proportion of its total flow derived from groundwater, due to its chalk geology. (Much of the lower River Frome flows across sand and gravel deposits). The middle reach will therefore have a more consistent nutrient supply throughout the year and less diffuse runoff during storm events. This study has shown that the use of mass balances can be successfully extended to groundwater-dominated chalk rivers. The approach developed in this
paper will provide a useful tool that will enable nutrient dynamics to be studied in these environmentally important habitats. Acknowledgements We thank the Environment Agency (UK) for supplying the river discharge data, and Grady Hanrahan (University of Plymouth) and Rebecca Coward (CEH Dorset) for help with the river sampling. GIS assistance was provided by Duncan Hornby (CEH Dorset). References Berrie AD. The chalk-stream environment. Hydrobiologia 1992; 248(1):3 – 9. Berrie AD, Wright JF. The winterbourne stream. In: Whitton BA, editor. Ecology of European rivers. Oxford: Blackwell; 1984. p. 179 – 206. Bowes MJ, House WA. Phosphorus and dissolved silicon dynamics in the River Swale catchment. UK: a mass-balance approach. Hydrol Process 2001;15(2):261 – 80. Bowes MJ, House WA, Hodgkinson RA. Phosphorus dynamics along a river continuum. Sci Total Environ 2003;313(1 – 3): 199 – 212. Brunet RC, Astin KB. Variation in phosphorus flux during a hydrological season: the River Adour. Water Res 1998;32(3):547 – 58. Brunet RC, Astin KB. A 12-month sediment and nutrient budget in a floodplain reach of the River Adour, southwest France. Regul Rivers Res Manage 2000;16(3):267 – 77. Casey H. Discharge and chemical-changes in a chalk stream headwater affected by the outflow of a commercial watercress bed. Enviro Pollut., Ser B 1981;2(5):373 – 85. Casey H, Farr IS. The influence of within-stream disturbance on dissolved nutrient levels during spates. Hydrobiologia 1982; 91-2(JUL):447 – 62. Casey H, Neal C. Abiological controls on silica in chalk streams and groundwaters. In: Sly PG, editor. Sediment and water interactions. New York: Springer-Verlag; 1986. p. 331 – 42. Casey H, Newton PVR. The chemical composition and flow of the River Frome and its main tributaries. Freshw Biol 1973;3: 317 – 33. Casey H, Smith SM. The effects of watercress growing on chalk headwater streams in Dorset and Hampshire. Environ Pollut 1994;85(2):217 – 28. Casey H, Clarke RT, Marker AFH. The seasonal-variation in silicon concentration in chalk-streams in relation to diatom growth. Freshw Biol 1981;11(4):335 – 44. Casey H, Clarke RT, Smith SM. Increases in nitrate concentrations in the River Frome (Dorset) catchment related to changes in land use, fertiliser applications and sewage input. Chem Ecol 1993; 8:105 – 17.
M.J. Bowes et al. / Science of the Total Environment 336 (2005) 225–241 Dorioz JM, Cassell EA, Orand A, Eisenman KG. Phosphorus storage, transport and export dynamics in the Foron river watershed. Hydrol Process 1998;12(2):285 – 309. Edwards AMC. Silicon depletions in some Norfolk rivers. Freshw Biol 1974;4:267 – 74. Eisenreich SJ, Bannerman RT, Armstrong DE. A simplified phosphorus analytical technique. Environ Lett 1975;9(4):5 – 53. Fuller RM, Groom GB, Jones AR. The land-cover map of GreatBritain—an automated classification of landsat thematic mapper data. Photogramm Eng Remote Sensing 1994;60(5):553 – 62. Haygarth PM, Ashby CD, Jarvis SC. Short-term changes in the molybdate reactive phosphorus of stored soil waters. J Environ Qual 1995;24(6):1133 – 40. Hetling LJ, Jaworski NA, Garretson DJ. Comparison of nutrient input loading and riverine export fluxes in large watersheds. Water Sci Technol 1999;39(12):189 – 96. Heywood MJT, Walling DE. Suspended sediment fluxes in chalk streams in the Hampshire Avon catchment, UK. Hydrobiologia 2003;494(1 – 3):111 – 7. Hill AR. Nitrate – nitrogen flux and utilisation in a stream ecosystem during low summer flows. Can Geogr 1981;25:225 – 39. Hill AR. Phosphorus and major cation mass balances for 2 rivers during low summer flows. Freshw Biol 1982;12(3):293 – 304. House WA, Denison FH. Exchange of inorganic phosphate between river waters and bed-sediments. Environ Sci Technol 2002; 36(20):4295 – 301. House WA, Warwick MS. Intensive measurements of nutrient dynamics in the River Swale. Sci Total Environ 1998a;210(1 – 6): 111 – 37. House WA, Warwick MS. A mass-balance approach to quantifying the importance of in-stream processes during nutrient transport in a large river catchment. Sci Total Environ 1998b;210(1 – 6): 139 – 52. House WA, Denison FH, Warwick MS, Zhmud BV. Dissolution of silica and the development of concentration profiles in freshwater sediments. Appl Geochem 2000;15(4):425 – 38. House WA, Duplat D, Denison FH, Henville P, Dawson FH, Cooper DM, et al. The role of macrophytes in the retention of phosphorus in the River Thame, England. Chem Ecol 2001a; 17:271 – 91. House WA, Leach DV, Armitage PD. Study of dissolved silicon and nitrate dynamics in a freshwater stream. Water Res 2001b; 35(11):2749 – 57. Jain CK. Application of chemical mass balance approach to determine nutrient loading. Hydrol Sci J 2000;45(4):577 – 88. Mann RHK. The populations, growth and production of fish in four small streams in southern England. J Anim Ecol 1971;40: 155 – 90.
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Marker AFH, Casey H, Rother JA. The seasonal cycle of diatoms in chalk streams. Verh.-Int Ver Theor Angew Limnol 1984;22(3): 1949 – 52. May L, House WA, Bowes M, McEvoy J. Seasonal export of phosphorus from a lowland catchment: upper River Cherwell in Oxfordshire, England. Sci Total Environ 2001;269(1 – 3): 117 – 30. Milan DJ, Petts GE, Sambrook H. Regional variations in the sediment structure of trout streams in southern England: benchmark data for siltation assessment and restoration. Aquat Conserv 2000;10(6):407 – 20. Murphy J, Riley JP. A modified single solution method for the determination of phosphorus in natural waters. Anal chim acta 1962;12:31 – 6. Owens M, Garland JHN, Hart IC, Wood G. Nutrient budgets in rivers. Symp Zool Soc London 1972;29:21 – 40. Owens PN, Walling DE, Carton J, Meharg AA, Wright J, Leeks GJL. Downstream changes in the transport and storage of sediment-associated contaminants (P, Cr and PCBs) in agricultural and industrialized drainage basins. Sci Total Environ 2001; 266(1 – 3):177 – 86. Paolillo SAG. Hydrogeology of the River Frome catchment (Southern England). Memorie note dell instituto di geologia aplicata. Napoli 1969;11:1 – 69. Pattinson SN, Garcia-Ruiz R, Whitton BA. Spatial and seasonal variation in denitrification in the Swale – Ouse system, a river continuum. Sci Total Environ 1998;210(1 – 6):289 – 305. Tecator. Determination of reactive phosphorus in water by flowinjection analysis. York, England; 1983a. Application note ASN 60-01/83 JM/ML. Tecator. Sum of nitrate and nitite by flow-injection analysis. York, England; 1983b. Application note ASN 62-02/83. Tecator. Determination of silica in water by flow-injection analysis. York, England; 1998. Application note ASTN 5616. Welton JS, Beaumont WRC, Ladle M. Timing of migration and changes in age structure of Atlantic salmon, Salmo salar L., in the River Frome, a Dorset chalk stream, over a 24-year period. Fisheries Manag Ecol 1999;6(6):437 – 58. Wood PJ, Armitage PD. Sediment deposition in a small lowland stream—management implications. Regul Rivers Res Manage 1999;15(1 – 3):199 – 210. Woodruff SL, House WA, Callow ME, Leadbeater BSC. The effects of a developing biofilm on chemical changes across the sediment – water interface in a freshwater environment. Int Rev Hydrobiol 1999;84(5):509 – 32. Wright JF. Spatial and temporal occurrence of invertebrates in a chalk stream, Berkshire, England. Hydrobiologia 1992;248(1): 11 – 30.
Seasonal nutrient dynamics in a chalk stream: the River Frome, Dorset, UK M.J. Bowes *, D.V. Leach, W.A. House Centre for Ecology and Hydrology, Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD, UK Received 24 October 2003; received in revised form 19 May 2004; accepted 22 May 2004
Abstract Chalk streams provide unique, environmentally important habitats, but are particularly susceptible to human activities, such as water abstraction, fish farming and intensive agricultural activity on their fertile flood-meadows, resulting in increased nutrient concentrations. Weekly phosphorus, nitrate, dissolved silicon, chloride and flow measurements were made at nine sites along a 32 km stretch of the River Frome and its tributaries, over a 15 month period. The stretch was divided into two sections (termed the middle and lower reach) and mass balances were calculated for each determinand by totalling the inputs from upstream, tributaries, sewage treatment works and an estimate of groundwater input, and subtracting this from the load exported from each reach. Phosphorus and nitrate were retained within the river channel during the summer months, due to bioaccumulation into river biota and adsorption of phosphorus to bed sediments. During the autumn to spring periods, there was a net export, attributed to increased diffuse inputs from the catchment during storms, decomposition of channel biomass and remobilisation of phosphorus from the bed sediment. This seasonality of retention and remobilisation was higher in the lower reach than the middle reach, which was attributed to downstream changes in land use and fine sediment availability. Silicon showed much less seasonality, but did have periods of rapid retention in spring, due to diatom uptake within the river channel, and a subsequent release from the bed sediments during storm events. Chloride did not produce a seasonal pattern, indicating that the observed phosphorus and nitrate seasonality was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors. D 2004 Elsevier B.V. All rights reserved. Keywords: Chalk stream; Mass balance; Nutrient dynamics; Phosphorus; Nitrate; Silicon
1. Introduction Chalk streams are one of the most productive and environmentally important riverine habitats, due to their unique physical characteristics. They are pre-
* Corresponding author. Tel.: +44-1305-213500; fax: +441305-213600. E-mail address: [email protected] (M.J. Bowes). 0048-9697/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2004.05.026
dominantly groundwater fed, resulting in a relatively low fluctuation in flow and water temperature (Berrie, 1992), and low suspended sediment concentration (Heywood and Walling, 2003). This stable flow, temperature, and resulting clear waters support large populations of fish (Mann, 1971) and invertebrates (Wright, 1992). The sources of these chalk streams tend to seasonally migrate up and down their valleys with fluctuating water-table level, which produces a unique assemblage of invertebrates which are able to
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withstand the summer drought conditions in these ‘winterbourne’ headwaters (Berrie and Wright, 1984). The high nutrient status and stable flow regimes of chalk streams provide excellent conditions for the growth of aquatic plants (Berrie, 1992). These rivers tend to have distinctive macrophyte assemblages, dominated by Ranunculus penicillatus in the middle channel and cresses along the margins. The very factors that make chalk streams unique and environmentally important also make them particularly susceptible to human impact. Their high productivity and the high nutrient quality of the water have meant that many have been impacted by fish farming and watercress production (Casey, 1981; Casey and Smith, 1994). Intensification of agriculture on the fertile flood meadows has increased the diffuse input of phosphorus and nitrate (Casey et al., 1993), which can lead to eutrophication. Drinking water is often abstracted from chalk aquifers, thereby reducing river discharge, resulting in increases in the concentration of pollutants and silt accumulation within gravel beds, which can destroy salmon and trout spawning grounds and reduce invertebrate biodiversity (Wood and Armitage, 1999; Milan et al., 2000). Chalk streams in the United Kingdom are found in an area stretching from Dorset in southern England, through the North Downs and Norfolk, to as far north as Humberside. Perhaps one of the most intensely studied of these rivers is the River Frome, Dorset, which has had its flora, fauna and water chemistry routinely monitored since 1965 (Casey and Newton, 1973; Casey et al., 1993; Welton et al., 1999). The objective of this study was to determine the spatial and temporal pattern of nutrient retention and export along the River Frome. Nutrient mass balances (the mass of a nutrient exported from a river section minus the total mass of nutrient entering that river section from surface waters) have been used in previous nutrient dynamics studies (Hill, 1982; Brunet and Astin, 1998, 2000; House and Warwick, 1998b; Hetling et al., 1999; Jain, 2000; Bowes and House, 2001). However, in chalk rivers, there will also be a significant input of nutrients from groundwater, and so a traditional mass-balance approach, based on surface water inputs, would not be suitable. This study aimed to include estimates of this groundwater supply of nutrients into the mass-balance calculation, thereby
extending this technique to be applicable to chalkriver environments. In addition, this paper aimed to present mass-balance data that was more comprehensive than most previous studies, by studying the dynamics of the three major plant nutrients (phosphorus, nitrate and silicon) simultaneously, alongside a conservative ion (chloride), at a relatively short (weekly) time interval for a 15 month period.
2. Study area The River Frome catchment, Dorset, UK, covers an area of 414 km2 (Casey and Newton, 1973), extending from Evershot (National Grid Reference ST 047576) on the Dorset – Somerset border, to Poole Harbour (Fig. 1). The dominant rock type for the study area is chalk, which outcrops over 46% of the River Frome catchment (Paolillo, 1969). In addition, there are areas of cretaceous greensand (River Hooke catchment), with fluvial sands and gravels in the lower reaches, downstream of Dorchester (Casey et al., 1993). The land use within the catchment is primarily agricultural, mainly grassland and cereals (Casey et al., 1993). The town of Dorchester is the only significant urban area in the study reach. The sewage treatment works (STW) serving the town has a population equivalent of 27,000 and discharges its treated effluent directly into the River Frome, ca. 30 m downstream of the Louds Mill gauging station, Dorchester (sampling site 5). The locations of other STW with population equivalents greater than 1000 are shown in Fig. 1.
3. Materials and methods 3.1. Fieldwork Water samples (1 l) were collected at weekly intervals from nine sampling sites along the River Frome catchment, between Frampton (site 1) and East Stoke (site 9) (Fig. 1), from June 1999 to October 2000. The samples were collected in acid-washed plastic sample bottles. After rinsing three times with river water, the bottles were immersed at least 30 cm below the water surface (when the depth allowed), capped to exclude air, and then returned to the laboratory for analysis.
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Fig. 1. The River Frome catchment, showing location of sampling sites.
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Water depths and flow velocities (at 60% of the total depth, using a SENSA RC-2 ultrasonic flow meter) were measured at the time of sampling, at the midpoint across each sampling site. In addition, comprehensive depth and flow velocity measurements were acquired at 0.5 m intervals across each sampling site, up to nine times throughout the sampling period (under a range of flow conditions and seasons), to allow estimates of river discharge to be made from the weekly spot measurements. Additional river discharge data were obtained from the Environment Agency (UK) gauging stations located at Louds Mill (sampling site 5), East Stoke (sampling site 9) and Sydling St. Nicholas (Fig. 1). The daily amount of precipitation was monitored using a rain gauge sited at East Stoke. The main river sampling sites were located at three points along the River Frome, at Frampton, Dorchester and East Stoke. This allowed the river to be divided into a ca. 12 km middle reach (between Frampton and Dorchester) and a ca. 22 km lower reach (between Dorchester and East Stoke). These river section boundaries are shown in Fig. 1. Due to the River Frome’s high sinuosity/branched course in its lower reaches, two branches of the river needed to be sampled at Dorchester [site 4 (Stinsford Road Bridge) and site 5 (Louds Mill)] to enable the total discharge and load of the river at that point to be calculated. The other weekly sampling sites were located on the five tributaries that enter the River Frome within the study reach. A number of samples from the STW outfalls at Bradford Peverell, Dorchester and Wool were also taken during the study. Information on the areas, river lengths and landuse for the catchment were obtained from the Centre for Ecology and Hydrology (CEH) digitised 1:50,000 scale Intelligent River Network and the 1990 CEH Land Cover Map of Great Britain (Fuller et al., 1994), using GIS software (ARCVIEW), and was based on 25 land cover types at 25 m grid resolution. 3.2. Chemical analysis A 100 ml aliquot of each sample was filtered through a 0.45 Am cellulose nitrate membrane, within 6 h of sampling. The unfiltered and filtered samples were analysed for total phosphorus (TP) and total
dissolved phosphorus (TDP) respectively, by digesting with acidified potassium persulphate in an autoclave at 121 jC for 40 min, then reacting with acid ammonium molybdate reagent to produce a molybdenum –phosphorus complex. This intensely coloured compound was then quantified spectrophotometrically at 880 nm (Eisenreich et al., 1975). Each batch of samples were analysed alongside a quality control (QC) standard, produced by dissolving analyticalgrade potassium dihydrogen orthophosphate (dried to constant weight at 105 jC) in deionised water. The concentrations of soluble molybdate-reactive phosphorus (SRP), nitrate and dissolved reactive silicon were determined using Flow Injection Analysis (Foss-Tecator FIAStar model 5012). Each sample was analysed in triplicate. SRP was determined spectrophotometrically by a method modified from Murphy and Riley (1962) (Tecator, 1983a). Sample filtering and SRP analysis was completed within 8 h of sampling, to minimise errors associated with sample instability (Haygarth et al., 1995; House and Warwick, 1998a). The combined nitrate/nitrite concentration of filtered samples was determined by reducing the nitrate present to nitrite by passing the sample through a cadmium column, and then reacting with sulphanilamide and N-(1-naphthyl)-ethylenediamine dihydrochloride. The resultant azo-dye was then measured spectrophotometrically at 540 nm (Tecator, 1983b). Silicon concentration was determined by reacting the filtered sample with ammonium molybdate and then oxalic acid, to produce molybdosilic acid, which was then reduced using tin(II) chloride to produce a blue complex which was analysed at 720 nm (Tecator, 1998). Quality control standards were analysed alongside each batch of flow injection analysis. Phosphorus and nitrate QC standards were produced from dried analytical-grade potassium dihydrogen orthophosphate and sodium nitrate respectively. The silicon QC standard was produced by diluting a 1000 mg l-1 standard solution (Spectrosol silicon standard solution; BDH laboratory supplies, Poole, UK). Chloride concentrations were determined by ion chromatography (Dionex, model DX100) using an IonPac AS14 analytical column (4 250 mm) and an ASRS-Ultra 4 mm suppressor with sodium carbonate/bicarbonate eluent. A chloride quality control standard (produced using dried, analytical-grade so-
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dium chloride, dissolved in deionised water) was analysed with each batch of samples. 3.3. River discharge estimates The water depths and flow velocities (measured at 60% of the total depth) of each ungauged sampling site were measured at 0.5 m intervals across the river, at ca. eight times throughout the study period, under a range of different flow conditions and seasons. These comprehensive measurements were used to calculate river discharge by multiplying the cross-sectional area of each 0.5 m section by the mean water velocity of that section, and then summing the discharge from each section to get the total river discharge. The total measured discharge from these comprehensive measurements was then regressed against the discharge measured at the central 1 m of the river. These correlations between total discharge and midpoint discharge were then used to estimate the total discharge at the time of sampling, using the weekly spot measurements of depth and water velocity taken at the midpoint of the river. 3.4. Theory The concentration of each sample and the corresponding river discharge estimate were used to calculate instantaneous loads for each determinand, as follows: Li ¼ Ci Qi
ð1Þ
where Li, Ci and Qi are the instantaneous load, concentration and river water discharge at the ith time period. For each study reach, the load of a particular determinand entering the section was subtracted from the load measured at the downstream site, in order to produce a mass balance for that determinand for that particular sampling day. Mj;Downstream ¼ Mj;Upstream þ RMj;Tributaries þ RMj;STW þ Mj;Residual
ð2Þ
Mj,Upstream represents the load at time j of the River Frome site at the upstream end of the section,
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Mj,Downstream is the load exported from the section, Mj,Tributaries is the load from the tributaries entering the reach, and Mj,STW is the estimated input from sewage treatment point sources entering directly into the River Frome within the section. In the case of the middle study reach, Mj,Downstream is the measured load at Dorchester (i.e., the combined load at sites 4 and 5). Mj,Upstream is the River Frome load entering the section at Frampton (site 1). Mj,Tributaries is the combined load entering the middle reach from the two tributaries, Sydling Water (site 2) and the River Cerne (site 3). Mj,STW is the estimated input from the Bradford Peverell STW (Fig. 1). Mj,Residual is the mass-balance residual at time j, and is composed of four components:
Diffuse nutrient inputs and small unmonitored point sources (i.e., field drains and septic tanks) within the study reach. The internal riverine processes that are occurring within the river section. Errors in sampling, chemical analysis and river flow estimates. Errors associated with river transit time. These have been minimised in this study by eliminating data gathered during sharp, intense storm events. In the absence of errors in the load estimates, if the inputs to a section equal the outputs, then the Mj,Residual will be zero. However, if the mass of determinand being exported from a section is less than is entering from upstream, then the Mj,Residual will be negative, indicating nutrient storage within the channel. Conversely, a positive Mj,Residual will indicate a net-export of determinand from the reach, and will consist of diffuse inputs and any remobilisation of stored nutrients from the river channel. This approach has been used successfully in previous studies in catchments with small diffuse inputs (House and Warwick, 1998b; Bowes and House, 2001). However, because chalk rivers are predominantly groundwater-fed, they receive a constant and significant supply of water and associated diffuse nutrients. In order to use a mass-balance approach for chalk streams, this source of subsurface, diffuse input needs to be estimated and included as an extra component in Eq. (2). The diffuse
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groundwater contribution in this study was estimated by first quantifying the amount of water being generated within each study reach. This was done by calculating the water mass-balance residual, using
Eq. (2), for each sampling day. This quantifies the difference between the amount of surface water entering and flowing out of the study reach. It would be expected that the surface water entering the study
Fig. 2. Relationships between spot measurements and total measured discharge.
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reach would always be less than the discharge at the downstream extent of the study reach, and this extra water, the water residual, Wj,residual, at time j, would consist of
successfully in a previous study (May et al., 2001). These groundwater estimates were then used to refine the mass-balance equation for chalk rivers [Eq. (2)]. Mj;Downstream ¼ Mj;Upstream þ RMj;Tributaries
Wj;residual ¼ Wj;groundwater þ Wj;runoff þ Wj;seepage þ Wj;interception þ Wj;errors
231
ð3Þ
þ RMj;STW þ Mj;groundwater þ Mj;Residual
where Wj,groundwater, Wj,runoff, Wj,seepage and Wj,interception are the discharges of groundwater, runoff, soil through-flow/seepage and direct rainfall interception, respectively, entering the section. During dry periods, the water residual will approach the groundwater input (plus errors in river discharge estimates), as runoff and seepage are likely to be minor, and interception will be zero. The quantity of water derived from groundwater was estimated using River Frome hydrographs from the Dorchester (Louds Mill) and East Stoke gauging stations. It was assumed that on ‘dry’ sampling days, the water mass-balance residual would consist entirely of groundwater. These ‘dry’ data points were used to estimate the percentage contribution of the groundwater to the total River Frome flow throughout the year. This approach has been used
ð4Þ
where Mj,groundwater is the mass of determinand entering the study reach via the groundwater. This was calculated by multiplying the estimated volume of groundwater by the concentration of that determinand in the groundwater, obtained from chemical analysis of borehole samples (Fig. 1, site 10) from a previous study (Casey and Neal, 1986). This borehole was sited at Waterston Manor in the adjacent River Piddle catchment, and was only 5 km north east of the River Frome at Dorchester. It is assumed in this study that the groundwater entering the River Frome will have similar concentrations of nutrients. Mj,Residual in Eq. (4) now consists of within-stream processing, surface diffuse inputs and unmonitored point sources, and errors associated with flow gauging and chemical analysis.
Fig. 3. Hydrograph from the River Frome at Louds Mill, Dorchester (site 5) during the study period, showing sampling dates (n), ‘dry’ sampling dates with no rain for 4 days previously (5) and sampling dates that were later removed from the data set, due to coinciding with sharp storm events (D). The dashed line indicates the quantity of the total river flow estimated to be due to groundwater input.
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produce total discharge estimates for all sampling sites. The hydrograph of the River Frome at Louds Mill, Dorchester, including sampling points, is shown in Fig. 3. The unfilled symbols (5) represent sampling days where no rainfall was recorded at East Stoke for at least 4 days previously. During the summer months, it was assumed that on these dry sampling days, all of the river flow was due to groundwater input. The discharges on these dry sampling points were connected to produce estimates of the groundwater contribution, as shown by the dashed line in Fig. 3. During periods of high rainfall, the contribution of groundwater to the total river flow was more difficult to assess, but was again estimated by connecting the hydrograph discharges measured on dry days before and after the rainfall events (dashed line in Fig. 3). Samples taken during
4. Results and discussion 4.1. River discharge and load estimates Regressions of the measured discharge in the centre of the river channel and the total discharge (Fig. 2) produced robust relationships for all sites, except site 2, Sydling Water at Grimstone. Problems with the flow gauging at the Sydling Water site were probably caused by widely varying river flow patterns throughout the year, as a result of excessive plant growth at the sampling site. The discharge at this site was estimated by regressing the measured discharges and the recorded discharge from the Sydling St. Nicholas gauging station, 5 km upstream (Fig. 1). The linear regressions produced in Fig. 2 were applied to the spot measurements of depth and velocity taken at the time of sampling to
Table 1 Summary of nutrient concentrations and discharges at each sampling site Site name (National Grid Reference)
Site number
River Frome, Frampton (SY 623949)
1
Sydling Water (Sy 640945)
2
River Cerne (SY 678936)
3
River Frome, Stinsford Road Bridge (SY 703909)
4
Frome, Louds Mill (SY 707903)
5
South Winterborne (SY 724896)
6
Tadnoll Brook (SY 811882)
7
River Win (SY 832869)
8
River Frome, East Stroke (SY 866867)
9
Load (mol h-1)
Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum Minimum
TP
TDP
SRP
NO3
Si
Cl
34.5 504 7.7 4.5 44 0.6 8.9 92 1.4 9.6 45 1.2 56.7 494 12.7 1.4 10 0.05 13.0 74 1.6 3.5 113 0.02 131 487 44
20.0 144 6.7 2.5 10 0.6 5.7 27 1.2 6.3 21 1.2 37.7 231 12.1 1.1 9 0.01 7.3 34 0.6 1.0 31 0.01 90 192 37
17.8 107 6.3 2.3 10 0.4 5.4 24 1.2 5.9 21 1.2 33.7 160 11.9 1.0 8 0.01 6.7 31 0.6 0.7 22 0.01 84 191 35
1747 6031 772 537 1857 154 651 2585 73 786 2418 119 3998 12817 1209 533 2108 39 721 1620 115 126 935 1.4 7577 26371 2313
752 3106 212 184 597 61 208 789 27 275 826 32 1425 4363 360 101 435 10 214 538 41 38 267 2.3 2500 8520 760
2789 15630 789 634 2033 190 831 2854 113 1080 3184 191 5672 17052 1694 582 2297 42 1353 3642 258 329 3045 17 11995 37953 4174
Discharge (m3 s-1) 1.45 7.45 0.46 0.38 1.12 0.14 0.47 1.57 0.07 0.56 1.56 0.11 2.97 9.97 0.98 0.23 0.85 0.02 0.46 1.14 0.09 0.10 0.93 0.01 5.13 15.60 1.87
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three particularly sharp storm events (represented by the symbol D in Fig. 3) were removed from the data set, as samples taken at these times would be at different points in the hydrograph, causing inconsistent results. For instance, if the upper part of the study-reach was sampled during a sharp storm event, and the site at the downstream extent of the reach was sampled before this flood-wave reached it, the mass-balance residuals would be large and negative. Alternatively, if the upper reach was sampled after the storm event had passed, and the lower reach was sampled at the height of the storm event, then the mass balance residuals would be large and positive. It was therefore deemed necessary to remove these three particular data points to avoid these transit-time errors. A summary of the flow and loads for each sampling site is given in Table 1. The mean concentration
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of all nutrients in the River Frome increased downstream. Approximately 55% to 80% of the phosphorus load was in dissolved form at all sampling sites (except the River Win), and 90% to 95% of this dissolved phosphorus was SRP. The River Win had much less phosphorus load in dissolved form (31%), and less dissolved phosphorus as SRP (64%). This is probably due to the absence of STW input to the River Win. 4.2. Mass balances 4.2.1. Water The water mass-balance residuals for the two study reaches, derived from Eq. (2), are shown in Fig. 4(a). The gaps in the data are due to sampling not taking place during those periods, and also data points coinciding with three particularly sharp storm events
Fig. 4. Water mass-balance residuals, shown as (a) cubic meters per second and (b) mass balance (including estimated groundwater input), normalised to the total discharge from each study section.
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(Fig. 3) being removed. Mass-balance residuals were positive throughout the study period, showing that additional water was always entering the River Frome from groundwater and unmonitored surface inputs (such as drainage ditches, runoff and soil seepage). The proportion of this water residual accounted for by groundwater input was estimated using Fig. 3. This estimated volume of groundwater was then included as an input in the mass-balance calculation [Eq. (4)]. The resulting discharge residual was divided by the total discharge at the downstream site, in order to normalise the data [Fig. 4(b)], and shows the proportion of the total discharge derived from diffuse inputs and unmonitored point sources (i.e., drainage ditches). Fig. 4(b) highlights that diffuse inputs can comprise over 30% of the total flow during periods of sustained heavy rainfall.
4.2.2. Phosphorus The TP mass-balance residuals [Eq. (4)] showed a clear seasonal trend, particularly in the lower reach, with a net-retention in the summer months and export during the rest of the year [Fig. 5(a) and (b)]. The large negative TP residuals (over 80% of the total load for the lower reach) during the summer period showed that the majority of phosphorus entering the reach was being retained within the river channel, due to processes such as sequestering of dissolved phosphorus from the water column by bed sediments (House and Denison, 2002), deposition of particulate-bound phosphorus (PP) during periods of low-flow (Owens et al., 2001), and bioaccumulation of phosphorus within aquatic plants and epilithic biofilms during the growing season (Woodruff et al., 1999; House et al., 2001a). The net export of phosphorus from each reach
Fig. 5. Total phosphorus (TP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total TP load exported from the section.
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during the autumn to spring period was a result of remobilisation of bed-sediment PP, release of phosphorus-rich sediment pore-water (Casey and Farr, 1982), loss of P from decaying biota and the input of diffuse phosphorus from the catchment. Similar seasonal patterns have been observed previously (Dorioz et al., 1998; Bowes and House, 2001; Bowes et al., 2003). The TDP and SRP mass-balance residuals are shown in Figs. 6 and 7, respectively. They also show retention of dissolved phosphorus during low-flow periods and a net-export in winter. The patterns and rates of TDP and SRP retention and export are very similar, indicating that transformation from SRP to soluble unreactive forms within this section of the catchment is negligible. The middle reach was a net-source of all forms of phosphorus, with an average net export of 9.4 mol h-1 TP over the course of the study period (equivalent to
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0.78 mol h-1 TP per km of river), indicating that diffuse phosphorus inputs are significant between Frampton and Dorchester throughout the year. The lower reach was a net-sink for phosphorus, retaining on average 14.1 mol h-1 TP over the study period (equivalent to 0.64 mol h-1 TP per km of river), which may be a result of the large-scale overbank flooding that regularly occurred in the flood meadows of the lower section, resulting in transfer of phosphorus from the channel to the floodplain. The average rate of TP, TDP and SRP retention in the lower reach, between July –September 1999 and 2000, was relatively uniform at ca. 30 – 40 mol h-1 Figs. 5(a), 6(a) and 7(a)]. Almost all of the phosphorus sequestered from the water column was in soluble reactive form. Surface diffuse inputs of phosphorus during these dry periods are negligible, and so the rate of retention is almost entirely within-channel storage
Fig. 6. Total dissolved phosphorus (TDP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total TDP load exported from the section.
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Fig. 7. Soluble reactive phosphorus (SRP) mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total SRP load exported from the section.
as a result of adsorption to bed-sediments and bioaccumulation. The lower study reach had an average phosphorus retention rate of ca. 83 Amol m-2 h-1 throughout these summer months, assuming an estimated channel-bed area of 418,000 m2. Similar retention rates (57 Amol m-2 h-1) have been observed in the River Swale, northern England, during summer lowflow conditions (Bowes and House, 2001). The middle reach also showed a marked seasonal pattern in TP residuals [Fig. 5(b)], with rates of export in the winter of 1999 and release in the summer of 2000 that were almost identical to the rates observed in the lower reach. However, there was very little seasonality in the TDP and SRP residuals in the middle reach [Figs. 6(b) and 7(b)]. These results demonstrate that the retention of ca. 40% of the phosphorus entering the middle reach during the summer of 2000 was predominantly in particulate-bound form.
Over the course of the study, there was a net retention of phosphorus from the lower reach and approximately equal inputs and outputs from the middle reach. However, as the mass-balance residual is a product of both within-stream processing and diffuse nutrient sources, there should be a net loss from the system (as extra nutrients are being added to the catchment in the form of fertiliser). During the wet periods between autumn and spring, there will be large, episodic inputs of diffuse phosphorus and rapid remobilisation of sediment-bound within-channel phosphorus (Brunet and Astin, 1998; Bowes and House, 2001). These rapid exports of phosphorus will be underrepresented by our weekly sampling regime, especially as we have had to remove data associated with sudden, sharp, storm events, due to transit-time errors. If more frequent sampling was adopted during high-flow events, then the phosphorus residuals
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would be expected to be large and positive during these winter storms. 4.2.3. Nitrate Net exports of up to 6 kmol h-1 nitrate were observed in both study reaches during periods of heavy rainfall in January 2000 [Fig. 8(a)], mainly as a result of surface diffuse inputs (runoff and soil seepage) to the river. These diffuse inputs would be exacerbated by the lack of vegetation cover in the winter months in many land-use categories. There is a clear seasonal pattern in nitrate residuals for both study reaches. The middle reach showed a net export for most of the year, with the highest mass-balance residuals in the winter and spring. The lower reach only showed a net export during periods of heavy rainfall, and a net retention at other times. The rate of nitrogen storage was greatest during periods of summer low-flow, where ca. 1 kmol h-1 nitrate was being
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stored within the channel, which was equivalent to 2.4 mmol m-2 h-1. Very similar nitrate retention rates (2.0 mmol m-2 h-1) were measured in the River Frome at East Stoke under low-flow conditions in 1992 (House et al., 2001b). Studies on other rivers have estimated retention rates ranging from 0.12 mmol m-2 h-1 (Hill, 1981) to 4.2 mmol m-2 h-1 (Owens et al., 1972). This negative nitrate residual was probably a result of bioaccumulation by aquatic plants and epilithic algae during the growing season, a lack of diffuse inputs from the catchment during dry periods, and could also indicate that microbial denitrification was occurring in the anaerobic bed sediments (Pattinson et al., 1998). 4.2.4. Dissolved silicon Silicon mass-balance residuals showed very little seasonal pattern (Fig. 9), compared with nitrate and phosphorus. However, there were short periods of
Fig. 8. Nitrate mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total nitrate load exported from the section.
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Fig. 9. Dissolved silicon mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total dissolved silicon load exported from the section.
major retention of dissolved silicon in the spring/early summer (of up to 500 mol h-1) in both reaches, equivalent to ca. 20 –30% of the expected load. Retention rates may have been even greater during March and April 2000, but samples were not taken, or had to be rejected due to coinciding with sharp storm events. There were also short periods of silicon storage between July and September. This pattern of major silicon storage in the spring, followed by smaller and episodic silicon-stripping in the summer has been observed previously in the River Frome (Casey et al., 1981; Marker et al., 1984; House et al., 2001b) and other UK rivers (Edwards, 1974). The removal and storage of dissolved silicon from the water column is a result of assimilation by diatoms (Edwards, 1974; Woodruff et al., 1999; House et al., 2001b), and, to a smaller extent, chemisorption to minerals in the bed
sediments (Casey and Neal, 1986). The silicon retention rate during the initial spring diatom bloom was 1.24 and 0.88 mmol m-2 h-1 for the lower and middle reaches, respectively, which was comparable to the rates observed in a previous study of the River Frome at East Stoke (between 1.9 and 2.6 mmol m-2 h-1; House et al., 2001b) and a study of an adjacent chalk stream catchment (between 1.5 and 4.5 mmol m-2 h-1 in the Bere Stream, Dorset, UK) (Casey et al., 1981). Major positive mass-balance residuals occurred in each study-reach in September and October 1999, and January 2000 [Fig. 9(a) and (b)], coinciding with periods of heavy rainfall/high river discharge (Fig. 3), which was equivalent to approximately a 20– 30% increase in silicon load. This export of silicon during storm events is most likely caused by the disturbance of the upper bed sediments by the increasing flow
M.J. Bowes et al. / Science of the Total Environment 336 (2005) 225–241
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Fig. 10. Chloride mass-balance residuals, shown as (a) a rate and (b) the rate, normalised to the total chloride load exported from the section.
velocity, resulting in the release of silicon-rich porewaters (produced by the dissolution of diatom frustules since the end of the growing season) (Woodruff et al., 1999; House et al., 2000). This additional silicon may also be derived from diffuse inputs from the floodplain, directly entering the River Frome. Similar patterns of silicon release during storms have been observed in previous studies (Casey and Farr, 1982; Bowes and House, 2001; House et al., 2001b). 4.2.5. Chloride Chloride residuals were small, relative to those of phosphorus and nitrate, and did not show a seasonal pattern [Fig. 10(b)]. However, there was a large (30 – 40%) generation of chloride during periods of high rainfall in December 1999, and May 2000, possibly due to atmospheric inputs and diffuse inputs from the floodplain, directly into the River Frome. The application of rock salt to roads during winter may also
contribute to this increased chloride load during the winter of 1999. During the rest of the monitoring period, the chloride residuals are relatively stable, varying within a range of ca. 20%. This contrasts strongly with the variation observed in the TP [Fig. 5(b)] and nitrate [Fig. 8(b)] residuals in the lower study reach, which varied by 100% and 50%, respectively. These seasonal trends in nitrogen and phosphorus must therefore be a result of internal riverine processing and diffuse nutrient inputs from the catchment, rather than an artefact of errors associated with discharge measurement, transit time or sampling.
5. Conclusions This use of mass-balance residuals has provided a valuable insight into the annual phosphorus, nitrate and silicon dynamics of the River Frome. All nutrients
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showed a net retention within the channel during the dry, summer months, as a result of bioaccumulation and, in the case of phosphorus, adsorption to bed sediments. During the wetter autumn to spring periods, there was a net generation of nutrients within each study reach, as increased runoff transported nutrients from the floodplain into the river channel. The high river discharges during these wet periods also remobilised phosphorus and silicon stored in the bed sediment, and this release of nutrients back into the water column also contributed to a positive massbalance residual. The chloride residuals did not show a seasonal pattern, indicating that the seasonality of the phosphorus and nitrate dynamics was a product of annual variation in diffuse inputs and internal riverine processes, rather than an artefact of sampling, flow gauging and analytical errors. The seasonal pattern in phosphorus and nitrate residuals was more pronounced in the lower reach, compared with the middle study reach. There are a number of possible explanations for this. First, it may be a result of changes in land-use down the catchment. The lower reach has a higher proportion of tilled land (4%, as opposed to 1% in the middle reach). This will have low diffuse nutrient losses during crop production in the summer, but high losses after harvesting. The lowland also has 10% less grassland and seminatural meadow (which will have more consistent nutrient losses throughout the year). Therefore, the diffuse nutrient losses from the lower reach would be expected to have a higher seasonality. Changes in the rates of within-channel nutrient processing downstream will also contribute to the increased seasonality in the lower reach. Higher biomass and an increasing proportion of fine sediment in the lower reach (due to attrition of the sediment load as it is transported downstream) will result in enhanced nutrient storage in summer and subsequent loss during the autumn – spring storms. Finally, the middle reach has a higher proportion of its total flow derived from groundwater, due to its chalk geology. (Much of the lower River Frome flows across sand and gravel deposits). The middle reach will therefore have a more consistent nutrient supply throughout the year and less diffuse runoff during storm events. This study has shown that the use of mass balances can be successfully extended to groundwater-dominated chalk rivers. The approach developed in this
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