Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events

Journal of Hydrology (2008) 350, 203– 214

available at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/jhydrol

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events M.I. Stutter *, S.J. Langan, R.J. Cooper The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK

KEYWORDS Phosphorus; Sediments; Storms; Hysteresis; Transport; Spatial scales

Summary Concentrations of suspended particulate matter (SPM), NO3-N and P fractions: PO4-P, dissolved organic P (DOP), particulate P (PP) and bioavailable exchangeable P were examined over 5 storm events in two nested agricultural catchments in NE Scotland: a (51 km2) catchment and its headwater (4 km2). NO3-N showed anticlockwise hysteresis for all storms in both catchments. In contrast, the headwater showed strong clockwise hysteresis of SPM, dissolved and particulate P concentrations, but which weakened through summer to spring. Less pronounced hysteresis of P forms in the larger catchment was attributed to a combination of factors: a less energetic system, nutrient leaching from the floodplain, a point source of a small sewage treatment works and the occurrence of coarser soil and sediment parent materials with less P adsorption and transport capacity. The headwater exhibited a strong ‘first flush’ effect of sediment and dissolved P, particularly following dry conditions, received a significant transfer of readily-solubilized organic P from the surrounding soils in late summer and after manure applications in winter, and was the likely cause of large sediment associated P signals observed in the 51 km2 catchment. Our results suggest that steeper gradient headwaters should be targeted for riparian improvements to mitigate soil erosion from headwater fields. The efficiency of riparian erosion controls is also dependant on the size of the store of fine sediment material within the stream channel and this may be large. ª 2007 Elsevier B.V. All rights reserved.

Introduction

limiting nutrient to primary productivity. Increasing losses of P from terrestrial to aquatic ecosystems are frequently associated with agricultural intensification and the resulting increases in soil P status, land application of animal wastes

The concentration and bioavailability of phosphorus (P) in surface waters governs eutrophication, since P is often the * Corresponding author. Tel.: +44 (0) 1224 498200. E-mail address: [email protected] (M.I. Stutter).

0022-1694/$ - see front matter ª 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jhydrol.2007.10.045

204 and catchment and channel erosion. The protection of waters from P enrichment is challenging due to the complex physical and biogeochemical processes which control the environmental interactions of particulate, inorganic and organic forms of P in the aqueous environment. The greatest changes in stream water P concentrations often occur over storm events (Evans and Johnes, 2004; Haygarth et al., 2005), especially for particulate associated forms (McDowell and Sharpely, 2002). Particulate P may be transported as a result of soil eroded from field slopes or channel banks, or remobilization of stream sediments. Hence, testing soils for P enrichment must be coupled with a spatial and temporal understanding of sediment sources within the catchment (Scanlon et al., 2004). The storm response in dissolved P includes processes of desorption from mobilized sediments, surface runoff and leaching (KoskiVa ¨lha ¨la ¨ and Hartikainen, 2001; Evans et al., 2004). These transport processes from the land to receiving waters and along stream networks are important in determining the relative loads of P forms, the transport potential downstream, the timing and duration of the P transport and the residual effects of the storm on the in stream P chemistry. An understanding of the changes in P transport and reactivity over sequences of events across catchment scales is therefore required to relate water quality problems with P sources. During storms, stream water concentrations of different P forms may evolve at different rates with transport from varying source regions by different pathways, for example surface runoff, flushing of soil water reservoirs, or particle erosion and resuspension (House and Warwick, 1998). Many studies have documented that the peak concentration of sediment and sediment associated constituents coincides with or precedes peak flow and that these constituents exhibit a hysteresis effect with decreasing flow (e.g. Robertson and Roerish, 1999; Harmel and King, 2005). Analyses of concentration – discharge hysteresis have been widely used to discriminate diffuse and in-channel sources of sediments and P (McDiffett et al., 1989; Williams, 1989; House and Warwick, 1998). These studies often conclude that P supply is governed by resuspension of streambed sediments in downstream catchments. Studies where dissolved and particulate P forms are considered together are limited in number, and it is only recently that consideration has been given to the spatial connectivity of the observations and processes. Bowes et al. (2005) contrasted the anticlockwise hysteresis of P forms in an upland headwater, with clockwise trajectories in the agricultural lowlands where channel storage and mobilization processes increased in importance. Haygarth et al. (2005) showed that the processes controlling dissolved and particulate P dynamics differed most at times of maximum hydrological energy between differing catchment scales. A catchment’s temporal response in P concentrations during storm events represents a complex set of interactions between land use and management, soils, antecedent conditions and event characteristics. An understanding of these interactions provides an important means of assessing the hydrological connectivity of the land to the stream network. This study describes the response of suspended particulate matter, particulate and dissolved P forms to storm events in an agricultural catchment and headwater. We utilize analyses of the rate of concentration change, hysteresis behavior and cumulative mass and discharge rela-

M.I. Stutter et al. tionships to determine the temporal variability and lags in loads and concentrations of phosphorus in the two nested catchments. From an understanding of this spatio-temporal variability we test the hypothesis that the mobilization of catchment sources of P varies according to a different set of processes at different catchment scales and discuss the implications for management of such systems. This study of the high resolution storm response of this catchment system accompanies another paper (Stutter et al., 2008) which focuses on the seasonal response and ecological impacts of nutrients in the same tributary and further downstream into a major river system.

Materials and methods Study area The catchments studied comprise a first-order agricultural headwater (Millhead Burn) flowing into a larger agricultural stream (Tarland Burn). The Tarland system drains the most westerly area of intensive agriculture in the River Dee catchment, NE Scotland. Several reports have noted that the generally nutrient-deficient waters of the River Dee are at risk due to diffuse pollution inputs (Langan et al., 1997; www.sepa.org.uk/ WFD, 2005) The physical setting of the study catchments is detailed in Stutter et al., 2008 together with a map of the sampling locations and land use (summarized again here in Table 1). The agricultural soils in the headwater are medium loam to clay loams derived from mixed acid and basic tills (MISR, 1957). The Tarland Burn sampling site is located downstream of a broad floodplain of alluvial soils which are used for cereal growing. In this part of the downstream catchment soils are coarser and less clay enriched than on the surrounding slopes, being derived from fluvio–glacial sands and gravels (MISR, 1957). The village of Tarland (population approximately 600) is served by a sewage treatment works (STW) 3 km upstream of the Tarland Burn sampling point and there are numerous septic tanks in use in the rural area; however, in general, the population density is small (Table 1).

Hydrological measurements and water sample collection Water samples were collected four-hourly during storms as part of an intensive hydrochemical sampling campaign throughout April 2004–May 2005 (for details see Stutter et al., 2008). Non-refrigerated autosamplers (ISCO 3700, Teledyne-Isco, Lincoln, USA) were located under trees for shade and the water samples were collected every few days during the storm periods. Hydrological data was generated by logging stage heights using a pressure transducer and recorded half-hourly at Millhead Burn and hourly at Tarland Burn (means of 1 min data; CR10X, Campbell Scientific, Loughborough, UK). The discharge was calibrated using a trapezoidal glassfibre flume in the Millhead Burn and, in the Tarland Burn, velocity area profiling integrated across the channel at 60% depth.

Analyses of waters Concentrations of suspended particulate matter (SPM) were determined gravimetrically by filtration through preweighed

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water Table 1 Characteristics and land use of the study catchments

Catchment area (kmb) Gauging/sampling point Altitude range (m a.s.l.) Land usea (% of area) Arable Improved grassland Rough grass/moor/montane Woodland Urban Population density (no./kmb) Pollution point sources a b

Millhead burn

Tarland burn

4.3 NJ 474060 190–619

51 NJ 511025 133–619

21 25 33 21 0 approx. 3

25 35 18 21 0.7 approx. 20 STWb (serving 650)

MLURI (1993). Swege Treatment Works.

membrane filters (Whatman, 0.45 lm cellulose acetate) and air-drying. Filters were not oven-dried to protect the integrity of the SPM for subsequent chemical analyses. However, on a subset of filters, we noted that further moisture loss from oven-drying (105 C for 2 h) previously air-dried SPM filters was less than 5% of the air-dried SPM mass. The air-dried filters were then archived in a desiccator for subsequent extractions. The filtrates were analysed by segmented flow automated colorimetry using the manufacturer’s standard procedures (San++ analyser, Skalar, The Netherlands) for NO3-N and molybdate reactive P (MRP) then for total dissolved P (TDP) following an automated persulphate/UV digestion procedure. Molybdate unreactive P (MUP) was determined as the difference between TDP and MRP concentrations. We subsequently use the terms PO4-P instead of MRP and DOP instead of MUP for simplification, even though we are aware that these are merely the most dominant fractions (see Stutter et al., 2008). To determine total P, unfiltered portions of thoroughly mixed original samples were manually digested (along with standards and QC samples) using a persulphate autoclave procedure (Williams, 1989) and analysed for MRP. The QC samples of tripolyphosphate (50 lg l1) gave mean recoveries of 96%. The difference between the digested unfiltered sample total P and TDP of the filtered samples was taken to be particulate P (PP).

Chemical analyses of sediments The bioavailable exchangeable P was extracted from SPM on filter papers using an iron oxide paper P sink (manufactured according to Chardon et al., 1996). This fraction is termed FeO-P. A 1 cm2 test paper (2 cm2 of reactive surface) was shaken in a temperature-controlled room (20 C for 16 h) with each filter paper in 10 ml 0.01 M CaCl2. The FeO papers were then washed with deionised water to remove sediment particles, then the adsorbed P was extracted into 10 ml 0.1 M H2SO4 (4 h) and analysed for MRP. Triplicate quality control samples of 10 mg of a reference soil material on

205

filter papers gave coefficient of variation of 21%. Varying masses of SPM on the filter papers (1–300 mg) gave a wide range of sediment to solution ratios. According to Chardon et al. (1996), a large ratio (i.e. a small mass of SPM) will promote desorption of P from the sediment by dilution, but in extreme cases can decrease sorption onto the FeO paper as the diffusion distance increases. The principal error in this technique would be if the bioavailable P in larger masses of SPM on filter papers at the peak of storms exceeded the available FeO sink provided by the test paper. However, we found no evidence of this using 10–1000 mg of agricultural topsoil standard materials with a 1 cm2 FeO paper size. The FeO-P values are represented in two ways: as the bioavailable P content of the SPM (mg P kg1 SPM dry mass) and as the concentration of bioavailable P in the water column (lg P l1) by multiplying the mg kg1 value by the SPM concentration in the water column at that time. Stream bed sediment samples taken adjacent to the sampling points were collected on 3/8/2004 to characterize stream bed sediments after a period of base flow. Sediments were collected from the top 3 cm of the bed material, mid channel, over an area of bed approximately 1 m2 using a plastic scoop with a lid that closed to prevent loss of fines. Particle size distributions (<2 mm fraction) were determined by laser diffractometry (Malvern 2000 analyser) following dispersion pretreatment of 16 h end over end shaking in 3% ammonia solution. The <250 lm fraction of the air-dried sediments was further analysed since it represents the material potentially mobilized during storms. Organic C and N were determined using a Thermo-Finnigan CN analyser. Oxalate extractable Fe, Al and P were determined according to Farmer et al. (1983). Sorption isotherms (16 h equilibration at 5 C) at solution concentrations of 0, 10 and 20 lM PO4-P were performed (1:25 w/v, 0.01 M CaCl2 matrix). From these data the Equilibrium Phosphate Concentration (EPC0) was calculated using the Langmuir isotherm method according to House and Denison (2002) using the FeO-P content as the adsorbed sediment ‘native’ P.

Data analyses Five storm cycles were identified (Fig. 1) over different seasons and antecedent flow conditions and where adequate samples were available to describe the hydrochemical changes over the hydrograph. Clogging of the sampler inlet tube led to insufficient samples being taken at the Tarland Burn site over the course of March storm 1 to allow analysis of stream chemistry. Patterns of change in stream water dissolved and particulate P concentrations were assessed by classifying hysteresis plots as either no hysteresis, weak, or strong hysteresis in a clockwise or anticlockwise direction. The extreme hysteresis form of the sediment constituents precluded the type of numerical analysis used by Bowes et al. (2005). Further analyses were undertaken using the ‘pollutogram’ approach developed by Rossi et al. (2005) approximated by the relationship: FðxÞ ¼ x b ;

ð1Þ

where F(x) is the fraction of the total mass of the determinant during the storm event and x is the total mass of water during the event. The parameter b is a coefficient repre-

206

M.I. Stutter et al. 0.7

4

storm represented the first rewetting of the catchment after a period of prolonged baseflow and gave the greatest discharge at both sites which dominated the hydrographs (Fig. 1). A further period of late summer flow recession ended with several October storms, of which storms 1 and 2 were selected as the two first discharge peaks. A number of smaller storms occurred from November through the winter until a period of elevated discharge, related to snow melt, occurred in March. Compared to the headwater the downstream catchment demonstrated a greater number of storm events of similar magnitude with longer recession times. The evolution of event stream chemistry and discharge at the Millhead and Tarland Burns over this period is depicted (Figs. 2 and 3) and concentration ranges given in Table 2.

3

Headwater hydrochemistry

Millhead Burn August

0.6

Q (m3 s-1)

0.5 0.4

March 2

October 2

0.3

October 1

March 1

0.2 0.1 0.0

5

Tarland Burn

October 2

March 2

October1 2

Q (m3 s-1)

August

1

Apr-05

Mar-05

Feb-05

Jan-05

Dec-04

Nov-04

Oct-04

Sep-04

Aug-04

Jul-04

0

Month

Figure 1 Hydrological context of the studied storm events in the Millhead Burn (headwater) and Tarland Burn (downstream) catchments.

senting the relationship between the mass and water volume over time which may be plotted as the cumulative proportion of the total mass transported against the cumulative proportion of water transported. Values of b of <1 and >1 indicate that the determinant mass arrived predominantly towards the start, or end of the event, respectively. A value of b = 1 denotes either that the pollutant mass and water volumes are proportional, or that the pollutant concentrations stay constant over the event.

Hydrochemical changes in the Millhead Burn headwater are presented in Fig. 2, with concentration ranges and instantaneous fluxes in Tables 2 and 3. The August storm in the headwater gave the greatest rates of concentration increase, maximum concentrations and instantaneous fluxes of SPM, PP and FeO-P for any storm in either catchment. Peak concentrations of all these sediment determinants strongly preceded maximum discharge, but rapid declines in SPM relative to PP and FeO-P meant that sediment P contents (mg kg1 basis) increased during the flow recession. Peak concentrations of DOP and PO4-P preceded maximum discharge in August, but occurred at maximum discharge during October storms. There was a lag in the decrease of DOP concentrations relative to those of PO4-P after the August and October storms. During the course of the October storms peak concentrations of SPM, PP, FeO-P and DOP declined, whilst those of PO4-P increased. Initially, snow fall in early March brought no rise in the hydrograph until a small melt started 07-Mar-05. Rain falling onto frozen ground led to rapid, but small, simultaneous increases in SPM, PP, FeO-P and PO4-P during March storm 1, then greater concentrations during March storm 2 as the melt progressed. Concentration increases in DOP lagged behind those of PO4-P, but were of a greater magnitude.

Results Tarland burn hydrochemistry The results are presented in terms of the context of the storms in the annual hydrographs, descriptions of the concentration changes over the course of the events, followed by analyses of the form of the hysteresis and cumulative pollutant mass to water volume relationships. Analyses of a relatively limited number of storms provides only limited data on which to comment on seasonal variability in nutrient and sediment response to storms, although gaining further data at high temporal resolution is costly. However, our data provides a good basis on which to address the scaling issues in sediment and P sources to, and transport between catchments.

Hydrological context of the storm events During the spring of 2004 (data not shown) a steady flow recession occurred in both catchments. The August 2004

Maximum concentrations of SPM, PP, FeO-P and PO4-P in the downstream catchment (Tarland Burn) occurred simultaneously with the peak in August discharge (Fig. 3). The maximum instantaneous fluxes of PP, DOP and PO4-P were comparable with the August storm in the headwater, but those of SPM and FeO-P were smaller (Table 3). A value of 1500 mg FeO-P kg1 occurred prior to the August storm at a time of very small SPM concentration. Maximum SPM, PP, FeO-P and dissolved P concentrations followed the peak discharge of October storm 1, but concentration changes in FeO-P were damped compared to SPM and PP. During October storm 2 SPM, PP, PO4-P and DOP concentrations were smaller than during October storm 1, despite greater peak discharge. March storm 2 in the Tarland Burn triggered a sharp rise and slow decline in PP concentrations, but conversely a slow rise to maximum SPM and FeO-P concentra-

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water Rainfall Q

August storm 0.7

10

2

NO 3-N

250 200 150 100 50

FeO-P (µg l-1)

FeO-P (mg kg-1) 15-Oct-04

0 18-Aug-04

0.1

20-Aug-04

1

DOP

300

19-Aug-04

10

PO 4-P

1

FeO-P (mg kg--1)

FeO-P (µg l--1)

0 100

09-Mar-05

4

08-Mar-05

6

07-Mar-05

NO3-N (mg l--1)

8

0.1 100

PO4-P, DOP (µg l--1)

0.0 10

1

06-Mar-05

0.1

March storm 2

10

05-Mar-05

0.2

100

04-Mar-05

0.3

SS PP March storm 1

17-Oct-04

0.4

October storm 2

16-Oct-04

Q (m3 s-1)

0.5

1000

PP (µg l--1), SPM (mg l-1), Rainfall (mm h-1)

0.6

October storm 1

207

Figure 2 Evolution of the discharge and hydrochemical changes in the headwater (Millhead Burn) during the course of the monitored storm events.

tions with sharp declines. At both sites, changes in concentrations of NO3-N during the storms were small when compared to those of dissolved P and, with the exception of March storm 1 in the headwater, showed a dilution on the hydrograph rise.

Event hysteresis and b relationships The directions of clockwise and anticlockwise concentration hysteresis indicate concentrations that are, respectively greater, or lesser for a given discharge, on the rising limb of the storm hydrograph than on the recession. The strength of the hysteresis is dependant on the relative concentration difference between these points. The prominent features of the analyses of hysteresis (Table 4) were: (1) consistent anticlockwise hysteresis in nitrate concentrations in the headwater and downstream catchments in accordance with initial dilution, then slow leaching, (2) strong clockwise hysteresis patterns for SPM, PP and FeO-P concentrations in the headwater during all the storms (although weaker for SPM and FeO-P during March storm 2), (3) changing patterns of hysteresis in the headwater in PO4-P and DOP concentrations, predominantly clockwise then tending to anticlockwise hysteresis in DOP during the March storms, (4) similarities in the clockwise hysteresis response of SPM and all P forms in the headwater and downstream catch-

ments during the August and October 2 storms, but (5) contrasting hysteresis between the catchments during October storm 1 (limited hysteresis in the downstream catchment) and March storm 2 (anticlockwise hysteresis in SPM and sediment associated P concentrations). Plots of cumulative mass versus cumulative discharge (Figs. 4 and 5) and values of b (Table 4) showed for SPM and sediment P forms: (1) an extreme ‘first flush’ effect (b values of 0.06–0.12) associated with the August storm in the headwater, (2) a slower response downstream in the Tarland Burn catchment to the August storm (b values of 0.50–0.56), (3) a general increase in b values through summer to autumn to spring in the headwater whilst mostly remaining <1 and (4) b values of SPM and sediment P forms were smaller than those for DOP and PO4-P with the exception of March storm 2 in the Tarland Burn. In both catchments similar values of b for PO4-P and DOP over individual storms indicated common sources and transport pathways. There were a couple of exceptions when b values for PO4-P were smaller than those of DOP in the headwater: during the August storm (0.60 PO4-P and 0.76 DOP) and during the initial snowmelt of March storm 1 (1.08 PO4-P and 1.33 DOP), both being times when DOP concentrations were slow to decline over the storm recessions. Small values of b for dissolved P forms in the headwater (0.61–0.77) during August and October storms became >1

208

M.I. Stutter et al.

October storm 1

August storm 5

PP (µg l--1), SPM (mg l-1), Rainfall (mm h-1)

4

1000

Rainfall Q

October storm 2

SS PP

March storm 2

Q (m3 s-1)

100

3 2 1

8

NO3-N (mg l--1)

1

0.1 100

PO4-P, DOP (µg l--1)

0 10

10

6 4 2

10

NO 3-N

10

1

PO 4-P

DOP

1 600

FeO-P (mg kg--1)

FeO-P (µg l--1)

0 100

FeO-P (µg l-1)

500

FeO-P (mg kg-1)

400 300 200 100

09-Mar-05

08-Mar-05

07-Mar-05

06-Mar-05

05-Mar-05

04-Mar-05

17-Oct-04

16-Oct-04

15-Oct-04

20-Aug-04

19-Aug-04

0 18-Aug-04

0.1

Figure 3 Evolution of the discharge and hydrochemical changes in the downstream catchment (Tarland Burn) during the course of the monitored storm events.

Table 2

Discharge and concentration ranges in response to the storms SPM (mg l1)

PP (lg l1)

FeO-P (lg l1)

PO4-P (lg l1)

DOP (lg l1)

NO3-N (mg l1)

(i) Millhead burn Aug-04 0.03 –0.42 Oct-04 (1) 0.04–0.15 Oct-04 (2) 0.12–0.25 Mar-05 (1) 0.06–0.13 Mar-05 (2) 0.08–0.19

3–933 14–121 14–116 8–37 3–89

8.0–1026 6.8–158 29.4–211 12.2–51.9 1.3–136

0.6–71.1 1.8–22.1 2.5–12.4 1.0–3.2 0.6–4.1

5.3–44.3 6.5–23.4 12.3–30.7 5.9–9.0 5.7–11.6

6.8–44.7 10.7–34.3 18.5–32.3 10.6–17.0 8.2–42.5

1.3–2.1 3.0–4.0 3.3–3.9 3.2–4.2 3.4–4.2

(ii) Tarland burn Aug-04 0.30–4.40 Oct-04 (1) 0.63–2.59 Oct-04 (2) 1.62–3.51 Mar-05 (2) 1.33–2.06

1–262 12–112 10–68 9–320

10.0–649 34.4–255 23.2–133 13.0–152

1.1–14.0 2.2–7.8 2.3–9.5 0.8–8.4

19.3–94.0 18.0–41.5 23.2–36.1 8.4–15.8

0–52.0 14.6–40.6 21.6–34.7 16.7–25.9

2.0–2.9 3.5–4.7 4.2–4.7 4.2–4.9

Storm

Q (m3 s1)

during the March storms. In the Tarland Burn catchment a ‘first flush’ of dissolved P (b values <<1) was only observed during the August storm and for subsequent storms dissolved P values approximated to unity. This seasonal intra-storm variation in the rate of storm dissolved P mass transport was in contrast to larger, relatively stable values of b for NO3-N (1.02–1.20). The greater b values showed that

changes in the pathway of NO3-N transport occurred during the August storm and the initial snowmelt of March storm 1 (Table 4). Discrepancies between the cumulative mass and discharge relationships for the headwater and downstream catchments during consecutive storms indicated different transport pathways operating at different catchment scales.

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water Table 3

209

Maximum instantaneous fluxes of stream water determinants over the storm periods

Storm

Maximum instantaneous fluxes Q max (l ha

SPM

1 1

s )

(mg ha

PP

FeO-P

1 1

s )

(lg ha

PO4-P

DOP

1 1

s )

(i) Millhead burn Aug-04 Oct-04 (1) Oct-04 (2) Mar-05 (1) Mar-05 (2)

0.97 0.34 0.58 0.29 0.45

222 34.5 67.6 10.6 39.8

244 45.3 123 14.2 60.7

16.9 6.31 7.20 0.68 1.84

42.8 7.65 17.9 2.62 4.99

43.3 10.3 18.8 4.67 18.4

(ii) Tarland burn Aug-04 Oct-04 (1) Oct-04 (2) Mar-05 (2)

0.86 0.50 0.68 0.40

84.6 56.4 43.3 108

206 129 85.4 60.7

6.71 3.90 5.30 2.84

45.8 18.6 24.2 6.09

40.8 20.5 22.2 10.0

Table 4 Storm

Modeled values of b and forms of hysteresis (in parentheses) for the pollutants over the storms SPM (mg l1)

(i) Millhead burn Aug-04 0.06 Oct-04 (1) 0.30 Oct-04 (2) 0.32 Mar-05 (1) 1.13 Mar-05 (2) 0.72

(C) (C) (C) (C) (c)

(ii) Tarland burn Aug-04 0.50 (C) Oct-04 (1) 0.88 (N+) Oct-04 (2) 0.55 (c) Mar-05 (2) 2.05 (A)

PP (lg l1)

FeO-P (lg l1)

PO4-P (lg l1)

DOP (lg l1)

NO3-N (mg l1)

0.12 0.47 0.35 0.82 0.68

(C) (C) (C) (C) (C)

0.09 0.34 0.47 0.72 0.81

(C) (C) (C) (C) (c)

0.61 0.76 0.69 1.08 1.03

(c) (c) (C) (c) (N+)

0.76 0.76 0.77 1.33 1.18

(N+) (c) (C) (a) (A)

1.20 1.02 1.07 1.16 1.08

(a) (a) (A) (a) (a)

0.52 0.86 0.57 1.23

(C) (N+) (C) (a)

0.56 1.00 0.64 1.50

(c) (N+) (C) (a)

0.79 1.02 0.85 0.91

(C) (N+) (c) (c)

0.74 1.07 0.87 0.94

(C) (a) (c) (N+)

1.02 1.13 1.04 1.05

(a) (A) (a) (a)

Visual interpretations of hysteresis form: C/c = strongly/weakly clockwise; A/a = strongly/weakly anticlockwise; N+ = no hysteresis with a +ve relationship with Q.

The rapid mobilization of SPM and sediment P in the headwater during October storm 1 (b values 0.30–0.47) compared to a slower reaction of sediment mass transport downstream (b values 0.88–1.00) indicated a lag in the transport of sediments into the downstream catchment. However, decreases b values (0.55–0.64) in the Tarland Burn for the October storm 2 indicated that sediments were more rapidly mobilized from the catchment or stream channel. Also, b values >1 for sediment determinants in the Tarland catchment during the snowmelt period of March storm 2 indicated a substantial lag in transport relative to the headwater which was more pronounced for total SPM (b value 2.05) than for P enriched fraction of SPM (b values 1.23– 1.50).

Stream sediment properties The bed sediments prior to the August storm event showed important physico–chemical differences between the sites that would affect transport and P complexation capacity (Table 5). The bed sediment was much finer in texture in the headwater with a greater mass content of oxalate

extractable Fe and Al (Feox, Alox) and FeO-P than at the Tarland Burn site. Bed sediments also generally had smaller FeO-P contents (8–12 mg kg1) than the suspended sediments during the storms (Figs. 2 and 3).

Discussion Storm response in P forms down the river system The response of stream water dissolved and particulate loads during storms represents a combination of the speed of transport pathways and the number and location of the sources (within the channel and across the catchment). The dominance of clockwise hysteresis trajectories and b values <1 in the headwater demonstrated the rapid mass transport of particulate P, and to a lesser extent dissolved P, in relation to the cumulative volume of discharge during the events. This suggests that the headwater is a high energy system with sources which act quickly in response to rainfall. Rapidly mobilized sources of particulates include the resuspension of stream sediments, the erosion of stream banks and near-channel field soils. Transport pathways for

August storm

1 0.8 0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8

1

October storm 1

1 0.9 0.8 0.7 0.6 0.6

0.7

March storm 1 1

0.8

0.6

0.4 0.4

0.6

0.8

0.9

1

October storm 2 1 0.8 0.6 0.4 0.2 0

0

0.2

0.8

1

0.4

0.6

0.8

1

Cumulative discharge

Cumulative discharge

Cumulative pollutant mass

Cumulative pollutant mass

Cumulative discharge

Cumulative pollutant mass

M.I. Stutter et al. Cumulative pollutant mass

Cumulative pollutant mass

210

March storm 2 1 0.8

SPM PO 4 -P DOP PP FeO - P

0.6 0.4 0.2 0 0

0.2

Cumulative discharge

0.4

0.6

0.8

1

Cumulative discharge

August storm 1 0.8 0.6 0.4 0.2 0

0

0.2

0.4

0.6

0.8

1

Cumulative pollutant mass

Cumulative pollutant mass

Figure 4 Relationships between cumulative mass transport and cumulative discharge volume for sediment and dissolved P forms during the storm events in the headwater (Millhead Burn). The dashed line shows the 1:1 relationship.

October storm 1

1 0.8 0.6 0.4 0.2 0 0

0.2

October storm 2

1 0.8 0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.6

0.8

1

0.8

1

March storm 2

1 0.8 0.6 0.4 0.2 0

0

0.2

0.4

0.6

0.8

1

Cumulative discharge

Cumulative discharge SPM

0.4

Cumulative discharge Cumulative pollutant mass

Cumulative pollutant mass

Cumulative discharge

PO 4 -P

DOP

PP

FeO-P

Figure 5 Relationships between cumulative mass transport and cumulative discharge volume for sediment and dissolved P forms during the storm events in the downstream catchment (Tarland Burn). The dashed line shows the 1:1 relationship.

material eroded from the adjacent agricultural land include overland flow and delivery via field drains. However, data from Stutter et al., 2008 suggest that three field drains within this headwater made a small contribution of 3% SPM and 4% PP to the annual stream load. Well connected and fast responding pathways of dissolved P in the headwater include shallow surface flow through soils, deeper flow through field drains and desorp-

tion from mobilized particles. Storm responses in the mobilization of PO4-P and DOP were fastest during summer to autumn storms in the headwater (b values <1). This suggests that soil water flow after periods of hydrological inactivity mobilizes a flush of solutes, released by biogeochemical processes during summer. However differences in the response of PO4-P, DOP, and to a greater extent NO3-N, suggest differences in sources and pathways. Soil

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water Table 5 Chemical properties of bed sediments (<250 lm fraction) from the two sites sampled on 3-Aug-04 Properties % particles <250 lm e.s.d % organic C C:N Alox (mg kg1)b Feox (mg kg1) EPC0 (lg l1)c FeO-P (mg kg1)d

a

Millhead burn

Tarland burn

81.4 1.2 10 1822 8635 7.2 12.3

21.9 3.0 12 1009 5512 3.1 7.9

a From particle size analysis of the <2 mm fraction (e.s.d, Equivalent spherical diameter). b Acid ammonium oxalate extractable metals. c Equilibrium P concentration. d Iron oxide test P.

drying-rewetting cycles are known to cause DOP release (Turner and Haygarth, 2001). This may explain similar concentrations and fluxes of DOP to PO4-P during storms after dry periods, followed by declining maximum DOP concentrations through autumn storms. Lags in the declines of DOP relative to PO4-P concentrations in the headwater indicate that DOP, but not PO4-P leaching is maintained as soil water flow switches from surface to deeper pathways during storm recessions. Additionally, PO4-P mobilization response may reflect a rapid desorption from mobilized sediments, as suggested by smaller b values for PO4-P than DOP in August (at a time of extreme SPM hysteresis). Bowes et al. (2005) and Jordan et al. (2005) observed that as near-channel P sources became depleted during consecutive storm events, clockwise hysteresis of P forms weakened and tended toward anticlockwise hysteresis. However, clockwise hysteresis and b values <1 for particulate and dissolved P in the headwater of the current study were similar from October storm 1 and 2 whilst maximum instantaneous fluxes were nearly doubled. Hence, near surface soils seem to provide a reservoir of dissolved P, accumulated during the summer. However, as the catchment wets up the initial flush of DOP becomes depleted and PO4-P increasingly dominates. This has also been observed by Haygarth et al. (2005). In the present study, the contrasting storm behavior of NO3-N, with b values close to unity, was characteristic of slower flushing of deeper soil waters. The initial decrease in NO3-N on the rising hydrograph indicated dilution by surface soil and overland flow. However, a general increase in NO3-N concentrations from summer to winter showed either that NO3-N concentrations in soiland groundwater increased after the summer period of biological uptake, or that soil waters contributing to storm flow became more mixed as the catchment wet up. In summary, the supply of sediment associated P by erosion of headwater field slope soils may be readily controlled through field margin buffer zones. However, the runoff of dissolved P through the upper soil layers may only be managed by longer-term reductions in soil fertility, but such management may best limit storm pulses of dissolved P if targeted to riparian soils. Data from an accompanying study (Stutter et al., 2008) observed lags over seasonal timescales in the transport of

211

sediments and associated P from the headwater downstream. This indicated retention of material in the downstream catchment over summer to autumn, then flushing over winter to spring. Data from the present study suggests that such lags also occur over shorter intra-storm timescales. The Tarland Burn catchment shows a more rapid mobilization of sediments and associated P (with lower b values and strengthening clockwise hysteresis) during October storm 2 relative to storm 1. This may, in part, reflect the contribution during storm 2 of material eroded from the headwater in the previous event and the timescale of transport down the system. Additionally, there may be a wetting up of the downstream catchment during the first October storm, with subsequently more rapid transport occurring in response to the second storm. Haygarth et al. (2005) characterized such behavior of connected agricultural catchments as a ‘piston effect’ of diffuse pollution transport. These authors observed that this effect was especially pronounced at: (i) at smaller scales, (ii) during times of maximum hydrological energy, and (iii) for sediment associated rather than dissolved forms. Hence, sediment associated pollutants delivered to the downstream catchment during a storm may not have originated from the headwaters during that event but may represent a mixture of material stored in the channel system from previous erosion events. Therefore, management of soil erosion in the headwaters during particularly sensitive times of year (e.g. cultivation times) would be necessary for protection of downstream ecosystems over a number of successive storm and intra-storm periods. The timescales of transport down the catchment depend on seasonal characteristics of event frequencies and magnitudes. However, there is an increasing residence time of sediment associated pollutants with increasing catchment scale, suggesting that even P forms of smaller bioavailability will have an increasing eutrophication potential downstream (Hilton et al., 2006). Bowes et al. (2005) previously observed anticlockwise hysteresis of P forms in an upland headwater (rough grazing land use) with clockwise hysteresis in an agricultural catchment downstream. Conversely, the two catchments in the present study, in this case with similar agricultural land use, showed weakening clockwise hysteresis and less extreme b values downstream. This is attributed to a number of factors including changes in topography, decreasing erosion energy downstream and lags in transport as a result of the headwater being a source of sediments to the downstream catchment. In addition, Stutter et al., 2008 suggested that certain nutrient pathways damped hydrochemical changes in the downstream catchment relative to the headwater over seasonal timescales and these are also likely to affect storm responses. Such damping may result from the point source input of the small sewage treatment works in the downstream catchment and greater nutrient leaching and stream-groundwater interactions promoted by the deeply incised stream channel through the alluvial soil floodplain. Groundwater has previously been associated with PO4-P removal in catchments (Jarvie et al., 2005), but this was related to CaCO3-P co-precipitation in chalk catchments. In the present study the slow storm response of dissolved P in the downstream catchment seems characteristic of the flushing of a large reservoir of groundwater sustaining P leaching through the year.

212 Evidence of this mechanism is that comparison of the Tarland site relative to the headwater shows: (i) pre-storm PO4-P concentrations were greater (Table 2), (ii) storm hysteresis in DOP and PO4-P was weaker with a greater degree of concentration tailing, and (iii) b values for DOP and PO4-P remained more constant. An increase in the ‘first flush’ revealed by the b relationships for SPM and PP occurred between March events 1 and 2 in the headwater. Earlier in March soil erosion was likely to have been mediated by the frozen ground, whilst the subsequent snow melt event (March storm 2) would have increased the potential for surface runoff and soil erosion. Slow release of DOP may have indicated soil organic P solubilization by freeze-melt mechanisms (Turner and Haygarth, 2001). However, the magnitude of DOP release in the headwater during March storm 2 was likely to have resulted from manure following application to the arable fields during winter. The additional loss of particulate organic P was indicated by increased PP: FeO-P ratios (Table 6) and large concentrations of PP relative to SPM (Table 2). Strongly tailing PP concentrations downstream during March storm 2 indicated the prolonged effect of continued snow melt on particulate organic P loss from field slopes.

The nature of sediment associated P and its potential for P exchange in the water column The PP fraction describes the sediment P load but provides a poor indicator of P bioavailability compared to the fraction desorbable to a standardized FeO sink (Chardon et al., 1996). Differences in the bioavailable P content of stream sediments is complex and varies with mineralogical controls on particle reactivity, the source of materials from fertilized topsoil or subsoil (McDowell and Wilcock, 2004), selective processes of erosion and transport and seasonal patterns in biological cycling (Steegen et al., 2001). Larger storm FeOP concentrations and fluxes in the headwater (Tables 2 and 3) suggest either that: (i) sediments from the headwater catchment have greater inorganic P complexation capacities than those originating from the downstream catchment, or (ii) a number of physical and biogeochemical processes alter the sediment during transport downstream. Soils in the headwater (Tarves Association; MISR, 1957) from mixed acid and basic parent materials had greater clay contents and finer textures than the coarser, sand dominated fluvio–glacial materials in the downstream catchment (Boyndie and Corby Associations and alluvium). Stream sediments in the downstream catchment comprise a mixture of locally-derived coarser sands and material transported from the more erodible headwater soils. At the times of peak instantaneous fluxes of PP and FeO-P the amounts of P associated with the SPM (mg kg1 basis) were at a minimum. Hence, although the first flush carried the greatest mass of P enriched sediments it simultaneously mobilized material of smaller P content from other sources, such as stream bank subsoil. Increases in FeO-P and PP mass contents during receding flows indicates that P enriched, fine suspended sediment remained in suspension after coarser loads had been deposited. Greater PP mass contents, smaller FeO-P concentrations and greater PP: FeO-P ratios in summer–autumn at the Tarland site than in the headwa-

M.I. Stutter et al. ter suggested that biological processing in the downstream catchment enhanced the organic forms of particulate P. Seasonal and antecedent conditions have been shown to affect the physical and chemical properties of the channel sediments available for resuspension (McDowell and Sharpely, 2002) and the relationship between the storm response of SPM and the PP and FeO-P fractions. McDowell and Wilcock (2004) observed in a dairy catchment (New Zealand) that FeO-P and total P contents of suspended sediments were greater in summer (173 and 2228 mg P kg1, respectively) than in winter (6 and 711 mg P kg1, respectively). The <250 lm fraction of the bed sediment material sampled in August represents the portion most likely to have been mobilized during the August storm. The exchangeable P contents of the bed sediment (indicated by the FeO-P) is related to the surface binding capacity indicated by the oxalate extractable Fe and Al (Feox and Alox) which complex a pool of P thought to be entirely desorbable (Lookman et al., 1995). The much finer texture, greater surface area and greater FeO-P content of the bed sediment in the headwater contributed to the extreme positive b relationships in August and to a lesser degree over the October 1 storm when compared to later discharge events or events downstream. The mixing of particulate material (from channel or terrestrial sources) within the stream can increase the stream load of total P; however the effects on dissolved P concentrations are often contradictory (Koski-Va ¨lha ¨la ¨ and Hartikainen, 2001). The bed sediment EPC0 (Table 5) denotes the stream water PO4-P concentration at which there is no net exchange of PO4-P between the sediment and the water column. The EPC0 value of the headwater bed sediment equaled the 48th percentile stream water concentration of PO4-P (Apr-04 to May-05; data from Stutter et al., 2008). The bed sediment EPC0 value at Tarland was below the minimum stream water concentrations indicating a sink for stream water PO4-P. However, summer anoxic conditions may promote PO4-P release from sediment surfaces into pore waters (House and Denison, 2002). It is possible that there is also an exchange between particulate organic and dissolved organic P pools in the stream which may have contributed to the rapid DOP storm flushes.

Conclusions The results suggest a rapid delivery of sediments during storms in the headwater from arable fields in riparian areas, bank erosion and sediment resuspension. Sediments sourced from the headwater field slopes in this study are enriched in bioavailable P and have a high potential for inorganic P complexation. Further downstream stream sediments differed due to changes in parent materials and a greater degree of biological processing. The magnitude of variations in dissolved P concentrations also became damped downstream with decreasing erosion energy, a point source input and greater leaching contributions from groundwater in the alluvial soils. There appeared to be varying timescales in the transport of material and associated P eroded from the high energy headwater to the downstream catchment over inter- and intra-event timescales related to sediment suspension, deposition and remobilization. Hence, management of headwater field slopes during critical times of

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water

213

Table 6 Suspended particulate matter dry mass contents (mg P kg1) of persulphate digest P (PP) and iron oxide paper test extractable P (FeO-P) Storm

(i) Millhead burn

(ii) Tarland burn

PP content

FeO-P content

Ratio PP: FeO-P

PP content

FeO-P content

Ratio PP: FeO-P

Aug-04 Oct-04 (1) Oct-04 (2) Mar-05 (1) Mar-05 (2)

1100 1313 1819 1339 1524

76 182 107 63 46

14 7 17 21 33

2482 2280 1975 – 382

54 69 81 – 26

46 33 24 – 15

Mass contents of P are determined at the time of maximum sediment transport for each storm.

erosion susceptibility can have prolonged benefits to sediment mobilization and P availability downstream. Management for improvement of riparian areas to mediate erosion transfer of sediments and P should be targeted in steeper gradient headwater reaches. However, the success of such measures is, in part, related to the size of the channel reserves of fine particulates which may be remobilized. The most consequential impacts on water quality may arise during summer periods of greatest biological sensitivity where isolated storm events mobilize large amounts of reactive fine material previously deposited during dry periods.

Acknowledgements This work was funded by the Scottish Executive Environment and Rural Affairs Department. We would like to acknowledge C. Taylor, L. Clark, L. Johnston, Y. Cook and H. Watson for field work and laboratory assistance.

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