Evaluating diffuse and point phosphorus contributions to river ...

Evaluating diffuse and point phosphorus contributions to river

transfers at different scales in the Taw catchment, Devon, UK

F.L. Wooda,b,1, A.L. Heathwaiteb,*, P.M. Haygartha

aInstitute of Grassland and Environment Research, North Wyke Research Station, Okehampton, EX20 2SB, UKbCentre for Sustainable Water Management, The Lancaster Environment Centre, Lancaster University, Lancaster LA1 4YQ, UK

Received 30 November 2003; revised 1 May 2004; accepted 1 July 2004

Abstract

This paper describes an empirical study carried out to improve understanding of how small scale transfers of mobilised

phosphorus (P) recorded at plot to field scale are mirrored by P fluxes measured at the river catchment scale. Phosphorus transfers

were studied in partially nested sites that range in area from 30 m2 to 834 km2. The work was carried out within the grassland

dominated catchment of the River Taw, south west England. The research sought to determine the characteristics of the link

between plot and river scale P fluxes for different P forms. The highest point source inputs of P were generally recorded in the

largest sub-catchments of the river. However, diffuse sources of P were estimated to constitute at least 60% of the annual P flux at

all sites. The magnitude of the diffuse P input varied with catchment land use. Detailed studies focused on the lowland catchments

showed that diffuse fluxes can be well represented by plot and field scale P transfers. The research produced no evidence for

significant inputs of P from river channel banks. However, at the hillslope scale, additional transfers of P from ‘incidental’ sources

such as re-seeded fields, open access ditches, and farmyard point sources were recorded. These inputs did not appear to

significantly affect river P fluxes, suggesting that they are either of low frequency, or that there is a corresponding decrease in

overall P transfer as the research scale is increased from field through to sub-catchment. Additional evidence of scaling was

observed where riverine P fluxes were diluted by upland or groundwater flows.

q 2005 Elsevier B.V. All rights reserved.

Keywords: Phosphorus; Diffuse pollution; Agriculture; Grassland; Scaling; Eutrophication

1. Introduction

The policies and implementation strategies designed to protect European waterbodies from P pollution are under

rapid development (e.g. Environment Agency, 2000a). To focus these policies, the processes that initiate the

Journal of Hydrology 304 (2005) 118–138

www.elsevier.com/locate/jhydrol

0022-1694/$ - see front matter q 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jhydrol.2004.07.026

Abbreviations: P, Phosphorus; TP, Total phosphorus; RP, Molybdate reactive phosphorus; UP, Phosphorus not determined by molybdatereaction (unreactive); !0.45, Less than 0.45 mm in size; STW, Sewage Treatment Works; FYM, Farm yard manure; HOST, Hydrology of SoilTypes; FAO, Food and Agriculture Organisation; USDA, United States Department of Agriculture; l.o.d., Limit of detection.

* Corresponding author. Address: Centre for Sustainable Water Management, The Lancaster Environment Centre, Lancaster University,Lancaster LAI 4YQ UK. Tel.: C44 1524 510204; fax: C44 1524 5101217.

E-mail address: [email protected] (A.L. Heathwaite).1 Present address: Institute of Grassland and Environment Research, Plas Gogerddan, Aberystwyth, SY23 3EB, UK.

http://www.elsevier.com/locate/jhydrol

F.L. Wood et al. / Journal of Hydrology 304 (2005) 118–138 119

mobilisation and drive the transfer of P from diffuse

sources to receiving waters have been investigated

extensively over the last ten years (e.g. Boesch et al.,

2001; Haygarth and Jarvis, 1999; Heathwaite and Dils,

2000; McDowell and Sharpley, 2001). Haygarth and

Condron (2004) argue that separating initial P release

(mobilisation) from subsequent P transfer (transport)

aids our understanding of the main drivers of diffuse P

losses because these two mechanisms operate at

different scales: the soil profile and plot scale,

respectively. Phosphorus mobilisation describes the

initial separation of P molecules from their source

through either solubilisation or detachment mechan-

isms,dependingonthephysical sizeof thePcompounds

that are mobilised. There is evidence to suggest that the

potential for P solubilisation increases with increased

soil P status (e.g. Heckrath et al., 1995; McDowell and

Sharpley, 2001) and that the degree of P saturation may

be used to predict the risk of P transfer by surface or

subsurface hydrological flow paths. Detachment of soil

particles and associated P is often linked to soil erosion,

which provides a physical mechanism for mobilising P

from soil into waters (Kronvang, 1990; Sharpley and

Smith, 1995).

Research on P mobilisation and transfer has resulted

in far better understanding of the processes of P

transport at the small plot to field scale. However,

better understanding of the scaling of diffuse P

transport from plot to catchment scale is still required.

This scaling is important because most of the research

examining diffuse P sources has been conducted in

intensively managed agro-ecosystems using exper-

imental plots under controlled conditions. Thus,

although there is evidence that continued inputs of P

in the form of mineral fertilisers and imported animal

feed have resulted in significant accumulation of P in

topsoil (e.g. Haygarth et al., 1998; Withers et al., 2001;

Sibbesen and Runge-Metzger, 1995; Sims et al., 2000)

there remains poorer understanding of the routing of P

through the landscape and of its contribution to P fluxes

in receiving waters. There is some evidence from

modelling (e.g. McGuckin et al., 1999; Viney et al.,

2000; Wade et al., 2002) and field studies (e.g. Vighi

and Chiaudani, 1985; Hunsaker and Levine, 1995;

Smith et al., 1995; Heathwaite et al., 1996; Hooda et

al., 1997) that diffuse P transfer significantly affect

river P fluxes. However, the routing of P from the point

of mobilisation to the point of delivery to water is

complex. It embraces: (i) differences in the relative

importance of the controls on P transfer at different

scales from point to plot, field, hillslope and catch-

ment, (ii) differences in the relative importance of

these controls over time—e.g. resulting from seaso-

nal variation, crop rotation, and as a consequence of

longer term climate change, and (iii) differences in

the relative contribution from different P fractions

according to the pathway of transfer and the

environmental controls on movement. Further com-

plexity arises because P is subject to deposition and

resuspension/dissolution during transport, and can

change form in space and time. Once transferred P

reaches surface waters, the extent, nature and

dynamics of interactions between soluble and

particulate P in water and sediments play a critical

role in the delivery and impact of P downstream

(Baldwin et al., 2002; McDowell et al., 2001).

This paper describes an empirical study carried out

to examine the transfer of P at a range of scales within

the grassland-dominated Taw river basin in southwest

England. The aim was to evaluate the link between plot

scale P transfers and river P fluxes, and to analyse any

variation in the forms of P at different scales.

2. Materials and methods

2.1. Study sites

The work reported here was undertaken at different

scales of investigation, using a nested approach where

feasible to reduce the number of variables that need to

be investigated in order to explain changes in P

transfer resulting from each incremental increase in

drainage basin area. The research was undertaken in

the 1242 km2 catchment of the River Taw in Devon,

in South West England. The southern headwaters of

the river basin rise over granite uplands c.600 m

above sea level. The lowlands and northern head-

waters flow over sandstones, a narrow east-west band

of breccias and conglomerates, shales and siltstones,

into the Bristol Channel. Most of these rocks are of

low permeability and porosity (Environment Agency,

2000b). Away from the main river valley, the

topography consists of steep sided, but rounded

hills. In the uplands, soils are peat, podzols, brown

earths, and slowly permeable gleys. Clay-rich gley

soils and typical brown earths occur over the central

F.L. Wood et al. / Journal of Hydrology 304 (2005) 118–138120

strata, with a belt of well-drained, gritty reddish loam

over the breccia (Environment Agency, 2000b).

Rainfall ranges from over 2200 mm yrK1 in the

south, and 1600 mm yrK1 in the northern uplands, to

less than 940 mm yrK1 in the lowland areas. Catch-

ment hydrology is dominated by surface water due to

the low groundwater storage capacity. There are no

major aquifers in the catchment, but groundwater,

inferred to be held in fractures in the rock bodies, is

observed to maintain river baseflow in periods of dry

weather. River discharge responds rapidly to rainfall.

Biological water quality in the catchment is generally

good, although the Taw estuary has been identified as

being eutrophic (Environment Agency, 2000b). A

sewage treatment works on the coast is the major

cause of this. (National Rivers Authority, 1995;

Environment Agency, 2000b).

Fifteen sub-catchments within the catchment of the

River Taw were studied (Fig. 1); with 6 of these sites

studied in detail (see Table 1). ‘Plot’ scale is defined

here as 30 m2, field scale lysimeters were of 1 ha size,

‘hillslope’ scale is defined as 1 ha up to 25 ha, and

‘catchment’ scale as larger than 25 ha. The plot, field

and hillslope scale sites were centred on the Rowden

drainage experiment, located in the southern part of

the Taw catchment (see Fig. 1). This facility consists

of fourteen, 1 ha, hydrologically isolated grassland

paddocks on land sloping at 5–10% (Scholefield et al.,

1993). Seven of the paddocks are mole and tile

drained to 85 cm, and this drainage water is routed

from the area separately from near-surface water,

which is constrained by an impermeable clay layer at

30 cm depth (see Armstrong et al., 1984 for a full

description of the facility, Haygarth et al., 1998 for

studies on P transfers). Adjacent to these 1 ha

lysimeters, there are twenty-four undrained 30 m2

plots, hydrologically isolated in a similar manner

(Preedy et al., 2001). The ditch draining a 19.9 ha

hillslope consisting of half of the 1 ha lysimeters and

part of an adjoining field (undrained) was also

monitored.

Soils are mainly of the Hallsworth series (FAO

dystric gleysol; USDA Typic Haplaquepts), with

small areas of Halstow series (FAO gleyic cambisol;

USDA Dystrochrepts). Both are slowly permeable,

clayey soils derived from shale dominated rock. The

subsoil lies at c. 30 cm depth, and is coarsely

structured and impermeable. The soils behave in a

similar way hydrologically, and have a hydrology of

soil types (HOST) drainage classification of 24

(Hallsworth) and 21 (Halstow; Boorman et al.,

1995). These, together, comprise the most common

hydrological soil type in England and Wales, at 14%

of the land area (Boorman et al., 1995; Fraser, 2000).

The characteristic of slowly permeable subsoil

horizons, which severely restrict the downward

movement of water is found in an additional 10% of

soils in England and Wales (Fraser, 2000). Rainfall is

c. 1067 mm per year, measured at the North Wyke

meteorological station, 2 km away.

The 1 ha lysimeters were grazed by steers/heifers

at a stocking density of 2.80 livestock units (l.s.u.) per

hectare from mid April to September, and subject to a

range of N input regimes (inorganic N: none up to

275 kg haK1, farmyard manure: none up to 10 tonnes

haK1). Inorganic P input was 26 kg haK1, applied in

early spring. The additional land contributing to the

monitored hillslope was similarly under a mixture of

low N and conventional management. The six 30 m2

plots used were hand cut, and managed convention-

ally with respect to inputs. Up to summer 1998, 10

tonnes haK1 of FYM were applied, and NPK inputs

were 275, 26, and 50 kg haK1 respectively. From

spring 1999, the plots carried a dairy slurry/inorganic

fertilizer treatment equivalent to 29 kg P haK1.

A further 9 hillslope scale sites were also studied;

their land use and soil type details are given in Table 2.

These sites were selected on the basis of representative

soil types and typical land management practice for the

area. Additional criteria were: simple topography,

single ownership, and the lack of obvious point-source

inputs. All 9 hillslope sites have Hallsworth and/or

Halstow soil series and predominantly grassland land

use that ranged in intensity from unimproved grass-

land, and beef and sheep pastures similar to Rowden, to

more intensively managed dairy farm pastures; an

arable field was included at one site. The area of the

hillslopes ranged from 5 to 18 ha and all were within 10

miles of the Rowden experimental site.

2.2. Sample and data collection

There were three consecutive phases of water

sampling to estimate diffuse P fluxes within

the catchment over a total period of three and a half

years: I (November 1996–March 1998), II (April

Fig. 1. Location of the catchment scale sampling sites and the plot to hillslope scale study areas in the River Taw catchment, Devon, UK.


1998–March 1999), and III (August 1999–July 2000).

Samples were collected manually in rinsed 125 ml

polyethylene sample bottles and stored at 4 8C in the

dark prior to analysis (Haygarth et al., 1995). All

analyses were carried out within 24 h of sample

collection. For each phase, water samples were

collected on the same day from a different subset of

the study sites. In Phase I samples were collected

fortnightly from the 15 catchment scale sites, the

Rowden 30 m2 and 1 ha lysimeters, and the Rowden

hillslope site (Fig. 1; Table 1). The samples were

analysed for TP and RP !0.45. In Phase II, samples

were collected at weekly intervals from the 6 of the 15

catchment scale sites because these sites had Environ-

ment Agency discharge data for P flux calculation

(Table 1); water samples were also taken from

Table 1

Soils, land use and known point source inputs for the 6 sub-catchments of the River Taw studied in Phase II

Catchment Area

(km2)

Soils Land use (expressed

as % of catchment

area)a

Catchment stocking density (head haK1) Known point-

source inputsDairy Beef Sheep Pig

Leeham

Ford

17.6 60% well-drained

podzol, 20% Poorly

drained peaty

podzols, 20% brown

earth

80% grassland, 18%

rough grazing

– 0.14 4.06 – None

Veraby 61.0 40% well-drained

brown earths, 30%

peaty but permeable

podzol, 30% peaty

gley

71% grassland, 21%

rough grazing, 3%

cereals

0.11 0.18 6.67 – Descriptiveb inputs

only-insignificant

Taw Bridge 72.4 Very mixed; well-

drained gritty loams,

slowly permeable

clays, earths, podzols

48% grassland, 29%

rough grazing, 11%

cereals, 2% other

crops/set aside

0.14 0.16 3.15 0.17 Sewage Treatment

Works (STW),

cheese factory

Collard

Bridge

79.6 50:50 well-drained

podzols and well

drained brown earths

86% grassland, 5%

rough grazing, 4%

cereals, 2% other

crops/set aside

0.21 0.17 7.86 0.05 STW, 3 fish farms

Woodleigh 328 60% well-drained

brown earth, 10%

well drained podzol,

10% poorly drained

peaty podzol, 20%

gleys.

76% grassland, 10%

rough grazing, 7%

cereals, 3% other

crops/set aside

0.19 0.14 6.54 0.04 STW, 2 fish farms,

1 wood processing

plant

Umberleigh 834 Mixed, 60% well-

drained brown-earths

and podzols, 30%

gleys.

71% grassland, 8%

rough grazing, 11%

cereals, 4% other

crops/set aside

0.27 0.11 4.93 0.15 STW, 2 cheese fac-

tories, 3 fish farms,

1 wood processing

plant, cider factory

(until Dec. 1998)

a Catchment woodland area ranges from 2 to 4% of total area.b ‘Descriptive’ STW refers to those with an effluent discharge of less than 1 m3 per day.


the Rowden 30 m2 and 1 ha lysimeters, and the

Rowden hillslope site throughout Phase II and at the

same interval as the 6 catchment sites. For the Phase II

samples, P was fully fractionated. For Phase III,

samples were collected at weekly intervals from all

the hillslope scale sites (Fig. 1 and Tables 1 and 2),

and all P forms determined. This phase was

undertaken to confirm that the Rowden hillslope

generated P transfers typical of the hillslope scale.

For Phase I and II, river discharge data was obtained

from the Environment Agency. Discharge at Rowden

was estimated as hydrologically effective rainfall

(HER)—rainfall minus potential evapotranspiration

during the field capacity season. Potential evapotran-

spiration from grassland was estimated on a weekly

basis from open water evaporation data, using

seasonal correction factors (Penman, 1952). The

proportion of HER routed along undrained, drained

near-surface, and drained drainflow lysimeter path-

ways were 70.5, 9.5, and 66.1%, respectively

(Scholefield et al., 1993). For the hillslope scale, all

HER was assumed to exit the catchment via the basin

outlet. For Phase III, discharge at the 9 hillslope scale

sites was determined from stage readings at manually

calibrated timber/plywood weirs, installed to British

Standard 3680 4A with respect to uniformity of

channel approach. Soil samples were collected from

the 9 hillslope scale sites using a stratified grid

sampling approach. Twenty-five samples to 2 cm

depth were collected with a 25 cm2 square section

Table 2

Characteristics of the hillslope scale sites in the River Taw catchment

Site Area

(ha)

Slope

(%)

Soil series Landuse Other

characteristics

Inorganic

NPK input

(kg ha-1)

Other nutrient

inputs

Olsen P

(mg kgK1)a

H1 5.5 7 Hallsworth Unimproved sheep/

beef pasture.

Annual cutting of

rushes.

Elongated along

direction of stream

channel

0-0-0 Calcified seaweed

0.6 tonnes haK1

22.7

H2 4.8 4–9 Halstow Sheep Sampled below

farm track

92-11-10 Intermittent FYM/

slurry application,

depending on

winter ground

conditions

22.9

H3 18.1 6–12 Hallsworth/

Halstow

Sheep/beef Organically

managed

17-10-27 4 tonnes haK1

FYM plus dirty

water

44.3

H4 7.1 4 Hallsworth Beef Cattle access to

ditch

109-6-11 25 tonnes haK1

slurry

39.1

H5 19.9 5–10 Hallsworth Beef Rowden hillslope,

partly compart-

mentalised into

1 ha plots

117-24-45 5 tonnes haK1

FYM

46.0

H6 6.8 6–12 Halstow Dairy/sheep One field reseeded

in late August 1999

344-27-51 36 tonnes haK1

slurry on silage

fields

36.4

H7 12.7 10 Halstow Sheep/silage 222-24-91 32 tonnes haK1

slurry

48.7

H8 8.6 15 Halstow Dairy Dirty water from

farmyard found to

be entering runoff

195-20-54 Intermittent 86.1

H9 7.1 8 Hallsworth Barley/maize, plus

25% ‘set aside’

alongside stream

margin

Sampled from tile

drain

73-18-58 17 tonnes haK1

slurry

34.5

Data protection prevents release of farm locations.a Olsen values are relatively high; soils are acid (pH 4.9–5.9), reducing the reliability of this standard soil P test.


corer, to give a total sample volume of 1250 cm3.

Samples were stored for up to five days at 4 8C in the

dark. Plant matter and stones were removed prior to

passing the samples through a 2 mm sieve. The

samples were then dried at 30 8C for 12 h (equivalent

to air drying).

Estimates were made of the point-source inputs of

P to the 6 catchment scale sites monitored in Phase II.

This was undertaken using data that is publicly

available from the UK Environment Agency on

discharges from sewage treatment works and from

the main industrial discharges to rivers in the

catchment. These data were supplemented with

observations on industrial point sources noted in

Table 1 although effluent volumes are not available.

The most important of these are the fish farms. Other

point sources include small hotel and local council-

maintained sewage treatment works, and private

septic tank systems. There were no known point

sources of P into the smaller study sites (Table 1).

Accurate calculation of point source P loads is not

usually possible because records of P concentrations

and actual discharges are not recorded or are not

publicly available. Here, we calculate P loads from

sewage inputs per unit catchment area as the product

of the total daily permitted volume of effluent for

the STWs in each catchment and a P concentration of

10 mg lK1. Clearly this concentration is the upper


boundary of the typical range of P concentrations in

effluent where there is no tertiary treatment, as in the

Taw (Brett et al., 1997; T. Griffiths, South West

Water, pers. comm.). Smaller treatment works, termed

‘descriptive’, have an effluent discharge of less than

1 m3 per day and were assumed to be discharging at

this maximum rate. Intermittent sewage discharge

data (R. Harwood, Environment Agency, pers.

comm.) indicate that these estimates are largely

accurate. The main industrial source (a cheese

factory) discharges at a relatively constant daily

rate, discharge and P concentration figures available

for 1998–1999 were used to calculate mean daily flux,

and thus the annual P input per ha. For the purposes of

comparing diffuse-source inputs with total catchment-

scale P export, estimates of point-source inputs were

subtracted from measured TP exports to give an

estimate of the diffuse export. Septic tanks are the

most commonly used method of disposing of human

waste in rural parts of Britain but P loads from this

source were not accounted for separately in our study

because no accurate means of accounting for their

contribution was available.

2.3. Analytical methods

(i) Water samples were shaken, and 50 ml vacuum

filtered (at less than 60 cm Hg, or 80 kPa) through

0.45 mm pore size cellulose nitrate filter papers (de

Haan et al., 1984). The Murphy and Riley (1962)

colorimetric technique was used to determine the

concentration of P in all the samples. The concen-

tration of TP was determined for both unfiltered and

filtered samples following an acid persulphate digest

(Rowland and Haygarth, 1997) and samples were read

at 880 nm using a manual Unicam Vis 2 spectro-

photometer. Reactive P was determined on filtered

and unfiltered samples by flow injection methods

(Karlberg and Pacey, 1989) and samples were read at

690 nm using a Tecator 5020 flow injection analyser

with autosampler. The concentration of UP in the

samples was determined by difference (TP-RP).

‘Blind’ analytical quality controls were determined

with each sample run as part of an internal QC scheme

and also as part of the external ‘Aquacheck’ Scheme

and only batches which fell within the tolerance

boundaries were accepted. Limits of detection (l.o.d.)

for the P analyses were determined every four months.

The l.o.d. was calculated as the mean blank value plus

three standard deviations of the mean (Analytical

Methods Committee, 1987). The l.o.d. calculated

from the combined datasets over the period of

sampling were 14.8 mg lK1 for TP, and 4.2 mg lK1

for RP !0.45. Concentrations of derived P forms

were only calculated where the relevant raw data were

above the l.o.d.

(ii) Soil samples were analysed for plant-available

P using Olsen soil P (Olsen et al., 1954) determined on

triplicate sub-samples of bulked hillslope site

samples. The colorimetric reagents were: (a) 6.0G0.001 g ammonium molybdate and 0.15G0.001 g

antimony potassium tartrate dissolved in 300 ml of

deionised water, to which was slowly added 74 ml of

conc. H2SO4, and the solution diluted to 500 ml. For

the working reagent, 200 ml of this was diluted to

1600 ml with deionised water (b) 7.5G0.001 g

ascorbic acid made up to 500 ml. To analyse for P,

1 ml of sample was pipetted into a test tube and 200 ml

of 1.5 M H2SO4 slowly added and mixed. Four ml of

working ammonium molybdate reagent and 1 ml of

ascorbic acid were then added. The solution was

agitated thoroughly and left to stand for 30 min to

allow colour development. Absorption was read at

880 nm on a Unicam Vis 2 spectrophotometer.

2.4. Statistical analyses

Daily mean values of the P concentration at each

scale of investigation were calculated as the arith-

metic mean of the concentrations recorded at each

replicate site. Linear regression analysis was then

used to compare the log-transformed daily mean

concentrations recorded at different scales. The

regression analysis required the P data to be log-

transformed as the time series data were not normally

distributed. Regression parameters were confirmed by

Bootstrap re-sampling (Efron and Tibshirani, 1993).

Two methods were used to calculate the mean annual

P concentration at a site, and thence the annual P flux

by multiplying by total annual discharge. First, for use

in comparisons between scales, the mean concen-

trations of the log-transformed P data over the time

series were calculated for each replicate site at a given

scale. Here, the values after re-transformation are

refered to as the antilog means. Second, where the

estimation of the diffuse contribution to river


catchments was made, it was possible to use flow

weighted mean concentrations, as instantaneous

discharge data were available at this scale. This

method gives a more accurate estimate of P flux in

catchments where P transfer varies with discharge

(Yaksich and Verhoff, 1983). Statistical comparison

of mean annual TP concentrations and fluxes was

performed using one way Analysis of Variance.

3. Results

The results are shown in Figs. 2–7 and Tables 3–6

and are separated here into a consideration of ‘point’

and ‘diffuse’ sources at different scales within the Taw

catchment.

3.1. Point source inputs

Large increases in RP !0.45 concentrations

occurred in some rivers when discharge was low.

However, as these high concentrations were most

pronounced when there is little or no runoff from the

land, they were taken as an indication of point-source

dominance. The summer: winter ratio of RP !0.45

concentration in the April 1997–March 1998 dataset

was used to rank the sites by a ‘point-source influence

index’. The mean seasonal concentrations of RP

!0.45 were calculated from log-transformed data

Fig. 2. Time series of total phosphorus concentration for sub-catchments of

for the period 1996–1999.

because of the log-normal frequency distribution of

the time series data. Forty-year mean monthly rainfall

data from the North Wyke meteorological station was

used to define the summer and winter periods, April–

September being designated as summer. The ratio of

summer: winter RP !0.45 concentrations for each of

the catchment scale sites is presented in Table 3. The

sites with an index greater than the median value for

the group of catchments are indicated.

Estimated point source inputs and annual catch-

ment scale TP transfers were calculated for the 1998–

1999 year (Table 4). Point source inputs were inferred

to constitute c.16% of the TP transfer for the

Umberleigh catchment, which is the largest catchment

monitored. In the Taw Bridge catchment this

contribution reached 38%. The P flux for the Taw

Bridge catchment was enriched in RP !0.45, and to a

lesser extent the other P forms, relative to the

grassland dominated sites. Transfers from Umber-

leigh were enriched in both RP O0.45 and UP O0.45.

3.2. Diffuse phosphorus transfers

Investigations on diffuse P transfers were carried

out in the sites dominated by diffuse sources, i.e. the

plot, field, and hillslope scale sites, and for the

catchment scale sites identified in Table 3. Of the 15

catchments initially monitored in Phase I, 8 catch-

ments that appeared to be principally affected by

the River Taw dominated by P inputs derived from diffuse sources,

Fig. 3. Time series of: (a) total P, and (b) reactive P!0.45 at scales ranging from 30 m2 to catchment scale for the period 1996–1999. Error bars

represent one standard error.


diffuse inputs of P (mainly the smaller ones) were

selected for the further analysis of diffuse P transfers.

For Phase II, 4 of the 6 sites monitored were from this

group.

Within these 8 sites, TP concentrations in the

three catchments containing large proportions of

unimproved land (Belstone, Leeham Ford, and

Brayford) were low (Fig. 2). Similarly, in Phase II,

TP concentrations and annual flux in the upland

Leeham Ford catchment were low compared to the

three lowland sites (Fig. 2; Table 4), although the

proportions of different P forms exported did not

change with increased intensity of management

(Fig. 6). These less intensive upland catchments

were therefore also excluded from the study of

diffuse P transfer because the land use was different

from the smaller scale study sites. This left 5

catchment scale sites in Phase I and three in Phase

II. The selection of these catchments affected the

nested relationship with the plot and hillslope scale

sites in Phase II.

Time series data for the 4 scales of study (30 m2

plots, 1 ha field lysimeters, hillslope, and catchment)

are presented in Fig. 3 (TP and RP !0.45 from Phase

I and Phase II) and Fig. 4 (all the P forms determined

in Phase II). The 1 ha dataset is represented by the

mean of the daily undrained lysimeter and drained

lysimeter drainflow pathways-these being the domi-

nant flow pathways in the two drainage treatments.

Gaps in the record indicate periods without drainage

or interruptions in sampling (April 1999). Discharge

at Umberleigh is presented for reference. At all sites,

field observation of river discharge at the time of

sampling suggested that where samples were taken

during periods of elevated discharge, high P concen-

trations were recorded. This relationship is not,

however, strongly apparent in the time series of P

concentration and daily rainfall, or discharge. Weak

relationships between TP concentration and discharge

were identified at some sites when regression analysis

was applied to data from a 1 ha lysimeter and the

Phase II rivers (maximum r2 of 0.35).

Fig. 5. The percent annual export of total P derived from different P

forms at scales ranging from 30 m2 up to catchment scale for sub-

catchments representing diffuse P sources and dominated by

lowland grassland land use. The results are calculated from antilog

mean concentrations and cover the period April 1998–March 1999.

3

Fig. 6. The percent annual total P export derived from different P

forms at catchment scale for the period April 1998–March 1999.


In general, there is a strong similarity in the

temporal patterns of concentration of all forms of P at

all scales. This was confirmed for TP by linear

regression analysis (Table 5). Regression of the P

concentrations recorded at the hillslope scale on those

at the field scale yielded a significant trend. At the 5%

level, the value of the slope parameter was indis-

tinguishable from ‘one’, and intercept indistinguish-

able from ‘zero’. The regression of catchment scale

concentrations on field scale was also significant. The

value of the slope parameter here was less than ‘one’.

The intercept parameter was not zero. At the hillslope

scale, the regression accounted for 57% of the

variance in the observed log transformed P concen-

trations, and at the catchment scale, 40%. The

concentration of UP O0.45 closely follows that of

TP (Fig. 4b), RP !0.45 and UP !0.45 show the least

temporal variation.

One way analysis of variance on the 1998–1999

data-set showed that there were no significant

differences at the 5% level in annual TP export

from the 30 m2 plots, 1 ha undrained lysimeters,

and lumped hillslope-catchment scale sites. Exports

from the 1 ha drained lysimeters were lower. By

contrast, the mean P concentrations recorded at the

hillslope-catchment scale were similar to those of

Fig. 4. Time series of different P fractions at scales ranging from 30 m2 to

RPO0.45 (d) UP!0.45 (e) RP!0.45 (note the difference in axes for the

the 1 ha drained lysimeters, whilst those from

30 m2 plots were similar to the 1 ha undrained

lysimeters. The dominant P form in terms of both

concentration and load (Tables 6 and 7; Fig. 5) was

UP O0.45 at the plot, field, and hillslope scales.

Although the TP O0.45 transfer appears to fall

below 50% of TP at the catchment scale when

estimated from antilog means, the proportions

calculated from the flow weighted mean concen-

trations are: UP O0.45 49%; RP O0.45 14%; UP

!0.45 11% and RP !0.45 26%.

During Phase III of the study, when the Rowden

hillslope appeared to generate P transfers typical of

the three other grassland hillslopes in which there

was minimal channel disturbance or point sources

(Fig. 7a). By contrast, P transfers from of the

remaining hillslopes studied in Phase III (Fig. 7b)

often recorded elevated P export.

4. Discussion

4.1. Sampling interval

For the Taw catchment, the concentration of P

varies with discharge. For reliable estimates of export

coefficients and annual mean concentrations to be

made, a sampling frequency greater than the 14–21

day interval regime sufficient for stable response

catchment scale for the period 1998–1999 (a) TP (b) UPO0.45 (c)

P fractions). Error bars represent one standard error.

Fig. 7. Time series of total P flux from hillslope scale sites (a) Rowden hillslope and 3 sites not influenced by point-sources or tillage (b) Rowden

hillslope and 5 sites influenced by disturbance, tillage or point-source inputs.


systems (Treunert et al., 1974) is preferable. In the

discussions that follow, the Phase II (1998–1999)

dataset used a weekly time step. Errors (expressed as

the 90% confidence interval of the mean measured

value) were found to be similar at both 1 ha and

834 km2 scale, over a six month period. Weekly

sampling was found to give rise to errors in antilog TP

exports of up to 20% when compared to daily

sampling, and errors of K46% to C121% when

compared to the flux calculated from flow weighted

mean concentrations. Estimates of P exports using

flow weighted mean concentrations were subject to an

uncertainty of K71% to C56% when calculated from

weekly rather than daily data.

Table 3

Summer to Winter ratios of P concentration used to determine point source influences for the catchment scale sites in the River Taw catchment

Site Area (km2) Mean RP !0.45

concentrationa,

summer

Mean RP !0.45

concentrationa,

winter

Point source index

(summer:winter)

Index higher than

median value

Belstone 11.1 2.36 5.01 0.47

Leeham Ford 17.6 6.92 6.18 1.12

Bow 34.7 51.26 46.93 1.09

Brayford 40.2 5.27 8.37 0.63

Veraby 61.0 11.25 11.61 0.97

Taw Bridge 72.4 396.25 178.92 2.21 †

Collard Bridge 77.4 35.15 33.89 1.04

Chumleigh 125 42.42 28.79 1.47

Lapford 127 222.60 86.90 2.56 †

George Nym 209 77.66 38.75 2.00 †

Woodleigh 328 48.17 36.49 1.32

Bridge Reeve 435 215.34 99.68 2.16 †

Kings Ford 802 95.12 56.03 1.70 †

Umberleigh 834 84.83 54.20 1.57 †

New bridge 914 81.89 53.63 1.53 †

Median value 1.47

a Mean concentration values derived from log-transformed data.


4.2. Point sources

In this study, high RP concentrations recorded at

times of low river discharge were taken as an

indication of point-source influence. These high

concentrations could alternatively arise if at these

times there were inputs of solute-rich, long residence

time throughflow to the river, subject to only low rates

of dilution (e.g. Hooda et al., 1997). There was

however little evidence of such pathways and inputs

in the Taw catchment, owing to its low storage

capacity. It was therefore concluded that nearly all the

catchment scale study sites in the Taw catchment

showed some evidence of point source influence, with

Table 4

Point source P inputs, measured total P export and estimated diffuse P expo

1998–March 1999)

Catchment Area km2 Measured expor

(kg haK1 yrK1)

Leeham Ford 17.6 0.62

Veraby 61.0 0.94

Taw Bridge 72.4 4.25

Collard Bridge 79.6 1.30

Woodleigh 328 1.37

Umberleigh 834 2.12

the greatest influence found in the largest catchments.

There are no recorded algal problems associated with

these intermittently high P concentrations, even in

summer when there is a high potential for biological

activity.

It is rarely possible to calculate the actual point

source P load to receiving waters from STWs

because accurate records of P concentrations and

effluent discharge are not normally recorded or are

unavailable. Bennion et al. (2002) calculated the

point source P load from STWs by taking the total

human population within a catchment and applying a

per capita P export coefficient to the resident popula-

tion. In this study, we assumed a P concentration

rt from the catchment scale sites in the River Taw catchment (April

t Point-source input

(kg haK1 yrK1)

Diffuse export

(kg haK1 yrK1)

0.00 0.62

0.00 0.94

1.63 2.62

0.08 1.22

0.36 1.01

0.34 1.78

Table 5

Regression analysis of hillslope total P concentrations on field scale

concentrations, and catchment scale total P concentrations on field

scale concentrations

Hillslope on

field scale

Catchment on

field scale

n 52 54

Fprobability !0.001 !0.001

Adjusted r2 0.57 0.40

Slope 0.94 0.55

Intercept 0.340 1.749

tprobability that slope is 0 !0.001 !0.001

tprobability that slope is 1 0.299 !0.001

Bootstrapped 95%

confidence intervals:

Slope 0.783, 1.082 0.403, 0.754

Intercept K0.364, 0.982 0.963, 2.432

All concentration data are log transformed.


from STWs of 10 mg lK1 and used the total

permitted daily volume of effluent for the STW in

each catchment to get an estimate of the P load from

this source. Both approaches whilst providing an

estimate of the point source P load from the human

population of a catchment, have a number of

potential sources of error. Using permitted volumes

leads to an over-estimation of flux, but omits the

contribution from septic tanks; using export coeffi-

cients: (i) the method may not distinguish between

people connected to mains sewerage systems and

those served by septic tanks; (ii) no account is taken

of differences in the degree of P removal (secondary,

tertiary) from one STW to another, and (iii) no

allowance can be made for the transfer of sewage

from one catchment to another for treatment.

Furthermore, if export coefficients are used, the

accuracy of the P load estimate is dependent on the

value selected and there is no universally agreed

figure. Johnes (1996), for example, used a coefficient

of 0.38 kg P capitaK1 yK1 following secondary

treatment, whilst Carvalho et al. (2003) quote values

ranging from 0.14 kg P capitaK1 yrK1 to 1.55 kg P

capitaK1 yrK1.

In most areas of rural Britain, septic tank systems

are the main method of disposal of human waste. The

effluent from these tanks usually drains to a soakaway

where, it has always been assumed, the nutrients that

it contains are dissipated into the soil (Harper, 1992).

Recent work by Carvalho et al. (2003) suggests that P

loads from septic tank systems may be higher than

expected, primarily due to lack of maintenance. For

example, a recent survey of 24 septic tanks within the

Lough Leane catchment, in Ireland (LLCMMS,

2000), suggests that discharges from septic tanks

may account for 10–20% of the P load to some lakes

from external sources. Other researchers have

attempted to derive coefficients for the export of P

from septic tanks to surface waters; these range from

0.33 kg P capitaK1 yr1 for septic tanks within the

catchment of Loch Leven, Scotland (Frost, 1996),

0.4 kg RP capitaK1 yrK1 for septic tanks within the

catchment of the River Main, Ireland (Foy and

Lennox, 2000) and 0.26 kg RP capitaK1 yrK1 in the

Lough Leane catchment. As this methodology devel-

ops, it will be possible to refine estimates of point

source contributions.

Other point sources of increasing importance in

catchments such as the Taw include fish farms.

Although effluent P concentrations are usually low

(Dumas et al., 1998; Jokela et al., 2001), fisheries can

have a significant effect on P concentrations in

receiving waters, and on local benthic invertebrates

(Kendra, 1991).

4.3. Diffuse phosphorus transfers at the catchment

scale

Diffuse P inputs were estimated to have accounted

for at least 60% of the annual P flux of the River Taw;

rising to 84% in the largest catchment monitored, and

virtually all the flux in the Leeham Ford and Veraby

catchments. In light of the uncertainties discussed

above, the error associated with these estimates may

be of the order of a 20% underestimation of the point

source contribution to measured P flux from a

catchment, if septic tank discharges were of the

order of those reported in LLCMMS (2000).

Atmospheric P sources are generally small with

typical concentrations of P in precipitation of

c.10 mg lK1 (Jordan and Smith, 1985). However, dry

deposition rates of up to 0.97 kg P haK1 yrK1 have

been recorded in some agricultural areas (review by

Ryding and Rast, 1989). In the Taw catchment, a

positive relationship between soil type/land use and P

flux was found. This relationship has been observed

previously at scales up to two or three km2 e.g. Hooda

Table 6

Mean and range of P concentrations (mg lK1) from 30 m2 to catchment scale for diffuse-source dominated sites (1997–1999)

Scale April 1997–March 1998 April 1998–March 1999

TP RP!0.45 mm TP PO0.45 mm RPO0.45 mm UPO0.45 mm RP!0.45 mm UP!0.45 mm

30 m2 Mean 93 24 133 83 15 68 23 20

95% confidence

interval

(72–115) (16–32) (94–171) (58–109) (9–20) (48–87) (12–33) (13–28)

Range 23–451 7–84 51–363 35–224 0–63 29–202 l.o.d.–149 0–35

1 ha

Undrained Mean 96 37 123 68 22 41 16 29

95% confidence

interval

(52–139) (19–52) (86–160) (45–91) (13–30) (25–57) (12–19) (16–43)

Range 6–493 l.o.d.–188 16–857 0–822 0–216 2–606 l.o.d.–155 0–192

Drained Meana 55 16 58 26 9 12 11 12

95% confidence

interval

(42–69) (13–19) (47–68) (19–32) (6–11) (8–16) (7–14) (8–17)

Rangea – – – – – – – –

Hillslope mean 48 31 66 37 9 3 13 15

95% confidence

intervalb– – – – – – – –

Range 14–173 7–96 15–375 6–338 1–96 4–242 l.o.d.–99 1–114

Catchment Mean 55 34 60 23 5 8 24 8

‘97 (nZ5)

‘98(nZ3)

95% confidence

interval

(26–86) (16–52) (15–104) (11–34) (4–6) (0–19) (0–51) (4–11)

Range 4–284 4–157 l.o.d.–407 1–319 0–63 0–256 l.o.d.–91 0–200

a Range not possible to calculate as mean drained concentration estimated on an annual basis as total load (using anitlog means) from near-surface and drainflow pathways,

divided by total discharge. Estimation of the range of concentrations recorded in the mixed outflows on each sample date would require instantaneous discharge measurements.b Confidence interval not applicable, sample size of one.

F.L

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det

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/Jo

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32

Table 7

Estimated annual export coefficients (kg haK1 yrK1) for different P fractions at scales ranging from 30 m2 up to catchment scale calculated from antilog mean concentrations (April

1997–March 1998 and April 1998–March 1999)

Scale April 1997–March 1998 April 1998–March 1999

Total

phosphorus

Reactive

phosphorus

!0.45 mm

Total

phosphorus

Phosphorus

O0.45 mm

Reactive

phosphorus

O0.45 mm

Unreactive

phosphorus

O0.45 mm

Reactive

phosphorus

!0.45 mm

Unreactive

phosphorus

!0.45 mm

30 m2 Mean 0.68 0.17 0.83 0.52 0.09 0.43 0.14 0.13

(nZ6) 95% confidence

interval of mean

(0.52–0.83) (0.12–0.23) (0.59–1.08) (0.36–0.69) (0.05–0.13) (0.30–0.55) (0.08–0.21) (0.08–0.17)

1 ha

Undrained Mean 0.69 0.26 0.77 0.43 0.14 0.26 0.10 0.19


interval of mean

(0.38–1.01) (0.14–0.38) (0.54–1.01) (0.28–0.57) (0.08–0.19) (0.16–0.36) (0.08–0.12) (0.10–0.27)

Drained Mean 0.43 0.13 0.39 0.17 0.06 0.08 0.07 0.08


interval of mean

(0.32–0.54) (0.10–0.15) (0.32–0.46) (0.13–0.22) (0.04–0.08) (0.05–0.11) (0.05–0.10) (0.05–0.11)

Hillslope Mean 0.49 0.32 0.59 0.33 0.08 0.03 0.12 0.13


intervala– – – – – – – –

Catchment Mean 0.40 0.27 0.83 0.32 0.07 0.2 0.33 0.10


interval of mean

(–0.01–0.81) (–0.07–0.61) (0.27–1.38) (0.13–0.50) (0.05–0.09) (0.03–0.38) (–0.03–0.70) (0.07–0.14)

a Confidence interval not applicable, sample size of one.

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det

al.

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81

33


et al. (1997) and Withers et al. (1999). However, such

a link between land use and P transfer is less

commonly observed in larger basins, for example

the 902 km2 River Vilaine catchment in France

(Moreau et al., 1998), and the 109 km2 Gjern A

catchment in Denmark (Svendsen et al., 1995). This

may be because direct inputs to the river system from,

for example, channel or bank erosion might contribute

significantly to the diffuse P load as was the case for

the Gjern A (Svendsen et al., 1995). Other expla-

nations may be that there was little variation in small-

scale P transfers from different land uses, or that any

such differences were dampened as the scale of

observation increased. In the River Taw, the differ-

ence between upland and lowland P transfers may

have more parallels with comparisons between

forested (i.e. non-agricultural) and agricultural catch-

ments (e.g. Prairie and Kalff, 1986). For the Taw, the

average discharge per hectare is higher in the upland

catchments than in the lowland catchments. This

suggests that, unlike highly conservative and mobile

elements such as nitrate (e.g. Heathwaite et al., 1996)

P export processes specific to soil type and land use

may control P transfer rather than the volume of

discharge.

4.4. Diffuse phosphorus transfers at the 30 m2 plot

and 1 ha scale

Further insight into the likely sources and transport

dynamics for diffuse P can be gleaned by looking a P

transfer at the different scales investigated in this

study. Here, over 50% of the P transferred from both

30 m2 (undrained) plots and 1 ha (undrained) field

lysimeters is accounted for by the PO0.45 fraction,

with unreactive P forming the majority of P

transferred. Although the database for the 30 m2

plots was small compared to that generated at 1 ha

scale due to the less frequent generation of runoff from

the small plots, the temporal dynamics of P transfer

and statistical similarity in annual mean TP concen-

trations and export coefficients suggest that rapid

connectivity and similar patterns of P transport up to

the 1 ha field scale are possible. There are no exposed

channel faces to form an additional source area for P

at this scale of study, and it seems unlikely that

retention combined with additional mobilisation

processes would cancel out to produce a similar

response at the two scales. The results contrast with

research reported by Dils and Heathwaite (1996), who

recorded an increase in the downslope concentration

of TP in throughflow samples for grassland

catchments.

4.5. Diffuse phosphorus transfers from plot to

hillslope scale

At the hillslope scale, the Rowden study site

consists of a mixture of drained and undrained land.

This is typical of the Taw catchment where c.70% of

the land has been drained at some point. Several

factors suggest that the connectivity of P transport

from plot and field to hillslope may be high. For the

two years of research reported here, mean TP

concentrations and P export at the hillslope scale

lies between that recorded for the drained and

undrained 1 ha lysimeters. The regression of log-

transformed hillslope on field scale P concentrations

is also highly significant (Table 5). The regression

parameters (slope c. 1, intercept c. 0) imply that P

concentrations at the two scales are approximately

equal. However, the TP concentration recorded at the

hillslope scale during the winter of 1996–1997 was

often higher than that recorded at other scales

(Fig. 3a). Furthermore, for both years, the export of

UP O0.45 and RP !0.45 was higher than that

anticipated from the plot and field studies. The high P

concentrations and P transfer recorded for certain

period of the year may be indicative of additional

inputs of P from the channel bed or stream banks or

from unaccounted point sources such as septic tanks.

Short-term storage of P in channel beds has been

inferred in other studies (e.g. Pommel and Dorioz,

1997; Walling et al., 1998), and the major form of the

additional P transferred in 1997–1998 during the

concentration peaks (forms other than RP !0.45)

supports this interpretation. At both the field and

hillslope scales, there were pronounced peaks of P in

the UP !0.45 fraction and to a lesser extent the RP

!0.45 fraction from June to August 1998 and during

a storm in early March 1999; this contrasts with the

relatively constant concentrations of these forms

recorded at other times of the year. Drying and re-

wetting of soils, which can release large amounts of

particularly organic P to soil solution (e.g. Turner and

Haygarth, 2001) and may be a cause of this release in


the summer (e.g. Dils and Heathwaite, 2000). The

March release may result from freeze-thaw or from a

rapid surface soil temperature rise from 1.4 to 9.1 8C

that occurred during the 4 day period of rainfall.

In summary, the connectivity of P transfers from

plot-field up to hillslope scale may be rapid, with

little of the sediment retention that has been

observed elsewhere (Withers et al., 1998), but plot

and field scale transfers may sometimes be aug-

mented by other inputs and processes; the system is

not conservative, and plot-field scale rates of P

transfers will not accurately predict transfers at

hillslope scale.

4.6. Diffuse phosphorus transfers from plot to

catchment scales

The mean TP export from the Taw catchment

calculated from flow weighted mean concentrations,

is 1.2 kg haK1 yrK1. In contrast to the findings of

other studies in untilled sites (e.g. Sharpley et al.,

1992; Tattari and Barlund, 1998; Ekholm et al.,

1999), P O0.45 was the dominant form of P

transferred from diffuse sources in the Taw at all

scales. A significant regression relationship was

found between catchment and field scale log-

transformed TP concentrations, though it was

weaker than that recorded between hillslope and

field scale TP concentrations. The annual TP export

at the catchment scale was not significantly

different to that recorded from the undrained plots

and lysimeters. These data suggest that P flux at

the catchment scale may have contributions from P

inputs from other sources. The mean catchment

scale export of RP !0.45 (0.33 kg haK1 yrK1) is c.

three times higher than that of the mean 1 ha

concentration (0.09 kg haK1 yrK1). This difference

could account for the variation in TP export between

catchment and plot and field scales, and may be

accounted for by point source inputs at catchment

scale that are estimated to be around 0.15 kg haK

1 yrK1. Site specific inputs of P of the types observed

at the hillslope scale in Phase III of the study do not

appear to affect catchment P fluxes. Either they are of

low enough frequency to be diluted by mixing with

water of lower P concentrations or there is a decrease

in P export with increasing scale (e.g. Prairie and

Kalff, 1986). Phosphorus concentrations at the

hillslope to catchment scale are significantly lower

than those recorded for the undrained field lysimeters

but similar to P concentrations recorded in runoff

from the drained lysimeters. The lower P concen-

trations recorded at the catchment scale, despite

similar exports of most P forms, are consistent with

there being dilution of plot-field scale P transfers.

The regression equation of catchment on field scale

concentrations similarly implies that catchment scale

TP concentrations are proportional to approximately

the square root of those at field scale. The major

source of this dilution is likely to be water from the

high rainfall upland areas of the Taw catchment,

known to be low in P (antilog mean TP concen-

trations for 1997–1998 at Belstone, Leeham Ford and

Brayford were 2, 9, and 11 mg lK1, respectively; and

for 1998–1998 at Leeham Ford: 18 mg lK1).

In addition to the point source and dilution effects

discussed above, the dataset was examined for other

instances where the plot to hillslope P concentrations

or fluxes do not appear to predict catchment P

transfers. Disregarding occasional mismatches

between scales that could result from time-lags in

sampling, the time series for UP !0.45 displays a

consistent lack of response at catchment scale to

smaller scale peaks in concentration. There is only

one occasion (late April 1998) when there is

significant variation in catchment UP !0.45 con-

centration. This appears to originate in the Woodle-

igh catchment, where an unusually high

concentration of 200 mg lK1 was recorded. This

event was still apparent downstream at Umberleigh,

where the concentration had fallen to 96 mg lK1.

Significant organic P sources have been identified in

water extracts from pasture soils and have been

shown to be rapidly hydrolysable phosphodiesters or

mononucleotides (Guggenberger et al., 1996; Turner

et al., 2002). Hydrolysis of organic P forms may

account small contribution made by UP !0.45 in

downstream samples and may also contribute to the

increased export of RP !0.45 seen at the catchment

scale. The process of hydrolysis was not evaluated in

this study but several researchers have demonstrated

phosphodiesterase enzyme activity in vertical

throughflow and river waters (Christmas and Whit-

ton, 1998; Turner, 2000).

Excepting drainflow at 1 ha scale, mean TP

concentrations recorded at all scales exceed the


OECD 35 mg lK1 threshold (OECD, 1982) for

increased risk of eutrophication in standing waters,

though not the proposed 60 mg lK1 mean annual RP

!0.45 river threshold for mesotrophic rivers

(Environment Agency, 2000a).

5. Conclusion

Point inputs of P were estimated, using the best

methods and understanding available, to account

for up to 38% of annual P fluxes at catchment

scale in the River Taw, the point source influence

tending to be greater in the larger sub-catchments.

However, the majority of the P transfer (from c.

60% to almost the entire annual P flux) at

catchment scale is estimated to be of diffuse

origin. Diffuse transfers of P measured at plot

and field scale from agricultural grassland can

account for the amount and composition of P

transfer observed at the catchment scale in diffuse

source dominated rivers, thus transport of P from

plot to catchment scale in general appears to be

conservative. There is little evidence for a signifi-

cant input of P from river channel banks in the

Taw, although there is evidence that several other

processes do affect the scaling of P transfers: the

appearance at hillslope scale of site specific source

areas such as bare soil, open access ditches, and

farm point sources; the possible hydrolysis of UP

!0.45; and the dilution of catchment scale P

concentrations by upland or groundwater sources.

These findings from the River Taw catchment are

relevant to the 14–24% of land in England and Wales

sharing similar soils and or hydrology. The P transfers

observed are not of immediate concern in the Taw

catchment, although the sediment they are associated

with might be. In the neighbouring Torridge and Tamar

catchments, however, there is no dilution from upland

headwaters, and algal blooms occur occasionally in the

Torridge and annually in the Upper Tamar Lake

(Environment Agency, 1998, 1999). Discharges from

STWs and specific farms have been identified as prime

causes of the associated eutrophication, but wide-

spread diffuse inputs of the magnitude observed in the

Taw catchment could also be contributing to the

problem.

Acknowledgements

The research was funded by a MAFF Studentship

to Fiona Wood at the University of Sheffield and

carried out at the Institute of Grassland and Environ-

mental Research North Wyke. Thanks are due to the

landowners who gave access to their farms; also to

Alison Coombs, Liz Williams, Neil Preedy, Rachel

Matthews, Trish Butler and Andrew Bristow for

technical assistance. The Environment Agency pro-

vided river hydrometric data.

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Baldwin, D.S., Mitchell, M.A., Olley, J.M., 2002. Pollutant-

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