Delivery and cycling of phosphorus in rivers: A reviewudel.edu/~inamdar/nps2007/Withers.pdfDelivery and cycling of phosphorus in rivers: A review P.J.A. Withersa,⁎, H.P. Jarvieb

S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 0 ( 2 0 0 8 ) 3 7 9 – 3 9 5

ava i l ab l e a t www.sc i enced i rec t . com

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Delivery and cycling of phosphorus in rivers: A review

P.J.A. Withersa,⁎, H.P. Jarvieb

aEnvironment Group, ADAS UK Limited, Gleadthorpe, Meden Vale, Mansfield, Nottinghamshire NG20 9PF, United KingdombCentre for Ecology and Hydrology, Maclean Building, Crowmarsh Gifford, Wallingford, Oxfordshire OX10 8BB, United Kingdom

A R T I C L E D A T A

⁎ Corresponding author.E-mail address: [email protected] (P

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.08.002

A B S T R A C T

Article history:Received 27 May 2008Received in revised form1August 2008Accepted 4 August 2008Available online 19 September 2008

Phosphorus (P) supply (concentration and flux) is an important driver for biological activity inflowing waters and needs to be managed to avoid eutrophication impacts associated withurbanisationandagricultural intensification.Thispaper examines the roleof in-streamretentionand cycling in regulating river P concentrations in order to better understand the links between Psources and their ecological impacts. In termsof their composition (solubility and concentration),patterns of delivery (mode and timing) and therefore ecological relevance, P sources enteringrivers are best grouped into wastewater discharges N runoff from impervious surfaces (roads,farmyards) N runoff from pervious surfaces (forestry, cultivated land and pasture). The localizedimpacts of soluble P discharges during ecologically sensitive periods can be distinguished fromthe downstream impacts associated with particulate P discharges under high flows due to thedifferent processes bywhich these sources are retained, transformed and assimilatedwithin theriver channel. The range of physico-chemical processes involved in P cycling and the variableimportance of these processes in different river environments according to stream size, streamgeomorphology and anthropogenic pressures are summarised. It is concluded that the capacityto retain (process) P within the river channel, and hence regulate the downstream delivery of Pwithout stressing the aquatic communities present, is considerable, especially in headwaters. Tohelp achieve goodwater quality, there is scope to better manage this ecosystem service throughregulation of P supplywhilst optimising in-streamP retention according to subsidy-stress theory.Further research isneeded todevelop in-streammanagementoptions formaximisingPsubsidiesand to demonstrate that regulation of downstream P delivery will reduce the incidence ofeutrophication in connected waterbodies.

© 2008 Elsevier B.V. All rights reserved.

Keywords:PhosphorusRiversIn-stream processesRetentionCyclingSubsidy-stress theory

1. Introduction

Urbanisation and agricultural intensification have led towidespread P enrichment of surface waters causing a rangeof environmental, social and economic problems at regionaland local levels encompassed under the term eutrophication.These problems relate to human health (algal toxins), speciesabundance and diversity, amenity value and costs of watertreatment for drinking. Rivers are particularly vulnerable dueto their proximity to population centres, sensitivity to land usechange and ubiquitous exploitation (Malmqvist and Rundle,2002; Walsh et al., 2005). In faster flowing rivers, increased

.J.A. Withers).

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periphyton growth including filamentous green algae (e.g.Cladophera spp.) and epiphytic diatoms, and reduced abun-dance of shallow rooting submerged plants (e.g. Ranunculusspp., Challitriche spp.) are of primary concern (Hilton et al.,2006). In slower moving rivers and standing waters, growth ofphytoplankton leads to reduced light penetration, increasedincidence of surface algal scums and deoxygenation causingfish kills (Smith, 2003). The incidence and severity of eutro-phication can be quite variable fromyear to year, depending onanumber of site factors other than P (flow regime,water depth,water clarity, temperature, shading, grazing pressures andsupplies of carbon (C), nitrogen (N) and silica (Si)) (Søballe and

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Kimmel, 1987; Dodds, 2007). However, managing the P (or bothP and N) supply can be an effective means of reducing theprimary production that drives eutrophication impacts inrivers (e.g. Chételat et al., 1999; Bowes et al., 2007; Elser et al.,2007).

With the recent introduction of the EuropeanWater Frame-work Directive (WFD) and its regulatory requirement to achievegood ecological status in a range ofwaterbodies by 2015, there isa major and urgent need to understand the delivery mechan-isms and ecological relevance of the multiple sources of Pentering rivers and how they might be managed (Neal andJarvie, 2005). The relationships between P supply, ambient Pconcentration in the water-column and ecological response arecomplex. Residence times can fluctuatewidely and the capacityof riverineecosystems toassimilateP isspatiallyand temporallyvery variable (Fisher et al., 1998; Edwards and Withers, 2007).Conceptually, the response of streambiota to additionsof P (andothernutrients) canbeexpected to benon-linearasdescribedbysubsidy-stress theory (Odum et al., 1979; Niyogi et al., 2007).Increased P supply may initially enhance the biomass anddiversity of the ecosystem (‘subsidy’), whilst longer-termexposure to excessive P supply will ‘stress’ the ecosystem bystimulating the algal growth that leads to undesirable eutro-phication. In some cases, it may be important to maintain anutrient subsidy; for example in preserving fish stocks thatmight otherwise decline if P supply is reduced too much; theconcept of cultural oligotrophication (Stockner et al., 2000).

The transition from subsidy to stress reflects a thresholdabove which the ecosystem can no longer buffer the impacts ofthePsupply.This is effectivelya lossof ecosystemfunctionwhichhas implications not only locally, but also over longer distances,affecting water quality further downstream (Marti et al., 2004). Inprinciple, therefore, the aquatic ecosystem must be managedsuch that the subsidy-stress threshold is not exceeded. Thesubsidy-stress threshold will be influenced by both the nature ofthePsupply (speciation,modeand timingofdelivery) and the fateof this P on entering the water-column and during transport(cycling and retention). Cycling of P encompasses the manytransformations thatPundergoesdue toa rangeofdynamicbioticand abiotic processes operating within the water-column orreach: uptake/release, adsorption/desorption, precipitation/dis-solution and advection/diffusion. The balance of these processeswithin different environments determines P retention and thebiological response, which can be both localized and landscapedependent (Vannoteetal., 1980;Bryceetal., 1999;Allan, 2004).Theneteffect of theamountof Penteringand leavinganecosystemasquantified by a mass balance provides a measure of P retention(e.g. during low stream flow) or P export (e.g. during high streamflow) over a given spatial and temporal scale (Meyer and Likens,1979; House, 2003).

Riverine processing is therefore an important aspect of thetransport of P from source to sea that is often not adequatelyrepresented in catchment management, where the emphasisis on major source reductions. Simply reducing P supply (e.g.load)maynot achieve the desired improvements in riverwaterquality due to a lack of appreciation of the ecological relevanceof this supply and how it is modified by in-stream processes.This information is needed for accurate targeting of sourcereduction strategies in catchments and sustainable manage-ment of riverine ecosystems. In this review, we examine the

role and importance of P cycling/retention within the water-column of streams/rivers in different environments in order toprovide a better understanding of how P supply and river Pconcentrations might bemanaged according to subsidy-stresstheory. Key aspects that are addressed include (a) theecological relevance of different P sources and how theymight be best characterized, (b) the role of in-streamprocessesin buffering the downstream transport and therefore poten-tial ecological impacts of these P sources in different streamenvironments and (c) what implications does this have forcatchment management and the development of controlstrategies to meet ecological improvements required for theWFD. The cycling and retention of P within the riparianecotone, in connected wetlands or standing waters, or duringtransition to estuarine and coastal ecosystems are also impor-tant aspects of river basin management but are outside thescope of this review.

2. Phosphorus sources and their delivery

A variety of natural and anthropogenic sources contribute P tostreams and rivers via a number of different pathways and atdifferent times of the year. Sources differ with respect to theircomposition (concentration, speciation and bioavailability), andmode and timing of delivery (continuous or episodic discharge,seasonality). The relative importance of the different P sourcesfor total P flux varies greatly between different catchmentsdepending on anthropogenic pressures and discharge, and istherefore highly site specific. Flux data for some sources arehard tomeasurewithout continuousmeasurements (e.g. Jordanet al., 2007) and source apportionment often relies heavily onmodelling approaches (Alexander et al., 2004; Kronvang et al.,2007). Such approaches tend to lose the field scale resolutionnecessary to understand the specific ecological relevance of thevarious combinations of individual sources, or source areas thatoccur in small catchments (Withers et al., in press).

2.1. Natural sources

Natural (or background) sources of P entering rivers areimportant where anthropogenic pressures are low/absent andcan include the natural weathering of soil parentmaterials andthe atmosphere (Holtan et al., 1988), riparian vegetation (Meyerand Likens, 1979), migratory fish that return to their spawninggrounds (Nislow et al., 2004) and river bank erosion (Wallinget al., 2008). These sources supply generally very small amountsof P to rivers (b0.1 kg P ha−1 yr−1) and are predominantly inparticulate form (soil anddustparticles, leaf litter, fish carcases),but still provide important substrates for microbial synthesis inupland, forestedandoligotrophicstreams.Forexample, Jonssonand Jonsson (2003) calculated a net import of fish-derived Pequivalent to 17% of the P flux in a Norwegian river.

2.2. Anthropogenic sources

Anthropogenic inputs are conventionally distinguished eitheras ‘point sources’ which are highly concentrated in soluble Pand pipe-discharged continuously from discrete points, or‘diffuse sources’ which typically have a greater proportion in

Table 1 – Various anthropogenic sources of phosphorus discharged to water and their compositional and deliverycharacteristics (adapted from Edwards and Withers, 2008)

Source type/source Delivery Chemicalcomposition

TP Reference

Discharge Rainfalldependency

Range(mg L−1)

Mean(mg L−1)

WastewaterSTW1/industry Continuous Low Concentrated 0.2–17.1 6.63 Neal et al. (2005)

bDL–13.1 2.90 Edwards and Withers (2008)CSOs2 Episodic High Concentrated n.d n.d.Septic tanks Episodic to semi-continuous Low Variable 1.0–22.0 10.24 Edwards and Withers (2008)

0.17–6.5 1.625 Withers et al. (in press)

Impervious surfacesRoad/track runoff Episodic High Variable (high SS)3 0.11–16 2.396 Withers et al. (in press)

0.03–4.7 0.297 Kayhanian et al. (2007)Farmyards Episodic to semi-continuous Low–high Variable 0.02–247 30.8 Edwards and Withers (2008)

0.1–3.3 0.778 Edwards et al. (2007)

Pervious surfacesField surface runoff Episodic High Variable (high SS)3 0.17–6.8 1.29 Withers et al. (in press)Field tile drains Episodic to semi-continuous Low–high Variable 0.02–6.2 0.76 Withers et al. (in press)Field sub-surface runoff Episodic High Dilute n.d. n.d.

1STW, Sewage Treatment Works; 2CSOs, Combined Sewer Overflows; 3SS, Suspended Solids. 4Direct discharge; 5Diluted by storm runoffcontaining other sources. 6Rural roads; 7Urban highways; 8Roof runoff; n.d. no data.

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particulate (N0.45 μm) form, arrive only during storm eventsin surface and sub-surface runoff and from a number ofscattered entry points within the catchment. While pointsources are usually synonymous with wastewaters/indus-trial effluents and diffuse sources are invariably linked withagriculture, there are a number of individual sources that havecharacteristics intermediate between those of point anddiffuse sources (Haygarth et al., 2004; Neal et al., 2004; Jordanet al., 2007; Edwards and Withers, 2008). These intermediatesources of P include road and track runoff, septic tankdischarges and farmyard runoff. A better grouping of anthro-pogenic sources in relation to ecological impacts can there-fore be related to their different compositional and deliveryattributes (Table 1). This grouping distinguishes the contin-uous, or semi-continuous, discharges of wastewater effluentswhich are largely storm independent, from storm depen-dent runoff from impervious surfaces (roads, roofs, yardsand driveways) which may occur all year round and stormdependent runoff from pervious surfaces (forestry, cultivatedland and pasture) which exhibits a more seasonal deliverypattern due to summer evapotranspiration by growing crops.

2.2.1. WastewatersDomestic and industrial wastewater effluents are dischargedto surface waters from sewage treatment works (STWs), orindustrial premises, after various degrees of treatment toremove toxic contaminants (metals and organics) and P. Theconcentrations and loads of P discharged depend on thesewered population in the catchment area, the extent ofindustrial activity and the methods of treatment deployed.After treatment, mean effluent TP concentrations dischargedto watercourses generally range from b1 to 20 mg L−1, withlowest values recordedat themajor STW ineutrophic sensitivewaters where tertiary treatment (P stripping) is carried out.

Neal et al. (2005) found a mean TP concentration for 6 smallSTW in the rural sub-catchments of the upper Thames (KennetandDun) of 6.6mg L−1 (range 0.2–17.1mgL−1) ofwhich 91%wasin soluble reactive P (SRP) form, 6% in a dissolved hydrolysableform (DHP) form and 3% in particulate P (PP) form. Fluxes of Pfrom these STW to the river ranged from only 0.1–2.9 t P yr−1

(ca. 0.5 kg person yr−1), but can be much greater. For example,Bowes et al. (2005) found P loads from STWs in one largeEnglish catchment (Warwickshire Avon) were in the range 0.1–548 t yr−1 and represented between 9 and 93% of the total Pload. Streams receiving wastewater effluent typically show acharacteristic pattern of high P concentrations during summerlow flows and more diluted concentrations during winterstorm events. Sewage-impacted streams also show diurnalvariationswith peak P concentrations occurring atmidday andduring the evening, reflecting typical patterns in domesticwater usage in the earlymorning and late eveningwith a time-lag of a fewhours for transit through the STW (Moss et al., 1988;Palmer-Felgate et al., 2008).

In rural catchments, a significant proportion of the popula-tion uses septic tank systems for waste disposal. For example,septic tankdensities of 3–14km−2 are reportedbyArnscheidt etal. (2007) in Ireland. Many older septic tank systems eitherdischarge directly to the stream, are not emptied regularlyenough, or do not have adequate soakaway facilities, andtherefore can make a significant contribution to stream Pconcentrations (Butler and Payne, 1995; Arnscheidt et al., 2007;Jarvie et al., 2008a). In reviewing potential treatment systemsfor septic tank effluent, Edwards and Withers (2008) report arange in TP concentrations of 1–22 mg L−1 (mean 10.2 mg L−1).Septic tank discharges would therefore appear to be asconcentrated a P source as STW effluent despite the muchsmaller volumes of effluent discharged. However, the potencyof septic tank discharges may become diluted when mixed


withother rural P sources suchas roadand farmyard runoff; forexample, Withers et al. (in press) report a much smaller rangein TP concentrations (0.2–6.5 mg L−1) in mixed source stormrunoff with just over 50% in dissolved form. Arnscheidt et al.(2007) considered the ecological relevance of septic tanks to bedisproportionally high compared to agricultural runoff in theIrish Oona catchment due to the chronic andmore continuousnature of P delivery to the stream.

2.2.2. Runoff from impervious surfacesA number of studies have shown that stream ecosystemsbecome impaired when as little as 10–15% of the catchmentarea is occupied by impervious surfaces (e.g. Kleine, 1979;Wang et al., 2000). Impervious surfaces are generally asso-ciated with urban areas but clearly also must include hardsurfaces in rural areas such as farmyards. Urban stormwater isa composite mixture of runoff from roads, roofs, residentialdriveways, parking lots, construction sites and gardens.Sources of P mobilized in urban runoff include atmosphericdeposition, leaf fall, industrial debris, urban litter, residentialactivities (e.g. car washing, domestic sink water), bird, dogand livestock excreta, domestic and industrial fertilizers, soilparticles deposited by vehicles and eroded from roadsideverges, detergents and lubricants (Bannerman et al., 1993;Duncan, 2005; Dunk et al., 2007). The various combinations ofthese multiple sources and the differential routing and speedof flow depending on the types of impervious surface and theirslopes typically gives rise to very large variation in both eventmean concentrations (EMCs) and in the form of P transported(Table 1). In reviewing data for 765 separate urban catchments,Mitchell (2001) found mean EMC values for TP of between 0.2and 0.4 mg L−1 with about 40–50% in SRP form. Very similarmean values are reported by Duncan (2005) for residentialareas and Kayhanian et al. (2007) for highways. Runoff fromurban surfaces is often connected to the sewerage systemgenerating combined sewer overflows, butmay also be directlyconnected to streams and rivers via surface water outfalls(Dunk et al., 2007). In rural areas,Withers et al. (in press) foundmuch larger mean TP concentrations (N2 mg L−1) in road andtrack runoff entering streams with the majority (90%) inparticulate form (Table 1). The pattern of runoff generation isstorm dependent, but nevertheless represents a semi-contin-uous supply of rapidly-mobilized P throughout the year,because ‘impervious’ surfaces have no, or very limited,infiltration capacity.

Runoff and piped discharges from farmyards also show alarge degree of temporal variability ranging from dilute yardwater to highly contaminated pipe effluent, particularly whereanimals congregate (Wilcock et al., 1999; Edwards et al., 2007).Farmyard runoff is often a composite mixture of various sour-ces includingparlourwashings, runoff fromsilage andmanurestores on concrete, livestock sheds, roof water and domesticwastewater (septic tank) discharges as well as open yardrunoff. Edwards et al. (2007) found storm runoff from farm-yards in Scotland containedvery largeTP concentrations (up toN200 mg L−1) (Table 1), with the majority in SRP form. Unlikeroof runoff, there was no apparent dilution of this P signal asthe storm progressed. Edwards and Hooda (2007) measured upto 51 mg TP L−1 in flow from two farmyard drains (mean 1 and6mgL−1) and estimated that the farmyard area contributed 25–

30% of annual downstream P loads and therefore of highecological significance locally. Hively et al. (2005) concludedthat the relatively impervious surfaces on farms such as cowpaths and farmyards maybe of minor spatial extent, but thehighly concentrated runoff generated from these areasmay bemuch more significant to summertime P loadings. Mean TPconcentrations for cowpath and farmyards were 0.99 and13.2 mg L−1, respectively. While cowpath runoff was domi-nated by particulate P (82%), farmyard runoff was dominatedby dissolved P (88%).Withers et al. (in press) similarly recordedup to 15 mg L−1 in storm runoff from three farmyards withlargest concentrations in runoff from a sheep shed and over50% in dissolved form. Further data on fluxes of P associatedwith runoff from impervious sources are required tomore fullyunderstand their relative importance and contribution tostream P loads within rural catchments.

2.2.3. Runoff from pervious surfaces (forestry, cultivated landand pasture)As rainwater passes over, or through, the soil at various ratesit mobilizes and transports inorganic and organic P alongsurface and sub-surface hydrological pathways to the water-course. Sources of P at the land surfacewhichmay be released,or detached, to runoff include soil, fertilizers, livestock faecesand vegetation (Haygarth and Jarvis, 1999; Hodgkinson andWithers, 2007). During storm events, variable combinations ofthese sources become mobilized and transported from vari-able areas within fields, farms and catchments providing alarge degree of both spatial and temporal variability. Thisvariability reflects the frequency and distribution of stormevents, the dominant pathway of P transport and certainaspects of land management; for example nutrient manage-ment (soil P saturation; P inputs and how they are managed),crop and livestock management (crop types, crop residuemanagement, animal numbers and stocking rates) and soilmanagement (type, method and timing of cultivations).

Recently, Withers et al. (in press) recorded concentrationsof TP up to 6 or 7 mg L−1 directly entering streams as bothsurface runoff and runoff through field tile drains (Table 1) andthis range is quite typical of runoff from a range of field andcatchment studies. Lower average TP concentrations (0.8 vs.1.3 mg L−1) and a higher proportion in dissolved form (41 vs.26%) in drain runoff as compared to surface runoff reflects thegreater opportunity for adsorption of SRP and filtering of SSand PP aswater percolates through the soil (e.g. Haygarth et al.,1998). Where fertilizers and manures have been recentlyapplied, P concentrations in both surface runoff and drainflowcan be well over 10 mg L−1 (e.g. Withers et al., 2003). Whilstrunoff from grassland fields generally contains a greater pro-portionof P in dissolved form than runoff fromarable land, thisis by nomeans universal (Douglas et al., 2007; Hodgkinson andWithers, 2007). Catchment P fluxes originating from agricul-tural land are in the range b0.1–6 kg P ha−1 (Ryden et al., 1973;Haygarth and Jarvis, 1999; Kronvang et al., 2007).

In themajority of the UK, runoff from agricultural land largelyoccurs duringwinter when soils are fully wetted and under theseconditionsmost P is transportedduring a fewmajor stormevents(e.g. Jordan et al., 2007). Large P concentrations may also bedelivered in small amounts of storm water during the moreecologically sensitive periods where field underdrainage systems

Fig. 1 –Seasonality in export and flow-weighted concentrations (FWC) of total dissolved P (TDP) and particulate P (PP) from a tiledrain in an arable field at Rosemaund (a and c) contrasts with the continuous delivery of P from an outfall of a village sewagetreatment works (b and d) in the Wye river basin (data adapted from Withers, 2008).


provide a degree of connectivity that would otherwise not exist.Data comparing P export and flow-weighted concentrations fromafielddrainandaSTWoutfall are shown inFig. 1. Export of P fromthe field drain is largely restricted to the main period of flow(December to April) (Fig. 1a), but large flow-weighted concentra-tions also occurred outside of the main drainage period (Fig. 1c).This contrasts strongly with the more continuous discharge ofboth export and flow-weighted concentrations from the STW(Fig. 1b and d). The highly soluble nature of the STW effluentmakesa strongcontrastwith the largelyparticulatenatureof fielddrain runoff. For agricultural P sources to cause eutrophication,there must be either contamination of the groundwater thatcontributes to lowsummer flows, a significant loss of bioavailableP during the biologically active summer period, or P transportedduring the winter may be stored within the channel and

Fig. 2 –Conceptualised diagram of in-stream processe

remobilized during the summer. In-stream processing thereforehas a particularly critical role in determining the ecologicalsignificance of P sources derived from agriculture.

3. In-stream P retention and cycling

Many mass balance studies have shown that P fluxes enteringrivers do not correspond with those measured at the reach orcatchment outlet and that P tends to be retained within riversystems, particularly under low flows during spring and summeri.e. at times of greatest eutrophication risk (e.g. Svendsen et al.,1995; Reddyet al., 1999; Bukaveckas et al., 2005; Jarvie et al., 2006a;Némery andGarnier, 2007). Phosphorus retention in rivers occursas a result of a combination of biogeochemical and physical

s influencing P concentrations in flowing waters.


processes which temporarily remove and/or transform P duringdownstream transport allowing opportunity for biotic and abioticassimilation (Fig. 2). Retention therefore provides two keyecosystemfunctions: (a) allowsnutrient-poorpristineanduplandstreams to develop and sustain a productive community (Triskaet al., 1989a,b) and (b) buffers the impacts of anthropogenic Pinputs on downstream communities during the critical summerperiod by temporarily removing P (i.e. a damping effect),(Svendsen et al., 1995). Phosphorus retention in rivers is alsohighly variable in time and space (e.g. Marti and Sabater, 1996;Bowes andHouse 2001; Bowes et al., 2003) and not all P entering astreamwill playanequalpart inbiogeochemical cycleswithin theecosystem: thiswill dependon the form that P enters the river, itsreactivity and the water residence time. For example, most Pinputs delivered to streams under very high flows will be flushedthrough without entering the stream biogeochemical pathways.Retention rates from b10% to over 30% were recorded under arange of flowconditions byHouse (2003)whilst Jarvie et al. (2002a)recorded up to 60% net retention under low flow in the RiverKennet, England downstream of a STW. House and Casey (1989)report that 50%of the SRP load could be removed during spring inthe R. Frome, England due to biological uptake.

Not onlydoesP retention result in changes to thequantity andtiming of P fluxes to downstream aquatic systems, but may alsoalter the formof P transported downstream,with implications forP bioavailability. For example, highly bioavailabledissolvedPmaybe converted into less-bioavailable particulate and organicfractions following uptake onto sediments or by biota (e.g.MeyerandLikens,1979; Svendsenetal., 1995),withcorrespondingincreases in the P content of the (re)-suspended particulate load(Némery and Garnier, 2007). Conversely, deoxygenation ofstreamwaters (enhancedmicrobial activity linked to high organicmatter (OM) inputs) and associated development of reducing

Table 2 – Stream uptake lenghs

Stream/catchment type Ran

First order/upland/‘pristine’First order deciduous woodland stream, eastern Tennessee 22–First order forested streams, upper Peninsula, Michigan, USA 98–Streamside artificial stream experiments, Coweeta,North Caroline, USA

b10

Forested upland streams (Hubbard Brook Experimental Forest,New Hamphire, USA)

2–8

First order upland stream, Scotland, UK c.8First order streams 24–

Second-fourth order/rural/agriculturalSecond order rural watershed, Wisconsin, USA 169Second order stream 26–Third order stream 99–Fourth order stream 71–Streams in NW Kentucky and SE Indiana, USA 137Streams in NW Kentucky and SE Indiana, USA 380Streams in NW Kentucky and SE Indiana, USA 518Sawtooth Mountain Lake district, Idaho, USA. 877Sawtooth Mountain Lake district, Idaho, USA. 781

Fifth order/large river/urban and agricultural influenceFifth order stream; agricultural and urban influence 414Fifth order stream 16,Large urban river, Georgia, USA 11,

conditions at the sediment–water interface may result inreductivedissolutionofFe III phosphatesandFe III oxyhydroxidesin surface bed sediments and release of highly bioavailable SRPinto the overlying streamwater (House, 2003; Jarvie et al., 2008a).

The need to characterize and quantify the removal, trans-formation and release of P within a stream (i.e. P cycling) hasresulted in the development of the ‘spiralling’ concept (New-bold et al., 1983; Fisher et al., 1998). A P spiral is defined as thedistance required for a phosphate ion to complete one cyclefrom dissolved form to particulate/organic transformationand back to dissolved ionic form. The distance a phosphate iontravels in solution before being removed by biological assim-ilation, or sorption/precipitation to particulate form, is definedas the ‘uptake length’. The distance travelled in particulate/organic form before release as a phosphate ion is defined asthe ‘turnover time’. Shorter P uptake lengths are indicative ofgreater efficiency of P retention and more rapid P cycling;example values obtained across different stream environ-ments are shown in Table 2. The most direct method formeasuring P uptake lengths is using a spiked addition of a32PO4–P radiotracer (Mulholland et al., 1985, 1997); thesestudies have demonstrated that P uptake, turnover andregeneration times in streams can be rapid (seconds/min-utes/hours), and may not be detected from measurement ofsmall changes in SRP concentrations and loads along the sameriver reaches. Other studies have used non-radiolabelledsingle, or multiple, spiked SRP additions in combination witha conservative chemical tracer to assess SRP loss along a riverreach, relative to the tracer (e.g. Demars, 2008). However,without an isotopic tracer, these measurements provide onlyan indication of the net effects of uptake and release along ariver reach, rather than a direct measurement of phosphateion uptake lengths.

ge in SRP uptake lengths (Sw) (m) Reference

97 Mulholland et al. (1985)530 Hoellein et al. (2007)–140 D'Angelo et al. (1991)

5 Hall et al. (2002)

5–c.115 (read from graph) Demars et al. (2008)161 Ensign and Doyle (2006)

–1667 Orr et al. (2006)223 Ensign and Doyle (2006)743 Ensign and Doyle (2006)907 Ensign and Doyle (2006)0 (channelised) Bukaveckas, 2007(restored) Bukaveckas, 2007(reference) Bukaveckas, 2007

–3460 (streams with no lake influence) Arp and Baker (2007)–848 (streams downstream of lakes) Arp and Baker (2007)

0–367,000 Doyle et al. (2003)175–61,350 Ensign and Doyle (2006)000–85,000 Gibson and Meyer (2007)


Both mass balance studies and estimates of P uptake lengthshave highlighted the temporal and spatial variability in P cyclingand flux transformations at the river reach and catchment scales,but do not provide any direct information about the processescontrolling within-river P cycling and their relative importance.Indeed, uncertainties associated with mass balance calculationsalong river reaches and wider errors and methodologicaldiscrepancies in calculation of catchment P budgets must beborne in mind when interpreting mass balance data in terms ofnet in-streamPcycling (Moss et al., 1988). In order to ascertain thekey processes controlling P cycling and flux transformationswithin rivers, direct measurements are required (e.g. Svendsenand Kronvang, 1993; Haggard et al., 2001a,b, Jarvie et al., 2005,2006a, 2008b; Demars, 2008). Such direct measurements arehighly labour-intensive and time-consuming and, given thehigh degree of heterogeneity in P cycling processes at the riverreach and microhabitat scales (Pretty et al., 2006), a key andpressing challenge is how process-based measurements andresults might be applied and scaled up to wider river system,catchment and basin scales.

4. Mechanisms of P uptake and releasein rivers

Themainphysico-chemical andbiological processescontrollingPuptake and release in rivers have been reviewed elsewhere (e.g.Reddy et al., 1999; House, 2003; Haggard and Sharpley, 2007) andso only a brief outline of these processes is provided below. Themajor focus of this review is discussion of how these keyprocesses of P uptake and release vary in different riverenvironments, and the implications of this variability in P cyclingfor river ecology and for catchment management.

4.1. Physico-chemical controls on P cycling in rivers

4.1.1. Sorption/desorption reactionsDissolved P interacts strongly with particulates such as alu-mino-silicates (clays) (House et al., 1998), andmetal oxides andhydroxides, particularly of iron (Fe) and aluminium (Al)(Lijklema, 1980, Zhang and Huang, 2007). Sorption is a two-step process involving (i) surface adsorption (or desorption) ofphosphate ions to (or from) the solids phase and (ii) diffusioninto or out of the solid phase as a result of either absorption ordesorption within the solid phase itself. Adsorption is con-trolled by mechanisms such as ligand-exchange on Me–OH2

+

and Me–OH sites, electrostatic attraction and displacement byanions competing for surface sites on sedimentparticles,whilethe amount of surface sites available for sorption are influ-enced by pH, redox conditions and particle–OM complexes(Froelich, 1988, Reddy et al., 1999; Zhou et al., 2005). Sediment Puptakes rates of 0.16 gm–2 day–1 were calculated by House andCasey (1989), twice the rate of P assimilation by algae.

4.1.2. Mineral precipitation and dissolutionMineral precipitation reactions that fix P in sediments alsoregulate dissolved P concentrations in interstitial waters andsediments (House, 2003). These precipitation reactions include (i)co-precipitation of phosphate with calcite, which is typicallykinetically controlled and can occur within the water-column

(requires suspended nucleation sites), in groundwaters or withinphotosynthetic (algal) biofims and within the photic zone ofsediments in hardwater systems (House and Denison, 1997; Nealet al., 2002), (ii) precipitation of iron, hydroxide and phosphate inoxic porewaters to produce ferric Fe III hydroxides and phos-phates (Fox, 1989, 1993) and (iii) precipitation of the ferrousphosphate mineral, vivianite, under anaerobic conditions inhighly eutrophic waters, with high dissolved P concentrations inporewaters and plentiful OM supply, often linked to sewageeffluent discharges (House, 2003).

4.1.3. Advection and diffusionThe primary mechanisms of P transport in rivers are advection,linked to fluctuations inwater flowalong thestreamchannel, anddiffusion through zones of P storage with water high residencetimes, such as bed sediment porewaters. Advection can result inmobilisation (entrainment, resuspension) of particulate P to andfrom bed sediments with associated release of dissolved P fromporewaters (Keup, 1968; Casey and Farr, 1982). Localized watercurrents generated throughmacrophyte stands (e.g. via bioturba-tion) may also cause release of P from fine sediments entrappedwithin the canopy structure (Gainswin et al., 2006). Thismixing ofwater and sediment at the benthic interface, increases thepotential for P sorption interactions. Flow velocities and degreeof water turbulence vary according to channel morphology andon seasonal and event timescales (Reddy et al., 1999). Hydrostaticgradients in groundwater or sediments may result in movementof dissolved P and porewaters. Molecular diffusion of dissolved Pis driven by steep gradients in porewater dissolved P concentra-tions within the sediment profile and concentration gradientsbetween surface sediments and the overlying bulk streamwater.Within sediments, variations in porewater concentrations arisefrom mineralization of OM, biological assimilation and P-solubility controls linked to precipitation/dissolution/sorptionreactions under varying redox and pH conditions (e.g. Lijklema,1993; Moore and Reddy, 1994; House and Denison, 2002).

4.2. Biological controls on P cycling in rivers

4.2.1. Periphyton and phytoplanktonPeriphyton and phytoplankton contribute to streamwater Pdynamics through their ability to assimilate both organic andinorganicP fractions, especially inshallowstreams (Bentzenetal.,1992; Whitton et al., 1998; Dodds, 2003). House and Casey (1989)suggest 10–15%of riverine P fluxmay be accounted for by benthicand/or phytoplankton communities. Benthic periphyton influ-ence P cycling by (i) removing P from the water-column andintercepting P diffusing from the sediments (Dodds, 2003); (ii)slowing water exchange across the sediment–water interface,which can decrease advective P transport away from the sedi-ments (Dodds, 1991), increase residence time for P uptake(Mulholland et al., 1994) and trap suspended particulate P (Adeyet al., 1993); (iii) throughdaytimephotosynthesis,whichproducesbiogeochemical conditions favourable for P-removal at thesediment–water interface by (a) reductions in pH promotingCaCO3–P co-precipitation (Hartley et al., 1997; Jarvie et al., 2002b)and (b) oxygen production promoting P sorption to ferric oxides/hydroxides and/or precipitation of ferric phosphates (Moore andReddy, 1994), and the development of an ‘oxic cap’ of surfacesediment capable of sorbing SRP from the overlying river water


and inhibiting the upward diffusion of SRP from sedimentporewaters (Jarvie et al., 2008b). Whilst, epiphythic, epilithic andfloating periphytonmats and phytoplankton remove dissolved Pdirectly from the water-column, filamentous assemblages mayalso retard advective transport of P through the water-columnand act as ‘filters’, removing suspended particulate P from thewater-column (Dodds, 1991).

4.2.2. MicroorganismsBacteria and fungi (hyphomycetes) are abundant in flowingwaters and contribute to P cycling via two mechanisms. Firstly,the decomposition of dissolved, fine particulate and coarseparticulate OM from autochthonous and allochthonous sources(Pusch et al., 1998). Secondly, regulation of the flux of P across thesediment–water interface either throughactiveuptake, alterationof redox conditions at the sediment surface, or production ofrefractory organic P compounds that become terminally buried(Gachter and Meyer, 1993; McDowell, 2003). Microbes bothassimilate and release P as part of the decomposition processproviding an important link (themicrobial loop) in the consumerfood chain within various lotic micro-ecosystems and typicallyresulting in a dominance of respiration (heterotrophy) overphotosynthesis (autotrophy) in most pristine streams (Meyer,1994; Rier and Stevenson, 2001; Dodds, 2007).

Whilst fungal hyphae have a dominant role in the ‘condition-ing’ of coarse leaf litter andwoody debris tomore labile forms forfurther decomposition, heterotrophic bacteria are the mostprolific consumers of fine particulate matter and dissolvedcarbon.Microbial activity therefore relies heavily onOMretentioneither within bed sediments, within biofilms at the benthicinterface, orwithin solids suspended in thewater-column and assuch shows considerable seasonality (Gachter and Meyer, 1993;Lock, 1993; Droppo, 2001). In open stream reaches, autrotrophiccarbon rather than leaf litter breakdown becomes the preferred Csource for heterotrophic bacteria and biofilms (mixtures ofmicrobes, algae and particulate matter within a polysaccharidematrix) become by far themost dominant sites for accumulationof microbial biomass, rapid P cycling and grazing activitydepending on flow, inorganic nutrient supply and temperature(Vannote et al., 1980; Bott and Kaplan, 1985; Pusch et al., 1998;McCutchanandLewis, 2002). The relative importanceofmicrobialP cycling in terms of riverine P flux is therefore unclear anddifficult to quantify.

4.2.3. MacrophytesMacrophytes are an integral part of intermediate and higherorder stream ecosystems and contribute to P cycling via (i) Puptake during the growing season and release of SRP duringdecay; (ii) oxygenation of the water-column and sedimentsinto which they root, (iii) reductions in water current velocitynear the streambed allowing increased sediment deposition,sieving of course particulate OM and fine suspended sedi-ments, and (iv) provision of increased surface area forepiphytic associations and a substrate/refuge for invertebratesand larger grazers (Denny, 1980; Gregg and Rose, 1982;Carpenter and Lodge, 1986; Sand-Jensen, 1998). The relativeinfluence of macrophytes on these processes depends on thespecies and its form (canopy structure), distribution and abun-dance (biomass), the concentration of P in the sediment and toa lesser extent in the water-column, the amount of alloctho-

nous inputs of organic and inorganic particulates and thegeomorphology (particularly the flow regime) of the stream(Gregg and Rose, 1982; Pelton et al., 1998; Flynn et al., 2002; Carret al., 2003; Demars and Edwards, 2007). Consequently, ma-crophytes generally tend to be relatively insensitive to changesin water-column P concentrations, and the relative influenceof macrophytes on P retention through P uptake is generallysmall (1–5% of total P flux) (e.g. House and Casey, 1989).

5. Importance of riverine processes indifferent environments

The balance between physico-chemical and biological processesand their relative importance in terms of P fluxmodification, thetiming of downstream P flux delivery, P fractionation, in-streamconcentrations and ecological impacts vary in space and timewithin the context of a wide range of factors. Some examples ofthe key characteristics of river environments which alter therelative contribution of various in-stream processes are given inTable 3 and are discussed in more detail below.

5.1. Catchment scale, from headwaters to large rivers

Vannote et al. (1980) observed that innate changes in thebiological structures and functioning in rivers often follow asimilar pattern as catchment size increases from headwaterstreams to intermediate streams to large rivers, which theydescribed in the River ContinuumConcept (RCC). This conceptpredicts the relative distribution of producer and consumercommunities in response to the progressive downstream gra-dient in river channel physiography which influences energyand nutrient sources, their availability and assimilation. Inparticular, the RCC recognized that the efficiency of energyand nutrient cycling upstream greatly influences the down-stream ecological response.

More specific studies have examined the longitudinalvariations innutrient retention and cycling along river continuafrom source to basin outlet (e.g. Svendsen et al., 1995;Bukaveckas et al., 2005; Némery and Garnier, 2007). Smallheadwater streams exhibit lower water volume to bed sedi-ment/benthic area ratio, which provides greater potential forboth physico-chemical and biological P exchange processes atthe benthic interface and also within sub-surface sediments(hyporheic zone) (Bukaveckas et al., 2005). The loss of SRP andtransformation to organic P formsbybiological assimilation andbed sediment uptake in headwaters can restrict downstream Psupply and potential ecological response (Svendsen et al., 1995;Bukaveckas et al., 2005). In larger rivers with higher watervolume to bed sediment/benthic ratios, water-column pro-cesses linked to suspended sediments (sorption to metaloxyhydroxides, e.g. Froelich, 1988; Fox, 1993, and co-precipita-tion with calcite, House and Donaldson, 1986; Neal et al., 2002)and uptake by phytoplankton tend to assume greater signifi-cance for P cycling and may considerably augment bedsediment uptake and benthic algal P assimilation. In largerriver systems, P retention within the ‘hydrographic network’,particularly floodplains and reservoirs can be highly significant,accounting for betweenc. 10%and 50%of total P inputs (Némeryand Garnier, 2007).

Table 3 – Prevalence of various attributes and in-stream processes in different stream environments

Streamenvironment

Key attributes and processes for in-stream cycling of phosphorus

Physical Chemical Biological

GeologyPermeable Low stream density Co-precipitation of P with calcite removes

P from the water-columnSubmerged macrophyte species flourish

Large baseflow index; damped flowregime

Carbonate deposits on leaves provide asubstrate for algae and microbes

Biofilms and bed sediments dominate Pexchange within the water-column

Low suspended sedimentconcentrationsHyporheic zone allows transientstorage and increased P retention

Impermeable High stream density Sediment P interactions governed moreby Fe/Al sorption and precipitationreactions

Macrophytes help to reduce flow and trapincoming sedimentsFlashier flow response

High suspended sedimentconcentrations

Stream sizeLow order Low water volume to bed ratio Efficient bed sediment P exchange Greater benthic and epilithic algal growth

(due to light penetration)Shallow streams allow lightpenetration through water-columnto stream bed

Greater self-cleansing capacityGravel substrates provide anchorage pointsfor benthic algae

Gravel and boulder substrates Heterotrophic activity dependent on riparianvegetationWater temperatures more

responsiveHigh order High water volume to bed ratio P exchange dominated more by

interaction with suspended sediments inthe water-column

Phytoplankton become more dominant aswater depth and residence time increases.Poorer degree of light penetration

(deeper water)Smaller self-cleansing capacity

Corresponding decrease in significance of Puptake by benthic algae, periphyton andmacrophytes.

Fine sediment substrate

Submerged macrophytes root in marginalsediments

Water temperature less responsiveLonger water retention timesFloodplains and open waterexpand P retention zones

Trophic stateOligotrophic Good light penetration Terrestrial suspended sediments may

become a P source rather than a P sinkBiofilms dominate internal P cycling

Greater self-cleansing capacityEutrophic Algal growth reduces light

penetrationBed sediments increasingly act as a sinkwhile water-column P concentrationsremain high.

Increasing nutrient loadings leads toproliferation of algal growth and decouplingof the microbial loop.

Smaller self-cleansing capacity Saturation of sediments eventually leadsto P release into the water-column

Macrophytes become colonised by epiphytesand may decrease in abundance

Anoxia in bed sediments leads totemporary release of SRP into the water-column

Phytoplankton increase where residencetimes are low

Vivianite precipitates out P in reducingconditions


Several studies (e.g. Alexander et al., 2000; Bukaveckas et al.,2005) state that the high processing potential of lower-orderstreams accounts for a disproportionate fraction of nutrientuptake within river networks. This is clearly evident in thecomparison of P uptake lengths in Table 2. However, thereappears to be no systematic study of the relative efficiency ofnutrient retention in small shallow vs larger deeper riversystems. In particular, the trade-off between the greater degreeof benthic processing in small shallow streams vs higher waterretention times, proportionally lower benthic processing buthigher water-column P processing and hydrographic network

deposition/floodplain P retention which prevail in larger low-land river systems.

5.2. Catchment geology

Catchment geology exerts a fundamental control on themineralogy and geochemistry of river sediments and thebaseline river water chemistry, which has implications formechanisms of P incorporation into the sediments and theiravailability for inorganic release processes and uptake by biota(House and Denison, 2002; Evans et al., 2004). Catchments


where bedrock weathers to produce clay, or fine silty, soilswith high concentrations of Fe and Al (hydro)oxides thereforemight be expected to generate sediments with high capacityfor P uptake by sediments. Concentrations of Fe and Ca in bedsediments across a range of geological source types in lowlandUK rivers have been found to have positive correlations withtotal P concentrations in the sediments (House and Denison,2002; Demars and Harper, 2002; Evans et al., 2004).

Particle size also appears to exert an important control onphosphate sorption: sedimentswith ahigherproportionof clay-size (b2 µm) particles and associated amorphous Fe and Al hydr(oxide) coatings have a larger specific surface area for P sorption(Horowitz and Elrick, 1987). In waters with relatively low Caconcentrations, amorphous FeIII hydroxides rather than Alhydroxides tend to regulate phosphate concentrations, becauseFehasagreateraffinity forphosphate, FeIII saltshydrolysemorereadily thanAl salts and riverwater is closer to equilibriumwithamorphous FeIII hydroxide than amorphous Al hydroxide (Fox,1993). Although there is considerable literature on interactionsof phosphate with individual components of sediments: Fe andAl hydroxides (Parfitt et al., 1975; Lijklema, 1980) and clays(kaolinite, goethtite, montomorillonite and gibbsite; Bar-Yosefand Rosenberg, 1988; Papadopoulos et al., 1998; Van Emmerik etal., 2007), applicationof results has been limitedbecausenaturalfluvial sediments have highly heterogeneous composition,including complex mixtures of minerals, detrital organicmaterial and microbes. Phosphorus exchange is thereforesubject to more complex controls than those exerted bysediment mineralogical composition alone (Reddy et al., 1995),particularly in flowing waters where microbial and algalbiofilms, hydrodynamic factors, bioturbation, redox conditionsand pH play a critical role in P exchange at the sediment–waterinterface (House, 2003). Indeed for bed sediment P exchange, therole of catchment geology and sediment mineralogy mayassume secondary importance to more localized biologically-mediated physico-chemical effects and hydrodynamic factors.

In catchments with permeable geologies (sandstones, car-bonates), the presence of a hyporheic zone of groundwater/surface water mixing can also have an important influence onin-river P cycling (Dahm et al., 1998). Hyporheic zones arepartially isolated from the flowing water, allowing temporarystorage of streamwater (thus increasing residence time forinteraction within sediments, biota and mixing with ground-water). Mulholland et al. (1997) revealed greater rates ofmetabolism and P uptake within hyporheic zones, linked togreater availability of labile OM and higher rates of microbialdemand for streamwater P. These ‘transient storage zones’ cantherefore be important sites of nutrient cycling and temporary Pretention. In hardwater areas, macrophytes species can utiliseHCO3 in addition to CO2 in the water-column for photosynth-esis, and the carbonate-P deposits that accumulate on macro-phyte leaves in alkaline streams provides additional substratefor algae and microorganisms (Carpenter and Lodge, 1986).

5.3. Channel hydrology, morphology and modifications

River hydrology and geomorphology have a major influenceon P cycling because they control the residence time of waterwithin the reach, the flow velocity and the contact timebetween the streamwater and P exchange surfaces (sediment,

biofilms, and periphyton). River flow velocities vary throughtime, for example in relation to seasonal changes in baseflowvolumes and episodic effects of rainfall events. Flow velocitydistributions vary spatially at the reach scale and along theriver continuum from the headwaters to the basin outlet,linked to interactions between factors such as channel shape,frictional resistance, turbulence and fluid dynamics (Keup,1968). Higher water velocities promote particle entrainmentand transport and reduce water residence times for uptake ofP by bed sediments and biota. Reduced flow velocities favourenhanced nutrient retention as a result of greater waterretention times within a reach, longer contact times betweenstreamwater and substrates, higher rates of periphytongrowth and therefore biological uptake and sedimentation ofparticulates.

At the river reach scale, channel morphology influencesthe spatial distribution of flow velocities and zones of‘transient storage’; areas of short-term water retention, suchas pools, eddies or the partially isolated hyporheic zone,allowing temporary trapping of the flowing water (Buka-veckas, 2007). Zones of transient storage allow higher resi-dence time for interaction within sediments and biota (andmixing with groundwater in the case of the hyporheos) andcan constitute chemical/biological ‘hotspots’ for nutrientcycling and temporary P storage (Meals et al., 1999). However,Hall et al. (2002) showed that, for a forested upland streamwith rapid uptake of nutrients from the streamwater, tran-sient storage accounted for only a small fraction of thevariance in nutrient retention; and Doyle et al. (2003) indicatedthat channel morphology may be of secondary importance forP retention, compared with the high background variability inbiochemical P transformations as a result of heterogeneity insediment type and biology in river systems; they suggest thathydromorphological variability only has an impact at thereach scale where P uptake is very high and/or has limitedvariability.

At the larger catchment scale, deposition and retentionof particulate P on the channel bed and floodplain are ofsignificance in terms of annual catchment P budgets (Wallinget al., 2003, Collins and Walling, 2007). Bed sediment P storagetends to increase downstream as a result of increases inchannel width and surface area of the channel bed and higherwater retention times in lower reaches, which help promotesedimentation. In a small lowland Chalk catchment (Piddle),Collins and Walling (2007) found that bed sediment storageaccounted for up to 57% of the annual SS flux at the catchmentoutlet. However, in-channel storage can assume much lowersignificance in larger river systems; for example in the R. Aireand R. Swale, in-channel storage accounted for b3% of theannual TP flux and a much higher proportion of annual TPlosses (up to15% for the Swale) were to the floodplain (Wallinget al., 2003). Indeed, deposition of particulate P on floodplainsduring overbank flow events provides a much longer-termterm sink for P (from decades to centuries) in lower reaches oflarge lowland river systems.

Channel modifications can have an important impact onthe hydrological and biological properties of rivers, withimplications for nutrient retention and ecosystem function-ing (Osmundson et al., 2000; Doyle and Stanley, 2006). Forexample, channelization (straightening and incising river


channels to improve efficiency of water transit) diminishesthe diversity in velocity and substrates, reducing transientstorage and eliminating the connectivity between the channeland its floodplain at all but the very highest flows, thusreducing the capacity for in-channel and overbank nutrientretention (Bukaveckas, 2007; Gibson and Meyer, 2007).

Weirs and impoundments (including ponds and reservoirs)promote siltationand long-termparticulate P retention (Demarset al., 2005), although large pools of P retained in lakes andbehindweirs and impoundments are subject to biogeochemicalcycling processes and can act as a source of SRP to the water-column (Svendsen et al., 1995), especially if accumulatingsediments are influenced by a point source inputs (e.g. Demarsand Harper, 2002). Weirs, impoundments, ponds and reservoirsconnected to the river, but with much higher water residencetimes, promote growth of phytoplanktonwhichmay thenact asinnocula to the main river, increasing rates of phytoplanktongrowth and biological P uptake (Neal et al., 2006).

5.4. Agricultural land use and other anthropogenic effects

The relative importance of different primary producers (algae,macrophytes) and heterotrophic microbes for P cycling instreams changes along the river continuum depending onland use, organic C inputs, nutrient availability/loadings andenvironmental conditions, particularly light (Vannote et al.,1980). In low-order streamswith riparian forest vegetation thatlimits light penetration, P cycling is driven by heterotrophicactivity that reliesheavily on leaf litter andother organicdebrisfrom the riparian zone (Pusch et al., 1998). In more openoligotrophic waters, land runoff provides the allochthonousinputs of C, N and P required to drive autotrophic and he-terotrophic activity in streams which in many cases in mostlimited by P due to the low concentrations of phosphate innatural systems (Dodds, 2007). As ambient P concentrationsincrease in response to land use change, algal dependency onmicrobial P release disappears and autotrophs become domi-nant (Scott et al., 2008), with the relative abundance of benthicalgae and phytoplankton being determined by the peakflow magnitude/frequency, water residence time, and grazingactivities (Biggs, 2000; Biggs et al., 2000; Hillebrand, 2002). Asthe elemental composition of the periphyton increases, sodoes the invertebrate community that feeds on them, resultingin increased excretion rates of P as invertebrates maintaina strict stoichiometric C:N:P balance (e.g. James et al., 2007).Increased dominance by more adaptive species reducesspecies diversity and studies have shown a general deteriora-tion in the biological integrity and sustainability of streamecosystemswith increasing land use intensification and urbanpressures (e.g. Roth et al., 1996; Harding et al., 1999).

Anthropogenic activities and discharges therefore greatlyinfluenceP cycling through (a) increasing loadings of nutrients,suspended sediment (SS) and OM to streams; (b) alteringstreamhydrology, water residence times and dilution capacityand (c) changing riparian vegetation and shading. Thesedischarges can influence P cycling and biological responseboth locally (e.g. direct cattle access to streams or via farmyarddischarges), or at the larger scale through the intensity of aparticular landusewithin the catchment area (Roth et al., 1996;Buck et al., 2004; Edwards and Hooda, 2007; Niyogi et al., 2007).

In rural catchments with variable intensities of agriculturalland use, land runoff has a varied composition (e.g. Table 1)with substantial quantities of fine-textured inorganic soilparticles and entrained OM, and P in both particulate anddissolved forms. The relative concentrations of dissolved andparticulate P in runoffwill vary depending on landuseand land(soil, livestock and crop) management and with transportpathways and transit times through the landscape and withinthe river channel (Hodgkinson andWithers, 2007;Monaghan etal., 2007). Typically, the export of P in small agriculturalcatchments is often dominated by particulate P, whilstdissolved P forms are prevalent in larger catchments due tothe influence of downstream wastewater discharges andincreased significance of deposition of SS as residence timesdecrease (Jarvie et al., 2006b, 2008c).

Suspended sediments in land runoff enter into sorptionreactions with SRP both during transport and on entering thewater-column and have important localized impacts on waterP concentrations depending on their particle size and Psorption properties (Evans et al., 2004; Stutter et al., 2007). Forexample, McDowell et al. (2002) found that the distribution ofSS derived from adjacent land uses and farms had a markedimpact on sediment equilibrium phosphorus concentrations(EPCo) and P concentrations in theWinnoski River in Vermont,USA, which otherwise tended to naturally decrease down-stream in response to fluvial dynamics which favoureddeposition of fine material with a higher P sorption capacity.Some agricultural systems (e.g. livestock) can lead to highorganic loadings as well as P which impacts on redoxconditions within deposited sediments promoting anoxia. Aswell as causing direct ecological impacts on fish egg survivalrates in gravels (Greig et al., 2005), anoxia leads to reductivedissolution of Fe III compounds and release of SRP from bedsediments, although re-precipitation of FeIII in the water-column may also act as a sink for in-stream SRP (Jarvie et al.,2008a). Concentrated wastewater discharges of P (mostly SRP)greatly increase P uptake by all aquatic communities andstream bed sediments just downstream from STWs (OwensandWalling, 2002; Gucker et al., 2006). When these dischargesare considerably reduced by P stripping, the P-enriched bedsediments then become sources of P to the water-column(Haggard et al., 2005; Jarvie et al., 2006a).

Land use and management also has an important influenceon the ratio of stormflow to base flow in streams. Waterabstraction for crop irrigation reduces summer flow rates andthe dilution capacity of baseflow contributions to streams. Incontrast, removal of the riparian vegetation, introduction oftramlines in fields as part of crop management, installation ofunderdrainage systems and increased density of impervioussurfaces in catchments have all increased the hydrologicalconnectivity between fields and the watercourse leading toflashier runoff, increasedscouring, erosionand siltation (Rothetal., 1996;Withers et al., 2006). Increased flow leads to loss of bothsubstrate and biomass (Biggs and Close, 1989), whilst removingriparian vegetation removes shading and causes an increase inwater temperatures leading to increased autrotrophic activityproviding nutrients are not limiting (Francoeur et al., 1999; Zhouet al., 2005). Such measures can lead to a significant increase(N100%) in P retention capacity where downstream standingwaters need to be protected (Zalewski, 2000).


5.5. Environmental change

Biological communities in flowing waters and the processesthat sustain them are clearly highly sensitive to a number ofenvironmental factors that are influenced by our changingclimate; e.g. flow, temperature, solar (UV) radiation andallochthonous inputs of SS, OM and nutrients. An increase inaverage air temperatures and more extreme variation in theamounts and intensity of precipitation are widely predicted,although the local effects of climate change on the loticenvironment, stream communities and P cycling are still farfrom clear and likely to be quite variable (Schindler, 1997;Meyer et al., 1999; Malmqvist and Rundle, 2002; Huntington,2006). This reflects the interdependent nature of the physicalenvironment and its impact on biotic communities (e.g.temperature vs. discharge effects) and the confoundinginfluences of anthropogenic activities such as effluent dis-charges, land use, water abstraction and species invasion.

The potential consequences of climate change for P cyclingin streams include (i) increasing prevalence of droughts andextreme summer low flows causing a reduction in baseflowdilution capacity, increased P retention during summer asresidence times increase and a greater frequency of anoxia(Caruso, 2002; Van Vliet and Zwolsman, 2008), (ii) changes inmagnitude and frequency of extreme high flows and floodscausing reduced river P retention capacity and net in-channelloss of P under eutrophic conditions, greater seasonalvariability in runoff volumes, carbon and nutrient inputsfrom terrestrial sources (e.g. more winter runoff and lesssummer runoff), scouring of streams and more frequentflushing of storm sewer overflows (Newson and Lewin, 1991;Schindler, 1997; Biggs et al., 2000; Bouraoui et al., 2002;Wilby etal., 2006a), (iii) greater range and higher average air tempera-tures causing warming of water temperatures in shallowstreams, increasing the time window of biological activity,higher rates of primary production, increased soil wetting/drying cycles, greater rates of OM mineralization and greaterdissolved organic carbon (DOC) concentrations reaching thestream with impacts on microbial populations and metabolicrates (Wilby et al., 2006b; Durance and Ormerod, 2007;Harrison et al., 2008). Additional exacerbating anthropogenicinfluences include (a) increasing water abstraction to meetdemands of increasing populations and for crop irrigation willreduce summer flows, (b) expansion of the rural populationwill result inmore ubiquitous effluent discharges (e.g. Evans etal., 2003) and (c) increased intensification of land use (urbanand agricultural) to meet increasing resource demands willincrease P loads in runoff from impervious and pervioussurfaces.

6. Conclusions and implications forcatchment management

In-stream P cycling and retention provide a major ecosystemservice by transforming and regulating downstream deliveryof nutrients (Meyer, 1997; Gibson and Meyer, 2007) and mo-difying the timing of delivery in a way that reduces ecologicalimpacts to downstream reaches (Svendsen et al., 1995). The

variable importance of the specific processes involved in Pcycling varies according to river size, the type of streamenvironment and the supply of autochthonous and allochtho-nous inputs (Table 3). Supplies of P are very varied in terms oftheir composition, mode and timing of delivery; factors whichhave a large influence on how they are assimilated within theriver system and whether their ecological impacts are locali-zed or downstream. In particular, there is a clear distinctionbetween the localized impacts associated with soluble Pdischarges during ecologically sensitive periods and moredownstream impacts associated with particulate P inputstransported during winter storm events. Climate change,expansion of the population, agricultural intensification andincreased water demand for domestic, agricultural and in-dustrial purposes will place increasing pressure on river waterquality, requiring more stringent source control and improvedecosystem management.

Along with controlling nutrient inputs to rivers, thecapacity of rivers to retain and transform excess nutrients isan important aspect of the maintenance of river water qualityto meet biological quality targets and the requirements of theWFD, and to protect and conserve habitats. Many of thestudies of nutrient retention have been conducted in clean/near-pristine river systems in North America (e.g. Meyer andLikens, 1979, Mulholland et al., 1985, 1997, Pelton et al., 1998;Fellows et al., 2006) and comparative studies on pollutedstreams (e.g. Haggard et al., 2001a; Marti et al., 2004) haveindicated that the efficiency of net nutrient retention is greatlydiminished in polluted urban systems. The subsidy-stressthreshold has effectively been exceeded causing a loss inecosystem function and increased risk of downstream eutro-phication. These observations have important managementimplications in relation to preserving and promoting in-stream P retention to protect downstream ecosystems, andthe significance of thresholds and alternative stable states innutrient uptake/release and ecological responses.

Even before a subsidy-stress threshold tips toward reducedefficiency of P retention as nutrient loadings increase, en-hanced rates of primary productivity may be highly effectivein assimilating P, but may result in changes in communitystructure, reductions in biodiversity and degradation ofhabitats for aquatic biota such as invertebrates and fish. Itshould be noted that even highly eutrophic lowland UK riverswith high agricultural and sewage effluent loadings have alarge capacity for P retention (e.g. Boar et al., 1995; Demarset al., 2005; Jarvie et al., 2005, 2006a,b). However, the high levelsof P retention in these lowland rivers by no means negatesthe need to control P inputs from anthropogenic sources,which are required to improve the ecological quality andmeettargets of good ecological status (Demars et al., 2005; Main-stone et al., 2008).

It should also be noted that, for rivers subject to urban,industrial and agricultural influences, nutrient loadings and“subsidy-stress” are by no means the only factors which maycontribute to reduced efficiency of nutrient retention: dischargesof other toxic contaminants may severely impact on theecosystemfunctioningof the river, asmay channelmodificationssuch as channelization, as discussed above. Gibson and Meyer(2007) suggest that channel modifications which enhancebiological activity, such as engineering solutions to increase the


surface area of substrates for biofilm/periphyton growth, increas-ing water residence times and reducing diurnal variabilitythrough management of urban water discharges may provideanopportunity to enhancenutrient retention in urban rivers. Thekey challenge for catchmentmanagers is to balance the compet-ing demands for water abstraction, waste disposal, nutrientassimilation and flood alleviation, with meeting targets fornutrient mitigation, protecting priority habitats and improvingriver ecological status.

Phosphorus cycling encompasses a wide range of processeswhichassumevaryingsignificancealong thecontinuumbetweensmall headwaters streams and large lowland rivers. Moreinformation is needed on the relative importance (magnitude)of theseprocesses indifferentenvironmentsso thatmanagementoptions to increase nutrient retention and protect downstreamreaches can be effectively designed and explored. Effectivemanagement of P and associated issues of siltation and rivereutrophication also requires an understanding of the pathwaysandtimingsof inputs to theriverand their storage transformationand delivery in relation to times of ecological sensitivity andprioritisation and targeting ofmanagement solutions accordingly(Jarvie et al., 2006b; Stutter et al., 2008). For catchment managers,an appreciation of the buffering capacity of rivers to amelioratewater quality by limiting downstream P export during periods ofgreatest ecological risk, does not imply derogation of duty toreduce inputs of P to rivers. On the contrary, evidence frommorepristine stream ecosystems suggests that by reducing P inputs,the retention capacity might even be enhanced (a positive feed-back effect), as riversmove back along the trajectory from ‘stress’to ‘subsidy’. Further research is needed to explore managementoptions to reduce P spiralling lengths and enhance the naturalretention capacity of streams using ‘soft’ engineering solutions,such as creation of debris dams and baffles to increase the rangeof habitats and flow velocities in streams, enhancing the waterretention timesand transient storage,whicharehavebeen linkedto enhanced nutrient retention as well as improved ecologicaldiversity. The relevance of greater P retention efficiency in head-waters for downstream ecological response must therefore bequantified so that the beneficial effects of in-stream processingcan more adequately incorporated into river basin managementplans.

Acknowledgements

We thank the two anonymous referees for their very usefulcomments and contributions to the manuscript.

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Documents

Delivery and cycling of phosphorus in rivers: A reviewudel.edu/~inamdar/nps2007/Withers.pdfDelivery and cycling of phosphorus in rivers: A review P.J.A. Withersa,⁎, H.P. Jarvieb