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 6 9 – 3 7 8

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

www.e l sev i e r. com/ loca te / sc i to tenv

Review

Flow controls on lowland river macrophytes: A review

Paul Franklina,⁎, Michael Dunbarb, Paul Whiteheada

aAquatic Environments Research Centre, Department of Geography, University of Reading, Whiteknights, Reading, RG6 6AB, UKbCentre for Ecology and Hydrology, Wallingford, Oxfordshire, OX10 8BB, UK

A R T I C L E I N F O

⁎ Corresponding author.National Institute ofWfax: +64 7 856 0151.

E-mail address: p.franklin@niwa.co.nz (P. F

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.06.018

A B S T R A C T

Article history:Received 6 April 2008Received in revised form10 June 2008Accepted 12 June 2008Available online 21 July 2008

We review the current status of knowledge regarding the role that flow parameters play incontrolling the macrophyte communities of temperate lowland rivers. We consider bothdirect and indirect effects and the interaction with other factors known to controlmacrophyte communities. Knowledge gaps are identified and implications for themanagement of river systems considered.The main factors and processes controlling the status of macrophytes in lowland rivers arevelocity (hence also discharge), light, substrate, competition, nutrient status and rivermanagement practices. We suggest that whilst the characteristics of any particularmacrophyte community reflect the integral effects of a combination of the factors,fundamental importance can be attributed to the role of discharge and velocity incontrolling instream macrophyte colonisation, establishment and persistence. Velocityand discharge also appear to control the relative influence of some of the other controllingfactors.Despite the apparent importance of velocity in determining the status of macrophytecommunities in lowland rivers, relatively little is understood about the nature of theprocesses controlling this relationship. Quantitative knowledge is particularly lacking.Consequently, the ability to predict macrophyte abundance and distribution in rivers is stilllimited. This is further complicated by the likely existence of feedback effects between thegrowth of macrophytes and velocity.Demand for water resources increases the pressure on lowland aquatic ecosystems. Despitegrowing recognition of the need to allocate water for the needs of instream biota, theinability to assess the flow requirements of macrophyte communities limits the scope toachieve this. This increases the likelihood of overexploitation of the water resource as otherusers, whose demands are quantifiable, are prioritised.

© 2008 Elsevier B.V. All rights reserved.

Keywords:MacrophytesFlowDischargeVelocityLowland riversHabitat

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3702. Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3703. Factors controlling aquatic plants in lowland rivers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

ater and Atmospheric Research, PO Box 11 115, Hamilton, New Zealand. Tel.: +64 7 859 1882;

ranklin).

er B.V. All rights reserved.

370 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 6 9 – 3 7 8

3.1. The effects of flow on macrophytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3713.1.1. Hydrological regime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3713.1.2. Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

3.2. Other controls on the growth of instream macrophytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3723.3. Feedback relationships . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374

4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375

1. Introduction

Freshwater environments face increasing pressures from factorssuch as land use change associated with urbanisation andagriculture, point and diffuse pollution, increased demand forwater supply to support population growth and greater uncer-tainties related to climate change (Wheater et al., 2006). There isalso increasing recognition of a responsibility to maintain riverflows for the needs of the biota (e.g. Acreman and Dunbar, 2004).These, often conflicting, pressures are a challenge for catchmentmanagement and are particularly acute in lowland rivers wheredevelopment pressures are often greatest.

Aquatic macrophyte communities constitute a fundamen-tal component of many lowland river ecosystems. They cancontribute significantly to the physical and biological diver-sity, habitat structure and ecological functioning of thesesystems (Haslam, 1978; Hearne and Armitage, 1993; Baattrup-Pedersen et al., 2002; Mainstone and Parr, 2002; Hatton-Elliset al., 2003; Lacoul and Freedman, 2006). Understanding thekey processes and factors that control and regulate thedynamics of river macrophytes is therefore important forfurthering knowledge of the structure and functioning oflowland river communities as a whole.

In this paper we review current understanding of the mainprocesses controlling the status of macrophytes in temperatelowland rivers. In particular we consider the role of watervelocity in determining macrophyte growth dynamics inriverine environments and reflect on the potential implica-tions for management of lowland rivers.

2. Background

Within temperate lowland freshwater habitats macrophytesare considered to be fundamental to the structure andfunctioning of the ecosystem and are an important controlover the ecological stability of these systems (Butcher, 1933;Sand-Jensen, 1998; Baattrup-Pedersen and Riis, 1999; Pedersenet al., 2006; Westwood et al., 2006). River plants are sensitiveindicators of the conditions in which they live (Haslam, 1978;Flynn et al., 2002) and the physical stream environment has amajor impact upon the development and growth of submersedmacrophytes (Barko et al., 1986; Stephan and Wychera, 1996;Baattrup-Pedersen and Riis, 1999; Lacoul and Freedman, 2006).

Some of the most important physical variables controllingmacrophytes include discharge and velocity (Butcher, 1933;Haslam, 1978; Wilby et al., 1998; Riis and Biggs, 2003), lightavailability (Sculthorpe, 1967; Barko et al., 1982; Sand-Jensen,1989), substrate (Butcher, 1933; Barko and Smart, 1986; Gurnell

et al., 2006) and nutrient availability (Carignan and Kalff, 1980;Madsen and Sand-Jensen, 1991; Jones et al., 2002; Mainstoneand Parr, 2002). Superimposed upon this are biotic interac-tions, principally competition (Barko et al., 1986; Wilby et al.,1998; Flynn et al., 2002; Jones et al., 2002) plus physical rivermanagement practices (Dawson, 1976; Ham et al., 1982a;Sand-Jensen et al., 1989; Baattrup-Pedersen and Riis, 2004;Pedersen et al., 2006). However, Sculthorpe (1967) points outthat as much as a plant community is a product of itsenvironment, the environment is also partially a product ofthe plant communities that reside there. Aquaticmacrophytescan act as “biological engineers” substantially altering thestreamenvironment (Gregg and Rose, 1982; Sand-Jensen, 1998;Cotton et al., 2006; Wharton et al., 2006) affecting velocities(Watson, 1987; Gurnell and Midgley, 1994; Riis and Biggs, 2003;Green, 2005a), water depth (Wilcock et al., 1999; Champion andTanner 2000), sediment patterns (Sand-Jensen and Madsen,1992; Riis, 2000; Baattrup-Pedersen et al., 2002), water quality(Sculthorpe, 1967; Riis and Biggs, 2003) and providing struc-tural habitat diversity (Westlake, 1975; Hearne and Armitage,1993; Baattrup-Pedersen et al., 2003). The key role of aquaticmacrophytes in structuring and influencing the streamenvironment and ecology emphasises the importance ofknowledge of the factors regulating them (Riis, 2000).

Anthropogenically induced changes in flow regimes (Petts,1984; Wright and Berrie, 1987; Wright et al., 1999; Riis andSand-Jensen, 2006), stream structural diversity (Baattrup-Pedersen and Riis, 1999; Baattrup-Pedersen et al., 2002) andnutrient status (Cranston and Darby, 2002; Mainstone andParr, 2002), coupled with climate variability (Flynn et al., 2002;Hatton-Ellis et al., 2003), have contributed to an overall declinein macrophyte species richness and diversity and a homo-genisation of communities within European and other low-land streams in recent decades (Riis and Sand-Jensen, 2001;Baattrup-Pedersen et al., 2003; Hatton-Ellis et al., 2003). Thekey role that macrophytes play in the structure and functionof lowland streams implies that such changes in macrophyteassemblages could have far reaching consequences for therest of the stream biota and that if one is to be preserved, theother must be also. In order to both prevent further deteriora-tion and support restoration of instream communities, suchas required by the EUWater Framework Directive (2000/60/EC),it is necessary to identify the dominant drivers of the observedchanges. The key to achieving this is to recognise andunderstand the fundamental processes controlling the statusof aquatic macrophytes in rivers.

Whilst the dynamics of macrophyte communities in lakeshave received considerable attention in the scientific litera-ture, riverine communities have been comparatively

371S 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 6 9 – 3 7 8

neglected (Sand-Jensen et al., 1989; Riis and Biggs, 2003; Hiltonet al., 2006). Research on lake systems has typically empha-sised the importance of light availability (Barko et al., 1982;Chambers, 1987; Schwarz et al., 2000; Herb and Stefan, 2003;Zhu et al., 2008), nutrient status (Barko and Smart, 1986;Chambers, 1987; Lau and Lane, 2002) and biological interac-tions (Phillips et al., 1978; Scheffer et al., 1993; Lau and Lane,2002; Jones and Sayer, 2003), amongst other factors, indetermining macrophyte community establishment, compo-sition and persistence. In riverine environments it is, however,argued that the importance of these factors in determiningmacrophyte community characteristics and dynamics isoverridden by the influence of water velocity.

Water movement has long been proposed as a prime factorregulating the growth and distribution of aquaticmacrophytesin rivers (Butcher, 1933; Haslam, 1978; Wilby et al., 1998; Largeand Prach, 1999; Madsen et al., 2001). It has been argued that,assuming the availability of plant seeds or propagules forcolonisation, discharge and velocity affect, and may entirelydetermine, the species present in a habitat, their performanceand the interactions between them (Haslam, 1978; Riis andBiggs, 2003). Despite this recognition, relatively little work hasbeen undertaken to assess the nature of the relationshipsbetween flow parameters and instream macrophytes. In thefollowing sectionswe assimilate and review the current statusof knowledge regarding the role that flow parameters play incontrolling the instream macrophyte communities of tempe-rate lowland rivers. We consider both direct and indirecteffects and the interaction with other factors known to impactonmacrophyte communities.We identify knowledge gaps andconsider implications for the management of river systems.

3. Factors controlling aquatic plants in low-land rivers

3.1. The effects of flow on macrophytes

The effects of flow onmacrophytes are typically considered asa function of discharge and/or velocity. The effects may bedirect through mechanical damage and uprooting (Butcher,1933; Madsen et al., 2001), or indirect due to changes in, forexample, sediment characteristics (Haslam, 1978; Madsenet al., 2001; Riis and Biggs, 2003), nutrient and gas exchanges(Sand-Jensen, 1989; Madsen et al., 2001) and competitiveinteractions (Ham et al., 1981; Wilby et al., 1998; Flynn et al.,2002). Unfortunately, the term flowhas often been usedwithinthe literature, without clear definition, tomean both dischargeand/or velocity. This can lead to confusion and ambiguity,therefore discharge (volume per unit time) and velocity(distance travelled per unit time)will be used here tominimiseany potential ambiguity.

3.1.1. Hydrological regimeThe hydrological regime of a river is considered to be of majorimportance to the vegetation, affecting both the speciespresent and their abundance (Biggs, 1996; Wilby et al., 1998;Riis and Biggs, 2003; Riis et al., 2008). The plant species presentin any particular reach will be those that can, in the long run,tolerate the full range of discharge conditions that character-

ise the reach (Haslam, 1978). It has been suggested thatsuccessful colonisation of macrophytes is controlled by floodfrequency because macrophyte immigration and growth ratesare relatively slow, and thus prolonged periods of hydrologicalstability are required for macrophyte propagules to arrive anddevelop to substantial levels of cover (Biggs, 1996; Wilby et al.,1998; Riis and Biggs, 2003; Riis et al., 2008). Riis and Biggs (2003)proposed that significant macrophyte development in NewZealand rivers was restricted to streamswhich experienced anaverage of less than 13 flood events per year (where a flood isdefined as an event exceeding 7 times the median discharge).Furthermore, vegetation abundance and species diversitywithin stream reaches in New Zealand were negativelycorrelated with flood frequency. For Danish streams, Riiset al. (2008) also found a relationship between vegetation coverand flood events. However, in Danish streams the magnitudeof flood event with the strongest relationship to vegetationcover was of a lower intensity (NQ25) compared to NewZealand. They proposed that this was a result of moresandy, and thus less stable, substrates in Danish streamsand the lower frequency of high magnitude flood events.

It has also, however, been shown that low flow eventscaused by drought can have a negative impact upon macro-phyte success with the seasonal patterns in growth of Ranun-culus spp. observed to change under drought conditions(University of Reading, 1977). The seasonal wetting and dryingof ephemeral winterbourne rivers, where on average dis-charge only occurs from late autumn/winter to early spring,have also been shown to have a significant impact upon theresident macrophyte communities. The communities areshown to become adapted to the periodic drying of the riverchannel (Westwood et al., 2006). There is also evidence linkingmacrophyte growthwith the discharge regime at certain timesof the year. For example, the successful growth of Ranunculusspp. in UK chalk streams has been linked to higher dischargein spring and summer, and specifically summer flushingevents that can remove competing algal communities (Hamet al., 1981; Ham et al., 1982b; Wright et al., 1982; Wilby et al.,1998). The hydrological regime has therefore been illustratedto exert a significant influence over the establishment andsuccess of instream aquatic plant communities.

3.1.2. VelocityOnce communities are established, Riis and Biggs (2003)argued that reach scale velocity should be an importantcontroller of local biomass. Several studies have argued thatwater velocity is the main factor in regulating aquaticmacrophyte distribution, composition, biomass and metabo-lism in rivers (Butcher, 1933; Westlake, 1967, 1975; Haslam,1978; Barko et al., 1986; Marshall and Westlake, 1990;Chambers et al., 1991; Cranston and Darby, 2002; Dodds andBiggs, 2002; Riis and Biggs, 2003). Typically both the abundanceand diversity ofmacrophytes are stimulated at low tomediumvelocities, and growth restricted at higher velocities (Madsenet al., 2001). Riis and Biggs (2003) illustrated a negativequadratic relationship in hydrologically stable streamsbetween vegetation abundance and mean water velocity invegetation free space, with vegetation abundance peaking at0.3–0.5 m s−1. They hypothesised that this relationship is areflection of the conflicting processes of mass transfer and

372 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 6 9 – 3 7 8

drag disturbances. Biggs et al. (2005) discuss this interaction ingreater detail, suggesting that drag disturbance processes aremost strongly related to higher magnitude, longer returninterval, discharge events, whilst mass transfer processes arecontrolled by low magnitude, high frequency events understable discharge conditions.

Water velocity has an indirect effect on photosynthesisthrough the regulation of the availability of dissolved substances(Westlake, 1967; Madsen and Søndergaard, 1983; Sand-Jensen,1989; Madsen et al., 2001). It has been shown that the photo-synthetic and nutrient uptake rates of freshwater macrophytesare positively correlated with low water velocities (0–0.1 m s−1)(Westlake, 1967; Madsen and Søndergaard, 1983; Marshall andWestlake, 1990; Madsen et al., 2001). This positive relationship atlow velocities is related to the impacts of velocity on masstransfer: plantmetabolism is chiefly limitedby the rate of transferof carbon to the leaf surfaces (Westlake, 1967). As velocityincreases, the diffusive boundary layer around the plant surfacesis gradually eroded. This reduces the restriction on the flux ofmetabolites and dissolved substances to the plant for use inphotosynthesis, thus enhancing growth (Westlake, 1967, 1975;Sand-Jensen, 1989; Madsen and Sand-Jensen, 1991; Madsen et al.,2001; Hatton-Ellis et al., 2003). At higher water velocities, theeffects of water movement on photosynthesis appear to reduceand stabilise (Westlake, 1967). Madsen and Søndergaard (1983)also reported a slight reduction in photosynthetic rates of Calli-triche stagnalis as velocity was increased to 0.04 m s−1 if plantstrands became agitated in the water column.

Aswatervelocity increases the florabecomesmore restrictedto species capable of resisting the current and when velocity israpid only a few species can survive (Westlake, 1975; Riis andBiggs, 2003). Chambers et al. (1991) suggest that when velocityexceeds 1 m s−1, aquatic macrophytes are only present innegligible quantities or are completely absent. The increasingvelocity leads to physical andmechanical stresses on the plantsdue to increased drag forces, which can lead to stem breakageand uprooting (Butcher, 1933; Haslam, 1978; Riis and Biggs,2003). The species most susceptible to the effects of increasedvelocity are those with high hydraulic resistance and lowanchoring strength e.g. Myriophyllum spicatum and Rorippanasturtium aquaticum (Haslam, 1978). In response to this effect,many aquatic plants have morphological adaptations and thecapability to re-configure to reduce their susceptibility to thedrag forces e.g. several Ranunculus species and Sparganiumemersum (Sand-Jensen, 2003; O'Hare et al., 2007).

Water velocity can also have notable indirect impacts uponinstream macrophyte communities through its effects onother factors that regulate macrophyte establishment, growthand persistence. Substrate (Haslam, 1978; Madsen et al., 2001;Riis and Biggs, 2003) and the competitive interaction withepiphytes (Ham et al., 1981, 1982b; Wilby et al., 1998) are, forexample, known to be influenced by variations in velocity.These indirect effects will be considered where appropriate inthe following section discussing the other significant controlson the growth and success of macrophytes.

3.2. Other controls on the growth of instream macrophytes

Whilst hydrological controls are considered significant indetermining the status of macrophytes in rivers, it is acknowl-

edged that a wide variety of other environmental factors canalso act to influence the distribution and abundance of aquaticplants in these systems. Lacoul and Freedman (2006) provide abroad review of the environmental influences on aquaticplants in freshwater systems, but in the context of temperate,lowland river ecosystems the most important controls can beconsidered to be light availability (Sculthorpe, 1967; Barkoet al., 1982; Sand-Jensen, 1989), substrate (Butcher, 1933; Barkoand Smart, 1986; Gurnell et al., 2006), nutrient availability(Carignan and Kalff, 1980; Madsen and Sand-Jensen, 1991;Jones et al., 2002; Mainstone and Parr, 2002), competition(Barko et al., 1986; Wilby et al., 1998; Flynn et al., 2002; Joneset al., 2002) andmanagement (Dawson, 1976; Hamet al., 1982a;Sand-Jensen et al., 1989; Baattrup-Pedersen and Riis, 2004;Pedersen et al., 2006). Their influence must therefore beconsidered alongside that of discharge and velocity.

The quality and quantity of light available within the riversystem have an important influence on the growth anddevelopment of submerged macrophytes (Owens and Edwards,1961; Barko et al., 1986; Chambers, 1987; Sand-Jensen et al., 1989).Macrophytes require light for photosynthesis, thus if lightavailability is limited e.g. by turbidity or epiphyte growth, therewillbea resultingnegative impactongrowthrates (Haslam,1978;Barko et al., 1982; Sand-Jensen, 1989; Large and Prach, 1999).

Due to the link between light availability and photosynth-esis, seasonal changes in biomass are found to be dependenton the availability of light and seasonal changes in thephotoperiod (Barko et al., 1986; Haury and Aidara, 1999).However, superimposed upon the general trend can be morelocal effects of shading which may be caused by siltation(Haslam, 1978; Ham et al., 1982b), epiphyte growth (Ham et al.,1981; Flynn et al., 2002; Hilton et al., 2006), the presence ofriparian vegetation (Ham et al., 1982b; Wright et al., 2003) oreven the presence of other submerged macrophytes(Sculthorpe, 1967; Ham et al., 1981).

Macrophyte species are also shown to vary in theirresponses to sediment conditions (Barko and Smart, 1986).Haslam (1978) suggests that, all other factors being equal,similar types of substrate, if equally stable, will bear similarvegetation. Substrate stability is a significant controllingfactor because a stable substrate allows rooting and establish-ment of macrophyte communities (Butcher, 1933; Haslam,1978; Riis and Biggs, 2003). Mobile substrates prevent this,resulting in a limited potential for community development.Even once established, communities can be damaged oruprooted by sediment erosion if the soil around the roots issufficiently disturbed (Haslam, 1978; Riis and Biggs, 2003).

Aquatic plants are also shown to respond to sedimentcomposition. Different macrophyte species are known to beassociated with different substrata types (Barko et al., 1986;Barko and Smart, 1986; Clarke and Wharton, 2001). Addition-ally, whilst capable of obtaining nutrients from the watercolumn, rooted macrophytes obtain most of their requirednitrogen and phosphorus from the sediments (Carignan andKalff, 1980; Carpenter and Lodge, 1986; Chambers et al., 1991;Carr and Chambers, 1998; Clarke andWharton, 2001) and thusvariation in sediment composition has the potential to impacton macrophyte productivity (Madsen et al., 2001).

It appears therefore that the substrate plays an importantrole in controlling the distribution and establishment of

373S 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 6 9 – 3 7 8

aquatic plant communities. However, the spatial couplingsand causal relationships between macrophyte communitiesand substrate conditions are perhaps more complex thaninitially apparent due to the interactions betweenmacrophytegrowth and the physical environment (Baattrup-Pedersen andRiis, 1999). Velocity is an important control on substratestability and composition and submerged macrophytes arealso capable of affecting a pronounced change in sedimentcomposition (Barko et al., 1986; Marshall and Westlake, 1990;Cotton et al., 2006; Wharton et al., 2006). Therefore, althoughmacrophyte species have been shown to be associated withdifferent substrata, this may be coincidental, instead reflect-ing differences in velocity tolerances between species and theeffects of macrophytes on instream hydraulics and sedimen-tation processes.

The assessment of the effects of nutrients on the status ofmacrophytes in running waters is confounded by the syner-gistic effects of the other environmental and biotic variablesthat affect growth (Carignan and Kalff, 1980; Demars andHarper, 1998; Clarke and Wharton, 2001; Cranston and Darby,2002). Consequently conclusions regarding the effects ofnutrients on plant growth, abundance and distribution areambiguous (Demars and Harper, 1998; Hilton et al., 2006).

Evidence for the onset of eutrophication in many lowlandriver systems has focused research on the impacts of nutrientenrichment on aquatic communities. The effects of eutrophi-cation on aquatic macrophytes are documented as usuallycausing a shift in community composition and an increase inbiomass (Hatton-Ellis et al., 2003). However, enrichment of thewater and/or sediments has been shown in some situations tohave limited or no effect on growth rates (Brierley et al., 1989;Madsen and Cedergreen, 2002), or to result in a degradation oreven loss of the higher plant community to the expense ofalgal dominated systems (Spink et al., 1993; Cranston andDarby, 2002; Mainstone and Parr, 2002).

Hilton et al. (2006) argue that there is currently noconceptual understanding of how eutrophication develops inrivers, with the majority of studies focused on standing waterbodies. They present a conceptual model based on retentiontime. They hypothesise that rivers with short retention timesrespond to increasing nutrients by increased growth ofbenthic and epiphytic algae, whereas rivers with long reten-tion times are dominated by phytoplankton. In order forphytoplankton levels to reach sufficient concentrations toachieve dominance and drive the positive feedback requiredto inhibit macrophyte populations, as has been seen in lakesystems, retention times need to be greater than cell doublingtime. However, this is not often encountered in river systemsand thus they hypothesise an alternative path resulting in thedominance of benthic and epiphytic algae in rivers. This ideais supported by the evidence presented by Spink et al. (1993),Flynn et al. (2002), Mainstone and Parr (2002), Cranston andDarby (2002), and Neal et al. (2006).

Biotic interactions can also be important in influencingmacrophyte communities under certain conditions (Haslam,1978). Competition between species can impact upon spatialarrangement and dominance patterns within a community(Dawson et al., 1978; Cranston and Darby, 2002). One of the keycompetitive relationships identified as impacting on macro-phyte dynamics is the interaction with epiphytic algal

communities (Barko et al., 1986; Wilby et al., 1998; Flynnet al., 2002;Wade et al., 2002a; Riis and Biggs, 2003, Hilton et al.,2006). Epiphytic algae accumulate on the surface of leaves,suppressing growth through restriction of carbon uptake andshading of the plant surfaces. However, the importance of thisrelationship is still apparently strongly influenced by environ-mental conditions, with velocity, retention time and nutrientstatus key controlling factors (Biggs and Close, 1989; Horneret al., 1990; Flynn et al., 2002; Wade et al., 2002b; Hilton et al.,2006). Ham et al. (1981, 1982b) andWilby et al. (1998) identifiedrelationships between velocity and the interactions betweenmacrophyte and epiphyte growth in lowland rivers of the UK.Under lowwater velocity conditions, epiphytic algaewere ableto colonise macrophyte leaves, limiting growth by restrictinglight availability to the leaf surface and consequently reducingphotosynthesis. However, higher velocities appeared to limitthe development of epiphytic algal blooms, leading toincreased macrophyte growth. Jones et al. (2002) have alsosuggested that the density of grazing invertebrates could be animportant factor in controlling competitive impacts of algalcommunities. Grazing birds (Mitchell and Perrow, 1998) andfish (Rowe and Schipper, 1985) have also been shown to havelocalised impacts on macrophytes.

Inter-specific competition between different macrophytescan also influence community character. The presence of aspecies at a particular location requires it to be able to grow aswell as or better than other species present (Haslam, 1978).However, within riverine communities this is again generallydetermined by the ability of a species to cope with theprevailing environmental conditions rather than direct com-petition. For example, Berula erecta is more shade tolerant thanRanunculus spp. and thus in shaded areas Berula can overcomethe normal dominance of Ranunculus (Wright et al., 1982). Itmay also reflect the superior ability of a species to recoverfrom disturbance, either natural or anthropogenic, and thushigh growth rates and good dispersal ability can be of keyimportance (Riis and Sand-Jensen, 2006).

Overall, the natural disturbance regime driven by hydro-logical variability maintains communities at an earlier succes-sional stage where inter-specific competitive strategy is muchless important in rivers compared to lentic or terrestrialenvironments (Haslam, 1978; Sand-Jensen et al., 1989;Baattrup-Pedersen and Riis, 1999).

The management of freshwater riverine ecosystems canhave a profound effect on aquatic plant communities andPedersen et al. (2006) argue that the influence of habitat onmacrophyte communities is suppressed in streams withfrequent management. Management effects may either bedirect i.e. the removal of plants for flood defence, navigation orfisheries maintenance purposes (Soulsby, 1974; Ham et al.,1982a; Wade, 1994; Wilby et al., 1998), or indirect, for examplethrough the impacts of flow regulation on plant communities(Petts, 1984; Wright and Berrie, 1987).

Management of macrophytes in streams can be consideredas a disturbance imposed upon a system that is naturallydisturbed (Baattrup-Pedersen and Riis, 2004). The effects ofmanagement will depend on the type of disturbance, char-acterised by its frequency and intensity, and the ability of theecosystem to recover, its resilience (Sand-Jensen et al., 1989;Milner, 1994; Underwood, 1994). The greater the frequency and

374 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 6 9 – 3 7 8

intensity of the disturbance and the lower the resilience of thesystem, the greater the impact is likely to be (Milner, 1994).

Frequent disturbance through management is likely tokeep plants in an early successional stage and reduce thelikelihood that the macrophyte community will reflect theoutcome of competitive interactions (Sand-Jensen et al., 1989;Baattrup-Pedersen and Riis, 1999). One of the key features indetermining the response of the community to amanagementdisturbance is its resilience. This means that importantspecies traits, for macrophytes that are cut or disturbedregularly are rapid growth, high dispersal capability andshort life cycles (Wade 1994; Baattrup-Pedersen et al., 2002;Riis and Sand-Jensen 2006).

Management in the form of macrophyte cutting is acommon component of river management regimes and willinfluence the maximum area and biomass achieved (Wrightet al., 1982; Wade, 1994). Baattrup-Pedersen et al. (2002)suggest that comprehensive weed cutting over periods ofyears can help explain the general decline in species diversityin Danish streams. Pronounced negative impacts on macro-phyte diversity and composition have been shown (Riis et al.,2000; Baattrup-Pedersen et al., 2002; Baattrup-Pedersen et al.,2003). It has also been illustrated in the UK that the type andtiming of cutting can have a significant impact on thecommunity response changing growth patterns and max-imum biomass (University of Reading, 1977; Ham et al., 1982b).

Given the key role of water velocity in influencing macro-phytes, manipulation of the discharge regime through regula-tion, augmentation or abstraction is also likely to have animpact on aquatic plant communities (Petts, 1984). Baattrup-Pedersen and Riis (1999) show that whilst total macrophytecoverage may be similar in regulated and unregulatedstreams, species richness and diversity are greater and thespatial distribution of macrophytes on the stream bottom ismore heterogeneous in unregulated streams.

3.3. Feedback relationships

It is widely recognised that as well as being controlled mainlyby velocity, plants also alter the velocity pattern of the streamand can produce substantially different conditions in streamhabitat compared to unvegetated reaches (Butcher, 1933;Haslam, 1978; Gregg and Rose, 1982; Stephan and Wychera,1996; Green, 2005a). Seasonal increases in biomass changewater velocity and modify velocity patterns (Marshall andWestlake, 1990; Hearne and Armitage, 1993; Cotton et al., 2006;Gurnell et al., 2006; Wharton et al., 2006). As macrophytebiomass proliferates, water resistance increases, restrictingdischarge, increasing depth, reducing mean velocity andchanging sediment dynamics (Dawson, 1976; Powell, 1978;Dawson and Robinson, 1984; Watson, 1987; Sand-Jensen et al.,1989; Wilby et al., 1998; Champion and Tanner, 2000; Cottonet al., 2006; Green, 2006; Gurnell et al., 2006; Wharton et al.,2006).

The key factors governing the impact of aquatic plants onphysical habitat conditions are their structural propertiesincluding size (Green, 2005b); morphology (Stephan andWychera, 1996; Large and Prach, 1999; Dodds and Biggs, 2002;Green, 2005a); flexibility (Green, 2005a; Gurnell et al., 2006);turgidity (Powell, 1978; Watson, 1987); density (Large and

Prach, 1999); spatial arrangement (Fisher and Reeve, 1994;Green, 2006) and biomass (Dawson, 1978; Stephan andWychera, 1996). As structural complexity, biomass and spatialheterogeneity increase, resistance is increased through theintroduction of turbulence and greater drag forces (Large andPrach, 1999). This effect is shown to be greater at lowerdischarges because the proportion of the stream that isimpacted is higher (Watson, 1987; Sand-Jensen et al., 1989).

Whilst these effects are widely acknowledged, they areoften only described qualitatively and understanding of theprecise relationships, particularly with respect to large aquaticplants such as those common in lowland rivers, is relativelylimited (Dawson, 1978; Watson, 1987; Madsen et al., 2001;Fisher and Dawson, 2003). These effects also raise the issue ofthe potential for important feedback relationships to occur,whereby the growth of macrophytes affects velocity, which isin itself also an important control on macrophyte growth.Such feedback processes have been observed to operate inbeds of Ranunculus spp. in UK rivers (Cotton et al., 2006;Franklin, 2007). Where individual Ranunculus stands amalga-mate into a large homogenous stand, velocity is reducedthroughout the reach and there is a negative impact on Ra-nunculus growth. However, where plant stands remain sepa-rated in a heterogeneous mosaic, velocity around the plants isobserved to accelerate in compensation to the flow restrictioncaused by plants, resulting in improved growth of Ranunculus.The changes in velocity induced by macrophyte growth alsoinfluence local sediment dynamics in terms of stability andcomposition (Barko et al., 1986; Marshall and Westlake, 1990;Cotton et al., 2006;Wharton et al., 2006). This could in turn alsolead to changes in the structure and functioning of macro-phyte communities via influences on nutrient dynamics andsubstrate stability. There are few studies documenting orinvestigating these important potential feedback effects, butthe occurrence of these effects has important consequencesfor understanding the growth of plants and for the ecology ofstreams in general (Dodds and Biggs, 2002), especially in riverswith an artificially modified discharge regime.

4. Discussion

As pressures on water resources increase, the importance ofbeing able to understand and predict the likely response ofaquatic systems to increased exploitation becomes greater.The important role of macrophytes in the structure andfunctioning of lowland river ecosystems therefore makesunderstanding the factors controlling the status of macro-phytes essential for developing future management strategiesfor these systems.

The main factors and processes involved in controlling thestatus ofmacrophytes in temperate lowland rivers are typicallyconsidered to include: discharge/velocity (e.g. Butcher, 1933;Haslam, 1978;Wilby etal., 1998; Riis andBiggs, 2003), light/shade(e.g. Sculthorpe, 1967; Barko et al., 1982; Sand-Jensen, 1989),substrate/siltation (e.g. Butcher, 1933; Barko and Smart, 1986;Gurnell et al., 2006), competition (e.g. Barko et al., 1986; Wilbyet al., 1998; Flynn et al., 2002; Jones et al., 2002), nutrient status(e.g. Carignan and Kalff, 1980; Madsen and Sand-Jensen, 1991;Jones et al., 2002; Mainstone and Parr, 2002) and management

375S 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 6 9 – 3 7 8

(e.g. Dawson, 1976; Ham et al., 1982a; Sand-Jensen et al., 1989;Baattrup-Pedersen and Riis, 2004; Pedersen et al., 2006). In thisreview, we have considered current knowledge of the relativeroles of each of these main factors in determining the status ofmacrophytes in temperate lowland river systems. The evidencesuggests that whilst the characteristics of any particularmacrophyte community reflect the integral effects of a combi-nation of the factors, fundamental importance can be attributedto the role of discharge and velocity in controlling instreammacrophyte colonisation, establishment and persistence.

Assuming that the availability of viable seeds or propagules isnot limited, the hydrological regime of a river is likely to be theprimarycontrol on theestablishmentandsuccessofmacrophytecommunities, with reach scale velocity characteristics acting todetermine local scale variability in biomass and speciesdynamics. In addition to these direct effects, it also seems likelythat discharge and velocity play an important indirect role indetermining macrophyte community composition within low-land river reaches through their influence in controlling therelative impact of other factors.Ofparticularnote is the influenceon substrate stability, character and composition and on thecompetitive balance between macrophytes and epiphytes.

Despite the apparent importance of water movement inregulating macrophyte communities in temperate lowlandrivers, the nature of the processes and the factors controllingtheir dynamics are not well understood. Existing data on theinteractions between instream velocity and plants show greatdifferences between rivers, sites, growth forms and species,both spatially and temporally. Consequently, the ability topredict macrophyte abundance and distribution in rivers isstill relatively limited.

The mechanisms behind the hydrological control ofmacrophyte biomass can be divided into biomass loss andbiomass gain processes (Haslam, 1978; Biggs et al., 2005; Riiset al., 2008). Loss processes are caused by increased shearstress resulting in breakage and dislodgement of plants, whilstbiomass gain processes i.e. colonisation and growth aretypically optimised under stable conditions. Discharge mag-nitude, variability, timing and frequency will all thereforecontribute towards determining the relative balance betweenthese biomass loss and gain processes and thus the characterand dynamics of riverinemacrophyte communities. Dischargemagnitude and flood frequency have often been found toaffect the distribution of macrophytes (e.g. Riis and Biggs,2003; Riis et al., 2008). However, little attention has been givento the timing of high or low discharge periods and eventsduring the year, which have the potential to have animportant influence on community dynamics (e.g. Wilbyet al., 1998). There is therefore a need to improve theunderstanding of how different components of the hydro-logical regime and their temporal dynamics can influencemacrophyte communities. As well as improving understand-ing of these broader scale controls onmacrophyte communitystructure and composition, there is a need to enhanceknowledge of the mechanisms by which the effects of thesecontrols are manifested at the local reach scale. This requiresbetter understanding of the interactions between macro-phytes and instream velocity.

Many of the interactions between macrophytes andvelocity, describing the processes of biomass gain and loss,

have been expressed qualitatively (e.g. Butcher, 1933; Haslam,1978; Cranston and Darby, 2002), but typically quantitativeknowledge is relatively limited and/or restricted to artificialenvironments under laboratory conditions (e.g. Stephan andGutknecht, 2002; Sand-Jensen, 2003). In the past, progress insome cases has been limited by the capabilities and resolutionof available methods for measuring and modelling fine scaletemporal and spatial variations in water velocity in andaround vegetation. However, advancements in methodssuch as 3D acoustic Doppler velocimetry, hot wire anemome-try (Sand-Jensen and Mebus, 1996) and computational fluiddynamics modelling approaches (e.g. Naden et al., 2004) nowallow better characterisation of the interaction dynamics.Progress now needs to be made in improving quantification ofthe interactions between velocity and instream vegetationwith respect to the processes of biomass gain and loss. Thereis also a need to investigate the transferability of laboratorybased experimental results to real world situations.

A particular area requiring further research is the nature ofpotential feedback relationships between macrophyte growthand velocity. There appears to be very little reference to thisinteraction within the literature, partially reflecting an appar-ent division between ecologists focusing on the effects ofvelocity and discharge on macrophytes and hydrologists orengineers concentrating on the impacts of plant growth onwater depth and velocity. The failure to consider thisinteraction is a considerable shortcoming in developingunderstanding of the dynamics of the relationship betweenmacrophytes and flow parameters and emphasises the needfor interdisciplinary approaches.

The inability to be able to assess accurately the flowcontrols on macrophyte communities has implications fordetermining management strategies in lowland rivers, parti-cularly with the increasing acknowledgement of the need toallocate flows for instream ecological needs. If the ecologicalneeds are not understood, accurate assessment of ecologicalflow requirements is difficult and alternative options must beidentified for allocating water resources. Where demand forwater resources is high, as is frequently the case in lowlandrivers, alternative usesmay be prioritised over instreamneedsresulting in overexploitation of the resource. Alternatively, aprecautionary approach to allocating flows may result inwater not being available to other users. Either way, the lack oftools to predict effects of flow changes on macrophytes couldlead to potentially significant economic, social and environ-mental costs. This is a significant gap limiting progress in themanagement of lowland waterways.

Acknowledgements

This work was supported by a NERC CASE studentship jointlywith the Centre for Ecology and Hydrology.

R E F E R E N C E S

Acreman MC, Dunbar MJ. Defining environmental river flowrequirements — a review. Hydrol Earth Sys Sci 2004;8:861–76.

376 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 6 9 – 3 7 8

Baattrup-Pedersen A, Riis T. Macrophyte diversity and compositionin relation to substratum characteristics in regulated andunregulated Danish streams. Freshwat Biol 1999;42:375–85.

Baattrup-Pedersen A, Riis T. Impacts of different weed cuttingpractices on macrophyte species diversity and composition ina Danish stream. River Res Appl 2004;20:103–14.

Baattrup-Pedersen A, Larsen SE, Riis T. Long-term effects ofstream management on plant communities in two Danishlowland streams. Hydrobiologia 2002;481:33–45.

Baattrup-Pedersen A, Larsen SE, Riis T. Composition and richness ofmacrophyte communities in small Danish streams — influenceof environmental factors and weed cutting. Hydrobiologia2003;495:171–9.

Barko JW, Smart RM. Sediment-related mechanisms of growthlimitation in submersed macrophytes. Ecology 1986;67:1328–40.

Barko JW, Hardin DG, Matthews MS. Growth and morphology ofsubmersed freshwater macrophytes in relation to light andtemperature. Can J Bot 1982;60:877–87.

Barko JW, AdamsMS, Clesceri NL. Environmental factors and theirconsideration in the management of submersed aquaticvegetation: a review. J Aquat Plant Manage 1986;24:1–10.

Biggs BJF. Hydraulic habitat of plants in streams. Reg River ResManage 1996;12:131–44.

Biggs BJF, Close ME. Periphyton biomass dynamics in gravel bedrivers: the relative effects of flows and nutrients. Freshwat Biol1989;22:209–31.

Biggs BJF, Nikora VI, Snelder TH. Linking scales of flow variabilityto lotic ecosystem structure and function. River Res Appl2005;21:283–98.

Brierley SJ, HarperDM, BarhamPJ. Factors affecting the distributionand abundance of aquatic plants in a navigable lowland river;the River Nene, England. Reg River Res Manage 1989;4:263–74.

Butcher RW. Studies on the ecology of rivers: I. On the distributionof macrophytic vegetation in the rivers of Britain. J Ecol1933;21:58–91.

Carignan R, Kalff J. Phosphorus sources for aquatic weeds: water ofsediments? Science 1980;207:987–9.

Carpenter SR, Lodge DM. Effects of submersed macrophytes onecosystem processes. Aquat Bot 1986;26:341–70.

Carr GM, Chambers PA. Macrophyte growth and sedimentphosphorus and nitrogen in a Canadian prairie river. FreshwatBiol 1998;39:525–36.

Chambers PA. Light and nutrients in the control of aquatic plantcommunity structure. II. In situ observations. J Ecol1987;75:621–8.

Chambers PA, Prepas EE, Hamilton HR, Bothwell ML. Currentvelocity and its effect on aquatic macrophytes in flowingwaters. Ecol Appl 1991;1:249–57.

Champion PD, Tanner CC. Seasonality of macrophytes andinteraction with flow in a New Zealand lowland stream.Hydrobiologia 2000;441:1–12.

Clarke SJ, Wharton G. Sediment nutrient characteristics andaquatic macrophytes in lowland English rivers. Sci TotalEnviron 2001;266:103–12.

Cotton JA,WhartonG, Bass JAB,Heppell CM,WottonRS. The effectsof seasonal changes to in-stream vegetation cover on patternsof flow and accumulation of sediment. Geomorphology2006;77:320–34.

Cranston E, Darby EJ. Ranunculus in Chalk rivers: phase 2. Bristol:Environment Agency; 2002.

Dawson FH. The annual production of the aquatic macrophyteRanunculus penicillatus var. calcareus (R.W. Butcher) C.D.K. Cook.Aquat Bot 1976;2:51–73.

Dawson FH. The seasonal effects of aquatic plant growth on theflow of water in a stream. EWRS 5th symposium on aquaticweeds. Amsterdam: EWRS; 1978.

Dawson FH, Robinson WN. Submerged macrophytes and thehydraulic roughness of a lowland chalkstream. Verh Int VerTheo Ang Limnol 1984;22:1944–8.

Dawson FH, Castellano E, Ladle M. Concept of species successionin relation to river vegetation and management. Verh Int VerTheo Ang Limnol 1978;20:1429–34.

Demars BOL, Harper DM. The aquatic macrophytes on an Englishlowland river system: assessing response to nutrientenrichment. Hydrobiologia 1998;384:75–88.

Dodds WK, Biggs BJF. Water velocity attenuation by streamperiphyton and macrophytes in relation to growth form andarchitecture. J N Am Benthol Soc 2002;21:2–15.

Fisher KR, Reeve CE. Impact of aquatic vegetation cutting. In:White WR, Watts J, editors. 2nd international conference onriver flood hydraulics. York, England. Chichester: John Wiley &Sons; 1994.

Fisher KR, Dawson FH. Reducing uncertainty in river floodconveyance — roughness review. London: DEFRA; 2003.

Flynn NJ, Snook DL, Wade AJ, Jarvie HP. Macrophyte andperiphyton dynamics in a UK Cretaceous chalk stream: theRiver Kennet, a tributary of the Thames. Sci Total Environ2002;282–283:143–57.

Franklin, P.A. Dynamic analysis and modelling of hydroecology intwo chalk streams. University of Reading. PhD thesis. 2007.

Green JC. Comparison of blockage factors in modelling theresistance of channels containing submerged macrophytes.River Res Appl 2005a;21:671–86.

Green JC. Modelling flow resistance in vegetated streams: reviewand development of new theory. Hydrol Proc 2005b;19:1245–59.

Green JC. Effect of macrophyte spatial variability on channelresistance. Adv Wat Res 2006;29:426–38.

Gregg WW, Rose FL. The effects of aquatic macrophytes on thestream microenvironment. Aquat Bot 1982;14:309–24.

Gurnell AM, Midgley P. Aquatic weed growth and flow resistance:influence on the relationship between discharge and stage overa 25 year river gauging station record. Hydrol Proc 1994;8:63–73.

Gurnell AM, Van Oosterhout MP, De Vlieger B, Goodson JM. Reach–scale interactions between aquatic plants and physical habitat:River Frome, Dorset. River Res Appl 2006;22:667–80.

Ham SF, Wright JF, Berrie AD. Growth and recession of aquaticmacrophytes on an unshaded section of the River Lambourn,England, from 1971 to 1976. Freshwat Biol 1981;11:381–90.

Ham SF, Cooling DA, Hiley PD, McLeish PR, Scorgie HRA, Berrie AD.Growth and recession of aquatic macrophytes on a shadedsection of the River Lambourn, England, from 1971 to 1980.Freshwat Biol 1982a;12:1–15.

Ham SF, Wright JF, Berrie AD. The effect of cutting on the growthand recession of the freshwater macrophyte Ranunculuspenicillatus (Dumort.) Bab. var. calcareus (R. W. Butcher) C. D. K.Cook. J Environ Manage 1982b;15:263–71.

Haslam SM. River plants: the macrophytic vegetation of water-courses. Cambridge: Cambridge University Press; 1978.

Hatton-Ellis TW, Grieve N, Newman J. Ecology of watercoursescharacterised by Ranunculion fluitantis and Callitricho-Batrachionvegetation. Conserving Natura 2000 rivers ecology series, vol. 11.Peterborough: English Nature; 2003.

Haury J, Aidara LG. Macrophyte cover and standing crop in theRiver Scorff and its tributaries (Brittany, northwestern France):scale, patterns and process. Hydrobiologia 1999;415:109–15.

Hearne JW, Armitage PD. Implications of the annual macrophytegrowth cycle on habitat in rivers. Reg River: Res Manage1993;8:313–22.

Herb WR, Stefan HG. Integral growth of submersed macrophytesin varying light regimes. Ecol Model 2003;168:77–100.

Hilton J, O'Hare M, Bowes MJ, Jones JI. How green is my river? Anew paradigm of eutrophication in rivers. Sci Total Environ2006;365:66–83.

Horner RR,Welch EB, Seeley MR, Jacoby J. Responses of periphytonto changes in current velocity, suspended sediment andphosphorus concentration. Freshwat Biol 1990;24:215–32.

Jones JI, Sayer CD. Does the fish–invertebrate–periphyton cascadeprecipitate plant loss in shallow lakes? Ecology 2003;84:2155–67.

377S 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 6 9 – 3 7 8

Jones JI, Young JO, Eaton JW, Moss B. The influence of nutrientloading, dissolved inorganic carbon and higher trophic levelson the interaction between submerged plants and periphyton.J Ecol 2002;90:12–24.

Lacoul P, Freedman B. Environmental influences on aquatic plantsin freshwater ecosystems. Environ Rev 2006;14:89–136.

Large ARG, Prach K. Plants and water in streams and rivers. In:Baird AJ, Wilby RL, editors. Eco-hydrology: plants and water interrestrial and aquatic environments. London: Routledge; 1999.

Lau SSS, Lane SN. Nutrient and grazing factors in relation tophytoplankton level in a eutrophic shallow lake: the effect oflow macrophyte abundance. Wat Res 2002;36:3593–601.

MadsenTV,CedergreenN. Sourcesofnutrients to rootedsubmergedmacrophytes growing in a nutrient-rich stream. Freshwat Biol2002;47:283–91.

Madsen, T.V., Søndergaard,M., The effects of current velocity on thephotosynthesis of Callitriche stagnalis scop. 1983; 15: 187–193.

Madsen TV, Sand-Jensen K. Photosynthetic carbon assimilation inaquatic macrophytes. Aquat Bot 1991;41:5–40.

Madsen JD, Chambers PA, James WF, Koch EW, Westlake DF. Theinteraction betweenwatermovement, sediment dynamics andsubmersed macrophytes. Hydrobiologia 2001;444:71–84.

Mainstone CP, Parr W. Phosphorus in rivers — ecology andmanagement. Sci Total Environ 2002;282–283:25–47.

Marshall EJP, Westlake DF. Water velocities around plants in chalkstreams. Folia Geobot Phytotaxon 1990;25.

Milner AM. System recovery. In: Calow P, Petts GE, editors. Therivers handbook, vol. 2. Oxford: Blackwell Scientific; 1994.

Mitchell SF, Perrow MR. Interactions between grazing birds andmacrophytes. In: Jeppesen E, Sondergaard M, Sondergaard M,Christoffersen K, editors. The structuring role of submergedmacrophytes in lakes. New York: Springer; 1998. p. 175–96.

Naden PS, Rameshwaran P, Vienot P. Modelling the influence ofinstreammacrophytes onvelocity and turbulence. In: Garcia de JalD, ó n, Martinez PV, editors. Proceedings of the fifth internationalsymposium on ecohydraulics, vol. 2. IAHR; 2004. p. 1118–22.

Neal C, Hilton J, Wade AJ, Neal M,WickhamH. Chlorophyll-a in therivers of eastern England. Sci Total Environ 2006;365:84–104.

O'HareM, Hutchinson KA, Clarke RT. The drag and reconfigurationexperienced by five macrophytes from a lowland river. AquatBot 2007;86:253–9.

OwensM, Edwards RW. The effects of plants on river conditions: II.Further crop studies and estimates of net productivity ofmacrophytes in a chalk stream. J Ecol 1961;49:119–26.

Pedersen TCM, Baattrup-Pedersen A, Madsen TV. Effects of streamrestoration and management on plant communities inlowland streams. Freshwat Biol 2006;51:161–79.

Petts GE. Impounded rivers. Chichester: John Wiley & Sons; 1984.Phillips GL, Eminson D, Moss B. A mechanism to account for

macrophyte decline in progressively eutrophicatedfreshwaters. Aquat Bot 1978;4:103–26.

Powell KEC. Weed growth — a factor of channel roughness. In:Herschy RW, editor. Hydrometry. Chichester: John Wiley andSons; 1978.

Riis T. Distribution and abundance of macrophytes in Danishstreams. National Environmental Research Institute. PhDthesis. 2000.

Riis T, Biggs BJF. Hydrologic and hydraulic control of macrophyteestablishment and performance in streams. Limnol Oceanogr2003;48:1488–97.

Riis T, Sand-Jensen K. Historical changes in species compositionand richness accompanying perturbation and eutrophicationof Danish lowland streams over 100 years. Freshwat Biol2001;46:269–80.

Riis T, Sand-Jensen K. Dispersal of plant fragments in smallstreams. Freshwat Biol 2006;51:274–86.

Riis T, Sand-Jensen K, Vestergaard O. Plant communities inlowland Danish streams: species composition andenvironmental factors. Aquat Bot 2000;66:255–72.

Riis, T., Suren, A.M., Clausen, B., Sand-Jensen, K., Vegetation andflow regime in lowland streams. Freshwat Biol 2008; firstonline.

Rowe DK, Schipper CM. An assessment of the impact of grass carp(Ctenopharyngodon idella) in New Zealand waters. Fisheriesenvironmental report, vol. 58. Fisheries Research Division ofthe New Zealand Ministry of Agriculture and Fisheries; 1985.

Sand-Jensen K. Environmental variables and their effect onphotosynthesis of aquatic plant communities. Aquat Bot1989;34:5–25.

Sand-Jensen K. Influence of submerged macrophytes on sedimentcomposition and near-bed flow in lowland streams. FreshwatBiol 1998;39:663–79.

Sand-Jensen K. Drag and reconfiguration of freshwatermacrophytes. Freshwat Biol 2003;48:271–83.

Sand-Jensen K, Madsen TV. Patch dynamics of the streammacrophyte, Callitriche cophocarpa. Freshwat Biol1992;27:277–82.

Sand-Jensen K, Mebus JR. Fine-scale patterns of water velocitywithin macrophyte patches in streams. Oikos 1996;76:169–80.

Sand-Jensen K, Jeppesen E, Nielsen K, Van Der Bijl L, Hjermind L,Nielsen LW, et al. Growth of macrophytes and ecosystemconsequences in a lowland Danish stream. Freshwat Biol1989;22:15–32.

Scheffer M, Hosper SH, Meijer ML, Moss B, Jeppesen E. Alternativeequilibria in shallow lakes. Trends Ecol Evol 1993;8:275–9.

Schwarz AM, Howard-Williams C, Clayton J. Analysis ofrelationships between maximum depth limits of aquaticplants and underwater light in 63 New Zealand lakes. N Z J MarFreshwat Res 2000;34:157–74.

Sculthorpe CD. The biology of aquatic vascular plants. London:Edward Arnold; 1967.

Soulsby PG. The effect of a heavy cut on the subsequent growth ofaquatic plants in a Hampshire chalk stream. J Inst Fish Manage1974;5:49–53.

Spink AJ, Murphy KJ, Smith SM, Westlake DF. Effects ofeutrophication on Ranunculus and Potamogeton. J Aquat PlantManage 1993;31:113–7.

Stephan U, Gutknecht D. Hydraulic resistance of submergedflexible vegetation. J Hydrol 2002;269:27–43.

Stephan U, Wychera U. Analysis of flow velocity fluctuations indifferent macrophyte banks in a natural open channel. In:Leclerc M, Capra H, Valentin S, Boudreault A, Cote Y, editors.2nd IAHR symposium on habitats hydraulics, ecohydraulics2000. Quebec: IAHR; 1996.

Underwood AJ. Spatial and temporal problems with monitoring.In: Calow P, Petts GE, editors. The rivers handbook, vol. 2.Oxford: Blackwell Scientific; 1994.

University of Reading. Ecological study of chalk streams — reportfor the period October 1973 to September 1977. Reading:University of Reading; 1977.

Wade PM. Management of macrophytic vegetation. In: Calow P,Petts GE, editors. The rivers handbook, vol. 2. Oxford,: BlackwellScientific; 1994.

Wade AJ, Whitehead PG, Hornberger GM, Jarvie HP, Flynn N. Onmodelling the impacts of phosphorus stripping at sewageworks on in-stream phosphorus and macrophyte/epiphytedynamics: a case study for the River Kennet. Sci Total Environ2002a;282–283:395–415.

Wade AJ, Whitehead PG, Hornberger GM, Snook DL. On modellingthe flow controls on macrophyte and epiphyte dynamics in alowland permeable catchment: the River Kennet, southernEngland. Sci Total Environ 2002b;282–283:375–93.

Watson D. Hydraulic effects of aquatic weeds in UK rivers. RegRiver: Res Manage 1987;1:211–27.

Westlake DF. Some effects of low-velocity currents on themetabolism of aquatic macrophytes. J Exp Bot 1967;18:187–205.

Westlake DF. Macrophytes. In: Whitton BA, editor. River ecology.California: University of California Press; 1975.

378 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 6 9 – 3 7 8

Westwood CG, Teeuw RM, Wade PM, Holmes NTH, Guyard P.Influences of environmental conditions on macrophytecommunities in drought-affected headwater streams. RiverRes Appl 2006;22:703–26.

Wharton G, Cotton JA, Wotton RS, Bass JAB, Heppell CM, TrimmerM, et al. Macrophytes and suspension-feeding invertebratesmodify flows and fine sediments in the Frome and Piddlecatchments, Dorset (UK). J Hydrol 2006;330:171–84.

Wheater HS, Neal C, Peach D. Hydro-ecological functioning of thePang and Lambourn catchments, UK: an introduction to thespecial issue. J Hydrol 2006;330:1–9.

Wilby RL, Cranston E, Darby E. Factors governing macrophytestatus in Hampshire chalk streams: implications for catchmentmanagement. J CIWEM 1998;12:179–87.

Wilcock RJ, Champion PD, Nagels JW, Croker GF. The influence ofaquatic macrophytes on the hydraulic and physico-chemicalproperties of a New Zealand lowland stream. Hydrobiologia1999;416:203–14.

Wright JF, Berrie AD. Ecological effects of groundwater pumpingand a natural drought on the upper reaches of a chalk stream.Reg River Res Manage 1987;1:145–60.

Wright JF, Cameron AC, Hiley PD, Berrie AD. Seasonal changes inbiomass of macrophytes on shaded and unshaded sections ofthe River Lambourn, England. Freshwat Biol 1982;12:271–83.

Wright JF, Gunn RJM, Winder JM, Blackburn JH, Wiggers R. Aninvestigation of ecological change in the Rivers Kennet andLambourn. Bristol: Environment Agency; 1999.

Wright JF, Clarke RT, Gunn RJM, Winder JM, Kneebone NT,Davy-Bowker J. Response of the flora and macroinvertebratefauna of a chalk stream site to changes in management.Freshwat Biol 2003;48:894–911.

Zhu B, Mayer CM, Rudstam LG, Mills EL, Ritchie ME. A comparisonof irradiance and phosphorus effects on the growth of threesubmerged macrophytes. Aquat Bot 2008;88:358–62.