Does water abstraction from unregulated streams affect aquatic macrophyte assemblages? An evaluation based on comparisons with reference sites

ECOHYDROLOGYEcohydrol. 1, 67–75 (2008)Published online in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/eco.8

Does water abstraction from unregulated streams affectaquatic macrophyte assemblages? An evaluation based on

comparisons with reference sites

Bruce C. Chessman,1* Meredith J. Royal2 and Monika Muschal3

1 New South Wales Department of Environment and Climate Change, Parramatta NSW 2124, Australia2 New South Wales Department of Water and Energy, University of New England, Armidale, New South Wales 2351, Australia

3 New South Wales Department of Water and Energy, Dangar, New South Wales 2309, Australia

ABSTRACT

Aquatic and amphibious macrophytes were surveyed during two seasons at 85 sites on unregulated streams in northeasternNew South Wales, Australia, during a period of prolonged and recurring drought. Fifty-four of these sites were designated asreference sites with respect to water abstraction because upstream entitlement for abstraction was less than 1% of their meanannual flow (MAF). The remaining sites had an average of 4% of MAF licensed for upstream abstraction (range 1–20%). Nostatistically significant overall difference in macrophyte assemblage diversity (number of taxa) or composition was detectedbetween reference and non-reference sites. When each non-reference site was compared with those particular reference sitesthat it most resembled in non-hydrological environmental features relevant to macrophyte assemblages, the similarity betweenobserved and reference data was unrelated to the amount of upstream entitlement for abstraction. The lack of any evidentimpact on macrophyte assemblages was attributed mainly to the relatively small proportion of flow licensed for abstraction, thefact that the study sites did not dry completely, and the resilience of stream macrophytes to drought. However, the difficultyin distinguishing abstraction impacts from high background spatial variability in macrophyte assemblages may also have beeninfluential. Copyright 2008 John Wiley & Sons, Ltd.

KEY WORDS freshwater; macrophyte; reference condition; river; stream; water abstraction

Received 13 November 2007; Accepted 31 January 2008

INTRODUCTION

The effects of river impoundment and flow regulationon aquatic vegetation have been studied extensively (e.g.Rørslett et al., 1989; Garcia de Jalon and Sanchez, 1994;Baattrup-Pedersen and Riis, 1999; Blanch et al., 2000;Bernez et al., 2004), but little attention has been paid tothe impact of water abstraction from unregulated streams.In Australia, such abstraction is widespread and provideswater supplies for towns, Aboriginal communities, farm-steads, livestock watering, irrigation and other industries.In an era of frequent and prolonged drought, concernabout anthropogenic climate change, and desire for moreeconomically efficient use of water, policy and plan-ning controls on abstraction from both regulated andunregulated streams are becoming more detailed andwidespread (Arthington and Pusey, 2003). These controlsaim to protect aquatic ecosystems as well as to allo-cate water equitably among potential abstractive users. Inorder to achieve the former aim, a better understandingof the impacts of current abstraction and the sensitiv-ity of ecosystem components to alternative managementregimes is needed.

* Correspondence to: Bruce C. Chessman, New South Wales Departmentof Environment and Climate Change, Parramatta NSW 2124, Australia.E-mail: [email protected]

We have recently described a new method to com-pare a biological assemblage between a stream reachexposed to a stressor of concern (an exposure reach)and reference reaches with little, or preferably no, expo-sure to that stressor, while taking account of the role ofother environmental factors that are associated with spa-tial variation in the assemblage (Chessman et al., 2008).This method, named the limiting environmental differ-ence (LED) approach, takes a set of reference sites andselects those that are most appropriate for comparisonwith a particular exposure site. This is done by consid-ering multiple environmental factors that may limit thesimilarity of biological assemblages among sites (Thom-son et al., 1996). Scatterplots of relationships betweenassemblage similarities and environmental differences forall possible pairs of reference sites are used to set themaximum allowable environmental differences (MAEDs)for a reference site to qualify for comparison with a par-ticular exposure site.

We applied this method to compare fish assemblagesin unregulated streams in northeastern New South Wales(NSW), Australia, between sites with an appreciable vol-ume of water licensed for potential upstream abstractionand reference sites with little upstream entitlement forabstraction. This comparison revealed an apparent impactof abstraction on fish at some sites (Chessman et al.,2008). Here, we use the same approach to assess the

Copyright 2008 John Wiley & Sons, Ltd.

68 B. C. CHESSMAN, M. J. ROYAL AND M. MUSCHAL

effect of abstraction on aquatic and semi-aquatic macro-phytes at the same sites. Abstraction from unregulatedstreams in this region would be expected to reduce flowvolume and velocity, water depth and wetted area, partic-ularly at times when demand for water is high and flowis naturally low. We hypothesized that these hydrologi-cal and hydraulic changes would alter species abundancesand assemblage composition, because variation in streammacrophyte assemblages can be strongly associated withantecedent water regimes (Westwood et al., 2006a,b).

METHODS

Study area

Northeastern NSW is a diverse area with elevations fromsea level up to 1500 m and a mix of land uses rangingfrom national parks and other protected areas to timberharvesting from native eucalypt forests and exotic pineplantations, grazing of sheep and cattle on native orexotic pastures, cropping and urbanization. Eighty-fivesites were selected at stream gauging stations (Figure 1)so that a record of the flow regime was available for eachsite. Sites were chosen both east of the Great DividingRange (draining eastward to the South Pacific Ocean) andwest of the range, draining inland to the Murray-DarlingBasin.

Designation of reference sites

A database of water abstraction licences was used tocalculate the total annual volume of water licensed forabstraction from all streams upstream of each site. Thisvolume does not necessarily coincide with actual use,since water users may not pump their full entitlement.However, data on actual use were not available for mostlicences. Sites with a total upstream annual entitlement of<1% of the long-term MAF were designated as referencesites. Ideally, reference sites would have been confined tostreams with no upstream entitlement, but this stricter cri-terion would not have allowed a sufficiently wide range

Figure 1. Location of reference and exposure sites in northeastern NSW.

of reference sites. Sites designated as reference sites onthis criterion may nevertheless be subject to appreciableanthropogenic stress from other factors. Sites not des-ignated as reference sites were regarded as exposed topotential stress from water abstraction (hereafter, expo-sure sites).

Macrophyte surveys

Each site was surveyed twice: once in spring (October–December) and once in autumn (March–May). About athird of the sites were surveyed each year from 2001 to2003; resources were not sufficient to survey all sites in asingle year. These were years of extended and recurringdrought in the study area, punctuated by occasionalintervals of wetter weather. For each survey, macrophytespecies were recorded by a careful search of four largequadrats at each site. Regardless of channel size, eachquadrat measured 20 m in the downstream direction andextended laterally across the streambed and out to 20 mfrom the edge of the bed on each bank. The quadratswere separated by 40-m gaps. Each species observedwas identified by reference to published descriptions andkeys, and in many cases specimens were pressed forlater examination by expert botanists. The abundance ofeach species was scored for each quadrat as absent (0),isolated (1), scattered (3), forming beds or stands (5), orovergrowing or filling the channel (7). In some cases, adecision between adjacent categories was difficult, andan intermediate score was used (e.g. isolated-scatteredscored 2). Scores were averaged for each site over thetwo surveys and four quadrats per survey.

Only herbaceous aquatic and semi-aquatic vascularplants are considered for this analysis. Aquatic speciesare defined as those that grow only in, emerging from,or floating on the surface of water bodies. Amphibiousspecies are defined as those that grow in or on thesurface of water bodies and also on land. Specieswere assigned to these categories on the basis of theauthors’ experience and habitat information providedby the PlantNET online botanical information system(http://plantnet.rbgsyd.nsw.gov.au/).

Environmental data

Data were acquired on potentially limiting environmen-tal variables, or likely surrogates for such variables, thatare either unaffected by upstream water abstraction orunlikely to be appreciably affected by abstraction relativeto the role of other factors (Table I). Environmental vari-ables that potentially were greatly affected by abstractioncould not be used to match exposure sites to appropriatereference sites because we had no way to estimate thevalues of such variables that would have been observedat exposure sites in the absence of upstream abstraction.For example, it would have been inappropriate to com-pare an exposure site with reduced wetted area as a resultof upstream abstraction with reference sites with natu-rally low wetted area. The comparison should insteadhave been with reference sites of a larger wetted area,

Copyright 2008 John Wiley & Sons, Ltd. Ecohydrol. 1, 67–75 (2008)DOI: 10.1002/eco

EFFECT OF WATER ABSTRACTION ON AQUATIC MACROPHYTES 69

representing the wetting expected at the exposure site inthe absence of upstream abstraction. However, we didnot know what this larger wetted area would be. Withthis limitation, the variables chosen covered most of thetypes of environmental factors that are known to associatewith spatial variation in stream macrophyte assemblages(see reviews by Barendregt and Bio (2003) and Lacouland Freedman (2006)). Geographical coordinates (eastingand northing) were included in order to address possiblespatial autocorrelation in macrophyte assemblages relatedto dispersal and colonization processes rather than spatialpatterns of habitat suitability (see Bell, 2005). The inclu-sion of easting and northing allowed matched referencesites to be limited to those within a certain geographicaldistance of an exposure site.

Sites were located on topographic maps, from whichgeographical coordinates and altitudes were determined.Slopes were calculated from the distance along thethalweg between the contour lines crossing the streamupstream and downstream of each sampling site. Afew sites were so close to sea level that there was nodownstream contour, and in these cases, approximatetidal limits were taken as having an altitude of zero.Upstream drainage areas were obtained from a databaseof hydrographic stations, or determined by regionalagency staff from geographic information systems.

Reach-scale habitat data were obtained by visualassessment and scoring of the abundances of habitatattributes for the four quadrats at the time of each macro-phyte survey. Abundances were scored as absent (0),isolated (1), scattered (3), common (5), or abundant (7).Intermediate scores were used for borderline cases (e.g.isolated-scattered scored 2). The attributes considered rel-evant to macrophytes were bedrock, boulders, cobbles,pebbles, granules, fine inorganic substrata (fines), and

shading by the riparian canopy. Scores were averagedfor the four quadrats and two surveys.

The effects of grazing and watering livestock wereassessed by observation of pruning of vegetation, faeces,pugmarks and erosion of access tracks. The severity ofeach of these four types of impact was scored for eachside of the stream in each quadrat on a scale from 0 (none)to 5 (severe). Scores were added for the four impact typesand then averaged across both banks, all four quadrats,and the two surveys.

The electrical conductivity (EC) and pH of the streamwere measured at each site during each survey with aHydrolab Datasonde 4 multiprobe and Surveyor 4 datalogger. Water alkalinity was measured onsite with Titretshand-held titration kits. Water samples were collectedin Terumo 60 ml disposable plastic syringes, filteredthrough Sartorius Minisart 0Ð45 µm filters into Sarstedt30 ml plastic screw-cap tubes and frozen for transportto an analytical laboratory where they were analysedfor oxidized nitrogen and filterable phosphorus. Waterquality data were averaged for the two survey occasions.

Data analysis

Separate analyses were done with mean abundancescores for macrophyte taxa and with binary macrophytedata–presence or apparent absence (i.e. non-observation)of each taxon at each site. In each case, relationships forreference sites between similarity in macrophyte assem-blages and differences in environmental variables thatare unlikely to be appreciably affected by abstraction(Table I) were used to define criteria with which to selecta sub-set of relevant reference sites for comparison witheach exposure site. Similarities of macrophyte assem-blages between the two members of each possible pair ofreference sites were calculated with the Bray-Curtis (BC)

Table I. Hydrological characteristics (upstream water entitlement and minimum daily flow) and environmental variables used forreference-site matching (remaining variables), showing means and ranges for reference and exposure sites. Ł, variable transformed

to natural logarithm for calculation of environmental differences.

Variable Reference sites (n D 54) Exposure sites (n D 32)

Upstream water entitlement (% mean annual flow) 0Ð3 (0Ð0–1Ð0) 4Ð4 (1Ð0–20Ð3)Minimum daily flow 2001–2003 (Ml) 1Ð3 (0Ð0–16Ð9) 1Ð6 (0Ð0–19Ð1)Easting (m) 413021 (213600–548600) 364214 (244200–501000)Northing (m) 6649925 (6405300–6863600) 6645051 (6470500–6789200)Altitude (m) 377 (<10–1280) 457 (<10–1225)SlopeŁ (m/km) 4Ð2 (0Ð8–23Ð5) 2Ð0 (0Ð2–5Ð6)Drainage areaŁ (km2) 475 (9–2491) 1313 (76–5648)Bedrock (score) 2Ð2 (0Ð0–6Ð3) 2Ð2 (0Ð0–5Ð8)Boulders (score) 2Ð3 (0Ð0–5Ð3) 1Ð7 (0Ð0–5Ð1)Cobbles (score) 3Ð9 (0Ð0–6Ð8) 3Ð8 (0Ð4–6Ð1)Pebbles (score) 3Ð9 (0Ð0–6Ð9) 4Ð0 (1Ð3–6Ð6)Granules (score) 4Ð1 (0Ð3–7Ð0) 4Ð1 (1Ð8–7Ð0)Fines (score) 4Ð4 (1Ð3–7Ð0) 4Ð6 (1Ð0–7Ð0)Shading (score) 2Ð5 (0Ð0–5Ð7) 2Ð1 (0Ð1–4Ð8)Livestock impact (score) 4Ð0 (0Ð0–10Ð0) 3Ð6 (0Ð0–10Ð5)Electrical conductivity (µS/cm) 264 (20–1081) 461 (32–1367)pH 7Ð5 (6Ð4–8Ð8) 7Ð7 (6Ð3–8Ð8)Alkalinity (mg/l CaCO3) 89 (5–485) 145 (5–338)Filterable oxidized nitrogenŁ (mg/l) 0Ð097 (0Ð008–0Ð760) 0Ð155 (0Ð005–0Ð840)Filterable phosphorusŁ (mg/l) 0Ð057 (<0Ð001–0Ð340) 0Ð122 (0Ð004–1Ð980)



measure. Environmental differences were calculated asthe absolute value of the difference in each environmen-tal variable between the two members of a pair, exceptin the case of strongly skewed variables, which weretransformed to natural logarithms before differences werecalculated.

Scatterplots were produced of similarity of macrophyteassemblages against environmental differences for allreference-site pairs and each environmental variable. Foreach environmental variable, a difference value wasidentified above which every pair of reference siteshad a similarity of macrophyte assemblages below athreshold value. This was treated as the MAED betweenan exposure site and those reference sites with whichit could be compared. MAEDs were calculated for theabundance and binary analyses, and for thresholds in therange 0Ð50–1Ð00, in order to test the effect of varying thesimilarity threshold. A threshold of zero was also tested,which allowed every reference site to qualify.

The MAEDs were used to select case-specific sub-sets of reference sites that were considered appropriatefor comparison with each exposure site in turn. Foreach environmental variable, the difference was calcu-lated between an exposure site and each reference site.Only those reference sites for which the difference wasbelow the MAED for every environmental variable wereselected for comparison with that exposure site. Thiscomparison was made in each case by calculation of theBC similarity between the exposed assemblage and theaverage assemblage of the applicable sub-set of referencesites. Because each exposure site differed in its environ-mental variables, the selected sub-set of reference siteswas potentially different in each case. Each reference sitewas compared by the same process with a sub-set of theremaining reference sites.

Overall comparisons of reference and exposure sites,for both abundance and binary macrophyte data, weremade with two-dimensional non-metric multidimen-sional scaling (NMS) in the PC-ORD statistical package(McCune and Mefford, 1999). The multi-response per-mutation procedure (MRPP) in PC-ORD was used totest for significant overall differences between the twotypes of sites. This analysis generates a probability (p)value and an A-statistic (the chance-corrected within-group agreement), which expresses effect size. The A-statistic reaches its maximum possible value of 1 ifentities are identical within each group but non-identicalamong groups, and can assume negative values if thereis less similarity within groups than expected by chance.

RESULTS

Fifty-four of the 85 sites were designated as referencesites. Reference and exposure sites were geographicallyinterspersed (Figure 1) and overlapped substantially intheir ranges of environmental variables. However, thereference sites tended to have steeper slopes, while onaverage the exposure sites had larger drainage areas and

were more chemically enriched (Table I). Minimum dailyflows during the study period (2001–2003) had similarranges and averages at reference and exposure sites, beingas low as zero in some sites of both types (Table I).

Sixty-two taxa of herbaceous aquatic or amphibiousvascular plants were recorded from the 85 sites, compris-ing 56 species and 6 genera (Table II). Generic groupingswere used for Lemna spp., Myriophyllum spp. (otherthan M. aquaticum), Nymphoides spp., Typha spp., Utric-ularia spp. and Vallisneria spp. because these generawere not always identified to species level. IdentifiedMyriophyllum spp. included M. gracile, M. latifolium,and, commonly, M. variifolium and M. verrucosum.Nymphoides spp. were mainly or possibly wholly N.indica, Typha spp. were mainly or wholly T. domingien-sis, and Vallisneria spp. comprised V. americana and V.nana. The number of taxa recorded per site averaged 9Ð8(s.d. 5Ð4) at reference sites and 9Ð9 (s.d. 4Ð6) at exposuresites.

MRPP analysis found no statistically significant overalldifference in assemblage composition between referenceand exposure sites for either abundance data (A D 0Ð003;p D 0Ð14) or binary data (A D 0Ð001; p D 0Ð31). Consis-tent with this result, NMS ordination did not show anyevident segregation of reference and exposure sites inordination space (Figure 2).

The BC similarity for pairs of reference sites was ratherlow, averaging 0Ð20 (s.d. 0Ð15) for abundance data and0Ð31 (s.d. 0Ð17) for binary data, i.e. the observed macro-phyte assemblages differed substantially among referencesites. Figure 3 illustrates how BC similarities among ref-erence sites were used to set MAEDs for particular sim-ilarity thresholds and environmental variables. As thethreshold rose from zero towards 1, the MAEDs becamesmaller, and hence, the environmental criteria for match-ing reference sites to each exposure site became morestringent. Thus, the average number of qualifying refer-ence sites per exposure site declined, as did the num-ber of exposure sites that had any qualifying referencesites (Figure 4). At thresholds of 0Ð70 and above forabundance data, or 0Ð85 and above for binary data, noexposure site had any qualifying reference sites. As thesimilarity threshold for setting MAEDs rose, the averagesimilarity between observed macrophyte data at exposuresites and corresponding reference data also rose, thoughmore noticeably for binary data than for abundance data(Figure 5).

The similarity between observed and reference data forexposure sites was not significantly related to the degreeof upstream entitlement for water abstraction (Figure 6),either for abundance data or binary data (Pearson correla-tion, p > 0Ð05). Nor was the average observed-referencesimilarity at exposure sites significantly different fromthe average similarity between observed data at referencesites and reference data for each reference site obtainedfrom other reference sites (t-test, P > 0Ð05).



Table II. Means and maxima of abundance scores for macrophytetaxa at reference and exposure sites. Minimum scores were zero

for all taxa in both types of sites.

Species Reference Exposure

Mean Maximum Mean Maximum

Alismaplantago-aquatica

0Ð00 0Ð13 0Ð00 0Ð00

Aponogeton elongatus 0Ð00 0Ð25 0Ð07 1Ð38Azolla filiculoides 0Ð29 4Ð00 0Ð70 4Ð00Azolla pinnata 0Ð21 3Ð38 0Ð18 2Ð00Bolboschoenus

fluviatilis0Ð00 0Ð00 0Ð13 2Ð13

Cabomba caroliniana 0Ð06 3Ð00 0Ð00 0Ð00Callitriche muelleri 0Ð12 3Ð38 0Ð24 4Ð88Ceratophyllum

demersum0Ð08 2Ð75 0Ð00 0Ð00

Crassula helmsii 0Ð15 2Ð00 0Ð27 3Ð25Cyperus difformis 0Ð03 0Ð38 0Ð05 0Ð75Cyperus eragrostis 1Ð38 4Ð63 1Ð44 4Ð13Cyperus exaltatus 0Ð27 2Ð50 0Ð20 1Ð88Echinochloa

telmatophila0Ð09 1Ð50 0Ð00 0Ð00

Egeria densa 0Ð10 2Ð38 0Ð07 2Ð25Eichornia crassipes 0Ð00 0Ð00 0Ð04 1Ð13Elatine gratioloides 0Ð22 3Ð38 0Ð13 1Ð25Eleocharis sphacelata 0Ð09 2Ð25 0Ð20 4Ð25Elodea canadensis 0Ð22 5Ð38 0Ð10 3Ð00Gratiola peruviana 0Ð16 3Ð75 0Ð07 0Ð88Hydrilla verticillata 0Ð38 5Ð00 0Ð08 2Ð50Hydrocotyle tripartita 0Ð93 3Ð63 1Ð40 5Ð00Isachne globosa 0Ð18 3Ð13 0Ð18 3Ð00Isolepis fluitans 0Ð09 2Ð00 0Ð25 2Ð75Juncus articulatus 0Ð21 1Ð88 0Ð07 0Ð63Lemna spp. 0Ð05 1Ð38 0Ð04 1Ð13Lepironia articulata 0Ð04 2Ð38 0Ð00 0Ð00Lilaeopsis polyantha 0Ð16 4Ð00 0Ð04 0Ð88Limosella australis 0Ð02 0Ð50 0Ð06 2Ð00Ludwigia peploides 0Ð60 3Ð38 0Ð42 3Ð88Marsilea hirsuta 0Ð00 0Ð00 0Ð02 0Ð50Marsilea mutica 0Ð04 1Ð75 0Ð00 0Ð00Myriophyllum

aquaticum0Ð10 3Ð13 0Ð08 1Ð80

Myriophyllum spp. 1Ð34 4Ð75 1Ð07 5Ð00Najas tenuifolia 0Ð06 2Ð50 0Ð00 0Ð00Nymphaea caerulea 0Ð00 0Ð25 0Ð00 0Ð00Nymphoides spp. 0Ð10 1Ð50 0Ð02 0Ð63Ottelia ovalifolia 0Ð09 2Ð13 0Ð02 0Ð50Paspalum distichum 1Ð54 6Ð13 2Ð03 5Ð75Philydrum

lanuginosum0Ð01 0Ð25 0Ð00 0Ð00

Phragmites australis 0Ð37 4Ð25 0Ð63 4Ð25Potamogeton crispus 0Ð16 2Ð63 0Ð27 2Ð75Potamogeton

javanicus0Ð19 1Ð88 0Ð04 0Ð63

Potamogetonochreatus

0Ð39 4Ð88 0Ð24 2Ð38

Potamogetonpectinatus

0Ð16 2Ð50 0Ð08 2Ð50

Potamogetonperfoliatus

0Ð37 5Ð25 0Ð20 2Ð88

Potamogetontricarinatus

0Ð08 3Ð13 0Ð09 1Ð50

Potamophilaparviflora

0Ð72 4Ð75 0Ð59 3Ð75

Ranunculus inundatus 0Ð12 2Ð88 0Ð00 0Ð00Ranunculus sceleratus 0Ð00 0Ð13 0Ð00 0Ð00Rorippa microphylla 0Ð07 1Ð63 0Ð05 1Ð00

Table II. (Continued ).

Species Reference Exposure

Mean Maximum Mean Maximum

Rorippa nasturtium-aquaticum

0Ð22 2Ð63 0Ð27 3Ð50

Rorippa palustris 0Ð01 0Ð38 0Ð00 0Ð13Salvinia molesta 0Ð02 1Ð00 0Ð00 0Ð00Schoenoplectus

mucronatus0Ð22 2Ð00 0Ð13 2Ð00

Schoenoplectusvalidus

0Ð61 4Ð75 1Ð14 4Ð50

Spirodela punctata 0Ð28 3Ð13 0Ð25 4Ð13Triglochin procerum 0Ð14 3Ð38 0Ð30 3Ð75Triglochin striatum 0Ð02 1Ð13 0Ð00 0Ð00Typha spp. 0Ð13 5Ð75 0Ð07 2Ð25Utricularia spp. 0Ð01 0Ð63 0Ð00 0Ð00Vallisneria spp. 1Ð09 4Ð88 1Ð67 5Ð25Veronica

anagallis-aquatica0Ð03 1Ð00 0Ð14 2Ð38

DISCUSSION

This analysis provided no evidence that water abstrac-tion from these unregulated streams was impacting onassemblages of aquatic and amphibious macrophytes incontrast to a previous analysis for the same sites basedon fish (Chessman et al., 2008). The average taxonomicrichness of macrophyte assemblages was virtually iden-tical between reference and exposure sites, and MRPPand NMS did not reveal any overall compositional dif-ference between the two types of sites. When exposuresites were individually matched to selected referencesites through the LED procedure, the difference betweenobserved and reference data was not significantly relatedto the upstream entitlement for water abstraction, andthe average similarity between observed and referencedata did not differ between sites with little or no licensedupstream abstraction and sites with appreciable licensedupstream abstraction.

Irrespective of the stringency of the criteria used tomatch exposure sites to appropriate reference sites, theBC similarity between reference macrophyte data andobserved data at exposure sites was rather low, usu-ally between 0Ð3 and 0Ð4. The average similarity withinpairs of reference sites was even lower, around 0Ð2–0Ð3.Thus, large-scale spatial variation in macrophyte surveydata was very high, even in the absence of appreciableupstream water abstraction. For fish, the average BC simi-larity among the same sites was somewhat greater (Chess-man et al., 2008). This does not necessarily translate tolower spatial variability of fish because measures of bio-logical similarity among sites are dependent on samplingmethod and effort (Kerans et al., 1992; Cao et al., 2002).However, the difference is not unexpected. Macrophytedistributions in river systems can be strongly influencedby stochastic dispersal and colonization processes inde-pendent of local habitat suitability (Honnay et al., 2001;Demars and Harper, 2005), and such processes may resultin patchy distributions at small to medium spatial scales.



Figure 2. Two-dimensional NMS ordination of macrophyte abundance data (a) and binary data (b) for reference (R) and exposure (E) sites(stress D 0Ð27 and 0Ð28 respectively).

Figure 3. Examples of relationships between BC similarity of macrophyte assemblages (abundance data) and differences in values of environmentalvariables for all possible pairs of reference sites. Vertical dashed lines indicate the MAED established for each variable for a similarity threshold of

0Ð50 (a, c) and 0Ð70 (b, d). Thresholds are represented by horizontal solid lines.

Fish, being capable of active movement, are likely to bedistributed more uniformly at these scales, and hence, tohave greater assemblage similarity among sites. Thus, theeffects of abstraction on macrophytes may be intrinsicallymore difficult to detect than effects on fish, because of

the difficultly in separating such effects from high spatialvariability caused by other factors.

Nevertheless, the lack of evident impact on macrophyteassemblages is not greatly surprising. In regulated rivers,the flow regime can be heavily altered by the capture



Figure 4. Effect of varying the similarity threshold for setting MAEDs on the percent of exposure sites with matched reference sites (ž) and themean number of matched reference sites per exposure site (�) for macrophyte abundance data (a) and binary data (b).

Figure 5. Effect of varying the similarity threshold for setting MAEDs on the mean (š s.d.) of BC similarity between observed and referencemacrophyte abundance data (a) and binary data (b) for exposure sites.

of freshes and floods in reservoirs and the releaseof large volumes of reservoir water at times whenflow would be naturally low, as well as by waterabstraction. In unregulated systems without reservoirs,the capture and release of high flows is not possible,and flow alteration is principally through abstraction.For the exposure sites that we studied, the upstreamentitlement for water abstraction averaged only 4% ofMAF, and the maximum was 20%. Moreover, becausedata were not available on water use, it is possible

that actual abstraction was less than these amounts.McKay and King (2006) detected statistically significantimpacts on macroinvertebrate assemblages when theyexperimentally diverted flow from unregulated streams inVictoria, Australia. However, they diverted 28–97% oftotal discharge, far more than the annual entitlement forupstream abstraction at our sites. Moreover, McKay andKing (2006) did not observe any statistically significantimpact on the cover of filamentous algae or othersubmerged vegetation.



Figure 6. Relationships between entitlement for water abstractionupstream of exposure sites and BC similarity between observed and refer-ence macrophyte assemblages for abundance data (a) and binary data (b).Reference sites were selected with MAEDs based on a similarity thresh-old of 0Ð55 for abundance data and 0Ð70 for binary data, the highestthresholds for which all exposure sites had matched reference sites. Hor-izontal lines represent mean similarity between observed and reference

data for reference sites.

Although upstream entitlement at our exposure sitesaveraged only 4% of MAF, a much greater proportionof flow may be abstracted during periods of dry weather.However, this intensity of impact is transient and thestream biota, adapted to natural drought (McMahon andFinlayson, 2003), may be resistant to it or resilient.Although several of our study streams ceased flowingduring the study, none dried completely, and hence,some residual wetted habitat for aquatic macrophyteswas always present. In most cases, although flowing-water habitats such as riffles dried, large pools remained.In addition, many riverine macrophytes can survivetemporary drying as resistant life-history stages (rhizomesand seeds), to germinate and grow when wetter conditionsreturn. Hence, macrophytes may recover rapidly whenflow resumes (Wright and Berrie, 1986; Holmes, 1999;Westwood et al., 2006a,b). Fish, for which we detectedsome apparent impact of water abstraction (Chessmanet al., 2008), are not so well adapted to drought, sincenone of the freshwater fish species in northeastern NSWis known to be capable of aestivation. Disturbance byfloods may be more limiting to aquatic macrophytes thandrought, since a high frequency of high-flow disturbancecan eliminate aquatic macrophytes altogether (Riis and

Biggs, 2003). However, flood frequency is not affectedby water abstraction from unregulated streams.

ACKNOWLEDGEMENTS

We gratefully acknowledge the following persons fortheir assistance with this project: Jo Clarke, Neal Fos-ter, Michelle Glover, David Hohnberg, Irene Jarosinski,Bronwen Jones, Warren Martin, Warwick Mawhinney,Geoff McDonald, Patrick Pahlow, Sue Powell and Nir-vana Searle for their participation in the fieldwork; ChrisNadolny, Greg Steenbeeke and the Royal Botanic Gar-dens, Sydney, especially Barbara Wiecek, for identifica-tion of plant specimens; John Brayan and staff at theNew South Wales Department of Water and Energy lab-oratory for analyses of water chemistry; Sue Rea and JonSayers for information on water abstraction and drainageareas; and an anonymous reviewer for comments on themanuscript.

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Does water abstraction from unregulated streams affect aquatic macrophyte assemblages? An evaluation based on comparisons with reference sites