assessing riparian quality using two complementary sets of bioindicators

7/30/2019 Assessing Riparian Quality Using Two Complementary Sets of Bioindicators

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Abstract Biological indicators are being increasingly used to rapidly monitorchanging river quality. Among these bioindicators are macroinvertebrates. A short-coming of macroinvertebrate rapid assessments is that they use higher taxa, andtherefore lack taxonomic resolution and species-specific responses. One subset ofinvertebrate taxa is the Odonata, which as adults, are sensitive indicators of bothriparian and river conditions. Yet adult Odonata are not necessarily an umbrella

taxon for all other taxa. Therefore, we investigated whether the two metrics ofaquatic macroinvertebrate higher taxa and adult odonate species might complementeach other, and whether together they provide better clarity on river health andintegrity than one subset alone. Results indicated that both metrics provide a similarportrait of large-scale, overall river conditions. At the smaller spatial scale of parts ofrivers, Odonata were highly sensitive to riparian vegetation, and much more so thanmacroinvertebrate higher taxa. Odonate species were more sensitive to vegetationstructure than they were to vegetation composition. Landscape context is alsoimportant, with the odonate assemblages at point localities being affected by theneighbouring dominant habitat type. Overall, benthic macroinvertebrates and adult

Odonata species provide a highly complementary pair of metrics which togetherprovide large spatial scale (river system) and small spatial scale (point localities)information on the impact of stressors such as riparian invasive alien trees. As adultOdonata are easy to sample and are sensitive to disturbance at both small and largespatial scales, they are valuable indicators for rapid assessment of river condition andriparian quality.

J. SmithSchool of Botany and Zoology, University of KwaZulu-Natal, P/Bag X01, Pietermaritzburg,South Africa

Biodivers Conserv (2007) 16:26952713DOI 10.1007/s10531-006-9081-2

O R I G I N A L P A P E R

Assessing riparian quality using two complementary sets

of bioindicators

Jenny Smith Michael J. Samways Stuart Taylor

Received: 18 February 2005 / Accepted: 29 May 2006 / Published online: 12 July 2006 Springer Science+Business Media B.V. 2006


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Keywords Riparian ecosystems Bioindicators Benthic macroinvertebrates Adult Odonata Complementarity

Introduction

River quality throughout the world is deteriorating, with stream biotas being alteredin numerous ways (Davies and Day 1998). Yet aquatic ecosystems are highly com-plex, and incorporate interactions between physical, chemical and biological entities.This makes them difficult and expensive to monitor by traditional physico-chemicalmeans. In contrast, organisms, being biological endpoints, are good indicators ofriver quality, and reflect the overall ecological integrity of their environments(Wright et al. 1984; Rosenberg and Resh 1993; Metcalf-Smith 1994; Smith et al.1999) and enable management decisions to be made (Karr 1991; Norris and Norris1995; Karr and Chu 1999; Mancini et al. 2005). However, not all organisms areequally sensitive to disturbance (Lammert and Allen 1999; Dovciak and Perry 2002;Heino et al. 2005).

Among the biological indicators that have been used for assessing river quality insouthern Africa, as elsewhere, are benthic macroinvertebrates (Dallas 1997; Brown2001; Dickens and Graham 2002). Macroinvertebrate assemblages have been used forcomparing river systems, as well as for monitoring long-term trends. However, thesemacroinvertebrate metrics are a coarse method, as they are intended to be a rapidbioassessment method that is field based to reduce the time needed to process

samples. Furthermore, the taxonomic resolution of samples is limited to taxonomiclevels above species (Dickens and Graham 2002). They generally do not provide ameasure of ecological integrity at the species level and therefore would not be able togive information relating to conservation issues of concern at this level (Brown 2001).In particular, as named species are not used, macroinvertebrate higher taxa do notgenerate assessments of the ways in which endemic species are being affected relativeto more geographically widespread species. Monitoring abundant resident speciesmay be important for detecting the early decline of a habitat (Hawking and New2002). However, monitoring rare species is also important as they can be indicative ofrelict or undisturbed conditions and used to rate the importance of any site within its

habitat groups (Eyre et al. 1986). It is also important to identify species that arerestricted to a narrow range of conditions as they may be good indicators of change.

One subset of aquatic macroinvertebrates is the Odonata, an insect order whichoccupies a spectrum of aquatic habitats (Corbet 1999). Odonates are sensitive tochanges in water quality and to landscape disturbance and thus they reflect to someextent the ecological conditions of their habitats (Samways and Steytler 1996;Chovanec and Waringer 2001). Odonata are relatively well studied, and there is asufficient number of species in most localities to give manageable number of speciesfor assessments (Samways 1993). Adults are large and conspicuous, and most species

are easily identified in the field, even by relative non-experts. These odonatecharacteristics, coupled with their long ontogenetic development, suggest they couldbe valuable for medium- or long-term monitoring However using only Odonata as

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organization. Conversely, not all benthic macroinvertebrate taxa can be monitoredat the species level as this would be too time consuming, labour intensive, expensiveand often the individual species cannot be identified.

Determination of the habitat requirements of Odonata species allows develop-

ment of characteristic assemblages of Odonata species, which can be used to monitorchanges in the environment (Clark and Samways 1996; Osborn and Samways 1996).Environmental disturbances alter odonate assemblages in terms of both speciescomposition and abundance. If the habitat preferences of certain species are known,a change in species composition is likely to be an indication of a type of disturbance.Species with more specific habitat preferences are more susceptible to certain typesof disturbance (Clark and Samways 1996). In this regard, rare and threatened sun-loving species are likely to be very indicative of invasion by alien woody plants witha dense canopy.

As no single indicator can give a full picture of the particular environmental state

of a river, it is necessary to look for complementarities among indicator metrics, andto identify indicators of change in structural, functional and compositional diversityat a range of scales and levels of organization (Rogers and Biggs 1999). One solutionwould be to combine a metric of higher benthic macroinvertebrate taxa, as ameasure of ecosystem health, with the Odonata as a measure of ecological integrityat the species level. This is done here in an attempt to provide a more comprehensivepicture of river health and conservation status. Special emphasis is given to thestructure and composition of the riparian canopy, an ecosystem that is being severelymodified in many parts of the world. The results are then used to assess the merits of

using both or either metric for determining the effects of alien vegetation changeupon the stream fauna.

Sites and methods

Sites and vegetation sampling units

This study was done on three permanent rivers, the Msunduzi, Dorpspruit andTownbush in the Pietermaritzburg basin, South Africa 3020E, 2936S (660690 ma.s.l.). A sampling unit (SU) was 10 m of river together with the 1 m wide strip of

vegetation on either bank. Within each 10 m stretch, was a glide and a riffle toensure that all habitats were included, minimizing variation (Dickens and Graham2002). River depth was < 0.75 m. SUs along each river included extremes ofstructural diversity (shaded versus sunlit) and compositional diversity (alien versusindigenous plants). It was hypothesized that using such a range of environmentalconditions would place extreme demands on the assemblages at point localities,while the three different river systems provided variation on a broader scale. Ouraim was to stretch the two metrics as far as possible, and to see whether they bothresponded in the same or in different ways to a spectrum of environmental condi-tions. To this end, 14 SUs were along the Msunduzi (M), 28 SUs along the Town-bush (T) and 28 SUs along the Dorpspruit (D) giving 70 SUs in all. Each SU wasdivided into four categories according to its combination of vegetation structure and

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Sampling of benthic macroinvertebrates and Odonata adults

Sampling of benthic macroinvertebrates was standardized according to the SouthAfrican Scoring System (SASS5) protocol, and included as much microhabitat

variation as possible within each SU (Dickens and Graham 2002). Both emergentmarginal and submerged vegetation were sampled. Samples were obtained usingthe kick-method, where rocks and other benthic material were disturbed by foot toflow downstream into a soft, 1 mm mesh net, 30 cm square. The content of eachsample was washed to the bottom of the net and then tipped into a water-filled,white tray.

Macroinvertebrates were identified to family level or above, while for the sensi-tive families Baetidae and Hydropsychidae, the number of species within each familywas also recorded. Identification of the macroinvertebrates in white, plastic trays wasfor


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abundance data were log transformed to down-weight the more abundant speciesand allow the rarer species to exert more influence on the similarity calculation(Clarke and Warwick 2001). An additional cluster analysis, using the same coef-ficient and transformation, was carried out to determine the role of both vegetation

structure and composition on SU similarity in terms of adult Odonata, macroin-vertebrate assemblages and their different sensitivity scores.

To ascertain whether Odonata abundance and species richness varied in the samedirection and to the same extent as macroinvertebrate scores, a regression analysiswas carried out. A similar comparison was done by regressing weighted Odonatascores against macroinvertebrate scores. Odonata were weighted using a rating basedon their national abundance and endemism status (Samways 1999). For approxi-mate abundance, the criteria were: 15 national records scored 5; 610 records scored4; 1120 records scored 3; 2130 records scored 1; 41 + records scored 0, while thecriteria used for endemism status were the largest areas for all records combined:

recorded from


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geographically closest together, overall had the most similar macroinvertebrateassemblages.

The Dorpspruit SUs clustered with 60% similarity. However, three Dorpspruitindividual SUs clustered with other river systems (Fig. 1). The macroinvertebrateassemblages at these SUs differed from the other Dorpspruit SUs by lackingOligochaeta and Tricorythidae. Significantly, D49 had an open canopy, but wasbetween two closed indigenous canopy SUs. These results illustrate that the contextof the SU is important in addition to its individual features. Msunduzi, Dorpspruitand half of the Townbush SUs clustered together, with 54% similarity. However,T22T30 were separate from this large cluster. Most of these SUs were in the higheststretch of the river, emphasizing again that physical closeness of sites results insimilar macroinvertebrate assemblages.

Table 1 Odonata species sampled in the three river systems

Msunduziriver

Townbushriver

Dorpspruitriver

ZygopteraSynlestidaeChlorolestes tessellatus (Burmeister, 1839) A A

PlatycnemididaeAllocnemis leucosticta Selys, 1836 A A

CoenagrionidaeCeriagrion glabrum (Burmeister,1839) L A LPseudagrion hageni Karsch, 1893 L APseudagrion kersteni (Gerstacker, 1869) A L A A LPseudagrion salisburyense Ris, 1921 A L A L A LPseudagrion sublacteum (Karsch, 1893) A

Ischnura senegalensis (Rambur, 1842) AChlorocyphidaePlatycypha caligata (Selys, 1853) A A A

AnisopteraGomphidaeCrenigomphus hartmanni (Forster, 1898) AParagomphus cognatus (Rambur, 1842) L A A L

AeshnidaeAnax imperator Leach, 1815 LAnax speratus Hagen, 1867 A

LibellulidaeOrthetrum julia Kirby, 1900 A A ACrocothemis erythraea (Brulle, 1832) A L ATrithemis arteriosa (Burmeister, 1839) ATrithemis dorsalis (Rambur, 1842) LTrithemis furva Karsch, 1899 A A ATrithemis stictica (Burmeister, 1839) LZygonyx natalensis (Martin, 1900) A

A = Adult, L = Larva



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Table 2 Benthic macroinvertebrate taxa sampled in the three river systems

Msunduzi river Townbush river Dorpspruit river

Coelenterata

Turbellaria Oligochaeta Hirudinea CrustaceaAmphipoda Potamonautidae Atyidae Palaemonidae Hydracarina PlecopteraPerlidae Ephemeroptera

Baetidae Caenidae Heptageniidae Leptophlebiidae Oligoneuridae Tricorythidae OdonataChlorocyphidae Synlestidae Coenagrionidae Platycnemidae Aeshnidae

Corduliidae Gomphidae Libellulidae HemipteraBelostomatidae Corixidae Gerridae Naucoridae Nepidae Notonectidae Pleidae Veliidae

MegalopteraCorydalidae TrichopteraHydropsychidae Philopotamidae Hydroptilidae Leptoceridae Pisuliidae ColeopteraDytiscidae Elmidae Gyrinidae

Hydraenidae Hydrophilidae Diptera



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Nevertheless, there was also a tendency for SUs in close proximity to group together.Vegetation structure did, however, have some effect on macroinvertebrate assem-blages on the Msunduzi, with similar vegetation structures and macroinvertebrateassemblages clustering together.

Further cluster analyses were done, but this time of vegetation composition onmacroinvertebrate assemblages. The results were non-significant. The Msunduzi SUsM14M12, clustered with a similarity of 65%. Despite, similar vegetation compo-sition, there were two clusters of Townbush SUs. Also, the Msunduzi and DorpspruitSUs were similar despite differences in vegetation composition, with site locationand canopy structure being important. The Townbush clusters, T42T24, (with a55% similarity) and the Dorpspruit clusters, D64D52 (with a 60% similarity) wereall characterized by alien vegetation. The SUs with the highest percentage similarity

(D61(0) and D58(0); M11(0) and M5(0); D67(1) and D45(0); D66(1) and D60(0);T42(0) and T18(1)) differed in vegetation composition again indicating that vege-tation composition had a minimal effect on these assemblages.

Along the Townbush, overall there was no discernable clustering of SUswith similar vegetation composition. Results thus indicate that clustering ofmacroinvertebrate assemblages did not depend strongly on indigenous versus alienvegetation. Similarly, along the Dorpspruit there was no distinct clustering. Thus, onthis river too, indigenous versus alien vegetation composition was not significant fordetermining macroinvertebrate assemblages.

Benthic macroinvertebrate scores and habitat types

Table 2 continued

Msunduzi river Townbush river Dorpspruit river

Muscidae

Simuliidae Tabanidae Tipulidae

Table 3 Mean ( 1 SE) benthic macroinvertebrate score, macroinvertebrate abundance andmacroinvertebrate taxa richness per sampling unit in the three river systems

Meanmacroinvertebratescore

ZScore

Meaninvertebrateabundance

ZScore

Meaninvertebratefamily richness

ZScore

Msunduzi 24.21 2.57 40.00 0.94 20.71 0.40Townbush 77.64 4.74 5.081 24.50 1.34 5.631 13.42 0.74 5.291

Dorpspruit 78.89 2.91 5.232 28.79 1.20 4.052 14.14 0.53 4.982

Statistically significant (P < 0.05) Z scores from KruskalWallis tests are also shown. Differencesbetween Msunduzi and Townbush are indicated by superscript 1 and superscript 2 indicates dif-ferences between Msunduzi and Dorpspruit, df = 2. There were no significant differences foundbetween Townbush and Dorpspruit



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similarity), which were 57% similar to each other. There was no discernable clus-tering of SUs with similar vegetation structure or with vegetation composition. Thus,macroinvertebrates with similar sensitivity scores occurred in the same river systems

rather than at sites with similar vegetation structure and composition, which may ormay not be in different systems. In other words, for macroinvertebrates, theparticular river system at the larger spatial scale overrides the vegetational featuresat the smaller, local scale along the rivers.

Variation in adult Odonata assemblages

There were significant differences in mean adult Odonata abundance betweenMsunduzi and Townbush and between Msunduzi and Dorpspruit, but not betweenTownbush and Dorpspruit (Table 4). There were significant differences in Odonataspecies richness between the Msunduzi and Townbush and between Msunduzi andDorpspruit), but not between the Townbush and Dorpspruit (Table 4). The Monte

T30(1)T17(0)T29(1)T28(0)T23(0)T34(1)T16(0)T22(0)D511()M12(1)M6(1)M4(1)M5(1)M11(1)M7(1)M3(1)M8(1)M2(1)M1(1)M10(1)M9(1)M13(1)M14(1)D52(0)D65(0)D69(0)D55(0)D53(0)D56(0)D57(0)D64(0)D44(1)D45(1)D67(0)D68(0)D70(0)D60(1)D66(0)D46(1)D43(1)D47(1)D54(0)D58(1)D61(0)D48(1)D59(1)D62(1)D63(1)T38(1)T24(0)T31(0)T37(0)T40(0)T26(0)T39(0)T21(0)T27(0)T18(0)T42(0)T32(1)T33(0)T35(0)D50(1)T36(1)T15(0)T20(0)T41(1)T19(0)T25(0)D49(1)

40 60 80 100

Similarity %

D

T

D

T

D

M

D

T

Fig. 1 Cluster analysis of benthic macroinvertebrate assemblages at all 70 sampling units in thethree rivers. M = Msunduzi river, T = Townbush river, D = Dorpspruit river



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significant in accounting for variation among the adult Odonata assemblages

(abundance and species richness) (F = 3.59, P 0.005) although not to the sameextent as vegetation structure.

Msunduzi SUs clustered together, M6 being the exception, with a similarity ofapproximately 45% (Fig. 2). M6 was one of the SUs with the least vegetation alongits bank compared to the other Msunduzi SUs, making conditions for Odonataspecies less favourable at this site. Townbush and Dorpspruit SUs were interspersed

Table 4 Mean ( 1 SE) Odonata abundance and species richness per sampling unit in the threeriver systems

Mean Odonataabundance

Z Score Mean Odonataspecies richness

Z Score

Msunduzi 21.86 1.76 6.07 0.35Townbush 7.57 0.83 5.171 2.46 0.18 5.741

Dorpspruit 8.14 0.97 4.892 3.21 0.25 4.252

Statistically significant (P < 0.05) Z scores from KruskalWallis tests are also shown. Differencesbetween Msunduzi and Townbush are indicated by superscript 1 and superscript 2 indicates dif-ferences between Msunduzi and Dorpspruit, df = 2. There were no significant differences foundbetween Townbush and Dorpspruit

T23T16T22T25T17T26T28D64T27T20T24D65D70T19D56D66D69T21D67T15D68M1M7M13M2M14M3M8M9M11M12M5M4M10D46

D57T35D62T40T42T33D63M6D45T18T37T30T36T29T34D47D43D48T31T32D61T41D60D58D44T38T39D59D50D52

D54D49D51D53D55

D & T

M

D & T

M

D & T



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among each other. Three Townbush SUs (T22, T16 and T23) separated out, andwere 95% dissimilar from the other SUs (Fig. 2). These SUs were geographicallyisolated from other Townbush SUs in a relatively forested area. Another cluster,D55D50 was 85% dissimilar from the other SUs (Fig. 2). These SUs were next to

each other on the same stretch of river. Overall, Odonata were thus sensitive to boththe larger spatial scale of river system type and smaller spatial scale effects, such asvegetation structure and composition.

To further determine the role of vegetation structure on Odonata assemblages,SU characteristics (i.e. whether a SU had an open or closed canopy) were incor-porated into Fig. 2. Adult Odonata assemblages along the three river systems wereclearly affected by vegetation structure with tight clusters of SUs. D55D50 (with a25% similarity) were closed canopy sites, D59M1 (with a 50% similarity), wereopen canopy sites and D68T23 (with 50% similarity) were all closed canopy sites.These closed canopy sites were divided into two groups which were less than 20%

similar. However, the outlier Townbush SUs (T16, T22 and T23) had open canopies.The cluster of dissimilar Dorpspruit SUs (D55D50) was characterized by closedcanopies except for the outlier D49, which was positioned between these SUs. Thisindicates that vegetation structure has a greater influence on Odonata assemblagesthan does river system.

D49, an open canopy, and T18, a closed canopy site, were also outliers. Theyclustered with SUs of different vegetation structure. D49 was an open canopy SUwas between closed canopy SUs and was therefore influenced by their characteris-tics. A few SUs (T22 and T16; T26 and T17; D64 and T28) were 100% similar with

each pair having the same vegetation structure. This indicates that vegetationstructure has a major edge-effect or halo effect on Odonata assemblages, indicatingthat landscape context is important.

Adult Odonata assemblages did not respond to differences in vegetation com-position, although there were two clusters. One was composed of alien vegetation(D68T23), with 50% similarity, and the other was composed of indigenous vege-tation (Msunduzi SUs) with 50% similarity. A few SUs were 100% similar (T22 andT16; T26 and T17; D64 and T28). These pairs all had the same vegetation compo-sition. The cluster of dissimilar Dorpspruit SUs (D55D50) had different vegetationcompositions, as did the cluster of dissimilar Townbush SUs (T16, T22 and T23).

Overall, vegetation composition at the fine scale of alien or indigenous vegetationhas a small affect on adult Odonata assemblages, but not as great as that of vege-tation structure.

Benthic macroinvertebrate scores in relation to adult Odonata assemblages

Benthic macroinvertebrate scores were positively and highly significantly correlatedwith adult Odonata abundance (r2 = 0.486, df = 68, P < 0.005) (Fig. 3) and withOdonata species richness (r2 = 0.402, df = 68, P < 0.005) (Fig. 4). Variation in

macroinvertebrate scores was in a similar direction to that of the Odonata indices,suggesting that both were responding in a similar way to environmental variablesalong the river system.



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(r2 = 0 393 df=68 P < 0 005) (Fig 5) Variation in macroinvertebrate scores were in

0

5

10

15

20

25

30

35

40

0 50 100

Macroinvertebrate score

Odonataabundance

150

Fig. 3 Linear regression of adult Odonata abundance in each sampling unit against benthicmacroinvertebrate score. Y = 6.850 + 0.2002x

0

1

2

3

4

5

6

7

8

9

10

0 50 100 150


Odon

ataspeciesrichness

Fig. 4 Linear regression of adult Odonata species richness in each sampling unit against benthicmacroinvertebrate score. Y = 0.2639 + 0.4287x



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Taxonomic groups and variation in benthic macroinvertebrate scores

Of the 51 benthic macroinvertebrate taxa in the three river systems, 41 were in theMsunduzi, 37 in the Townbush and 35 in the Dorpspruit. About 10 of the 51 highertaxa were unique to the Msunduzi river (Table 2). Five of these ten taxa had highsensitivity scores (Palaemonidae (10); Hydracarina (8); Heptageniidae (13); Oligo-neuridae (15) and Corduliidae (8)), the other five had low sensitivity scores (Hiru-dinea (3); Belastomatidae (3); Pleidae (4); Culicidae (1) and Tabaenidae (5)). Threeof the taxa were unique to the Townbush river (Table 2). These taxa (Hydroptilidae(6); Dyticidae (5); Hydraenidae (8)) had average to low sensitivity scores. Only, twoof the taxa were unique to the Dorpspruit river (Table 2). These taxaincluded Coelenterata (1) and Hydrophilidae (5) which had low sensitivity scores.Macroinvertebrate scores were high along the Msunduzi river, because of greaternumbers of taxa present at each SU and because certain families (Perlidae,Heptageniidae, and Chlorocyphidae) with very high sensitivity scores (10) were atvery few SUs (Table 5).

Heptageniidae, (at 85.7% of the SUs) were only along the Msunduzi river.Perlidae (at 85.7% of the SUs along the Msunduzi) and Chlorocyphidae (at 78.6% ofthe SUs along the Msunduzi) were also along the Townbush river, but at very few ofthe SUs (3.6% and 25.0% respectively). The Msunduzi SUs were all characterized

by open canopies and indigenous vegetation. Perlidae were at one Townbush SU,which had a closed canopy and alien vegetation. Chlorocyphidae, along the Town-b h i t t SU ith i d i di t ti t SU

0

10

20

30

40

50

60

70

80

0 50 100 150


We

ightedtotalendemismandabundance

Fig. 5 Linear regression of total weighted adult Odonata endemism and abundance in eachsampling unit against benthic macroinvertebrate score. Y = 3.592 + 0.4110x



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Taxonomic groups and variation in adult Odonata assemblages

Odonata assemblages (abundance and species richness) varied significantly betweenriver systems. Mean abundance and species richness per SU were both higher alongthe Msunduzi river (Table 5). Seventeen Odonata species were along the three riversystems. Of these, 11 were along the Msunduzi river, with Pseudagrion sublacteum,Crenigomphus hartmanni, Anax speratus, Trithemis arteriosa and Zygonyx natalensisbeing unique to this river. Nine species were along the Townbush river (Ceriagrionglabrum being the only species unique to this river), and 11 species were along theDorpspruit river (P. hageni and Ischnura senegalensis were the two species unique to

this river) (Table 1).The differences in abundance and species richness scores from one river to

another was because of the frequency at which the species were found at each SUalong them. Most species along the Msunduzi river, occurred at most of theSUs, whereas along the other two river systems most of the species occurred at fewSUs (Table 6). P. kersteni, Platycypha caligata and T. furva, were found at most of

Table 5 Percentage of sampling units (along the three river systems) which had very high (10)benthic macroinvertebrate sensitivity scores

Taxon Msunduzi Townbush Dorpspruit

Amphipoda(13) 21.4 3.6Palaemonidae(10) 7.1 Perlidae(12) 85.7 3.6 Heptageniidae(13) 85.7 Oligoneuridae(15) 7.1 Chlorocyphidae(10) 78.6 25.0 Platycnemididae(10) 28.6 10.7Philopotamidae(10) 21.4 32.1 Pisulidae(10) 3.6 3.6Athericidae(10) 14.3 32.1 35.7

Table 6 Percentage of sampling units (along the three river systems) where focal Odonata speciesoccurred

Odonata species Msunduzi Townbush Dorpspruit

Chlorolestes tessellatus 25.0 14.3Allocnemis leucosticta 10.7 35.7Ceriagrion glabrum 3.6 Pseudagrion hageni 28.6P. kersteni 100 89.3 78.6P. salisburyense 64.3 3.6 10.7P. sublacteum 78.6 Ischnura senegalensis 7.1Platycypha caligata 85.7 17.9 50.0Crenigomphus hartmanni 28.6

Paragomphus cognatus 17.9 Anax speratus 14.3 Orthetrum julia 78 6 71 4 46 4



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the Msunduzi SUs. At the SUs along the Townbush and Dorpspruit rivers, P.kersteni was the most common species (Table 6).

Of the 47 SUs along the Townbush and Dorpspruit rivers where P. kerstenioccurred, 19 had closed canopies and alien vegetation, nine had open canopies and

alien vegetation and 19 had open canopies and indigenous vegetation. P. caligataoccurred at 17 SUs along the Townbush and Dorpspruit rivers. Seven of these hadclosed canopies and alien vegetation, six had open canopies and alien vegetation andfour had open canopies and indigenous vegetation.

Chlorolestes tessellatus and Allocnemis leucosticta, both of which are SouthAfrican endemics, were along both the Townbush and the Dorpspruit rivers, but notalong the Msunduzi river. C. tessellatus was only at SUs with closed canopies wherethere was never more than two other species present. A. leucosticta occurred at 13SUs along these two rivers. Two of these 13 SUs had open canopies, the others wereall composed of closed canopies and all but two of these had two or less other species

present.

Discussion

Spatial scale: variation between and within river systems

A biological monitoring technique should reveal whether any anthropogenic impactis causing deterioration in river condition, and should provide some indication of the

severity of this impact. Use of several different techniques may enhance the inter-pretation of data collected as part of a biological monitoring programme, as it isunlikely that any single technique will fulfil all the above criteria on its own (Brown2001).

Our study showed that spatial scale (river system versus point localities) was asignificant variable for both the benthic macroinvertebrate and adult Odonatametrics. At the large spatial scale of a river, macroinvertebrate abundance andspecies richness, as well as Odonata abundance and species richness, respondedsimilarly to the different river systems. In short, at the large spatial scale, themacroinvertebrate metric and adult dragonfly metric were comparable. Habitat

quantity, quality and diversity all affect macroinvertebrate scores, with Odonataspecies richness and abundance affected similarly. Odonata scores were high alongthe most sunlit river (Msunduzi), probably because adult Odonata are known tohave strong and species-specific sunlight versus shade preferences (Clark andSamways 1996; Stewart and Samways 1998), with most South African Odonataspecies avoiding closed canopy riparian vegetation (Pinhey 1984; Kinvig and Sam-ways 2000). Aquatic macroinvertebrates, which are less vagile than adult Odonata,were mostly affected by the overall canopy cover. In contrast, Odonata adults arehighly sensitive to local environmental conditions, and respond rapidly using flight to

seek suitable habitat (Samways et al. 1996). Thus, these vagile insects reflect theimmediate structure of the habitat along the river, as well as the general condition ofthe river.



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system, whereas species data are more sensitive to specific patterns in environmentalvariables.

Macroinvertebrate assemblages here were similar along whole river systems, withlittle response to closed versus open vegetation, nor to alien versus indigenous

vegetation. Therefore, macroinvertebrates were responding to average habitatcharacter (i.e. riparian zone vegetation) as well as likely to water quality. Odonataassemblages, in contrast, were particularly sensitive to vegetation changes, particu-larly structure over composition, as well as reflecting overall character of the river.Nevertheless, the context of the habitat at individual point localities was important,especially where there was high contrast from one point locality to the next invegetation structure. Where closed canopy dominated, with only short stretches ofriver with open canopy, the Odonata assemblage was largely of the closed canopytype. These results also support those from elsewhere, where structures inhibitedlinear movements of Odonata (Schutte et al. 1997) and riparian habitats were not

necessarily movement corridors (Rosenberg et al. 1997). Thus, the two metrics(benthic macroinvertebrates and adult Odonata) together provide a relativelycomprehensive synthesis of the biological conditions of a particular river system (orpoint locality at the smaller spatial scale).

Adult male Odonata can be used to characterize a habitat (Hawking and New2002). Furthermore, resident adults are likely to be more affected by changes in theimmediate environment than would transitory species, and therefore should have thegreatest value as indicators. Here, most Odonata larvae were too young to identifyto species level, meaning that resident status could not be categorically determined.

However, by inference at the generic level, the species encountered here were alllikely to be resident. Indeed, the two most sensitive species, Chlorolestes tessellatusand Allocnemis leucosticta, are both national endemics requiring natural forest.

Benthic macroinvertebrates and individual species

We highlighted here the difficulty of using Odonata larvae at the species level,principally because they could not be identified. Furthermore, Hawking and New(2002) found that the diversity and abundance of larvae varies considerably, even on

consecutive dates. This shortcoming is partly overcome by using a macroinvertebratemetric which specifies the habitats to be sampled, so that there is no chance sam-pling of different substrates. Nevertheless, it is still important to be aware that thelarvae present in a single sample may not be a true representation of the fullspectrum of local species.

Knowledge of the life histories of the macroinvertebrates used in the bioassess-ment could help interpret the state of the water body and its environment. This wassuggested by this study, where taxa with high sensitivity scores were more numerousalong one river yet poorer in the other two. This compares with taxa with lowsensitivity scores which were more common in these two other rivers, suggesting

perhaps some degradation of these two systems. This degradation was probablymostly due to alien trees shading the habitat. A cautionary note, which parallels the



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tree canopies. Nevertheless, when the invasive alien canopy becomes very dense, itextirpates sun-loving endemic odonate species (Samways and Taylor 2004).

Sensitivity scores of benthic macroinvertebrates can be an early indicator ofwhether or not rivers and their canopies are changing in quality. Along streams in

the USA, early signs of degradation were shown in the loss of intolerant and long-lived taxa. This was followed by an overall decrease in taxa richness. Heavilyaffected sites were dominated by a few, highly-tolerant taxa (Morley and Karr 2002).It is important to note the composition of macroinvertebrate assemblages in eachsample, as macroinvertebrate scores could still be relatively high if many highly-tolerant taxa are still present, despite degradation of the stream. It is importanttherefore, to also include the average score per taxon (ASPT), to give a moreaccurate picture of the true health of the river (Dickens and Graham 2002). With abetter understanding of the life histories of macroinvertebrates and their responsesto specific stressors, the diagnosis of causes of degradation, and not just the warning

of degradation, might be possible. This will greatly enhace restoration and conser-vation efforts.

Recommendations arising from this study

Our results support the findings from other continents (Wright et al. 1984; Rosen-berg and Resh 1993) that benthic macroinvertebrates are sensitive indicators of rivercondition, in this case overall character of the riparian zone. Furthermore, ourresults support those of Dickens and Graham (2002) and others, that the average

score per taxon (ASPT) is a more consistent metric than the benthic macroinver-tebrate score or number of macroinvertebrate taxa. Furhtermore, Odonata speciesrichness and abundance both supported the ASPT, suggesting that both metrics(benthic macroinvertebrate ASPT and adult Odonta) are equally good responders tothe general character of the riparian zone, and are highly complementary.

Additionally, adult Odonata species richness and abundance, and certain speciesin particular, were highly sensitive to point localities along the riparian corridor.Odonata species richness, abundance and assemblage composition were especiallysensitive to vegetation structure rather than composition. However, in the case ofriparian trees, the effects of vegetation composition and structure are compounded

when the alien trees form a dense canopy. Furthermore, the odonate assemblageswere so sensitive that they also responded to small-scale (only a few metres) frag-mentation of the riparian corridor. As adult odonates are relatively easy to sample(and only requiring close-focus binoculars), they make an excellent metric for rapidassessment of both river condition and riparian quality.

Acknowledgements Financial support was from the Working for Water Programme.

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assessing riparian quality using two complementary sets of bioindicators

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