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Effects of riparian grazing andchannelisation on streams inSouthland, New Zealand. 2. BenthicinvertebratesJohn M. Quinn a , R. Bruce Williamson a , R. Keith Smith a &Maggie L. Vickers aa DSIR Marine and Freshwater, Department of Scientific andIndustrial Research, Water Quality Centre, P. O. Box 11–115,Hamilton, New ZealandVersion of record first published: 29 Mar 2010.

To cite this article: John M. Quinn , R. Bruce Williamson , R. Keith Smith & Maggie L. Vickers(1992): Effects of riparian grazing and channelisation on streams in Southland, New Zealand. 2.Benthic invertebrates, New Zealand Journal of Marine and Freshwater Research, 26:2, 259-273

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New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26: 259-2730028-8330/2602-0259 © The Royal Society of New Zealand 1992

259

Effects of riparian grazing and channelisation on streams in Southland,New Zealand. 2. Benthic invertebrates

JOHN M. QUINNR. BRUCE WILLIAMSONR. KEITH SMITHMAGGIE L. VICKERS

Water Quality CentreDSIR Marine and FreshwaterDepartment of Scientific and Industrial ResearchP. O. Box 11-115Hamilton, New Zealand

Abstract A survey of benthic invertebrate faunasin riparian-protected, riparian-grazed, and channelisedreaches of five Southland streams with catchmentsizes of 3-37 km2 was carried out. It was part of awider investigation to assess the effects of ripariangrazing and channelisation on stream habitat andbiota. In small streams (catchment areas 3-10 km2;widths 1-4 m), channelisation or intensive grazingby cattle greatly reduced shading by riparianvegetation, resulting in substantial increases in dailymaximum temperatures during summer. Channel-isation also caused gross changes in channelmorphology and intensive grazing of a reach withmoist streamside soils was associated with increasedbed sedimentation and bank damage. Marked changesin invertebrate communities were associated wilhthese habitat modifications. In general, taxa favouredby cool water and low periphyton abundance (e.g.,Plecoptera, Paraleptamphopus caeruleus,Deleatidium sp., and Helicopsyche albescens)decreased in density, whereas densities of taxafavoured by an abundance of periphyton (e.g.,Chironomidae and Oxyethira albiceps) increased. Incontrast, differences in physical habitat andinvertebrate communities were minor between pairedgrazed and riparian-protected reaches of the largerstreams (catchment areas 10-33 km2; median widths6-16 m) where grazing had little or no effect on

M91053Received 16 August 1991; accepted 3 December 1991

stream shading. These results indicate that in smallstreams, with median natural channel widths belowc. 6 m, the effects on benthic invertebrates decreasein the following order, channelisation > intensivegrazing by cattle > extensive grazing by cattle and/orsheep. Shade provided by riparian vegetation appearsto play a vital role in maintaining cool, headwater,stream habitats for benthic invertebrate communitiesin these streams.

Keywords benthic invertebrates; agriculture;riparian protection; grazing; channelisation; stream;temperature

INTRODUCTION

A recent survey of invertebrates in 88 New Zealandrivers indicated that the degree to which catchmentsare developed to improved pasture (i.e., pastureformed by ploughing, fertilising, and sowing exoticgrasses) has an important influence on the type andbiomass of the benthic invertebrate community (Quinn& Hickey 1990a, 1990b). Sites draining catchmentswith low-moderate levels of improved pasture ( 1 -30% of catchment area) had higher invertebratebiomass but similar community types compared withsites with undeveloped catchments. However, where>30% of the catchment Avas improved pasture,invertebrate community diversity and biomass of taxasensitive to eutrophication were significantly lower,but biomass of taxa favoured by periphyton increased.These findings and those of previous studies (Allen1959; Dance & Hynes 1980; Lenat 1984, 1988),indicating loss of sensitive taxa with agriculturaldevelopment, are particularly important for streammanagement in New Zealand because about 36% ofthe land area has been developed as improved pasture(Rutherford et al. 1987).

The detrimental effects of agricultural land usecan be diminished through management of streamriparian zones by fencing off berms (e.g., Karr &Schlosser 1978), and this technique is widely acceptedas a water and soil conservation practice in New

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260 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

Zealand (Dixie 1982; Rowell 1983). As well aspreventing grazing of riparian vegetation, bermfencing prevents stock from trampling stream banksand the stream bed, and from degrading water qualityby direct input of excreta (e.g., Platts 1979).Depending on its composition and structure, riparianvegetation can:

• have a major influence on the amount and type ofenergy input to the stream (Fisher & Likens 1973;Minshall 1978; Cummins 1974,1986);

• reduce temperature fluctuations by providingshade (Graynoth 1979; Skovlin 1984; Beschta &Taylor 1988);

• promote stream bank and channel stability (Smith1976; Elmore & Beschta 1987);

• maintain water quality by reducing nutrient andsediment inputs (Cooke & Cooper 1988; Smith1989); and

• provide habitat for birds, adult stages of aquaticinsects, and cover for fish (McDowall 1980;Wescheetal. 1987).

In addition to grazing impacts, agriculturallydeveloped streams are often modified by channel-isation (i.e., excavation to widen, deepen, andsometimes straighten channels) to improve drainageand assist livestock management. This can have asevere impact on biota by reducing habitat diversity,reducing riparian shade, and increasing bank erosion(e.g., Karr & Schlosser 1978; Swales 1982).

The aims of this study were to evaluate the effectsof riparian grazing by sheep and cattle andchannelisation on the benthic communities ofSouthland streams draining agriculturally developedland. We hypothesised that grazing effects woulddecrease with increasing stream size, because shadingand energy inputs of riparian vegetation become lessimportant (Vannote et al. 1980) and accessibility ofthe channel to stock decreases with increasing depth.Hence we compared paired reaches that were riparian-grazed and riparian-retired on streams ranging fromsmall, second-order streams to an open, fourth-orderstream. Riparian-retired reaches were fenced off toexclude stock from the stream and its riparian area.Our comparisons of channelisation were restricted tosmall (second- and third-order) streams, because therewere no suitable paired reaches on larger streams inthe study area. The effects of grazing andchannelisation on channel morphology are discussedin detail by Williamson et al. (1992). Effects of grazingon riparian vegetation and instream habitat arediscussed by Williamson et al. (1990).

DESCRIPTION OF STUDY AREA

The study area consisted of four catchments drainingMt Hamilton and nearby hills, and two draining hills50 km east in Southland (see fig. 1 in Williamson etal. 1992). All the streams had headwaters (max.elevation 663-1493 m) in tuffaceous greywackes ofMesozoic age. The lower parts of the catchments,that include the study reaches (elevations c. 300 m),drain across weathered outwash and till material ofthe Southland Plains. The larger streams had braidedchannels with shingle bars creating riffles that separatequieter runs and pools, although sandstone bedrockwas sometimes exposed in the channel. Stream erosionof the gravel layer has left much of the bank undercutand prone to collapse.

The native vegetation and the histories ofdevelopment and riparian protection of the studyreaches and their catchments are summarised inTable 1. Originally red tussock (Chionochloa rubra)was the dominant vegetation throughout the area(Williamson et al. 1992). The whole area wasextensively (i.e., lightly) grazed (mainly by sheep)from the 1850s. The three catchments near MtHamilton were settled and developed for agriculturebetween 1965 and 1975. The others were developedaround 1980, except for the lower Reed Burn (RU2)which was developed at the turn of the century.Development techniques varied from grazing thenative vegetation (SU1-3, and upper Mount HamiltonStream) to ploughing and sowing with exotic grasses(mainly rye grass, clover, and cooksfoot, Dactylisglomerata). Mole and tile drainage is extensive in theCentre Burn catchment but minimal in the others.

Table 1 Summary of main characteristics of studyreaches. +, feature present, -, feature absent; see fig. 1 inWilliamson et al. (1992, this issue) for stream names.

Riparian Extensive Intensive Channel-Reach protection grazing grazing isation

CP + -CU2 + +HP1 + -HU1 +MP + -MU - +RP1 + -RU1 - + +RU2 + +SU1 - +SU2 - + - +SU3 - +SU4 - - + -

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Quinn et al.—Grazing, channelisation and stream invertebrates 261

(Mole drains are formed by dragging a 0.1m diameterball through the soil at a depth of c. 1 m using avertical ploughshare). Channelisation of the developedreaches of the Reed and Centre Burns involvedwidening, deepening, and straightening the channelto increase drainage of the adjacent pasture, whereasin SU2, the stream was widened along its naturalmeander pattern to prevent sheep falling into thenarrow, incised channel.

Riparian retirement occurred between 1975 and1982 in response to concerns about the effects ofagricultural development on channel erosion, waterquantity and quality, and trout fisheries. Retirementtypically involved the fencing off of a strip 20 mwide or more along the channel. In some instances,all paddocks along the study reach had been retiredfrom farming. Vegetation at all retired sites wasdominated by 0.5-1 -m-high grasses, both nativetussock and introduced exotics. Grazed reaches wereimproved pasture (intensively grazed, typically 15stock units (s.u.) ha"1 (= 2.5 adult cattle ha"1)) oroversown tussock (extensively grazed, up to 10 s.u.ha"1). Parts of the lower Stag Stream (SU3, SU4) andthe protected reach of the Centre Burn were alsopartly lined with shrubs (chiefly Coprosma spp).The physical characteristics of the study reaches aresummarised in Table 2. Photographs of most reachesare given in Williamson et al. (1990) or Williamsonet al. (1992).

METHODS

Survey timing and reach selection

At most streams, geomorphically representativereaches (50-250 m long) of areas that were eitherriparian-protected or riparian-grazed (or grazed andchannelised) were selected for comparison (identifiedby P and U as final letter of sitecodes respectively).Exceptions were Mt Hamilton Stream, where thegrazed reach included a broad stock and vehiclecrossing, and thus represented the most damaged partof the reach; and Stag Stream, where no riparianprotection had taken place but a range of grazingintensities and both channelised and unchannelisedreaches occurred. In Stag Stream we compared: (1) areach that was extensively grazed by sheep (SU1) butlacked riparian protection with a reach that waschannelised and extensively grazed (SU2) in theadjacent paddock downstream; and (2) a reach (SU4)in improved pasture, that was intensively grazed bycattle for long periods (August-December), with anadjacent upstream reach in oversown tussock (SU3),lacking riparian protection, where cattle usually grazefor 1 month per year.

Selected reaches on four of the streams (see fig. 1in Williamson et al. 1992) were surveyed between 18and 22 November 1987, and the reaches on StagStream were surveyed on 10 and 11 December 1988.The flows in Hamilton Bum during the surveys were

Table 2 Summary of physical characteristics of study reaches. Shade ratio = bank plus vegetationheight/channel width (high value = high degree of shading); (a) = clay overlain with interspersedcobbles and gravel; (b) = predominantly small cobbles (64-128 mm); (c) = bedrock with 30% cover bycobbles and gravel in wetted areas. Details of measurements are given in Williamson et al. (1992).

Reach

SU1SU2SU3SU4RPRU1RU2CP

CU2MPMUHP1HU1

Catchmentarea (km2)

4.54.87.39.53.34.38.25

10.3

12.010.013.632.837.4

Channelslope

(m.nr1)

0.0180.0250.0220.0220.0100.0240.0100.038

0.0160.0180.0070.0060.007

Medianwidth (m)

0.73.31.31.91.34.32.14.0

9.36.49.9

15.925.7

Meansubstratesize (mm)

18501411(a)7348(b)

(c)32603830

Stream vegetation(cover, dominant type)

20% moss60% algal mats2% moss and films12% moss and silty films30% moss5% moss80% algal mats and filaments13% macrophytes and

filamentous algae65% algal mats02% algal mats00

Shade ratio

1.080.401.500.451.340.560.404 . 0

0.250.520.150.100.10

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262 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

between values exceeded 90 and 96% of the time(Southland Regional Council pers. comm.), indicatingbaseflow conditions in the stream in the study area.

Stream temperatures were monitored in pairedprotected, grazed, or grazed and channelised reachesfor 5-17 days during February-April 1989.Thermistors attached to stakes in the main flowrecorded water temperature at 30-min intervals ondata loggers (Tasman mini datalogger or SquirrelSQ8-1 pH/lU).

Invertebrates

Benthic invertebrates were collected using a Surbersampler (0.1 m2; 0.5 mm mesh net). In reaches of thelarger streams (Mt Hamilton and Hamilton), threesamples were collected at points selected at randomat five cross-sections that were equally spaced alongthe representative reaches. These 15 samples werepooled by combining one sample from each cross-section to give three, five-sample, composites perreach for analysis. In the smaller streams (Reed, Stag,and Centre) samples were collected at three or fourrandom points at three equally spaced cross-sections.These 9 or 12 samples were pooled, by combiningone sample from each cross-section, to give three(Reed, Centre) or four (Stag) composites (each ofthree samples) per reach for analysis. Samples werepreserved immediately in 10% formalin. In thelaboratory, invertebrates were identified to the lowestpracticable taxonomic level (usually species or genus,except for Chironomidae), following the keys listedin Quinn & Hickey (1990a), and counted. First, theless common taxa were counted, then densities ofabundant taxa (> 200 in a pooled sample) werecalculated from counts of two subsamples that eachcontained at least 25 individuals, obtained using aFolsom-type sample splitter.

In-stream vegetation

The percentage of the bed covered by periphyton andmacrophytes was assessed visually at each cross-section. In Stag Stream, periphyton biomass and finesediment content were also measured to quantify theobvious differences between the reaches. This wasachieved by scraping representative areas (8.5 cm2)from the upper surfaces of three randomly selectedstones at five equally spaced cross-sections in eachstudy reach. These 15 samples were pooled bycombining one sample from each cross-section togive three, five-sample, composites per reach foranalysis, and frozen. After thawing and removal ofmacroinvertebrates, the dry weight (105°C) and ash-

free dry weight (400°C) were measured to assess thetotal biomass and sediment content of the periphyton.

Statistical analyses

Overall tolerance of the invertebrate community topollution or enrichment was assessed by calculatingQuantitative Macroinvertebrate Community Index(QMCI) values (Stark 1985):

sQMCI = Z(/i,a¡)W

¿ = 1where n, = number of individuals of the ith scoringtaxon, a, = score of that taxon (assigned value from 1to 10, representing grossly polluted to very cleanconditions respectively), and N - total number ofindividuals collected.

Cochran's Q test (Pridmore 1985) was used toevaluate differences in presence or absence ofinvertebrate taxa within and between the pairedreaches. Differences in invertebrate communitydominance between paired reaches were investigatedby calculating the Spearman correlation coefficient(rs) (Zar 1984) for densities of common taxa (> 10 m2

in at least one reach of a comparison). The statisticalsignificance of differences in density of individualand total invertebrate taxa and QMCI values weretested using non-parametric ANOVA (Conover &Iman 1981) (two reach comparisons) or non-parametric ANOVA with the Tukey adjustment formultiple inference for intersite comparisons (> tworeach comparisons) (Zar 1984).

Similarities in the overall composition of theinvertebrate communities among the sites were alsoinvestigated by using detrended correspondenceanalysis (Hill 1979) to ordinate the sites. Log biomassand density scales were used to set pseudo-speciescut levels (Furse et al. 1984) (i.e., cut levels = 10,100,1000,10 000 nr2). Rare taxa were downweightedto reduce their influence in the ordinations.

RESULTS

Effects of intensive riparian grazing

Intensive grazing of riparian vegetation by cattleappeared to have the greatest impact on invertebratecommunities in small streams. This is illustrated bymarked differences between the intensively, cattle-grazed reach of the Stag Stream (Site SU4) and itsreference, extensively grazed, reach (Site SU3)compared wilh the minor differences between theriparian-protected and intensively grazed reaches ofthe larger Mt Hamilton Stream and Hamilton Burn.

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Quinn et al.—Grazing, channelisation and stream inveitebrates 263

Key to A:Bothers •Crustacea HTrichoptera

QoligochaetaD Diptera 0 Ephemeroptera

@ Mollusca EColeoptera • Plecoptera

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SU3SU4 MP MU HP1 HU1

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Fig. 1 Differences in invertebrates between referenceand intensively grazed reaches: A, relative contributionsof major taxonomic groups; B, invertebrate densities (mean+ SE); C, taxonomic richness (excluding Chironomidae,mean + SE); and D, QMCI values (mean + SE). See Table1 for riparian management.

The riparian area of SU3 comprised oversowntussock that was extensively grazed, predominantlyby sheep (Table 1). Riparian shrubs shaded the reach,and its gravel bed supported thin periphyton filmsfree of visible silt deposits (Table 2). Periphytonbiomass at the more open SU4 was twice that at SU3,but the thick films and isolated mosses were ladenwith fine sediment and consequently the periphytonAFDW/DW ratio (0.16) was 3-fold lower than inSU3. Despite the differences in shading, SU3 wasonly slightly (0.6°C on average) cooler than SU4 insummer, resulting from the carry-over of heat fromthe unshaded, channelised, reach upstream (SU2;Fig. 2). SU3 and SU4 had similar invertebrate densitiesand their taxonomic lists were similar (Q test;P > 0.05), as was expected for two reaches < 1 kmapart on the same stream. However, the densities of10 taxa differed significantly (Table 3) and samplesfrom the two reaches were clearly separated inordination space (Fig. 3A). Mean QMCI value was40% lower in RU4 (Fig. 1), indicating a shift to amore enrichment-tolerant fauna. Notably, the densitiesof the mayfly Deleatidium sp., the spiral-cased caddisHelicopsyche albescens, the clam S. novaezelandiae,Naididae, and the snail Potamopyrgus antipodarumwere significantly lower (P < 0.05) at SU4, whereasdensities of Chironomidae, and thecaddisfly Oxyethiraalbiceps were significantly higher (Table 3, Fig. 1).

In Mt Hamilton Stream the extensively sheep-grazed reach (MU) included a broad stock and vehiclecrossing, and thus represented the most damaged partof the grazed reach. MU was much more open tosunlight than the riparian-protected reach (MP),because of its wider channel and lower banks andriparian vegetation (i.e., short exotic pasture grassescompared with small shrubs, cooksfoot, and tussocksin the riparian protected reach; Table 2). Elsewherein the paddock that included MU, the channelmorphology was indistinguishable from the riparian-protected reach (Williamson et al. 1992). Althoughlight input was greater in MU, temperatures andwater chemistry would not be expected to differ fromMP because of the short distance (50 m) separatingthe reaches. These two reaches had very similartaxonomic lists and overall community composition,with only six taxa differing significantly in density(Table 3); the main difference was a 2-fold highertotal invertebrate density (P < 0.05) in the grazedreach (Fig. 1). The similarity of the invertebratecommunities between the riparian-grazed and-protected reaches is demonstrated by the overlap intheir distribution of replicate samples in ordinationspace (Fig. 3A).

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264 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

B

S!i

222018161412108

222018161412108

2422201816141210

222018161412108

12

11

10

9

8

7

6

150

0 48 96 144 192Time (hours from 31 Jan 1989)

48 96 144 192Time (hours from 31 Jan 1989)

CPCU2

48 96 144 192Time (hours from 13 Feb 1989)

RPRU2

24 48 72 96Time (hours from 13 Feb 1989)

RPRU2

120

24 48 72 96Time (hours from 30 March 1989)

Fig. 2 Comparison of water temperatures betweenreference and grazed or channelised reaches in summerand autumn 1989.

200

200

Fig. 3 Patterns of invertebrate community similarityamongst samples and reaches shown by ordination(detrended correspondence analysis). A, intensively grazedand reference reaches. B, channelised plus grazed andreference reaches. See Table 1 for riparian and channelmanagement.

In contrast to the marked effects of cattle grazingon invertebrate communities apparent in Stag Stream,few differences were observed between riparian-protected and intensively cattle-grazed reaches of thelarger Hamilton Burn (HP1 and HU1 ; median widths15.9 and25.7 m, respectively; Table 2). These reacheshad similar total invertebrate densities and overallcomposition (Fig. 1), with the samples occurringclose together in ordination space (Fig. 3A). Onlyfour, relatively uncommon, taxa differed signifi-cantly (P < 0.05) in density between the reaches(Table 3).

Effects of channelisation and riparian grazingThe physical and invertebrate community character-istics of the four channelised reaches that were grazed

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Quinn et al.—Grazing, channelisation and stream invertebrates 265

intensively (RU1, RU2, and CU2) or extensively The total densities, the invertebrate taxa present,(SU2) are summarised in Tables 2 and 4 and Fig. 3B taxa richness, and their order of abundance differedand 4, along with their paired unchannelised reaches, significantly {P < 0.05) between these reaches (Fig. 4)Two of the reference reaches were riparian-protected and their invertebrate communities were widely(RP and CP) and one was extensively grazed (SU1, separated in ordinarion space, reflecting grossTable 2). The effects of channelisation at RU1 and differences in the physical characteristics of the sitesCU2 have been exacerbated by subsequent erosion (Table 2). CU2 had significantly higher densities ofcausing channel-deepening and widening, but this taxa capable of exploiting the thick algal mats presentdid not appear to have occurred at SU2. (Table 2) and associated detritus (i.e., the algal piercer

Differences between (he paired reaches were most O. albiceps, the burrowing chironomidae, andmarked in Centre Burn. At CP, the stream meandered collector-gatherer Hydora sp.: Table 4). In contrast,across the floodplain and comprised gravel-bottomed the protected reach had high densities of the detrituspools linked by short cobbly riffles. Small shrubs feeding worm Lumbriculus variegatus and theshaded the channel margins, as indicated by the shade scraping snail P. antipodarum. Despite the proximityratio (Table 2). In contrast, the channelised and grazed of the two reaches, several taxa that were present atreach (CU2) had a > 2-fold wider, straight channel CP were not found in the channelised and grazedthat had eroded down to the mudstone bedrock. Here reach. These included the obligate shredderthe stream flowed in several shallow braids and Triplectides obsoleta, the facultative shredders 0.cobbles and gravel covered only 30% of the wetted feredayi, and A. cyrene, the predatory damselflyarea (Table 2). The more open nature of the channel- Xanthocnemis zealandica, the filtering bivalvesised reach was reflected in increases in daily maximum Hyridella menziesi and Sphaerium novaezelandiae,temperature of up to 5.1°C above the riparian- the omnivorous caddisfly Hudsonema amabilis, andprotected reach in late February 1989 (Fig. 2). crayfish Paranephrops zealandicus (Table 4).

Table 3 Mean densities (no. m 2) of taxa whose densities differed significantly (P < 0.05) between grazed reaches andreference riparian-protected, or lightly grazed, reaches (see Table 1). Functional feeding groups: C, collector; B,browser; F, filterer; P, predator; S, shredder; A, algal piercer; O, omnivore.

Reaches

Species (feeding group) SU3 SU4 MP MU HP1 HU1

OligochaetaLumbriculus variegatus (C)Naididae (C)MolluscaPotamopyrgus antipodarum (CB)Sphaerium novaezelandiae (F)CrustaceaParaleptamphopus caeruleus (C)EphemeropteraColoburiscus hume ralis (F)Deleatidium sp. (CB)Nesameletus sp. (CB)Rallidens mcfarianei (CB)PlecopteraStenoperia prasina (P)TrichopteraHelicopsyche albescens (CB)Oxyethira albiceps (A)Pycnocentria evecta (CB)Psilochorema leptoharpax (P)Polycentropodidae (P)Pynocentrodes sp. (CB)DípteraChironomidae (CB)Eriopterini (CB)Muscidae (P)

1304

141713

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375

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22

206

9.3

7.31.3

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18940

0

277.3

6.7

32

1.3 11

4.7 1.3

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266 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

Key to A :• Others BCrustacea UTrichoptera

QOIigochaetaD Diptera HEphemeroptera

SMollusca E3Coleoptera«piecoptera

SU1SU2 RP RU1 RU2 CP CU2B

30000

>• 20000inc"2. 10000CO

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—*1 T

\ASU1SU2 CPCU2RP RU1RU2

ReachFig. 4 Differences in invertebrates between referenceand channelised and grazed reaches: A, relativecontributions of major taxonomic groups. Pairs joined bya horizontal bar did not differ significantly (P > 0.05) intaxa presence/absence (Q test) or had significantlycorrelated (P< 0.05) taxa order of rank (Spearman rs); B,invertebrate densities (mean + SE); C, mean taxonomicrichness (excluding Chironomidae, mean + SE); and D,QMCI values (mean + SE). See Table 1 for riparian andchannel management

In Reed Burn, two channelised and grazed reaches(RU1 and RU2) were compared with the riparian-protected reach (RP). RU1 was only 100 mdownstream of the protected area, but had a wider,steeper channel and larger sediments than RP (Table2). High banks provided canyon shading for much ofthe day at RU1, but light input would have beenhigher than in RP where tussocks and exotic grassesover-hung the banks of the narrow channel. RU2 wasmore open than RU1 and periphyton mats covered80% of the gravel bed (Table 2). Although the taxapresent did not differ significantly between thesethree reaches (Q test, P > 0.05), order of abundance(rs; Fig. 4) and densities of many taxa (Table 4)changed markedly and the samples were clearlyseparated in the ordination (Fig. 3B). The densities ofmost of the numerically dominant taxa in RP weresignificantly lower in RU1; Deleatidium sp.,Pycnocentrodes aureola, and Psilochorema nemoralewere exceptions, being markedly more abundant(Table 4). The increase in Deleatidium density wasthe main cause of the higher QMCI values in RU1(Fig. 4). This was associated with a change in substratefrom clay overlain with interspersed gravel andcobbles at RP to a cobble bed at RU1 (Table 2).

RU2 was much more open than RP (Table 4), andthis was reflected in differences in water temperatures:mean and daily maximum temperatures were 3.6°Cand up to 10.2°C higher, respectively, at RU2 thanRP in summer (early February), and 0.5 and 2.2°Clower, respectively, in autumn (Fig. 2). The mainchanges in invertebrate community composition atRU2 compared with RP were: a 40% reduction inmean QMCI; increases in densities of Chironomidae,O. albiceps, and Deleatidium sp.; the elimination ofthe H. albescens and S. novaezelandiae; and themarked reduction in densities of the amphipodP. caeruleus (Table 4).

The channelised and extensively grazed reach ofStag Stream (SU2) was compared with an extensivelygrazed reach immediately upstream (SU1). Theextensive grazing of the riparian zone in SU1 appearedto have had little impact on the stream habitatconditions other than reducing vegetation overhang(Williamson et al. 1992). However, canyon shadingby the banks maintained a high shade ratio (Table 2)and restricted the solar input to skylight, rather thandirect sunlight. The channel works in SU2 involvedwidening, without significant channel straighteningor deepening. Average bed particle size increasedfrom medium gravels at SU1 to large gravels at SU2(Table 2). The reduction in shading at SU2 wasreflected in: (1) the change in periphyton from moss

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Quinn et al.—Grazing, channelisation and stream invertebrates 267

and thin diatom films at SUl to thick periphyton markedly in their density between reaches (Table 4).mats that covered c. 60 % of the cobble surfaces at The stoneflies M. diminuta and A. cyrene, S.SU2 (Table 2); and (2) higher mean and daily novaezelandiae, and P. caeruleus, that weremaximum water temperatures at SU2 (of 0.5°C and moderately common at SUl, were much less abundantup to 4.2°C, respectively cf. SUl) (Fig. 2). at SU2, or were not recorded. Species capable of

These two reaches did not differ significantly in exploiting the abundant periphyton mats, such as 0.total invertebrate density or taxa present, and had albiceps and Chironomidae, were much moresimilar faunal dominance (rs = 0.56, P < 0.05) (Fig. abundant at SU2, whereas densities of Deleatidium4). However, SU2 had significantly lower taxa and H. albescens were 3 times lower. Theserichness and QMCI values than SUl (Fig. 4), and all differences in invertebrate community compositionthe numerically dominant taxa, other than P. were reflected in the clear separation of samples fromantipodarum, Naididae, and L. variegatus, differed the two reaches in ordination space (Fig. 3B).

Table 4 Mean densities (no. rrr2) of taxa whose densities differed significantly (P < 0.05) betweenriparian-protected (RP, CP) or extensively grazed (SUl) reaches and paired, channelised and grazed(RUl and RU2, CU2, SU2), reaches. Underlined values did not differ significantly (P > 0.05) betweenReed reaches and bracketed values differed significantly between RUl and RU2 but did not differ fromRP; see Table 3 for feeding group key.

Species (feeding group)

OligochactaL variegatus (C)Naididae (C)MolluscaP. antipodarum ( CB)S. novaezelandiae (F)CrustaceaP. caeruleus (C)MegalopteraA. diversus (P)OdonataX. zealandica (P)EphemeropteraDeleatidium sp. (CB)PlecopteraAustroperla cyrene ( S, CB)M. diminuta (CB)TrichopteraH. albescens (CB)H. amabilis ( O)H. parumbripennis (P)O. albiceps (A)O.feredayi(S,CB)P. aureola (CB)P. evecta (CB)P. nemorale (P)Polycentropodidae (P)T. obsoleta ( S)HemipteraSigara sp. (CB)ColeopteraHelodidae (SB)Hydora sp.(CB)DipteraChironomidae (CB)A. neozelandica (CB)Muscidae ( P)

SUl

25

464

45

1547

3418

1937

27290

38

146

354467

SU2

2.5

1.7

10.8

553

4.20

663

1242300

173.3

19

2146175

RP

1653

640116

3635

447

659Q

26Ûß0

1.11

14

42

RUl

277

38Q

145

2153.

701211

434

2Q2

3.3

55

RU2

(550)

(556)Q

1

1843

3.3

037

(18)962626

2Q18

1

3342

CP

23570

1967341

10

0

5.6

11614

61

22

0

373

1

CU2

5612

2610

0

0

3790

0

1

110

1033

17

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268 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

DISCUSSION

The main effects of grazing and channelisation onstream habitat and invertebrate communities observedare summarised in Table 5. The results support thehypothesis that the effects on stream biota of stockgrazing of riparian vegetation decrease with increasingstream size. This suggests that programmes to monitoragricultural impacts on stream health should focus onsmall streams, rather than large rivers as is usualpractice. Differences in invertebrate communitiesbetween reaches that were intensively grazed andtheir reference, riparian-protected or extensivelygrazed, reaches were minor in the larger HamiltonBum but substantial in the smaller Stag Stream (Fig. 1and 3, Table 3). Two factors probably contributed tothis: (1) riparian vegetation in the protected reachesdid not shade the larger streams; and (2) HamiltonBurn was actively meandering within its flood plain(Williamson et al. 1992), masking any effects ofgrazing on channel morphology and benthiccommunities. Increase in solar radiation input throughreduction of shading vegetation appeared to be themain factor determining impacts of riparian grazingon the invertebrates in the smaller streams, and wasalso important at sites that were both channelised andgrazed. In the channelised reaches, stock grazingappeared to have a relatively minor effect oninvertebrate community structure compared to thephysical effects of channel widening and deepening

brought about by channelisation works and anysubsequent bank erosion.

Effects of shading

Reduction of shading as a result of grazing was mostmarked in lower Stag Stream. In SU1, the smalleststream examined, extensive grazing reduced the heightof riparian vegetation but the banks were not damaged(Williamson et al. 1992) and the small stream was sonarrow and incised that canyon shading kept lightinput low (Table 2). In the largest stream, HamiltonBurn, shading by riparian vegetation was minimal inprotected reaches (Table 2) so that riparian grazinghad no influence on light input. Channelisationresulted in marked reductions in stream shading in allof the small to intermediate-sized streams surveyed(Table 2).

The higher daily maximum water temperaturesduring summer and the greater diurnal temperaturevariability observed in the more open, grazed orchannelised reaches (Fig. 2), are consistent with otherobservations of the effects of shade removal (e.g.,Graynoth 1979; Skovlin 1984; Beschta & Taylor1988). The autumn temperature measurements at RPand RU2 (Fig. 2E) also indicate that unshaded reachesin the small streams show greater seasonal variabilitythan shaded reaches, being cooler in late autumn andwinter, and warmer in summer. This is consistentwith Graynoth's (1979) observations that mean daily

Table 5 Summary of main impacts of riparian grazing and channelisation on channel morphology (from Williamson etal. 1992) and invertebrates in the study streams (-, decreased; +, increased).

Activity

Extensive grazing

Extensive grazing

Intensive grazing

Intensive grazing

Grazing andchannelisation

Streamwidths

1-2 m

6m

1-2 m

16 m

1-4 m

Effects on physical habitat

- Vegetation overhang and shade insmall streams

Bank damage at stock crossingsSlight - shade-Shade- Vegetation overhang+ Temperature variabilityLocalised bank damage increasing to

severe damage and sedimentationwhere streamside soils moist

Bank damage at stock crossings

-Shade- Vegetation overhang+ Temperature variability+ Width-Depth+ Substrate size or erosion to bedrock

Effects on invertebrates

Minor

Minor+ Total density- Cool water taxa- Taxa favoured by low periphyton+ Chironomids and algal piercers- QMCI values

Minor

- Cool water taxa- Taxa favoured by low periphyton-Bivalves- Shredders+ Chironomids and algal piercers-QMCI values

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maximum water temperature in an open stream reach(forest clearfelled to stream edge) was 7.2°C higherin spring (October) and 3.6°C cooler in winter (July)than in an upstream shaded reach (logged catchmentwith forested buffer strip).

Increases in algal periphyton abundance andsummer water temperatures were usually observed inthe smaller streams where grazing or channelisationopened the stream to increased insolation (i.e., atRU2, SU2, SU4, and CU2) (Table 2, Fig. 2). Directsunlight is particularly favourable to filamentous greenalgae (Spencer et al. 1985; Steinman & Mclntire1987). Analyses of single water samples collectedduring our survey (authors' unpublished data) givean indication of the nutrient status of the streams. Inthe shaded reaches (RP, SUl, and SU3) of the smallerstreams, the concentrations of dissolved reactivephosphorus (16-23 mg m""3) and dissolved inorganicnitrogen (58-77 mg mr3) were at growth-saturatinglevels (Stockner & Shortreed 1978; Grimm & Fisher1986; Bothwell 1989, Homer et al. 1990; Stanley etal. 1990). This suggests that the increases intemperature and light, rather than changes in nutrient;,were primarily responsible for the higher periphytonbiomass in open reaches of these streams. By contrast,in the larger Mt Hamilton Stream and Hamilton Bum,low dissolved reactive phosphorus concentrations of2 and 5 mg m~3, respectively, suggest that nutrientlimitation contributes to the low periphyton cover atboth riparian-protected and -grazed sites.

Increased periphyton was generally reflected inchanges in the invertebrate communities. Taxafavoured by thick periphyton mats, such as the algal-piecer 0. albiceps, Chironomidae, Ostracoda (Allen1959; Towns 1981; Winterbourn 1981), increasedwith periphyton abundance. In contrast, taxa thatprefer to browse on thin periphyton films, such asDeleatidium sp. and H. albescens (Cowley 1978;Winterbourn 1981, Winterbournetal. 1984), generallydecreased where mats covered most of the bed (Tables3 and 4; Fig. 1 and 4). These changes to a moretolerant fauna were reflected in reductions in QMCIvalues, which have been shown to be negativelycorrelated with periphyton biomass and chemicalindicators of enrichment (Quinn & Hickey 1990a).

Similar increases in periphyton biomass andchanges in invertebrate abundances were observed inrivers (typical median flows 7-11 m3 s"1) havingmore than 30% of their catchment in improved pasture(Quinn & Hickey 1990b).

In the present study, improved pasture occupied34-60% of most of the study reaches' catchments(table 1 in Williamson et al. 1992). However, Hie

periphyton proliferation and associated changes inthe invertebrate faunas, expected on the basis ofQuinn & Hickey's (1990b) findings, only occurred inunshaded reaches where nutrient concentrations wereabove levels that limit periphyton growth (i.e., RU2,SU2, SU4, and CTJ2). This suggests thatbothnutrientsand riparian shading have important influences onperiphyton abundance and invertebrate communitycomposition.

The higher, more variable temperatures, probablyalso influenced the success and survival of someinvertebrate species. Temperature influences thegrowth, metabolism, reproduction, and emergence ofaquatic insects (e.g., Hynes 1970; Vannote & Sweeney1980), and > 20°C is lethal for some species ofPlecoptera and Ephemeroptera (Whitney 1939;Nebeker & Lemke 1968). hi New Zealand we haveindirect evidence that this also happens. Distributionpatterns of invertebrates amongst New Zealand rivershaving annual maximum temperatures above 19°Csuggest that these groups may be restricted by warmtemperatures; in particular, Plecoptera were absentfrom such sites in a broad survey of New Zealandrivers (Quinn & Hickey 1990a). The lower abundancesof Plecoptera in the warmer open reaches of thesmall-intermediate sized streams (i.e., SU2-SU4, RU2,and CU2; Tables 3 and 4) are consistent with atemperature effect.

In Mt Hamilton Stream, there was a slightreduction in shade at the grazed reach (Table 2), butno increases in temperature or nutrients were expectedbecause of the short (50 m) distance to the riparian-protected reach. Here periphyton cover andinvertebrate taxa composition were similar in bothreaches (Tables 2 and 3, Fig. 1 and 3), with only oneof the nine common taxa (>10 nr 2) differingsignificantly between the reaches. Nevertheless, totalinvertebrate density was 2-fold higher (P < 0.05) inthe grazed reach, suggesting that periphytonproductivity was greater in the more open reach.

Although the channelised reach at RU1 was lessshaded than the heavily overgrown, protected reach(RP), mechanical channel deepening and subsequenterosion resulted in substantial canyon shading, andperiphyton cover was low (Table 2). Here taxafavoured by dense periphyton mats had low densities,whereas Deleatidium sp., H. albescens, andPycnocentrodes sp. were moderate-high in abundance(Table 4).

Effects of sedimentation

Only one reach (SU4) showed a measurable increasein accumulated fine sediment in response to intensive

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270 New Zealand Journal of Marine and Freshwater Research, 1992, Vol. 26

riparian grazing. Along with the increase inperiphyton, this may have contributed to the markeddecreases in Deleatidium sp. and H. cdbescens betweenSU3 and SU4 (Table 3).Elsewherethesetaxadeclinedin response to experimental addition of fine sedimentto a stream in Otago (Ryder 1989) and clay dischargefrom placer gold mines to several west coast streams(authors' unpubl. data).

Effects of changes in streambed morphology as aresult of channelisationChannelisation of Centre Burn increased streambederosion. Photographs taken in 1980 showed a cobblebed in the channelised reach (CU2) (SouthlandAcclimatisation Society 1981), but by 1987 this haderoded down to the bedrock with cobbles restricted to30% cover (Table 2). The gross changes in bothriparian and instream habitat between the channelisedreach and the riparian-retired reach (CP) were reflectedin their invertebrate faunas (Fig. 3 and 4, Table 4).The abundance of deposit-feeding oligochaetes andthe relative abundance of shredders in the protectedreach indicate an abundance of both fine and coarsedetritus. The presence of waterboatmen, Sigara sp.,and the red damselfly larvae, Xanthocnemiszealandica (Table 4), that are typical of backwatersand ponds (Miller 1984), reflect the abundance ofpools. Although shredders are rarely more than aminor component of New Zealand stream faunas(Winterboum et al. 1981; Rounick & Winterbourn1982), their abundance has been found to correlatewith stream stability and retentiveness of coarsepaniculate organic matter (Rounick & Winterbourn1982, 1983). Thus, the reduction in oligochaetedensities, the loss of shredders and pond species, andthe lower taxonomic richness (Fig. 4) in thechannelised reach indicate a general reduction inorganic matter retention and habitat diversity andstability with channelisation.

Increases in sediment particle size of allchannelised reaches (particularly CU2) (Table 2) wereprobably also a key factor causing the absence ormarked reductions in densities of Hyridella menziesiand 5. novaezelandiae (Table 4), which prefer finegravel and sand sediments (e.g., James 1985).

General ecological role of riparian zones

Our comparisons of stream communities in reacheswith contrasting riparian vegetation and grazingpractices indicate that riparian zone management canhave an important influence on the quality and stabilityof stream habitat conditions for macroinvertebrates.

Shading by riparian vegetation emerged as the mainfactor; where the vegetation was sufficiently tall tosubstantially shade the stream, variations in watertemperature were reduced at the diurnal and annualscales (Fig. 2) and periphyton was limited to mossesand thin epilithic films. In more open reaches, thickmats developed under summer low-flow conditionsin the more open reaches where nutrient concen-trations appeared to exceed levels that limit biomassdevelopment These mats would be expected to behighly susceptible to sloughing in floods (Biggs &Close 1989; Quinn & Hickey 1990b). Hence foodresources in these open reaches are likely to be morevariable over a monthly-annual time scale, resultingin a corresponding decrease in stability of the animalcommunities.

Riparian vegetation in reaches that were notintensively grazed or channelised provided substantialshading under summer conditions when bare channelwidths were less than c. 6 m (i.e., at RP, SU1, SU3,and CP; Table 2). Intensive grazing by cattle andsheep greatly reduced this shading whereas extensivegrazing had less of an effect. Over a whole drainagebasin, this represents loss of a substantial amount ofthe cool, headwater, stream habitat, resulting in acorresponding reduction in biodiversity of the aquaticinvertebrate fauna.

The loss of shading from small headwater streamswould also be expected to increase maximum watertemperatures in a drainage basin. Two factorscontribute to this effect. First, the shallow headwaterreaches are more susceptible to solar heating than thedeeper, larger reaches (Mosley 1983) (i.e., at widthsof > c. 6 m) at which shading by ungrazed vegetationis lost. Second, the warmer water flowing from theunshaded small reaches into the larger, naturally open,reaches will increase maximum temperatures there.

The key role of riparian shading in conservinginvertebrates in headwater stream habitats suggeststhat in such streams riparian protection need onlyextend a few metres from the stream bank to protectinstream habitat conditions. However, such narrowstrips are not likely to substantially reduce sedimentand nutrient concentrations in water flowing offpastures (Phillips 1989; Nieswand et al. 1990). Hencewider strips are expected to be required whereprotection aims to reduce sediment and nutrientconcentrations in downstream reaches, lakes, orestuaries.

Where the riparian vegetation comprises trees,rather than the tussocks, grasses, and occasional smallshrubs of the Southland study streams, the size ofstream to which riparian shading has important

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Quinn et al.—Grazing, channelisation and stream invertebrates 271

influences on the invertebrates, organic matter inputs,and bank stabilisation, is expected to increase (e.g.,Sedell & Froggatt 1984). Our studies indicate thattree planting may be useful in mitigating the effectsof agricultural development in larger Southlandstreams. Further surveys, covering streams that includeriparian tree planting in addition to retirement, arerequired to provide a broader understanding of theinfluences of riparian management of agriculturallydeveloped catchments on channel morphology andstream ecology.

ACKNOWLEDGMENTS

We thank the Southland Regional Council for theirassistance in selection of the study sites and provision offlow data; Southland Fish and Game Council (particularlyMaurice Rodway) for their assistance in selection of thestudy sites, logistical support during our field work and thetemperature monitoring; John Oldman and Ron Ovendenfor technical assistance with physical habitat measurements;Geoff Latimer for assistance with temperature monitoringinstruments; and Bryce Cooper, Mike Winterbourn,Stephen Swales, Kevin Collier, and two anonymousreferees for their constructive reviews of the manuscript.

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