Hydrobiologia 518: 79–94, 2004.© 2004 Kluwer Academic Publishers. Printed in the Netherlands.

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The influence of stream habitat and water quality on macroinvertebratecommunities in degraded streams of northwest Mississippi

J. D. Maul1, J. L. Farris1, C. D. Milam1,2, C. M. Cooper3, S. Testa III3 & D. L. Feldman1,4

1Ecotoxicology Research Facility, Environmental Sciences Program, Arkansas State University,P.O. Box 847, State University, AR 72467, U.S.A.E-mail: jmaul@astate.edu2EA Engineering, Science & Technology, Inc., 15 Loveton Circle, Sparks, MD 21152, U.S.A.3USDA-ARS, National Sedimentation Laboratory, 598 McElroy Drive, Oxford, MS 38655, U.S.A.4Montana Department of Environmental Quality, 1520 East 6th Street, Helena, MT 59601 U.S.A.

Received 21 April 2003; in revised form 25 July 2003; accepted 28 October 2003

Key words: environmental relationships, physical habitat, macroinvertebrate communities, Mississippi, streamrestoration, water quality

Abstract

Streams in the loess hills of northwest Mississippi have undergone dramatic physical changes since Europeansettlement and both physical and water quality processes may play a role in influencing biotic communities ofthese stream systems. The objectives of this study were to identify the response of macroinvertebrate taxa to waterquality and habitat parameters in streams of northwest Mississippi, examine the efficacy of an a priori classificationsystem of stream channel evolution and condition class using macroinvertebrate communities, and examine short-term (<2 yr) temporal variation of macroinvertebrate communities. Separation of sites based on four conditionclassifications was not distinct. However, best attainable sites did plot together in an ordination analysis suggestingsimilarity in macroinvertebrate communities for least disturbed sites. Similarly, for stage of channel evolution,sites characterized by lack of bank failure and sinuous fluvial processes had relatively similar macroinverteb-rate communities. Ordination analysis also indicated high temporal variation of macroinvertebrate communities.Reference sites (best attainable and stable sites) had more similar communities between years than unstable andimpacted sites. Results of this study: (1) suggest total solids, total phosphorus concentration, percent substrate assand, ammonia concentration, and conductivity were important variables for structuring stream macroinvertebratecommunities in northwest Mississippi, (2) identify potential indicator taxa for assessing such streams based onwater quality and physical habitat, (3) provide support for current a priori site classifications at the best attainable(least impacted) category relative to the macroinvertebrate communities, and (4) demonstrate that between-yearvariation is an important factor when assessing streams of north Mississippi and this variability may be related tothe degree of stream degradation.

Introduction

Streams in the loess hill and bluffline region of north-west Mississippi have undergone dramatic physicalchanges that have altered habitat for stream macroin-vertebrates (Cooper & Knight, 1986). These alter-ations have occurred since European settlement andcan be attributed to forest clearing combined withexcessive sediment loads from row crop agriculture.

Processes such as channel incision (i.e., channelbed lowering and ultimately disconnection from thefloodplain) and stream widening have led to loss ofstream habitat heterogeneity, specifically loss of or-ganic debris, decline in mean water depth, and declinein pool habitat (Shields et al., 1994). These channeldegradation processes have caused bank failure andgully erosion that further impaired stream water qual-ity (e.g., increased within stream suspended sediment

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loads) and adversely impacted macroinvertebrate com-munities (Lenat, 1984). Both physical processes andwater quality likely play an important role in influ-encing biotic communities within stream systems ofnorthwest Mississippi (Shields et al., 1994).

Macroinvertebrate metrics are commonly used toassess stream and riverine systems and for comparingimpacted to least impacted sites (Kerans & Karr, 1994;Fore et al., 1996). Streams have also been classified bytheir physicochemical and habitat characteristics (Si-mon, 1989; Shields et al., 1998). Understanding therelationship between biotic and abiotic characteristicsis critical for ultimately differentiating levels of streamdegradation. In this study, seventeen commonly util-ized macroinvertebrate metrics were examined fortheir response direction relative to two habitat basedclassification schemes used for identifying stream im-pacts in northwest Mississippi. For the loess hills ofMississippi, invertebrate metrics and / or taxa use-ful for identifying stream degradation are relativelyunknown. Such information can be used in future de-velopment of an ecoregion specific invertebrate indexof biotic integrity.

The first objective of this study was to identifyinvertebrate taxa that responded to individual waterquality and physical habitat parameters from 26 northMississippi stream reaches, and ultimately identifypotential candidate taxa for inclusion into a regionalinvertebrate community index. The second objectivewas to examine biological communities of 44 streamreaches relative to a priori classifications of streamchannel evolution and channel condition status. Thepurpose of this analysis was to test the biologicalrelevance of these classification schemes and assistin determining if best attainable or least impactedstream reaches can be differentiated from impactedreaches using the macroinvertebrate community. Thefinal objective was to examine between-year variationof macroinvertebrate communities and physical hab-itat in 17 stream reaches sampled in 1999 and 2000.Previous studies have shown that benthic macroin-vertebrates may exhibit short-term (<2 yr) temporalvariation within a site (Kerans & Karr, 1994). Thiscomparison allowed for a test of the validity of 2-year studies, estimated variability of physical habitat,and identified which invertebrate metrics might beconfounded by temporal variation.

Study sites

Forty-four stream reaches (sites) were sampled from17 watersheds in the loess hill region of northwestMississippi between 14 June and 12 August 1999(Fig. 1). Current and historical landscape and physicalcharacteristics of these study watersheds and streamreaches have been described previously (Shields et al.,1995a–c). Each watershed was located east of the bluffline, which delineates the Mississippi Alluvial Plainfrom the loess hills that have highly erodible soil withextensive stream channel incision and bank failure. Inyear 2000, 17 stream reaches previously sampled in1999 were re-sampled between 20 June and 26 July2000.

Methods

Invertebrate sampling

Aquatic invertebrates were collected from a variety ofsubstrates at each study site including sand and gravelbed materials, large woody debris and snags withinthe stream channel, stone and rocks (riprap), leafpacks, and coarse particulate organic matter (CPOM).A qualitative multi-habitat composite sample was alsotaken. All samples were preserved in 95% ethanol withrose bengal dye added to facilitate processing.

Sediment cores (10 cm depth) were collected witha 5.08 cm diameter Lexan corer from three locationswith sand substrate and three with gravel at each site.In sand, cores were collected perpendicular to waterflow from a shallow area, run, and depositional areaof the reach and composited into a single container.In gravel beds, cores were taken in upstream, middle,and downstream sections of the bed (or from multiplegravel areas if those present were small) and compos-ited. During core sampling, excess water was removedcarefully to maintain the integrity of the collected sur-face material. Samples were tumbled after addition ofethanol/rose bengal to ensure even distribution of thepreservative.

Large woody debris or snag surface material wasbrushed into a standard D-frame net (0.8 × 0.9 mmmesh size) using a toothbrush until approximately1800 cm2 of area was sampled. Two 30 × 30 cmbrushings were targeted for each site; however, ifwood substrates were less than 30 × 30 cm, availableareas were brushed consecutively until the 1800 cm2

area was sampled.

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Figure 1. Locations of 44 stream reaches (�) from 17 watersheds in the loess hill region of northwest Mississippi where macroinvertebrate,habitat, and water quality data were collected in 1999 and 2000.

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Most encountered rock consisted of introducedlimestone riprap previously placed for improved chan-nel stability. Some sites also contained sandstone orlarge hardened clay pieces that resembled cobble orboulder material. Sampling was restricted to rocks thatwere fully submerged, but not embedded, and withinthe upper 50 cm of water. Rocks were sampled fromlotic and lentic areas and ranged from 10 to 30 cm indiameter. After a rock was selected for sampling, itwas transferred to a submerged plastic bucket wherea toothbrush and squirt bottle were used to removeall material from the rock surface. Material from thebucket was poured through a <0.5 mm mesh aquar-ium net and transferred to a sample jar containingpreservative. Multiple rocks were sampled until thevolume of sampled rocks was between four and fiveliters (usually 3–6 rocks) to facilitate similar samplingeffort among sites (i.e., sampled surface area of rocksof about 1800 cm2) (Cooper & Testa, 2001).

Two grabs of leaf pack and CPOM material fromdifferent locations in the stream reach were collec-ted and composited, yielding approximately 750–1000 cm3 volume of non-compacted material per site.Small twigs, exposed small roots, and submergedgrass were collected if leaf material was absent at thesite.

A 15 min qualitative sampling of the reach wasconducted after completion of quantitative samplingto selectively capture representatives from any ad-ditional taxa. Invertebrates were picked from rocks,gravel, clay, limbs, and other substrates and includedorganisms from the stream bottom, margin, and watersurface.

Invertebrate sorting and identification

The same team of technicians conducted macroinver-tebrate sorting so that experience bias was similaramong all samples and sites. Entire samples from thefive chosen microhabitats for each site were sorted. Asingle taxonomist identified all macroinvertebrates sovariation due to identification skill among samples andsites was evenly distributed. Organisms were identi-fied to the generic level, with the exception of thosebelonging to the family Chironomidae, those that weredamaged, very early instars, or non-insects (identi-fied to lowest most specific level of family, order, orclass). Data from each of the five sample types withina reach were combined to derive a taxon occurrenceand abundance list representative of the entire stream

reach accounting for multiple habitats (Kerans et al.,1992).

Water quality

The following water quality parameters were meas-ured twice monthly or monthly at the downstreamend of 26 study reaches: dissolved oxygen (mg l−1),conductivity (µS cm−1), temperature ( ◦C), pH, totalsolids (mg l−1), dissolved solids (mg l−1), suspen-ded solids (mg l−1), filtered ortho-phosphate (mg l−1),total phosphorus (mg l−1), ammonia (NH4) (mg l−1),nitrate (NO3) (mg l−1), total chlorophyll (mg l−1),fecal coliform bacteria (colony forming units (CFU)100 ml−1), and enterococci bacteria (CFU 100 ml−1).All water quality parameters were measured followingAPHA (1998) methods. The 12-month reach aver-ages of these variables were used for this study (i.e.,average of measurements commencing 1 year priorto biotic sampling and ending on the date of bioticsampling).

Physical habitat

Transects from left to right bank were establishedevery 10 m along each 200 m reach (i.e., 20 equidistanttransects) to systematically evaluate representativehabitat. Habitat characteristics recorded at each tran-sect included: (1) water surface width, (2) top bankwidth, (3) channel depth, (4) left and right bank angle,(5) percent canopy cover and (6) left and right bank ve-getation (% of trees, brush, Kudzu (Pueraria lobata),grass, and bare) (Table 1). Five equidistantly spacedpoints were identified on each transect resulting in agrid array of 100 total measurement points within eachsample reach. Water depth and bed substrate/habitattype (i.e., either clay, sand, gravel, riprap, woodydebris, or periphyton) were recorded at each gridpoint. Habitat richness values were calculated for eachreach by summing the number of each different sub-strate/habitat type among the grid points (Table 1).Percent representation of each bed type, number andtotal area of large woody debris (LWD) structures,length of riprap bank revetment and stone-toe, andnumber of dikes and beaver dams were also calculatedor recorded within a reach (Table 1).

Stream channel evolution stage and conditionclassifications

Stream reaches were classified based on their degreeof instability, which presents a potential source of

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Table 1. Physical aquatic and terrestrial habitat measured from 200 m reaches in streams of northwestMississippi.

Physical habitat parameter Measured as Source of data Code

Dikes No. Raw data DIKE

Rip-rap revetment Meter of bank line Raw data RRAP

Stone Toe Meter of bank line Raw data STON

Jacks Meter of bank line Raw data JACK

Grade-control structures No. Raw data GCSS

Beaver dams No. Raw data BEAV

Top-bank width Meters Mean from 20 segments of study reach TBWD

Channel depth Meters Mean from 20 segments of study reach CHDP

Left bank angle Degrees Mean from 20 segments of study reach LBAA

Right bank angle Degrees Mean from 20 segments of study reach RBAA

Canopy Percentage Mean from 20 segments of study reach CANO

Trees >5 m high Percent of bank Mean from 20 segments of study reach TREE

Brush <5 m high Percent of bank Mean from 20 segments of study reach BRSH

Kudzu Percent of bank Mean from 20 segments of study reach KDZU

Grass Percent of bank Mean from 20 segments of study reach GRSS

Bare Percent of bank Mean from 20 segments of study reach BARE

Mean water surface width Meters Mean from 20 transects of study reach WSMN

CV of surface width Unit less SD of surface width/WSMN × 100 WSCV

Max. water surface width Meters Max from 20 transects of study reach WSMX

Mean water depth Centimeters Mean from 100 grid points DPMN

CV of water depth Unit less SD of surface width/WSMN × 100 DPCV

Max. water depth Centimeters Max from 100 grid points DPMX

Habitat richness No. Habitat types observed at 100 grid points HBTR

% Substrate as clay Percentage No. grid points as clay/100 PCLY

% Substrate as wood Percentage No. grid points as wood/100 PDBR

% Substrate as gravel Percentage No. grid points as gravel/100 PGRV

% Substrate as rock Percentage No. grid points as rock/100 PRCK

% Substrate as sand Percentage No. grid points as sand/100 PSND

% Substrate as vegetation Percentage No. grid points as vegetation/100 PVGT

Woody debris structures No. Cumulative count throughout reach LWDN

Area of woody debris m2 Area calculated for each reach LWDA

Wood area/structure Percentage LWDA/LWDN WAWN

variation in stream macroinvertebrate communities ofthis region (Cooper et al., 1997). Incised streamsoften have sharper hydrographs, contain most floodflow within the channel (i.e., they are disconnectedfrom the floodplain and stream power is concentratedwithin the channel), and are characterized by lim-ited and highly volatile pool habitat (Shields et al.,1998), all of which may influence the stability of bioticcommunities (Shields et al., 1995b).

Two stream stability classifications were assignedto each reach by the same individual (5th author) basedon in-field examination and knowledge of restorationactivities. First, each reach was classified follow-

ing the channel evolution model (CEM) described bySimon (1989) and included: (1) pre-modified (char-acterized by lack of bank failure and sinuous fluvialprocesses), (2) degradation (down-cutting channel bederosion), (3) threshold (heightening and steepeningof stream banks), (4) aggradation (bank failure andchannel widening) and (5) re-stabilization (fluvial de-position and return of sinuosity) (Simon, 1989). De-scriptions of typical aquatic habitat conditions for eachstage are detailed in Shields et al. (1998).

The second classification was channel conditionstatus. Unstable/impacted (US/IM) sites were char-acterized by severely incised channels, active head-

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cutting, shallow water depths, and unstable sandy sub-strate conditions. Stable (ST) sites exhibited incisedchannels with non-active head-cutting and more slop-ing banks, thus decreased likelihood of bank failure.Sites with stream restoration activities (i.e., placementof stone toe revetments, weirs, willow post plant-ings, etc.) were classified as restored (RE) while bestattainable (BA) sites were those that were believedto be least impacted or closest to regional referenceconditions.

Invertebrate metrics

A series of commonly used macroinvertebrate metricswere calculated for each stream reach. These rangedfrom taxa richness measures to functional groupingsand were selected based on usefulness for reflectingstream conditions reported in previous studies (Lenat,1984, 1993; Kerans & Karr, 1994; DeShon, 1995;Muotka et al., 2002). Percent composition of variousfunctional groups was included because these metricsare indicative of trophic processes and has gained at-tention for their ability to convey important ecologicalinformation about a community (Cummins & Klug,1979; Cousins, 1991).

Analyses

Each stream reach was considered an individualsample or site. The first objective was addressed byconstructing site × taxa and site × environmentalvariable matrices for 26 sites where physical habitatand water quality data were collected. Taxa abund-ance and most physical and water quality data werelog-transformed (ln x + 1) while proportional datawere arcsine square root transformed. An indirectgradient multivariate analysis method, Detrended Cor-respondence Analysis (DCA) (Gauch, 1980), usingthe program PC-ORD (McCune & Mefford, 1999)was used for this analysis. DCA is based on unimodalmodels and describes the relationship among indi-vidual invertebrate taxa with a gaussian response curve(i.e., a bell-shaped response curve with an optimumrepresented as a point on an ordination diagram)(Jongman et al., 1995). Corresponding environmentalfactors and external information on site characterist-ics were then used to further interpret relationships.The non-linear multivariate approach was chosen be-cause it may be more ecologically appropriate thanlinear models (e.g., Principal Components Analysis)(ter Braak, 1986; Jongman et al., 1995) and necessary

for the simultaneous examination of multiple taxa par-ticularly since each taxon may respond differentiallyto environmental gradients (Lenat et al., 1984).

A subset of environmental variables (i.e., physicalhabitat and water quality variables) were selected thatfit into either of the following categories: (1) stronglycorrelated to either the first or second DCA axes, thus,explaining more of the variation associated with thataxis than other variables or (2) typically highly vari-able in a degraded stream (e.g., total suspended solids,Shields et al., 1997). This subset of variables wasthen used in a direct gradient analysis method, Ca-nonical Correspondence Analysis (CCA) (ter Braak,1986), to examine community patterns directly relatedto selected environmental variables (ter Braak, 1986;Palmer, 1993). Joint plots of species scores and en-vironmental variable scores were examined to identifyoptimal abundance positions of taxa along environ-mental gradients. For example, the intersection of aperpendicular line drawn from a species point to an en-vironmental axis is the weighted average of that taxonrelative to the environmental axis (ter Braak, 1986,1987).

The second objective was addressed using a site× taxa matrix of 44 sites sampled in 1999 with eachstream reach classified into CEM and condition cat-egories (see Methods) and analyzed with DCA. Over-lays of the classification variables were examined toexplore distribution of sites in macroinvertebrate or-dination space. Classifications were identified withsymbols and sites occurring near one another were in-terpreted as having similar invertebrate communities.Seventeen frequently used macroinvertebrate metricswere compared among CEM condition categories withone-way ANOVAs (PROC GLM, SAS Institute Inc.,1989) to determine which metrics best distinguishedstream classifications. A Bonferroni adjustment wasused (α = (0.05)/17 (tests) = 0.0029) since multipleinferential tests were performed on metrics derivedfrom the same biological samples.

The third objective was addressed by construct-ing a sample x taxa matrix of 34 entries. The overallmacroinvertebrate community patterns were first ex-plored using DCA and the classification overlay ofyear then examined. It was expected that the 1999and 2000 samples for each site would group closelywithin the ordination plot if low inter-annual variationof communities had occurred within a site. A secondoverlay of channel condition by year was constructedfor this DCA. The purpose was to examine the pat-terns of between-year variation of macroinvertebrate

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Table 2. Physical habitat and water quality variables from northwest Mississippi streamsincluded in the Canonical Correspondence Analysis and resulting intraset correlations and ca-nonical coefficients. Correlations that are bold indicate the five most important variables inpredicting macroinvertebrate community composition.

Variable Code Intraset Canonical

correlations (r) coefficients

Axis 1 Axis 2 Axis 1 Axis2

% occurrence of trees (ht >5 m) TREE −0.323 −0.248 −0.324 −0.406

% of Substrate as clay PCLY 0.067 −0.130 −0.085 0.018

% of Substrate as sand PSND −0.417 −0.122 −0.250 −0.146

Area of lg. woody debris LWDA −0.185 −0.175 −0.178 0.391

Avg. annual temperature YTMP −0.202 −0.279 −0.520 −0.621

Avg. annual conductivity YCND 0.446 0.288 0.025 0.478

Avg. annual pH YRPH 0.178 −0.317 0.361 0.187

Total solids TS 0.171 0.406 0.034 0.598

Total phosphorus TP −0.053 0.669 0.349 0.097

Ammonia NH4 −0.727 0.550 −0.794 0.706

Enterococci bacteria ENTERO 0.228 −0.014 0.082 −0.336

communities within each channel condition class. Atwo-way ANOVA was used to examine main and inter-action effects of stream condition class and year (α =0.0033; initial α = 0.05 with Bonferroni adjustmentfor 15 tests) (PROC GLM, SAS Institute Inc., 1989)on macroinvertebrate metrics after observing a shift instream samples due to year.

Results

Response of macroinvertebrate taxa to water qualityand physical habitat

From 26 sites sampled in 1999, 198 macroinvertebratetaxa were identified. Eigenvalues from the DCA foreach of the first two axes were relatively low (e.g.,0.183 and 0.130, respectively), indicating poor separ-ation of study reaches in macroinvertebrate ordinationspace. The gradient lengths for DCA axis I and IIwere 1.99 and 1.94, respectively. Pearson correlationcoefficients between environmental variables and thefirst and second ordination axis indicated that someenvironmental variables could be useful for explainingthe ordination axes. The 11 environmental variablesselected for inclusion in the direct gradient analysis(CCA) are shown in Table 2.

The CCA joint plot containing taxa scores and en-vironmental axes (indicated as lines extending fromthe plot origin) identified taxa responses to en-vironmental gradients (Fig. 2). Five environmental

variables had high intra-set correlations, thus weremore important in predicting community composition(Table 2). Total solids (TS) and total phosphorus (TP)were more strongly correlated with the second CCAaxis while percent sand (PSND), ammonia (NH4), andaverage annual conductivity (YCND) correlated withthe first CCA axis (Fig. 2). Longer solid lines ex-tending from the origin within the joint plot (Fig. 2)indicated greater importance of that environmentalvariable. The five environmental variables were rankedin order of importance to the invertebrate communitybeginning with ammonia (NH4) through total solids(TS) (Fig. 2; Table 2).

Along the ammonia axis (Fig. 2), genera such asNeogerris, Palmacorixa, and Ramphocorixa occurredat sites with the highest measured ammonia levelswhile Mochlonyx, Antocha, and Crangonyx were ob-served at sites with the lowest measured ammonialevels (Fig. 2). Taxa that occurred at the extremes ofthe five environmental axes (Table 3) would be of in-terest to water quality managers in Mississippi becausethey convey information on the sensitivity of taxa toenvironmental variable and could have potential useas indicators for particular aquatic conditions.

Efficacy of channel condition and CEM stage forclassifying stream reaches

From 44 sites sampled in 1999, 237 macroinvertebratetaxa were identified. The number of sites classified as

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Table 3. Summary results from the joint plot of the Canonical Correspondence Analysis(Fig. 1). Presented are the taxa with the greatest weighted taxa average at the high and lowextremes of five selected environmental gradients. Individuals not identified to a particulartaxonomic level are indicated (–).

Environmental Gradient Order Family Genera

gradient extreme

NH4 High Hemiptera Gerridae Neogerris

Hemiptera Corixidae Palmacorixa

Hemiptera Corixidae Ramphocorixa

Hemiptera Corixidae Hesperocorixa

Turbellaria Planaridae Phagocata

Low Diptera Chaoboridae Mochlonyx

Diptera Tipulidae Antocha

Diptera Tipulidae –

Diptera Culicidae –

Amphipoda Crangonytridae Crangonyx

Total solids High Coleoptera Hydrophilidae Helochares

Coleoptera Hydrophilidae Hydrobiomorpha

Diptera Tabanidae Tabanus

Decapoda Palaemonidae Macrobranchium

Annelida – –

Low Ephemeroptera Leptophlebiidae –

Ephemeroptera Ephemeridae Ephemera

Trichoptera Leptoceridae Nectophsyche

Diptera Simuliidae Ectemnia

Odonata Libellulidae Cannacria

Odonata Coenagrionidae Chromagrion

% of substrate High Ephemeroptera Leptophlebiidae –

as sand Odonata Libellulidae Cannacria

Odonata Coenagrionidae Chromagrion

Turbellaria Planaridae Phagocata

Hemiptera Gerridae Neogerris

Hemiptera Corixidae Palmacorixa

Hemiptera Corixidae Ramphocorixa

Hemiptera Corixidae Hesperocorixa

Low Coleoptera Hydrophilidae Hydrobiomorpha

Coleoptera Hydrophilidae Helochares

Diptera Tabanidae Tabanus

Decapoda Palaemonidae Macrobranchium

Odonata Coenagrionidae –

Conductivity High Coleoptera Hydrophilidae Hydrobiomorpha

Coleoptera Hydrophilidae Helochares

Coleoptera Hydrochidae Hydrochus

Decapoda Palaemonidae Macrobranchium

Diptera Tabanidae Tabanus

Low Trichoptera Leptoceridae Nectophsyche

Odonata Libellulidae Cannacria

Odonata Coenagrionidae Chromagrion

Diptera Simuliidae Ectemnia

Ephemeroptera Leptophlebiidae –

Ephemeroptera – –

Trichoptera – –

Continued on p. 87

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Table 3. Continued.

Environmental Gradient Order Family Genera

gradient extreme

Total High Coleoptera Hydrophilidae Helochares

phosphorus Coleoptera Hydrophilidae Hydrobiomorpha

Diptera Tabanidae Tabanus

Decapoda Palaemonidae Macrobranchium

Hemiptera Gerridae Neogerris

Hemiptera Corixidae Palmacorixa

Hemiptera Corixidae Ramphocorixa

Turbellaria Planaridae Phagocata

Low Ephemeroptera Leptophlebiidae –

Odonata Libellulidae Cannacria

Odonata Coenagrionidae Chromagrion

Trichoptera Leptoceridae Nectophsyche

Diptera Simuliidae Ectemnia

Ephemeroptera – –

Trichoptera – –

BA, US/IM, ST, and RE were 5, 24, 12 and 3, respect-ively. The number of sites classified as CEM stage1 (Pre-modified), 2 (Degradation), 3 (Threshold), 4(Aggradation), and 5 (Re-stabilization) were 4, 3,6, 27, and 4, respectively. Eigenvalues and gradientlengths for each of the first two axes from this DCAwere higher (Axis 1: eigenvalue = 0.237, gradientlength = 2.45; Axis 2: eigenvalue = 0.138, gradientlength = 1.65) than observed for objective 1, indicat-ing greater separation of study reaches in macroinver-tebrate ordination space, particularly along DCA axis1. Gradient lengths suggest taxa turnover occurred atsites positioned on either end of each axis but notto the extent to consider them completely dissimilarcommunities.

The four condition classifications did not separatedistinctly in the ordination analysis (Fig. 3A). Best at-tainable sites grouped closely together with most sep-aration occurring on the first axis. Unstable/impacted(US/IM) sites were the most variable in their macroin-vertebrate communities and occurred across the entireordination space, even plotting at each end of bothordination axes (Fig. 3A). Nine of 12 sites classifiedas stable (ST) did not vary greatly along DCA axis 2.However, most separation occurred on the first DCAaxis similar to BA sites. Interestingly, restored (RE)sites appeared to have greater separation along DCAaxis 2 than axis 1 (Fig. 3A) and differed from thepattern observed for BA and ST sites.

There were no discernible patterns for CEM classi-fied sites in the ordination analysis except for groupingof premodified sites (Fig. 3B), suggesting that thesesites shared similar communities. Also of interest isthat three degraded sites formed a loose groupingin the upper central portion of the ordination space(Fig. 3B). CEM classifications other than premodifiedand degraded were not useful for predicting macroin-vertebrate communities and ultimately the biotic statusof particular sites. Macroinvertebrate community met-rics did not differ between groupings of the con-dition status and CEM stage classification schemesdespite macroinvertebrate community patterns amongsome site classifications in the ordination analyses(Table 4).

Between-year variation of macroinvertebratecommunities

From 17 sites sampled both in 1999 and 2000, 210macroinvertebrate taxa were identified. Eigenvaluesand gradient lengths for each of the first two axes weresimilar to those observed for the DCA for objective 1(Axis 1: eigenvalue = 0.235, gradient length = 1.91;Axis 2: eigenvalue = 0.113, gradient length = 1.67),suggesting that sites at either end of each DCA axishad similar invertebrate communities. The overlay ofyear classification on sites indicated a distinct patternbetween sites sampled in 1999 to those sampled in2000 (Fig. 4A) with the former having lower axis 1

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Figure 2. Ordination diagram (Canonical Correspondence Analysis) with five environmental axis and taxa scores (+) plotted. Environmentalaxes extend increasingly in the direction of the arrow. Solid lines of the environmental axes that extend from the origin indicate the relativelengths (and subsequently importance) of each variable and the dotted lines have been extended in the positive direction and through the originfor axes NH4 and TS to facilitate identifying relationships of taxa to these axes. Taxa names have been included for those associated with sitesplotting at the high and low extremes on the NH4 and TS gradients. Taxa relationships to the other gradients can be found in Table 3.

scores and positioned on the left side of the diagram(Fig. 4A).

The distance between years for the same site in theordination diagram was considered the degree of com-munity change over time. Average distances between1999 and 2000 samples for the best attainable andstable sites were similar to distances between yearsfor unstable/impacted (Fig. 4B). However, variabilityin the degree of community change over time wasgreater among US/IM sites (coefficient of variation

(CV) = 33.48%) than among BA and ST sites (CV =18.33%). Of the two restored sites examined, only onecommunity remained similar between years (Fig. 4B).Best attainable and stable sites (i.e., sites 16a, 4a, 3a,and 7a) clustered well and the grouping was persistentbetween years (Fig. 4B), indicating similarity of themacroinvertebrate communities at these sites withineach year.

An interaction effect of condition class and yearon the macroinvertebrate community metrics was not

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Table 4. Results of ANOVA comparing selected macroinvertebrate metrics amongfive channel classifications in the channel evolution model and four channel conditionclasses used for a priori classification of stream sites in northwest Mississippi. F andP values are from seperate 1-way ANOVA tests for each of the classification schemes.

Invertebrate metric Code Channel condition Channel evolution

class model

F P F P

Total no. of taxa s 1.81 0.163 1.12 0.361

Simpson’s diversity index D 1.95 0.138 1.01 0.414

No. Ephemeroptera taxa Es 1.19 0.329 1.53 0.213

% Comp. of Ephemeroptera EN 0.21 0.888 0.49 0.742

No. Plecoptera taxa Ps 1.05 0.382 1.16 0.345

% Comp. of Plecoptera PN 1.12 0.355 0.98 0.431

No. Trichoptera taxa Ts 2.28 0.096 2.16 0.093

% Comp. of Trichoptera TN 2.24 0.099 2.37 0.071

No. Dipteran taxa Ds 1.07 0.373 0.55 0.697

% Comp. of Diptera DN 1.99 0.132 0.69 0.602

Functional group diversity FGD 1.21 0.321 1.93 0.126

% Taxa as collector/filterer CFs 0.54 0.656 0.64 0.638

% Taxa as collector/gatherer CGs 0.18 0.906 0.49 0.740

% Taxa as parasite PAs 0.75 0.529 0.55 0.697

% Taxa as predator PRs 0.80 0.501 0.72 0.587

% Taxa as scraper SCs 0.19 0.901 0.23 0.923

% Taxa as shredder SHs 0.06 0.981 0.86 0.495

detected (Table 5). However, the mean number of Eph-emeroptera and Diptera taxa observed in 1999 (5.35and 6.88, respectively) was greater than that observedin 2000 (2.53 and 4.18, respectively) (Table 5).

Discussion

Response of invertebrates to water quality andphysical habitat

In general, the 26 sites examined for this objectivehad similar macroinvertebrate communities based ongradient axis lengths of 1.99 and 1.94. Gradient axislengths that approach 4.0 units often indicate entirecommunity turnover between sites at either end of theaxis (Sabo, 1980; Jongman et al., 1995). These res-ults suggest that a few intolerant taxa or functionalgroups may be sensitive indicators when comparingcommunities of impacted or restored streams to thoseof best attainable streams. Taxa that are strongly re-lated to physical or water quality variables should beconsidered for inclusion in multimetric indices of bi-otic integrity (e.g., B-IBI) (Kerans & Karr, 1994) thatare ecoregion specific. Shields et al. (1995b) used a

Table 5. Results of two-way ANOVA testing for effects of year,channel condition, and their interaction on selected macroin-vertebrate metrics from stream reaches sampled in 1999 and2000 in northwest Mississippi. See Table 4 for definition ofinvertebrate metric codes. – = P > 0.0033.

Invertebrate Year Channel Year× channel

metric condition condition

F P F P F P

s 0.73 – 0.08 – 0.89 –

D 0.36 – 1.03 – 1.18 –

Es 20.98 0.0001 0.65 – 0.19 –

EN 2.49 – 0.04 – 0.71 –

Ts 0.23 – 0.97 – 0.99 –

TN 9.06 – 2.63 – 2.37 –

Ds 14.01 0.0009 0.35 – 0.91 –

DN 1.11 – 1.79 – 0.42 –

FGD 0.28 – 1.8 – 0.21 –

CFs 2.16 – 1.44 – 2.44 –

CGs 1.09 – 3.23 – 0.82 –

PAs 0.05 – 0.57 – 0.42 –

PRs 0.02 – 2.77 – 0.36 –

SCs 0.81 – 0.64 – 0.95 –

SHs 5.05 – 1.80 – 0.03 –

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Figure 3. Ordination (Detrended Correspondence Analysis (DCA)) of 44 stream reaches of northwest Mississippi based on their macroinver-tebrate community structure. Overlay of channel condition class (A) and channel evolution model (B) identifies each site as to the classificationthey were assigned. Ellipses encircling Best Attainable, Premodified, and Degraded sites are to facilitate interpretation of the diagram and donot represent a confidence level ellipse around a mean.

region specific index of biotic integrity (IBI) for fish toexamine physical aquatic habitat and found that onlylarge variation in physical habitat could be identifiedwith IBI scores. Water quality parameters were notmeasured in that study and provided potential con-founding effects. Physical habitat and water qualitymeasurements were analyzed simultaneously in the

current study and both categories of environmentalvariables were found to provide gradients that influ-enced macroinvertebrate taxa and probably importantfor shaping the invertebrate communities. Seven of the11 most important environmental variables from theinitial analysis were water quality parameters.

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Figure 4. Ordination (Detrended Correspondence Analysis (DCA)) of 17 stream reaches of northwest Mississippi based on their macroinver-tebrate community structure. Sites are individually numbered within the diagram and were sampled in either 1999 (a) or 2000 (b). Overlay ofyear (A) identifies each of the sites as to the year they were sampled. Overlay of channel condition class (B) identifies each of the sites as to theclassification they were assigned. Ellipses encircling Best Attainable and Stable sites for 1999 and 2000 are to facilitate interpretation of thediagram and do not represent a confidence level ellipse around a mean.

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Agriculture was an important land use categorywithin the watersheds examined. Consequently, manystreams received substantial agricultural input. Lenat(1984) showed that streams receiving agricultural run-off had greater suspended solids and sedimentation,increased particulate organic matter, elevated nutri-ent concentrations, and lower abundance of Ephem-eroptera, Plecoptera, and Trichoptera. Results fromthis study concur with Lenat et al. (1981) andLenat (1984). Families such as Leptophlebiidae, Eph-emeridae (Order: Ephemeroptera), and Leptoceridae(Order: Trichoptera) had greatest abundances at siteswith lowest total solids concentrations. Likewise,these taxa were most abundant when the greatestproportion of substrate was sand rather than clay orgravel. Reaches with the lowest total phosphorus con-centrations also had the greatest abundance of Eph-emeroptera and Trichoptera. This study confirms thatseveral of the intolerant taxa commonly used to assessstream conditions in other ecoregions (Lenat, 1984,1993; Kerans & Karr, 1994; DeShon, 1995; Foreet al., 1996) can be used in this ecoregion for the samepurpose.

Components of macroinvertebrate indicator met-rics include responses that are hypothesized to eitherincrease or decrease in response to degraded streamconditions (Kerans & Karr, 1994). Thus, attentionwas placed on taxa that responded positively (i.e.,increased abundance) to environmental conditionsthought to adversely impact invertebrates at elevatedconcentrations (e.g., suspended solids and nutrient).Genera within the orders of Hemiptera and Cole-optera were most abundant at sites with the greatestNH4, total phosphorus (TP), and total solids (TS) con-centration and highest conductivity. Similarly, Lenat(1984) reported that Coleoptera replaced sensitive taxa(e.g., Ephemeroptera, Plecoptera, and Trichoptera)in streams receiving agricultural runoff. Thus, Co-leoptera in North Carolina streams were ranked asrelatively more tolerant to agricultural inputs (Lenat,1993).

Diptera taxa tend to persist in aquatic communit-ies experiencing contaminant related stress (DeShon,1995). They are generally considered tolerant taxa(Lenat, 1993) and increased abundances of Dipterahave been used in the Ohio Environmental ProtectionAgency’s Invertebrate Community Index to indicatedegraded stream conditions (DeShon, 1995). The cur-rent analysis of the NH4 gradient contradicts this pat-tern for some Diptera taxa including members of thefamilies Tipulidae and Culicidae. The genus Antocha

and undetermined genera of the Tipulidae and Culi-cidae exhibited greatest abundances at the lower endof the NH4 concentration gradient. Perhaps Diptera ofstreams in the study region were intolerant to NH4,or some other unexplored relationship is present. Re-cent data has indicated that several taxa of MississippiTipulidae are very intolerant of pollutants (MississippiDepartment of Environmental Quality 2002, unpub-lished data), but no tolerance information for the genusAntocha was reported in a recent summary (Barbouret al., 1999).

Hemiptera such as Neogerris, Palmacorixa,Ramphocorixa, and Hesperocorixa also had weightedtaxa averages positioned near the high end of the nutri-ent gradients (NH4 and TP) and these taxa dominatedmacroinvertebrate communities at sites with greaterNH4 and TP concentrations. Results reported heresupport examination of these Hemiptera as possibleindicators of elevated (present or near past) nutrientlevels in stream systems in this ecoregion. One pos-sible reason for the observed relationship could bethat food / prey items for these Hemiptera are moreabundant at locations with elevated concentrations ofnutrients. For example, Corixidae feed on stream bot-tom organic material containing algae, diatoms, andother organic sources that could be more abundantwhere nutrient concentrations are high (Griffith, 1945)and Gerridae feed on small arthropods at or near thewater surface that could be more numerous in con-junction with elevated local nutrient concentrations(Blankenhorn, 1991). The response pattern of Hemi-ptera to NH4 and TP gradients occurred for generain more than one family (i.e., Gerridae and Corix-idae) and could warrant further investigation. Anyinvertebrate response that can be generalized at highertaxonomic levels is of interest to water quality man-agers because of the potential decreased time andeffort for generating such data.

Efficacy of channel condition and CEM stage forclassifying stream reaches

The utility of the channel condition classificationscheme may reside in its ability to group referenceor best attainable (BA) sites based on their macroin-vertebrate communities. These sites had the least sep-aration among site scores and thus had more similarmacroinvertebrate communities. This confirmation ofthe current best attainable site classification provides afoundation for selection of future best attainable sitesand an estimate of the macroinvertebrate community

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variation for best attainable sites. It is emphasized thatwatershed managers carefully observe physical attrib-utes of potential study sites in addition to biotic datawhen determining best attainable or least impactedsites. For example, several US/IM sites had similarcommunities to BA sites and were only differentiatedby physical attributes of a stream reach.

Commonly used macroinvertebrate metrics did notdiffer among either stream channel classifications orchannel evolution model stages; yet, patterns were ex-tracted using multivariate ordination methods (DCA).Therefore, it is suggested that both multivariate andmultimetric methods be employed for identifying dis-turbed streams rather than selecting only one approach(Fore et al., 1996). Multivariate methods have theirstrength in incorporation of community-wide patternswhile multimetric methods offer a focus on individualtaxa (Fore et al., 1996) or ecological process such asfunctionality (Cousins, 1991).

Between-year variation of macroinvertebrates

Temporal variation in biotic communities and IBIscores has been reported previously (Karr et al., 1987;Townsend et al., 1987; Kerans et al., 1992; Shieldset al., 1995b; Laasonen et al., 1998; Muotka et al.,2002). Similarly, community data here were clearlydefined between sites sampled in 1999 and 2000. Thegreater number of Ephemeroptera and Diptera taxadetected in the 1999 samples may explain some ofthis variation. There are several possible explanationsfor the mechanism of this temporal variability. First,perhaps an emergence of several taxa occurred inmany of the sites in one of the years, shifting thecommunity composition. If this was the case, thenmore emphasis should be placed on consideration ofmacroinvertebrate life history characteristics when at-tempting to compare modified or restored study sitesto a reference condition or best attainable site. Per-haps biotic evaluations should emphasize examiningorganisms that have a life history trait of persistencewithin a community rather than those with occur-rences that are episodic. Second, variability due tofield sampling and/or taxa identification skill may con-tribute to temporal variation. Although the same in-dividuals accomplished these tasks during both years,such precautions cannot fully eliminate this potentialsource of variation.

The temporal shift in macroinvertebrate com-munities observed between 1999 and 2000 was con-sistent for all stream channel classifications. It was

predicted that communities within US/IM sites wouldbe more dissimilar between years than BA sites dueto more frequent and severe habitat alterations fromheadcut migration, bank failure, and shifting bedmaterial. Indeed, greater variation in the degree ofcommunity change over time was observed for US/IMthan BA and ST sites and is consistent with greatertemporal variation in IBI scores reported for degradedsites (Karr et al., 1987).

Short-term temporal variation was examined inthis study to improve interpretation of the findings.Such a temporal scale may not fully reflect the ac-tual long-term patterns and variability occurring instream reaches, especially since water quality can becyclical in this region (Cooper & Knight, 1991) andin-stream processes in reference or best attainable sitesgenerally occur over longer periods. Future macroin-vertebrate studies in the ecoregion should examinelong-term temporal variation in communities amongsite classifications (e.g., reference, degraded, restored,etc.), particularly when assessing impacts of a ma-jor perturbation or the response of communities afterrestoration practices (Laasonen et al., 1998; Muotkaet al., 2002).

Acknowledgements

We thank the following for a significant contributionof effort to this project: L. Harding, J. Johnson, M.Moore, R. Raushenberger, W. Stephens, R. Winterrin-ger, and the staff at the USDA-ARS National Sedi-mentation Laboratory and Arkansas State UniversityEcotoxicology Research Facility. We also thank D.Harper for suggestions that improved the manuscriptand E. Ashcraft, J. Bouldin, R. Maul, A. McBride, S.McCord, J. Seagraves, M. Scott, B. Stephens, and B.Walker for reviewing earlier drafts of the manuscript.The U.S. Army Corps of Engineers, Vicksburg Districtprovided cooperative funding for this project.

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