Macroinvertebrate community structure in a regulated river segment with different flow conditions

RIVER RESEARCH AND APPLICATIONS

River Res. Applic. 18: 367–382 (2002)

Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/rra.679

MACROINVERTEBRATE COMMUNITY STRUCTURE IN A REGULATEDRIVER SEGMENT WITH DIFFERENT FLOW CONDITIONS

RUI MANUEL VITOR CORTES,a* MARIA TERESA FERREIRA,b SIMONE VARANDAS OLIVEIRAa

and DANIEL OLIVEIRAa

a Universidade de Tras-os-Montes e Alto Douro, Dept. Florestal, Apart. 1013, 5001-911 Vila Real Codex, Portugalb Instituto Superior de Agronomia, Dept. Eng. Florestal, Tapada da Ajuda, 1349-017 Lisboa, Portugal

ABSTRACT

Two types of modification of the hydrological system are present in the same regulated segment of the Lima River(NW Portugal): (a) a reduced and constant flow from hypolimnetic release; (b) an intense irregular flow (daily andseasonal). Using multivariate techniques it was possible to compare the effects of these two kinds of disturbance on themacroinvertebrate communities. The communities colonizing both sites exhibited a higher variation in composition anddiversity when compared to undisturbed sites. However, such variability was even more evident in the first case, in spiteof the stability of the environmental conditions. Such temporal replacement of species is linked to the dominance oftolerant taxa with short life cycles. In the regulated segment the poor water quality and the lack of litter input impactedmainly on the shredders group. This work shows the failure of the practice of releasing constant flows as an attempt tomitigate regulation impacts. Copyright 2002 John Wiley & Sons, Ltd.

KEY WORDS: regulation impacts; hydropower; hypolimnetic water; eutrophication; macroinvertebrates; diversity

INTRODUCTION

Most large impoundment structures in northern Portugal are placed for the purpose of electricity generation.The monomitic stratification patterns developing in the reservoirs and their discharge often impact in fourways on the biota downstream: (a) physico-chemical disturbance associated with the release of waters (andsediment) derived from the hypolimnion, which are frequently nutrient-rich, cold and thermally constant(Ward and Stanford, 1979; Craig and Kemper, 1987; Pozo et al., 1997); (b) modification of available foodresources, especially through changes in the downstream transfers of particulate organic matter (Andersen andCummins, 1979; Petts, 1984); (c) creation of a disruption in the hydrological patterns by daily hydropeaking,and a catastrophic increase in invertebrate drift (Gore et al., 1989; Lauters et al., 1996); and (d) diversionand water abstraction downstream of the dam, generally with flood suppression, drastically decreasing theabundance and diversity of benthic fauna (Morgan et al., 1991; Moog, 1993). Often there is a reduction ofavailable or suitable habitat, especially when the channel reflects hydrogeomorphic changes (Erskine et al.,1999). Intense and erratic fluctuations in water discharge are generally considered as producing the mostdramatic events, because the released water often erodes the river channel until armouring by medium and largecobbles occurs (Death and Winterbourn, 1995; Gore, 1996), often contributing to the loss of habitat diversity,with the above-mentioned implications for the decline in the number of species (Gumiero and Salmoiraghi,1994). Probably the habitats most severely affected by such flow changes are rapids and waterfalls (Englundand Malmqvist, 1996). However, species display different resilience curves, according to their capacity todevelop behavioural adaptations to intense water level fluctuations (Bunt et al., 1999) or depending on theirreproductive characteristics (e.g. aerial ovipositing adults; Palmer et al., 1992).

* Correspondence to: Rui Manuel Vitor Cortes, Universidade de Tras-os-Montes e Alto Douro, Dept. Florestal, Apart. 1013, 5001-911Vila Real Codex, Portugal. E-mail: [email protected]

Copyright 2002 John Wiley & Sons, Ltd.

Received 1 August 2001Revised 14 January 2002

Accepted 18 February 2002

368 R. M. V. CORTES ET AL.

In the lotic system under research–Lima River catchment–we intend to compare two different situationsoffered by two other types of flow pattern imposed by regulation: in one case where the release of wateris entirely hypolimnetic, but strongly reduced and with a practically constant flow; in the second case thehydrograph of peaking tailwaters is intermittent and is released from a more superficial layer. What also makesthis situation interesting is the fact that both dams are located in the same stream, only a few kilometresapart. This study was conducted also to determine whether a minimum and stable flow, such as the one wedescribe in the first case, and which represents an attempt for an experimental instream flow, was effectivein properly maintaining the aquatic life.

MATERIAL AND METHODS

Study area

The Lima River runs in a NE–SW direction for 135 km from the mountain of Saint Mamede (950 m) inSpain to the river mouth near Viana do Castelo in Portugal, being equally divided between both countries.The catchment area on the Portuguese side occupies an area of 1885 km2 (total catchment area 2535 km2)and is characterized by narrow and steep valleys of granite bedrocks, but with a relatively high run-off, aconsequence of an average yearly rainfall of 1950 mm, reaching approximately 2800 mm in the headwatersin the neighbouring country.

Near the border between the two countries two dams are located, 16.5 km apart: Alto Lindoso, upstream,and Touvedo. Alto Lindoso is one of the key electric power generators for the whole country, and produces upto 320 MW at full load, whilst the Touvedo, of a smaller size, acts as a tailwater reservoir, modulating the highflows released by the previous dam (Table I). Along the regulated river segment there are three sub-sectors,each with specific hydraulic conditions imposed by regulation (Figure 1): (a) an upstream reach of 7.0 kmfrom the Alto Lindoso dam to the outlet of the tailrace tunnel, which is fed by a constant hypolimnetic flowdesigned to provide the minimal flow requirements (0.5 m3 s−1 for the period from June to September and2.0 m3 s−1 for the remaining part of the year); (b) a reach of 9.5 km with a much higher streamflow (as itreceives the flow discharged by the above-mentioned tunnel) and partially occupied by the Touvedo reservoir;(c) downstream of the Touvedo dam where, in contrast, the valley widens greatly and the irregularity of flowis the main feature, as it was designed to modulate the high flows from upstream (see Figure 2, where naturaland modified regimes are compared), storing the water for flash releases in periods during the day whendemand is higher (the minimum flow was set here permanently at 4.0 m3 s−1).

Nutrient input in the streams is mainly derived from the drainage of the catchment, therefore, from diffusesources. In the studied part of the basin urban areas are scarce, industry virtually absent and arable landsare considerably reduced, being replaced by grasslands. Major nutrient losses are probably related to theforest (intensive pine tree afforestations and native oak forest), from clear-cutting to wild fires. Runoff in thiseroded soil always transports some phosphorus, because this element remains bound to the molecular latticeof soil particles (Harper, 1992). Another potential source of nutrient loading is related to the breakdown of

Table I. Descriptive data for the Alto Lindoso and Touvedo impound-ments and respective location in the Lima River catchment

Alto Lindoso Touvedo

LocationDistance from source (km) 71 87.5Catchment area above dam (km2) 1335 1705Altitude (m) 280 90

Dam and reservoir characteristicsSubmerged area (ha) 1072 172Total capacity (hm3) 379 15.5Maximum turbine flow (m3 s−1) 125 100

Copyright 2002 John Wiley & Sons, Ltd. River Res. Applic. 18: 367–382 (2002)

MACROINVERTEBRATE COMMUNITY STRUCTURE 369

AdrãoRiver

Tailracetunnel

St9

Touvedoreservoir

VideirinhasRiver

Vez River

Vade River

St3

St4

St1

St2

St5

St6St7

St8

St11

St10

FroufeRiver

AltoLindosoreservoir

N

Figure 1. Representation of the Lima River system, showing the relative position of the two impoundments and the sampling sites.These are located in the regulated segment of Lima River and in the main tributaries

submerged vegetation, which was not removed prior to the construction of both dams. Because water storagetook place in 1992, these impoundments are still in the maturation period.

Sampling sites and invertebrate sampling

The sites for collecting water and benthic invertebrates (Figure 1) were located in the main course of theregulated segment and in reference sites in the nearby unregulated tributaries. Particular attention was paid tothe sub-sector below Alto Lindoso (between this dam and the outlet of the tailrace tunnel). Here, sites 1, 2 and3 for sampling water and benthic fauna, and sites 5 and 6 exclusively for complementing the determinationof water quality were chosen. In site 7, water samples and invertebrates were collected downstream of thesecond dam (Touvedo). Finally, sites 4, 8, 9, 10 and 11 were selected as reference sites in different tributariesfor obtaining samples of water and fauna. Collections took place over a whole year in the following periods:January, April, June, July, August and October 1998, and January 1999. A selection of physical variablesto characterize the sites surveyed for benthic fauna appears in Table II. Invertebrates were sampled using a350 µm mesh net with a constant sampling time of 4 minutes (CPUE), and with an effort proportional to thesize of each habitat to obtain semi-quantitative data. Larger gravels and cobbles were separated and washed toremove any invertebrates. We believe that standardizing of these procedures in each site allowed us to comparethe abundance between sites, since the grain size of the channel bed sediment was relatively similar in this partof the catchment (at least 75% of the substratum was composed by particles >64 mm). The contents of thehand-net were preserved in 4% formol and later sorted at the laboratory. Wherever possible, organisms wereidentified to species level using the available keys (except Diptera and Oligochaeta). Stratification conditions



(a)

m3s-1

(b)

m3s-1

Figure 2. Daily discharge (m3 s−1) for an average year (1982/83) before (a) and after (b) regulation. In this case the natural regime isreplaced by an artificial sequence of flows varying between 4 m3 s−1 (instream flow) and 50 or 100 m3 s−1 corresponding to one or

two turbines operating, except in situations of maximum release

in both reservoirs were also characterized through the dissolved oxygen and temperature vertical profiles indifferent periods.

Data treatment

Several multivariate methods were used to detect the spatial and temporal patterns underlying abiotic andbiotic data. Discriminant function analysis (DA) and metric dimensional analysis (MDS) were used through thepackage STATISTICA 5.6 (StatSoft, 2000), and canonical correspondence analysis (CCA) through CANOCO(ter Braak and Smilauer, 1998). DA was used for chemical data and allowed us to classify the sites accordingto a priori number of groups: (a) mainstream and tributaries; (b) seasonal variation. Between groups vari-ances are maximized and variances within group are minimized by the axes (Manly, 1994). MDS produced



Table II. Physical measurements at each site. Mean and maximum values are referred tothe studied period. Stream substrate was based on the Wentworth scale

Site Substrate type (%) Mean width(m)

Depth (m) Mean currentvelocity (m s−1)

1 Bedrock (80) 33.0 Max. 1.5 1.20Boulders (20) Mean 0.9

2 Bedrock (40) 20.0 Max. 1.2 0.98Boulders (40) Mean 0.8Cobbles (10)Gravel/pebble (5)Sand (5)

3 Boulders (60) 12.0 Max. 1.5 2.05Cobbles (30) Mean 0.7Gravel/pebble (5)Sand (5)


7 Bedrock (75) 26.9 Max. 6.6 0.48Gravel/pebble (5) Mean 1.7Sand (20)

8 Boulders (40) 26.0 Max. 0.5 1.34Cobbles (40) Mean 0.4Gravel/pebble (10)Sand (10)


10 Bedrock (5) 11.8 Max. 0.6 1.17Boulders (80) Mean 0.2Cobbles (5)Sand (10)

11 Bedrock (50) 8.3 Max. 1.50 2.63Boulders (10) Mean 0.25Cobbles (30)Sand (10)

an ordination of sites based on a matrix of similarities of benthic data, computed from squared euclidiandistances. This technique is widely used with biotic data since it uses directly the distances or dissimi-larities between objects (sites) derived from its taxonomic composition, respecting the original matrix of(dis)similarities (Podani, 2000). Finally CCA is a method of direct gradient analysis; this is the ordination ofobjects (sites) and is based simultaneously on species data and on the environmental information (Jongmanet al, 1987; ter Braak and Prentice, 1988). In the present work it is allowed to relate two sets of scores (taxaversus descriptors of water quality). Data was transformed by linear methods for the chemical variables andby non-linear methods for taxa. Additionally, a two-way ANOVA with repeated measures was performed forspecies data, namely diversity and abundance, to test for differences between regulated and non-regulatedsites, with time the dependent variable.



RESULTS

Water quality

Released water from both dams displays a relative degradation when compared to the reference sites,which is related to the mesotrophic conditions (Table III). These are corroborated by a steady deoxygenationthroughout the year and by a higher load in organic matter (assessed by chemical oxygen demand) andnutrients, namely in nitrates. The stratification of oxygen and temperature in these monomitic reservoirsbecomes conspicuous in the summer period (Figure 3). In contrast to Alto Lindoso, the metalimnion is at alower depth in the Touvedo reservoir. We may assign this to the cooler tailwater released upstream whichmixes with the hypolimnetic layer and increases the metalimnion level. During the summer stagnation phase,the river segments below both dams are strongly influenced by the instream flow, which exhibits oxygendepletion, as it is released from the bottom outlets. This aspect is more critical downstream of the upperdam, not only because of its higher storage capacity and the longer residence time, but also especially as aconsequence of a lower and constant discharge.

DA technique was applied to the water quality data to detect the differences between regulated and non-regulated sites (spatial discrimination) and between sampling periods (temporal discrimination). The variablesconsidered were Cl−, Na+, Mg2+, Ca2+, NO3

−, NH4+, PO4

3+, SO42−, hardness, alkalinity, conductivity,

dissolved oxygen (DO) and chemical oxygen demand (COD), which were previously standardized to the unitof variance. Consequently, data were sorted into 14 groups (seven periods × two types of sites). Figure 4displays a clear difference in water quality between the sites impacted and non-impacted by regulation. Infact, the most distinct feature arising from the analysis is that the space defined by the first two axes ispractically divided by a diagonal separating those two groups. Table IV (case a) presents the standardizedcoefficients for the canonical variables defined by these two axes and highlights their ecological significance.Thus, the first axis (eigenvalue: 4.218) is directly related to NO3

−, conductivity, and COD, whereas the secondaxis (eigenvalue: 3.377) is mainly related to DO. Therefore, we may conclude that regulation favoured streameutrophication. When data are grouped only according to the sampling periods (seven sets of data) a temporalpattern becomes visible, meaning that each period is characterized by specific conditions of the water chemistry(Figure 5). A slight disagreement for January (second year) may be explained by an unusually dry month in1999. The first and second axes (with eigenvalues of 3.799 and 1.407) are, respectively, negatively relatedto conductivity and NO3

−, and positively with oxygen and Ca2+ (Table IV, case b). Such analyses suggest apoorer water quality in August and July, decreasing the nutrient content in the winter period.

Diversity and abundance

Diversity variation is shown in Figure 6a and illustrates two main shifts throughout the study period:(a) diversity is generally inferior in the four sites located in the segment impacted by regulation (LimaRiver); (b) this parameter shows a relatively higher range of variation in the three sites below Alto Lindoso,not reflecting the fact that the hydrological conditions remain practically stable in this reach. However, atwo-way ANOVA, where the seven sampling periods are considered as repeated measures for factor time,does not confirm that diversity is statistically different (F0.748; P > 0.05) between the samples from the mainstream and the tributaries (regulation is the independent variable). The mean density in the two consideredgroups of sites (regulated versus non-regulated) is remarkably similar (Figure 6b). A two-way ANOVA alsoshows no significant differences between such groups (F0.307; P < 0.05). However, invertebrate numbersdisplay a wider range of variation in the sites of the Lima River, again in apparent contradiction with thelower environmental variation that occurs in this sector. Nevertheless, Station 7, subjected to the highest flowfluctuations, exhibited the lowest range of mean abundance. In both cases (diversity and abundance), the effectof factor time or the interaction between the two factors was also not significant for the same statistical level.

Trophic groups

The most important biological traits, arising from the analysis of the relative proportions of the differenttrophic groups for the sites representative of the distinct environmental conditions, can be deduced from



Tabl

eII

I.C

hem

ical

char

acte

rist

ics

(mea

nan

dst

anda

rdde

viat

ion)

ofth

esa

mpl

ing

site

s

Sam

plin

gsi

tes

Con

duct

-iv

ity(µ

Scm

−1)

pHA

lkal

inity

(mg

HC

O3l−

1)

Har

dnes

s(m

gC

aCO

3l−

1)

Oxi

dabi

lity

(mg

O2l−

1)

Dis

solv

edO

2(m

gl−

1)

NO

3−

(mg

l−1)

NH

4+

(mg

l−1)

N-T

otal

(mg

l−1)

P 2O

5

(mg

l−1)

P-To

tal

(mg

P 2O

5l−

1)

SO4

2−(m

gl−

1)

Cl−

(mg

l−1)

1M

ean

44.1

76.

684.

375.

811.

799.

954.

280.

141.

390.

014

0.01

84.

334.

46Sd

ev3.

920.

302.

000.

760.

621.

512.

340.

140.

330.

011

0.01

81.

870.

552

Mea

n46

.13

6.52

4.97

8.79

1.80

10.3

53.

480.

030.

900.

005

0.01

32.

494.

87Sd

ev7.

530.

281.

486.

190.

321.

691.

890.

030.

530.

004

0.00

62.

071.

133

Mea

n41

.64

6.82

4.20

5.73

1.57

9.03

3.73

0.08

1.07

0.00

80.

037

4.22

4.39

Sdev

5.08

0.14

2.14

0.60

0.41

2.35

2.34

0.09

0.68

0.00

70.

037

1.64

0.54

4M

ean

25.0

46.

793.

273.

741.

099.

542.

190.

010.

580.

008

0.01

71.

633.

50Sd

ev4.

830.

330.

964.

120.

341.

931.

450.

010.

400.

006

0.01

41.

250.

755

Mea

n37

.87

6.64

3.75

4.73

1.57

9.28

2.83

0.03

0.80

0.00

80.

014

3.19

4.27

Sdev

8.49

0.33

1.78

0.69

0.40

1.58

1.52

0.03

0.42

0.00

70.

012

2.16

0.87

6M

ean

32.6

66.

464.

643.

843.

699.

972.

270.

020.

640.

014

0.02

22.

744.

07Sd

ev4.

800.

401.

570.

775.

441.

381.

610.

020.

480.

021

0.01

52.

300.

517

Mea

n36

.06

6.37

3.16

4.54

1.50

9.80

3.17

0.01

0.81

0.01

60.

066

3.42

4.66

Sdev

7.61

0.31

1.43

1.56

0.68

2.29

1.78

0.01

0.44

0.01

70.

103

1.59

0.73

8M

ean

38.3

66.

403.

344.

130.

6810

.88

379

0.02

1.04

0.00

70.

012

2.27

5.34

Sdev

5.21

0.74

1.23

0.97

0.38

2.31

2.47

0.02

0.74

0.00

50.

009

1.62

0.79

9M

ean

24.4

66.

593.

802.

441.

3910

.62

2.92

0.02

0.94

0.01

10.

030

3.15

4.43

Sdev

5.08

0.50

1.75

1.10

0.36

1.73

2.58

0.02

0.67

0.01

30.

026

2.01

0.71

10M

ean

24.9

76.

493.

702.

641.

1311

.09

3.53

0.02

1.13

0.01

30.

033

2.84

4.14

Sdev

6.66

0.48

1.24

1.47

0.24

2.08

3.26

0.02

0.86

0.01

70.

037

2.50

0.90

11M

ean

41.8

36.

804.

044.

960.

8811

.51

3.75

0.02

1.04

0.01

60.

028

2.34

5.31

Sdev

3.47

0.61

2.36

0.76

0.55

3.08

2.46

0.02

0.68

0.02

40.

035

2.05

0.45



100

80

60

40

20

0

0 2 4 6 8 10 12 14 16 18 20 22 24

(TJan)(TJan)

(LJul)

(TJul)(TJul)

(LJan)(LJul)

(LJan)

Temperature (°C)

Dep

th (

m)

2 4 6 8 10 12 14

DO (mg/ l)

Figure 3. Vertical profiles displaying temperature stratification (dashed line) and clinograde oxygen conditions (continuous line) in AltoLindoso (L) and Touvedo (T) reservoirs for January and July 1998

root 1

root

2

L

L

L

T

LT

T

T

T

L

L

L

T

L

T TTT

L

L

L

TL

TTT

T

L

L

L

T

L

T

T TT

L LL

T L

TT

T

T

L

L

L

T

L

TT

T

T

L

LL

TL

TTT

-6

-4

-2

0

2

4

6

-4 -2 0 2 4 6

River Lima

Tributaries

Figure 4. Discriminant function analysis (DA) of water quality data for the first two roots. Samples from the regulated segment of LimaRiver (L) are clearly separated from those from the reference sites in the tributaries (T). Notice that the line was arbitrarily placed and

is intended merely to emphasize such separation



Table IV. Standardized coefficients of the chemical variables for the first two canonicalvariables (axes) when data are grouped considering 14 groups (case a: two types of sitessampled seven times: case b: the seven sampling periods only). Case b is reported toFigure 5. These coefficients allow us to select the variables with a higher contributionfor the two discriminant functions

Variables Case a Case b

Axis 1 Axis 2 Axis 1 Axis 2

Cl− −0.372 0.612 −0.870 0.275Ca2+ −0.053 −0.951 0.804 0.774Mg2+ 0.440 −0.198 −0.387 −0.569Na+ −0.146 −0.034 0.154 0.019NO3

− 0.708 0.217 −0.780 −0.231NH4

+ −0.305 −0.014 0.365 −0.096PO4

3− 0.119 −0.131 0.061 −0.331SO4

3− −0.276 −0.371 0.462 0.147HARD −0.247 −0.038 0.296 0.223COD 0.668 −0.225 −0.568 0.187COND 0.678 0.215 −0.827 −0.157ALK 0.443 −0.005 0.387 −0.508D.O. 0.083 0.552 −0.188 0.892

JA

AP

JN

JL

AG

OU

JA2

root 1

root

2

-4

-3

-2

-1

0

1

2

3

4

5

6

-6 -5 -4 -3 -2 -1 0 1 2 3 4

Figure 5. DA of samples grouped according to the seven collection periods (JA, January; AP, April; JN, June; JL, July; AG, August;OU, October; JA2, January second year) displayed for the first two axes (labelled root 1 and root 2) based on water quality data

Figure 7: (a) shredders were under-represented in the regulated segment of the Lima River; (b) however,scrapers were relatively more abundant in the same sites, but constrained to some periods. These conclusionssuggest that the sites influenced by regulation were comparatively more autotrophic and hence the invertebratecommunity was less dependent on the input of particulate organic matter (POM). The nutrient enrichmentcaused by the waters released by the mesotrophic reservoirs and the low litter input from a scarce riparianband, probably direct the energetic functioning of the regulated segment towards a stronger dependence onthe organic seston and benthic algae.



(a)

±SE

Mean0.3

0.5

0.7

0.9

1.1

1.3

1.5

ST1 ST2 ST3 ST4 ST7 ST8 ST9 ST10 ST11

ST1 ST2 ST3 ST4 ST7 ST8 ST9 ST10 ST11

H'

(b)

-50

0

50

100

150

200

250

300

350N.

±sd

±SE

Mean

±sd

Figure 6. Means and dispersion measures of diversity (a) and abundance (b) along the sampling period (Jan. 98 to Jan. 99) for theinvertebrate assemblages collected in the studied sites. No replicates were taken

Species composition

Species data were treated through MDS using the squared euclidean distances on log-transformed data.The corresponding diagram appears in Figure 8, together with the Shepard diagram. This may be regarded asan indication that MDS was appropriate for the data set since the original inter-sample distances are linearlyrelated to the final distances in this ordination procedure and these distances fall close to the step-line. Thisfigure suggests that the two groups of sites (regulated versus non-regulated) support different assemblages.Therefore, faunal composition expresses the different environmental conditions linked to flow regime andwater quality. The macroinvertebrate community below Touvedo (site 7) is in a somewhat intermediateposition between these two groups of stations. The reference sites present, again, a narrower range of variationconsidering the seasonal pattern (expressed by the inter-site distance along the year), which means that theysupport communities with a relatively similar faunistic composition for that period.



Jan-

98

Apr-9

8

Jun-

98

Jul-9

8

Aug-9

8

Oct-98

Jan-

99

Jan-

98

Apr-9

8

Jun-

98

Jul-9

8

Aug-9

8

Oct-98

Jan-

99

Jan-

98

Apr-9

8

Jun-

98

Jul-9

8

Aug-9

8

Oct-98

Jan-

99

Jan-

98

Apr-9

8

Jun-

98

Jul-9

8

Aug-9

8

Oct-98

Jan-

990%

20%

40%

60%

80%

100%St1 St4 St7 St11

Shredders Gatherers Filt. Feeder Scrapers Predators

Figure 7. Comparison between the relative proportions of the function feeding groups between sites impacted by regulation (sites 1and 7) and non-impacted (sites 4 and 11)

Both sets of data (taxa and the water quality parameters) were treated simultaneously with CCA. Becausecollinearity was detected when fitting COD, this variable was excluded. The resulting diagrams are shownfor the first two axes on Figures 9 and 10 (eigenvalues: 0.339 and 0.222, which account for 39.7% ofthe species–environment variation). Here vectors represent the environmental variables, with a proportionallength to the effect of the variable on the species ordination (Jongman et al., 1987). The Monte Carlo testof significance of all canonical axes (to judge the significance of that relation) presented a P -value of 0.010.The biplot displayed in Figure 10 highlights the existence of a eutrophication gradient along its diagonal,discriminating also between the Lima River and the tributaries. Because site 7 lies between these two groupsof sites, we may infer that the disturbance caused by Touvedo on the benthic community, in spite of amore intense flow variation, is more reduced than the one caused by Alto Lindoso. Figure 10 illustrates theposition of the invertebrate assemblages along the same gradient: one of the extreme positions is occupiedby Diptera (families Ephydridae, Culicidae and Empididae) and Gastropoda (Physella acuta and P. jenkinsi ),which represent the most tolerant taxa, whereas the stoneflies (mainly Leuctra spp.) and caddisflies (Tinodesand Synagapetus spp.) characterize the less impacted reaches.

DISCUSSION

Water chemistry and temperature (linked to the depth of water release and trophic status), flow regime andhabitat modification, are the principal sources disturbing the biota following regulation. However, they mayinduce different patterns along the temporal scale. Petts and Greenwood (1985) postulated four stages ofchannel change after regulation: (a) a channel form stabilized by the controlled discharges but unaffected bysedimentation; (b) a channel where fine gravels and sands accumulate; (c) a gravel bed in a braided channelwith vertical and lateral accretion; (d) a new quasi-equilibrium single and narrow-thread channel. Even if wedoubt that all the adjustment phases were present since the dams’ construction (in narrow channels at leaststage c is virtually absent), we may accept that the regulated segment of the Lima River progressed to the



(a)

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Figure 8. Multidimensional scaling ordination of invertebrate data using squared euclidean distances (a) and Shepard diagram (b), whichplots the input data against the reproduced distances (D-hat values are the monotonically transformed input data as a step function),indicating the goodness of fit. The reference sites (encircled) exhibit a different composition from the most disturbed ones, displayed

on the left side of (a)

last phase. Our opinion relies also on the predominant natural substrate type that occurs in the main channel,composed essentially of large particle size, similar to the non-disturbed tributaries (see Table II).

Meanwhile, if theoretically more stable physical conditions are now becoming established, this work showsthat these dams, dramatically disrupting the natural discharge and temperature regimes, had an adverse impacton the ecological conditions. This is well illustrated by the replacement of the benthic fauna in the regulatedsegment, which exhibited a different composition from the non-regulated ones (see the performed ordinations



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Figure 9. Canonical correspondence analysis of sites based on invertebrate data and water quality. Each site is represented by therespective number and collection date, and chemical parameters by arrows

of Figures 8 and 9). The reported changes in the water quality created by both dams are thus an important factorin justifying such distinct faunistic assemblages. But we must take into account that the mentioned differenceswere also caused by two kinds of hydrological disturbance. In Alto Lindoso there was an intense physicalstability related to a constant flow overcoming the natural hydrological variation, whereas hydropeaking andassociated temperature variation characterized Touvedo. Some results may be surprising. For instance, it wasexpected (though not confirmed by the present data) that in the first case a narrower range of variation inthe structure and composition of the benthic fauna would be detected. This assumption was based on thelower temperature and thermal constancy, as a result of the constant input of cold hypolimnetic water, whichinvolves a general metabolism decrease, according to Brittain (1983). For a more consistent analysis of suchdifferences we have to look to the invertebrate composition, namely into the character of the taxa present ineach case: the regulated sites were colonized by more tolerant species (resistant to oxygen depletion and toreduced nutrients). Here, the lack of shredders (stoneflies and caddisflies) was filled by species with shortlife cycles. Such species are eurythermic and polyvoltine, and that may explain the large variation throughthe year of the assemblages at each site in this segment. But these taxa, clearly identified by CCA analysis(Figure 9), may be classed also as evidence of slow or standing water, against the ones that characterize thereference points, which are typical of rapid or moderate/fast streams (see scoring taxon associated with flowin Extence et al., 1999). So, in this situation, there is a compound impact of eutrophication and artificial lowflow continuous discharges.

The observed alteration of the trophic structure, the shredders being the group most adversely affected,has also previously been observed by various authors (e.g. Ward and Stanford, 1984; Camargo and Garcia



S lacustris

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StictotarsusL leptogaster

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ProtonemuraBeraea

Figure 10. Canonical correspondence analysis of invertebrate taxa and environmental variables

de Jalon, 1990; Cortes et al., 1998), who related such a change to the eutrophic conditions created by thereservoirs. This is particularly important in these types of heterotrophic streams, running through siliceousbedrock, as the structure of the benthic communities is strongly regulated by the transportation of particulateorganic food (Cortes, 1992; Cortes et al. 1995).

The above-mentioned impacts were also expected to have a detrimental effect on species richness, in onecase (Alto Lindoso) caused by the constancy of environmental conditions (Ward and Stanford, 1983), and inthe other (Touvedo), linked to catastrophic drift associated with flushing during peak flows (Cereghino andLavandier, 1998; Greenwood et al., 1999). We observed, indeed, that diversity was generally lower in theimpacted sites but not significantly different from the reference ones. Therefore, one of the most relevantconclusions is the permanence of a high number of species in the regulated segment (lower, however, thanin the non-impacted sites), in spite of the residual flow (Figure 6a). This may be explained by the diversityof available habitats, which also explains that the abundance of benthic organisms was not significantlyaltered (Figure 6b). The permanence of the numerous habitats is linked to the bed of coarse sediments andto the heterogeneities of the marginal areas of the steep valley, which followed the described model of Pettsand Greenwood. This pattern displayed by diversity was also reported by Gaschignard and Berly (1987) whofound greater richness in the adjusting reach, which they linked to an increasing habitat complexity during thechannel adjustment which thus developed a greater hydraulic and substrate heterogeneity. The rarefaction ofcoarse sediment has been pointed out by various authors (e.g. Resh, 1977; Gaschignard and Berly, 1987) as areason for drastic reductions in the abundance of shelter-building invertebrates; this coincides with the work ofCortes et al. (1998), for whom the observed high resilience of some systems to damming is associated with themaintenance of substratum heterogeneity. On the other hand, Cauzobon and Giudicelli (1999), when studying



the effects of a residual flow, attributed the main causes controlling invertebrate composition and diversityboth to the hydrothermal variation (and its implications for organisms life cycle) and habitat availability.According to Pozo et al. (1997), temperature was the parameter that displayed the highest recovery distance,meaning that the longitudinal disruption from the outlet of a dam affected a relatively longer stretch (>10 kmwas the distance necessary for this variable to acquire the same values at the inflow).

However, abundance and richness have to be looked at carefully when assessing regulation effects, espe-cially when flow diversion takes place, because methodological problems may confound the main factors.Thus, Englund and Malmqvist (1996) observed that flow reduction may cause no direct consequences forthe community parameters such as density and richness: when the sampling unit is equal in all stations, itmeans that the sampled area is proportionally higher at sites with low flow. Besides, large streams can onlybe sampled close to the margins (such as below Touvedo, during the flash water releases), and the ability toreach all the habitats in the sites with a residual flow is comparatively much higher. Moreover, if we takeinto account the reduction in aquatic habitats (and in the total usable area) because of the water diversion,we have to conclude that there was an underestimation of the global impacts caused by regulation, whichconsiderably exceeded the effects when a scale of similar sampling units is the tool for comparison. Therefore,it is not surprising that some works (e.g. Raddum and Fjllheim, 1993; Rader and Belish, 1999) found thatduring periods of constant diversion, downstream densities and biomass could be even higher than before flowabstraction, even if reduced peak flows were presented as the main explanation by these authors. Besides, itis not difficult to conceive that when intense flow fluctuations take place, a reduction in the wetted perimeterfrequently leads to a higher concentration of invertebrates.

As we may conclude from the present work, abundance and diversity are probably not the most appropriateparameters for assessing the effects of regulation. Instead, replacement of species gives a better interpretationof the biota. Considering the community composition, we observed that the release of a constant flow didnot reduce the detrimental effects resulting from permanent reductions in discharge caused by huge dams,especially when such water has a hypolimnetic origin. However, even taking into consideration the contradic-tory response of abundance and diversity, we believe that habitat heterogeneity acts effectively as a buffer toregulation. Armitage (1995), observing faunal changes to flow manipulation, demonstrated that, despite lowdischarges, a high diversity could be achieved provided that habitat heterogeneity is maintained.

Consequently, mitigation from river impoundment with hydropower generation may be achieved in twoways: (a) the need to adopt an environmental flow strategy is necessary, not only to stabilize baseflows, butalso to reproduce the natural flow distribution and to allow for habitat and channel maintenance flows; (b) themaintenance of the habitat diversity, both in the reach (allowing a wide range of refuge for aquatic communi-ties) and in the segment. In this case, for instance, the construction of small impoundments along a segment thatfaces a low and permanent discharge proved, in northern European countries, beneficial to fish and invertebratepopulations, by raising the water level and contributing to the simultaneous presence of pool and riffle habitats(Fjllheim and Raddum, 1996). This may also be a management tool to be tested in the southern countries, inorder to increase water levels and the diversity of hydrophysical conditions and thus to mitigate the ecologicalimpacts of water diversion related to hydropower projects. Also, solution (b) may be an alternative when diffi-culties arise in implementing solution (a) because of losses of electricity production and associated high costs.

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Macroinvertebrate community structure in a regulated river segment with different flow conditions

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