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The impact of flow regulation by hydropower dams onthe periphyton community in the Soča River, SloveniaNataša Smolar-Žvanuta & Matjaž Mikošb

a Institute for Water of the Republic of Slovenia, Hajdrihova 28c, 1000 Ljubljana, Sloveniab Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, Ljubljana,SloveniaAccepted author version posted online: 19 Aug 2013.Published online: 29 Apr 2014.

To cite this article: Nataša Smolar-Žvanut & Matjaž Mikoš (2014): The impact of flow regulation by hydropower dams on theperiphyton community in the Soča River, Slovenia, Hydrological Sciences Journal, DOI: 10.1080/02626667.2013.834339

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The impact of flow regulation by hydropower dams on the periphytoncommunity in the Soča River, Slovenia

Nataša Smolar-Žvanut1 and Matjaž Mikoš2

1Institute for Water of the Republic of Slovenia, Hajdrihova 28c, 1000 Ljubljana, Slovenianatasa.smolar@izvrs.si2Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, Ljubljana, Slovenia

Received 10 April 2012; accepted 28 June 2013; open for discussion until 1 November 2014

Editor Z.W. Kundzewicz; Associate editor M. Acreman

Citation Smolar-Žvanut, N. and Mikoš, M., 2014. The impact of flow regulation by hydropower dams on the periphyton community inthe Soča River, Slovenia. Hydrological Sciences Journal, 59 (5), 1–14.

Abstract The effects of hydropower dams and, in particular, the impacts of reduced river flows on the periphytoncommunity were assessed in the Soča River, Slovenia. Sampling sites were selected upstream and downstream ofthe Podsela and Ajba dams. Sampling was carried out in 1998 during a period of low flows. Reaches downstreamfrom the dams experienced prolonged periods of reduced flows, and a corresponding decrease in flow velocityand water depth. The chain of hydropower dams has stopped sediment inflow from the upstream reach. Below thedams, the oscillations of water temperature, dissolved oxygen and oxygen saturation are much larger than atunregulated sites upstream. The impact of prolonged periods of reduced flows, a lack of sediment supply fromupstream and changes in physicochemical variables has caused high periphyton biomass, proliferation of greenalgae and increases in the number of periphytic algae species below the dams. This has significant implicationsfor the design of environmental flow strategies that provide a sediment supply to maintain a healthy periphytoncommunity.

Key words Central Europe; hydropower plants; periphyton; regulated rivers; Soča River; Slovenia; water abstraction

Impact de la régularisation du débit causée par les barrages hydroélectriques sur la communautépériphytique de la rivière Soča, SlovénieRésumé Les effets des barrages hydroélectriques, et en particulier les effets de la réduction du débit des rivièressur la communauté périphytique, ont été évalués dans la rivière Soča (Slovénie). Les sites d’échantillonnage ontété sélectionnés en amont et en aval des barrages Podsela et Ajba. L’échantillonnage a été effectué en 1998 aucours d’une période d’étiage. Les biefs en aval des barrages ont connu des périodes d’étiage prolongées et unediminution correspondante de la vitesse d’écoulement et la profondeur de l’eau. La chaîne de barrageshydroélectriques a arrêté l’apport de sédiments du bief amont. A l’aval des barrages les oscillations de latempérature de l’eau, de l’oxygène dissous et de la saturation en oxygène ont été beaucoup plus importantesqu’aux sites non régularisés de l’amont. L’impact des périodes d’étiage prolongées, l’absence d’apportsédimentaires de l’amont et les modifications des variables physico-chimiques ont provoqué l’apparition d’uneimportante biomasse de périphyton, la prolifération d’algues vertes et l’augmentation du nombre d’espècesd’algues périphytiques en aval des barrages. Ceci a des implications importantes pour la conception destratégies concernant les débits réservés devant fournir un apport de sédiments suffisant pour maintenir unecommunauté périphytique saine.

Mots clefs Europe centrale ; centrales hydroélectriques ; périphyton ; rivières régularisées ; rivière Soča ; prélèvements d’eau

1 INTRODUCTION

Dams on rivers have significant effects on the down-stream hydrology and geomorphology (Graf 2006),modify the structure and dynamics of aquatic andriparian habitats (Poff and Hart 2002), change

physicochemical parameters and can cause changesin aquatic flora and fauna (Petts 1984). River regula-tion can have an impact on the flow regime in anumber of different ways, i.e. through changes inthe timing, magnitude and frequency of high and

Hydrological Sciences Journal – Journal des Sciences Hydrologiques, 2014http://dx.doi.org/10.1080/02626667.2013.834339

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low flows (Magilligan and Nislow 2005, Graf 2006),and this hydrological alteration can cause changes inperiphyton communities (Biggs 2000). Periphyton(microscopic and macroscopic algae, includingCyanobacteria) is widely considered to be both themain source of energy for higher trophic levels instreams and a valuable indicator of environmentalchange. In stable-flowing, nutrient-enriched streams,it can proliferate and cause water management pro-blems (Biggs 1996); become a nuisance, degradingswimming and fishing spots and clogging irrigationand water supply intakes; have aesthetic impacts andreduce recreational and biodiversity values (Biggs2000). The most important factors that influence thegrowth and development of periphyton include light,water temperature, the nature of the substrate, flowvelocity and turbulence, pH, alkalinity, hardness,nutrients and other dissolved substances, salinity,oxygen and carbon dioxide (Hynes 1979). Thepresence, abundance, composition and growth ofperiphyton are also controlled or influenced by envir-onmental variations, such as disturbances, stressors,nutrients, hydraulic conditions and biotic interactions(Larned 2010).

Several studies have examined the effects ofriver flow regulation on periphyton (Lowe 1979,Biggs 1987, Valentin et al. 1995, Bergey et al.2010, Tang et al. 2013). Flow regulation can influ-ence periphyton species composition and the abun-dance of individual species (Biggs 1996), causingtheir numbers to increase or decrease depending onsite-specific characteristics and the exact nature ofhydrological change. For example, regulated flowconditions may provide new habitats or conditionswhich are suitable for species that did not previouslyoccur in the stream (Growns and Growns 2001).Suren and Riis (2010) found that the longer theduration of low flow, the more the algal communitywill change, and in turn, the more the habitat qualitywill change. Flow regulation that creates reducedflow variability and increased bed stability canincrease periphyton biomass, whereas increasedflow variability typically decreases biomass (Biggs2000). Maximum periphyton biomass usually occursat low flow velocities (Biggs et al. 1998). Low flowsenhance plant biomass through changes to hydraulicparameters, light and temperature conditions (Surenand Riis 2010). Many studies have also reported highperiphyton biomass, expressed as chlorophyll-a andorganic matter, measured downstream of dams (Lowe1979, Bundi and Eichenberger 1989, Smolar 1997,Koudelkova 1999).

In Slovenia, 36 large dams on rivers have beenbuilt, 20 of them for electricity production by hydro-power plants (HPPs) (six with a bypass reach). Butthe reservoir capacities in Slovenia are relativelysmall and flood frequencies remain largely unaltered.The effects of reduced flow caused by water intakefor selected HPPs in Slovenia during periods ofdrought showed changes in the periphytic speciescomposition in alpine (Smolar-Žvanut 2001, Smolaret al. 2005, Smolar-Žvanut and Krivograd-Klemenčič2011), lowland and karst rivers (Smolar et al. 1998),respectively.

Many studies that have examined the impact ofriver regulation due to dam operation have focusedspecifically on diatom communities (e.g. Wu et al.2010, Tang et al. 2013), but not on whole periphyticcommunities and their biomass. Most work has alsofocused on rivers where the discharges have beenreduced across the whole flow regime, includingreduced flood occurrence. The main aim of ourstudy was to detect the downstream effects of analtered flow regime due to water diversion for HPPson the periphyton community in the alpine SočaRiver, Slovenia. This site provides a case study toillustrate the impacts on the periphyton of a largelyunaltered high flow frequency and significantlyreduced low flow occurrence. We compared periphy-tic algae composition and periphyton biomassbetween upstream and downstream reaches of twodams on the Soča River. Moreover, we measured themain hydrological, morphological and physicochem-ical parameters to examine their influence on thegrowth and development of periphyton. We expectedto observe an increase in periphyton biomass and adecrease in species diversity downstream from thedams.

2 HYDROPOWER PLANTS ON THE SOČARIVER AND THEIR CHARACTERISTICS

The Soča River is a typical European alpine river thatremains largely unregulated in its upper course. Itrises in the Slovenian Alps, flowing for 95 kmthrough Slovenia before crossing into Italy and dis-charging into the Adriatic Sea. It has a catchmentarea of 1576 km2 and is predominantly underlain bylimestone, but the lower parts of the river run overflysch and Quaternary gravels. The Soča River has atorrential flow regime, with high flows occurring atany time of year. The lowest flows are experiencedboth in summer and in winter months, with generally

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higher snow-fed flows in spring and rain-fed flows inautumn (Maddock et al. 2008).

The flow in the middle course of the Soča Riverin Slovenia is highly regulated by a chain of threelarge (and one small) HPPs:

– Doblar HPP (built in 1938) is fed by a diversionfrom Podsela Dam, a 55 m high concrete arch-gravity dam, the second highest in Slovenia;

– Plave HPP (built in 1940) is fed by a diversionfrom the Ajba Dam, which is 4.5 km downstreamfrom the Doblar HPP;

– Solkan HPP is fed by Solkan Dam (built in 1984)(Fig. 1) and

– HPP Ajba (built in 1975) is a small HPP and islocated at the Ajba Dam.

Water is abstracted from the reservoir upstream ofeach dam. It then flows through a bypass tunnel tothe HPP and is subsequently augmented back to theSoča River further downstream. Therefore, bypassedriver sections with reduced flows exist below eachdam. The river section with diverted water from thePodsela Dam to the Doblar HPP is 4320 m long, andthat from the Ajba Dam to the Plave HPP is 7950 mlong. At the Podsela Dam, the highest permittedwater abstraction for the Doblar HPP is 96 m3 s-1.

At the Ajba Dam, the highest permitted abstractionfor the Plave HPP is 75 m3 s-1. The HPP Solkan is arun-of-the river hydropower station, with an installa-tion capacity of 180 m3 s-1, and is located 7500 mdownstream of the HPP Plave.

If river discharge is high and the reservoirs arenot being filled, water spills over the dams. In recentdecades, the measured discharge duration curves ofthe Soča River have shown there is no obviouspattern to the frequency of dam spilling. The averagespilling frequency is 78 days a year for the PodselaDam (discharge >96 m3 s-1) and 130 days for theAjba Dam (discharge >75 m3 s-1). The spilling isquite frequent and can occur at most times duringthe year (e.g. in 1998, the Podsela Dam spilled on 81days (3 days in January, 18 days in April, 4 days inMay, 1 day in June, 9 days in July, 13 days inSeptember, 24 days in October and 8 days inNovember)) due to the flashy flow regime of theSoča River. Spilling is more frequent for the AjbaDam than the Podsela Dam due to the lower per-mitted abstraction. From the measured dischargeduration curves of the Soča River, one can alsoestimate that the minimum flow downstream ofboth dams is less than 10% of the naturalized flow.The lowest natural low flows were measured in the

Fig. 1 The Soča River and sampling sites, SO1–SO4.

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period from December to April and in the summermonths (normally in August), which is typical ofalpine rivers with a nival–pluvial flow regime(Frantar 2005). Consequently, at this time of year,spilling over the dams is also quite rare.

In the Soča River downstream of the PodselaDam, the only water in the channel is derived fromone tributary (the Ušnica Stream, see Fig. 1) with anaverage flow of 0.05 m3 s-1 and small quantities ofwater from a gate leakage at the Podsela Dam hav-ing an average flow of 0.20 m3 s-1. In the dryseason, most other tributaries dry up because oftheir karst catchments. Downstream of the PodselaDam, a minimum environmental flow requirementwas not prescribed. In the river reach between theAjba Dam and the outlet of the Plave HPP, there areonly small tributaries that contribute less than 1% ofthe Soča River flow. The minimum environmentalflow released into the river channel downstream ofthe Ajba Dam is 0.50 m3 s-1, as prescribed in thewater management operating licence for thePlave HPP.

With regard to the frequency and magnitude offloods, the dams have no significant impact on thehigh flow regime along the Soča River in its middlecourse. The total storage volume of PodselaReservoir is 5 800 000 m3 (effective volume is3 600 000 m3), with a maximum oscillation inwater level of 2 m. The total storage volume ofAjba Reservoir is 1 650 000 m3 (effective volume960 000 m3), with a maximum oscillation in waterlevel of 4 m. The maximum flow (for the period1961–1995) observed at Podsela Dam was2140 m3 s-1 (the same for the natural and regulated

flow regime), while the Q1% (the discharge that isequalled or exceeded for 1% of the time) for thenatural flow regime was 298 m3 s-1 and for theregulated flow regime was 293 m3 s-1. The reservoirsupstream from the diversion dams are not used forflood control purposes; the dams spill during thesetimes and their impact on reducing flood peaks is notsignificant. Therefore, at times of very high flow, theSoča River becomes a homogenous hydrological unitalong its middle course.

3 MATERIALS AND METHODS

3.1 Sampling sites

Sampling sites were selected upstream (labelled SO1)and downstream (SO2, SO3) of the first reservoir inthe chain, namely the Podsela, and then downstream(SO4) of the second reservoir, i.e. below the AjbaDam. Additionally, a sampling site was selected onthe Ušnica Stream (USN), an unregulated tributarywhich flows into the Soča River just below thePodsela Dam (Fig. 1). A description of the studysites is presented in Table 1.

Due to field restrictions, not all samplingactivities were possible at all sites at all times. InFebruary 1998, no sampling was possible at thesampling site USN due to the Ušnica Streambeing frozen. The sampling in August 1998 atSO4 was restricted due to high flows in thisreach (whilst the turbines in the HPP Plave werebeing renovated); therefore, we could not sampleperiphyton and measure hydrological parameters onthat occasion.

Table 1 Characteristics of the sampling sites in the Soča River.

Samplingsite

Gauss-Krügercoordinates, x; y

Riverbedwidth (m)

Substrata Riverbank Riverbank vegetation

SO1 116 242; 401 294 50 Gravel, sandand silt

Low angle or flat river bank;alluvial plain; natural

Alluvial forest; most common species: Salixalba and Salix glutinosa

SO2 110 693; 401 294 45 Coursegravel andboulders

River bank angle steeperthan1:1; rocks; natural

Dense forest vegetation: Ostryo-Fagetum,Cytisantho-Ostryetum in Ornithogalo-Carpinetum

SO3 109 992; 400 651 35 Coarsegravel andcobbles

River bank angle steeper than1:1; rocks; natural

Dense forest vegetation: Ostryo-Fagetum,Cytisantho-Ostryetum in Ornithogalo-Carpinetum

SO4 106 637; 395 199 60 Coursegravel andsinglelargeboulders

River bank man-made withrip-rap

Shrubs and trees: most common species:S. alba, Salix caprea, Salix eleagnos,Salix purpurea, Populus nigra andPopulus tremula

USN 111 314; 401 106 2 Smallbouldersandcobbles

River bank angle steeper than1:1; rocks; natural

Hardwood: most common species areS. alba, Alnus glutinosa and Corylusavellana

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3.2 Periphyton sampling procedure andlaboratory analyses

Sampling of periphyton for taxonomic and biomassanalyses was carried out in 1998 during the periodof low flows, i.e. in February, May, August andNovember.

The periphyton was sampled at several sam-pling points along a river cross-section. The numberof sampling points was determined according to theriverbed width, substrata structure, water depth andflow velocity. The maximum number of points wasfive. Where the selected cross-section was domi-nated by stagnant water, sampling points in thenearest upstream and downstream pools were alsoadded.

Periphyton samples intended for a qualitativeanalysis were taken by scraping off the surface areaof pebbles, stones, rocks, sand, macrophytes andsunken wood found at the sampling site. The sampleswere preserved with formaldehyde in the field, sothat the final concentration of formaldehyde in thesamples was 5%. Periphyton samples intended forquantitative analysis of the periphyton biomass(including attached algae, protozoa, bacteriaand fungi) were taken from river sediments of50–200 mm in diameter. At each sampling point,samples were scraped from five sediment particlesover a surface area of 200 mm2.

Dry weight (DW) and organic matter (ash-freedry weight, AFDW) were assessed in the laboratoryusing the APHA technique (1992). The chlorophyll-aconcentration was determined by the use of filtrationthrough Whatman GF/C filters and extraction withhot methanol (Vollenweider 1974). The values ofperiphyton biomass were calculated per squaremetre of river bottom sediment. In the laboratory,periphytic algae were examined under a lightmicroscope Nikon Eclipse E400 (Nikon, Tokyo,Japan) by means of phase-contrast optics for magni-fications of ×1000 in order to assess the frequency ofindividual recognized taxa (species) (1 = rare, 3 = fre-quent, 5 = abundant) (Pantle and Buck 1955).

3.3 Hydrological parameters

The current velocity and discharge were measuredwith a SEBA Mini Current Meter MI. At all theperiphyton sampling points, the current velocity wasmeasured at 3 cm above the bottom (v3cm). The meancolumn velocity (�vv) was also measured above thesampling points at 0.4 of the water depth above the

river bed. Flow velocity was averaged over a 1-min-ute period per sampling point.

The catchment areas of sampling sites (F in km2)were determined, and the following values were calcu-lated: �Qd (mean flow); �Qmin (mean minimum flow);Qmin (minimum flow); Qmax (maximum flow); Q82

(flow in m3 s-1 equalled or exceeded 82% of the time)andQ95 (flow in m3 s-1 equalled or exceeded for 95% ofthe time), for the duration of the flow record.

The aim of analysing the hydrological data wasto determine the hydrological characteristics of theSoča River regime in the sections downstream of thePodsela and Ajba dams. The analysis of hydrologicaldata was performed for single hydrological cross-sections under the following water regimes:

– without abstractions: the flows were determinedon the supposition that there were no abstractionsfor the HPP in the reaches under considerationand

– with abstractions: the flows were determined forthe operation of the HPP at that time in the SočaRiver in the reaches under consideration.

3.4 Physical and chemical parameters

Concurrent with periphyton sampling, measurementsof the following physical and chemical parameterswere performed with portable multimeters (WTWMultiline/F, Germany):

– electrical conductivity and temperature were mea-sured with a MA 5950 (Iskra, Kranj, Slovenia);

– pH was measured with a MA 5721 (Iskra) and– dissolved oxygen content and oxygen saturation

were measured using an OM 8 oxymeter (WTW,Munich, Germany).

3.5 Granulometric analysis of the Soča Riversediments

In July 1999, a grain-size analysis of river sedimentsfrom the sampling sites was performed using theWolman sampling strategy. This is a method of sur-face sampling in which all particles of river sedi-ments found under a selected line parallel to theflow direction are sampled. It is recommended thatat least 100 particles should be sampled. Theapproach of Anastasi (1984) and Fehr (1987) wasadopted. We used a non-flexible combination of theWolman samples for coarser particles from the riv-erbed surface, and the theoretical Fuller curve as an

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approximation for finer particles from the subsurfaceof the riverbed. As a result of the granulometricanalysis, we obtained the particle-size distributionsof the surface of the riverbed at the sample sites.From the resulting particle-size curves, we deter-mined different particle sizes, e.g. d90, d84, d16, arith-metic mean dm, and the particle sortingcoefficient σ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

d84=d16p

.

3.6 Shear stress and shear velocity

Shear velocity was determined as v* ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffig � h � Ip

inm s-1 and shear stress as τ = γ · g · h · I in N m-2,where g is acceleration due to gravity (9.81 m s-2), his flow depth in m and I is water level gradient. If theriver bottom is very uneven, the river bottom gradientis hard to determine. Therefore at low flows, waterlevel gradient was used instead. In a chosen cross-section, the water level gradient was determined in areach length three times the width of the cross-sec-tion. Hence, the average values of shear velocities v*and shear stresses τ at each sampling site weredetermined.

3.7 Statistical analyses

A comparison of periphytic algae communitieswithin individual sampling points and within sam-pling sites was made using the data on species com-position and the relative frequency of species found.Similarities and differences among periphytic algaewere assessed by means of a multivariate clusteranalysis (Bray-Curtis coefficient of similarity; Clark

and Warwick 1990) and a Slovenian national data-base of algae (the DABA data system; Vrhovšeket al. 1998).

The Pearson correlation coefficient and the t testwere used to assess significant differences withinsampling sites and between them for the followingbiological, hydraulic, physical and chemical para-meters: periphyton DW, periphyton organic matter,chlorophyll-a, number of periphyton taxa, flow velo-city 3 cm above the river bottom, mean columnvelocity at the sampling point, flow depth, shearvelocity, shear stress, water temperature, dissolvedoxygen content and oxygen saturation.

4 RESULTS AND DISCUSSION

4.1 Effects of dams on hydrological parameters

Dams typically have a profound effect on riverhydrology (O’Reilly and Silberblatt 2009), and thiswas also observed in the Soča River. The results ofthe hydrological measurements and analyses of meanannual flow, �Qd, mean minimum flow, �Qmin andminimum flow, Qmin, as well as of Q82 and Q95, arepresented in Table 2. The results show substantialchanges in the hydrological regime of the SočaRiver downstream of the Podsela and Ajba damsfor all indices except peak flows. The changes con-cerned reduced flows, especially at lower discharges,resulting in a decrease in flow velocity and in flowdepth, i.e. an altered flow duration curve.

A comparison of hydrological parameters for theSoča River for the period 1961–1995 shows a decreasein the values of �Qmin and Qmin at sampling sites SO2

Table 2 Hydrological parameters in the selected reaches of the Soča River and its tributary, the Ušnica Stream, withoutwater abstraction and in the selected water abstraction reaches as determined from the official hydrological data for theperiod 1961–1995.

Hydrological cross-section Samplingsite

F (km2) �Qd

(m3 s-1)�Qmin

(m3 s-1)Qmin

(m3 s-1)Qmax

(m3 s-1)Q82

(m3 s-1)Q95

(m3 s-1)

Soča River at gauge station Kobarid 434.7 34.1 7.9 4.6 664 12.1 8.9Soča River at the Podsela Dam 1244 80 16 10 2140 27.4 20.0Ušnica Stream at the Ušnik Dam USN 9.43 0.60 0.03 0.02 - - -Soča River downstream of the Podsela Dam,

without abstractionSO2 1254 80.6 16.0 10.0 2140 27.5 20.1

Soča River downstream of the Podsela Dam,with abstraction

SO2 1254 22.5 0.13 0.12 2140 0.64 0.25

Soča River downstream of the Ajba Dam,without abstraction

SO4 1345 86.2 17.8 10.5 >2140 29.7 21.2

Soča River downstream of the Ajba Dam, withabstraction

SO4 1345 33.1 1.0 1.0 >2140 1.4 1.1

Note F: catchment area; �Qd: mean flow (=arithmetic average of mean daily flows in m3 s-1 for all days in the considered period); �Qmin: mean minimumflow (=the arithmetic average of minimum measured daily flows for every year in the period considered); Qmin: minimum flow; Qmax: maximum flow; Q82:flow equalled or exceeded 82% of the time; Q95: flow equalled or exceeded 95% of the time.

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and SO4 by over 90% that can be attributed to waterdiversion from the reservoirs through the bypass tunnelto the Doblar and Plave HPP (Table 2). Tang et al.(2013) reported that, due to water abstraction in HongKong streams in a monsoonal climate, the downstreamdischarge declined by 71% during the wet season and54% during the dry season, but current velocitydecreased by 46% in the wet season and by 18% inthe dry season. In the Soča River, in a temperateclimate the changes in hydrological variables weremuch higher for low flows, but remained largelyunchanged for high flows. The measured flows ofthe Soča River downstream of the Podsela Dam wereat least 96.7% lower, and those downstream of theAjba Dam at least 81.3% lower than the flows atreference sampling site SO1 (Table 2). Hence, lowerlocal flow velocities, both v3cm and �vv were evident atthe sampling sites downstream of the dams (Table 3).Owing to the low flows, water stagnated downstream ofthe dams. The measured flow velocities v3cm and �vv atsampling points where pools were present were equal to0 in the sampling sites SO2, SO3 and SO4. The highestvalue, v3cm = 0.86 m s-1, was measured at SO1, wherethe highest average value (v3cm = 0.44 m s-1) was alsomeasured. The highest and lowest measured averagevalues for �vv were measured at SO1 (�vv = 0.71 m s-1)and SO2 (�vv = 0.16 m s-1), respectively. The highestlocal water depth in a vertical of 0.63 m wasrecorded at site SO1. The average water depth ofthe USN sampling site in the Ušnica Stream was0.15 m.

4.2 Effects of dams on river sediments

Backwater effects and water abstraction from awatercourse normally cause a significant decrease ofhigh flows in the downstream reach (Ward andStanford 1995, Erskine et al. 1999, Magilligan andNislow 2005). This decrease in water flow may alsoreduce the sediment transport (Ward and Stanford1995, Graf 2006). However, as shown in Section4.1, high flows have not been significantly reducedin the bypassed reaches on the Soča River. Restrictedsediment transport in the Soča River in the reachesdownstream from the diversion dams is due primarilyto reservoir sedimentation and restriction of transportpast the dam wall (especially in the large reservoirupstream of Podsela Dam) rather than due to a reduc-tion in the number of peak flow events capable oftransporting sediment in the bypassed reaches.

In gravel-bed rivers that have experienced sev-eral years of hydropower operation, a stable riverbedor ‘bed armouring’ and downstream coarsening hasoften been reported downstream of the dam (latenterosion) (e.g. Sear 1995, Graf 2006). In the SočaRiver, the chain of hydropower dams has stoppedsediment inflow from the upstream reach.Therefore, normal sediment transport in the river isinterrupted, causing high flows with reduced sedi-ment transport and also substantially altering sus-pended load dynamics. This is why downstream ofthe Podsela Dam and the Ajba Dam, the riversediments are predominantly cobble size with an

Table 3 Measured hydrological and physicochemical parameters at sampling sites on the Soča River and Ušnica Streamduring four field surveys in 1998 (February, May, August and November).

Site Value Q v3cm �vv h dm d90 σ T DO OS pH EC DWt OM Chl.-a(m3 s-1) (m s-1) (m s-1) (m) (mm) (mm) (-) (°C) (mg L-1) (%) (μS cm-1) (g m-2) (g m-2) (mg m-2)

SO1 Minimum 11.3 0.11 0.12 0.13 58.5 137.9 5.32 5.4 11.7 100 7.7 217 9 5 6Maximum 16.9 0.86 1.65 0.63 72.4 183.2 5.54 13.5 13.5 122 8.3 245 324 67 153Average 13.9 0.44 0.71 0.34 - - - 9.2 12.9 110 8.1 230 56 16 42

SO2 Minimum 0.23 0 0 0.03 92.2 219.2 5.28 2.6 8.1 87 7.3 260 5 2 24Maximum 0.27 0.41 0.42 0.34 155.0 358.7 4.85 19.8 13.2 121 8.5 297 352 72 283Average 0.25 0.14 0.16 0.19 - - 10.6 11.6 99 8.0 280 85 24 89

SO3 Minimum 0.23 0 0 0.13 83.5 210.0 5.27 2.1 9.4 97 7.4 258 14 4 25Maximum 0.27 0.25 0.34 0.35 133.5 301.4 5.05 20.2 14.5 142 8.4 299 283 84 260Average 0.25 0.09 0.17 0.25 - - - 11.2 12.6 110 8.1 278 114 40 123

SO4 Minimum 0.86 0 0 0.12 - - - 3.8 11.5 100 7.5 232 12 5 11Maximum 2.23 0.28 0.53 0.55 - - - 17.5 13.6 123 8.6 273 76 21 130Average 1.4 0.11 0.19 0.32 190.4 498.9 5.15 9.1 12.9 109 8.2 251 37 13 57

USN Minimum 0.05 0.06 0.17 0.15 - - - 7.4 10.3 102 7.4 310 28 8 34Maximum 0.07 0.14 0.22 0.16 - - - 15.5 12.9 115 8.2 338 45 22 147Average 0.05 0.11 0.20 0.15 - - - 11.5 12.0 107 7.9 325 38 14 91

Note v3cm: flow velocity at 3 cm above the river bottom; �vv: mean flow velocity in vertical; h: water depth; dm: arithmetic mean particle size, d90: 90% ofbed load grain; σ: particle sorting coefficient; T: water temperature; DO: dissolved oxygen; OS: oxygen saturation; EC: electrical conductivity; DWt: dryweight; OM: organic matter; Chl.-a: chlorophyll-a.

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abundance of large boulders. The arithmetic meangrain size, dm, downstream of both dams was muchlarger than at their upstream cross-section (Table 3),showing downstream coarsening instead of thedownstream fining of river sediments normally pre-sent under natural conditions (e.g. Mikoš 1994). Theparticle sorting coefficient of river sediments σ waslargely unaffected, having a value close to 5 at allsites (Table 3). The results of our study have shownthat the arithmetic mean particle size dm downstreamof the Podsela and Ajba dams of more than 100 mmare typical of a very coarse and stable surface layerbed load. They are substantially larger than the arith-metic mean sizes of 35–40 mm of comparableSlovenian gravel-bed rivers (Mikoš 2000).

4.3 Effects of reduced flow on physical andchemical parameters

Below the Podsela and Ajba dams, the oscillations ofwater temperature, dissolved oxygen and oxygensaturation are much larger in comparison to relatedsites upstream of the dams, and this can be attributedto the large abstraction of water and significantlyreduced flows (Table 2).

The higher values of pH were measured belowthe dams in the Soča River in spring and summer,possibly due to active photosynthesis, when algae usecarbon and release oxygen into the water and withthis process increase pH. The higher values of elec-trical conductivity below the dams could be due tothe relative importance of inflow from the Ušnicatributary which has a calcareous substratum that dis-solves readily and creates the highest conductivityvalues of all the sampling sites (Table 3). The highestvalues were in the spring and autumn, when higherflows might increase the leaching of ions from thesubstrata.

Temperature is one of the most important physi-cal parameters of water, because it affects other phy-sical and chemical parameters of running water, andthe distribution and ecology of periphytic algae.Abstraction and diversion of flows during summerhas the potential to cause increases in water tempera-ture (Biggs 2000). In the summer, when the watertemperature in the Soča River at the reference siteSO1 was 13.5°C, water temperature in the abstractionsection (SO3) was higher by 6.7°C (Table 3). Similarvalues were reported for the Schächenbach River inSwitzerland (Bundi and Eichenberger 1989), wherean increase in water temperature of 6°C was found inthe abstraction reach.

Below the Podsela and Ajba dams, lower con-centrations of oxygen were recorded at all times ofthe year in river pools compared to sampling sites inthe same reaches with flowing water. This can beattributed to the absence of turbulence in the poolswith only minor aeration present. However, the high-est values of oxygen below the Podsela Dam are thereason for the greater photosynthetic activity there,because of algae proliferation.

Due to reduced flows, the physicochemical com-position of water in the river reach influenced bythem is no longer defined by conditions in theupper reaches, but by inflows downstream of thedam (Bundi and Eichenberger 1989). This statementis supported by the results of the present study.

4.4 Periphytic species responses to flowreduction

The Soča River and its tributary, the Ušnica Stream,are distinguished by a highly diverse periphytoncommunity. Altogether, 127 taxa were identified inthe periphyton samples, of which 126 were found inthe Soča River and 32 in the Ušnica Stream. In allsamples, the highest number (66) of species wasfound to belong to Bacillariophyta, as has also beenreported by other investigations of algae in Slovenianrivers (e.g. Smolar-Žvanut 2000, Smolar et al. 2005).The results also identified 33 taxa of Chlorophyta; 23taxa of Cyanophyta and 2 taxa of Rhodophyta,Chrysophyta and Xanthophyta. A detailed list of allthe identified taxa is published elsewhere (Smolar-Žvanut 2000).

The highest number of taxa belonging toBacillariophyta was determined at sites SO2 (58 spe-cies) and SO3 (52 species). The following specieswere found at all sampling sites: Achnanthes lanceo-lata, A. sp., Cymbella affinis, Cymbella ventricosa,Diatoma hiemale v. mesodon, Diatoma vulgare,Gomphonema intricatum, Meridion circulare,Navicula gracilis, Nitzschia fonticola, Nitzschialinearis, Pinnularia interrupta and Synedra ulna.Species Amphipleura pellucida, Fragillaria sp.,Gomphonema sp., Gyrosigma attenuatum, Nitzschiaangustata, Rhoicosphaenia curvata and Tabellariaflocculosa were found only at site SO2 and speciesFragillaria crotonensis, Navicula placentula,Nitzschia sigmoidea, Pinnularia microstauron andSurirella angusta only at sampling site SO3. Themost abundant species were Cocconeis pediculusrecorded in May and August 1998 at sampling site

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SO4 and S. ulna in August 1998 at samplingsite SO1.

In the Soča River, 32 species of Chlorophytawere observed, mostly at sites SO2 (22 taxa) andSO3 (21 taxa), as compared to only three taxa inthe Ušnica Stream. Cladophora glomerata was abun-dant in May 1998 and August 1998 at sites SO2, SO3and USN. Species from the genus Closterium andScenedesmus were rarely found. In November 1998,Ulotrix zonata algae were found in high abundance atsampling sites SO2 and SO3.

In the Soča River, 23 species of Cyanophytawere determined, as compared to only five taxa inthe Ušnica Stream. At all sampling sites, the genusChrococcus and Phormidium were observed, as wellas the species Phormidium inundatum, which wasabundant at all sampling times. The speciesChamaesiphon fuscus and Phormidium tenue wereonly determined at sampling site SO1. At samplingsite SO1 in May 1998, species Lyngbya kützingii andOscillatoria limosa were identified.

At all sampling sites in the Soča River andUšnica Stream, the species Audiouinella chalybeawas found, but Bangia artropurpurea was onlyfound in the Soča River sampling sites. Both speciesbelong to Rhodophyta.

At all sampling sites in the Soča River, thespecies Hydrurus foetidus (Chrysophyta) was found,but the genus Characiopsis was only found at siteSO2. The H. foetidus species was abundant in winterand autumn at site SO1.

Only two genus of Xanthophyta were found,Tribonema sp. found in the samples from sites SO3and SO4, and algae Tribonema aequale found inFebruary 1998 at site SO4.

The modified flow regime below a dam usuallyresults in changes to the periphyton communities(Biggs 2000). Suren and Riis (2010) reported thatmore significant changes in the algal communitywere associated with longer periods of low flow.Factors which have resulted in a greater number ofperiphytic algae being determined downstream ratherthan upstream of the dams include the stability ofhydrological factors downstream of the Podsela andAjba dams (i.e. a prolonged periods of low flow andthe occurrence of high flows), the low-to-moderateflow velocities which make the colonization by greenalgae possible, a substratum of Bacillariophyta andadequate light. Biggs and Smith (2002) reported thatthe highest taxonomic richness occurred in streamswith low-to-intermediate frequencies of flood distur-bance (up to 10 bed-moving events per year) andintermediate-to-high concentrations of mats. Wealways sampled periphyton at least 3 weeks afterthe last flood. It is known that periphytic algae diver-sity is significantly limited in watercourses with afrequent increase in flow (Clausen and Biggs 1997).Periphytic algae will be torn from the substratum byfast flows, and, in particular, damage to algae will becaused by the transport of larger sediment grains. Thehighest number of periphytic algae species were allobserved at site SO2 and the lowest number of spe-cies were found at reference site SO1 (Fig. 2). Theexception was in May 1998, when the highest num-ber was determined at site SO4. In all samples, dia-toms represented at least 50% of all species of algaepresent. Despite the fact that seasonal patterns inperiphyton communities in rivers appear to be mostlymediated by river hydrology, seasonality in grazeractivity or seasonality in light regime and water tem-perature (Biggs 1996), the highest number of

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BACILLARIOPHYTA CYANOPHYTA CHLOROPHYTA

CHRYSOPHYTA XANTHOPHYTA RHODOPHYTA

Fig. 2 Composition of periphytic algae at sampling sites in the Soča River and Ušnica Stream.

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Cyanophyta and Chlorophyta were present in sum-mer samples, when we determined 14 species ofChlorophyta at sampling sites SO2 and SO3 and 11species of Cyanophyta at site SO3. Higher numbersof green algae downstream from the dams at that timeare probably due to the presence of higher watertemperatures and more stable low flows. If we com-pare the number of green algae species present duringthe year in the Soča River, we can support the resultsof previous studies that Cyanophyta develop in thesummer and filamentous algae in late summer (Biggs1996). Bergey et al. (2010) reported that filamentousgreen algae form dense growths in regulated rivers,and this is also supported by the results from ourstudy. The lowest number of species was alwaysdetermined in the Ušnica Stream, because it is amuch smaller water course than the Soča River andhas uniform habitats with very low water velocities.

According to the literature, the Chrysophyta spe-cies H. foetidus only proliferates in the winter at lowwater temperatures and during constant flow condi-tions (Ward 1974, Valentin et al. 1995, Smolar 1997).Traaen and Lindstrom (1983) reported that 90% ofthe H. foetidus algae occurred at velocities of over0.8 m s-1 (measured 1 cm above the bottom). Theresults from this study in the Soča River confirmthese findings. The species only proliferated at thereference site upstream from the dams and only inwinter time. Downstream from the dams, the specieswas rarely present, probably due to the occurrence oflower velocities in these reaches.

The Bray-Curtis coefficient of similarity showsqualitative changes in the structure of a periphyticalgae community and provides an indication of simi-larities and differences amongst communities. Thehighest similarities were observed among samplescollected at sites which are affected by flow regula-tion and have similar hydrological and physicochem-ical variables, whilst the community of periphyticalgae at reference site SO1 was at most times of theyear significantly different to the communities down-stream of both dams (Fig. 3).

The number of taxa of periphytic algae wasnegatively correlated with flow velocity at 3 cmabove the river bottom (v3cm) in the Soča River(Fig. 4). This can be explained by the findings ofPeterson and Stevenson (1989), who reported that athigh flow velocity the algae colonization is limitedby shear stress.

In the Soča River, downstream from the dams,more algae species common to polluted waters wereidentified, such as Stigeoclonium tenue, N. angustata,

Cyclotela meneghiniana, O. limosa and Oscillatoriatenuis. This is probably due to the presence of lowerflow velocities at these sites, which results in thealtered physicochemical conditions of the water.

4.5 The impact of flow regulation on periphytonbiomass

Flow regulation associated with reduced flows andflow variability, and therefore increased bed stabilitycan increase biomass (Biggs 2000). During low flows,periphyton biomass accrual processes dominate,

SO3N98SO4N98SO2N98SO4F98SO2F98SO3F98SO2M98SO3M98SO2A98SO3A98SO1A98USNM98USNA98SO1M98SO4M98SO1F98SO1N98USNN98

00.10.20.30.40.50.60.70.80.91

Fig. 3 Comparison of the Bray-Curtis coefficient-of-simi-larity indices for the Soča River (SO1–SO4) and UšnicaStream (USN) sampling sites for all sample times (M98:May 1998; N98: November 1998; A98: August 1998;F98: February 1998).

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Fig. 4 Correlation between the flow velocity at 3 cmabove the river bottom (v3cm) and the number of taxa ofperiphytic algae found in the Soča River and UšnicaStream (N = 74, r = –0.43, p < 0.001).

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because loss processes associated with high velocitiesand shear stress, substrate movement and abrasion donot occur (Biggs 2000, Suren and Riis 2010). Lowperiphyton biomass in regulated rivers can be main-tained by providing sequential floods (Mannes et al.2008). In the Soča River and the Ušnica Stream, thevalues of DWand the organic matter of the periphytonwere highest at sampling sites with lower flow velo-cities. The Pearson correlation coefficients were sig-nificant at the p < 0.05 level and were –0.30 and –0.31for DW and flow velocities v3cm and �vv, respectively,and –0.28 for organic matter of the periphyton andthese two flow velocities (Table 4). At times of con-stant discharge, periphyton usually attains a high bio-mass on large pebbles and stones, particularly due tolimited sediment transport at the bottom of a water-course (Biggs 1996). High biomass of periphyton candevelop only after an extended period of habitat sta-bility (Biggs 1996, Biggs et al. 2001, Suren and Riis2010). Low flows, favourable physicochemical condi-tions and appropriate sediment size were factors thatmade the proliferation of algae possible in the SočaRiver below both dams.

The reduced flow in the Soča River, downstreamof the Podsela and Ajba dams, resulted in hydrolo-gical and physicochemical changes in the river,which in turn led to an increased periphyton biomass(Table 3). Biological changes were most striking inthe overgrowth of the river bottom, with green algaeat this location in the Soča River, which can poten-tially alter habitat quality and cause degradation ofthe ecosystem structure (Suren et al. 2003).

The high periphyton biomass downstream fromdams, expressed as chlorophyll-a and organic matter,

has also been reported by other studies (Lowe 1979,Bundi and Eichenberger 1989, Smolar 1997,Koudelkova 1999, James and Suren 2009). Thismay be primarily attributed to minor fluctuations inthe water temperature, the alteration of the flowregime to one without distinct seasonal fluctuations,an increase in the concentration of nutrients and theirabsorption by algae (Koudelkova 1999) and to thepresence of large, immobile sediments (Biggs et al.2001, Suren and Riis 2010). In addition to flowvelocity, at locations with low flow velocities inunshaded watercourses, a high periphyton biomassalso depends on the water chemistry, which in thisstudy was found to be of lower quality downstreamof the Podsela and Ajba dams. Figure 5 illustrates thenegative correlation between the flow velocity at3 cm above the river bottom, v3cm, and the chloro-phyll-a in the Soča River and Ušnica Stream.

Table 4 Pearson correlation coefficients for biological, hydrological and physicochemical parameters of the samples fromthe Soča River and the Ušnica Stream.

Parameter DWt OM Chl.-a N v3cm �vv H v* τ T DO OS

DWt 1OM 0.93 1Chl.-a 0.61 0.61 1N 0.27 0.24 0.43 1v3cm –0.30 –0.28 –0.34 –0.43 1�vv –0.31 –0.28 –0.34 –0.39 0.88 1H –0.20 –0.20 –0.34 –0.30 0.36 0.49 1v* 0.18 0.21 0.05 0.41 –0.25 –0.22 0.25 1τ 0.20 0.22 0.09 0.41 –0.28 –0.26 0.19 0.96 1T 0.52 0.64 0.23 0.11 - - - - - 1DO –0.52 –0.49 –0.20 –0.15 - - - - - –0.68 1OS 0.03 0.23 0.03 –0.11 - - - - - 0.47 0.31 1

Notes DW: dry weight; OM: organic matter; Chl.-a: chlorophyll-a; N: number of taxa; v3cm: flow velocity 3 cm above the river bottom; �vv: mean flowvelocity in vertical; h: water depth; v*: shear velocity; τ: shear stress; T: temperature; DO: dissolved oxygen; OS: oxygen saturation.Significant correlations at 95% significance level using the t test in bold font.

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Fig. 5 Correlation between the flow velocity at 3 cmabove the river bottom (v3cm) and the chlorophyll-afound in the Soča River and Ušnica Stream (N = 74,r = –0.34, p < 0.01).

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No species of green algae proliferated at sam-pling site SO1 upstream from the dams, which ischaracterized by a mobile substratum and high cur-rent velocities. This study highlights evidence for theimpact of reduced flows due to hydropower schemeoperation on the proliferation of green algae in theSoča River, a factor not found by studies on otherSlovenian rivers with water diversions (Smolar-Žvanut 2000).

5 CONCLUSIONS

A field analysis of the impacts of flow regulationdownstream of the Podsela and Ajba dams in theSoča River has shown changes in hydrological, phy-sical and chemical variables that affect the structureof the periphyton community. In the Soča River, highflows occur frequently and, due to the limitations inoperation of the HPPs at these flows, and the fact thatthe dams spill during peak flows, floods in the regu-lated reaches have occurred at the same time andfrequency as in the upstream, unregulated riverreach. Yet, significant differences occur in the per-iphyton communities of the regulated and unregu-lated reaches. We can conclude that floods are notthe most important factor for maintaining the periph-yton community in the Soča River, but the combina-tion of hydraulic habitat variables, sediment size andlack of sediment mobility and physicochemical vari-ables may explain why periphyton biomass and num-ber of periphytic algae were increased downstream ofthe dams.

The results of our study have shown that, tounderstand periphyton dynamics in lotic ecosystems,especially those impacted by flow regulation, adetailed understanding of the nature and timing ofhydrological alteration is needed. During periods oflow constant flows downstream of both dams, theperiphyton is producing high biomass on large cob-bles and boulders, especially due to the stablehydraulic habitat conditions (with low velocity),low mobility of the bedload and, consequently, abra-sion of the sediment grains’ surface. Therefore, it isnot sufficient to design and implement an environ-mental flow strategy that restores flow variabilitydownstream of the dams. It is also important toestablish some degree of continuous sediment trans-port through the chain of reservoirs. If this cannot beachieved, artificial feeding of sediments may berequired for reaches downstream of the dams.

In accordance with the requirements of EuropeanUnion Water Framework Directive (EU WFD 2000),

further research is necessary to examine the relation-ship between the periphyton community and thenutrients downstream of the dams in the SočaRiver. This is needed to define environmental flowsand sediment transport requirements necessary tomaintain a healthy periphyton community, typicalfor this alpine river with a torrential character.

Acknowledgements The authors would like to thankDanijel Vrhovšek for his contribution to the draftversion of this article. The authors would also liketo thank Dušan Rebolj, Peter Muck, Darko Burja andDarko Anzeljc, all from the Institute for Waters of theRepublic of Slovenia in Ljubljana, for helping withfield work and hydrologic analyses. The SočaElectricity Board from Nova Gorica kindly helpedwith relevant technical and hydrological data on theHPPs under investigation. The article was greatlyimproved by insightful comments of two anonymousreviewers and Ian Maddock’s comments and recom-mendations. Hydrological data were obtained fromthe archives of the Slovenian Environment Agency.

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