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

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1 This article was downloaded by: [University of Ljubljana], [Matjaz Mikos] On: 01 May 2014, At: 07:16 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Hydrological Sciences Journal Publication details, including instructions for authors and subscription information: The impact of flow regulation by hydropower dams on the periphyton community in the Soča River, Slovenia Nataša Smolar-Žvanut a & Matjaž Mikoš b a Institute for Water of the Republic of Slovenia, Hajdrihova 28c, 1000 Ljubljana, Slovenia b Faculty of Civil and Geodetic Engineering, University of Ljubljana, Jamova 2, Ljubljana, Slovenia Accepted author version posted online: 19 Aug 2013.Published online: 29 Apr To cite this article: Nataša Smolar-Žvanut & Matjaž Mikoš (2014): The impact of flow regulation by hydropower dams on the periphyton community in the Soča River, Slovenia, Hydrological Sciences Journal, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Hydrological Sciences Journal Journal des Sciences Hydrologiques, The impact of flow regulation by hydropower dams on the periphyton community in the Soča River, Slovenia Nataša Smolar-Žvanut 1 and Matjaž Mikoš 2 1 Institute for Water of the Republic of Slovenia, Hajdrihova 28c, 1000 Ljubljana, Slovenia natasa.smolar@izvrs.si 2 Faculty 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., The impact of flow regulation by hydropower dams on the periphyton community in the Soča River, Slovenia. Hydrological Sciences Journal, 59 (5), Abstract The effects of hydropower dams and, in particular, the impacts of reduced river flows on the periphyton community were assessed in the Soča River, Slovenia. Sampling sites were selected upstream and downstream of the Podsela and Ajba dams. Sampling was carried out in 1998 during a period of low flows. Reaches downstream from the dams experienced prolonged periods of reduced flows, and a corresponding decrease in flow velocity and water depth. The chain of hydropower dams has stopped sediment inflow from the upstream reach. Below the dams, the oscillations of water temperature, dissolved oxygen and oxygen saturation are much larger than at unregulated sites upstream. The impact of prolonged periods of reduced flows, a lack of sediment supply from upstream and changes in physicochemical variables has caused high periphyton biomass, proliferation of green algae and increases in the number of periphytic algae species below the dams. This has significant implications for the design of environmental flow strategies that provide a sediment supply to maintain a healthy periphyton community. 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 communauté périphytique de la rivière Soča, Slovénie Résumé Les effets des barrages hydroélectriques, et en particulier les effets de la réduction du débit des rivières sur la communauté périphytique, ont été évalués dans la rivière Soča (Slovénie). Les sites d échantillonnage ont été sélectionnés en amont et en aval des barrages Podsela et Ajba. L échantillonnage a été effectué en 1998 au cours d une période d étiage. Les biefs en aval des barrages ont connu des périodes d étiage prolongées et une diminution correspondante de la vitesse d écoulement et la profondeur de l eau. La chaîne de barrages hydroélectriques a arrêté l apport de sédiments du bief amont. A l aval des barrages les oscillations de la température de l eau, de l oxygène dissous et de la saturation en oxygène ont été beaucoup plus importantes qu aux sites non régularisés de l amont. L impact des périodes d étiage prolongées, l absence d apport sédimentaires de l amont et les modifications des variables physico-chimiques ont provoqué l apparition d une importante biomasse de périphyton, la prolifération d algues vertes et l augmentation du nombre d espèces d algues périphytiques en aval des barrages. Ceci a des implications importantes pour la conception de stratégies concernant les débits réservés devant fournir un apport de sédiments suffisant pour maintenir une communauté 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 downstream hydrology and geomorphology (Graf 2006), modify the structure and dynamics of aquatic and riparian habitats (Poff and Hart 2002), change physicochemical parameters and can cause changes in aquatic flora and fauna (Petts 1984). River regulation can have an impact on the flow regime in a number of different ways, i.e. through changes in the timing, magnitude and frequency of high and 2014 IAHS Press

3 2 Nataša Smolar-Žvanut and Matjaž Mikoš low flows (Magilligan and Nislow 2005, Graf 2006), and this hydrological alteration can cause changes in periphyton communities (Biggs 2000). Periphyton (microscopic and macroscopic algae, including Cyanobacteria) is widely considered to be both the main source of energy for higher trophic levels in streams and a valuable indicator of environmental change. In stable-flowing, nutrient-enriched streams, it can proliferate and cause water management problems (Biggs 1996); become a nuisance, degrading swimming and fishing spots and clogging irrigation and water supply intakes; have aesthetic impacts and reduce recreational and biodiversity values (Biggs 2000). The most important factors that influence the growth and development of periphyton include light, water temperature, the nature of the substrate, flow velocity and turbulence, ph, alkalinity, hardness, nutrients and other dissolved substances, salinity, oxygen and carbon dioxide (Hynes 1979). The presence, abundance, composition and growth of periphyton are also controlled or influenced by environmental variations, such as disturbances, stressors, nutrients, hydraulic conditions and biotic interactions (Larned 2010). Several studies have examined the effects of river flow regulation on periphyton (Lowe 1979, Biggs 1987, Valentin et al. 1995, Bergey et al. 2010, Tang et al. 2013). Flow regulation can influence periphyton species composition and the abundance of individual species (Biggs 1996), causing their numbers to increase or decrease depending on site-specific characteristics and the exact nature of hydrological change. For example, regulated flow conditions may provide new habitats or conditions which are suitable for species that did not previously occur in the stream (Growns and Growns 2001). Suren and Riis (2010) found that the longer the duration of low flow, the more the algal community will change, and in turn, the more the habitat quality will change. Flow regulation that creates reduced flow variability and increased bed stability can increase periphyton biomass, whereas increased flow variability typically decreases biomass (Biggs 2000). Maximum periphyton biomass usually occurs at low flow velocities (Biggs et al. 1998). Low flows enhance plant biomass through changes to hydraulic parameters, light and temperature conditions (Suren and Riis 2010). Many studies have also reported high periphyton biomass, expressed as chlorophyll-a and organic matter, measured downstream of dams (Lowe 1979, Bundi and Eichenberger 1989, Smolar 1997, Koudelkova 1999). In Slovenia, 36 large dams on rivers have been built, 20 of them for electricity production by hydropower plants (HPPs) (six with a bypass reach). But the reservoir capacities in Slovenia are relatively small and flood frequencies remain largely unaltered. The effects of reduced flow caused by water intake for selected HPPs in Slovenia during periods of drought showed changes in the periphytic species composition in alpine (Smolar-Žvanut 2001, Smolar et al. 2005, Smolar-Žvanut and Krivograd-Klemenčič 2011), lowland and karst rivers (Smolar et al. 1998), respectively. Many studies that have examined the impact of river regulation due to dam operation have focused specifically on diatom communities (e.g. Wu et al. 2010, Tang et al. 2013), but not on whole periphytic communities and their biomass. Most work has also focused on rivers where the discharges have been reduced across the whole flow regime, including reduced flood occurrence. The main aim of our study was to detect the downstream effects of an altered flow regime due to water diversion for HPPs on the periphyton community in the alpine Soča River, Slovenia. This site provides a case study to illustrate the impacts on the periphyton of a largely unaltered high flow frequency and significantly reduced low flow occurrence. We compared periphytic algae composition and periphyton biomass between upstream and downstream reaches of two dams on the Soča River. Moreover, we measured the main hydrological, morphological and physicochemical parameters to examine their influence on the growth and development of periphyton. We expected to observe an increase in periphyton biomass and a decrease in species diversity downstream from the dams. 2 HYDROPOWER PLANTS ON THE SOČA RIVER AND THEIR CHARACTERISTICS The Soča River is a typical European alpine river that remains largely unregulated in its upper course. It rises in the Slovenian Alps, flowing for 95 km through Slovenia before crossing into Italy and discharging into the Adriatic Sea. It has a catchment area of 1576 km 2 and is predominantly underlain by limestone, but the lower parts of the river run over flysch and Quaternary gravels. The Soča River has a torrential flow regime, with high flows occurring at any time of year. The lowest flows are experienced both in summer and in winter months, with generally

4 The impact of flow regulation by hydropower dams 3 higher snow-fed flows in spring and rain-fed flows in autumn (Maddock et al. 2008). The flow in the middle course of the Soča River in Slovenia is highly regulated by a chain of three large (and one small) HPPs: Doblar HPP (built in 1938) is fed by a diversion from Podsela Dam, a 55 m high concrete archgravity dam, the second highest in Slovenia; Plave HPP (built in 1940) is fed by a diversion from the Ajba Dam, which is 4.5 km downstream from 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 is located at the Ajba Dam. Water is abstracted from the reservoir upstream of each dam. It then flows through a bypass tunnel to the HPP and is subsequently augmented back to the Soča River further downstream. Therefore, bypassed river sections with reduced flows exist below each dam. The river section with diverted water from the Podsela Dam to the Doblar HPP is 4320 m long, and that from the Ajba Dam to the Plave HPP is 7950 m long. At the Podsela Dam, the highest permitted water abstraction for the Doblar HPP is 96 m 3 s -1. At the Ajba Dam, the highest permitted abstraction for the Plave HPP is 75 m 3 s -1. The HPP Solkan is a run-of-the river hydropower station, with an installation capacity of 180 m 3 s -1, and is located 7500 m downstream of the HPP Plave. If river discharge is high and the reservoirs are not being filled, water spills over the dams. In recent decades, the measured discharge duration curves of the Soča River have shown there is no obvious pattern to the frequency of dam spilling. The average spilling frequency is 78 days a year for the Podsela Dam (discharge >96 m 3 s -1 ) and 130 days for the Ajba Dam (discharge >75 m 3 s -1 ). The spilling is quite frequent and can occur at most times during the year (e.g. in 1998, the Podsela Dam spilled on 81 days (3 days in January, 18 days in April, 4 days in May, 1 day in June, 9 days in July, 13 days in September, 24 days in October and 8 days in November)) due to the flashy flow regime of the Soča River. Spilling is more frequent for the Ajba Dam than the Podsela Dam due to the lower permitted abstraction. From the measured discharge duration curves of the Soča River, one can also estimate that the minimum flow downstream of both 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.

5 4 Nataša Smolar-Žvanut and Matjaž Mikoš period from December to April and in the summer months (normally in August), which is typical of alpine 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 Podsela Dam, the only water in the channel is derived from one tributary (the Ušnica Stream, see Fig. 1) withan average flow of 0.05 m 3 s -1 and small quantities of water from a gate leakage at the Podsela Dam having an average flow of 0.20 m 3 s -1. In the dry season, most other tributaries dry up because of their karst catchments. Downstream of the Podsela Dam, a minimum environmental flow requirement was not prescribed. In the river reach between the Ajba Dam and the outlet of the Plave HPP, there are only small tributaries that contribute less than 1% of the Soča River flow. The minimum environmental flow released into the river channel downstream of the Ajba Dam is 0.50 m 3 s -1,asprescribedinthe water management operating licence for the Plave HPP. With regard to the frequency and magnitude of floods, the dams have no significant impact on the high flow regime along the Soča River in its middle course. The total storage volume of Podsela Reservoir is m 3 (effective volume is m 3 ), with a maximum oscillation in water level of 2 m. The total storage volume of Ajba Reservoir is m 3 (effective volume m 3 ), with a maximum oscillation in water level of 4 m. The maximum flow (for the period ) observed at Podsela Dam was 2140 m 3 s -1 (the same for the natural and regulated Table 1 Characteristics of the sampling sites in the Soča River. Sampling site Gauss-Krüger coordinates, x; y Riverbed width (m) flow regime), while the Q 1 % (the discharge that is equalled or exceeded for 1% of the time) for the natural flow regime was 298 m 3 s -1 and for the regulated flow regime was 293 m 3 s -1. The reservoirs upstream from the diversion dams are not used for flood control purposes; the dams spill during these times and their impact on reducing flood peaks is not significant. Therefore, at times of very high flow, the Soča River becomes a homogenous hydrological unit along 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 in the chain, namely the Podsela, and then downstream (SO4) of the second reservoir, i.e. below the Ajba Dam. Additionally, a sampling site was selected on the Ušnica Stream (USN), an unregulated tributary which flows into the Soča River just below the Podsela Dam (Fig. 1). A description of the study sites is presented in Table 1. Due to field restrictions, not all sampling activities were possible at all sites at all times. In February 1998, no sampling was possible at the sampling site USN due to the Ušnica Stream being frozen. The sampling in August 1998 at SO4 was restricted due to high flows in this reach (whilst the turbines in the HPP Plave were being renovated); therefore, we could not sample periphyton and measure hydrological parameters on that occasion. Substrata Riverbank Riverbank vegetation SO ; Gravel, sand and silt SO ; Course gravel and boulders SO ; Coarse gravel and cobbles SO ; Course gravel and single large boulders USN ; Small boulders and cobbles Low angle or flat river bank; alluvial plain; natural River bank angle steeper than1:1; rocks; natural River bank angle steeper than 1:1; rocks; natural River bank man-made with rip-rap River bank angle steeper than 1:1; rocks; natural Alluvial forest; most common species: Salix alba and Salix glutinosa Dense forest vegetation: Ostryo-Fagetum, Cytisantho-Ostryetum in Ornithogalo- Carpinetum Dense forest vegetation: Ostryo-Fagetum, Cytisantho-Ostryetum in Ornithogalo- Carpinetum Shrubs and trees: most common species: S. alba, Salix caprea, Salix eleagnos, Salix purpurea, Populus nigra and Populus tremula Hardwood: most common species are S. alba, Alnus glutinosa and Corylus avellana

6 The impact of flow regulation by hydropower dams Periphyton sampling procedure and laboratory analyses Sampling of periphyton for taxonomic and biomass analyses was carried out in 1998 during the period of low flows, i.e. in February, May, August and November. The periphyton was sampled at several sampling points along a river cross-section. The number of sampling points was determined according to the riverbed width, substrata structure, water depth and flow velocity. The maximum number of points was five. Where the selected cross-section was dominated by stagnant water, sampling points in the nearest upstream and downstream pools were also added. Periphyton samples intended for a qualitative analysis were taken by scraping off the surface area of pebbles, stones, rocks, sand, macrophytes and sunken wood found at the sampling site. The samples were preserved with formaldehyde in the field, so that the final concentration of formaldehyde in the samples was 5%. Periphyton samples intended for quantitative analysis of the periphyton biomass (including attached algae, protozoa, bacteria and fungi) were taken from river sediments of mm in diameter. At each sampling point, samples were scraped from five sediment particles over a surface area of 200 mm 2. Dry weight (DW) and organic matter (ash-free dry weight, AFDW) were assessed in the laboratory using the APHA technique (1992). The chlorophyll-a concentration was determined by the use of filtration through Whatman GF/C filters and extraction with hot methanol (Vollenweider 1974). The values of periphyton biomass were calculated per square metre of river bottom sediment. In the laboratory, periphytic algae were examined under a light microscope Nikon Eclipse E400 (Nikon, Tokyo, Japan) by means of phase-contrast optics for magnifications of 1000 in order to assess the frequency of individual recognized taxa (species) (1 = rare, 3 = frequent, 5 = abundant) (Pantle and Buck 1955). 3.3 Hydrological parameters The current velocity and discharge were measured with a SEBA Mini Current Meter MI. At all the periphyton sampling points, the current velocity was measured at 3 cm above the bottom (v 3cm ). The mean column velocity (v v ) was also measured above the sampling points at 0.4 of the water depth above the river bed. Flow velocity was averaged over a 1-minute period per sampling point. The catchment areas of sampling sites (F in km 2 ) were determined, and the following values were calculated: Q d (mean flow); Q min (mean minimum flow); Q min (minimum flow); Q max (maximum flow); Q 82 (flow in m 3 s -1 equalled or exceeded 82% of the time) and Q 95 (flow in m 3 s -1 equalled or exceeded for 95% of the time), for the duration of the flow record. The aim of analysing the hydrological data was to determine the hydrological characteristics of the Soča River regime in the sections downstream of the Podsela and Ajba dams. The analysis of hydrological data was performed for single hydrological crosssections under the following water regimes: without abstractions: the flows were determined on the supposition that there were no abstractions for the HPP in the reaches under consideration and with abstractions: the flows were determined for the operation of the HPP at that time in the Soča River in the reaches under consideration. 3.4 Physical and chemical parameters Concurrent with periphyton sampling, measurements of the following physical and chemical parameters were performed with portable multimeters (WTW Multiline/F, Germany): electrical conductivity and temperature were measured 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 River sediments In July 1999, a grain-size analysis of river sediments from the sampling sites was performed using the Wolman sampling strategy. This is a method of surface sampling in which all particles of river sediments found under a selected line parallel to the flow direction are sampled. It is recommended that at least 100 particles should be sampled. The approach of Anastasi (1984) and Fehr (1987) was adopted. We used a non-flexible combination of the Wolman samples for coarser particles from the riverbed surface, and the theoretical Fuller curve as an

7 6 Nataša Smolar-Žvanut and Matjaž Mikoš approximation for finer particles from the subsurface of the riverbed. As a result of the granulometric analysis, we obtained the particle-size distributions of the surface of the riverbed at the sample sites. From the resulting particle-size curves, we determined different particle sizes, e.g. d 90, d 84, d 16, arithmetic mean pd m, and the particle sorting coefficient σ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi d 84 =d Shear stress and shear velocity p Shear velocity was determined as v * ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffi g h I in ms -1 and shear stress as τ = γ g h Iin N m -2, where g is acceleration due to gravity (9.81 m s -2 ), h is flow depth in m and I is water level gradient. If the river bottom is very uneven, the river bottom gradient is hard to determine. Therefore at low flows, water level gradient was used instead. In a chosen crosssection, the water level gradient was determined in a reach length three times the width of the cross-section. Hence, the average values of shear velocities v * and shear stresses τ at each sampling site were determined. 3.7 Statistical analyses A comparison of periphytic algae communities within individual sampling points and within sampling sites was made using the data on species composition and the relative frequency of species found. Similarities and differences among periphytic algae were assessed by means of a multivariate cluster analysis (Bray-Curtis coefficient of similarity; Clark and Warwick 1990) and a Slovenian national database of algae (the DABA data system; Vrhovšek et al. 1998). The Pearson correlation coefficient and the t test were used to assess significant differences within sampling sites and between them for the following biological, hydraulic, physical and chemical parameters: periphyton DW, periphyton organic matter, chlorophyll-a, number of periphyton taxa, flow velocity 3 cm above the river bottom, mean column velocity at the sampling point, flow depth, shear velocity, shear stress, water temperature, dissolved oxygen content and oxygen saturation. 4 RESULTS AND DISCUSSION 4.1 Effects of dams on hydrological parameters Dams typically have a profound effect on river hydrology (O Reilly and Silberblatt 2009), and this was also observed in the Soča River. The results of the hydrological measurements and analyses of mean annual flow, Q d, mean minimum flow, Q min and minimum flow, Q min, as well as of Q 82 and Q 95, are presented in Table 2. The results show substantial changes in the hydrological regime of the Soča River downstream of the Podsela and Ajba dams for all indices except peak flows. The changes concerned reduced flows, especially at lower discharges, resulting in a decrease in flow velocity and in flow depth, i.e. an altered flow duration curve. A comparison of hydrological parameters for the Soča River for the period shows a decrease in the values of Q min and Q min at sampling sites SO2 Table 2 Hydrological parameters in the selected reaches of the Soča River and its tributary, the Ušnica Stream, without water abstraction and in the selected water abstraction reaches as determined from the official hydrological data for the period Hydrological cross-section Sampling site F (km 2 ) Q d (m 3 s -1 ) Q min (m 3 s -1 ) Q min (m 3 s -1 ) Q max (m 3 s -1 ) Q 82 (m 3 s -1 ) Q 95 (m 3 s -1 ) Soča River at gauge station Kobarid Soča River at the Podsela Dam Ušnica Stream at the Ušnik Dam USN Soča River downstream of the Podsela Dam, SO without abstraction Soča River downstream of the Podsela Dam, SO with abstraction Soča River downstream of the Ajba Dam, SO > without abstraction Soča River downstream of the Ajba Dam, with abstraction SO > Note F: catchment area; Q d : mean flow (=arithmetic average of mean daily flows in m 3 s -1 for all days in the considered period); Q min : mean minimum flow (=the arithmetic average of minimum measured daily flows for every year in the period considered); Q min : minimum flow; Q max : maximum flow; Q 82 : flow equalled or exceeded 82% of the time; Q 95 : flow equalled or exceeded 95% of the time.

8 The impact of flow regulation by hydropower dams 7 and SO4 by over 90% that can be attributed to water diversion from the reservoirs through the bypass tunnel to the Doblar and Plave HPP (Table 2). Tang et al. (2013) reportedthat,duetowaterabstractioninhong Kong streams in a monsoonal climate, the downstream discharge declined by 71% during the wet season and 54% during the dry season, but current velocity decreased by 46% in the wet season and by 18% in the dry season. In the Soča River, in a temperate climate the changes in hydrological variables were much higher for low flows, but remained largely unchanged for high flows. The measured flows of the Soča River downstream of the Podsela Dam were at least 96.7% lower, and those downstream of the Ajba Dam at least 81.3% lower than the flows at reference sampling site SO1 (Table 2). Hence, lower local flow velocities, both v 3cm and v v were evident at the sampling sites downstream of the dams (Table 3). Owing to the low flows, water stagnated downstream of thedams.themeasuredflowvelocitiesv 3cm and v v at sampling points where pools were present were equal to 0 in the sampling sites SO2, SO3 and SO4. The highest value, v 3cm =0.86ms -1, was measured at SO1, where the highest average value (v 3cm =0.44ms -1 ) was also measured. The highest and lowest measured average values for v v were measured at SO1 (v v =0.71ms -1 ) and SO2 (v v =0.16ms -1 ), respectively. The highest local water depth in a vertical of 0.63 m was recorded at site SO1. The average water depth of the USN sampling site in the Ušnica Stream was 0.15 m. 4.2 Effects of dams on river sediments Backwater effects and water abstraction from a watercourse normally cause a significant decrease of high flows in the downstream reach (Ward and Stanford 1995, Erskine et al. 1999, Magilligan and Nislow 2005). This decrease in water flow may also reduce the sediment transport (Ward and Stanford 1995, Graf 2006). However, as shown in Section 4.1, high flows have not been significantly reduced in the bypassed reaches on the Soča River. Restricted sediment transport in the Soča River in the reaches downstream from the diversion dams is due primarily to reservoir sedimentation and restriction of transport past the dam wall (especially in the large reservoir upstream of Podsela Dam) rather than due to a reduction in the number of peak flow events capable of transporting sediment in the bypassed reaches. In gravel-bed rivers that have experienced several years of hydropower operation, a stable riverbed or bed armouring and downstream coarsening has often been reported downstream of the dam (latent erosion) (e.g. Sear 1995, Graf 2006). In the Soča River, the chain of hydropower dams has stopped sediment inflow from the upstream reach. Therefore, normal sediment transport in the river is interrupted, causing high flows with reduced sediment transport and also substantially altering suspended load dynamics. This is why downstream of the Podsela Dam and the Ajba Dam, the river sediments are predominantly cobble size with an Table 3 Measured hydrological and physicochemical parameters at sampling sites on the Soča River and Ušnica Stream during four field surveys in 1998 (February, May, August and November). Site Value Q v 3cm v v h d m d 90 σ T DO OS ph EC DWt OM Chl.-a (m 3 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 Maximum Average SO2 Minimum Maximum Average SO3 Minimum Maximum Average SO4 Minimum Maximum Average USN Minimum Maximum Average Note v 3cm : flow velocity at 3 cm above the river bottom; v v : mean flow velocity in vertical; h: water depth; d m : arithmetic mean particle size, d 90 : 90% of bed load grain; σ: particle sorting coefficient; T: water temperature; DO: dissolved oxygen; OS: oxygen saturation; EC: electrical conductivity; DWt: dry weight; OM: organic matter; Chl.-a: chlorophyll-a.

9 8 Nataša Smolar-Žvanut and Matjaž Mikoš abundance of large boulders. The arithmetic mean grain size, d m, downstream of both dams was much larger than at their upstream cross-section (Table 3), showing downstream coarsening instead of the downstream fining of river sediments normally present under natural conditions (e.g. Mikoš 1994). The particle sorting coefficient of river sediments σ was largely unaffected, having a value close to 5 at all sites (Table 3). The results of our study have shown that the arithmetic mean particle size d m downstream of the Podsela and Ajba dams of more than 100 mm are typical of a very coarse and stable surface layer bed load. They are substantially larger than the arithmetic mean sizes of mm of comparable Slovenian gravel-bed rivers (Mikoš 2000). 4.3 Effects of reduced flow on physical and chemical parameters Below the Podsela and Ajba dams, the oscillations of water temperature, dissolved oxygen and oxygen saturation are much larger in comparison to related sites upstream of the dams, and this can be attributed to the large abstraction of water and significantly reduced flows (Table 2). The higher values of ph were measured below the dams in the Soča River in spring and summer, possibly due to active photosynthesis, when algae use carbon and release oxygen into the water and with this process increase ph. The higher values of electrical conductivity below the dams could be due to the relative importance of inflow from the Ušnica tributary which has a calcareous substratum that dissolves readily and creates the highest conductivity values of all the sampling sites (Table 3). The highest values were in the spring and autumn, when higher flows might increase the leaching of ions from the substrata. Temperature is one of the most important physical parameters of water, because it affects other physical and chemical parameters of running water, and the distribution and ecology of periphytic algae. Abstraction and diversion of flows during summer has the potential to cause increases in water temperature (Biggs 2000). In the summer, when the water temperature in the Soča River at the reference site SO1 was 13.5 C, water temperature in the abstraction section (SO3) was higher by 6.7 C (Table 3). Similar values were reported for the Schächenbach River in Switzerland (Bundi and Eichenberger 1989), where an increase in water temperature of 6 C was found in the abstraction reach. Below the Podsela and Ajba dams, lower concentrations of oxygen were recorded at all times of the year in river pools compared to sampling sites in the same reaches with flowing water. This can be attributed to the absence of turbulence in the pools with only minor aeration present. However, the highest values of oxygen below the Podsela Dam are the reason for the greater photosynthetic activity there, because of algae proliferation. Due to reduced flows, the physicochemical composition of water in the river reach influenced by them is no longer defined by conditions in the upper reaches, but by inflows downstream of the dam (Bundi and Eichenberger 1989). This statement is supported by the results of the present study. 4.4 Periphytic species responses to flow reduction The Soča River and its tributary, the Ušnica Stream, are distinguished by a highly diverse periphyton community. Altogether, 127 taxa were identified in the periphyton samples, of which 126 were found in the Soča River and 32 in the Ušnica Stream. In all samples, the highest number (66) of species was found to belong to Bacillariophyta, as has also been reported by other investigations of algae in Slovenian rivers (e.g. Smolar-Žvanut 2000, Smolar et al. 2005). The results also identified 33 taxa of Chlorophyta; 23 taxa of Cyanophyta and 2 taxa of Rhodophyta, Chrysophyta and Xanthophyta. A detailed list of all the identified taxa is published elsewhere (Smolar- Žvanut 2000). The highest number of taxa belonging to Bacillariophyta was determined at sites SO2 (58 species) and SO3 (52 species). The following species were found at all sampling sites: Achnanthes lanceolata, A. sp., Cymbella affinis, Cymbella ventricosa, Diatoma hiemale v. mesodon, Diatoma vulgare, Gomphonema intricatum, Meridion circulare, Navicula gracilis, Nitzschia fonticola, Nitzschia linearis, Pinnularia interrupta and Synedra ulna. Species Amphipleura pellucida, Fragillaria sp., Gomphonema sp., Gyrosigma attenuatum, Nitzschia angustata, Rhoicosphaenia curvata and Tabellaria flocculosa were found only at site SO2 and species Fragillaria crotonensis, Navicula placentula, Nitzschia sigmoidea, Pinnularia microstauron and Surirella angusta only at sampling site SO3. The most abundant species were Cocconeis pediculus recorded in May and August 1998 at sampling site

10 The impact of flow regulation by hydropower dams 9 SO4 and S. ulna in August 1998 at sampling site SO1. In the Soča River, 32 species of Chlorophyta were observed, mostly at sites SO2 (22 taxa) and SO3 (21 taxa), as compared to only three taxa in the Ušnica Stream. Cladophora glomerata was abundant in May 1998 and August 1998 at sites SO2, SO3 and USN. Species from the genus Closterium and Scenedesmus were rarely found. In November 1998, Ulotrix zonata algae were found in high abundance at sampling sites SO2 and SO3. In the Soča River, 23 species of Cyanophyta were determined, as compared to only five taxa in the Ušnica Stream. At all sampling sites, the genus Chrococcus and Phormidium were observed, as well as the species Phormidium inundatum, which was abundant at all sampling times. The species Chamaesiphon fuscus and Phormidium tenue were only determined at sampling site SO1. At sampling site SO1 in May 1998, species Lyngbya kützingii and Oscillatoria limosa were identified. At all sampling sites in the Soča River and Ušnica Stream, the species Audiouinella chalybea was found, but Bangia artropurpurea was only found in the Soča River sampling sites. Both species belong to Rhodophyta. At all sampling sites in the Soča River, the species Hydrurus foetidus (Chrysophyta) was found, but the genus Characiopsis was only found at site SO2. The H. foetidus species was abundant in winter and autumn at site SO1. Only two genus of Xanthophyta were found, Tribonema sp. found in the samples from sites SO3 and SO4, and algae Tribonema aequale found in February 1998 at site SO4. Number of species SO1 SO2 SO3 SO4 SO1 USN SO2 SO3 SO4 The modified flow regime below a dam usually results in changes to the periphyton communities (Biggs 2000). Suren and Riis (2010) reported that more significant changes in the algal community were associated with longer periods of low flow. Factors which have resulted in a greater number of periphytic algae being determined downstream rather than upstream of the dams include the stability of hydrological factors downstream of the Podsela and Ajba dams (i.e. a prolonged periods of low flow and the occurrence of high flows), the low-to-moderate flow velocities which make the colonization by green algae possible, a substratum of Bacillariophyta and adequate light. Biggs and Smith (2002) reported that the highest taxonomic richness occurred in streams with low-to-intermediate frequencies of flood disturbance (up to 10 bed-moving events per year) and intermediate-to-high concentrations of mats. We always sampled periphyton at least 3 weeks after the last flood. It is known that periphytic algae diversity is significantly limited in watercourses with a frequent increase in flow (Clausen and Biggs 1997). Periphytic algae will be torn from the substratum by fast flows, and, in particular, damage to algae will be caused by the transport of larger sediment grains. The highest number of periphytic algae species were all observed at site SO2 and the lowest number of species were found at reference site SO1 (Fig. 2). The exception was in May 1998, when the highest number was determined at site SO4. In all samples, diatoms represented at least 50% of all species of algae present. Despite the fact that seasonal patterns in periphyton communities in rivers appear to be mostly mediated by river hydrology, seasonality in grazer activity or seasonality in light regime and water temperature (Biggs 1996), the highest number of SO1 USN SO2 SO3 Feb 1998 May 1998 Aug 1998 Nov 1998 SO1 USN SO2 SO3 SO4 BACILLARIOPHYTA CYANOPHYTA CHLOROPHYTA CHRYSOPHYTA XANTHOPHYTA RHODOPHYTA Fig. 2 Composition of periphytic algae at sampling sites in the Soča River and Ušnica Stream.

11 10 Nataša Smolar-Žvanut and Matjaž Mikoš Cyanophyta and Chlorophyta were present in summer samples, when we determined 14 species of Chlorophyta at sampling sites SO2 and SO3 and 11 species of Cyanophyta at site SO3. Higher numbers of green algae downstream from the dams at that time are probably due to the presence of higher water temperatures and more stable low flows. If we compare the number of green algae species present during the year in the Soča River, we can support the results of previous studies that Cyanophyta develop in the summer and filamentous algae in late summer (Biggs 1996). Bergey et al. (2010) reported that filamentous green algae form dense growths in regulated rivers, and this is also supported by the results from our study. The lowest number of species was always determined in the Ušnica Stream, because it is a much smaller water course than the Soča River and has uniform habitats with very low water velocities. According to the literature, the Chrysophyta species H. foetidus only proliferates in the winter at low water temperatures and during constant flow conditions (Ward 1974, Valentin et al. 1995, Smolar 1997). Traaen and Lindstrom (1983) reported that 90% of the H. foetidus algae occurred at velocities of over 0.8 m s -1 (measured 1 cm above the bottom). The results from this study in the Soča River confirm these findings. The species only proliferated at the reference site upstream from the dams and only in winter time. Downstream from the dams, the species was rarely present, probably due to the occurrence of lower velocities in these reaches. The Bray-Curtis coefficient of similarity shows qualitative changes in the structure of a periphytic algae community and provides an indication of similarities and differences amongst communities. The highest similarities were observed among samples collected at sites which are affected by flow regulation and have similar hydrological and physicochemical variables, whilst the community of periphytic algae at reference site SO1 was at most times of the year significantly different to the communities downstream of both dams (Fig. 3). The number of taxa of periphytic algae was negatively correlated with flow velocity at 3 cm above the river bottom (v 3cm ) in the Soča River (Fig. 4). This can be explained by the findings of Peterson and Stevenson (1989), who reported that at high flow velocity the algae colonization is limited by shear stress. In the Soča River, downstream from the dams, more algae species common to polluted waters were identified, such as Stigeoclonium tenue, N. angustata, SO3N98 SO4N98 SO2N98 SO4F98 SO2F98 SO3F98 SO2M98 SO3M98 SO2A98 SO3A98 SO1A98 USNM98 USNA98 SO1M98 SO4M98 SO1F98 SO1N98 USNN98 Fig. 3 Comparison of the Bray-Curtis coefficient-of-similarity indices for the Soča River (SO1 SO4) and Ušnica Stream (USN) sampling sites for all sample times (M98: May 1998; N98: November 1998; A98: August 1998; F98: February 1998). Number of taxa v 3cm (m s -1 ) Fig. 4 Correlation between the flow velocity at 3 cm above the river bottom (v 3cm ) and the number of taxa of periphytic algae found in the Soča River and Ušnica Stream (N = 74, r = 0.43, p < 0.001). Cyclotela meneghiniana, O. limosa and Oscillatoria tenuis. This is probably due to the presence of lower flow velocities at these sites, which results in the altered physicochemical conditions of the water. 4.5 The impact of flow regulation on periphyton biomass Flow regulation associated with reduced flows and flow variability, and therefore increased bed stability can increase biomass (Biggs 2000). During low flows, periphyton biomass accrual processes dominate,

12 The impact of flow regulation by hydropower dams 11 because loss processes associated with high velocities and shear stress, substrate movement and abrasion do not occur (Biggs 2000, Suren and Riis 2010). Low periphyton biomass in regulated rivers can be maintained by providing sequential floods (Mannes et al. 2008). In the Soča River and the Ušnica Stream, the values of DW and the organic matter of the periphyton were highest at sampling sites with lower flow velocities. The Pearson correlation coefficients were significant at the p < 0.05 level and were 0.30 and 0.31 for DW and flow velocities v 3cm and v v, respectively, and 0.28 for organic matter of the periphyton and these two flow velocities (Table 4). At times of constant discharge, periphyton usually attains a high biomass on large pebbles and stones, particularly due to limited sediment transport at the bottom of a watercourse (Biggs 1996). High biomass of periphyton can develop only after an extended period of habitat stability (Biggs 1996, Biggs et al. 2001, Suren and Riis 2010). Low flows, favourable physicochemical conditions and appropriate sediment size were factors that made the proliferation of algae possible in the Soča River below both dams. The reduced flow in the Soča River, downstream of the Podsela and Ajba dams, resulted in hydrological and physicochemical changes in the river, which in turn led to an increased periphyton biomass (Table 3). Biological changes were most striking in the overgrowth of the river bottom, with green algae at this location in the Soča River, which can potentially alter habitat quality and cause degradation of the ecosystem structure (Suren et al. 2003). The high periphyton biomass downstream from dams, 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). This may be primarily attributed to minor fluctuations in the water temperature, the alteration of the flow regime to one without distinct seasonal fluctuations, an increase in the concentration of nutrients and their absorption by algae (Koudelkova 1999) and to the presence of large, immobile sediments (Biggs et al. 2001, Suren and Riis 2010). In addition to flow velocity, at locations with low flow velocities in unshaded watercourses, a high periphyton biomass also depends on the water chemistry, which in this study was found to be of lower quality downstream of the Podsela and Ajba dams. Figure 5 illustrates the negative correlation between the flow velocity at 3 cm above the river bottom, v 3cm, and the chlorophyll-a in the Soča River and Ušnica Stream Table 4 Pearson correlation coefficients for biological, hydrological and physicochemical parameters of the samples from the Soča River and the Ušnica Stream. Parameter DWt OM Chl.-a N v 3cm v v H v * τ T DO OS Chlorophyll-a (mg m -2 ) v 3cm (m s -1 ) Fig. 5 Correlation between the flow velocity at 3 cm above the river bottom (v 3cm ) and the chlorophyll-a found in the Soča River and Ušnica Stream (N = 74, r = 0.34, p < 0.01). DWt 1 OM Chl.-a N v 3cm v v H v * τ T DO OS Notes DW: dry weight; OM: organic matter; Chl.-a: chlorophyll-a; N: number of taxa; v 3cm : flow velocity 3 cm above the river bottom; v v : mean flow velocity 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.

13 12 Nataša Smolar-Žvanut and Matjaž Mikoš No species of green algae proliferated at sampling site SO1 upstream from the dams, which is characterized by a mobile substratum and high current velocities. This study highlights evidence for the impact of reduced flows due to hydropower scheme operation on the proliferation of green algae in the Soča River, a factor not found by studies on other Slovenian rivers with water diversions (Smolar- Žvanut 2000). 5 CONCLUSIONS A field analysis of the impacts of flow regulation downstream of the Podsela and Ajba dams in the Soča River has shown changes in hydrological, physical and chemical variables that affect the structure of the periphyton community. In the Soča River, high flows occur frequently and, due to the limitations in operation of the HPPs at these flows, and the fact that the dams spill during peak flows, floods in the regulated reaches have occurred at the same time and frequency as in the upstream, unregulated river reach. Yet, significant differences occur in the periphyton communities of the regulated and unregulated reaches. We can conclude that floods are not the most important factor for maintaining the periphyton community in the Soča River, but the combination of hydraulic habitat variables, sediment size and lack of sediment mobility and physicochemical variables may explain why periphyton biomass and number of periphytic algae were increased downstream of the dams. The results of our study have shown that, to understand periphyton dynamics in lotic ecosystems, especially those impacted by flow regulation, a detailed understanding of the nature and timing of hydrological alteration is needed. During periods of low constant flows downstream of both dams, the periphyton is producing high biomass on large cobbles and boulders, especially due to the stable hydraulic habitat conditions (with low velocity), low mobility of the bedload and, consequently, abrasion of the sediment grains surface. Therefore, it is not sufficient to design and implement an environmental flow strategy that restores flow variability downstream of the dams. It is also important to establish some degree of continuous sediment transport through the chain of reservoirs. If this cannot be achieved, artificial feeding of sediments may be required for reaches downstream of the dams. In accordance with the requirements of European Union Water Framework Directive (EU WFD 2000), further research is necessary to examine the relationship between the periphyton community and the nutrients downstream of the dams in the Soča River. This is needed to define environmental flows and sediment transport requirements necessary to maintain a healthy periphyton community, typical for this alpine river with a torrential character. Acknowledgements The authors would like to thank Danijel Vrhovšek for his contribution to the draft version of this article. The authors would also like to thank Dušan Rebolj, Peter Muck, Darko Burja and Darko Anzeljc, all from the Institute for Waters of the Republic of Slovenia in Ljubljana, for helping with field work and hydrologic analyses. The Soča Electricity Board from Nova Gorica kindly helped with relevant technical and hydrological data on the HPPs under investigation. The article was greatly improved by insightful comments of two anonymous reviewers and Ian Maddock s comments and recommendations. Hydrological data were obtained from the archives of the Slovenian Environment Agency. REFERENCES Anastasi, G., Geschiebeanalysen im Felde unter Berücksichtigung von Grobkomponenten. Mitteilungen der VAW ETH Zürich, 70, (in German). APHA (American Public Health Association), SI: standard methods for the examination of water and wastewater. 18th ed. Washington, DC: American Public Health Association. 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