Science of the Total Environment 407 (2009) 5694–5700

Contents lists available at ScienceDirect

Science of the Total Environment

j ourna l homepage: www.e lsev ie r.com/ locate /sc i totenv

Effects of sediment deposition on periphytic biomass, photosynthetic activity andalgal community structure

Oihana Izagirre a,⁎, Alexandra Serra b, Helena Guasch b, Arturo Elosegi a

a Department of Plant Biology and Ecology, Faculty of Science and Technology, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spainb Department of Environmental Sciences, Institute of Aquatic Ecology, Faculty of Sciences, University of Girona, Campus Montilivi, 17071 Girona, Spain

⁎ Corresponding author. Tel.: +34 946015514; fax: +E-mail address: gvbizigo@lg.ehu.es (O. Izagirre).

0048-9697/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.scitotenv.2009.06.049

a b s t r a c t

a r t i c l e i n f o

Article history:Received 12 May 2008Received in revised form 7 January 2009Accepted 10 June 2009Available online 9 August 2009

Keywords:ClaySedimentPeriphytonPhotosynthetic activityAlgae community

Suspended solids and siltation are among the most prevalent problems in streams and rivers of the world;however, because they are often associated with other stresses such as increased nutrient concentrations orchanges in channel form, their impacts on the biota and on ecosystem functioning are not fully understood.To assess the effects of pulse sediment deposition on periphyton, we applied an exponential gradient of clayconcentration (from 0 to 54.7 gL−1) for three days to eleven artificial indoor channels precolonized by algae(three controls+eight treatments). This resulted in a gradient of inorganic particulate matter in the bottomfrom two to over 200 gm−2. Periphytic biomass, photosynthetic activity and algal communities were studiedduring the following four weeks. High sediment loads (N6 gL−1) initially reduced algal growth but by theend of the experiment periphytic biomass was similar in all channels. Under high sediment load, algalphotosynthetic efficiency showed a quick decrease after three days of exposure, followed by a delayedincrease in chlorophyll a contents. After two weeks signs of adaptation were observed, first as an increase inphotosynthetic efficiency and then as an increase in pigment concentration. Siltation led to changes incommunity structure; diatoms increased in high silt treatments although green algae still dominated.Overall, the accumulation of fine sediment affected periphytic biomass, photosynthetic activity andcommunity composition. Periphyton adaptation reduced the initial impact, reaching almost full compensa-tion in terms of chlorophyll a and photosynthetic activity; however, algal community composition did notrecover within the time frame of this study. Thus, the frequent siltation pulses observed in many streamsthroughout the world may have an important impact on the periphyton, which would in turn affect streamecosystem structure and functioning.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Soil erosion is an important issue in many parts of the worldbecause about 75 billion tons of soil is eroded per year, mostly due tohuman activities such as agriculture or forest clearing (Pimentel andKounang, 1998). A large fraction of these materials are transported byrivers (Lal, 2003), and thus, suspended solids are rated among themost prevalent types of freshwater pollution (USEPA, 2000). Modelsof soil use in relation to scenarios of global warming and increasedhuman population suggest that erosion is likely to increase sharply inthe future (Yang et al. 2003).

Siltation (fine sediment deposition) occurs when tractive forcesexerted upon the transported grains are lower than the settlingvelocity (Richards, 1982). It is affected by factors such as particleshape, water temperature, flocculation and turbulence (Carling and

34 946013500.

ll rights reserved.

Reader, 1982; Norwell and Jumars, 1984). In streams, sedimenttransport is highest during storms, and deposition typically occursin pulses in the receding phase of floods. Sedimentation pulses alsooccur in the absence of floods, such as when heavy machinery (forinstance, during forest operations) crosses the channel. Whatever themode of input, deposited sediment can remain for a long time duringbaseflow periods, thus affecting the biota long afterwards. Therefore,siltation is typically a pulse disturbance (as opposed to a press orramp, Downes et al., 2002).

Fine sediments, deposited as well as suspended, alter the physicaland chemical characteristics of rivers, in addition to their bioticstructure and ecosystem functioning (Crowe and Hay, 2004). Someauthors have studied the effects of soil use and catchment geology ondeposition, turbidity and particle size in rivers (e.g., Owens andWalling, 2002; Boer et al., 2005), whereas others have assessed theimpacts of solids on the biota. Sediments impact both invertebratesand fish directly because of abrasion (NewCombe and MacDonald,1991) and indirectly by affecting their habitat and food (Ryan, 1991;Wood and Armitage, 1997), or by reducing the connectivity betweenepibenthos and hyporheos (Waters, 1995).

5695O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

Siltation can also have profound impacts on primary producers,and thus can be especially detrimental in areas where primary pro-duction is the main basis of food webs (Ryan, 1991).

The effects of sediments on periphyton are complex, as they includelight attenuation (Waters, 1995; Wood and Armitage, 1997), abrasion,reduction of hard substrata available for colonists (Biggs, 1995), anddecreased hydraulic connectivity with the hyporheos. Studies havefocused on both the effect of suspended (Davies-Colley et al., 1992;Parkhill and Gulliver, 2002) and deposited sediments (Graham, 1990;Yamada and Nakamura, 2002). Decreased light availability can reducephotosynthetic activity (Van Nieuwenhuyse and La Perriere, 1986;Davies-Colley et al., 1992) and affect algal community composition(NewCombe and MacDonald, 1991). However a reduced biomass ofprimary producers does not necessarily imply a reduction in whole-stream primary production because the photosynthetic efficiency ofalgae can quickly change (Parkhill and Gulliver, 2002). Similarly,Burkholder (1992) described the physiological adaptations of phyto-plankton to turbidity in reservoirs where phosphate retention by algaeincreased when clay was added.

Most rivers in developed areas are affected simultaneously bymany stresses, from pollutants to water withdrawal and changes inchannel form. This makes it difficult to discern the effects of eachimpact (Downes et al., 2002). Artificial channels are one of the bestways to assess the impacts of single stressors, because their designmakes it possible to isolate particular effects from complex influences(Shriner and Gregory, 1984). Our objective here is to assess the effectsof pulse deposition of fine sediments on periphyton biomass,physiology and community structure. We hypothesized that sedi-ments will decrease the biomass and photosynthetic activity ofperiphyton in the short term (i.e., days), but that in the long term(weeks) algae will adapt (either by changes in physiology or shifts incommunity composition) to the new conditions, and thus compensateat least in part for the impact of siltation.

2. Methods

2.1. Experimental setting

The experiments were carried out in eleven indoor channels at theFaculty of Sciences, University of Girona. Each channel,made of Perspex,was 170 cm long and 9 cmwide and had a closed circuit inwhich 10 L ofwater were recirculated at a rate of 0.068 Ls−1 by centrifuge pumps,from a carboy through the channel and again into the carboy. Light wasprovided by halogen lamps (80–100 µmol m−2s−1) with a 12/12 light/dark cycle. The temperature was kept at 20 °C with a refrigerated bath.Tap water, filtered through active carbon to keep chlorine values wellbelow toxic limits, was used for the experiments and renewed everythree days. Because tap water at the University of Girona has highconcentrations of nitrate, 0.18 mg P-PO4L−1 was added as a source ofphosphorus and to avoid nutrient limitation.

The channel bottoms were covered with etched glass substrata(2×8.5 cm), previously incubated for one month in an aquariumwithrecirculating stream water and an aliquot of periphyton from theLlémena, a small tributary of the Ter River (north–east Spain). Waterin the aquarium was replaced every week, and light and temperaturewere kept as in the channels. After a month of colonization, glasssubstrata were introduced into the channels and conditions were keptconstant for one week so that periphyton could acclimatize.

After the acclimatisation period, three of the eleven channels wereleftwithout clayas controls and the other eight channelswere subjectedto a pulse of clay. Because adding clay caused an increase in dissolvedsalts (detected as an increase in water conductivity), two controlsreceived just tap water (C1 and C2) while the other one received waterfiltered froma clay–watermixture (Cf). Thiswasused to check the effectof conductivity on the periphyton. Clay (Perlisur® VS, expandedvermiculite, ground on a DFH48LL grinder with a 0.5-mm sieve) was

added to the eight treatment (T) carboys in an exponential gradient ofconcentration (0.03 (T1), 0.08 (T2), 0.23 (T3), 0.68 (T4), 2.03 (T5), 6.08(T6), 18.23 (T7) and 54.68 (T8) g L−1), and kept for three days, stirringconstantly to avoid clay decantation inside the carboys. After this pulseof siltation, water was renewed every three days by adding phosphatebut no additional silt, so clay remaining in suspension after three dayswas eliminated.Water in the control Cfwasfiltered from the clay–watermixture. The experiment lasted for 30 days, during which the glasssubstrata on the stream bottom remained silted.

2.2. Water quality

We measured conductivity (WTW Cond 340i), temperature,dissolved oxygen concentration and saturation (WTW Oxi 340i),and pH (WTW pH 340i) in all carboys every three days, before andafter water renewal. In addition, on days 0, 6 and 30 of the experiment,before and after water renewal, water samples were taken from onecontrol (C1), one channel with intermediate clay addition (T6) andone with high clay addition (T8). Samples were filtered immediatelythrough 0.2 µm pore nylon filters (Whatman GD/X) for analysis ofanions (ion chromatography, Metrohm Ltd., Herisau Switzerland) andphosphate (Murphy and Riley, 1962).

2.3. Periphyton

On days 0, 1, 3, 5, 7, 14 and 21 of the experiment, three colonizedglass substrata were randomly sampled from each channel. Basalfluorescence (Fo) and effective quantum yield of photosyntheticenergy conversion in PSII (Y) were measured by pulse amplitudemodulation fluorometry (Phyto-PAM, Heinz Walz GmbH, Effeltrich,Germany). Phyto-PAM is a non-intrusive method (Schreiber et al.2002) that measures fluorescence at four wavelength signals (470 nm(Fo1), 520 nm (Fo2), 645 nm (Fo3) and 665 nm (Fo4)) and thereforeshows the contribution of various types of pigments. Samples werekept in the dark for 10min prior to themeasurement of Fo. Light pulsesof 0.15 µmol m−2s−1 were used to measure basal fluorescence, sotheir actinic effect could be neglected, as electrons do not accumulateat the acceptor side of PSII. Y, a measure of photosynthetic efficiency ofthe community, was calculated as (Fm−F)/F, where Fm is themaximalfluorescence yield measured by applying a saturating pulse, and F isfluorescence measured shortly before the saturating pulse at120 µmol m−2 s−1 of PAR. Two of the three samples used forfluorescence were scraped, filtered (Whatman GF/F) and frozen tomeasure chlorophyll a and clay content. Chl a was analyzed using aspectrophotometer after extraction in hot ethanol (Sartory andGrobbelaar, 1984). Clay content was determined by ashing filters at500 °C for 4 h. At days 0, 14 and 30 two additional glass substrataweresampled per stream to study periphytic communities. Algae wereidentified under an optical microscope (400 and 1000×), to genuslevel and grouped by morphology (non-filamentous green algae,filamentous green algae, unicellular diatoms, unicellular cyanobac-teria, filamentous cyanobacteria), and at least 300 cells were counted.

In addition, on days 0, 14 and 21, three periphyton samples werecollected per channel to investigate the photosynthesis/irradiancerelationships. The phyto-PAM automatically increased light intensityfrom 0 to 295 µmol m−2s−1 in 12 steps, followed by a decline inintensity in seven steps. F and Y of the sample were measured in eachstep. Photosynthetic parameters were calculated from these curves:the initial slope of the curve (α); themaximum electron transport rate(ETR max) and the irradiance of half-saturation (IK).

2.4. Data analysis

To detect differences in water quality between treatments, one-way ANOVA and Fisher's Post-Hoc tests were performed three times(Statview 5.0.1.): One ANOVA compared the “initial” characteristics of

Fig.1. Amount of clay deposited for each treatment, measured as ash content (g m−2) ofperiphyton, as a function of initial clay concentration in water during the pulse ofsiltation. Mean values and standard errors are shown.

5696 O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

water (i.e., water recently changed), another the “final” characteristicsof water (water that had recirculated for 3 days), and the third the“average” water characteristics (average of “initial” and “final”).

As fluorescence is a non-destructive method and quicker to measurethan chlorophyll concentration, the relationshipbetweenfluorescenceat665 nm (Fo4) and chl awas calculated from linear regression with datafrom 67 samples. The regression model was significant (Chl a (Fo4)=0.021⁎Fo4+1.1706, r2=0.7873, pb0.0001) so fluorescence was used(instead of spectrophotometry data) to estimate chlorophyll content. Toensure that sediment content did not influence fluorescence measure-ments, channels were grouped into four groups: control (0 gL−1), lowsiltation (0.03–0.23 gL−1), medium siltation (0.68–6.08 gL−1) and highsiltation (18.23–54.68 gL−1). For each of the groups a linear regressionbetween Fo4 and Chl a was analyzed. All groups were comparedstatistically by ANCOVA (Zar, 1999). Results showed that there were nosignificant differences (p=0.3581) and that sediments did not affectfluorescence and consequently algal biomass estimates.

For each channel, the growth of algaewas characterized by plottingChl a (Fo4) against time, and fitting it to a sigmoid growth curve withthree parameters: time to highest growth rate X0, saturation value aand initial slope 1/b.

The signal obtained at different wavelengths (Fo1 and Y1 at470 nm; Fo2 and Y2 at 520 nm; Fo3 and Y3 at 645 and Fo4 and Y4 at

Table 1Water conductivity (µScm−1), oxygen concentration (mg L−1), temperature (°C) and pH infinal are mean values of water prior to next change.

Treatment Conductivity (µScm−1) Oxygen (mg L−1)

Initial Final Average S.D. Initial Final Average

C1 480.1 409.5a 444.8 48.3 9.29 9.40 9.29C2 477.4 374.4a 425.9 63.6 10.00 10.03 10.01Cf 486.4 406.4a 446.1 50.4 9.41 9.91 9.65T1 (0.03 g/L) 480.7 387.8a 435.2 60.6 9.48 9.57 9.52T2 (0.08 g/L) 478.4 384.7a 431.4 60.9 9.68 9.72 9.70T3 (0.23 g/L) 477.7 374.8a 425.3 63.9 9.75 9.89 9.81T4 (0.68 g/L) 478.0 364.2a 420.6 73.5 9.79 10.00 9.89T5 (2.03 g/L) 478.4 379.3a 429.9 61.8 9.81 9.97 9.89T6 (6.08 g/L) 480.8 400.6a 443.0 52.6 9.76 9.86 9.81T7 (18.23 g/L) 484.1 432.4b 460.5 38.1 9.56 9.67 9.61T8 (54.68 g/L) 487.4 461.4b 476.3 33.1 9.51 9.56 9.53ANOVA n.s. ⁎ n.s. n.s. n.s. n.s.

ANOVA results, ⁎ means pb0.05 and ⁎⁎pb0.0001, n.s. means non significant. T means treaSuperscripts a, b, c show post-hoc grouping results.

665 nm) depends on pigment composition and can be used to detectchanges in the algal community structure. In this study, the ratio Fo1/Fo3 was used as an indication of dominance of green algae (highvalues) vs. dominance of cyanobacteria (low values).

For each sampling date, the measured parameters Chl a (Fo4), Y, α,ETRmax, IK and Fo1/Fo3 were plotted against deposited clay content. Datawere adjusted to a dose–response curve and used to estimate the claycontent which resulted in a 50% reduction of the measured parameter(EC50) (SigmaPlot). The effect of sediment deposition on algal commu-nities was analyzed using a simple regression between percentageabundance of algal groups and deposited clay (Statview 5.0.1.).

3. Results

3.1. Experimental conditions

The proportion of clay deposited on the substrata at the end of the3-day pulse depended on the treatment. In channels subject toconcentrations ≤0.23 gL−1 (6T1–T3) all clay settled, whereas in therest of the channels the final proportion of clay deposited ranged from66% (T4) to 6% of the initial clay (T8). As a result, clay deposition at theend of the experiment (measured from the ash content of periphyton)showed an exponential gradient, rising from 4.94 to 241.85 gm−2

with the clay concentration during the siltation pulse (Fig. 1).The physico-chemical characteristics of the water were monitored

at the onset (initial) and at the end (final) of each three day period,i.e., just after refilling with new water, and just prior to the nextchange. There were no significant differences in the initial conditionsof temperature, oxygen, conductivity and pH between treatments(Table 1, ANOVA, pN0.05). After three days of water recirculationconductivity decreased; however, it decreased less with high claydoses. Therefore, there were significant differences at the end of thethree day periods (ANOVA, pb0.05), and post-hoc analyses showedthat conductivity was significantly higher at treatments T7 and T8.Oxygen concentrations showed slight increases in all channels, but nosignificant differences between treatments were observed (ANOVA,pN0.05). Temperature also showed no significant variations betweentreatments (ANOVA, pN0.05), and almost no temporal variations.However, pH increased in all channels, and resulted in significantdifferences between treatments, as it was lower at high clay doses(ANOVA, pb0.05); post-hoc analyses showed that pH was signifi-cantly lower at treatments T6, T7 and T8. Nevertheless, differenceswere generally slight, and average conductivity, oxygen, temperatureand pH conditions (calculated as the average of all measurementstaken before and after water renewal) were similar betweentreatments (ANOVA, pN0.05). The three control channels (two

experimental canals. Initial are mean values of water recently changed (every 3 days),

Temperature (°C) pH

S.D. Initial Final Average S.D. Initial Final Average S.D.

1.05 22.5 21.0 21.7 1.75 7.93 8.93b 8.45 0.551.26 20.5 20.8 20.7 1.60 8.07 9.37b 8.73 0.701.17 21.3 20.6 20.9 1.64 8.04 9.10b 8.58 0.571.13 21.3 20.9 21.1 1.46 7.96 9.24bc 8.59 0.701.16 20.9 20.8 20.9 1.52 7.98 9.27bc 8.64 0.711.18 20.6 20.8 20.7 1.59 8.01 9.35c 8.70 0.731.25 20.6 20.8 20.7 1.60 8.05 9.31b 8.69 0.681.20 20.5 20.7 20.6 1.67 8.06 9.14b 8.61 0.581.25 20.5 20.8 20.7 1.58 8.05 8.83a 8.45 0.461.15 20.6 21.0 20.8 1.66 8.06 8.61a 8.35 0.391.15 20.8 20.8 20.8 1.84 8.07 8.59a 8.33 0.45

n.s. n.s. n.s. n.s. ⁎⁎ n.s.

tment, C means control, Cf is the control with water filtered from clay water mixture.

Table 2Mean and standard deviation of concentrations of PO4, NO2, NO3, SO4 and Cl at three of the channels.

Treatment PO4 (ppm) NO2 (ppm) NO3 (ppm) SO4 (ppm) Cl (ppm)

Initial Final Average S.D. Initial Final Average S.D. Initial Final Average S.D. Initial Final Average S.D. Initial Final Average S.D.

C1 0.17 0.011 0.09 2.85 bdl 0.13 0.07 0.17 13.35 2.27 7.81 6.75 35.13 38.89 37.01 10.98 29.84 35.90 32.87 8.43T6 ( 6.08 g/L) 0.19 0.004 0.10 3.37 bdl 0.18 0.09 0.20 14.25 6.88 10.57 6.07 37.74 41.64 39.69 13.31 32.20 36.77 34.49 8.77T8 (54.68 g/L) 0.12 0.002 0.06 2.09 bdl 0.36 0.18 0.34 15.70 12.03 13.86 6.16 31.15 51.35 41.25 14.48 31.61 43.35 37.48 9.00ANOVA ⁎ ⁎ n.s. n.s. n.s. n.s. n.s. ⁎ ⁎ n.s. n.s. n.s. n.s. n.s. n.s.

Initial are mean values of water recently changed (every 3 days), final are mean values of water prior to next change. Bdl=below detection limit.To calculate average values, bdl were considered as 0.00. ANOVA results, ⁎ means pb0.05 and ⁎⁎pb0.0001, n.s. means non significant.

5697O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

unsilted (C1 and C2) and one receiving the clay–water mixture (Cf))showed no significant differences in average conductivity, oxygenconcentration and temperature (ANOVA, pN0.05), but pH differedsignificantly between them (ANOVA, pb0.05). Post-hoc analysesshowed all of them to differ significantly from each other, the highestpH being at C2, followed by Cf and C1 (Table 1).

Nitrite, chloride and sulphate concentrations increased after threedays of recirculation in all treatments (Table 2), whereas nitrate andphosphate concentrations decreased. The decrease in nitrate wasgreatest in the least silted channels, and the opposite trend wasobserved for phosphate, which showed a marked decrease in the highsilt treatments.

3.2. Effects of clay on the periphyton

Algal biomass (Chlorophyll a calculated from Fo4) increased duringthe experiment at all channels, controls and treatments (Fig. 2). Ingeneral, temporal changes followed a sigmoidal curve. We thereforeestimated three parameters for each channel: the time to reachhighest growth rate X0, the saturation value a, and the initial slope 1/b(Table 3). The increase in algal biomass over time was slower underhigh clay doses (T6–T8) and a clear pattern appeared when plottingX0 vs clay concentration, with the shortest time to highest growth rateat medium concentrations (T2–T4) (Fig. 3). The initial slope of thegrowth curve (1/b) was higher at medium clay doses. The saturation

Fig. 2. Chl a (Fo4) sigmoid growth curves for all treatments.

value (a) was similar in all the channels except in T6 and T8, wherealgal growth curves did not reach total saturation.

Differences of algal biomass between channels changed with time.After three days of experiment, algal biomass did not show any trendrelated to clay deposition. After five days of experiment the high claytreatments had lower algal biomass values than either low- or no-claytreatments (Table 4). After 14 days, treatment effects were clear,showing no effect of clay under 35 gm−2 and a 50% reduction of algalbiomass at 59.03gm−2.

Clay addition also affected photosynthetic activity. Initially (days 3and 5) photosynthetic efficiency (Y) decreased in treatments with thehighest clay doses (Table 4). Afterwards photosynthetic efficiency athigh clay doses increased. By day 14 values were 10–20% higher in themost silted channels than in the controls (Table 4).

The effects of clay addition on the P–I relationships were slight.The initial slope (α) showed a slight increase with clay deposition atday 21. ETRmax and IK showed no clear trend (Table 5).

The Fo1/Fo3 ratio, indicating the proportion of cyanobacteria vsgreen algae, was clearly affected by the addition of clay (Table 4),decreasing at high doses after day 5. Amarked reductionwas observedat days 5, 7, 14 and 21. The clearest pattern was observed at day 14. Aninhibition curve fitted to the data showed the clay deposition causinga 50% reduction in the ratio to be 38.12 gm−2 (pb0.0001). Cellcounting under an optical microscope showed that the algalcommunity was initially dominated by unicellular and colonialgreen algae (Ankistrodesmus sp., Scenedesmus sp., Cosmarium sp., Pe-diastrum sp.), plus some filamentous greens (Oedogonium sp.), smalldiatoms and unicellular cyanobacteria (Fig. 4). After 14 days, theabundance of filamentous greens increased in most channels. In themost silted channels (treatments T6 and T7) the diatom percentagewas high (31.06%, 36.31%), and in the treatment of 54.68 gL−1

cyanobacteria were especially abundant (23.36%). After 30 days greenalgae dominated in the controls and low dose treatments, whereas thehighest dose treatment (T8) was dominated by diatoms (77.9%).Linear regression showed a significant positive relationship between

Table 3Results of sigmoid growth curves to model temporal changes in periphyton basalfluorescence Fo. Growth parameters Xo, a and 1/b refer to time to reach highest growthrate, saturation value, and the initial slope, respectively.

Treatment X0 a 1/b r2 p

C1 6.72 42.87 0.19 0.785 0.021C2 7.61 44.30 0.21 0.914 0.003Cf 9.75 45.82 0.28 0.755 0.027T1 ( 0.03 g/L) 5.53 42.21 0.18 0.724 0.034T2 ( 0.08 g/L) 8.86 40.47 0.44 0.922 0.003T3 ( 0.23 g/L) 3.85 42.44 0.21 0.780 0.021T4 ( 0.68 g/L) 1.53 36.30 0.76 0.796 0.018T5 ( 2.03 g/L) 2.79 39.31 0.48 0.682 0.045T6 ( 6.08 g/L) 29.20 – 0.12 0.911 0.003T7 (18.23 g/L) 11.11 35.44 0.31 0.848 0.010T8 (54.68 g/L) 42.97 – 0.14 0.765 0.025

T means treatment, C means control, Cf is the control with water filtered from claywater mixture.

Fig. 3. Time to reach highest growth rate (X0, days) of Chl a (Fo4) vs. the deposited clayin experimental canals. Arrows show how response time decreased with low claysiltation and increased at higher doses. Labels show treatment references (C1, C2, Cf,T1…).

5698 O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

the percentage of unicellular diatoms and deposited clay (r2=0.77;p=0.0002).

4. Discussion

4.1. Experimental constraints

Our experimental design was aimed at assessing the short- andlong term effects of a pulse of sediment deposition on periphyticbiomass, photosynthetic efficiency and community structure. Itmimicked both the pulsed dynamics and the amounts of silt oftenfound in rivers. Our highest deposition values resulted in a sedimentthickness of ∼1 cm, which is well below the 10 cm recorded byBrookes (1986) in canalized UK streams, and thus, the impactsdetected could be considered as conservative.

Table 4Periphytic biomass Chl a (Fo4), photosynthetic efficiency (Y) and Fo1/Fo3 ratio (indication ofand days.

Treatment /day

C1 C2 Cf T1(0.03 g/L)

T2(0.08 g/L)

T3(0.23 g/L)

T4(0.6

Chl a (Fo4) 0 5.1 3.6 3.1 4.8 3.2 9.8 51 7.8 10.2 5.0 17.5 5.9 16.7 183 24.5 17.3 17.6 14.4 5.2 26.3 235 14.1 16.3 8.8 28.6 7.9 16.3 397 21.8 16.6 4.5 19.3 8.4 29.8 30

14 33.5 37.2 41.0 32.9 41.0 38.4 4121 41.0 41.0 41.0 41.0 37.0 41.0 34

Y 0 0.33 0.33 0.31 0.35 0.28 0.33 01 0.26 0.39 0.32 0.36 0.38 0.38 03 0.30 0.33 0.30 0.35 0.35 0.34 05 0.31 0.33 0.33 0.33 0.35 0.31 07 0.16 0.23 0.23 0.26 0.27 0.27 0

14 0.28 0.29 0.27 0.34 0.29 0.31 021 0.20 0.25 0.28 0.29 0.28 0.27 0

Fo1/Fo3 0 94.4 108.9 96.7 93.4 107.0 119.7 1211 84.6 109.0 106.4 98.3 109.7 74.6 993 100.1 100.4 99.5 91.4 77.6 110.6 995 91.0 109.0 100.0 120.1 89.6 110.2 1347 108.4 100.0 91.7 71.4 101.5 133.4 130

14 91.0 104.3 104.7 93.0 102.8 119.1 10421 99.98 100.01 100.01 100.01 82.20 100.01 92

T means treatment, C means control, Cf is the control with water filtered from clay water mixvalue), only the statistically significant ones are shown.

It is often difficult to discern the effects of sediments from those ofother stressors associated with them, such as changes in hydrology,nutrient dynamics or in aquatic habitats (Martin et al., 2000; Swanket al.,2001).We used artificial streams to keep conditions other than sedimentloads constant, but sediments influenced phosphorus dynamics in ourchannels, as has been described elsewhere (Newbold,1992). Thus, one ofthe effects of sediment deposition was immobilization of phosphate,which resulted in decreased SRP concentration in the high sedimenttreatments. This contrasted with the dynamics of nitrate, which is lessreactive to sediments: nitrate decline was presumably due to biologicaluptake, as it decreased in the low siltation, high chlorophyll treatments.Clayalso affected conductivityandpH. In theunsilted canals pH increasedmore and conductivity decreased more than in the silted ones. Probablyboth trends occurred because of higher photosynthetic activity, which isknown to raise pH and induce precipitation (Wetzel, 2001). However,frequent renewal of water ensured that whatever the level of sedimentdeposition, nutrients were available to the periphyton during the wholeexperiment, and that there were no significant differences in waterchemistry between treatments. The addition of clay contributed salts tothe water and potentially might have affected the periphyton, but thecontrol with water filtered from thewater–clay mixture (Cf), showed noeffect of increased conductivity on the periphyton. Therefore, resultsshould be interpreted as a direct effect of sediments on the periphyton.

During the experiment, algal biomass was estimated indirectly bymeasuring fluorescence using Phyto-PAM. To test whether sedimenton the algae interfered with fluorescence measurements, weperformed independent calibrations between the fluorescence signaland chlorophyll concentration of the periphyton differing in their siltcontent. All calibrations gave a similar result showing that the amountof sediment present in this experiment was not interfering with thefluorescence signal.

4.2. Effects of sediment deposition on periphyton

Sediment deposition affects primary producers in several ways: itreduces the availability of stable attachment surfaces (Wood andArmitage, 1997), smothers them directly (Brookes, 1986) and reduceslight availability (Davies-Colley et al., 1992). As a result, a reduction inboth algal biomass (Davies-Colley et al., 1992; Yamada and Nakamura,

dominance of green algae vs. dominance of cyanobacteria) values for all the treatments

8 g/L)T5(2.03 g/L)

T6(6.08 g/L)

T7(18.23 g/L)

T8(54.68 g/L)

p-value EC50 pEC50

.5 3.4 4.2 8.5 3.1

.7 13.3 5.0 3.4 15.0

.7 28.7 2.2 5.8 2.1

.1 17.6 4.7 3.0 1.9

.0 41.4 16.1 3.2 2.4.0 36.4 18.9 27.4 13.1 0.0055 59.03 g/m2 b0.0001.1 41.0 41.0 33.0 39.9.35 0.35 0.47 0.38 0.29.33 0.32 0.30 0.15 0.27.33 0.34 0.20 0.34 0.13 b0.0001.32 0.40 0.34 0.20 0.11 0.0005.27 0.30 0.30 0.22 0.16.32 0.30 0.30 0.31 0.32.31 0.29 0.30 0.29 0.28.0 111.7 114.5 113.5 118.8.2 92.9 95.4 57.5 104.6.7 98.8 93.1 54.7 107.4.9 54.7 43.2 53.6 29.0 0.0033.8 118.0 47.9 23.6 26.4 0.0069.7 92.0 61.6 62.2 60.6 0.0013 38.12 g/m2 b0.0001.07 99.05 100.01 69.35 67.70 0.0058

ture. p-value, of the dose response curve, EC50 and pEC50 (statistical significance of EC50

Table 5Photosynthetic parameter values, initial slope of the curve (α); maximum electron transport rate (ETR max) and irradiance of half-saturation (IK), for all the treatments and days.

Treatment/day C1 C2 C( f ) T1 (0.03 g/L) T2 (0.08 g/L) T3 (0.23 g/L) T4 (0.68 g/L) T5 (2.03 g/L) T6 (6.08 g/L) T7 (18.23 g/L) T8 (54.68 g/L)

α 0 0.28 0.26 0.2614 0.18 0.26 0.21 0.27 0.27 0.27 0.24 0.26 0.26 0.27 0.2721 0.15 0.23 0.20 0.24 0.25 0.26 0.27 0.25 0.26 0.27 0.26

ETRmax 0 19.3 23.9 21.514 16.7 19.7 18.9 22.4 17.5 22.3 19.4 18.3 18.5 25.6 22.921 11.2 15.9 26.9 16.2 20.6 19.0 19.3 20.4 21.1 28.4 17.5

IK 0 69.8 94.1 74.814 91.4 75.1 91.1 82.0 65.4 82.8 79.4 70.2 69.6 94.7 86.212 73.0 70.4 133.5 66.7 80.9 75.0 73.1 82.4 81.6 105.8 67.5

T means treatment, C means control, Cf is the control with water filtered from clay water mixture.

Fig. 4. Cell number % of major algal groups, at days 0, 14 and 30 after clay addition at allthe treatments.

5699O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

2002; Graham, 1990) and productivity (Davies-Colley et al., 1992;Lloyd et al., 1987; Van Nieuwenhuyse and La Perriere, 1986) has beenreported at sites affected by sediments. In the present study, twoweeks after sediment addition, significant decreases in chl a wereregistered at depositions over 35 gm−2 of clay (50% reduction at59.03 gm−2). Nevertheless, the impact was temporary, as one weeklater periphyton chl a increased in the high load treatments. Thissuggests that periphytic biomass can adapt to sediment depositionand show full compensation, at least in our experimental range.

The possibility of sediments being a source of nutrients that couldfavour algae growth is ruled out.

As discussed above, phosphate concentrations decreased at highdose treatments although algal growth was low, thus suggesting theimmobilization of phosphate by sediments. Photosynthetic efficiencyshowed adaptation to clay deposition. After three days, photosyn-thetic efficiency decreased significantly at the two highest sedimentdoses, but by day 14 differences between treatments disappeared.Parkhill and Gulliver (2002) reported that turbidity did not affectwhole-stream primary productivity in artificial streams, and deducedthat an increase in photosynthetic efficiencymight be the cause, as theinitial slope of the P–I curve (α) increased in the high sedimentstream. In our study, α showed only a slight but non significant trendto increase with clay content.

At least three possibilities could account for the growth ofperiphyton after the pulse of siltation: a) vertical migration of buriedcells to the sediment surface; b) physiological adaptation to lightattenuation; and c) shifts in community composition. In our case,although we cannot rule out vertical migration and physiologicaladaptation, shifts in community composition are responsible for atleast a part of the observed adaptation. The ratio Fo1/Fo3, which is anindex of the relative contribution of green algae vs cyanobacteria tothe pigment pool, showed an increased proportion of cyanobacteria inhigh sediment treatments after two weeks, but a slight recovery tolower values after three weeks. An increase in the presence ofcyanobacteria with clay was also seen in cell counts, and at the end ofthe experiment diatoms became dominant at high clay doses. Thus,the combination of fluorescencemeasurements and direct communityobservations showed that highest sediment depositions affectedgreen algae the most. Cyanobacteria and diatoms adapted better tosiltation, perhaps due to their gliding motility (Hoiczyk, 2000;Dickman et al., 2005).

Thus our experiment shows that the response of the periphytonto siltation depends on the amount of sediment deposited. Lowdeposition rates stimulate algal growth, while high deposition clearlyhas detrimental effects: algal biomass recovers in some weeks, butcommunities remain affected. Similarly, Gray and Ward (1983)reported an increase in stream algae with silt; however, this wasexplained in terms of elevated nutrient levels. In our experimentnutrients were available in all treatments, so that cannot account forthe stimulation of periphytic growth at low depositions. Perhaps theresult can be explained in terms of biofilm architecture: particlesembedded in the periphytonmatrix act as “stones” for building the 3Dstructure of the biofilm. Particles are always present in flowing water

5700 O. Izagirre et al. / Science of the Total Environment 407 (2009) 5694–5700

and it is not surprising that the periphyton may take advantage of thissituation.

In summary, siltation pulses produce short term impacts onperiphytic biomass, photosynthetic activity and community structure.Photosynthetic activity and periphytic biomass recover to pre-distur-bance values in a few weeks, but community composition remainsaffected if the substrate remains silted. Therefore, recurrent siltationpulses, which occur often in streams impacted by fine sediments, couldresult in long term impacts on periphytic communities.

Acknowledgements

This research was funded by the Spanish Department of Scienceand Technology, the University of the Basque Country, and theEuropean Regional Development Fund, through projects CGL 2006-12785/HID, 9/UPV00118.310-14476/2002 and BOS2003-04466.Oihana Izagirre carried out this work thanks to a pre-doctoral grantby the Basque Government. Helen Read (Corporation of London)checked English style.

References

Biggs BJ. The contribution of disturbance, catchment geology and landuse to the habitattemplate of periphyton in stream ecosystems. Freshw Biol 1995;33:419–38.

Boer DH, Hassan MA, MacVicar B, Stone M. Recent (1999–2003) Canadian research oncontemporary processes of river erosion. Hydrol process 2005;19:265–83.

Brookes A. Response of aquatic vegetation to deposition downstream from riverchannelisation works in England and Wales. Biol Conserv 1986;38:352–67.

Burkholder JM. Phytoplankton and episodic suspended sediment loading: phosphatepartitioning and mechanisms for survival. Limnol Oceanogr 1992;37:974–8.

Carling PA, Reader NA. Structure, composition and bulk properties of upland streamgravels. Earth Surf Processes Landf 1982;7:349–65.

Crowe A, Hay J. Effects of fine sediment on river biota. Cawthron; 2004. Report No. 951.Davies–Colley RJ, Hickey CW, Quinn JM, Ryan PA. Effects of clay discharges on streams.

Hydrobiologia 1992;248:215–34.Dickman MD, Peart MR, Yim WW. Benthic diatoms as indicators of stream sediment

concentration in Hong Kong. Int Rev Hydrobiol 2005;90:412–21.Downes BJ, Barmuta LA, Fairweather PG, Faith DP, Keough MJ, Lake PS, et al. Monitoring

ecological impacts. Concepts and practice in flowing waters. Cambridge UniversityPress; 2002. 446 pp.

Graham AA. Siltation of stone-surface periphyton in rivers by clay-sized particles fromlow concentrations in suspension. Hydrobiologia 1990;199:107–15.

Gray LJ, Ward JV. Response of lotic algae to sediment release from a reservoir. SouthwestNat 1983;28:239.

Hoiczyk E. Gliding motility in cyanobacterial: observations and possible explanations.Arch Microbiol 2000;174:11–7.

Lal R. Soil erosion and the global carbon budget. Environ Int 2003;29:437–50.Lloyd DS, Koenings JP, LaPerriere J. Effects of turbidity in freshwaters of Alaska. North

Am J Fish Manage 1987;7:18–33.Martin CW, Hornbeck JW, Likens GE, Buso DC. Impacts of intensive harvesting on

hydrology and nutrient dynamics of northern hardwood forests. Can J Fish AquatSci 2000;57:19–29.

Murphy J, Riley JP. A modified single solution method for the determination of phosphatein natural waters. Anal Chim Acta 1962;27:31–6.

Newbold JD. In: Calow P, Petts GE, editors. Cycles and spirals of nutrients. The rivershandbookOxford, England: Blackwell Scientific; 1992. p. 379–408.

Newcombe CP, MacDonald DD. Effects of suspended sediments on aquatic ecosystems.North Am J Fish Manage 1991;11:72–82.

Norwell ARM, Jumars PA. Flow environments of aquatic benthos. Annu Rev Ecol Syst1984;15:303–28.

Owens PN, Walling DE. Changes in sediment sources and floodplain deposition rates inthe catchment of river Tweed, Scotland, over the last 100 years: the impact ofclimate and land use change. Earth Surf Processes Landf 2002;27:403–23.

Parkhill KL, Gulliver JS. Effect of inorganic sediment on whole-stream productivity.Hydrobiologia 2002;472:5-17.

Pimentel D, Kounang N. Ecology of soil erosion in ecosystems. Ecosystems 1998;1:416–26.Ryan PA. Environmental effects of sediment on New Zealand streams: a review. N Z J

Mar Freshw Res 1991;25:207–21.Sartory DP, Grobbelaar JE. Extraction of chlorophyll a from freshwater phytoplankton

for spectrophotometric analysis. Hydrobiologia 1984;114:177–87.Schreiber U, Gademann R, Bird P, Ralph PJ, Larkum AWD, Kühl M. Apparent light

requirement for activation of photosynthesis upon rehydration of desiccatedbeachrock microbial mats. J Phycol 2002;38:125–34.

Shriner C, Gregory T. Use of artificial streams for toxicological research. Crit Rev Toxicol1984;13:253–81.

Swank WT, Vose JM, Elliot KJ. Long-term hydrologic and water quality responsesfollowing commercial clearcutting of mixed hardwoods on a southern Appalachiancatchment. For Ecol Manag 2001;143:163–78.

Richards K. River: form and process in alluvial channels. London: Methuen; 1982. 361 pp.USEPA. Nutrient criteria technical guidancemanual: Lakes and Reservoirs. Office ofWater,

Office of Science and Technology, Report EPA-822-B00-001, Washington, D.C; 2000.Van Nieuwenhuyse EE, La Perriere JD. Effects of placer gold mining on primary

production in subarctic streams of Alaska. Water Resour Bull 1986;22:91–9.Waters TF. Sediment in streams: sources, biological effects and control. Bethesda, Md:

American Fisheries Society; 1995.Wetzel RG. Limnology: lake and river ecosystems. 3rd edition. San Diego, CA: Academic

Press; 2001.Wood PJ, Armitage PD. Biological effects of fine sediment in the lotic environment.

Environ Manag 1997;21:203–17.Yamada H, Nakamura F. Effect of fine sediment deposition and channel works on

periphyton biomass in the Makomanai River, northern Japan. River Res Appl2002;18:481–93.

Yang X, Zhang X, Fang H. Erosion losses of black soil under different management inJilin. Chin J Soil Sci 2003;34:389–92.

Zar JH. Biostatistical analysis. 4th Edition. Upper Saddle River, NJ: Prentice-Hall, Inc.;1999. 931 pp.