nt2 – nam theun 2 - 5 years of monitoring of zooplankton … · 2020. 5. 24. · 2 material and...

Hydroécol. Appl.© EDF, 2016DOI: 10.1051/hydro/2015009

5 years of monitoring of zooplankton communitydynamics in a newly impounded sub-tropical reservoirin Southeast Asia (Nam Theun 2, Lao PDR)

Suivi de cinq ans de la dynamique des communautészooplanctoniques d’un réservoir subtropical nouvellement misen eau dans le sud-est Asiatique (Nam Theun 2, Laos)

S. Descloux (1), M. Cottet (2)

(1) EDF – Hydro Engineering Centre Sustainable Development Department Savoie-Technolac,73373 Le Bourget-du-Lac, [email protected]

(2) Nam Theun 2 Power Company Limited (NTPC), Environment & Social Division – Water Qualityand Biodiversity Dept.– Gnommalath Office, PO Box 5862, Vientiane, Lao PDR

Abstract – The zooplanktonic community of the Nam Theun 2 Reservoir, newly impoun-ded, located in Lao PDR was monitored from 2009 to 2013. Five sampling stations distribu-ted on the surface of the reservoir were sampled on a monthly basis. Results wereinterpreted to assess the spatial and temporal variability of the communities and to identifyenvironmental factors such as physical or water chemical parameters influencing communi-ties. The zooplanktonic community was dominated by Copepods over the study period andwas characterized by low densities and biomasses reflecting low productivity conditions.The spatial heterogeneity of the zooplanktonic community observed along a longitudinalgradient disappeared rapidly after the beginning of the operational phase in 2010 andtended to stabilize. The results also highlighted a seasonal variability explained by a phyto-plankton-based food chain. Zooplankton communities were significantly and negatively dri-ven by hydraulic factors, such as the mixing period, or water quality factors, such as watertemperature or the depth of the oxycline. This zooplankton community must be evaluated inorder to anticipate the food availability for zooplanktivorous fish before developing a fishmanagement plan.

Keywords– sub-tropical reservoir,SoutheastAsia,Copepods,spatial heterogeneity,mixing

Résumé – La communauté zooplanctonique du réservoir de Nam Theun 2, nouvellementmis en eau et situé dans la République démocratique du Laos, a été suivie de 2009 à 2013.Cinq stations d'échantillonnage réparties sur la surface du réservoir ont été échantillonnéesà une fréquence mensuelle. Les résultats ont été interprétés pour évaluer les variations spa-tiales et temporelles de la communauté et d’identifier les facteurs environnementaux tels que

Article publié par EDP Sciences

http://publications.edpsciences.orghttp://dx.doi.org/10.1051/hydro/2015009

2 S. Descloux and M. Cottet

les paramètres physiques ou chimiques de l’eau influençant les communautés. La commu-nauté zooplanctonique était dominée par les Copépodes pendant la période d'étude et a étécaractérisée par de faibles densités et biomasses reflétant des conditions de faible produc-tivité. L'hétérogénéité spatiale de la communauté zooplanctonique observée le long d'un gra-dient longitudinal a disparu rapidement après le début de la phase d'exploitation en 2010 etla communauté a eu tendance à se stabiliser. Les résultats ont également mis en évidenceun effet saisonnier expliqué par une chaîne alimentaire basée sur le phytoplancton. La com-munautézooplanctoniqueaétésous l’influencesignificativede facteurshydrauliquescommela période de mélange des eaux ou de facteurs de qualité d’eau comme la température del'eau ou de la profondeur de l'oxycline. Cette communauté zooplanctonique doit être évaluéeafin d’anticiper la nourriture disponible pour les poissons zooplanctophages et avant la miseen place d’un programme de gestion de la ressource piscicole.

Mots clés – réservoir sub-tropical, Asie du sud-est, Copépodes, hétérogénéité spatiale,mélange

1 INTRODUCTION

Freshwater zooplankton communi-ties have been described in tropical andsub-tropical areas in numerous studiesin South America (Matsumura-Tundisi& Tundisi, 1976; Bini et al., 1997;Starling, 2000; Fernando, 2002;Bonecker et al., 2007; Lansac-Tôhaet al., 2008; Sousa et al., 2014), Africa(Uku & Mavuti, 1994; Mageed & Heikal,2006; Fetahi et al., 2011) and Asia(Dussart et al., 1984). They are, in lenticzones, generally driven by environmen-tal factors such as turbidity, nutrient anddissolved oxygen concentrations orphytoplankton density and diversity(Hart, 1990; Schulz & Sterner, 1999).These factors fuel the spatial heteroge-neity in a water body and enhance tem-poral variations of the zooplanktoncommunities (Bini et al., 1997; Seda &Devetter, 2000; Espindola et al., 2000;Bernot et al., 2004).

The few available data in Asiamainly focused on systematics. Dussartet al. (1984) indicated that Rotifera,

Cladocera and Copepods are com-posed of few species and that theCyclopoida Copepod could representhigh densities. They also specified thatthe common limnetic species can usethe entire range of habitat types. How-ever, scarce data on zooplankton com-munity dynamics exists in SoutheastAsia (Dussart et al., 1984), and evenless in the Mekong River and its tribu-taries. Studies on reservoirs remainedalso inexistent in Lao PDR.

The number of new reservoirs inSoutheast Asia is increasing to face thegrowing energy demand. In 2009, TheMekong River Commission (MRC) esti-mated that more than 70% of a total 124existing, under construction and poten-tial projects in the Lower Mekong Basinwill be located in Lao PDR (MRC, 2010).The impoundment of the Nam Theun 2(NT2) Reservoir in 2008 led to a pro-found changes in aquatic communities(Cottet et al., same issue; Martinet et al.,same issue) resulting in a modificationof the trophic web structure. Phyto-plankton and zooplankton, respectively

5 years of monitoring of zooplankton community dynamics in a newly impounded sub-tropical reservoir 3

primary and secondary producers arekey components of this new ecosys-tem, influencing the entire food web(Hairston & Fussman, 2002; Rennella &Quirós, 2006; Araujo et al., 2008). Theunderstanding of the temporal and spa-tial evolution of zooplankton communityappears all the most important thatnewly created reservoirs play an impor-tant role for the local populationsdepending primarily on fish as proteinresources. It is highly important tounderstand and assess the availabilityof zooplankton for the trophic chain andthus studies are required before imple-menting any fisheries managementprogrammes.

In this context, the first objective ofthe study was to assess the intra andinter-annual evolution of the zooplank-ton community since the NT2 Reservoirimpoundment based on density andbiomass estimations of the main taxo-nomic groups. The second objectivewas to identify the main environmentalparameters which influenced thesezooplankton communities.

2 MATERIAL AND METHODS

2.1 Sampling strategy

2.1.1 Study area

The NT2 Project is a trans-basinhydro-power scheme located in Kham-mouane Province, central Lao PDR(Fig. 1). The water coming from the NT2Reservoir (Nakai Plateau – Nam Theunwatershed) is diverted to the Xe BangfaiRiver southwards (Xe Bangfai water-shed). The impoundment of the reser-voir was initiated in April 2008. Com-mercial operation started in April 2010.

Since then, the average retention timeof the water in the reservoir is estimatedto be approximately 6 months on thebasis of the production of 6000GWh.year-1. The NT2 Reservoir coversa surface area of 489 km² at its full sup-ply level and potentially decreases to aminimum of 86 km² at the end of the dryseason (Descloux et al, same issue).The reservoir flooded almost 70% ofdense to light forest, 23% of degradedforest, riparian forest and agriculturalsoils and the last 7% were composed ofwater system, swamps and villages(Descloux et al., 2011). The reservoir isqualified as a dendritic and is relativelyshallow with an average depth of 8 m(Chanudet et al., 2012). The main trib-utaries of the reservoir are the Nam Xot,the Nam Theun and the Nam On Riv-ers. The climate is moist sub-tropicalwith a warm-wet (WW) season (June toOctober), a cool-dry (CD) season(November to February) and a warm-dry (WD) season (March to May). TheCD season is also characterized bywind events. During this season andbecause of the turbulences linked to thewind, the water column is mixed (mixingperiod). This mixing period occurs for afew weeks between November andFebruary. Then, the NT2 Reservoir isdefined as a monomictic reservoir withthermal and oxygen stratification occur-ring during the dry and warm seasons.Detailed features of this project aredescribed in Descloux et al. (same issue).

2.1.2 Monitored stations

The sampling strategy covers thespatial heterogeneity of the reservoir.Zooplankton was monitored in the NT2Reservoir on a monthly basis (Fig. 1)


Fig. 1. Zooplankton communities monitoring stations.

Fig. 1. Stations de suivi des communautés zooplanctoniques.


from February 2009 to December 2013at three stations (i) RES1 is the deepeststation situated just upstream to theNakai Dam (ii) RES4 is situated at theformer Nam Theun River thalweg, thearea is surrounded by submergedmedium forest and (iii) RES6 is locatedat the former Nam Theun River thalwegnear the North shore of the reservoirnear the headrace channel. Since May2013, RES6 station has been removedfrom the monitoring because of its sim-ilarity with RES8. Two stations wereadded in June 2009 to include a widerrange of habitat (i) RES3 is situated ina sheltered and shallow area in a sub-merged dense forest and (ii) RES8 issituated in an open area outside theformer Nam Theun River thalweg on thesouthern shore of the reservoir.

2.1.3 Environmental parameters

Since 2009, in situ physico-chemicalparameters (depth, oxycline depth, sur-face dissolved oxygen (DO), surfacewater temperature, and surface waterconductivity) were measured with amulti-parameter probe (Quanta, Hydro-lab; the analytical methods with theassociated limits of detection anduncertainties can be found in Chanudetet al., same issue). Water samples werecollected in the euphotic zone definedby the Secchi disk (2.5 x Secchi depthin m). A total of 5 samples of 1 L werecollected with a Niskin bottle at equidis-tant depths covering the entire euphoticzone and were mixed together. One litreof the mixed water was collected forchlorophyll a (Chlo a) analysis (Loren-zen’s method). A sample of 250 mLwas collected for chemical analyseswhich included Total phosphorus (Ptot,

spectrophotometry), Total nitrogen(Ntot, spectrophotometry). All of theanalyses followed the American Stand-ards for Water Quality (APHA stand-ards, 1995). Meteorological data (rain,wind) were recorded at RES4 and aredetailed in Descloux et al. (same issue).

2.1.4 Zooplankton sampling

Zooplankton were collected verti-cally with a plankton net (50 µm meshsize) from 1 m below the oxycline to thesurface to include organisms colonizingthis area. All samples were preservedin 75% alcohol for further laboratoryanalysis.

2.2 Laboratory analysis

2.2.1 Zooplankton analysis

At laboratory, samples were care-fully washed on a 50 µm mesh sizesieve, collected and diluted to a final vol-ume of 150 mL. This final volume ofsample was chosen because (i) it allowsa good repartition of individuals duringthe mixing step; (ii) it avoids aggrega-tion between individuals and then arapid sedimentation; (iii) it avoids anoverload of the sample in case of abun-dant and large phytoplankton cells (e.g.Peridinium spp.) and (iv) it allows agood constancy between subsamples,collected with a 5 mL micro-pipette.

The identification of the taxa wasrealized at relatively coarse level mainlybecause of the high number of samples.These groups are the Cyclopoids, Cala-noids, Cladocera, Nauplii stage (mainlycomposed by Copepods), Rotifer, andOstracoda.


2.2.2 Zooplankton Sub-countingtechnique

Seven sub-samples of 5 mL, col-lected from the homogenized 150 mLsample, were counted on a countingplate under stereomicroscope. Thistotal of 35 mL was chosen because itresulted in extrapolated densities withan error less than 5% for large individ-uals and 10% for small individuals com-pared to the real abundance (test real-ized with extrapolation on sub-countingand total sample counting: data notshown). This error range remainsacceptable compared to the literature(Frontier, 1972). Furthermore, thisnumber of sub-samples allows theobservation of rare groups.Finally, if lessthan 200 individuals in the first 35 mLfor a main group defined above werecounted; additional counts of 5 ml con-tinues until a minimum of 200 individualswas reached. Finally, total densities(ind.L-1) are reported according to thesub-counting factor and the total volumeof the water column sampled on site.

2.2.3 Zooplankton biomass estimation

An estimation of the mean dryweight (DW) of each zooplankton groupwas established for the NT2 Reservoir.This estimation was based on size ofindividuals which was defined as (i) themaximum length for Ostracoda andRotifera (ii) the length from the headuntil the tail for Copepods and Naupliiand finally (iii) length from the headuntil spine for Cladocera. For Rotifer,Nauplii, and Ostracoda all individualscollected for the biomass estimationwere measured, counted and kept in asample for further analysis. Copepods

and Cladocera were measured andseparated according to different sizeclasses (Tab. I). Each group samplewas dried at 60 °C during 24 hours andweighed (µg) on a micro-balance afterbeing cooled in a desiccator. Then, bio-masses were expressed as mean dryweight (DW) in µg.L-1. A comparisonwith literature data on zooplankton bio-masses showed that the estimations ofthe biomasses of the NT2 zooplanktongroups are reliable (Tab. I).

For monthly routine analysis, 30 in-dividuals of Cladocera, Cyclopoids andCalanoids were measured to estimatethe total biomass of each group. For theother groups, the mean DW was multi-plied by the number of the individualsfound in the sample. Finally, total bio-masses (µg.L-1 DW) are reported ac-cording to the sub-counting factor andthe total volume of the water columnsampled on site.

2.3 Data analysis

Densities, biomasses and relativedensities of each group were used todescribe the spatial and temporal evo-lution of the community (GraphPadSoftware). Non-parametric tests ofKruskal-Wallis were used to comparethe densities and biomasses amongmonitoring stations and years. Pair-wise comparisons were done followingthe Dunn procedure. Zooplankton com-munities and environmental relationswere studied through Principal Compo-nent Analysis (PCA). The PCA wasundergone on a matrix including thebiomass of the zooplankton groups,trophic factors (nutrient concentra-tions and Chlo a concentrations),


physico-chemical parameters of thewater (temperature, DO and depth ofthe oxycline), hydrometeorologyparameters (season, rain, water level,wind and mixing period) The signifi-cance level for all statistical analyseswas 0.05. Statistical analyses were per-formed using R statistical software (ver-sion n° 2.6.1, R Development CoreTeam 2011) and XLSTAT software.

3 RESULTS

3.1 Waterquality in theNamTheun 2Reservoir

Surface water temperature variedfrom 18.5 °C (at RES8 during the CD

Table I. Mean dry weight (µg) of zooplankton groupavailable dry weight in literature. * = according to thsize of individual used for dry weight estimation.Tableau I. Poids sec moyen (µg) des groupescomparaison avec les valeurs de poids sec disponide l’individu et d = taille moyenne de l’individu dans

ZooplanktonGroup

Sizeclass(mm)

Number ofindividual for

estimation(this study)

Calculateddry weight(µg) (this

study)

CladoceraLarge

> 0.5 1120 2.68

CladoceraSmall

≤ 0.5 2739 0.73

CyclopoidLarge

> 0.5 743 2.56

CyclopoidSmall

≤ 0.5 3624 0.72

Calanoid Large ≥1.2 615 16.75CalanoidMedium

0.75 ≤ xx


temporal variations (Fig. 2). At RES1,RES4 and RES6, the thickness of thesurface oxic layer increased from nearly0.5 m in 2008 to 5 m in 2013. In 2010and 2011, high floods induced a signif-icant increase of the oxygenation at theend of the year. RES3 which is an iso-lated sheltered area and RES8 which isunder the influence of tributaries,showed respectively a lower evolution

Fig. 2. Temporal evolution of main physico-chemicthe monitoring stations.

Fig. 2. Évolution temporelle des principaux paramè2013 aux stations du monitoring.

and no significant evolution of the thick-ness of the DO concentrations. The DOconcentrations in RES1 and RES3ranged from 0.5 mg.L-1 (5% saturation)to 9.6 mg.L-1 (115%) over the studyperiod. However, the low DO onlyoccurred for a short period in December2009 (water column mixing process).RES4, RES6 and RES8 had bettersurface oxygenation, ranging from

al parameters on sub-surface from 2009 to 2013 at

tres physico-chimiques en sub-surface de 2009 à


3.0 mg.L-1 (32%) to 8.7 mg.L-1 (105%).The conductivity in surface water wasalways very low (monthly average val-ues ranging from 11 at RES3 to32 µS.cm-1 at RES1). The conductivitydecreased during WW seasons andwhen the reservoir overturned. Thehighest values were recorded immedi-ately after the impoundment until 2011at RES1, RES3, RES4 and RES6 andslightly decreased the following years.In RES8, seasonal variation can beobserved due to the influence of annualfloods of the tributaries (Nam Theunand Nam On) but the range of concen-trations is close to the others stations.

Annual peaks of chl a concentra-tions are observed during the stratifica-tion period (up to 67.4 µg.L-1 at RES3)at the end of the WD season, with high-est concentrations during the first twoyears (Fig. 2) and at RES1 and RES3.The rest of the year, values remainedaround 10 µg.L-1. Ptot concentrationsmeasured in the reservoir were lowuntil 2011 (from 0.01 to 0.06 mgP.L-1;Fig. 3), expect during the first month

Fig. 3. Concentrations in Total Phosphorous (Ptot)monitored stations.

Fig. 3. Concentrations en phosphore total (Ptot) etmonitoring.

after impoundment in RES1 (up to0.27 mgP.L-1). In 2012, Ptot concentra-tions showed the highest values with amaximum of average value across sta-tions in June 2012 at 0.27 mgP.L-1.Then, Ptot concentrations decreasedagain in 2013 except in RES8. Ntot con-centrations were close to 0 mgN/L at thebeginning of the impoundment andincreased at the end of 2009. Meanmonthly Ntot concentrations rangedfrom 0 to 2.83 mgN.L-1 and no clearspatial pattern appeared among sam-pling stations. Two peaks occurred inJune 2011 in RES6 (7.1 mgN.L-1) andinDecember2012 inRES4(9.6mgN.L-1).After an upsurge period of two years(2009-2011), the NT2 Reservoirevolved to an oligo-mesotrophic status(Martinet et al., same issue).

3.2 Zooplankton populationdynamics

Figure 4 shows the temporal evolu-tion of total densities and biomasses for

and total Nitrogen (Ntot) from 2009 to 2013 at the

azote total (Ntot) de 2009 à 2013 aux stations du


each sampling station. Globally, bio-masses follow the same dynamic thanthe densities characterized by highmonthly variations. Each year, threepeaks were observed (up to 250 ind. L-1

and 250 µg DW L-1 in RES1) (i) aroundMarch (end of the CD season), (ii)around September (end of the WW sea-son) and (iii) in November (WD windyseason). The dry and windy seasonpeaks were more pronounced in 2013compared to 2012. An annual disap-pearance of the zooplankton commu-nity occurred in December 2010, 2011and 2013 and in January 2012 and canbe related to the destratification of thewater column (Fig. 4).

Fig. 4. Zooplankton biomasses and densities from

Fig. 4. Biomasses et densités du zooplancton de 2

At the reservoir scale, the meanannual densities and biomasses of thezooplankton community (Tab. II, Tab. IIIand Fig. 4) reached a maximum in 2010,two years after the reservoir filling. Theaverage densities and biomasses in2011, 2012 and 2013 were not signifi-cantly different but all were significantlylower than in 2010 (Kruskal-Wallis test,respectively p = 0.03; p = 0.02 and p =0.01 for densities and p =0.001; p =0.001 and p = 0.01 for biomasses).While 2009 had high density and bio-mass, only biomass values were signif-icantly higher than those in 2011 and in2012 (Kruskal-Wallis test, respectivelyp = 0.04; p = 0.03). Biomasses in 2010

2009 to 2013 for the monitored stations.

009 à 2013 pour les stations du monitoring.


were significantly higher than in otheryears for each sampling stations (seeSM 1). The maximum annual meandensity and biomass were reachedrespectively at RES3 in 2010 with49.7 ind. L-1 and at RES1 in 2010 with39.1 µg DW L-1 (Tab. II and Tab. III). Theminimum mean density and biomasswere recorded respectively at RES6 in2013 with 5.3 ind. L-1 and at RES1 in2011 with 4.2 µg DW L-1. The resultsalso highlight the spatial heterogeneityof the densities and biomasses into thereservoir during the first years of the

Table II. Annual average densities (ind.L-1 + SD) o

Tableau II. Densités moyennes annuelles (ind.L-1

entre 2009 et 2013.

RES1

(ind.L-1 + SD)

RES3

(ind.L-1 + SD)

RES4

(ind. L-1 + SD

2009 34.4 ± 52.6 24.9 ± 23.3(last 7 months)

9 ± 6.9

2010 40.8 ± 57.9 49.7 ± 36.9 16.9 ± 8.72011 6 ± 5.3 29.7 ± 33.9 7.2 ± 5.82012 10.5 ± 8.5 18.8 ± 15.2 10.4 ± 10.82013 8.6 ± 7.8 19.5 ± 17.7 13.8 ± 12.7

Table III. Annual average biomasses (µg DW.L-1 +2013.

Tableau III. Biomasses moyennes annuelles (µg DNT2 entre 2009 et 2013.

RES1

(µg DW L-1

+ SD)

RES3

(µg DW L-1

+ SD)

RES4

(µg DW L-1

+ SD)

2009 23.3 ± 34.7 15.9 ± 11.6(last 7 months)

9.7 ± 8.8

2010 39.1 ± 66.2 37.7± 31.0 14.7 ± 8.0

2011 4.2 ± 3.6 20.1 ± 20.3 5.2 ± 4.8

2012 7.9 ± 5.6 8.8 ± 7.3 7.1 ± 8.1

2013 8.4 ± 7.7 12.3 ± 11.9 11.3 ± 7.4

monitoring. Whereas in 2009 and 2010RES1 showed the highest densitiesand biomasses, since 2010, RES3 hadthe highest densities and since 2011had the highest biomasses. Neverthe-less, both densities and biomasseswere not significantly different amongstations and years in 2012 and 2013(Kruskal-Wallis test, p > 0.05; SM 1)underlining a homogenization of thecommunity at the reservoir scale sincethis date.

Relative densities (Fig. 5) and rela-tive biomasses (not shown) have a

f zooplankton in NT2 reservoir from 2009 to 2013.

+ SD) du zooplancton dans le réservoir de NT2

)

RES6

(ind. L-1 + SD)

RES8

(ind. L-1 + SD)

Annualaverage

(ind. L-1 + SD)

11.9 ± 10.5 13.6 ± 7.8(last 7 months)

18.7 ± 28.6

18.3 ± 14 16.5 ± 10.4 27.7 ± 33.58.9 ± 8.4 8.3 ± 11.5 12.4 ± 19

13.5 ± 11.5 10.3 ± 8.9 12.7 ± 11.45.3 ± 3.4

(first 4 months)9.7 ± 8.9 12.3 ± 12.4

SD) of zooplankton in NT2 reservoir from 2009 to

W.L-1 + SD) du zooplancton dans le réservoir de

RES6

(µg DW L-1

+ SD)

RES8

(µg DW L-1

+ SD)

Annual average

(µg DW L-1

+ SD)

10.4 ± 8.5 15.4 ± 11.2(last 7 months)

14.8 ± 18.9

12.9 ± 8.9 14.4 ± 6.6 23.3 ± 34.3

4.7 ± 4.6 5.2 ± 6.9 7.9 ± 11.6

8.0 ± 5.7 7.0 ± 5.7 7.8 ± 6.3

4.3 ± 2.8(first 4 months)

8.5 ± 8.4 9.7 ± 8.7

http://www.R-project.org/http://www.R-project.org/http://www.R-project.org/


similar pattern and highlight that thecommunity was dominated by Copep-ods and especially the naupliar stage(mainly composed of Cyclopoids) andadults of Cyclopoids. RES4, RES6 andRES8 had a more stable composition ofzooplankton than RES1 and RES3,where zooplankton group densities andbiomasses varied substantially until2011. At the inter-annual scale, Naupliicontributed between 46.0% (RES1) to54.4% (RES6) to the total zooplanktondensities (during the 5-year survey they

Fig. 5. Zooplanktonic groups relative density from 2

Fig. 5. Densité relative des groupes zooplanctomonitoring.

contributed to 49.0% + 18.2%). Naupliiindicated a constant recruitment of theCopepod group. A short decrease ofpercentage of Nauplii appeared inOctober 2009 (4%), March 2010(11.3%) and December 2010 (14.7%).Calanoids, which were the largest indi-viduals, had low average relative den-sities ranging from 3.2% at RES3 andRES8 to 4.2% at RES4 (mean value of3.7% + 5.6%). High relative densitieswere recorded in February 2009 atRES4 (29%) and RES6 (72.9%) for this

009 to 2013 for the monitored stations.

niques de 2009 à 2013 pour les stations du


group. In the following years, only smallincreases of this population wereobserved at the end of the CD seasons(up to 12% of relative densities) andduring the wet season 2013 which alsocontributed significantly to the biomass.Cladocera populations had a small con-tribution to the zooplankton community(9.2% + 10.4% relative abundance overthe study period), even though somepeaks of density and biomass wereobserved, e.g. in February 2009 atRES1 (80.0%) and RES4 (55.7%), inOctober and May 2010 at RES4(60.8%) and in September 2013 atRES8 (59.6%). The lowest relative den-sity of Cladocera was observed atRES3 (3.6%). Since 2001, the popula-tion declined excepted at RES4, RES6and RES8 where the populationremained stable. The total contributionof the Ostracoda population to the zoo-plankton abundances remained low forall the stations (0.8% + 3.6%) duringthe monitoring except at RES3 inDecember 2009 and January 2011where their contributions reachedrespectively 48% and 32.5%. Theseperiods corresponded to the mixing ofthe water column period. Finally, Rotif-ers, which are important as a foodresource for fish juveniles, showed arelatively low contribution to the zoo-plankton community with an averagevalue of 13.8% + 14.0% during thestudy period. The contribution of Rotif-ers ranged from 10.8% at RES6 to15.3% at RES3 and RES8. Stable, butlow densities and biomass wererecorded during the CD season justafter the mixing period (December-February).

3.3 Environmental driving factors

The zooplankton community wasexplored by PCA ordination based onbiomasses. The two first axesexplained a total variance of 44.4%(Fig. 6). The variables associated withthis analysis explaining a high and sig-nificant proportion of variance were (i)on the first axis, the depth of the oxy-cline (12.3%), the temperature (8.5%),the mixing period (8.1%) and the Chl aconcentration (7.1%) and (ii) on the sec-ond axis meteorological variables withthe mean monthly rain (17.9%) and theseason (12.8%). The zooplankton com-munity can be divided in two main clus-ters. The first cluster is composed of theCalanoids, Cyclopoids and Cladorecathat were negatively correlated to thedepth of the oxycline (as well as theNauplii; Fig. 6 and Tab. IV). The secondcluster is composed of Ostracoda,Rotifera and Nauplii positively associ-ated with temperature and Chl a andsensitive to physical variables (mixingperiod and wind). Rotifera populationwas also positively related to the sea-son. Surprisingly, nutrients (Ntot andPtot) and oxygen concentration varia-bles were not significantly related withthe zooplankton groups.

4 DISCUSSION

4.1 Zooplankton communitydynamics in the NT2 Reservoir

From the beginning of the monitor-ing in 2009, the zooplankton community


showed different phases of develop-ment mainly in line with the evolution ofthe reservoir water quality and hydrol-ogy. The first phase, occurring between2009 and 2010, was a productive periodlinked to the trophic upsurge phenome-

Mixing

W

NtPtot

Depth-Oxy

Wind

-1

-0,75

-0,5

-0,25

0

0,25

0,5

0,75

1

-1 -0,75 -0,5 -0,25

F2 (1

6,51

%)

F1

Fig. 6. Principal component analysis.

Fig. 6. Analyse en composante principale.

Table IV. Pearson correlation matrix from the PCAwith alpha = 0.05. WL = water level, Chlo a =temperature, Ntot = total nitrogen, Ptot = total phosTableau IV. Matrice des corrélations de Pearson de0 à un niveau de signification alpha=0,05. WL = nidissous, Temp = température, Ntot = azote total, Pl’oxycline.

Variables Mixing Season WL Chlo a DO

Calanoids -0.168 0.271 0.046 0.183 -0.03

Cyclopoids -0.218 0.270 0.081 0.278 -0.02

Nauplii -0.564 0.230 -0.291 0.431 0.15

Rotifera -0.267 0.332 -0.205 0.326 0.15

Cladocera -0.271 0.288 0.055 0.184 0.00

Ostracoda -0.046 0.066 0.005 0.538 0.06

non observed after the first filling of thereservoir. During these 2 years, zoo-plankton showed the highest densitiesand biomasses, especially in 2010 atthe end of the CD season. Populationsof Calanoids, and Rotifers, which are

Season

L

Chlo a

DO

Temp

ot

Rain

CalanoidsCyclopoids

Nauplii

Ro�fera

Cladocera

Ostracoda

0 0,25 0,5 0,75 1

(27,85 %)

. In bold, correlations significantly different from 0Chlorophyll a, DO = dissolved oxygen, Temp =horus, Depth-Oxy = depth of the oxycline.puis l’ACP. Les valeurs en gras sont différentes de

veau d‘eau, Chlo a = Chlorophylle a, DO = oxygentot = phosphore total, Depth-Oxy = profondeur de

Temp Ntot Ptot Depth-Oxy Rain Wind

3 0.100 -0.182 -0.140 -0.324 -0.033 -0.161

9 0.060 -0.126 -0.188 -0.381 -0.121 -0.252

4 0.490 0.139 -0.263 -0.613 0.062 -0.386

7 0.325 -0.065 -0.120 -0.259 0.018 -0.181

1 0.139 -0.265 -0.229 -0.413 -0.112 -0.208

2 0.161 -0.084 -0.295 -0.317 -0.036 -0.319


mainly herbivorous, and the omnivo-rous Cyclopoids were well-developed.Cladocera species, which are mainlydetritivorous, took advantage of a highamount of particles directly linked to ahigh amount of nutrients and organicmatter coming from the decompositionof drowned soil and vegetation. Ostra-coda were not abundant but these spe-cies are mainly benthic and are gener-ally under-represented in the watercolumn. This high zooplankton produc-tion was enhanced by a low waterrenewal rate in the reservoir at thisperiod (production started in March2010). The peak observed during theend of the CD season 2010 was relatedto a combination of abiotic factors e.g.high temperature, lake stratification,low water level and the end of the periodof the lowest water renewal rate, factorsalready qualified as important in zoo-plankton dynamics in shallow lakes(Rennella & Quirós, 2006). The secondphase occurred rapidly at the end of2010 – beginning of 2011, and washighlighted by a decrease of annualzooplankton densities and biomasses.The decrease of zooplankton appearedduring the last months of 2010 becauseof relatively low recruitment comparedto that observed the previous year,especially at RES1. Seasonal peaks ofdensity and biomass observed duringthe CD season (highest productivityperiod) decreased. This could beexplained by a higher water renewalrate at this time compared to the previ-ous years at the same season. But themain negative effect on zooplanktoncommunities appeared in 2011 after theexceptional rainy season. This highflooding event led to a faster renewalwater rate which is a factor already

known to affect zooplankton community(Nogueira, 2001) e.g. Rotifers or Cope-pods (Gonzales et al., 2008). Since2012 the zooplankton populationseems to have reached stabilization interms of densities, biomasses and pop-ulation composition. Zooplankton pop-ulations recovered from the flushingand dilution effects of the previous rainyseason.

More globally, the NT2 annual aver-age density was low in the main body ofthe reservoir (RES1, RES4, RES6 andRES8) compared to some other tropicalreservoirs. However, values found inthe NT2 Reservoir are in the range ofthe lowest values observed in the liter-ature. For instance, Pinto-Coelho et al.(2005) found a mean density of11.6 ind. L-1 for large oligotrophic trop-ical reservoirs. Other studies in tropicalreservoirs indicated larger range ofdensities such as the Corumba Reser-voir (Brazil) with densities between 0.5to 447 ind. L-1 (Lansac-Toha et al.,2008) or in the Jurumirim Reservoir, anoligotrophic reservoir in Brazil, withdensities between 1 to 1000 ind. L-1

(Matsumura-Tundisi & Tundisi, 1976).Biomass of the NT2 Reservoir couldbe compared to the biomass ofMourao Reservoir, an oligotrophicreservoir in Brazil (7.5 µg DW L-1;Bonecker et al., 2007) or 12.3 µg DW L-1

for large oligotrophic tropical reservoirs(Pinto-Coelho et al., 2005). Based onthe densities and biomasses found inthe NT2 Reservoir compared to otherstudies in tropical reservoirs, the NT2Reservoir can be qualified as oligo-trophic. This trophic state is also sup-ported by the observed size distributionof individuals. Zooplankton communi-ties in the NT2 Reservoir have small


body sizes, which is common in otheroligotrophic tropical reservoir and lakes(e.g. South America; Gliwicz, 1994). Forinstance, the NT2 zooplanktonic com-munity was dominated by small Cope-pods (>80% of the population andoccasionally by Cladocera up to 70% ofthe population) that could indicate anoligotrophic status (Matsumura-Tundisi& Tundisi, 1976).

The high density of naupliar stagescompared to the density of adultsunderlines that communities weremaintained by a constant recruitmentlikely as a response to predation, foodlimitation or an unstable environment(strategy r). The small size of individualsis also known to be related to a top-down predation effect (Fetahi et al.,2011). Amazonian reservoirs wereoften dominated by Rotifers the yearsfollowing the first filling (Arcifa, 1984)but they were not abundant in our sur-vey. It is likely that the high fish biomassobserved after the impoundment(Cottet et al., same issue) was sup-ported as a result of high Rotifer preda-tion. Nevertheless, the NT2 zooplank-ton community was dominated byCopepods (mainly Cyclopoida) whichseems to be common in Asia (Dussartet al., 1984). This result is also in linewith what has been observed by Horeauet al. (2005) in the Petit-Saut Reservoirin French Guiana.

4.2 Environmental parametersinfluencing zooplanktoncommunities

The results of the PCA have showna separation of the zooplanktonic com-munity in two different clusters. The firstcluster composed of Nauplii, Rotifer

and Ostracoda, is related to Chl a andtemperature. These populations aremainly composed of phytophage spe-cies and were linked to the phytoplank-ton production, itself highly seasonal.The Naupliar stage was also signifi-cantly negatively driven by the hydraulicfactors like mixing period or wind lead-ing to a nearly total disappearance ofthe population. Large mortalities ofzooplankton following annual over-turn have already been reported bySaunders & Lewis (1988). On anotherhand, short-term disappearance of thezooplankton community can also berelated to the decrease of the phyto-plankton biomass (Martinet et al., sameissue). The second cluster composed ofCladocera and Copepods (Cyclopoidsand Calanoids) was mainly influencedby the depth of the oxycline, e.g. thedilution of the epilimnion.

These abiotic factors led to a spatialand temporal heterogeneity of the res-ervoir particularly during the first fillingand the beginning of the operationphase. A clear longitudinal gradient wasobserved in 2009 and 2010. The RES3sampling site is isolated from the mainbody of the reservoir and was protectedfrom floods. The upstream samplingsites (RES6 and RES8) are clearlyunder the influence of the hydrologycompared to the most downstream sta-tions which are influenced by the mixingperiod, temperature and Chl a. No spa-tial difference was observed after 2011.This is in accordance with the study ofBonecker et al. (2007) that found nospatial variation of zooplankton bio-mass in relatively young Brazilian res-ervoirs (5, 12 and 40 years old). Thetheoretical model of Marzolf (1990) thatpredict higher zooplanktonic density


toward the dam seems to be valid onlyfor the first two years. This means thatthe influence of the current velocity waseffective along the reservoir (lower cur-rent velocity at the dam site) during thisperiod. Afterwards, other factors liketemperature or mixing of the water col-umn led to a homogenization of the hab-itat conditions. Bini et al. (1997) foundsimilar results on limnological parame-ters. For instance, DO, pH and temper-ature did not explain longitudinal heter-ogeneity into their reservoir because ofthe low variation of these parametersalong the axis. During the study period,no dramatic oxygen depletion eventsappeared in the NT2 Reservoir alsominimizing the influence of the DO.Finally, no significant relation has beenobserved between the zooplanktoncommunity and nutrients. This can bedue to low nutrient concentrations evenduring the peaks of zooplanktonic pro-duction. Nevertheless, densities andbiomasses were correlated to Chl a witha high seasonal effect underlying thedominance of phytophage within thezooplantktonic community and a phy-toplankton-based food chain in theNT2 Reservoir. As in these systemsmost of the energy transfer couldcome from both the phytoplanktonand bacterioplankton (Havens, 2002),monitoring of bacterioplankton in theNT2 Reservoir may be of interest. Therelationship of zooplankton communitydynamics with fish pressure was notassessed in this study, but the rate ofpredation may influence zooplanktondensity and biomass. This could be par-ticularly important during the beginningof the rainy season when zooplanktivo-rous fish focus on reproduction and

decrease the predation pressure onRotifera and Cladocera populations.

5 CONCLUSION

The filling of the NT2 Reservoir ledto a change in the zooplanktonic com-munity which is linked to the appear-ance of a thin oxygenated epilimnionand a large anoxic hypolimnion. Thecommunity was dominated by Copep-ods (including the Naupliar stage) overthe study period. The spatial heteroge-neity of the zooplanktonic communityobserved along a longitudinal gradientdisappeared rapidly after the beginningof the operation phase and tend to sta-bilize. Nevertheless, the results high-lighted a seasonal effect explained by aphytoplankton-based food chain. Thezooplankton community of the NT2Reservoir had low densities and bio-masses compared to other tropical res-ervoirs and reflected oligotrophic condi-tions. They were significantly andnegatively driven by the hydraulic fac-tors like mixing period, temperature ofthe water or depth of the oxycline.Finally, the zooplankton communitymust be evaluated in order to anticipatethe food availability for zooplanktivo-rous fish before developing a fish man-agement plan.

ACKNOWLEDGEMENT

This research was conducted at theAquatic Environment Laboratory ofNam Theun 2 Power Company in LaoPDR whose Shareholders are Électric-ité de France, Lao Holding State Enter-prise and Electricity Generating PublicCompany Limited of Thailand.


We would like to thank all the NTPCteams for their support during this mon-itoring program with a special thanks tothe hydrobiology team, especially MsSadavee S.Phabmixay, and chemistryteam from the WQB Dept for their sup-port in chemistry analysis. We wouldlike also to thank Mr Liankham Payas-ane (GIS team), the logistic team withtheir useful boat drivers and Ms LeahBêche who reviewed and improved thisversion of the manuscript as a nativeEnglish speaker.

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Supplementary Material 1

Comparison of average densities and biomasses oKruskal-Wallis test. Values in bold and grey are sig

Density

RES1 2009 2010 2011 2012 201

2009 1

2010 0.345 1

2011 0.020 0.001 1

2012 0.336 0.051 0.161 1

2013 0.115 0.010 0.440 0.529

RES3

2009 1

2010 0.129 1

2011 0.957 0.071 1

2012 0.694 0.029 0.692 1

2013 0.727 0.033 0.731 0.958

RES4

2009 1

2010 0.021 1

2011 0.603 0.004 1

2012 0.933 0.022 0.537 1

2013 0.294 0.195 0.109 0.324

RES6

2009 1

2010 0.280 1

2011 0.291 0.029 1

2012 0.889 0.336 0.222 1

2013 0.199 0.038 0.593 0.162

RES8

2009 1

2010 0.703 1

2011 0.045 0.005 1

2012 0.303 0.100 0.257 1

2013 0.224 0.063 0.359 0.829

Average value

2009 1

2010 0.151 1

2011 0.144 0.003 1

2012 0.439 0.024 0.483 1

2013 0.343 0.015 0.601 0.859

f each sampling site among years. P-values fromnificantly different.

Biomass

3 2009 2010 2011 2012 2013

1

0.308 1

0.012 0.000 1

0.288 0.033 0.134 1

1 0.268 0.030 0.147 0.962 1

1

0.095 1

0.852 0.033 1

0.219 0.001 0.224 1

1 0.424 0.005 0.475 0.615 1

1

0.074 1

0.145 0.001 1

0.420 0.008 0.506 1

1 0.361 0.373 0.015 0.079 1

1

0.399 1

0.058 0.005 1

0.646 0.183 0.142 1

1 0.152 0.040 0.938 0.264 1

1

0.878 1

0.006 0.001 1

0.068 0.021 0.273 1

1 0.115 0.044 0.165 0.769 1

1

0.232 1

0.041 0.001 1

0.035 0.001 0.953 1

1 0.200 0.011 0.433 0.399 1


Comparison of average densities and biomasses of each year site among sampling sites. P-valuesfrom Kruskal-Wallis test. Values in bold and grey are significantly different.

Density Biomass

2009 RES1 RES3 RES4 RES6 RES8 RES1 RES3 RES4 RES6 RES8

RES1 1 1

RES3 0.526 1 0.555 1

RES4 0.096 0.036 1 0.327 0.146 1

RES6 0.270 0.108 0.576 1 0.524 0.249 0.732 1

RES8 0.837 0.447 0.207 0.443 1 0.585 0.969 0.158 0.268 1

2010

RES1 1 1

RES3 0.272 1 0.146 1

RES4 0.196 0.020 1 0.384 0.022 1

RES6 0.209 0.022 0.971 1 0.157 0.005 0.586 1

RES8 0.165 0.015 0.923 0.894 1 0.425 0.027 0.942 0.538 1

2011

RES1 1 1

RES3 0.060 1 0.061 1

RES4 0.674 0.144 1 0.888 0.084 1

RES6 0.870 0.086 0.797 1 0.824 0.036 0.717 1

RES8 0.898 0.044 0.583 0.770 1 0.700 0.024 0.599 0.870 1

2012

RES1 1 1

RES3 0.157 1 0.852 1

RES4 0.674 0.067 1 0.368 0.277 1

RES6 0.632 0.350 0.368 1 0.981 0.870 0.356 1

RES8 0.852 0.109 0.815 0.505 1 0.674 0.543 0.632 0.657 1

2013

RES1 1 1

RES3 0.071 1 0.346 1

RES4 0.191 0.618 1 0.165 0.657 1

RES6 0.536 0.058 0.123 1 0.463 0.161 0.086 1

RES8 0.968 0.078 0.205 0.517 1 0.861 0.264 0.118 0.542 1

1 Introduction2 Material and methods2.1 Sampling strategy2.2 Laboratory analysis2.3 Data analysis

3 Results3.1 Water quality in the Nam Theun 2 Reservoir3.2 Zooplankton population dynamics3.3 Environmental driving factors

4 Discussion4.1 Zooplankton community dynamics in the NT2 Reservoir4.2 Environmental parameters influencing zooplankton communities

5 ConclusionAcknowledgementReferences

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