Egyptian Journal of Aquatic Research (2015) 41, 177–185

HO ST E D BYNational Institute of Oceanography and Fisheries

Egyptian Journal of Aquatic Research

http://ees.elsevier.com/ejarwww.sciencedirect.com

FULL LENGTH ARTICLE

Vertical distribution of zooplankton in Lake Nasser

* Corresponding author.

E-mail address: nehadkhalifa@hotmail.com (N. Khalifa).

Peer review under responsibility of National Institute of Oceanography

and Fisheries.

http://dx.doi.org/10.1016/j.ejar.2015.03.0021687-4285 ª 2015 National Institute of Oceanography and Fisheries. Hosting by Elsevier B.V.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Nehad Khalifa a,*, Khaled A. El-Damhogy b, M. Reda Fishar a, Amr M. Nasef b,

Mahmoud H. Hegab a

a National Institute of Oceanography and Fisheries, 101 Kasr El Aini Street, Cairo, Egyptb Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt

Received 13 January 2015; revised 1 March 2015; accepted 1 March 2015Available online 23 April 2015

KEYWORDS

Lake Nasser;

Vertical distribution;

Zooplankton;

Copepoda

Abstract The composition and distribution of zooplankton communities in three depths (surface,

10–5 m and 20–15 m depths) along main channel of Lake Nasser were studied in 2013. The density

of total zooplankton was increased to maximum during winter and autumn at surface water

(39,362 and 63,100 Ind. m�3, respectively) and gradually decreased with depth until attaining the low-

est average density at 20–15 m (12,460 and 8976 Ind. m�3). During spring and summer, zooplankton

was irregularly distributed through the water profile, where the highest average density was recorded

at 10–5 m depth (66,007 and 66,734 Ind. m�3). Copepoda was the dominant zooplankton group at all

depths, it represented about 70–76.2% of the total zooplankton count. Cladocera formed about

13.4%, 14.5% and 11% of total zooplankton density for surface, 10–5 m and 20–15 m depth. It

was decreasedwith increasing depth duringwinter and autumn; however it attained itsmaximumden-

sity at 10–5 m depth during spring and summer. Rotifera average density decreased with increasing

depth. The dominant zooplankton species inhabiting Lake Nasser were strongly temperature-depen-

dent. The study recommends the introduction of some pelagic fish species to consume the high persis-

tence of zooplankton community at the upper 10 meters of water column.ª 2015 National Institute of Oceanography and Fisheries. Hosting by Elsevier B.V. This is an open access

article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Introduction

The construction of the Aswan High Dam in southern Egypt

resulted in the creation of the longest man-made lake in theworld. The major portion of this lake is extending in Egyptfor about 300 km and is known as Lake Nasser and for

180 km further south in Sudan as Lake Nubia. Lake Nasserextends between 22� 31to 23� 45N and 31� 30 to 33� 15 Eand filled during late 1970s (El-Shabrawy, 2009).

The role of zooplankton not only regulates the aquatic pro-ductivity by occupying intermediate position in the food chain,

but also by indicating environmental status in a given time(Xie et al., 2008). Many studies dealt with the seasonal varia-tion and surface distribution of zooplankton in Lake Nasser(Gaber, 1982; Zaghloul, 1985; Iskaros, 1993; Mohamed,

1993; Taha and Mageed, 2002; Mokhtar, 2003; El-Shabrawyand Dumont, 2003; Mageed and Heikal, 2005; Ali et al.,2007; El-Enany, 2009; El-Serafy et al., 2009). While the vertical

distribution of zooplankton was concerned by Samaan (1971)who estimated the standing crop in the upper 10 meters duringMarch 1970 from three localities. Samaan and Gaber (1976)

studied the plankton population of Lake Nasser at ten meterwater depth during March and August. Mageed (1995),

178 N. Khalifa et al.

El-Shabrawy (2000), Abdel Mola (2012) compared betweenthe distributions of zooplankton at different water levels ofthe upper ten meters of the lake. The concentration of

zooplankters in the water column (0–20 m) was variablethroughout the year at six sampling sites from El-Ramla inthe north to Abu Simbel in the south along the main channel

of Lake Nasser (Habib, 2000).The vertical position of different zooplankton groups was

explained by specific combinations of biotic and abiotic fac-

tors. Even though the vertical position of individual zooplank-ton groups was quite variable, the study of Wissel andRamacharan (2003) found consistent patterns among lakes,sampling dates, and zooplankton groups. Also, the relative

vertical positions of groups of different body size suggestedthat avoiding predation by planktivorous fish was the majorconstraint that shaped this behavior.

The objective of the present study is to investigate the tem-poral and spatial distribution patterns of zooplankton organ-isms at different water levels in Lake Nasser.

Material and methods

Sampling program

Seasonal samples were collected at nine sites in Lake Nasser

(Fig. 1) from February 2013 to November 2013. Surface watersamples were collected to estimate temperature, pH, trans-parency and dissolved oxygen by using the standard methods

of APHA (1995).

Figure 1 Lake Nasser map showing the selected study sites.

Table 1 Ranges and averages of physico-chemical parameters in L

Winter

Temperature (�C) Range (average) 18.1–21.5 (19.6)

pH Range (average) 8.2–9.01 (8.6)

Trans. (cm) Range (average) 180–430 (320)

DO (mg/l) Range (average) 5–8.1 (6.5)

Zooplankton analysis

For zooplankton quantitative analysis, three zooplanktonsamples were collected from each site, one from the surfaceand the second from 10 to 5 m depth, while a third sample

from 20 to 15 m depth. Thirty liters were taken from surfacewater at each sampling site by filtering through a zooplanktonnet of 55 lm mesh diameter and vertical samples were col-lected by vertical tows. Collected samples were kept in plastic

bottles with some lake water to which 4% formalin was addedas a preservative. Samples were studied under the compoundmicroscope and specimens identified at the species level when

possible. Zooplankton numbers were expressed as number oforganisms per cubic meter. Many publications and taxonomicreferences were used for zooplankton identification

(Edmondson, 1963; Bick, 1972; Ruttner-Kolisko, 1974;Koste, 1978; Pontin, 1978; Dodson and Frey, 1991; Husseinet al., 1991; Einsle, 1996; Foissner and Berger, 1996).

Data analysis

Shannon-Wiener diversity, species richness, evenness, Simpsondiversity and similarity index were calculated using the Primer

5 program. The correlation coefficient was done using theSPSS program version 16 between species of zooplanktonand the different environmental variables. A two-way analysis

of variance was calculated to find out the significance of thedifferences in density of the zooplankton groups among differ-ent sites at the three studied depths of the main channel.

Results

The ranges and averages of physico-chemical characteristics of

the lake water at different seasons are shown in Table 1.Zooplankton in Lake Nasser comprised Copepoda,

Cladocera and Rotifera and Protozoa, while meroplanktonic

organisms were rarely recorded. Four copepods, seven clado-cerans, thirty rotifers and five protozoans were detected fromthe three depths (Table 2).

The seasonal average density of zooplankton was nearly

equal at both surface and 10–5 m depth (50,789 and49,114 Ind. m�3) and dropped to the least value of14,527 Ind. m�3 at 20–15 m depth. During winter and autumn,

zooplankton density decreased with depth and maximumaverages of 39,362 and 63,100 Ind. m�3 at surface water, anddecreased to the least averages densities of 12,460 and

8976 Ind. m�3 at 20–15 m depth. During spring and summer,the highest average density of zooplankton was detected at10–5 m depth (66,007 and 66,734 Ind. m�3), and it decreasedto the lowest density of 23,651 and 13,021 Ind. m�3 at 20–

15 m depth, respectively (Fig. 2).

ake Nasser.

Spring Summer Autumn

23.7–33.9 (28.3) 27–32.4 (29.6) 22.7–26.8 (24.4)

7.9–8.6 (8.4) 7.1–8.8 (8.1) 7.1–8.5 (8.1)

190–300 (230) 150–525 (271) 150–500 (350)

3.5–7.6 (5.7) 2.9–6.9 (4.7) 5.2–6.8 (6.6)

Table 2 The occurrence of different zooplankton taxa at different water depths in Lake Nasser.

Surface 10–5 m 20–15 m

Protozoa

Acropisthium mutabile � + �Arcella dentata + � �Centropyxis aculeata + + �Epistylis sp. + + +

Vorticella campanula + + +

Rotifera

Asplanchna girodi + + +

Ascomorpha ecaudis + + +

Brachionus patulus + + +

B. calyciflorus + + +

B. falcatus + + +

B. caudatus + + +

B. plicatilis + + +

Conochilus hippocrepis + + +

Conochiloides sp. + + �Collotheca sp. + + +

Cephalodella catalina + + �Euchlanis dilatata + + �Epiphanus sp. � + +

Filina longiseta � + +

F. opoliensis + + +

Hexarthra mira + + +

Keratella cochlearis + + +

K. tropica + + +

Lecan luna + + �L. bulla + + +

L. closterocerca � � +

Lepadella ovalis � + �Mytilina ventralis + � +

Philodena sp. + + �Pompholyx complanata � + �Synchaeta oblonga + + �Trichocerca longiseta + + +

T. similis + + +

Trichocerca Sp. + + +

Trichotria tetractis + � �

Copepoda

Nauplius larvae + + +

Cyclopoid copepodite + + +

Calanoid copepodite + + +

Mesocyclops ogunnus + + +

Thermocyclops neglectus + + +

Thermodiaptomus galebi + + +

Harpacticoida + + �

Cladocera

Alona sp. + + +

Bosmina longirostris + + +

Chydorus sphaericus + + +

Ceriodaphnia dubia + + +

Diaphanosoma mongolianum + + +

Daphnia longispina + + +

Macrothrix spinosa � + �

Meroplankton

Annelid larvae + + �Ostrachoda sp. + + �Microdalyellia sp. � + +

Free-Living Nematodes � + +

Chironomus larvae + � �

Vertical distribution of zooplankton in Lake Nasser 179

Figure 2 Seasonal distributions of total zooplankton and total Copepoda with its main forms at different water levels in Lake Nasser.

180 N. Khalifa et al.

Copepods were abundant throughout the year; it wasmainly represented by the calanoid Thermodiaptomus galebi,in addition to the cyclopoids Thermocyclops neglectus and

Mesocyclops ogunnus. Copepoda was highly abundant at 10–5 m depth during the study except during autumn where itflourished at surface water (Fig. 2). Copepoda presented meandensities of 35,500 and 36,200 Ind. m�3 at the surface and 10–

5 m depth respectively and it decreased to 11,100 Ind. m�3 at

20–15 m depth. Nauplius larvae were the main bulk ofCopepoda at all depths of main channel, its density variedfrom 64% to 70% of total Copepoda at the three depths with

the highest average of 23,070 Ind. m�3 at 10–5 m. These larvaedecreased vertically with increasing depth during autumn,while attained its highest crop at 10–5 m depth in other peri-ods. Copepodites decreased with increasing depth during win-

ter and autumn and attained high crop at 10–5 m depth during

Vertical distribution of zooplankton in Lake Nasser 181

summer and autumn (Fig. 2). Thermodiaptomus galebi waspeaked during spring with its highest density average of9300 Ind. m�3 at 10–5 m depth. Thermocyclops neglectus was

flourished during summer at 10–5 m depth (Fig. 2), contraryto Mesocyclops ogunnus which increased during winter at thesame depth.

Figure 3 Seasonal distributions of total Cladocera and i

Cladocera ranged from 11% to 14.5% of total zooplanktondensity at different water levels during the study. It attained itshighest average density of 7000 Ind. m�3 at 10–5 m and the

lowest of 1600 Ind. m�3 at 20–15 m depth. Its crop decreasedwith increasing depth during winter and autumn and attainedits maximum density at 10–5 m depth during spring and

ts main taxa at different water levels in Lake Nasser.

182 N. Khalifa et al.

summer (Fig. 3). Diaphanosoma mongolianum was the mostdominant species at the surface and 10–5 m depth with averagecrop of 2750 and 2340 Ind. m�3. Ceriodaphnia dubia was the

main cladoceran at 20–15 m depth with average density of2219 Ind. m�3. Bosmina longirostris average density variedfrom a minimum of 290 Ind. m�3 at 20–15 m depth to a maxi-

mum of 1560 Ind. m�3 at surface water. Its highest standingcrop was harvested from surface water during winter andautumn and from 10–5 m depth during summer and spring.

Daphnia longspina and Chydorus sphaericus were high in coldseasons especially at the surface (Fig. 3).

Rotiferan’s average density decreased with increasingdepth, it attained the highest averages of 5742 Ind. m�3 at

the surface and it decreased to 4019 Ind. m�3 at 10–5 m andthe least count of 1341 Ind. m�3 at 20–15 m. While its percent-age to the total zooplankton increases with depth, it formed

about 11.3% at surface water and it increased to form about21% of the total zooplankton count at 20–15 m depth. The dis-tribution of Rotifera at different water levels was very obvious

during autumn and winter, where the surface water recordedthe highest averages of 6800 and 4362 Ind. m�3, respectivelyand it gradually decreased with increasing depth. During sum-

mer and spring, no remarkable variation was noted betweenrotifer distributions at the upper 10 meters, however it

Figure 4 Seasonal distributions of total Rotifera and it

decreased to its minimum average (1495 & 1970 Ind. m�3) at20–15 m depth (Fig. 4). Keratella, Collotheca and Brachionuswere the most abundant genera of rotifers. K. cochlearis was

flourished at surface samples during winter with average den-sity of 3239 Ind. m�3 and K. tropica was more frequent duringsummer at surface samples (average 3645 Ind. m�3). The dis-

tribution of Collotheca sp. at different water levels was notice-able during autumn, where its highest average density(3190 Ind. m�3) was recorded at the surface then decreased

to 870 Ind. m�3 at 10–5 m and attained its lowest averagecount of 325 Ind. m�3 at 20–15 m depth. Spring was the mostproductive season for Collotheca sp. with the highest averagedensity of 3710 Ind. m�3 at surface water.

Brachionus patulus dominated genus Brachionus and flour-ished during summer at the surface with the highest averagedensity of 895 Ind. m�3.

Protozoa was frequently recorded during this study, its den-sity decreased with increasing depth, its average density was2500 Ind. m�3 at surface water and decreased to

506 Ind. m�3 at 20–15 m depth. Protozoa was represented by5 species belonging to Ciliophora and Rhizopoda (Table 2).Meroplankton was very rare and occurred with the highest

annual average (190 Ind. m�3) at surface water and decreasedwith the depth.

s main taxa at different water levels in Lake Nasser.

No.

ofspecies

Den

sity

(Ind.10

3 m-3)

AS N

50 70

40 6050

30 4020 30

2010 10

0 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

surface 10-5m 20-15m

B d J H′4

3

2

1

0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

surface 10-5m 20-15m

Figure 5 Diversity indices of zooplankton community in Lake Nasser. (A) S = number species and N= total density of zooplankton.

(B) d= species richness, J= evenness, H0 = Wiener index of diversity.

Vertical distribution of zooplankton in Lake Nasser 183

Zooplankton community analysis

Zooplankton population density and number of species

detected in the study fluctuated at sites of surface water andat 10–5 m depth and it decreased with depth to the least countsat 20–15 m depth (Fig. 5A). The highest diversity index was

2.238 at surface water of site 8 and the lowest of 1.446 wasdetected at 20–15 m of site 5 (Fig. 5B). The species richnessvalue was the highest (3.775) at 10–5 m depth of site 1 whileits lowest one (1.863) was detected at 20–15 m depth of site

6. The evenness values of diversity were ranged from 0.4678to 0.6954 (Fig. 5B).

ANOVA (2-way)

Analysis of variance was performed to assess the significantdifference of zooplankton faunal abundance existing in rela-

tion to sampling sites at different depths (Table 3).Insignificant difference was obtained between surface sitesand 10–5 m depth according to abundance of zooplankton

and its group except with total Rotifera. While, there are sig-nificant difference between sites of the surface and 20–15 mdepth, also between 10–5 m and 20–15 m depth.

Table 3 The analysis of variance (ANOVA) partitioning the total

depths in Lake Nasser.

Depths T. zooplankton T. Copepoda

F P F P

Surface · 10–5 m depth sites 0.87 0.44 0.32 0.

Surface · 20–15 m depth sites 110.6 0.008 100.4 0.

10–5 m · 20–15 m depth sites 87.86 0.011 76.17 0.

Discussion

Zooplankton is a major component of aquatic ecosystem andchanges in their abundance, species composition and seasonalvariation have cascading effects at higher trophic levels. The

zooplankton population in Lake Nasser is mainly representedby typical limnoplankton forms, dominated by Copepoda(4 species), Cladocera (7 species) and Rotifera (30 species) in

addition to rare forms of Protozoa and meroplankton.Samaan (1971) detected 15 species (9 Crustacea and 6Rotifera); Rzoska (1976) recorded 13 species (mainly

Crustacea); and Samaan and Gaber (1976) recorded 15 species(9 Crustacea and 6 Rotifera). At different sampling sites duringthe study, zooplankton at the three studied depths attained thehighest average during spring and summer (48,135 and

41,910 Ind. m�3), while the least frequency prevailed duringwinter (28,250 Ind. m�3). Zaghloul (1985) mentioned that mostof zooplankton species inhabiting Lake Nasser are warm water

eurythermic forms which can tolerate a wide range of tempera-ture, so the maximum persistence of dominant species occurredduring spring and summer when water temperature was over

25 �C. El-Shabrawy (2000) recorded zooplankton peak duringspring (88,000 Ind. m�3), and dropped during winter to

variance in zooplankton and its main groups at different water

T. Cladocera T. Rotifera T. Protozoa

F P F P F P

63 0.13 0.75 38.8 0.025 3.34 0.209

009 62 0.016 49.2 0.02 12.08 0.073

012 64.7 0.015 53.7 0.018 90.29 0.01

184 N. Khalifa et al.

32,000 Ind. m�3. Iskaros et al. (2008) mentioned that, zoo-plankton populations in Lake Nasser were more abundant dur-ing spring. Studies conducted since 1971 have shown that the

standing stock of zooplankton increased during the first 10–15years after damming and has since stabilized (FAO, 2011).

Zooplankton was more abundant in the upper 10 meter, its

density nearly equal at both surface and 10–5 m depth anddropped to the least value at 20–15 m depth during the study.The most zooplankton density and biomass reside in the upper

ten meters (euphotic layer) of Lake Nasser (El-Shabrawy,2000; El-Shabrawy and Dumont, 2003). The vertical dis-tribution of different zooplankton species in euphotic zone(from the surface to 20 m) of the main channel of the lake

and its relation with thermocline which takes place in LakeNasser during summer season was discussed by Abdel Mola(2012) and recorded that the highest density and number of

zooplankton species were between 2 and 5 meters. Theabundance of zooplankton in the upper water of LakeNasser may be due to flourishing of phytoplankton. The

highest values of Chlorophyll a concentrations were recordedat the upper layers as a result of photosynthetic activity ofphytoplankton (Habib, 1992) and also a high positive

correlation was found between phytoplankton andzooplankton population density in main channel of LakeNasser (Taha and Mageed, 2002). The phytoplankton abun-dance and biomass showed pronounced high occurrence in

the euphotic zone than in the non-euphotic layer in the mainchannel of Lake Nasser during the highest flood season duringautumn 1999 (Gharib and Abdel-Halim, 2006).

The highest occurrence of Copepoda and Cladocera wasdetected at the surface and 10–5 m depth. While Rotiferaand Protozoa attained its highest average density at surface

water and its density decreased with increasing depth. Theanalysis of variance which performed to assess the significantdifference of total zooplankton and its main groups existing

in relation to different depths found insignificant differencebetween surface sites and 10–5 m depth except with Rotifera(F = 38.8, P = 0.025). While, there are significant differencebetween sites of the surface and 20–15 m depth, also between

10–5 m and 20–15 m depths. Thus samples from the surfaceand 10–5 m water levels are more similar in species com-position to each other than to other sampling sites.

In the present study, Copepoda was the most dominantzooplankton group, although it attained the least species diver-sity. Copepoda was more abundant at 10–5 m depth during

spring and summer, they may migrate to this depth to avoidhigh temperature and light intensity during these periods.The low diversity in copepods and cladocerans in the mainchannel of Lake Nasser may be due to high temperature

(El-Shabrawy, 2000). Nauplius larvae were the mainCopepoda form at all depths of main channel, its densityranged from 63.8% to 70.3% of the copepods. Iskaros et al.

(2008) and El-Enany (2009) refereed the dominance ofCopepoda in Lake Nasser to the abundance of nauplius larvaewhich mainly feed on phytoplankton. Nauplius larvae

increased at surface water during winter and autumn andflourished at 10–5 m depth during spring and summer. Amoderate positive relationship was detected between copepod

nauplii and surface water temperature (r= 0.634) and a nega-tive weak correlation with water transparency (r= �0.351).Adult forms of copepoda were higher at 10–5 m depth whichmay be due to that they can freely swim in water column.

Ali et al. (2007) mentioned that Copepoda are good swimmerscompared to the other groups. A positive relationship wasrevealed between surface water temperature and total

copepods (r = 0.465) and a high negative one was detectedwithM. ogunnus (r = �0.985). Regarding the relation betweenwater transparency and copepods, there was a moderate nega-

tive correlation with nauplius larvae (r= �0.351), while pHwas positively correlated with T. galebi and M. ogunnus(r = 0.475 and 0.656) and appeared negative correlation with

T. neglectus (r = �0.768).Cladocera attained its lowest density of 1600 Ind. m�3 at

20–15 m depth, it seems that the vertical distribution ofCladocera were accompanied with high values of dissolved

oxygen as estimated by Zaghloul (1985), who illustrated thatthe limnoplanktonic forms of Cladocera require high oxygenconcentration. El-Shabrawy (2009) recorded the highest den-

sity of Cladocera between 15 and 20 m in Lake Nasser, andalmost all Cladocera disappeared below 20–15 m during sum-mer, due to a lack in oxygen. Cladocera increased during

spring and summer seasons due to flourishing ofDiaphanosoma mongolianum. Temperature seems to be one ofthe main factors controlling the occurrence of D. mongolianum

which is supported by its high positive correlation with surfacewater temperature (r = 0.866). Both D. mongolianum andD. brachyurum are characteristic species of warm waters, andare preponderant elements of the summer zooplankton

communities of Spanish reservoirs (Jaume, 1991). Daphnialongspina and Chydorus sphaericus were higher during coldseasons especially at the surface and showed a high negative

correlation with temperature (r = �0.907 and �0.851).Rotifera was more abundant at surface water samples and

all of its dominant species detected at 10–5 m depth and to

the least extent at 20–15 m depth. These differences weresignificant (P = 0.25, P = 0.02 and P = 0.018, ANOVA) forrotifers between the three depths and the density of rotifers

decreased with increasing depth. The vertical distribution ofzooplankton in Lake Nasser during spring 1997 revealed thatmembers of rotifers including K. cochlearis, Proalides sp. andSynchaeta oblonga migrate over large amplitudes of depth

(El-Shabrawy, 2000). Rotifera attained its highest density dur-ing spring and summer due to flourishing of Brachionus speciesand Keratella tropica which have the ability to tolerate high

temperature. Keratella tropica is a characteristic species oftropical water and B. caudatus and B. angularis are warmstenothermal forms (Winner, 1975). Abdel Mola (2012) found

Rotifera at 0–2 m in Lake Nasser during summer formed72% of total zooplankton due to the abundance of B. calcifloris,K. cochlearis and B. patulus which can tolerate the warm water inthis layer and recorded a high positive correlation with water tem-

perature. During the present study surface water temperature waspositively correlated with total Rotifera, B. patulus, K. tropica,collotheca sp. and A. girodi. The top temperatures in Lake

Nasser are tropical, whereas the minima remain high enough toallow zooplankton production to proceed, albeit at a slower pace(El-Shabrawy and Dumont, 2003). The vertical distribution of

zooplankton is dependent on environmental parameters whichinclude abiotic factors such as temperature, light intensity anddissolved oxygen, in addition to biotic factors including food

resources and avoids predations (Lampert, 1989).The study concluded that the dominant zooplankton spe-

cies inhabiting Lake Nasser were strongly temperature-depen-dent. Since the early filling of Lake Nasser extensive and

Vertical distribution of zooplankton in Lake Nasser 185

comprehensive studies was carried out on the various aspectsof the Lake by many researchers and authorities. However,these studies are inadequate and more studies needed due to

the importance of the lake as a main reservoir for water Nilein Egypt. Future studies recommend detecting the expectedchanges in Lake Nasser ecosystem as a result of the construc-

tion of the Renaissance Dam in Ethiopia. Also, the studyrecommends the introduction of some pelagic fish species toconsume the high persistence of zooplankton community at

the upper 10 meters of water column.

Acknowledgment

The authors are most grateful to Inland Water andAquaculture Branch, National Institute of Oceanographyand Fisheries, Cairo, Egypt which funded this work.

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