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Vertical distribution of zooplankton in Lake Nasser
Egyptian Journal of Aquatic Research (2015) 41, 177–185
H O S T E D BY
National Institute of Oceanography and Fisheries
Egyptian Journal of Aquatic Research http://ees.elsevier.com/ejar www.sciencedirect.com
FULL LENGTH ARTICLE
Vertical distribution of zooplankton in Lake Nasser Nehad Khalifa a,*, Khaled A. El-Damhogy b, M. Reda Fishar a, Amr M. Nasef b, Mahmoud H. Hegab a a b
National Institute of Oceanography and Fisheries, 101 Kasr El Aini Street, Cairo, Egypt Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt
Received 13 January 2015; revised 1 March 2015; accepted 1 March 2015 Available 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. m3, respectively) and gradually decreased with depth until attaining the lowest average density at 20–15 m (12,460 and 8976 Ind. m3). 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. m3). 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 decreased with increasing depth during winter and autumn; however it attained its maximum density 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-dependent. The study recommends the introduction of some pelagic fish species to consume the high persistence 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 the world. The major portion of this lake is extending in Egypt for about 300 km and is known as Lake Nasser and for 180 km further south in Sudan as Lake Nubia. Lake Nasser extends between 22 31´to 23 45´N and 31 30´ to 33 15´ E and filled during late 1970s (El-Shabrawy, 2009). * Corresponding author. E-mail address: [email protected] (N. Khalifa). Peer review under responsibility of National Institute of Oceanography and Fisheries.
The role of zooplankton not only regulates the aquatic productivity 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 variation and surface distribution of zooplankton in Lake Nasser (Gaber, 1982; Zaghloul, 1985; Iskaros, 1993; Mohamed, 1993; Taha and Mageed, 2002; Mokhtar, 2003; El-Shabrawy and 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 during March 1970 from three localities. Samaan and Gaber (1976) studied the plankton population of Lake Nasser at ten meter water depth during March and August. Mageed (1995),
http://dx.doi.org/10.1016/j.ejar.2015.03.002 1687-4285 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/).
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El-Shabrawy (2000), Abdel Mola (2012) compared between the distributions of zooplankton at different water levels of the upper ten meters of the lake. The concentration of zooplankters in the water column (0–20 m) was variable throughout the year at six sampling sites from El-Ramla in the 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 factors. Even though the vertical position of individual zooplankton groups was quite variable, the study of Wissel and Ramacharan (2003) found consistent patterns among lakes, sampling dates, and zooplankton groups. Also, the relative vertical positions of groups of different body size suggested that avoiding predation by planktivorous fish was the major constraint that shaped this behavior. The objective of the present study is to investigate the temporal and spatial distribution patterns of zooplankton organisms at different water levels in Lake Nasser.
Zooplankton analysis For zooplankton quantitative analysis, three zooplankton samples were collected from each site, one from the surface and the second from 10 to 5 m depth, while a third sample from 20 to 15 m depth. Thirty liters were taken from surface water at each sampling site by filtering through a zooplankton net of 55 lm mesh diameter and vertical samples were collected by vertical tows. Collected samples were kept in plastic bottles with some lake water to which 4% formalin was added as a preservative. Samples were studied under the compound microscope and specimens identified at the species level when possible. Zooplankton numbers were expressed as number of organisms per cubic meter. Many publications and taxonomic references were used for zooplankton identification (Edmondson, 1963; Bick, 1972; Ruttner-Kolisko, 1974; Koste, 1978; Pontin, 1978; Dodson and Frey, 1991; Hussein et al., 1991; Einsle, 1996; Foissner and Berger, 1996). Data analysis
Material and methods Sampling program Seasonal samples were collected at nine sites in Lake Nasser (Fig. 1) from February 2013 to November 2013. Surface water samples were collected to estimate temperature, pH, transparency and dissolved oxygen by using the standard methods of APHA (1995).
Shannon-Wiener diversity, species richness, evenness, Simpson diversity and similarity index were calculated using the Primer 5 program. The correlation coefficient was done using the SPSS program version 16 between species of zooplankton and the different environmental variables. A two-way analysis of variance was calculated to find out the significance of the differences in density of the zooplankton groups among different sites at the three studied depths of the main channel. Results
Figure 1
Table 1
Lake Nasser map showing the selected study sites.
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 cladocerans, thirty rotifers and five protozoans were detected from the three depths (Table 2). The seasonal average density of zooplankton was nearly equal at both surface and 10–5 m depth (50,789 and 49,114 Ind. m3) and dropped to the least value of 14,527 Ind. m3 at 20–15 m depth. During winter and autumn, zooplankton density decreased with depth and maximum averages of 39,362 and 63,100 Ind. m3 at surface water, and decreased to the least averages densities of 12,460 and 8976 Ind. m3 at 20–15 m depth. During spring and summer, the highest average density of zooplankton was detected at 10–5 m depth (66,007 and 66,734 Ind. m3), and it decreased to the lowest density of 23,651 and 13,021 Ind. m3 at 20– 15 m depth, respectively (Fig. 2).
Ranges and averages of physico-chemical parameters in Lake Nasser.
Temperature (C) pH Trans. (cm) DO (mg/l)
Range Range Range Range
(average) (average) (average) (average)
Winter
Spring
Summer
Autumn
18.1–21.5 (19.6) 8.2–9.01 (8.6) 180–430 (320) 5–8.1 (6.5)
23.7–33.9 (28.3) 7.9–8.6 (8.4) 190–300 (230) 3.5–7.6 (5.7)
27–32.4 (29.6) 7.1–8.8 (8.1) 150–525 (271) 2.9–6.9 (4.7)
22.7–26.8 (24.4) 7.1–8.5 (8.1) 150–500 (350) 5.2–6.8 (6.6)
Vertical distribution of zooplankton in Lake Nasser Table 2
179
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
+ + +
+ + + +
+ +
180
Figure 2
N. Khalifa et al.
Seasonal distributions of total zooplankton and total Copepoda with its main forms at different water levels in Lake Nasser.
Copepods were abundant throughout the year; it was mainly 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 it flourished at surface water (Fig. 2). Copepoda presented mean densities of 35,500 and 36,200 Ind. m3 at the surface and 10– 5 m depth respectively and it decreased to 11,100 Ind. m3 at
20–15 m depth. Nauplius larvae were the main bulk of Copepoda at all depths of main channel, its density varied from 64% to 70% of total Copepoda at the three depths with the highest average of 23,070 Ind. m3 at 10–5 m. These larvae decreased vertically with increasing depth during autumn, while attained its highest crop at 10–5 m depth in other periods. Copepodites decreased with increasing depth during winter and autumn and attained high crop at 10–5 m depth during
Vertical distribution of zooplankton in Lake Nasser summer and autumn (Fig. 2). Thermodiaptomus galebi was peaked during spring with its highest density average of 9300 Ind. m3 at 10–5 m depth. Thermocyclops neglectus was flourished during summer at 10–5 m depth (Fig. 2), contrary to Mesocyclops ogunnus which increased during winter at the same depth.
Figure 3
181 Cladocera ranged from 11% to 14.5% of total zooplankton density at different water levels during the study. It attained its highest average density of 7000 Ind. m3 at 10–5 m and the lowest of 1600 Ind. m3 at 20–15 m depth. Its crop decreased with increasing depth during winter and autumn and attained its maximum density at 10–5 m depth during spring and
Seasonal distributions of total Cladocera and its main taxa at different water levels in Lake Nasser.
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summer (Fig. 3). Diaphanosoma mongolianum was the most dominant species at the surface and 10–5 m depth with average crop of 2750 and 2340 Ind. m3. Ceriodaphnia dubia was the main cladoceran at 20–15 m depth with average density of 2219 Ind. m3. Bosmina longirostris average density varied from a minimum of 290 Ind. m3 at 20–15 m depth to a maximum of 1560 Ind. m3 at surface water. Its highest standing crop was harvested from surface water during winter and autumn and from 10–5 m depth during summer and spring. Daphnia longspina and Chydorus sphaericus were high in cold seasons especially at the surface (Fig. 3). Rotiferan’s average density decreased with increasing depth, it attained the highest averages of 5742 Ind. m3 at the surface and it decreased to 4019 Ind. m3 at 10–5 m and the least count of 1341 Ind. m3 at 20–15 m. While its percentage to the total zooplankton increases with depth, it formed about 11.3% at surface water and it increased to form about 21% of the total zooplankton count at 20–15 m depth. The distribution of Rotifera at different water levels was very obvious during autumn and winter, where the surface water recorded the highest averages of 6800 and 4362 Ind. m3, respectively and it gradually decreased with increasing depth. During summer and spring, no remarkable variation was noted between rotifer distributions at the upper 10 meters, however it
Figure 4
decreased to its minimum average (1495 & 1970 Ind. m3) at 20–15 m depth (Fig. 4). Keratella, Collotheca and Brachionus were the most abundant genera of rotifers. K. cochlearis was flourished at surface samples during winter with average density of 3239 Ind. m3 and K. tropica was more frequent during summer at surface samples (average 3645 Ind. m3). The distribution of Collotheca sp. at different water levels was noticeable during autumn, where its highest average density (3190 Ind. m3) was recorded at the surface then decreased to 870 Ind. m3 at 10–5 m and attained its lowest average count of 325 Ind. m3 at 20–15 m depth. Spring was the most productive season for Collotheca sp. with the highest average density of 3710 Ind. m3 at surface water. Brachionus patulus dominated genus Brachionus and flourished during summer at the surface with the highest average density of 895 Ind. m3. Protozoa was frequently recorded during this study, its density decreased with increasing depth, its average density was 2500 Ind. m3 at surface water and decreased to 506 Ind. m3 at 20–15 m depth. Protozoa was represented by 5 species belonging to Ciliophora and Rhizopoda (Table 2). Meroplankton was very rare and occurred with the highest annual average (190 Ind. m3) at surface water and decreased with the depth.
Seasonal distributions of total Rotifera and its main taxa at different water levels in Lake Nasser.
Vertical distribution of zooplankton in Lake Nasser
183 N 70 60 50 40 30 20
40 30 20 10 0
10 0
Density (Ind.103m-3)
50 No. of species
S
A
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
d
B
4
J
H′
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.
Zooplankton community analysis
Discussion
Zooplankton population density and number of species detected in the study fluctuated at sites of surface water and at 10–5 m depth and it decreased with depth to the least counts at 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 was detected at 20–15 m of site 5 (Fig. 5B). The species richness value was the highest (3.775) at 10–5 m depth of site 1 while its lowest one (1.863) was detected at 20–15 m depth of site 6. The evenness values of diversity were ranged from 0.4678 to 0.6954 (Fig. 5B).
Zooplankton is a major component of aquatic ecosystem and changes in their abundance, species composition and seasonal variation have cascading effects at higher trophic levels. The zooplankton population in Lake Nasser is mainly represented by 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 6 Rotifera); Rzoska (1976) recorded 13 species (mainly Crustacea); and Samaan and Gaber (1976) recorded 15 species (9 Crustacea and 6 Rotifera). At different sampling sites during the study, zooplankton at the three studied depths attained the highest average during spring and summer (48,135 and 41,910 Ind. m3), while the least frequency prevailed during winter (28,250 Ind. m3). Zaghloul (1985) mentioned that most of zooplankton species inhabiting Lake Nasser are warm water eurythermic forms which can tolerate a wide range of temperature, so the maximum persistence of dominant species occurred during spring and summer when water temperature was over 25 C. El-Shabrawy (2000) recorded zooplankton peak during spring (88,000 Ind. m3), and dropped during winter to
ANOVA (2-way) Analysis of variance was performed to assess the significant difference of zooplankton faunal abundance existing in relation to sampling sites at different depths (Table 3). Insignificant difference was obtained between surface sites and 10–5 m depth according to abundance of zooplankton and its group except with total Rotifera. While, there are significant difference between sites of the surface and 20–15 m depth, also between 10–5 m and 20–15 m depth.
Table 3 The analysis of variance (ANOVA) partitioning the total variance in zooplankton and its main groups at different water depths in Lake Nasser. Depths
Surface · 10–5 m depth sites Surface · 20–15 m depth sites 10–5 m · 20–15 m depth sites
T. zooplankton
T. Copepoda
T. Cladocera
T. Rotifera
T. Protozoa
F
P
F
P
F
P
F
P
F
P
0.87 110.6 87.86
0.44 0.008 0.011
0.32 100.4 76.17
0.63 0.009 0.012
0.13 62 64.7
0.75 0.016 0.015
38.8 49.2 53.7
0.025 0.02 0.018
3.34 12.08 90.29
0.209 0.073 0.01
184 32,000 Ind. m3. Iskaros et al. (2008) mentioned that, zooplankton populations in Lake Nasser were more abundant during spring. Studies conducted since 1971 have shown that the standing stock of zooplankton increased during the first 10–15 years 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 and dropped 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 distribution 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 Lake Nasser 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. The abundance of zooplankton in the upper water of Lake Nasser may be due to flourishing of phytoplankton. The highest values of Chlorophyll a concentrations were recorded at the upper layers as a result of photosynthetic activity of phytoplankton (Habib, 1992) and also a high positive correlation was found between phytoplankton and zooplankton population density in main channel of Lake Nasser (Taha and Mageed, 2002). The phytoplankton abundance and biomass showed pronounced high occurrence in the euphotic zone than in the non-euphotic layer in the main channel of Lake Nasser during the highest flood season during autumn 1999 (Gharib and Abdel-Halim, 2006). The highest occurrence of Copepoda and Cladocera was detected at the surface and 10–5 m depth. While Rotifera and Protozoa attained its highest average density at surface water and its density decreased with increasing depth. The analysis of variance which performed to assess the significant difference of total zooplankton and its main groups existing in relation to different depths found insignificant difference between surface sites and 10–5 m depth except with Rotifera (F = 38.8, P = 0.025). While, there are significant difference between sites of the surface and 20–15 m depth, also between 10–5 m and 20–15 m depths. Thus samples from the surface and 10–5 m water levels are more similar in species composition to each other than to other sampling sites. In the present study, Copepoda was the most dominant zooplankton group, although it attained the least species diversity. Copepoda was more abundant at 10–5 m depth during spring and summer, they may migrate to this depth to avoid high temperature and light intensity during these periods. The low diversity in copepods and cladocerans in the main channel of Lake Nasser may be due to high temperature (El-Shabrawy, 2000). Nauplius larvae were the main Copepoda form at all depths of main channel, its density ranged from 63.8% to 70.3% of the copepods. Iskaros et al. (2008) and El-Enany (2009) refereed the dominance of Copepoda in Lake Nasser to the abundance of nauplius larvae which mainly feed on phytoplankton. Nauplius larvae increased at surface water during winter and autumn and flourished at 10–5 m depth during spring and summer. A moderate positive relationship was detected between copepod nauplii and surface water temperature (r = 0.634) and a negative weak correlation with water transparency (r = 0.351). Adult forms of copepoda were higher at 10–5 m depth which may be due to that they can freely swim in water column.
N. Khalifa et al. Ali et al. (2007) mentioned that Copepoda are good swimmers compared to the other groups. A positive relationship was revealed between surface water temperature and total copepods (r = 0.465) and a high negative one was detected with M. ogunnus (r = 0.985). Regarding the relation between water transparency and copepods, there was a moderate negative correlation with nauplius larvae (r = 0.351), while pH was 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. m3 at 20–15 m depth, it seems that the vertical distribution of Cladocera were accompanied with high values of dissolved oxygen as estimated by Zaghloul (1985), who illustrated that the limnoplanktonic forms of Cladocera require high oxygen concentration. El-Shabrawy (2009) recorded the highest density of Cladocera between 15 and 20 m in Lake Nasser, and almost all Cladocera disappeared below 20–15 m during summer, due to a lack in oxygen. Cladocera increased during spring and summer seasons due to flourishing of Diaphanosoma mongolianum. Temperature seems to be one of the main factors controlling the occurrence of D. mongolianum which is supported by its high positive correlation with surface water temperature (r = 0.866). Both D. mongolianum and D. brachyurum are characteristic species of warm waters, and are preponderant elements of the summer zooplankton communities of Spanish reservoirs (Jaume, 1991). Daphnia longspina and Chydorus sphaericus were higher during cold seasons 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 were significant (P = 0.25, P = 0.02 and P = 0.018, ANOVA) for rotifers between the three depths and the density of rotifers decreased with increasing depth. The vertical distribution of zooplankton in Lake Nasser during spring 1997 revealed that members of rotifers including K. cochlearis, Proalides sp. and Synchaeta oblonga migrate over large amplitudes of depth (El-Shabrawy, 2000). Rotifera attained its highest density during spring and summer due to flourishing of Brachionus species and Keratella tropica which have the ability to tolerate high temperature. Keratella tropica is a characteristic species of tropical water and B. caudatus and B. angularis are warm stenothermal forms (Winner, 1975). Abdel Mola (2012) found Rotifera at 0–2 m in Lake Nasser during summer formed 72% of total zooplankton due to the abundance of B. calcifloris, K. cochlearis and B. patulus which can tolerate the warm water in this layer and recorded a high positive correlation with water temperature. During the present study surface water temperature was positively 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 to allow zooplankton production to proceed, albeit at a slower pace (El-Shabrawy and Dumont, 2003). The vertical distribution of zooplankton is dependent on environmental parameters which include abiotic factors such as temperature, light intensity and dissolved oxygen, in addition to biotic factors including food resources and avoids predations (Lampert, 1989). The study concluded that the dominant zooplankton species inhabiting Lake Nasser were strongly temperature-dependent. Since the early filling of Lake Nasser extensive and
Vertical distribution of zooplankton in Lake Nasser comprehensive studies was carried out on the various aspects of 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 Nile in Egypt. Future studies recommend detecting the expected changes in Lake Nasser ecosystem as a result of the construction of the Renaissance Dam in Ethiopia. Also, the study recommends the introduction of some pelagic fish species to consume the high persistence of zooplankton community at the upper 10 meters of water column. Acknowledgment The authors are most grateful to Inland Water and Aquaculture Branch, National Institute of Oceanography and Fisheries, Cairo, Egypt which funded this work. References Abdel Mola, H.R., 2012. Vertical distribution of zooplankton in relation to thermocline at the main channel of Lake Nasser, Egypt. Egypt. Soc. Environ. Sci. 7 (1), 121–127. Ali, M.A., Mageed, A.A., Heikal, M., 2007. Importance of aquatic macrophyte for invertebrate diversity in large subtropical reservoir. Limn. 37, 155–169. American Public Health Association (APHA), 1995. Standard Methods of the Examination of Water and Waste Water, 19th ed. AWWA, WEF, New York, 1015 pp. Bick, H., 1972. Ciliated Protozoa: 198 pp. (An Illustrated Guide to the Species Used as Biological Indicators in Freshwater Biology), World Health Organization, Geneva. Dodson, S.I., Frey, D.G., 1991. Cladocera and other Branchiopoda. In: Ecology and Classification of North American Freshwater Invertebrates. Academic Press, New York, pp. 723–785. Edmondson, W.T., 1963. Fresh Water Biology, 2nd edition. John Wiley and Sons. Inc., New York and London, 1248 pp. Einsle, U., 1996. Copepoda. Cyclopoida Guides the Identification of the Micro Invertebrates of the Continental Waters of the World, Gustav Fisher Verlag, Stuttgart Jena New Yark, 81pp. El-Enany, H.R., 2009. Ecological studies on planktonic and epiphytic microinvertebrates in Lake Nasser, Egypt. Ph. D. Zool. Dept. Thesis, Fac. Sci. Banha Univ., 311pp. El-Serafy, S.S., Mageed, A.A., El-Anany, H.R., 2009. Impact of flood water on distribution of zooplankton in the main lake Nasser Khors. Egypt. J. Energy. Acad. Soc. Environ. Develop. 10 (1), 121–141. El-Shabrawy, G.M., 2000. Seasonal and spatial variation in zooplankton structure in Lake Nasser. J. Egypt. Acad. Soc. Environ. Develop. 1 (1), 19–44. El-Shabrawy, G.M., 2009. Lake Nasser-Nubia. In: Dumont, H.J. (Ed.) The Nile: Origin, Environments, Limnology and Human Use, Springer Science+Business Media, 125–156. El-Shabrawy, G.M., Dumont, H.J., 2003. Spatial and seasonal variation of the zooplankton in the coastal zone and main khors of Lake Nasser (Egypt). Hydrobiology 491, 119–132. FAO, 2011. Review of tropical reservoirs and their fisheries – The cases of Lake Nasser, Lake Volta and Indo-Gangetic Basin reservoirs. van Zwieten. In: Be´ne´, P.A.M., Kolding, J., Brummett, R., ValboJørgensen, J., (eds.) FAO Fisheries and Aquaculture Technical Paper. No. 557. Rome, 148 pp. Foissner, W., Berger, H., 1996. A user-friendly guide to the ciliates (Protozoa, Ciliophora) commonly used by hydrobiologists as bioindicators in rivers, lakes and waste waters with notes on their ecology. Fresh. Biol. 35, 375–482. Gaber, M.A., 1982. Report on Phytoplankton Investigations in Lake Nasser During October 1979, Report, Aswan Regional Planning. Water Resources Department. 56pp.
185 Gharib, S.M., Abdel-Halim, A.M. (2006). Spatial variation of phytoplankton and some physico-chemical variables during the highest flood season in Lake Nasser (Egypt). Egypt. J. Aquatic Res., 32(1), 246–263. Habib, O.A., 1992. Distribution of Chlorophyll-a Along the Main Channel of the High Dam Lake as an Index of Primary Productivity of Phytoplankton. Work. Rep. Fish. Mang. Cent., High Dam Lake Development Auth., Aswan, 1, 127–134 Habib, O.A., 2000. Zooplankton distribution. In: Craig J.F. (ed.), Sustainable Fish Production in Lake Nasser: Ecological Basis and Management Policy. ICLARM Conference Proceedings, 67–74 pp. Hussein, M.A., Obuid-Allah, A.H., Mohamad, A.H., 1991. Redescription of the cyclopoid copepod Mesocyclops ogunnus Onabamiro (1957) collected from Assiut, Egypt. Bull. Fac. Sci., Assiut University, 20 (1E), 119–133. Iskaros I.A., 1993. Biological Studies on Bottom Fauna of Lake Nasser and Adjacent Water, M.Sc. Thesis, Faculty of Science. Alex. Univ., 184pp. Iskaros, I.A., Bishai, R.M., Mokhtar, F.M., 2008. Comparative study on zooplankton in Aswan reservoir and River Nile at Aswan, Egypt. Egypt. J. Aquatic Res. 34 (2), 260–284. Jaume, D., 1991. The genus Diaphanosoma (Ctenopoda: Sididae) in Spain. Hydrobiology 225, 23–35. Koste, W., 1978. Rotatoria Die Ra¨dertiere Mitteleuropas Ein Bestimmungswerk, begru¨ndet von Max Voigl. U¨berodnung Monogonota. Vol. 1–2, 673 pp. + 234 pl. Lampert, W., 1989. The adaptive significance of dial vertical migration of zooplankton. Funct. Ecol. 3, 21–27. Mageed, A.A., 1995. Studies on Zooplankton from Lake Nasser (Egypt), Ph.D. Thesis, Fac. Sci., Al-Azhar Univ., Cairo, 243. Mageed, A.A., Heikal, M.T., 2005. Water quality and zooplankton dynamic in Lake Nasser, Egypt. Egypt J. Aquat. Biol. Fish. 9 (3), 1–24. Mohamed, I.O., 1993. Seasonal variations of chlorophyll a in the main channel of the High Dam Lake in 1986-1987. Work. Rep. Fish. Mang. Cent., High Dam Lake Develop. Auth., Aswan. 2, 133-154 Mokhtar, F.M., 2003. Ecological studies on the distribution of zooplankton in Lake Nasser and Adjacent waters. Ph.D. Thesis, Fac. Sci., Mansoura Univ., Egypt, 361pp. Pontin, R. M., 1978. A Key to the Freshwater Planktonic and Semiplanktonic Rotifera of the British Isles, 178 pp. Freshwater Biological Association, Scientific Publication 38. Ruttner-Kolisko, A., 1974. Plankton Rotifers, Biology and Taxonomy. 146 pp. E. Schweize bartsche Verlagsbuch hand lung. (Na¨gel, u. Obermiller). Stuttgart. Rzoska, J. 1976. Zooplankton of the Nile System. In J. Rzoska, ed. The Nile: biology of an ancient river, pp. 333–343. The Hague, Dr. W. Junk, b.v. Publishers. Samaan, A.A., 1971, Report on trip to Lake Nasser to investigate its primary productivity during March 1971, Rep LUNDC., Aswan, 11pp. Samaan, A.A., Gaber, M.A., 1976. Report on plankton investigation in Lake Nasser during March and August, 1976. In: AFA Latif (Ed.). Reports on surveys to Lake Nasser & River Nile Project. Acad. Sci. Res. & Tech., Cairo, Egypt. 13pp. Taha, O.E., Mageed, A.A., 2002. Spatial distribution and relationship between phytoplankton and zooplankton in Lake Nasser (Egypt) after the flood season. Egypt J. Aquat. Biol. Fish. 6 (4), 265–281. Winner, J.M. 1975. Zooplankton, 155–169. In, River Ecology. Ed. Whitton, B.A., Blackwell Scientific Publications. Oxford, London, Edinburgh and Melbourne. Wissel, B., Ramacharan, C.W., 2003. Plasticity of vertical distribution of crustacean zooplankton in lakes with varying levels of water colour. J. Plankton Res. 25 (9), 1047–1057. Xie, Z., Xiao, H., Tang, X., Lu, K., Cai, H., 2008. Interactions between red tide microalgae and herbivorous zooplankton: Effects of two bloom-forming species on the rotifer Brachionus plicatilis (O.F. Muller). Hydrobiology 600, 237–245. Zaghloul, F.A., 1985. Seasonal Variations of Plankton in Lake Nasser, Ph.D. Thesis, Faculty of Sci., Suez Canal Univ., Egypt, 364pp.
H O S T E D BY
National Institute of Oceanography and Fisheries
Egyptian Journal of Aquatic Research http://ees.elsevier.com/ejar www.sciencedirect.com
FULL LENGTH ARTICLE
Vertical distribution of zooplankton in Lake Nasser Nehad Khalifa a,*, Khaled A. El-Damhogy b, M. Reda Fishar a, Amr M. Nasef b, Mahmoud H. Hegab a a b
National Institute of Oceanography and Fisheries, 101 Kasr El Aini Street, Cairo, Egypt Zoology Department, Faculty of Science, Al-Azhar University, Cairo, Egypt
Received 13 January 2015; revised 1 March 2015; accepted 1 March 2015 Available 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. m3, respectively) and gradually decreased with depth until attaining the lowest average density at 20–15 m (12,460 and 8976 Ind. m3). 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. m3). 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 decreased with increasing depth during winter and autumn; however it attained its maximum density 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-dependent. The study recommends the introduction of some pelagic fish species to consume the high persistence 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 the world. The major portion of this lake is extending in Egypt for about 300 km and is known as Lake Nasser and for 180 km further south in Sudan as Lake Nubia. Lake Nasser extends between 22 31´to 23 45´N and 31 30´ to 33 15´ E and filled during late 1970s (El-Shabrawy, 2009). * Corresponding author. E-mail address: [email protected] (N. Khalifa). Peer review under responsibility of National Institute of Oceanography and Fisheries.
The role of zooplankton not only regulates the aquatic productivity 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 variation and surface distribution of zooplankton in Lake Nasser (Gaber, 1982; Zaghloul, 1985; Iskaros, 1993; Mohamed, 1993; Taha and Mageed, 2002; Mokhtar, 2003; El-Shabrawy and 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 during March 1970 from three localities. Samaan and Gaber (1976) studied the plankton population of Lake Nasser at ten meter water depth during March and August. Mageed (1995),
http://dx.doi.org/10.1016/j.ejar.2015.03.002 1687-4285 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/).
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El-Shabrawy (2000), Abdel Mola (2012) compared between the distributions of zooplankton at different water levels of the upper ten meters of the lake. The concentration of zooplankters in the water column (0–20 m) was variable throughout the year at six sampling sites from El-Ramla in the 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 factors. Even though the vertical position of individual zooplankton groups was quite variable, the study of Wissel and Ramacharan (2003) found consistent patterns among lakes, sampling dates, and zooplankton groups. Also, the relative vertical positions of groups of different body size suggested that avoiding predation by planktivorous fish was the major constraint that shaped this behavior. The objective of the present study is to investigate the temporal and spatial distribution patterns of zooplankton organisms at different water levels in Lake Nasser.
Zooplankton analysis For zooplankton quantitative analysis, three zooplankton samples were collected from each site, one from the surface and the second from 10 to 5 m depth, while a third sample from 20 to 15 m depth. Thirty liters were taken from surface water at each sampling site by filtering through a zooplankton net of 55 lm mesh diameter and vertical samples were collected by vertical tows. Collected samples were kept in plastic bottles with some lake water to which 4% formalin was added as a preservative. Samples were studied under the compound microscope and specimens identified at the species level when possible. Zooplankton numbers were expressed as number of organisms per cubic meter. Many publications and taxonomic references were used for zooplankton identification (Edmondson, 1963; Bick, 1972; Ruttner-Kolisko, 1974; Koste, 1978; Pontin, 1978; Dodson and Frey, 1991; Hussein et al., 1991; Einsle, 1996; Foissner and Berger, 1996). Data analysis
Material and methods Sampling program Seasonal samples were collected at nine sites in Lake Nasser (Fig. 1) from February 2013 to November 2013. Surface water samples were collected to estimate temperature, pH, transparency and dissolved oxygen by using the standard methods of APHA (1995).
Shannon-Wiener diversity, species richness, evenness, Simpson diversity and similarity index were calculated using the Primer 5 program. The correlation coefficient was done using the SPSS program version 16 between species of zooplankton and the different environmental variables. A two-way analysis of variance was calculated to find out the significance of the differences in density of the zooplankton groups among different sites at the three studied depths of the main channel. Results
Figure 1
Table 1
Lake Nasser map showing the selected study sites.
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 cladocerans, thirty rotifers and five protozoans were detected from the three depths (Table 2). The seasonal average density of zooplankton was nearly equal at both surface and 10–5 m depth (50,789 and 49,114 Ind. m3) and dropped to the least value of 14,527 Ind. m3 at 20–15 m depth. During winter and autumn, zooplankton density decreased with depth and maximum averages of 39,362 and 63,100 Ind. m3 at surface water, and decreased to the least averages densities of 12,460 and 8976 Ind. m3 at 20–15 m depth. During spring and summer, the highest average density of zooplankton was detected at 10–5 m depth (66,007 and 66,734 Ind. m3), and it decreased to the lowest density of 23,651 and 13,021 Ind. m3 at 20– 15 m depth, respectively (Fig. 2).
Ranges and averages of physico-chemical parameters in Lake Nasser.
Temperature (C) pH Trans. (cm) DO (mg/l)
Range Range Range Range
(average) (average) (average) (average)
Winter
Spring
Summer
Autumn
18.1–21.5 (19.6) 8.2–9.01 (8.6) 180–430 (320) 5–8.1 (6.5)
23.7–33.9 (28.3) 7.9–8.6 (8.4) 190–300 (230) 3.5–7.6 (5.7)
27–32.4 (29.6) 7.1–8.8 (8.1) 150–525 (271) 2.9–6.9 (4.7)
22.7–26.8 (24.4) 7.1–8.5 (8.1) 150–500 (350) 5.2–6.8 (6.6)
Vertical distribution of zooplankton in Lake Nasser Table 2
179
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
+ + +
+ + + +
+ +
180
Figure 2
N. Khalifa et al.
Seasonal distributions of total zooplankton and total Copepoda with its main forms at different water levels in Lake Nasser.
Copepods were abundant throughout the year; it was mainly 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 it flourished at surface water (Fig. 2). Copepoda presented mean densities of 35,500 and 36,200 Ind. m3 at the surface and 10– 5 m depth respectively and it decreased to 11,100 Ind. m3 at
20–15 m depth. Nauplius larvae were the main bulk of Copepoda at all depths of main channel, its density varied from 64% to 70% of total Copepoda at the three depths with the highest average of 23,070 Ind. m3 at 10–5 m. These larvae decreased vertically with increasing depth during autumn, while attained its highest crop at 10–5 m depth in other periods. Copepodites decreased with increasing depth during winter and autumn and attained high crop at 10–5 m depth during
Vertical distribution of zooplankton in Lake Nasser summer and autumn (Fig. 2). Thermodiaptomus galebi was peaked during spring with its highest density average of 9300 Ind. m3 at 10–5 m depth. Thermocyclops neglectus was flourished during summer at 10–5 m depth (Fig. 2), contrary to Mesocyclops ogunnus which increased during winter at the same depth.
Figure 3
181 Cladocera ranged from 11% to 14.5% of total zooplankton density at different water levels during the study. It attained its highest average density of 7000 Ind. m3 at 10–5 m and the lowest of 1600 Ind. m3 at 20–15 m depth. Its crop decreased with increasing depth during winter and autumn and attained its maximum density at 10–5 m depth during spring and
Seasonal distributions of total Cladocera and its main taxa at different water levels in Lake Nasser.
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summer (Fig. 3). Diaphanosoma mongolianum was the most dominant species at the surface and 10–5 m depth with average crop of 2750 and 2340 Ind. m3. Ceriodaphnia dubia was the main cladoceran at 20–15 m depth with average density of 2219 Ind. m3. Bosmina longirostris average density varied from a minimum of 290 Ind. m3 at 20–15 m depth to a maximum of 1560 Ind. m3 at surface water. Its highest standing crop was harvested from surface water during winter and autumn and from 10–5 m depth during summer and spring. Daphnia longspina and Chydorus sphaericus were high in cold seasons especially at the surface (Fig. 3). Rotiferan’s average density decreased with increasing depth, it attained the highest averages of 5742 Ind. m3 at the surface and it decreased to 4019 Ind. m3 at 10–5 m and the least count of 1341 Ind. m3 at 20–15 m. While its percentage to the total zooplankton increases with depth, it formed about 11.3% at surface water and it increased to form about 21% of the total zooplankton count at 20–15 m depth. The distribution of Rotifera at different water levels was very obvious during autumn and winter, where the surface water recorded the highest averages of 6800 and 4362 Ind. m3, respectively and it gradually decreased with increasing depth. During summer and spring, no remarkable variation was noted between rotifer distributions at the upper 10 meters, however it
Figure 4
decreased to its minimum average (1495 & 1970 Ind. m3) at 20–15 m depth (Fig. 4). Keratella, Collotheca and Brachionus were the most abundant genera of rotifers. K. cochlearis was flourished at surface samples during winter with average density of 3239 Ind. m3 and K. tropica was more frequent during summer at surface samples (average 3645 Ind. m3). The distribution of Collotheca sp. at different water levels was noticeable during autumn, where its highest average density (3190 Ind. m3) was recorded at the surface then decreased to 870 Ind. m3 at 10–5 m and attained its lowest average count of 325 Ind. m3 at 20–15 m depth. Spring was the most productive season for Collotheca sp. with the highest average density of 3710 Ind. m3 at surface water. Brachionus patulus dominated genus Brachionus and flourished during summer at the surface with the highest average density of 895 Ind. m3. Protozoa was frequently recorded during this study, its density decreased with increasing depth, its average density was 2500 Ind. m3 at surface water and decreased to 506 Ind. m3 at 20–15 m depth. Protozoa was represented by 5 species belonging to Ciliophora and Rhizopoda (Table 2). Meroplankton was very rare and occurred with the highest annual average (190 Ind. m3) at surface water and decreased with the depth.
Seasonal distributions of total Rotifera and its main taxa at different water levels in Lake Nasser.
Vertical distribution of zooplankton in Lake Nasser
183 N 70 60 50 40 30 20
40 30 20 10 0
10 0
Density (Ind.103m-3)
50 No. of species
S
A
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
d
B
4
J
H′
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.
Zooplankton community analysis
Discussion
Zooplankton population density and number of species detected in the study fluctuated at sites of surface water and at 10–5 m depth and it decreased with depth to the least counts at 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 was detected at 20–15 m of site 5 (Fig. 5B). The species richness value was the highest (3.775) at 10–5 m depth of site 1 while its lowest one (1.863) was detected at 20–15 m depth of site 6. The evenness values of diversity were ranged from 0.4678 to 0.6954 (Fig. 5B).
Zooplankton is a major component of aquatic ecosystem and changes in their abundance, species composition and seasonal variation have cascading effects at higher trophic levels. The zooplankton population in Lake Nasser is mainly represented by 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 6 Rotifera); Rzoska (1976) recorded 13 species (mainly Crustacea); and Samaan and Gaber (1976) recorded 15 species (9 Crustacea and 6 Rotifera). At different sampling sites during the study, zooplankton at the three studied depths attained the highest average during spring and summer (48,135 and 41,910 Ind. m3), while the least frequency prevailed during winter (28,250 Ind. m3). Zaghloul (1985) mentioned that most of zooplankton species inhabiting Lake Nasser are warm water eurythermic forms which can tolerate a wide range of temperature, so the maximum persistence of dominant species occurred during spring and summer when water temperature was over 25 C. El-Shabrawy (2000) recorded zooplankton peak during spring (88,000 Ind. m3), and dropped during winter to
ANOVA (2-way) Analysis of variance was performed to assess the significant difference of zooplankton faunal abundance existing in relation to sampling sites at different depths (Table 3). Insignificant difference was obtained between surface sites and 10–5 m depth according to abundance of zooplankton and its group except with total Rotifera. While, there are significant difference between sites of the surface and 20–15 m depth, also between 10–5 m and 20–15 m depth.
Table 3 The analysis of variance (ANOVA) partitioning the total variance in zooplankton and its main groups at different water depths in Lake Nasser. Depths
Surface · 10–5 m depth sites Surface · 20–15 m depth sites 10–5 m · 20–15 m depth sites
T. zooplankton
T. Copepoda
T. Cladocera
T. Rotifera
T. Protozoa
F
P
F
P
F
P
F
P
F
P
0.87 110.6 87.86
0.44 0.008 0.011
0.32 100.4 76.17
0.63 0.009 0.012
0.13 62 64.7
0.75 0.016 0.015
38.8 49.2 53.7
0.025 0.02 0.018
3.34 12.08 90.29
0.209 0.073 0.01
184 32,000 Ind. m3. Iskaros et al. (2008) mentioned that, zooplankton populations in Lake Nasser were more abundant during spring. Studies conducted since 1971 have shown that the standing stock of zooplankton increased during the first 10–15 years 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 and dropped 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 distribution 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 Lake Nasser 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. The abundance of zooplankton in the upper water of Lake Nasser may be due to flourishing of phytoplankton. The highest values of Chlorophyll a concentrations were recorded at the upper layers as a result of photosynthetic activity of phytoplankton (Habib, 1992) and also a high positive correlation was found between phytoplankton and zooplankton population density in main channel of Lake Nasser (Taha and Mageed, 2002). The phytoplankton abundance and biomass showed pronounced high occurrence in the euphotic zone than in the non-euphotic layer in the main channel of Lake Nasser during the highest flood season during autumn 1999 (Gharib and Abdel-Halim, 2006). The highest occurrence of Copepoda and Cladocera was detected at the surface and 10–5 m depth. While Rotifera and Protozoa attained its highest average density at surface water and its density decreased with increasing depth. The analysis of variance which performed to assess the significant difference of total zooplankton and its main groups existing in relation to different depths found insignificant difference between surface sites and 10–5 m depth except with Rotifera (F = 38.8, P = 0.025). While, there are significant difference between sites of the surface and 20–15 m depth, also between 10–5 m and 20–15 m depths. Thus samples from the surface and 10–5 m water levels are more similar in species composition to each other than to other sampling sites. In the present study, Copepoda was the most dominant zooplankton group, although it attained the least species diversity. Copepoda was more abundant at 10–5 m depth during spring and summer, they may migrate to this depth to avoid high temperature and light intensity during these periods. The low diversity in copepods and cladocerans in the main channel of Lake Nasser may be due to high temperature (El-Shabrawy, 2000). Nauplius larvae were the main Copepoda form at all depths of main channel, its density ranged from 63.8% to 70.3% of the copepods. Iskaros et al. (2008) and El-Enany (2009) refereed the dominance of Copepoda in Lake Nasser to the abundance of nauplius larvae which mainly feed on phytoplankton. Nauplius larvae increased at surface water during winter and autumn and flourished at 10–5 m depth during spring and summer. A moderate positive relationship was detected between copepod nauplii and surface water temperature (r = 0.634) and a negative weak correlation with water transparency (r = 0.351). Adult forms of copepoda were higher at 10–5 m depth which may be due to that they can freely swim in water column.
N. Khalifa et al. Ali et al. (2007) mentioned that Copepoda are good swimmers compared to the other groups. A positive relationship was revealed between surface water temperature and total copepods (r = 0.465) and a high negative one was detected with M. ogunnus (r = 0.985). Regarding the relation between water transparency and copepods, there was a moderate negative correlation with nauplius larvae (r = 0.351), while pH was 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. m3 at 20–15 m depth, it seems that the vertical distribution of Cladocera were accompanied with high values of dissolved oxygen as estimated by Zaghloul (1985), who illustrated that the limnoplanktonic forms of Cladocera require high oxygen concentration. El-Shabrawy (2009) recorded the highest density of Cladocera between 15 and 20 m in Lake Nasser, and almost all Cladocera disappeared below 20–15 m during summer, due to a lack in oxygen. Cladocera increased during spring and summer seasons due to flourishing of Diaphanosoma mongolianum. Temperature seems to be one of the main factors controlling the occurrence of D. mongolianum which is supported by its high positive correlation with surface water temperature (r = 0.866). Both D. mongolianum and D. brachyurum are characteristic species of warm waters, and are preponderant elements of the summer zooplankton communities of Spanish reservoirs (Jaume, 1991). Daphnia longspina and Chydorus sphaericus were higher during cold seasons 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 were significant (P = 0.25, P = 0.02 and P = 0.018, ANOVA) for rotifers between the three depths and the density of rotifers decreased with increasing depth. The vertical distribution of zooplankton in Lake Nasser during spring 1997 revealed that members of rotifers including K. cochlearis, Proalides sp. and Synchaeta oblonga migrate over large amplitudes of depth (El-Shabrawy, 2000). Rotifera attained its highest density during spring and summer due to flourishing of Brachionus species and Keratella tropica which have the ability to tolerate high temperature. Keratella tropica is a characteristic species of tropical water and B. caudatus and B. angularis are warm stenothermal forms (Winner, 1975). Abdel Mola (2012) found Rotifera at 0–2 m in Lake Nasser during summer formed 72% of total zooplankton due to the abundance of B. calcifloris, K. cochlearis and B. patulus which can tolerate the warm water in this layer and recorded a high positive correlation with water temperature. During the present study surface water temperature was positively 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 to allow zooplankton production to proceed, albeit at a slower pace (El-Shabrawy and Dumont, 2003). The vertical distribution of zooplankton is dependent on environmental parameters which include abiotic factors such as temperature, light intensity and dissolved oxygen, in addition to biotic factors including food resources and avoids predations (Lampert, 1989). The study concluded that the dominant zooplankton species inhabiting Lake Nasser were strongly temperature-dependent. Since the early filling of Lake Nasser extensive and
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