Abstract: The phytoplankton community structure in Lake Nasser was qualitatively and quantitatively studied in a significant flood season. The most common planktonic algae were the cyanoprokaryotic species Anabaenopsis cunningtonii and Phormidium sp. in addition to the centric diatoms Aulacoseira granulata, Cyclotella meneghiniana and the green alga Ankistrodesmus falcatus. Quantitatively, eight species, Anabaenopsis cunningtonii, Lyngbya sp., Oscillatoria sp., Phormidium sp., Aulacoseira distans, Aulacoseira granulata, Cyclotella meneghiniana and Cyclotella ocellata were recorded as the major species. The data were analyzed by clustering and ordination multi variate techniques using the programmes TWINSPAN and DECORANA. Data analyses showed that water turbidity was the most important factor determining the distribution of phytoplankton in Lake Nasser during the flood. Among the other estimated water variables pH, oxygen contents, nitrate, silicate, phosphate, sodium and potassium seemed to be important in characterizing the phytoplankton community structure and its spatial distribution along the main water body of Lake Nasser during the flood period.
Introduction
Man-made lake ecosystems are often characterized by their own phytoplankton assemblages, referred to as plankton formations (Teiling, 1916). These phytoplankton assemblages can reflect the ecological status of lakes and respond both qualitatively and quantitatively to changes therein. The dynamics and species diversity of the phytoplankton are greatly influenced by water physico-chemistry and biological factors (Harris, 1986; Reynolds, 1986; Sommer, 1989) as well as the external physical disturbances (i.e. accidental water mixing, storms, floods). Thus, they must be taken into account when considering the reasons for fluctuations in phytoplankton (Jacobsen and Simonsen, 1993). The importance of such factors in controlling the biotic structure including phytoplankton assemblages of man-made lakes was discussed by Uhlmann (1998) and subsequently confirmed by Wilk-Wozniak and Kosinski (2001). However, due to the complex interactions, rapid changes and great variability of these factors no absolute standards for biological quality can be set (Commission of the European Communities, 1999).
Lake Nasser in south Egypt (Fig. 1) constitutes about 84% of the surface area of the total High Dam reservoir basin. There is a regular annual rhythmic fluctuation in water level caused by the famous Nile flood. The effect of flood is manifested in the arrival of turbid water in July-August (Entz, 1997) starting from the southernmost part of the reservoir. Then extended northwards and reaches only the southern sector of Lake Nasser (180-200 km south of the High Dam) in October-November. The water level in Lake Nasser is subjected to dramatic changes from one year to another due to the fluctuations in amount of flood water flowing into it. During the flood season of 1999 the water level increased remarkably to 181.6 m a.s.l. (above sea level), which is considered the highest record since the construction of the High Dam (Fig. 2A and B). The irregular hydrological events due to the strong water level fluctuations may influence the life conditions and produce an irregular pattern of variability in phytoplankton assemblages in the lake (Harris and Baxter, 1996).
Investigations of phytoplankton assemblages in Lake Nasser were initiated in the early 1970s by some sporadic works during the limnological studies supported by UNDP and FAO as well as the Egyptian government. In the late 1970s and the early 1980s some other fragmentary works were devoted to the lake phytoplankton and summarized by Bishai et al. (2000). Otherwise, the phytoplankton development was systematically and comprehensively investigated by Zaghloul (1985), Mohammed et al. (1989); El-Otify (1991), Abd El-Monem (1995) to follow the dynamics of seasonal succession and spatial variability of phytoplankton. During most of these systematic surveys, the planktonic cyanoprokaryote, Microcystis aeruginosa was one of the infrequent species. However, intermittent occurrence of water blooms caused mainly by this species were reported by Entz (1976), Mohamed (1993), Abd El-Monem (1995) in a limited area in the southern part of the lake and monitored using remote sensing by Hamed (2000).
The main task of this work was to investigate the species composition and relative abundance of phytoplankton during the highest water level reached in a significant flood period of 1999 in order to evaluate the main features of the phytoplankton assemblages in Lake Nasser during the flood.
Materials and Methods
The area of Lake Nasser where the present work was carried has been repeatedly described in previous studies (Ahmed et al., 1989; El-Otify, 1991; Entz, 1997; Bishai et al., 2000). It is necessary to emphasize that the large surface area (mean value: 3917 km2), elongated shape (length: > 300 km; average width: 13.5 km) and deepness (maximum depth: 100 m and mean depth: 23.4 m) represent the main features of the lake.
| Fig. 1: | Map showing he location of investigated sites along the main water body of Lake Nasser. The inset shows the location of Lake Nasser in Egypt. |
| Fig. 2: | Water level in Lake Nasser: A) Annual fluctuations during the period 1964-2001, B) Monthly fluctuations during 1999 |
The materials (water samples) for this study were collected once during October-November 1999 from 10 successive sites (I-X) along the main water body of the lake situated at variable distances: 3, 45, 85, 110, 190, 250, 270, 300, 320, 333 Km south of the High Dam, respectively (Fig. 1). Sites IX, 320 and X, 333 km were in the confluence of Lake Nubia. Samples were collected from the water surface in clean polyethylene bottles, each of one litre capacity and has a wide mouth. In addition, water samples were gathered from different depths by the water sampler, Nansen bottle (Goodwin and Goddard, 1974). The different depths include the subsurface water layer (1 m deep) and the other depths starting from 2.5 to 20 m in the intervals of 2.5 m. Water physico-chemistry was analyzed according to APHA (1992). Samples for chlorophyll-a determination were filtered onto SM 13400 Sartorius glass fibre filter buffered with magnesium carbonate. Phytoplankton chlorophyll-a concentrations were estimated according to the method of Parsons and Strickland (1965) after 12 h extraction in 90% acetone in a dark freezer. For phytoplankton qualitative and quantitative analyses, the samples were preserved with 4% formaline and concentrated by sedimentation (Stein, 1973) to aliquot volumes. Quantification of phytoplankton was performed microscopically using a counting chamber of 10–1 cm3. The cyanoprokaryotic and eukaryotic phytoplankton were identified according to the following references: Smith (1950), Prescott (1954), Kimor and Pollingher (1965), Weber (1971), Krienitz (1990), Guarrera and Echenique (1992). Then, the phytoplanktons were enumerated and their counts were expressed in terms of number of individuals (cells, colonies, coenobia and filaments) per litre.
Two-Way Indicator Species Analysis (TWINSPAN) as a classification technique and Detrended Correspondence Analysis (DCA) as an ordination technique were applied to the recorded 81 phytoplankton species from 100 collected water samples using the programmes TWINSPAN and DECORANA, respectively (Hill, 1979a,b). Species richness (alpha-diversity) of each phytoplankton cluster was calculated as the average number of species per sample and species turnover (beta-diversity) as the ratio between the total species recorded in a certain phytoplankton cluster and its alpha diversity (Pielou, 1975). The statistical computer programme MINITAB was used to study the correlation of the water variables with the first two axes of DCA.
Results and Discussion
A total of 81 phytoplankton species were recorded from Lake Nasser in this investigation and were listed with their authorities (Table 1). Here, the relative frequencies of the species are indicated by the number of occasions in which they were recorded out of the 100 investigated water samples. The most common species were; Anabaenopsis cunningtonii, Phormidium sp. (cyanoprokaryotes) and Aulacoseira granulata (diatom) that were present in all samples (100%), while two others, the diatom Cyclotella meneghiniana and the green alga Ankistrodesmus falcatus were present in more than 90% of the samples. Six species were recorded in ≥ 75% of the samples, Lyngbya sp., Merismopedia tenuissima (cyanoprokaryotes), Nitzschia holsatica, Synedra acus (diatoms), Chodatella ciliata and Oocystis sp. (green algae). Nine species were recorded in over 50% of the samples, Chodatella citriformis, Closterium venus, Crucigenia rectangularis, Dictyosphaerium pulchellum, Golenkinia radiata, Scenedesmus sp., Staurastrum paradoxum (green algae), Aulacoseira distans and Cyclotella ocellata (diatoms). Fourteen species appeared in over 25% of the samples: Anabaena sp., Anabaenopsis sp., Chroococcus sp., Oscillatoria sp. (Cyanoprokaryotes), Navicula exigua (diatom), Coelastrum reticulatum, Elakatothrix genevensis, Gloeocystis ample, Micraactinum sp., Pediastrum simplex, Pediastrum sp., Scenedesmus ecornis, Schroederia setigera and Staurastrum uniseriatum (green algae). A further 19 species were recorded in ≥ one-tenth of the samples. At the other extreme, the remaining 28 species, which appeared in less than one-tenth of the samples, especially the 16 that were recorded only once or twice, were regarded as the rare species. Majority of the recorded species were quantitatively of low contributions (<5%) to the total phytoplankton counts.
| Fig. 3: | Cluster diagram for the phytoplankton species distribution derived from TWINSPAN analysis. The 12 phytoplankton clusters are: I. Oscillatoria sp.- Scenedesmus acuminatus, II. Oocystis sp., III. Crucigenia rectangularis-Merismopedia tenuissima-Chodatella citriformis, IV. Elakatothrix genevensis-Closterium venus, V. Golenkinia radiata-Oscillatoria sp.- Pediasrum simplex, VI. Cymbella ventricosa- Elakatothrix genevensis, VII. Elakatothrix genevensis-Cosmarium depressum- Synedra ulna, VII. Aulacoseira distans, IX. Pediastrum simplex- Ankistrodesmus spiralis, X. Navicula exigua- Nitzschia holsatica- Cosmarium depressum-Oscillatoria sp., XI. Coelastrum reticlatum-Cyclotella ocellata- Peridinium sp., XII. Scenedesmus quadricauda-Scenedesmus ecornis. |
| Fig. 4: | Relationship between 12 phytoplankton clusters identified according to TWINSPAN analysis and the ordination of the first to axes derived from DCA analysis of the phytoplankton species distribution. |
Only eight of the species individually contributed > 5% of the phytoplankton population in at least one sample and were designated as the “major” species. Similar community structures and species combinations have been reported from Lake Nasser during the last three decades (Bishai et al., 2000), from the headwater lakes of the Nile (Talling, 1976; 1986; Hecky, 1993) and from other different African lakes (Hecky and Kling, 1987; Patterson and Kachinjika, 1993; Zohary et al., 1996). Microcystis aeruginosa was recorded in 22% of the collected water samples and its quantitative contribution was always less than 1% of the total phytoplankton counts. Due to the ability of buoyancy of this cyanoprokaryote it can move to the euphotic zone (Carr and Whitton, 1982; Van der Veer et al., 1995) and create blooms closer to the water surface in a limited area in Lake Nasser (Entz, 1976; Mohamed, 1993; Abd El-Monem, 1995; Hamed, 2000).
| Fig. 5: | Mean values (±SD) of water turbidity (N.T.U., Nephelometric turbidity unit) along the main water body of Lake Nasser during the flood period of 1999. |
Mass appearance of Microcystis aeruginosa was used for estimation of the degree of water eutrophication, signaling deterioration of water quality (Bucka, 2000) due to the production of toxins. Blooms were developed in a limited area of the south region of Lake Nasser only during exceptionally warm and calm conditions. Since these conditions are disturbed by the flood, it could be regarded as a factor involved in explanation for the irregularity of Microcystis aeruginosa blooms. This irregularity is a positive feature in the aspect of eventual eutrophication water and the toxic effect on the water of Lake Nasser.
Classification of the sampling sites and depths resulted in 12 clusters at level four of the hierarchy (Fig. 3). The clusters were named after the dominant species as follows: Oscillatoria sp.-Scenedesmus acuminatus (I), Oocystis sp. (II), Crucigenia rectangularis-Merismopedia tenuissima-Chodatella citriformis (III), Elakatothrix genevensis-closterium venus (IV) Golenkinia radiata-Oscillatoria sp.-Pediastrum simplex (V) and Cymbella ventricosa-Elakatothrix genevensis (VI). This group of clusters contains the south sites (250-333 Km. south of the High Dam) and at this point, the primary bifurcation of the diagram is reached. Then, the second part of the diagram contains all of the sampling sites in the north 250 Km. of the lake. The clusters of this part are named as follows: Elakatothrix genevensis-Cosmarium depressum-Synedra ulna (VII), Aulacoseira distans (VIII), Pediastrum simplex-Ankistrodesmus spiralis (IX), Navicula exigua-Nitzschia holsatica-Cosmarium depressum-Oscillatoria sp. (X), Coelastrum reticulatum-Cyclotella ocellata-Peridinum sp. (XI) and Scenedesmus quadricauda-Scenedesmus ecornis (XII). These results provide evidence that the phytoplankton community structure was subjected to spatial differentiation along the main water body of the lake depending on hydrological conditions. Since, the distribution of phytoplankton species in different clusters may reflect the influence of habitat disturbance in the south part of the lake due to the decisive effect of the strong water level fluctuation (Harris and Baxter, 1996). In contrast, the cluster analysis of the phytoplankton spatial distribution during the last three decades separated the main water body of the lake into three parts. No remarkable regular differences in depth distribution of phytoplankton could be recognized in this investigation. This could be related to the destruction of summer stratification conditions (Entz, 1997). In this respect, it was reported that the vertical variation of phytoplankton appeared concomitantly with the thermal stratification in Lake Nasser (Mohammed et al., 1989).
The most frequent phytoplankton group (Table 2) was the green algae (50%) followed by diatoms (28.39%), cyanoprokaryotes (18.52%) and dinoflagellates (2.47%).
| Table 1: | Mean values of the frequency of occurrence (%) of the different species belonging to the 12 phytoplankton clusters identified after the application of TWINSPAN analysis |
| *: Major species comprising > 5% of the total phytoplankton counts in at least one sample | |
| Table 2: | Presence contributions of the different phytoplankton groups, total species richness (species/sample) and species turnover belonging to 12 clusters identified after the application of TWINSPAN analysis |
| Table 3: | Correlation coefficient between water variables and the first two axes of DCA |
| *: P<0.05, **:P< 0.01, ***: P<0.001 | |
The highest value of species richness was that of cluster VI (34.25 species/sample), followed by cluster VII (30.4 species/sample). Otherwise, cluster II had the lowest value of species richness (13.71 species/sample).
In DECORANA analysis, the Eigenvalues of the axes were as follows: axis 1, 0.2; axis 2, 0.11; axis 3, 0.09; and axis 4, 0.07 (Fig. 4). Because the first two axes represented by far the largest portion of the variation, the other axes were not plotted. The sites of clusters I-VI (north sites) occupied the upper part of axis 1 and the sites of clusters VII-XII (south sites) occupied the lower part of the same axis. It is clear that axis 1 represents variation in distribution based on the site locations along the main water body of the lake. The southern sites were affected by the flood water and the northern ones were not. To emphasize this, the mean values of water turbidity in both of the north and south parts were determined (Fig. 5). Axis 2 showed more variations, as judged by the scores that ranged between-378 and 629, as compared with-281 and 468 for axis 1. The distribution of species along axis 2 indicated that the sites of clusters I-III occupied the right side of axis 2 and the sites of clusters IV-XII occupied the left side of the same axis.
The data concerning the correlation between the water variables and both of axis 1 and 2 (Table 3) revealed some significant correlation. The monovalent cations (Na+ and K+) correlated negatively with axis 1. The pH value, oxygen content, nitrate, silicate and sulphate correlated positively with axis 1. Turbidity and phosphate correlated positively with both of axis 1 and 2. The electrical conductivity, total phytoplankton counts and phytoplankton chlorophyll-a correlated negatively with axis 2. These results may indicate the importance of the aforementioned parameters in determining the structure and general pattern of the spatial distribution of phytoplankton communities in lake during the flood period. In this context, the turbulence caused by water turbidity could be regarded as a factor influencing water colour and light conditions that consequently affect the phytoplankton assemblages (Lepisto, 1999). Phytoplankton species composition and their relative abundance were shown to be consistently related with variables including physical disturbances (Padisak and Reynolds, 1998), pH values and nutrients (Wilk-Wozniak and Kosinski, 2001) particularly nitrate-nitrogen and phosphate-phosphorus (Pociecha and Wilk-Wozniak, 2000). It is worth mentioning here that in Lake Nasser, the main source of nutrient supply is the Nile river inflow which is controlled by the flood. Chlorophyll-a contents were estimated as the best descriptive parameter to indicate phytoplankton quantity in freshwater lakes (Voros and Padisak, 1991). Phytoplankton density and chlorophyll-a values in man-made lakes showed relatively good correlation (Bucka, 1998; Lepisto, 1999).
The results obtained from phytoplankton analyses, although based on once-collected samples, provided important information, stressing out the usefulness of investigations of its communities during the significant flood periods to evaluate the possible influences of the strong water level fluctuations on the development of phytoplankton in Lake Nasser.
Acknowledgment
The author acknowledges the High Dam Authority for providing facilities for collection of samples during the field trip to Lake Nasser. Thanks are also extended to Dr. Mohamed G. Sheded, Department of Botany, Faculty of Science, Aswan for his continuous encouragement and help.