1 Introduction

The world’s growing population experiences severe freshwater and nutrition shortages (FAO, 2019). By 2030, water demand will be intensified in all sectors, leading to a global water shortage of 40% (UNWWDR, 2015). Globally, freshwater aquaculture is one of the fastest-growing industries. Freshwater aquaculture production increased from 4.7 million tons in 1980 to 85.3 million tons in 2019 (FAO, 2019). However, this growth contributes to eutrophication and severe limitation in drinking water (Anufriieva, 2018). On the other hand, inland saline water bodies marginally share the aquaculture development and total fish production, despite making up 45.22% of the global inland waters (Williams, 1996) with a total volume of 104 × 103 km3 (Anufriieva, 2018).

Inland saline lakes (ISL) with salinity levels below 40‰ are crucial for aquaculture development. They can considerably share the total fish production and the aquaculture economic returns without conflicts with freshwater supply. Most of these lakes located in arid regions where residents have low incomes, and the growth of the fish industry could improve their living standards (Kavembe et al., 2016). Furthermore, saline lakes may be a keystone in the environmental management of these areas. Saline lakes do contribute to carbon and gas emissions sequestration, with an annual sequestration capacity of 0.21% of the global carbon emissions (Boyd et al., 2010). The carbon burial capacity of utilized small lakes and ponds can compensate for greenhouse gas emissions and can bury more carbon than unutilized inland lakes, at least, that resulted from their fish productions activities (Aralappanavar, et al., 2022).

However, commercializing fish production in inland saline lakes is constrained by salinity fluctuations and changes in ionic composition relative to seawater levels (Prangnell & Fotedar, 2006). Salinity is a core factor manipulating the life span of fish species, its fluctuation creates an osmotic stress, which can have drastic effects on the metabolism, growth, and reproduction and can threaten species fitness and survival (Leite et al., 2022). Euryhaline fishes can survive in environments with fluctuated salinity through osmoregulation strategies to maintain a virtually persistent blood osmolality. The patterns and mechanisms of osmotic and ionic regulation in euryhaline fishes have been well-documented in adult fish (Mozanzadeh et al., 2021), juveniles (Loi et al., 2022), early post-embryonic stages, larval, and post-larval stages (Anand et al., 2023).

Aquatic organisms require several ions at minimum levels with balanced ratios to maintain normal physiological functions. Both fresh and saline groundwater may depict by imbalanced levels of major ions which can be toxic, the physiological effects of rising ion concentrations on aquatic organisms are a problem of growing concern (Mishra & Mahapatra, 2021; Öztürk et al., 2019). While the toxic effects of changed ionic levels and ratios on freshwater organisms have been investigated (Leite et al., 2022), fewer works were dedicated to marine organisms (Doroudi et al., 2006).

Wadi El Natrun is an elongated narrow depression in the northeastern part of the Western Desert, parallel to the Nile Delta and about 50 km apart from Rosetta Branch. The depression is about 23 m below sea level, characterized by ancient alkaline salt lakes in its eastern part (Abu Zeid, 1984). Springs and wells are the sole source of water for these lakes with inflow restricted to the winter months when they become fullest by its end. As the temperature rises, the lake water evaporates rising the salts concentration. Small lakes’ water completely evaporates leaving sheets of mineral deposits on the sediment surface, while major lakes remain wet due to continuous spring flow (Shortland, 2004). These evaporitic deposits were rich sources of minerals and have been exploited for over 6000 years (Shortland, 2004). However, many of these lakes have dried up and disappeared, while others have experienced a decrease in mineral levels due to the continuous salt extraction and freshwater discharge, such as El Bieda Lake. After a long time of minerals extraction and discharging of agricultural drainage water, salinity and chemistry of El Bieda Lake have drastically changed compared to its background that was reported by Sayed and Abdo (2009). Consequently, the lake has undergone progressive changes in its biological features. This study aimed at mapping the current biological and chemical characteristics of El Bieda Lake and identifying its fisheries commercializing opportunities.

2 Materials and Methods

2.1 Site Description

El Bieda Lake is roughly square in shape with four short and wide arms, two in the east and two shorter in the west. The lake is surrounded by cultivated land in the north and eastern north, while the desert and ruins of mineral evaporation ponds surround the lake in the west, south, and southeast. The lake has a sandy bottom in the south and southeast and a muddy-clay bottom in the north and northwest. Three small drains discharge into the lake in the northern side (Fig. 1). The lake’s total area is about 4.26 km2 with a mean depth of 2.6 m.

Fig. 1
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Map of Lake El Bieda showing the sampling sites

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2.2 Sampling Program and Water Sampling

Fifteen subsurface water samples were collected from five selected sites in El Bieda Lake during late autumn 2021 and mid-winter and mid-summer 2022. A 2.5 L Ruttner sampler was used to collect the samples. The water samples were immediately transferred to dry, clean, and separately labeled polyethylene bottles for chemical and phytoplankton analysis. Zooplankton were collected by filtering 30 L through a plankton net with a mesh size of 55 μm. All planktonic samples were immediately preserved with 4% formaldehyde.

2.3 Chemical Analysis

Physical and chemical parameters were determined according to the methods of the American Public Health Association (APHA, 2017), unless otherwise noted. Nitrate was determined using the reduction method (Mullin & Riley, 1956).

2.4 Water Quality and Trophic State Indices

Water quality and trophic states were assessed using the Water Quality Index (WQI) and the Trophic Level Index (TLI). The WQI was calculated according to the Canadian Council of Ministers of the Environment (CCME, 2001), while the TLI was calculated according to Burns et al. (2005). Detailed calculations are found in the supplementary materials (Text S1 and Text S2).

2.5 Plankton Analysis

Phytoplankton species were identified and counted using an inverted microscope following the Utermöhl (1958) method. The currently accepted nomenclature was achieved according to AlgaeBase (2022). Three subsamples of zooplankton were examined separately under a binocular research microscope with a magnification of ×100 or ×400.

2.6 Chlorophyll a and Biochemical Analysis

Water samples were filtered through a Whatman GF/F glass fiber filter. Chlorophyll a was extracted using 90% acetone and measured according to APHA (2017) using a PerkinElmer (LS45) fluorescence spectrometer and compared to a standard curve as μg/L. Total protein was investigated using the Biuret method (David and Hazel, 1993), while total lipid was determined according to Chabrol and Castellano (1961).

2.7 Phytobenthic Diatoms Analysis

Repeated surface sediment samples were collected using an Ekman Grab until undisturbed samples were obtained. Phytobenthic diatom samples were collected from the uppermost millimeter, digested using strong acids, and permanent slides were prepared using Naphrax. Phytobenthic diatom samples were identified and at least 350 valves were enumerated using an inverted microscope.

2.8 Fisheries Sampling and Analysis

A total of 1008 fish samples were collected year-round from the trammel nets used in the commercial fisheries of El Bieda Lake. Fish length was measured from the tip of the snout to the tip of the longest caudal fin ray and expressed in mm. Total weight was measured in grams. Fulton’s coefficient of condition (Kc) was applied (Fulton, 1904):

$$Kc=\frac{W}{L^3}\times 100$$

where W is total or gutted body weight (gm) and L is the total body length (cm).

2.9 Statistical Analysis

A detrended multivariate analysis was used to relate the dominant phytoplankton, zooplankton, and phytobenthic diatoms species composition to the environmental variables and evaluate how much variance could be explained by these variables, Canonical Corresponding Analysis (CCA) was used (Ter Braak & Prentice, 1988). The significance of the CCA was verified with the Monte Carlo Permutation Test using 499 permutations. For the ordination analysis, data were log10 transformed, and the analysis was performed using CanoDraw v4.5. To signify the multivariate analysis, multi-collinearity test was performed for the available environmental dataset. Water temperature (W Temp), total dissolved solids (TDS), Biological Oxygen Demand (BOD), total alkalinity (Alk.), total phosphorus (TP), nitrogen-phosphorus ratio (N:P), and reactive silicate (SiO3) were retained, while other environmental variables were excluded.

One-way ANOVA was used to compare the seasons and sites of the lake. Pearson’s correlation analysis was used to test the pairwise relationships between chlorophyll a, lipid, protein, and environmental variables. Multi-collinearity, Pearson correlation coefficients, and ANOVAs were performed using XLSTAT v2016.

3 Results

3.1 Water’s Physical and Chemical Characteristics

Some physicochemical characteristics of El Bieda Lake are given in Table 1. Salinity, total dissolved solids, and electrical conductivity showed the same distribution pattern at all sampling sites. Their values ranged between 18.11 and 27.26 ‰, 18.57 and 27.94 g/L, and 26.0 and 39.12 mS/cm, respectively. Their maximum values were recorded during summer, while the lowest values were recorded during winter. The moderate salinity gradient classified the lake as polyhaline (18–30ppt). The lake water was slightly alkaline (pH range = 7.8–8.88). Carbonates and bicarbonates showed normal ranges, 20.0–60 and 100–212.5 mg/L, respectively.

Table 1 Range, mean, and standard deviation of physico-chemical characteristics of El Bieda Lake
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Dissolved oxygen (DO) values increased during winter and decreased during summer (11.4–6.0 mg/L, respectively) with oxygen saturation of 136.32% and 80.66%, respectively. The DO has an annual mean (116.62%) revealing well-oxygenated water. BOD and COD values varied within narrow ranges (4.8–9.8 and 4.8–21.8 mg/L, respectively). Nitrite values showed a narrow insignificant variation (12.3–20.9 μg/L); nitrate showed high levels during winter, while ammonium peaked during summer. Orthophosphate (PO4) and TP values showed similar distribution; their minimum values (24.6 and 43.9 μg/L, respectively) were found at site 4 during summer, while their maximum values (48.71 and 131.08 μg/L, respectively) were reported at site 1 during winter. Reactive silicate showed a climax (18.2 mg/L) at site 5 during summer and a minimum of 6.1 mg/L at site 2 during winter.

3.2 Ionic Composition of El Bieda Lake

Major cations (Na+, K+, Mg+2, and Ca+2) and major anions (Cl−, SO4−2, and HCO3−) were well represented in El Bieda Lake. TDS values were much lower than the surface seawater (Tables 1 and 2). Na+ and Cl− ions were obviously lower than the surface seawater at the equivalent TDS of 23.417 g/L, while all major ions were many folds signified. Major cations were arranged in the lake in the order of, Na+ > Mg+2 > K+ > Ca+2, while major anions were in the order of Cl− > SO4−2 > HCO3−.

Table 2 Mean concentrations of major ions (g/L) in El Bieda Lake (current study) compared with its background, seawater, Lake Qarun, and Bardawil Lagoon
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The abundance of the major cations in El Bieda Lake water showed that sodium ion was the most abundant cation (mean value 182.7 meq; 47.4%), followed by magnesium (mean value 28.9 meq; 28.9%), calcium (mean value 55.7 meq; 14.4%), and potassium (mean value 35.4 meq, 9.3%). Chloride ion was the most abundant anion (mean value 253.9 meq; 72 %), followed by sulfate (mean value 94.5 meq; 26.8 %) and bicarbonate (avr. value 3.3 meq; 0.9 %). Piper diagram demonstrates the dominance of alkali metals (Na + K) above earth elements (Ca + Mg) (Piper 1944). Moreover, Piper diagram (Fig. 2) indicated that El Bieda Lake samples belong to the hydrochemical type of (Na+K)-(Cl+SO4). Both the anion and cationic triangles of the diagram showed the high homogeneity of the lake water, which was reinforced by the minimal standard deviations and variation coefficients lower than 6%.

Fig. 2
figure 2

Piper diagram demonstrates the ionic composition of El Bieda Lake water samples

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3.3 Water Quality and Trophic State

WQI values showed non-significant fluctuation (67.73–71.09%), rating the lake at the fair level (65–80%, with the mean of 70.01 (Table 3). Site 1 showed the highest WQI value (71.09%), while site 4 represented the minimum value (67.73 %). TLI index classified El Bieda Lake at a mesotrophic water state (Table 3). TLI index varied within a very narrow range, from the lowest of 3.01 at station 2 to the highest of 3.22 at station 5.

Table 3 Water quality and Trophic Level indices (WQI and TLI) in El Bieda Lake
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3.4 Phytoplankton Composition, Biochemical, and Chlorophyll a Contents

A total of 62 species were identified in El Bieda Lake belonging to 7 phyla: Bacillariophyta (29 species), Cyanobacteria (20 species), Chlorophyta (4 species), Euglenozoa (4 species), Miozoa (3 species), Ochrophyta (1 species), and Cryptophyta (1 species) (Table 1S). A minimum mean phytoplankton density of 827 × 104 unit/L was found during summer compared to a maximum mean of 2177 × 104 unit/L in winter. Mean phytoplankton densities were minimum at the most eastern station (Site 2) and the most north-western station (Site 4). Bacillariophyta dominated the phytoplankton communities, contributing to 60.48%, followed by Miozoa (15.75%), Cyanobacteria (15.32%), and Chlorophyta (7.63%). Euglenophyta, Cryptophyte, and Ochrophyta were marginally represented (< 1). Additionally, phytoplankton communities were mainly represented by Aphanocapsa litoralis, Lindavia glomerata, Pantocsekiella kuetzingiana, Prorocentrum micans, and Pyramimonas diskoicola; altogether constituted 60.01% (in autumn) and 82.5% (in winter) of the total phytoplankton density (Table 1S). Lindavia glomerata and Pantocsekiella kuetzingiana flourished principally during winter, whereas other representative species slightly dominated in autumn.

The variance of phytoplankton species distribution explained by environmental variables through CCA analysis was 70.83% of the total variability (Fig. 3a). Monte Carlo Test indicated that TP, alkalinity, and N:P were the most important (P = 0.013, 0.031, and 0.042, respectively). The first two canonical axes explained 67.21% (axis 1: 44.4% and axis 2: 22.8%). Planktonic diatom distribution did not associate well with SiO3. Primarily Lindavia glomerata and Pantocsekiella kuetzingiana, and marginally Pantocsekiella ocellata and Pinnularia major, were collectively affected by all explanatory variables. In addition, cyanobacterial species except Chroococcus minor and Romeria leopoliensis were highly related to TDS and N:P ratios.

Chlorophyll a and protein contents were significantly varied between seasons, while lipid showed a non-significant temporal distribution (Fig. 4). Chlorophyll a value peaked in autumn (9.01 μg/L) compared to a minimum in summer (4.81 μg/L, P < 0.01). Protein contents were highest in winter (13.52 mg/L) and lowest in summer (3.638 mg/L, P < 0.005). Water temperature, TDS, and COD had negative effects on chlorophyll a (r = −0.65, −0.55, and −0.56, respectively, P < 0.05), while TP, alkalinity, and pH had positive impacts (r = 0.46, 0.53, and −0.69, respectively, P < 0.05). Furthermore, protein contents were negatively correlated with water temperature, TDS, N:P, and SiO3 (r = −0.45, P < 0.05, r = −0.71, P < 0.005, and r = −0.53, P < 0.005, respectively) but were positively associated with BOD (r = 0.73, P < 0.002), TP (r = 0.72, P < 0.004), alkalinity (r = 0.57, P < 0.05), and total phytoplankton (r = 0.51, P < 0.05). The mean content of lipid was highest in winter (0.24 mg/L) and lowest in autumn (0.1 mg/L). Lipid had no significant association with different environmental variables.

Fig. 4
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Chlorophyll a and protein (at the primary axis) and lipid (at the secondary axis) at different stations

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3.5 Phytobenthic Diatoms

Sixty-six taxa were identified: Sixty-three to the species or variety level and three to the genus level. A complete species list of identified species in El Bieda Lake is presented in Table 1S. The most frequent species (≥ 5%) contributed to the CCA analysis. Phytobenthic diatoms formed dens biofilm with a thickness of about 4 mm in the southern sandy bottom and a thinner one in the northern clayey bottom. El Bieda Lake comprised mostly of chain-forming centric diatoms, such as Aulacoseira, Thalassiosira, Lindavia glomerata, Pantocsekiella ocellata, and pinnate taxa such as P. rostratoholarcticum, Navicula, and Nitzschia spp. Six taxa were considered highly abundant (i.e., relative abundance ≥ 15% in at least one sample), P. rostratoholarcticum, Amphora lineolata, L. glomerata, Sellaphora pupula, Nitzschia palea, and Thalassiosira sp.

Some influential environmental variables are illustrated by the CCA bi-plot (Fig. 3b). These environmental variables could explain 79.89% of the phytobenthic diatom species variation. TDS, BOD, and water temperature were the most significant variables influencing species abundance (F = 2.68; P = 0.008) (Fig. 3b). SiO3 was the least associated with the species axes. N. palea was the most positively correlated with SiO3, while most other species showed partial or no association. The majority of phytobenthic diatom species were positively accompanying with total alkalinity, TP, and BOD but were negatively linked with water temperature and TDS.

Fig. 3
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CCA biplot of most important a phytoplankton, b microphytobenthic diatoms, and c zooplankton species with the most significant environmental factors, codes of species are represented in Table (1S)

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3.6 Zooplankton

Zooplankton were characterized by four rotifer species (Brachionus plicatilis, B. calyciflorus, B. angularis, and Hexarthra mira), two adult copepod species (Apocyclops panamensis and Euterpina acutifrons), in addition to the larval copepod forms. Rotifers and copepods shared the total zooplankton abundance with 52.07% and 47.93%, with annual average densities of 1,527,400 and 1,276,350 org. m−3, respectively (Table 1S). The rotifer species, B. plicatilis, was the most dominant and frequent; it shared the rotifer abundance with 96.4%. B. plicatilis was more abundant in summer, especially at sites 3 and 4 with densities of 120,000 and 110,000 org. m−3, respectively. Other rotifers were slightly recorded at all sites, completely disappeared at sites 1, 2, and 4 in winter. H. mira shared B. plicatilis in summer. The larval stages of copepods contributed to 81.23% of total Copepoda with an average density of 930,800 org. m−3. A. panamensis was found year-round except at site 1, contributing 8.7 % of the total mean copepod’s density. The highest density (700,000 org. m−3) was reported at site 4 in winter.

The clarified zooplankton species variation by the environmental variables through CCA analysis was 94.82% of the total variability (Fig. 3c). Monte Carlo Test indicated that all environmental variables highly explained zooplankton distribution (P < 0.035). The first two canonical axes explained 72.01% (axis 1: 44.61% and axis 2: 27.4%). Copepoda species showed cumulative response to different environmental variables, whereas most Rotifera spp. were highly correlated with TDS and water temperature.

3.7 Fisheries

More recently, El Bieda Lake was colonized by two euryhaline tilapia fishes; blue tilapia (Oreochromis aureus) and redbelly tilapia (Coptodon zillii). The total production of both species is still below the economic level and primarily used for recreational activities. The Nile tilapia (Oreochromis niloticus) and the invasive species sailfin molly (Poecilia latipinna) rarely appeared in the samples. Tens of kilos of C. zillii and O. aureus were irregularly caught in the lake by only one paddleboat using a trammel net. It was worth noticing that summer was characterized by very low mortality for O. aureus and poor representation for C. zillii, and both fishes shared the fisheries catch for the rest of the year.

Total weight and total length were used for calculating the condition factor (K) (Table 4). The condition factor slightly varied between fish species in El Bieda Lake. However, the K values were higher for C. zillii than those reported for O. aureus all seasons, except for summer when C. zillii nearly disappeared.

Table 4 Average condition factor (K) for dominant fish species in El Bieda Lake
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4 Discussion

Since the early study of Lucas (1912), Wadi El Natrun Depression has been studied until now. Most of these studies focussed on the geological structure, particularly the mineral composition of the surface and deeper sediments around and in the lakes (Soliman et al., 2021; El-Dars & Sami, 2020; Mashaal et al., 2020; Hussein et al., 2017). Most of these minerals were sodium complexes, e.g., natron, thermonatrite, trona, nahcolite, burkeite, thenardite, mirabilite, halite, and pirsonnite (Shortland, 2004).

Nakhla et al. (1986) dug 40 pits across El Bieda Lake, and a superficial layer of halite with an average thickness of 0.5 m was found throughout the lake. Underneath the halite layer, a black clay layer was present, followed by a layer with a thickness of 4 m composed of thenardite intercalated with black clay. Due to the continual extraction of the superficial halite layer, the underneath clay layer became permanently submerged with the continual discharge of agricultural and domestic drainage water resulting from the great expansion in agricultural land cultivation and massive increase in urbanization (Gad & El-Zeiny, 2016).

The continual extraction of the superficial sodium minerals and discharging of drainage water could explain the sharp decrease in major cations (Na+, K+, Mg++, and Ca++, sum≈108 g/L) and major anions (Cl−, SO4−, CO3−, and HCO3−, sum≈103.63 g/L) that were reported by Sayed and Abdo (2009) and the studies cited above (mean TDS was about 212 g/L). These salt reductions in El Bieda Lake could be confirmed by the stopping of mining by El-Nasr Salt Company due to the uneconomic levels of these evaporites. The inversion of Cl/Na ratio from 0.75:1 (Sayed & Abdo, 2009) to 2.24:1 in this study could be because most extracted minerals were sodium complexes, as mentioned above.

Piper trilinear diagram is one of the most common tools used for water classification based on ions relationship. Piper diagram identified one principal water type (Na + Cl Type) for different water samples. The Na + K cationic water type, Cl anionic type, and the dominance of sodium chloride type indicated the ancient groundwater source. The isotopic results of Mashaal et al. (2020) supported these findings. The results of Mashaal et al. (2020) indicated that there was a continual replenishment of Wadi El Natrun aquifer from the Nile Delta aquifer water contributing to 80% of total Wadi El Natrun aquifer water, whereas the paleowater contributed only 20%.

Compared to the equivalent sea water, El Bieda Lake has lower Na and Cl ionic concentrations equivalent to those of ~58.38% and 72.69%, respectively, but has higher Mg, Ca, and K equivalent to those of ~282.73, 424.0, and 591.35%, respectively. These imbalanced ion concentrations may negatively affect the biota of the aquatic ecosystem (Pillard et al., 2000). Many laboratory studies indicated that marine organisms could survive under higher concentrations of Mg, Ca, and K. Douglas et al. (1996) reported that tolerance ranges of Mysidopsis bahia against Ca, K, and Mg increased as salinity increased, reaching a maximum tolerance at the salinity of 25–30‰. Pillard et al. (2000) conducted similar studies for Cyprinodon variegatus and found that it could survive at Mg concentration of 4.8 g/L, Ca of 4.01 g/L, and K of 1.17 g/L. Rao (2020) demonstrated that the acute toxic effects of K on rainbow trout ranged from 2108 to 2737 mg/L. Furthermore, some ions could antagonize the toxic effect of a given ion in the marine environment. Pillard et al. (2000) found that four times increase of SO4 concentration could interact and eliminate the toxicity of five times Ca, rising the survival to 100%. The presence of sodium, magnesium, and calcium was shown to ameliorate the toxic effects of potassium (Borvinskaya et al., 2017). These findings are well established through many field observations. For example, Kavembe et al. (2016) stated that the East African Saline Lakes, including extreme saline lakes, support 71 fish species of considerable economic importance. Qarun Lake and Bardawil Lagoon are characterized by plentiful and diversified fish catches (GAFRD, 2017), even though they have SO4, Na, Mg, and Cl values (Table 2) much higher than the equivalent seawater values (Al-Afify et al., 2019; Ali et al., 2006). The current study indicated that El Bieda Lake itself successfully supported the growth of two euryhaline fishes, O. aureus and C. zillii, with condition factors comparable to other Egyptian lakes.

Phytoplankton in El Bieda Lake showed lower species richness than other Western Desert Lakes. Qarun Lake had a phytoplankton species richness of 197 spp. (Abd El-Karim, 2012), whereas Wadi El Rayan Lake showed a phytoplankton species richness of 92 spp. (Sabae and Mahmoud, 2021). On the other hand, the phytoplankton density in El Bieda Lake was comparable with, sometimes higher than, both Qarun and Wadi El Rayan Lakes. Qarun Lake harbored a phytoplankton density of 296.9 cells × 104/L (Abd El-Karim, 2012) and 1199.5 cells × 104/L (Flefil & Mahmoud, 2021). Phytoplankton abundance of 1450 cells × 104/L (Konsowa & Abd Ellah, 2002) and 801.8 cells × 104/L (Sabae & Mahmoud, 2021) was reported in Wadi El Rayan Lakes.

Zooplankton in El Bieda Lake were composed of Rotifera and Copepoda. Rotifera was represented mainly by Brachionus spp.; B. plicatilis was the most common among them. Similar results were recorded in Wadi El Rayan Lakes (El-Shabrawy, 1999; Khalifa & Abd El-Hady, 2010) and Qarun Lake (El-Shabrawy et al., 2015). El-Shabrawy (1999) and Khalifa and Abd El-Hady (2010) stated that Rotifera dominated zooplankton communities in the saline Wadi El Rayan Lake, prevailed with Hexarthra oxyuris in winter, and B. plicatilis in summer specifically in the highest saline area in the south. Similarly, El-Shabrawy et al. (2015) indicated that Rotifera dominated zooplankton with B. cf. rotundiformis, particularly in the highest saline area of Qarun Lake, comprising 49.7% of total zooplankton abundance. B. plicatilis had high protein and lipid contents of 51.6% and 33.01%, respectively, and high contents of several fatty acids and essential amino acids, specifically glutamic acid (Hegab et al., 2020).

The composition of Copepoda in El Bieda Lake was so different from the Western Desert Egyptian saline lakes (Qarun and Wadi El Rayan Lakes). The prevailed copepods, A. panamensis and E. acutifrons, in El Bieda Lake were not or partially recorded in Qarun and Wadi El Rayan Lakes (El-Shabrawy et al., 2015; Khalifa and Abd El-Hady, 2010). Additionally, the total zooplankton (2,566,136 org. m−3) in El Bieda Lake were many folds higher than the recorded densities in Wadi El Rayan (Khalifa & Abd El-Hady, 2010) and Qarun Lakes (El-Shabrawy et al., 2015).

O. aureus occurs predominately in fresh waters, exists in brackish, and seldom in marine waters as well (Trewevas, 1983). Adults forage primarily on microalgae and sometimes feed on zooplankton. O. aureus juveniles feed mostly on zooplankton and small arthropods (Trewevas, 1983; Buntz & Manooch, 1968;). C. zillii is euryhaline, inhabits freshwaters (Riehl & Baensch, 1991), and can tolerate salinity in the range of 29-45 (Lee et al., 1980). C. zillii has diversified food items (Shalloof et al., 2020). The average condition factors (K) were 1.82 and 2.03 for O. aureus and C. zillii, respectively. These values are close to those recorded for the same species in Lake Burullus (El-Haweet, 1991), Lake Edku (El-Sawy, 2006) and Lake Qarun (Azab et al., 2015) but are high when compared with those recorded in Lake Mariut (Abaza, 2003), indicating that these species grow well in El Bieda Lake.

The higher average densities and high nutritional values of phytoplankton and zooplankton communities accompanied by the nominal fish yield in El Bieda Lake compared with other older Western Desert Egyptian lakes, revealing that fishes colonize the lake are under their carrying capacity. Thereby, stocking more C. zillii and O. aureus or introducing more economical euryhaline fish species, e.g., mullets, is an imperious issue.

Mullets, e.g., Mugil cephalus, Chelon labrosus, C. ramada, C. auratus, and C. saliens, are natives of the Mediterranean Sea (Froese and Pauly, 2022; Gisbert et al., 1995). They are considerably caught from many Egyptian lagoons (GAFRD, 2017). M. cephalus and C. ramada are the most popular, efficaciously translocated and stocked in many inland lakes and successfully cultured in Egypt, representing 14.48% of the total fish cultured in 2017 (1,451,841 tons) (GAFRD, 2017). Members of the family Mugilidae are euryhaline and could tolerate a wide range of salinities (Cardona, 2006; Whitfield et al., 2012). The optimal salinity levels of Mugil cephalus fingerlings are at 10–20‰, which are required for high growth and metabolic rate (Barman et al., 2005). Adults always avoided freshwater areas and concentrated in oligohaline, mesohaline, and euhaline sites (Loi et al., 2022; Cardona, 2006, 2001). However, M. cephalus acclimated well to a gradual increase of salinity and could tolerate a level up to 126 ppt, and no stress sign or decrease in appetite appeared (Hotos & Vlahos, 1998). C. ramada could tolerate a wide range of salinity (Lafaille et al., 2002), and could exist at high salinity sites, but displayed a preference for low salinity sites (Cardona, 2006, 2000).

Grey mullets showed discrete diet changes during their life cycles since larvae and fry stages were planktivorous, specifically zooplanktivorous, and they shifted to herbivorous/detritivorous as they grew, finally they became bottom feeders (Blanco et al., 2003), whereas the prevalent prey in the M. cephalus fry’s diet were cyclopoids, calanoids, and cladocerans, rarely fed on detritus, C. ramada fry exploited cyclopoids and calanoids principally (Cardona, 2006), but both could shift to phytoplankton and filamentous green algae when zooplankton were scarce (Gisber et al., 1995).

M. cephalus and C. ramada adult fed mainly on benthic diatoms and detritus, together with foraminifera, filamentous algae, protists, meiofauna, small invertebrates, and bacteria (Lawson & Jimoh 2010; Mwandya et al. 2010). Many studies demonstrated that adults of M. cephalus could digest blue-green algae (Payne, 1976) but could be switched, with C. ramada, to less nutritious detritus when necessary (Cardona, 2006). Ecologically, grey mullets inhabit clear and turbid areas, sandy and muddy habitats, tolerate low oxygen levels, and can switch to anaerobic metabolism in hypoxic waters (Shingles et al., 2005;).

Finally, the well-oxygenated water in El Bieda Lake, rich with phytoplankton privileged with diatoms (60.48% of total phytoplankton) and zooplankton dominated with Copepoda (45.47% of total zooplankton), together with the rich benthic diatoms, sandy bottom in the south and muddy in the north, and low salinity (annual mean of 23.07‰) compared to seawater, evidenced that translocation and culturing of the euryhaline mullets in El Bieda Lake are very advised. M. cephalus or C. ramada are more favorable than C. zillii and O. aureus due to their wide tolerance range of the expected salinity changes and their commercial preferences due to their fast growth rate and high economic value (Hotos & Vlahos, 1998).

5 Conclusion

The chemistry of El Bieda Lake has drastically changed due to the prolonged extraction of salts and the continuous discharging of agricultural and domestic drainages water resulted from the great expansion in land cultivation and massive increase in urbanization. Consequently, TDS decreased from 212.714 g/L in 2009 to 23.417 g/L during this study. However, the lake still has higher concentrations of some major ions than the equivalent seawater TDS. Even though the recent origin of the lake, phytoplankton and zooplankton communities’ densities were much higher than many other older Egyptian lakes. The study concluded that plankton communities and the phytobenthic diatoms were slightly utilized by C. zillii and O. aureus which are under their carrying capacity. The well-oxygenated water, low salinity, high diatoms, copepods densities, and many other lake features encouraged us to recommend the translocation and culturing of the commercially and fast-growing M. cephalus or C. ramada. Further studies should be conducted to evaluate the effect of the higher load of major ions and their ratios on the growth and survival of different stages of introduced fishes, as well as fishes’ osmoregulation and histopathological, should be investigated as important bases for rational stocking and utilizing El Bieda Lake for commercializing fishes. Investigating pollutants’ levels, including metals and pesticides, and their impact on fish productivity is highly recommended.