Introduction

The Egyptian government, with its ambitious strategies for the future, plans to protect and conserve the coastal ecosystems for sustainable development. Goal #14 of the Egyptian National 2030 Agenda is “conserve and sustainably use of the oceans, seas, and marine resources.” Decision makers critically need fundamental data regarding the current conditions and predictions of future changes in ecosystems quality, to determine and mandate precautions regarding the hazardous effects of environmental deterioration. Lake Edku is a human-impacted Nile Delta Lake on the northern Mediterranean coast of Egypt. It supports a fishery that accounts for more than 5% of the Egyptian northern lakes fish production and provides habitat for both wintering and breeding water birds (Khalil et al., 2013). The environment of the lake has been substantially degraded after the construction of Aswan High Dam over the Nile River in 1965 (Zalat & Vildary, 2007) as it resulted in (1) checking the flow of water downstream, (2) drastic changes in the physico-chemical and biological parameters of the Nile ecosystem, and (3) the reduction in area of three critical lacustrine ecosystems of the Nile Delta including Edku Lake (Abdel-Satar et al., 2017; El-Shazly, 2019). The input of untreated wastes from many local pollution sources (agricultural, industrial, and urban effluents) along its eastern and southern margins was an additional threat on Edku Lake’s environmental quality (Badr & Hussein, 2010). Dickman (1998) concluded that assessments of the ecological quality of aquatic ecosystems should not depend on chemical measurements only, yet pollutant impacts can be observed directly by studying the affected biological communities.

Benthic foraminifera (BF) are single-celled microscopic organisms (Kingdom Protista), many of which construct shells (commonly called tests) made of calcium carbonate or by agglutinating sediment particles. Because BF are extremely diverse and cosmopolitan, some species can tolerate in almost any marine environment (Förderer et al., 2018). With relatively short life cycles, BF can respond quickly to changes in physical and geochemical factors including temperature, salinity, pH, oxygen variability, sediment texture, organic carbon, and inorganic sediment composition, through changing foraminiferal assemblages (Murray, 2006). In addition, changes in their community structure (e.g., assemblages) have been widely used as a tool to help in the interpretation and reconstruction of modern and ancient environments (e.g., de Jesus et al., 2020; Narayan et al., 2015; Reymond et al., 2013). An added advantage of BF is their high preservation potential in the sediment record and the abundance of their tests that provides comparative information to assess short-term environmental changes (years to decades). The responses of BF to environmental stress have been used for more than 60 years as indicators for characterization and monitoring anthropogenically impacted coastal systems (e.g., Ben-Eliahu et al., 2020; Chalkley et al., 2019; Dimiza et al., 2019; Resig, 1960). Benthic foraminifera respond to both natural and anthropogenic environmental gradients as evidenced in taxonomic structure, foraminiferal density and diversity, and increased occurrences of deformities (e.g., Coccioni et al., 2009; Elshanawany et al., 2011; Martins et al., 2016; Samir & El-Din, 2001; Yanko et al., 1998).

The main objectives of the present work were to (1) investigate the species composition and diversity of the benthic foraminiferal assemblages and (2) determine the foraminiferal assemblage response to wastewater discharges, including organic-carbon and heavy metals. Two ecological proxies, species richness and foraminiferal abnormality index (FAI), were utilized as environmental stress indicators.

Materials and methods

Data collection

Lake Edku is the third-largest, shallow-brackish coastal basin in northern Egypt (Fig. 1), situated west of the Nile Delta (latitudes 31° 11′ 30″ and 31° 18′ 00″ N, longitudes 31° 8′ 30″ and 31° 23′ 00″ E) and separated from the Mediterranean Sea a coastal by sand barrier. A narrow, 2 m depth entrance (Boughaz El-Maadia) on the west side allows hydrodynamic circulation between Lake Edku and Abu-Qir Bay. Abdel Halim et al. (2013) reported a very low average salinity (1.13%o ± 0.19) in the eastern basin of the lake, which is affected by discharges from the El-Khairy drain. Seawater enters the lake from Abu-Qir Bay through Boughaz El-Maadia, increasing the salinity to about 15‰ in the lake entrance (Abdel Halim et al., 2013). The water in the lake varies from clear to very turbid with sediment and plankton where its water depth ranges from 40 to 150 cm with an average of ~ 1 m (Abdallah, 2017). Lake Edku is divided into three main basins: western, central, and eastern. Two main drains; El-Khairy drain, which connects to three subdrains (Edku, El-Bousily and Damanhour), and the Barsik drain (Fig. 1), discharge domestic, agricultural, and industrial wastewater (Ossman & Badr, 2010), as well as the drainage water of more than 300 fish farms, into middle and eastern lake basins (Badr & Hussein, 2010). The lake receives total annual untreated drainage water (domestic, agricultural, and industrial) of 592 × 106 m3/year and 348 × 106 m3 from El-Khairy and Barsik drains, respectively (Morsy et al., 2020).

Fig. 1
figure 1

A Map showing location of Edku Lake northeast Nile Delta of Egypt. B Google map showing the sampling locations after BadrElDin et al. (2022)

Short sediment cores were collected in triplicate in 2020 from nine sites; one replicate was used for sedimentological and geochemical analyses and two replicates for foraminiferal assessment (Fig. 1). The sampling was undertaken using heavy duty PVC tubes (5 cm diameter and 50 cm height). The sediment cores were carefully extruded, preserved in clean labeled polyethylene bags, and were kept in ice tank until arrival at the laboratory. As the core samples lack any kind of stratification, they were sliced at 5-cm intervals to yield three or four subsamples (0–5 cm, 5–15 cm, 15–25 cm, and 25–35 cm).

Geochemical and contamination evaluation data

The mechanical sieving and pipette analyses were applied for grain size determination (Folk, 1980), and the percentages of sand (S%), silt (Z%), and clay (C%) were calculated. The total organic carbon (TOC%) was determined by loss in ignition (Heiri et al., 2001), and the total carbonate (TCO3%) was determined by the indirect method (Vogel, 1978). Concentrations of seven elements were analyzed (Cu, Pb, Zn, Cd, Cr, Ni, and As) using the method described by Liao et al. (2014). Three environmental quality indices were employed to evaluate sediment contamination: (1) contamination factor (CF) (Pekey et al., 2004), (2) degree of contamination (DC) (Håkanson, 1980), and (3) sediment quality guidelines (SQG) (Long et al., 1995; MacDonald et al., 2000). The methodologies used to determine the environmental variables and to calculate the environmental quality indices were fully described in BadrElDin et al. (2022). The base values that correspond to the terminologies used to describe the CF and DC are presented in Table 1.

Table 1 Terminology of pollution classes for contamination factor (CF) and degree of contamination (DC)

Foraminiferal data

For all samples, constant volumes of 50 cm3 were washed over 63-μm sieves, and coarse fractions were oven-dried at 50 °C. Samples were examined using a stereomicroscope, and ~ 100 foraminiferal tests were handpicked from each sample (the foraminiferal fauna of Lake Edku was made up of very few species; see also Fatela and Taborda (2002)). Specimens were identified to genera level following the generic classification of Loeblich and Tappan (1988) and to species level consistent mainly with Cimerman and Langer (1991) and a variety of sources. The name of each species was checked and revised in accordance with the online database WoRMS (World Register of Marine Species; Hayward et al., 2020). Species richness and foraminiferal abnormality index (FAI, Coccioni et al., 2005) were evaluated. Selected species and deformed specimens were photographed using scanning electron microscope (SEM) JSM–IT 200 in Faculty of Science, Alexandria University.

Statistical analyses

Species counts in each sample were converted to percent abundance (i.e., species relative abundance). The data set was the averages of the species relative abundance, and all determined environmental variables for each individual core and its related salinity (Table 2). The salinity data was provided by Prof. Nadia B.E. Badr (personal communication). The results of CF, DC, and FAI were not included in the statistical analyses. Analysis was applied to taxa that averaging ≥ 5% relative abundance in at least one core in the subsequent statistical analyses (Fishbein & Patterson, 1993). The infrequently occurring taxa do not significantly affect the formation of the major groups (Frezza & Carboni, 2009; Romano et al., 2008), and focusing on the most abundant taxa reduces background noise and reveals the underlying signatures of the assemblages (Fajemila et al., 2020). The data set was normalized using the equation (N = (value-mean)/standard deviation). One-way ANOVA analysis was performed to show the variability between the cores using Past program (V. 4.03). Q-mode cluster analysis (QCA) and principal component analysis (PCA) were carried out to identify cores characterized by similar assemblages and illustrate the key factors controlling the distribution of BF in core sediments (Chai et al., Full size image

Morphological abnormalities, evaluated using the foraminiferal abnormality index (FAI), ranged between 0 and 10% (average for each core 0–5%) and were only found in specimens of A. tepida (Table 5, Appendices 3, 4, 5, and 6). No aberrant specimens were found in western cores I–III, whereas the FAI increased gradually eastward in the central and eastern basins, reaching its maximum value in the upper 5 cm in core IX (10%, Tables 5 and 6). The percentages of deformed specimens decreased downcore in cores V–IX.

Ammonia tepida specimens showed a wide range of deformation. The recorded morphological abnormalities included aberrant chamber shape, abnormal chamber size, additional chambers, abnormal test growth, elongated axes of rotation, spiroconvex coiling, twisted test, excess deposition of calcium carbonate, irregular periphery, and complex deformities. We classified the degree of test deformation according to the following criteria: (1) mild deformation (group A, Appendix 3), where the test maintains the general characteristics of a normal test and could be identified easily; (2) moderate deformation (group B, Appendices 4 and 5), where tests exhibited more than one type of deformation, yet the test retains the general characteristics of normal tests; and (3) extreme deformation (group C, Appendix 6), where the tests show multiple complex deformations, such that species identification was very difficult in some instances. The degree of deformation increased toward the eastern basin of Edku Lake (Table 5, Fig. 8A). Abnormal specimens of groups A and B were represented in cores of the central and eastern lake basins (cores IVIX), whereas the specimens belonging to group C were found only in cores VIIIX. The spatial and vertical distributions of malformed specimens were consistent with contaminant concentrations.

Multivariate analyses

One-way ANOVA revealed that the foraminiferal assemblages and the determined variables were significantly different among the cores (f = 3.1; p = 0.003). A dendrogram generated by hierarchical Q-mode cluster analysis showed two groups of samples that reflected the regional distribution of cores (Fig. 8B). Cluster I included cores I, II, and III from the northeastern basin near Boughaz El-Maadia outlet, whereas cluster II grouped the central cores (IV–V) and the eastern cores (VI–IX) close to the major drains (El-Khairy, Edku, El-Bousily, and Barsik) (Figs. 1 and 8B).

The principal component analysis (PCA) was used to realize the environmental variables probably affecting the distribution of foraminiferal assemblages. The PCA revealed that the first two components clarify ~ 86.3% of the total data variance (Fig. 9, Appendix 7). The heavy metals, sand, silt, clay, TOC, and TCO3 are the predominant variables in the first component, whereas the major contributors to the second component are the foraminiferal species, salinity, Cd, and As. Ammonia tepida showed high abundances positively related to the lower salinities and the higher concentrations of Cd and As. The foraminiferal species, Ammonia parkinsoniana, Quinqueloculina auberiana, Cibicides refulgens, Asterigerinata mamilla, and Cribroelphidium excavatum, showed a preference for areas with marine influence and lower Cd and As concentrations.

Fig. 9
figure 9

Principal components analysis (PCA) for the average values for foraminiferal species abundance and the chemical and sedimentological variables for each core from Edku Lake. In frame the scree plot representing the number of components