Assessment of heavy metal pollution in seawater, benthic flora and fauna and their ability to survive under stressors along the northern Red Sea, Egypt

The study areas extended for about 290 km along the Red Sea coastline, from Ras Gharieb (150 km northern of Hurghada) to Qusier (140 km south of Hurghada). The selected study stations were located in tidal flat zones off the main cities of Ras Gharieb (n = 8), Hurghada (n = 11), Safaga (n = 10) and Qusier (n = 10) (Fig. 1). These cities are exposed to over-population and severe land-based activities involving oil exploration and production, maritime activities, tourist activities, marine wharves, marinas, shipyards, desalination plants, sewage treatment stations, fishing operations and harbours, land reclamation, mining and shipping operations, subsurface untreated sewage runoff and human waste dumping in the tidal flats, as well as temporary flash floods and non-point sources.

Figure 1

Thirty-nine seawater samples were collected semi-annually from the study stations with a water sampler (PVC tube with a capacity of approx. 3 liters) into acid-washed polyethylene bottles, then transported immediately in an ice box to the laboratory, where the pH of the samples was adjusted to 3–4 (Brown & Holley 1982). The seawater samples were filtered as soon as possible after collection through a 0.45-μm membrane to remove any suspended materials; their pH value was checked.

Heavy metals in the filtered seawater were pre-concentrated by complexing the metals with ammonium pyrrolidine dithiocarbamate (APDC); the complex compound was extracted into methyl isobutyl ketone and back-extracted into an acidic aqueous solution (Brewer et al. 1969). Five ml of the APDC suspension was added to 1 liter of the seawater sample, which was continuously shaken until chelation was complete (~5 min). A volume of 35 ml of methyl isobutyl ketone was then added and the solution mixed by magnetic stirrer for 5 min for complete extraction. The resulting organic complex layer was drawn by a separating funnel, evaporated until dry, dissolved into 2 ml of HNO₃, filtered and completed to 10 ml with double distilled water (DDW) prior to analysis (Boniforti et al. 1984). The concentrations of heavy metals were measured using a flame atomic absorption spectrophotometer (FAAS, GBC-932) at the National Institute of Oceanography and Fisheries in Egypt. The resulting data are expressed in μg l⁻¹.

The available benthic fauna was collected from each city by scuba diving and snorkelling. A total of 70 specimens representing 28 genera and 40 species of benthic fauna were collected semi-annually from the study stations (Supplementary Table 1). After the sampling process, all specimens were kept in polyethylene bags and transferred to the laboratory in an ice box. The collected specimens of marine fauna were identified according to Macfadyen (1936), Sung et al. (2009), Veron (2014) and Verseveldt (1982).

The benthic fauna samples were washed with fresh water several times to remove any adhering materials and then dried in direct sunlight. Approximately 10 g of each specimen was powdered using an automatic agate mortar. To measure the bio-available heavy metals (Fe, Mn, Zn, Cu, Ni, Pb and Cd), 0.5 g of each powdered sample was digested according to Chester et al. (1994) in nitric acid (HNO₃) and perchloric acid (HClO₄) (3:1) until completely dissociated. The samples were then digested on a hot plate. The residue of each sample was dissolved into 2 ml of 12N HNO₃, diluted to 25 ml with DDW, then filtered using a filter paper (Whatman, USA). The heavy metal concentrations were measured using an FAAS (GBC-932) at the National Institute of Oceanography and Fisheries, Egypt. The results are expressed as μg g⁻¹.

The study stations were surveyed and the available macro-algae and seagrasses were collected at each city by scuba diving and snorkelling. A total of 34 specimens representing 15 genera and 16 species were collected semi-annually from the study sites (Supplementary Table 2). After collection they were kept in polyethylene bags and transferred immediately to the laboratory in an ice box. The collected specimens of marine benthic flora were classified or identified according to El Shaffai (2016) and Jha et al. (2009).

The collected flora samples were washed several times with fresh water to remove any foreign adhering materials. The samples were air-dried and then powdered using an automatic homogeniser to assure complete homogeneity. To determine the bio-available Fe, Mn, Zn, Cu, Ni, Pb and Cd, 0.5 g of each powdered sample was digested using a 10-ml mixture of HNO₃ and HClO₄ (3:1) until complete (Chester et al. 1994). They were then evaporated and the residue was dissolved with 2 ml of 6N HNO₃, then diluted to 25 ml with DDW and filtered using a filter paper. The concentrations of bio-available heavy metals were determined using an FAAS, and the results are expressed as μg g⁻¹.

The data were statistically analysed in the software programme R version 4.1.3. The heavy metal concentrations in seawater were plotted in R script using the ‘geom_bar’ function in the ‘ggplot2’ package, version 3.3.6. All findings are expressed in tables and visualised in figures as means ± SD.

Heavy metal contamination in the marine environment represents a major worldwide environmental threat (Salah-Tantawy et al. 2022a). Its abundance implies dramatic changes in environmental conditions and provides the basis for identifying anthropogenic influences on marine environments (Al-Rousan et al. 2007; Fallon et al. 2002; Jayaraju et al. 2009). Heavy metal content in seawater is highly dependent upon some physicochemical characteristics: pH, salinity, suspended particulate matter and organic matter content (Hatje et al. 2003; Salah-Tantawy et al. 2022b).

3.1.1

Iron (Fe)

Iron is the fourth most abundant element in the earth's crust; it may be present in natural waters in varying quantities, depending on the geology of the area and other chemical components of the waterway. It has a terrestrial origin mainly derived from igneous, metamorphic and sedimentary rocks during erosion, weathering and chemical operations. Serving more biological roles than any other metal, it occurs in two main oxidation forms: an oxidising state (Fe^±3), which forms insoluble compounds, and a ferrous state (Fe^±2), which is soluble in aqueous media (USEPA 1986).

In this study, the average values of Fe in the seawater ranged between 7.86 and 27.95 μg l⁻¹, which is much higher than measurements from most seas (0.06–0.17 μg l⁻¹), including the Arabian Sea (Ferrier-Pagès et al. 2001; Measures & Vink 1999). The highest average values of Fe were recorded at Qusier and Ras Gharieb (Fig. 2A) in summer, due to increased terrestrial runoff from coastal activities and subsurface wastewater seepage. Whereas the Fe values were greater than those Al-Wesabi et al. (2015); Shriadah et al. (2004), while it consistent with Ali et al. (2011), Bazzi (2014), and El-Metwally (2015) (Supplementary Table 4).

Figure 2

The benthic fauna collected in this study showed varying abilities to accumulate heavy metal within their structures. Table 1 illustrates the concentrations of heavy metals in the collected fauna at Ras Gharieb. The lowest values of faunal diversity were recorded at Ras Gharieb: The benthic fauna was represented by six species, including soft corals, hard corals, echinoids and gastropods (Supplementary Table 1). Heteroxenia fuscescens (soft coral) demonstrated a great ability to accumulate significant amounts of Zn (446.59 μg g⁻¹), while Echinometra mathaei recorded significant Pb (65.55 μg g⁻¹) and Cd (5.50 μg g⁻¹) levels. Another species of soft coral, Sarcophyton trocheliophorum, accumulated 8.45 μg g⁻¹ of Ni. According to these estimated values, the accumulation sequence was in the following order: soft corals > echinoids > hard corals > Mollusca. The tidal zone of Ras Gharieb is highly impacted by dense petroleum pollution, which affects the faunal diversity and distribution. Consequently, the recorded species have a great deal of adaptability to survive under these severe amounts of pollution.

Additionally, the tidal flat of Hurghada suffers from different types of pollution from land-based activities that affect the benthic faunal types, distribution and diversity. Under these severe conditions, 22 species of adaptable, surviving organisms were collected – Porifera, soft coral, hard coral, Mollusca and echinoids (Supplementary Table 1). These species have varying abilities to accumulate heavy metals. As illustrated in Table 2, sponge species (Plakinastrella onkodes and Hyrtios protius) have a strong tendency to incorporate very high amounts of Fe (2011.31 and 852.10 μg/g, respectively) within their structures, followed by the echinoid species Clypeaster audouini (531.40 μg/g) and the Mollusca species Tridacna maxima (294.85 μg/g). Furthermore, Plakinastrella onkodes accumulated high Mn (75.75 μg/g) and Cu (22.95 μg/g) levels relative to the other benthic fauna, and the soft coral Dendronephthia hemprichi was found to have very high Zn content (196.95 μg/g) compared with the other species. Tectus dentatus showed high Pb accumulation, though it recorded an insignificant amount of Cd. Generally, the heavy metal content in the benthic fauna at Hurghada followed the descending bioaccumulation ability order: Porifera > soft corals > echinoids > Mollusca > hard corals.

Long ago, Safaga Fishing Harbour was used to finish and repair fishing boats. Moreover, it receives huge amounts of fine-particle sediments and heavy metals from trading and phosphate harbours as well as many other coastal activities. Twelve faunal species had adapted to these poor conditions, representing Porifera, hard corals, soft corals and Mollusca (Supplementary Table 1). Plakinastrella onkodes (Porifera) works as a scavenger of heavy metals from the surrounding environment, and has an outstanding ability to accumulate significant levels of Fe, Mn, Zn, Ni, Pb and Cd (1768.09, 246.30, 323.75, 14.80, 39.40 and 3.60 μg g⁻¹, respectively) (Table 3). In addition, the Mollusca species Murex tribulus was a scavenger of Cu (309.16 μg g⁻¹). The other benthic faunal communities recorded varying degrees of heavy metal accumulation. Heavy metal bioaccumulation in the benthic fauna at Safaga followed the descending ability order of Porifera > Mollusca > hard corals > soft corals.

A poor situation was also found in the tidal flat of Qusier, an area which suffers from underground wastewater seepage and high eutrophication, as well as fine sediment inputs from coastal-based activities (El-Metwally et al. 2017). Despite these challenging conditions, 17 benthic faunal species had adapted within this zone, representing Porifera, hard corals, echinoids and Mollusca (Supplementary Table 1). The Porifera species Plakinastrella onkodes was also found to be a powerful scavenger of Fe, Cu and Ni (1817.81, 78.60 and 31.95 μg g⁻¹, respectively), and Crassostrea sp. (a bivalve) was enriched with Zn (Table 4). Meanwhile, the hard coral species Acropora clathrata, Acropora valida and Galaxia fasicularis were highly enriched with Pb (131.55, 93.15 and 56.90 μg g⁻¹, respectively). Cd enrichment, meanwhile, was found in significantly high levels in the hard coral species Acropora valida, Acropora clathrate and Millopora dichtoma (12.80, 11.40 and 6.45 μg g⁻¹, respectively). Additionally, the echinoid species Tripneastus gratella and Diadema setosum recorded significantly high Fe levels. The tendency for heavy metal bioaccumulation in the benthic fauna at Qusier was in the following descending order: Porifera > Mollusca > echinoids > hard corals.

Porifera have a strong ability to concentrate metals in their tissues (Berthet et al. 2005; Cebrian et al. 2007; Johnston & Clark 2007), since they have numerous deep pores that can absorb metals in particle form and calcium can be replaced in their spines. The heavy metals found in the benthic faunal communities may have directly replaced calcium within the aragonite skeletal framework, may have been introduced into the skeletal pore spaces as suspended particulate matter (Dar 2004) or as metals incorporated inside the carbonate skeleton during biosynthesis (Ali et al. 2011; Sun et al. 2020). A previous study documented that heavy metals are not necessarily incorporated into the calcite structure, but can also be adsorbed onto the skeletal organic matrix or trapped as separate mineral phases. Additionally, the bioaccumulation processes within the benthic fauna were controlled by certain factors, including the bio-availability of the heavy metals, the surface area exposed to these metals, the degree of protection from the intensive wave action, turbidity limits and the varying abilities of these organisms to incorporate or assimilate heavy metals within their tissues or skeletons (Vander Putten et al. 2000). Another study summarised that the controlling factors for heavy metal bioaccumulation in the skeletal framework of corals were the exposed surface area for metal uptake, turbidity, overlying mucus thickness and the ability of the metals to substitute inside the crystal lattice of the hard corals (Abdel-Aziz & Dar 2010).

The heavy metal levels we measured in the tidal flat zones of the selected cities were higher than those recorded by Abd El-Wahab et al. (2005), Abdel-Aziz & Dar (2010), Dar et al. (2008), Dar & Abd El Wahab (2005), Dar & Mohammed (2009) and Madkour (2013) at different sites around the Red Sea.

Five seaweeds (Padina boryana, Digenea simplex, Sargassum cinereum, Coralline berteroi and Galaxaura marginata) and one seagrass species (Halodule pinifolia) were recorded at Ras Gharieb (Supplementary Table 2). P. boryana (a seaweed) is one of the more predominant species throughout the year. Significant concentrations of Fe, Mn, Zn, Cu and Ni were recorded in this species (2673.90, 188.35, 68.45, 28.70 and 50.75 μg g⁻¹, respectively). D. simplex had the highest accumulation of Pb (15.05 μg g⁻¹), while S. cinereum had the highest Cd concentration (2.20 μg g⁻¹) (Table 5). The recorded heavy metal values in the seagrass species H. pinifolia were significantly lower than all the seaweed species.

At Hurghada, nine seaweed species (Padina boryana, Digenea simplex, Sargassum cinereum, Galaxaura marginata, Halimeda tuna, Laurencia majuscula, Dictyopteris acrostichoides, Cystoseira indica and Caulerpa racemosa) and two seagrasses species (Halophila stipulacea and Halodule pinifolia) were collected (Supplementary Table 2). As shown in Table 6, L. majuscula was the highest accumulator for Fe, Mn and Zn (1848.39, 153.10 and 85.00 μg g⁻¹, respectively). P. boryana was found to be the best scavenger for Ni (25.25 μg g⁻¹) and S. cinereum for Pb (13.50 μg g⁻¹). Cu bioaccumulation was nearly equal in 4 seaweed and seagrass species: C. racemosa, H. pinifolia, H. tuna and L. majuscula (11.90, 10.90, 10.80 and 9.20 μg g⁻¹, respectively). The highest Cd bioaccumulation was recorded in L. majuscula (2.65 μg g⁻¹). In conclusion, the studied species showed different tendencies towards the different metals, with significant responses from L. majuscula and P. boryana.

Three species of seaweeds (Padina boryana, Galaxaura marginata and Halimeda tuna) and two seagrass species (Halophila stipulacea and Halodule uninervis) were collected in the extreme conditions of the Safaga tidal flat (Supplementary Table 2), where P. boryana, H. stipulacea, G. marginata and H. uninervis were found to have nearly equal high Fe bioaccumulation (1293.20, 1197.65, 1160.45 and 1074.10 μg g⁻¹, respectively). G. marginata was the highest accumulator species for Mn, Zn, Cu, Ni and Pb (88.70, 48.20, 18.75, 24.45 and 17.70 μg g⁻¹, respectively), while Cd was found in nearly equal amounts in most species (Table 7). Generally, seaweeds have a much more effective ability of heavy metal bioaccumulation than seagrasses, and the species collected in our study have adaptable mechanisms to survive under even extremely polluted conditions.

Additionally, the presence of Chaetomorpha crassa, Ulva lactuca and Cladophora sp. was a great indication of subsurface wastewater seepage at the Qusier tidal flat. Two seaweeds (Padina boryana and Sargassum cinereum) and two seagrasses (Halodule pinifolia and Halophila stipulacea) were recorded there (Supplementary Table 2). These flora species showed a high degree of adaptability. P. boryana was the highest bioaccumulator species for Fe, Mn and Zn (2589.64, 169.85 and 63.30 μg g⁻¹, respectively), while H. pinifolia was the highest accumulator for Ni (53.00 μg g⁻¹). The highest accumulation of Pb was found in Cladophora sp. (12.70 μg g⁻¹) while, S. cinereum was the highest accumulator for Cd (5.00 μg g⁻¹) (Table 8).

The wide range of heavy metal concentrations in different algal species reflects the importance of biochemical factors in affecting the relative tendency of different tissues to concentrate pollutants. Such biochemical or physiological differences may also play a major role in causing certain species to concentrate pollutants to a much higher degree than other organisms, regardless of the species’ relative position in the aquatic food chain (Steele et al. 2001). The bioaccumulation of Fe, Mn, Cu and Cd measured in the seaweeds and seagrasses at the megacity sites were lower than those reported by Kannan et al. (1992), Thangaradjou et al. (2013) or Thangaradjou et al. (2010) and higher than those reported by Al-Shwafi & Rushdi (2008), Dadolahi-Sohrab et al. (2011) or Qari and Siddiqui (2010) for most metals except Fe.