Bioaccumulation of Polycyclic Aromatic Hydrocarbons (PAHs) and Total Petroleum Hydrocarbons (TPHs) in Aquatic Taxa from the Niger Delta: A Review

Abstract

The Niger Delta region of southern Nigeria is home to one of the largest deposits of crude oil on the planet. This is having devastating effects on aquatic and ecosystem health in the region due to increasing pollution by petroleum hydrocarbons in the surrounding water bodies, which leads to bioaccumulation in the aquatic organisms across taxa that inhabit these water bodies. The bioaccumulative effects of these hydrocarbons could have long-term effects on the population, reproductive, behavioural, and hormonal health of these organisms, which can cause negative impacts for the human consumers in these regions that depend on these biota as a source of food and livelihood. The objective of this review is to examine the bioaccumulative potentials of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs) in different taxa across multiple regions of the Niger delta, with a focus on fish, amphibians, and invertebrates in lower trophic levels due to their high sensitivity to contaminants. They act as early warning indicators of degradation and play important roles in the aquatic food web. The key pollutant categories are PAHs and TPHs due to their carcinogenic and mutagenic properties. The bioaccumulation was mostly measured using Biota-sediment accumulation factors (BSaFs). A BSaF value greater than 1 indicates bioaccumulation in that species, and a value less than one means non-bioaccumulative potential. A trend across studies is that the PAH concentrations are reduced across trophic levels, with lower concentrations observed in vertebrates (fish) and an increase in concentration down the trophic level, with molluscs and crustaceans having greater concentrations of PAHs. The concentrations of PAHs were also observed to be higher in sediments compared to those in the biota. This implies that bioaccumulation of PAHs in aquatic taxa with concentrations of PAHs differed across trophic levels. Bioaccumulation in lower trophic levels might result in the ingestion of these hydrocarbons by humans, which can increase cancer risks and non-cancerous health hazards in consumers. This review can help inform local sensitisation activities and guide government policies to preserve biodiversity and promote ecosystem health and sustainability in the region.

1.    Introduction

Oil was first discovered in the Niger Delta by Shell on January 15, 1956, at Oloibiri in present-day Bayelsa State. According to the Organisation of the Petroleum Export Countries (OPEC), Nigeria currently has the world's tenth-largest crude oil reserves and is the world's thirteenth-largest producer of crude oil. According to the United Nations Environmental Program (UNEP), while oil exploration and production in the Niger Delta began in the late 1950s, operations were suspended in Ogoniland in the early 1990s due to disruptions from local public unrest. The oilfields and installations have since largely remained dormant. However, major oil pipelines still cross through Ogoniland, and oil spills continue to affect the region, due to factors such as the lack of maintenance and vandalism to oil infrastructure and facilities. Environmental contamination in Ogoniland from oil spills remains untreated or only partially remediated today.

The Niger Delta region has one of the largest wetland systems in the world and is a biodiversity hotspot comprising the greatest diversity of aquatic species in Africa (Zabby & Uhi, 2014). This means that the region is heavily dependent on fishing, farming and aquatic resources as a means of livelihood. It is also heavily tied to their cultural identities, with different villages organising seasonal festivities like the Nkoro fishing festival by the Ijaw people, the Unyeada fishing festival in Rivers state, and the Urie fishing festival in the Igbeda kingdom. The aquatic ecosystems are undoubtedly tied to their sustenance, income, transportation and way of life.

Ever since the discovery of crude oil in the region, it has been consistently plagued by oil spills and illegal and indiscriminate dumping of oil wastes in the water bodies. The major source of pollution is from the industrialisation of oil, which makes up about a third of the economy, but has paradoxically caused devastating effects on the lives of inhabitants. According to Amnesty International, people living in the oil-producing communities of the Niger Delta have had their human rights undermined by oil companies that the Nigerian government have failed to hold to account. Industrial pollution has led to oil spills and leaks contaminating soil and water, gas flaring, illegal refining, and heavy metal contamination. This has caused a reduction in agricultural and fishing activities and a massive health crisis, as even the drinkable water is being affected.

Crude oil is a source of petrogenic polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPHs) that have been classified as toxic, mutagenic and carcinogenic. Marine and terrestrial oil spills are a major source of PAH pollution, and they are persistent pollutants that remain in the environment long after contamination. PAH input into the aquatic environment could be from fuel (petrogenic), incomplete combustion process (pyrogenic), organic metabolism (biogenic), and through the transformation process in sediment (diagenetic). However, petrogenic and pyrogenic sources are noted as the most important contributors of PAHs into the aquatic environment (Ibor et al. 2026). Ecotoxicology matters in this region because it allows us to study and understand the source points of these PAH pollutants and their fate in the environment, the bioaccumulation of these chemicals in organisms that inhabit these water bodies, and how these contaminants disrupt the ecosystem and reduce biodiversity. These studies provide comprehensive information on the levels and presence of pollutants in the water, sediments and biota, using certain species as indicators of increasing degradation and pollution. This could help inform protection and mitigation strategies by governmental organisations and NGOs that can help improve environmental health and secure the livelihood of those dependent on these aquatic ecosystems for survival.

The main aim of this review is to understand the bioaccumulative effects of the PAHs in aquatic taxa across multiple publications, particularly in major aquatic biota in different regions across the Niger Delta affected by oil spillage. This includes fish, crustaceans, molluscs, and amphibians, because of their high sensitivity to pollutants and bioaccumulation potential. This review synthesises current knowledge across multiple regions, including port regions and coastal communities, and in different species of fish, crustaceans and amphibians, and provides a comparative analysis of the varying levels of PAH presence in water bodies and sediments across regions and the varying bioaccumulation in different aquatic taxa. It aims to provide insights into exposure quantification and assess bioaccumulation and subsequently biomagnification potentials across biota in different trophic levels and across multiple regions. It also hopes to provide insight into the possibility of bioaccumulation progressing to biomagnification in humans post-consumption. This review aims to emphasise the significant public health and food safety concerns that might result from the utilisation of these water bodies for domestic activities. It will also help provide a comparative summary on the information currently available on the presence and environmental impact of pollution in the Niger Delta water bodies and the subsequent effect of these contaminants on the biota and human populations in constant contact with these aquatic environments.

2. Study Area: Niger Delta

2.1 Geography

According to Britannica, the Niger Delta is a vast low-lying geographical region in southern Nigeria where the Niger River drains into the Gulf of Guinea. It consists of four ecological zones, which are the mangrove forest, freshwater swamp, and lowland rainforest, which are characterised by oxbow lakes and meandering rivers. It is a major global hydrocarbon reserve heavily impacted by pollution. For this review, the sample biota were collected from different regions across the Niger Delta, including study samples collected from Bodo town (including the northern and western parts), Forcados terminal river in Port Harcourt, Adiabo, Obutong, Nsidung regions, Ekpan, Niger state, and some regions in Calabar.

2.2 Major Pollution Sources

Estuarine regions in the Niger Delta are known sinks of contaminant sediments and a source of contamination for the adjacent marine habitats (Lindén & Pålsson, 2013). A recurring theme across all the papers and journals is that the presence and detection capacity of PAHs and TPHs in the port areas are elevated due to increased human activity like oil exploration, industrialisation, and the like. This increases the risk of exposure for aquatic biota and fauna found in these areas. The major sources of pollution are from the petroleum industry, including oil spills from pipelines, gas fields, oilfield operational discharges, unregulated industrial effluents, heavy metal contamination, and agricultural and urban waste disposal.

2.3 Ecological Importance

The Niger Delta region, which occupies the largest extension of freshwater swamps, is predominantly occupied by rural communities that depend solely on the natural environment for sustenance and livelihood (UNDP Report, 2006; Ogon, 2006). The original Niger delta region (about 29,900 km²) consists of areas covered by the natural delta of the river Niger and areas to the east and west that produce oil (Environmental Resources Management Ltd., 1997). According to the World Bank (1995), Nigeria has the third largest area of mangrove forests in the world and the largest in Africa, the majority of which are found in the Niger Delta (Ebeku, 2005). It is pertinent to note that the mangrove swamps lie at the centre of a complex and sensitive ecosystem, vital for fishing industries and sources of employment and income for local dwellers (Chidumeje et al., 2015). The Niger Delta region is very rich in aquatic resources with high diversity and an abundance of over 200 species of fishes (Uluocha and Okeke, 2004; Ebeku, 2004; Nwadiaro, 1984; Fentiman, 1996; NDWC, 1995). It has more species of freshwater fish (197) than any other coastal ecosystem in West Africa (Powell, 1993). Previous studies have revealed that about 16 species of the 200 species of fishes found in the Niger Delta have been identified as endemic to the region, while another 29 are near-endemic (Moffat and Linden, 1995; Ebeku, 2004; Niger Delta Wetland Centre, NDWC, 1995). The region is also of vital ecological importance because it helps in climate regulation by providing a good sink for greenhouse gases of CO2 and CH4 (Brooks et al., 2000), it improves hydrological flows by providing a buffer against natural disasters, including coastal erosion and regulates flood (Cugusi and Piccarozzi, 2009), and is a source of food and economic stability for inhabitants of the region.

3. Petroleum Hydrocarbons in Niger Delta Waters

3.1 Sources

A combination of sabotage, crude oil theft, artisanal refining and operational spills has resulted in impacts to land and water resources and aquatic ecosystems (Lindén and Pålsson, 2013; United Nations Environment Programme, 2011). Of particular concern are oil releases in the intertidal mangrove swamps of the Niger Delta, where tidal action can result in impacts over an extensive area (Gundlach, 2018). This crude oil serves as the primary source of Total Petroleum Hydrocarbons and Polycyclic Aromatic Hydrocarbons. Due to their potential rapid metabolism, these compounds are considered priority environmental contaminants due to their ability to bioaccumulate and biomagnify in aquatic food fish species and food webs. A report was issued by the ATSDR stating that the totality of petroleum hydrocarbons in the environment is becoming more alarming than any other contaminant. PAHs reported across multiple studies focused on the 16 priority PAHs defined by the US-EPA. They majorly include PAHs with high molecular weights like PHEN, five-ring PAHs including BkF, BbF, and DBahA and six-ring PAHs including BghiPRL and 1123 cdPYR. Some studies also investigated aliphatic hydrocarbons like decane, dodecane, hexane, etc.

3.2 Occurrence in Water and Sediment

Multiple studies examined the presence of hydrocarbons in water bodies and in sediments with reference to climate and seasonal changes. The effects of the presence of these hydrocarbons on the physico-chemical parameters of the water bodies and sediments across multiple regions were also studied. In a study conducted by Anyanwu et al. (2023) to determine the presence of Total Petroleum hydrocarbons in relation to seasonal dynamics in the Niger delta, it included collecting samples from three major ecosystems of the Niger delta coastal region - the Bonny estuary, the Imo river, and the Lagos lagoon - over a year, with stations representing upper, middle and lower reaches sampled in each site. The results showed the contamination levels in the Niger delta ecosystems with high contamination levels of Total Petroleum Hydrocarbons (TPHs) ranging from 17.38 to 889.10 mg/L (95.6–889.10 mg/L in estuary, 17.38–330.26 mg/L in river, 26.52–505.45 mg/L in lagoon) with mean values higher than the national and USEPA regulatory standards measured in each habitat. The estuary and upper reach of the Lagos lagoon reported elevated concentrations of TPHs (> 880 mg/L and > 500 mg/L), respectively, and this could be attributed to increased port activities like oil exploitation, petroleum loading and offloading, and increased industrial discharge.

Similarly, in a study carried out by Saunders et al. (2022) on the analysis of PAHs in surface sediments and edible aquatic species in an oil-contaminated mangrove in Bodo town, results showed that the PAHs measured, which included the 16 priority PAHs as defined by USEPA, were higher in sediments than in the biota. The average TPH concentration in sediment samples was 11,500 mg/kg with a range between 250 and 42,000 mg/kg. Average concentrations for individual PAHs in sediment ranged between <20 μg/kg for acenaphthylene (Acl) and 170 μg/kg for pyrene (PYR). The average sum of the 16PAHs was 461 μg/kg, and the maximum sum for 16PAHs was 2350 μg/kg. Both average and maximum concentrations are far below the Nigerian Environmental Guidelines and Standards for the Petroleum Industry (EGASPIN, DPR, 2018) Intervention Value of 40 mg/kg (which is defined for the sum of a subset of 10 PAHs: CRY, BaP, BghiPRL, BkF, I123cdPYR, N, A, PHEN, FLU, BaA). Comparatively, the mean concentrations of all PAHs and the sum of 16PAHs measured in this study were greatly lower than those recently reported from core samples collected near an active port in the Bonny Estuary, Niger Delta (Anyanwu et al., 2020). This confirmed the study by Anyanwu et al. (2022), in which the study was conducted across the Niger Delta coastal ecosystems, that the concentrations of TPHs and PAHs are elevated in port regions due to increased human activities.

Similarly, in a study conducted by Ibor et al. (2026) in Adiabo, Obutong, and Nsidung stations along the length of the Calabar River, which is a part of the Cross River Estuary, the results showed that lower molecular weight (LMW) and higher molecular weight (HMW) PAHs showed sediment occurrence at all sites, including Adiabo, which is the putative control site that showed no visible sign of oil pollution. Obutong demonstrated significantly higher levels of PAHs in sediments than Nsidung, which was higher than Adiabo. The higher PAH levels reported in Obutong compared to the two other sample sites could be attributed to the fact that it is a heavily industrialised site, receiving point and diffuse sources of effluents from petroleum industries/tank farms, cement factory and domestic waste effluents. The sum of total PAHs (Σ19PAHs) exhibited site-specific concentrations with higher levels recorded at Obutong (197.9 ± 5.9 ng/g) and Nsidung (61.5 ± 4.3 ng/g), compared with Adiabo (29.6 ± 2.3 ng/g). This confirms the results of the previous two studies of higher occurrence in regions with increased human activities.

Numerous studies also report an effect on the physicochemical parameters of water bodies due to the presence of these contaminants, and a variation in concentration across multiple sites and seasons. In the study by Anyanwu et al. (2023), elevated concentrations of TPHs (70.1–889.1 mg/L) were detected during dry seasons. Through seasons, the physico-chemical parameters like temperature, salinity, dissolved oxygen, conductivity, and total dissolved solids varied across all the sites. The variation in these parameters affects the mobilisation of TPHs across the river, lagoon and estuary sample sites. Climate parameters are also known to affect the fate, enrichment, speciation, transformation, and bioavailability of contaminants in the environment. In another study conducted by Ihunwo et al. (2021), on TPHs in surface water and sediments along the Woji creek of the Niger delta, the distribution and mobilisation of TPHs in the water body is strongly dependent on the water chemistry, i.e. the physico-chemical parameters of the water body, the flow and depth of the water and environmental factors like organic pollution. This affects how TPHs move, accumulate, and persist in both the surface water and sediment. Increased TPHs in the creek lead to an increase in temperature, and a decrease in DO, which is deadly for aquatic life. TPHs were found in higher concentrations in the sediments than in surface water because they act as a sink for these contaminants, and this can cause continuous exposure because they re-release the contaminants into the water body when disturbances occur. Increased rainfall and tides can also act as disturbances, which can increase pollutant mobilisation and redistribution in these water bodies.

4. Ecotoxicological Effects Across Taxa

4.1 Fish

Various species of fish were sampled across multiple studies and in different regions to determine the effects of these hydrocarbons on fish physiology and the bioaccumulative factor, which was calculated by comparing the concentration in the fish species to the concentration present in sediment samples. In the study by Oparaji et al. (2017), estimating the PAHs and TPHs levels in aquatic fauna, seven species of cultured fish samples were collected from the Forcados terminal river in the Delta, as they have the highest number of brackish water cages and since it is located near the exploration sites of most oil industries in rivers. This study was conducted as data on the accumulation of hydrocarbons in both aquaculture and cultured organisms were lacking. The cultured fish sample species were Ilisha africana, mackerel, barracuda, needle fish, cutlass fish, millet, and tilapia fish. The results showed that the relatively lighter members of the hydrocarbon homologous series predominated in almost all the aquatic fauna that were analysed, while the heavier members of the homologous family were not detected in almost all the sampled fauna species (Oparaji et al., 2017). This could be attributed to the same trend reported by Saunders et al. (2022), in their study, as they explained that lower chain hydrocarbons found their way into the systems of living organisms more readily than the heavier chain ones. The results of the study showed that TPH attained its highest concentrations in Mackerel and Ilisha africana fishes, with concentrations of 2.15 ± 0.42 and 3.64 ± 0.94 mg/kg, respectively.

In contrast, the lowest concentration was seen in millet fish, with a concentration of 0.08 ± 0.01 mg/kg. Aliphatic hydrocarbons also showed the same pattern, with the highest concentrations of Total Aliphatic Hydrocarbons (TAHs) measured in Mackerel and Ilisha africana, and the lowest concentration measured in millet fish (3.46 ± 0.91, 1.92 ± 0.34 mg/kg, and 0.04 ± 0.02 mg/kg, respectively). The studies concluded that in all the measured aquatic fauna, Ilisha africana and Mackerel have the highest bioaccumulation of petroleum hydrocarbons among all the sampled species. However, a gap in this study is that the concentrations of petroleum hydrocarbons in the sediments were not analysed or discussed, and as such, the biota-sediment accumulation factor (BSAF) or the Biota accumulation factor (BAF) for each species could not be calculated, as we don't know the concentration of TPHs in the sediments compared to that of the biota. BSaF is dependent on the concentration and the detection frequencies of PAHs across multiple sample sites in different regions.

It was observed across multiple studies that due to the high metabolic rates in vertebrates, they have the ability to process these hydrocarbons using enzymes that render these xenobiotic compounds harmless and reduce their bioaccumulative potential. This was demonstrated in the study by Saunders et al. (2022), which aimed to establish a relationship between PAH concentrations in sediments and biota, and to derive the biota-sediment accumulation factor (BSAF). The sample biota were commonly consumed aquatic fauna, including tilapia. The findings showed that Tilapia had the lowest average PAH concentration, and only PHEN, PYR, and FLU were detected above the LOQ compared to the invertebrates in the study samples. Also, statistically significant correlations (p < 0.05) were found between PAHs and the tissue weight of tilapia (R² = -0.69; p < 0.01). The negative correlation between tissue weight of tilapia and PAH concentration might be an artefact of the tissue pooling process or might indicate that smaller fish carry a relatively greater body burden of PAHs than larger fish. This means that as the tissue weight increases, PAH decreases, and smaller fish have a higher chance of bioaccumulation than larger fish.

Comparing these results with studies conducted by Udofia et al., (2021) to the east of the sample site of the Bodo region on the 16 priority PAHs, the concentrations were significantly higher than the PAHs levels detected in the Bodo study (10- to 1000-times higher), and this could be attributed to the fact that the sample site was located close to oil wells, commercial fishing regions and oil terminals. However, the most commonly detected PAHs in the biota in the Bodo study (PHEN, PYR, FLU) were not detected in catfish, which was also the only vertebrate in that study. The mean BSaF for tilapia ranged between 0.09 and 0.41 for the Bodo study, with only the BSaFs for PHEN, FLU, and PYR being calculable, while the value couldn't be calculated for other PAHs. This result is indicative of low bioaccumulation in tilapia. Metabolism is particularly relevant in the case of PAHs, as they are naturally occurring compounds which are ubiquitous in the environment. Many aquatic species, particularly vertebrates, express enzymes (e.g. Cytochrome p450) which detoxify PAHs (Rand, 1995), and this could explain the low levels of PAHs in vertebrates across all studies and their low bioaccumulative potentials compared to lower trophic levels.

While the results of this study by Saunders et al. (2022) differ from those by Oparaji et al. (2017) on the cultured fish species from the Forcados terminal, the disparities cannot be fully investigated as the BSaF was not calculated. Also, it is important to note that the studies from the Bodo region were on wild species, while those from Forcados were from cultured fish species, so any potential differences in the bioaccumulative potential between both species could be attributed to differences in feeding habits, physico-chemical parameters of the water bodies or environmental factors. All of these factors could influence the organism's metabolism and affect its ability to process or detoxify these chemicals.

This was demonstrated in a study by Ibor et al. (2026), which examines the species-specific toxicological responses in relation to body burden and bioaccumulation pattern of PAHs in Adiabo (a putative control site with no visible signs of oil pollution), Obutong (a heavy industrial site receiving point and diffuse sources of effluents from petroleum industries), and Nsidung (receiving effluents of agricultural and industrial origin). This study examined how exposure of aquatic organisms to these xenobiotics results in the activation of physiological processes that are targeted towards metabolising and possibly eliminating these chemical contaminants from the body in a process called biotransformation, which can help to better understand how important metabolism is at different trophic levels and under different environmental conditions to the bioaccumulative process of these chemicals. As mentioned in the study by Saunders et al. (2022), on the presence of CYP enzymes to help in the metabolism of these hydrocarbons in vertebrates, this study investigated oxidative stress, biotransformation and bioaccumulation of PAHs in Chrysichthys nigrodigitatus and other invertebrates in the three sample sites. Hepatic oxidative stress and biotransformation enzyme activities; glutathione peroxidase (Gpx), glutathione reductase (Gr), glutathione S-transferase (Gst), uridinediphosphate glucuronosyltransferase (Udpgt), 7-ethoxy-, methoxy-, pentoxy-, and bezyloxyresorufin O-deethylase (EROD, MROD, PROD and BROD), and PAHs levels were determined, as induction of biotransformation and oxidative stress enzymes were adopted as sensitive and reliable biomarkers of exposure to environmental pollution and ecotoxicological monitoring. The result showed that fish (Chrysichthys nigrodigitatus) samples taken from Adiabo showed comparatively high levels of muscle PAHs accumulation compared to samples taken from Obutong or Nsidung. The higher body burden of these PAHs probably reflects the omnivorous feeding habit of this species and associated biomagnification of PAHs from lower trophic levels to C. nigrodigitatus (Han et al. 2022; Rahmanpour, Farzaneh Ghorghani, and Lotfi Ashtiyani 2014). This is the first potential explanation for biotransformation and biomagnification of PAHs across food webs from all the reviewed papers.

4.2 Molluscs and Crustaceans

Crustaceans and molluscs are sensitive biomonitors and bioindicators of environmental pollution, and this makes them invaluable in ecotoxicological studies. Their unique biological characteristics, like their sessile lifestyles, specific feeding habits, and widespread distribution, allow them to accumulate chemicals, which is reflective of the health of aquatic ecosystems better than most organisms. They play the role of sentinel species by acting as early warning systems with their health, population, or behaviour signalling ecosystem decline, environmental hazards or pollution.

In the study by Saunders et al. (2022), in the Bodo region of the Niger Delta, results showed that PAH concentrations varied greatly across species with the lowest PAH concentrations found in the vertebrate samples (fish), however, the trend showed an increase in PAH concentration with a decrease in size and tissue weight with periwinkles having the highest concentration of PAH, followed by the estuarine shrimp, crabs, and then tiger prawn. The study also confirmed that the biotic concentrations of the samples taken from the Bodo region never exceeded the maximum concentrations for smoked fish and fishery products for BaP (used to measure carcinogenic risk factor) and the sum of BaP, BaA, BbF and CRY defined by the European Union (2011). The study also observed that organisms are exposed to greater concentrations of PAHs via sediment, which might lead to greater concentrations found in tissue samples. Higher molecular weight (HMW) PAHs, such as the five-ring BkF, BbF, and DBahA and the six-ring BghiPRL and I123cdPYR, were detected almost exclusively in periwinkles, while lower molecular weight PAHs were prevalent in other species. The greater frequency of detection and concentrations of HMW PAHs in periwinkles might be due to differences in exposure (exposure via sediment vs. water) and the lower metabolic capacity relative to other species (Zhang et al., 2020), possibly resulting in increased bioaccumulation of these chemicals.

The concentrations of PAHs were generally not correlated with several biometric measurements in the study; however, statistically significant correlations (P < 0.05) were found between PAHs and the length of periwinkles (R² = 0.49; p = 0.01). Length is a surrogate measurement for weight in periwinkles, which suggests that larger and older periwinkles accumulate greater concentrations of PAHs. Comparatively, in the study by Udofia et al. (2021), it was reported that of the 17 PAHs collected from prawns and periwinkles samples to the east of Bodo, trends of detection frequencies and concentrations of PAHs were markedly different. Of all the PAHs most commonly detected in biota in the present study, only PHEN was detected in prawns, and all three were detected in periwinkles collected from a single site. In addition, Nwaichi and Ntorgbo (2016) analysed PHEN, PYR, FLU, BbF, and BghiPRL in edible tissues of a different genus of periwinkle (Littorina littorea) collected from nine sites along Nigerian coastal regions approximately 5–10 km north of the Bodo site. Mean tissue concentrations of most PAHs were lower than those in the current study (10- to 100-times lower), though concentrations of BbF were approximately 20-times higher. The differences between the studies could be attributed to differences in analytical methods, as the concentrations reported in Nwaichi and Ntorgbo (2016) were generally 10-times below the LOQ in the study by Udofia et al. (2021).

The BSaFs for crabs, estuarine shrimp, tiger prawns, and periwinkles ranged between 0.01 and 0.42, 0.03–0.46, 0.11–0.64, and 0.42–1.73, respectively (Saunders et al., 2022).  For the most part, the results are indicative of low bioaccumulation in these organisms, as most of the BSaF values are < 1. However, only periwinkles showed a BSaF value greater than 1 for PHEN, PYR, and BkF, indicative of bioaccumulation. A meta-analysis by the Dutch National Institute for Public Health and the Environment (RIVM) concluded that PHEN is bioaccumulative in fish and invertebrates (crustaceans and molluscs), while PYR and BkF are non-bioaccumulative in fish, but are bioaccumulative in lower trophic level organisms, such as crustaceans and molluscs (Bleeker and Verbruggen, 2009). The greater BSaF value in periwinkles could be attributed to differences in exposure dynamics, as Niger delta periwinkles are deposit feeders and in constant contact with sediments, while other sample species reside in overlying water columns, and differences in metabolic capacities across trophic levels. However, none of the studies mentioned how the bioaccumulation of PAHs could be transferred across the aquatic food web and biomagnify in the organisms that feed on these deposit feeders.

In another study by Davies and Oladejo (2025) on bioaccumulation of TPHs in Tympanotonus fuscatus from coastal sediments in Rivers State, the samples were collected from three stations, with periwinkles from station 2 consistently exhibiting the highest TPH levels, while stations 1 and 3 recorded lower concentrations, suggesting that station 2 is more directly affected by pollution sources. Elevated TPH concentrations in station 2 could also be attributed to slower water circulation, promoting pollutant accumulation over time. Interestingly, results from the studies show that the highest BAF values for benzene (130), n-nonane (165), and n-pentadecane (245) were from samples taken in station 3, indicating greater exposure to bioavailable hydrocarbons. The study attributed this to greater proximity to pollution sources. Station 1 also displayed high BAFs for specific hydrocarbons such as n-pentadecane (480) and n-hexadecane (510), despite lower sediment TPH concentrations than Station 2. This may reflect environmental conditions or species-specific physiological traits that favour selective hydrocarbon uptake (Disner and Torres, 2020). Station 2, while showing lower BAFs for benzene (70) and n-pentadecane (235), exhibited the highest ethylbenzene bioaccumulation, indicating that local factors enhance the solubility and bioavailability of specific compounds. The results emphasised the importance of compound-specific bioaccumulation in ecological risk assessments. While the BAFs for TPHs in this study were significantly lower than those of Saunders et al., these could be attributed to differences in the class of the hydrocarbons, as well as species-specific differences in periwinkles.

4.3 Amphibians

Amphibians are organisms that have an amphibious or biphasic lifecycle. This means that their developmental stages are in two phases: Aquatic (eggs and tadpoles) and terrestrial (adults). There is little to no available study on the impact of PAHs and TPHs to the developmental lifecycle of amphibians, and this poses a threat to the survival of amphibian biodiversity and the ecosystem health of the Niger Delta, as these are important organisms that could help serve as biomarkers of environmental degradation due to their high sensitivity. Studies on the importance of these contaminants are also really important, as amphibians play a vital role in the food web and are essential in maintaining wetland balance. Amphibians are currently the most globally threatened group of vertebrates (approximately 41% of all species) (Hoffmann et al., 2010). Habitat destruction, the introduction of exotic species, emergent diseases, and the pollution of terrestrial and aquatic habitats have all been described as important threatening factors (Stuart et al., 2004; Wake and Vredenburgh, 2008). Most Amphibians are omnivores, consuming algae, macrophytes, detritus and small invertebrates, which may also act as a dietary source of aquatic contaminants resulting in bioaccumulation (Patterson, 2019). In spite of these threats and statistics, amphibians are still relatively understudied in ecotoxicology research in the Niger Delta. However, this could be due to the limited research and infrastructural capabilities of Nigeria as a developing country. Amphibian skins are permeable, with shell-less eggs that are directly exposed to these contaminants, yet there is little to no information on the effects of these contaminants on teratogenicity and developmental toxicity in the Niger Delta region.

In a study conducted by Faden et al. (2017) to assess PAHs levels in edible frogs, Hoplobatrachus occipitalis from Ekpan, Uvwie Local Government Area of Delta State, the results showed that a total of seven PAH compounds were detected in the muscle tissues of the frogs obtained from study sites. These include Fluorene, Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benzo(a)anthracene and Benzo(k)fluoranthene. The frogs sampled from remediated oil-spill sites in Uvwie, Delta State, exhibited detectable levels of several PAH compounds in their muscle tissues, while frogs from the control areas showed non-detectable concentrations. The study also noted that although some variation existed in the mean levels among sampling stations with generally higher PAH burdens in frogs from more contaminated sites, these differences did not reach statistical significance. In a similar study by Leney et al. (2006), they demonstrated that adult anurans (green and leopard frogs) have modest metabolic ability to eliminate PAHs, resulting in tissue persistence that makes them suitable biomonitors of hydrophobic organic pollutants. The detection of low- to mid-weight PAHs such as Fluorene and Phenanthrene in frog tissue from impacted sites mirrors contamination profiles noted in aquatic food webs after oil pollution events. A study conducted by Vives et al. (2005) examined PAHs across a freshwater food web and found that phenanthrene (a 3-ring PAH) commonly dominated organismal burdens in littoral organisms, matching observations from this study that phenanthrene and other 3–4 ring compounds were detected in frogs. It is worth noting that these studies were not conducted in the Niger Delta region.

Frogs showed uniformly low PAH levels across sites with no significant differences, likely due to effective remediation, low pollutant bioavailability, or limited biomagnification, contrasting with higher concentrations reported in sediments and fish from heavily polluted Niger Delta areas (Saunders et al., 2022; Ofori et al., 2021). This study also confirmed temporal variations in PAH levels, with peak concentrations observed during the rainy season. This could increase pollutant mobilisation and the risk of these chemicals bioaccumulating in these organisms. A similar case study by Regnault et al. (2014) on the impact of benzo[a]pyrene on liver function of Xenopus tropicalis showed that amphibians can biotransform and suffer metabolic and hepatic effects from exposure to PAHs such as benzo[a]pyrene and related compounds even when tissue concentrations are modest. While this study confirms the presence and concentrations of PAHs in this amphibian species, limited or no data are available on the ability of this species to biotransform these xenobiotics into harmless compounds. Although the concentration found in amphibians is lower than that found in other vertebrates like fish, it cannot be concluded whether this is due to their feeding habits or lower detection capacities of the PAHs, as the fish samples were taken from heavily polluted regions.

While limited data is available on the bioaccumulative potential of PAHs in the Niger delta region, other studies have shown that exposure to PAHs has demonstrated an impairment or disruption to the normal function of endocrine systems in amphibians, with the thyroid hormone receptor beta (TRB) expression being highly affected (Alderman et al., 2018; Alsaadi et al., 2018). Regarding developmental toxicity, alterations in metamorphosis timelines and developmental outcomes occur in L. sylvaticus raised in reclaimed metals contaminated with PAHs (Products et al., 2011; Whyte et al., 2000). This is explained by Alderman et al. (2018) that the normal activity of the hypothalamus-pituitary-thyroid (HPT) axis, which is essential for the reproduction, development and growth of amphibians, is disrupted by exposure to PAHs. Another study by Wallace et al. 2018 showed that exposure to certain PAHs like naphthalene could potentially cause asphyxiation as it can disrupt the normal gas exchange mechanism in amphibian integument, leading to decreased CO2 excretion rates. Studies have also shown that even low concentrations of PAHs can cause teratogenicity in amphibian embryos, and tadpoles that survived the PAHs exposures displayed behavioural and morphological abnormalities such as twisted spines and irregular swimming patterns, indicating chronic, sub-lethal effects from embryonic PAHs exposures (Campbell et al., 2019). The jelly coating around amphibians' eggs, known as the extracellular matrix, confers protection against the embryonic exposure of PAHs, acting as a sequestering agent to shield the embryos (Parrott et al., 2018). The targeted organ accumulation and toxicity are typically localised to the kidneys and liver, and compounds such as B(a)P cause metabolic and transcriptomic toxicity and carcinogenicity in amphibian hepatocytes (Lara-Jacobo et al., 2019; Harner et al., 2018). This data provides evidence that PAHs and other similar AhR antagonists in crude oil may lead to adverse biological responses in developing amphibians either through mechanisms of increased genotoxicity, narcosis or disrupted endocrine signalling (Campbell et al., 2019). In addition, high mortality of amphibians living in PAH-contaminated sediment ponds was documented (Johansen, 2013).

In spite of all of these results from numerous studies, there is very limited information on the effects of PAHs on developmental toxicity, endocrine disruption and population decline in amphibian populations in the Niger Delta region. The information on the bioaccumulative potential is also almost non-existent, and there are limited to no studies on the biotransformation capacity of amphibians at different developmental levels. The biomagnification across the food web between amphibians and other vertebrates and invertebrate species is also not available. Given the intensity and extent of oil pollution in the Niger Delta region and the persistence of these hydrocarbons in the ecosystem, as well as the sensitivity of amphibians to pollutants, there is an urgent need for research on the various effects of these xenobiotics on amphibians.

5. Biomarkers and Indicators of Pollution

Environmental pollution due to constant and unregulated release, uptake and bioaccumulation of contaminants in aquatic biota and sediments remains one of the biggest global banes of modern civilisation (Bhat et al. 2023; Qayoom et al. 2024). The environment plays a significant role in sustaining ecosystem life, and all biota are dependent on environmental resources for growth and development, reproduction and overt survival (Munang, Thiaw, and Rivington 2011).

In the study conducted by Ibor et al. (2026) on the Adiabo, Obutong and Nsidung regions, results showed an increase in the phase I- and II- biotransformation and oxidative stress biomarker responses with significantly higher concentrations of PAHs in the sediments and estuarine food webs (fish, crabs, prawns and periwinkles). Using principal component analysis (PCA), the results from this study showed a cause-and-effect relationship between PAH levels and biomarker responses, indicating a strong relationship between PAH occurrence in sediments and bioaccumulation and toxicological responses. The investigation noted a significant elevation in phase I (EROD, MROD, BROD, and PROD), II (UDPGT and GST) and oxidative stress (GPx and GR) enzymatic responses in all the biota at Obutong and Nsidung compared to Adiabo, which is the putative control site. The study also demonstrated site and species-specific responses in these biota that parallel biota and sediment PAHs concentrations. This is in agreement with other studies that biotransformation and oxidative stress induction are related to PAHs exposure (Fang et al. 2020; Gaber et al. 2021; Şeker 2012; Sun et al. 2020). The study suggests that the biotransformation mechanisms of PAHs were linked to aryl hydrocarbon receptor (AhR), which regulates several cellular activities, including xenobiotic metabolising enzymes. The binding of PAHs to the AhR initiates the induction of P450 enzymes, resulting in biotransformation processes and thus considered as the first step in PAHs biotransformation (Pampanin et al. 2016; Zhou et al. 2010).

Some levels of species and site-specific variability and dynamics were noted in the catalytic activities for EROD, BROD, MROD and PROD; however, this was not surprising considering the unique and different degrees of anthropogenic contamination of these sampling sites. Further, the observed variability and dynamics in CYP enzyme inductions found in this study may suggest the presence and occurrence of other CYP-inducing compounds in the studied environment. While this is among the first studies on biotransformation across all reviewed papers, there is no account for how co-occurring pollutants from the sample sites might have effects on the biotransformation processes and the presence of these enzymatic biomarkers. Also, the study mentioned a cause-and-effect relationship between PAH levels and biomarker responses, which could be translated as lower PAH concentration equals lower levels of enzymatic biomarkers. However, it has been established in previous studies that vertebrates like fish have CYP enzymes that aid these processes to help detoxify these xenobiotics, which are not present in lower trophic levels, like in periwinkles. The observations from the study, however, showed that the analysis of phase I and II enzyme levels demonstrated significant site- and species-related differences, with the highest recorded in C. nigrodigitatus and T. fuscatus. Previous studies have established the presence of biotransformative enzymes in vertebrates; however, the higher enzymatic levels recorded in periwinkles could be due to a variety of factors, such as the high levels of PAH concentrations in sediments. This is interesting and should prompt further investigations on the capability of invertebrates or certain invertebrates to carry out xenobiotic biotransformation when exposed to certain toxicants, as this would provide some interesting insight. It could be that some invertebrates, after a certain period of exposure, develop some ability to transform these toxicants or after reaching certain bioaccumulative thresholds. Further research into this could provide really interesting insights into the dynamics of biotransformation at lower trophic levels.

In a study conducted by Achuba and Ekute (2017) to determine the effect of exposure to chronic petroleum pollution on biomarkers of oxidative stress in African toad (Bufo regularis) in the Ekpan area of Delta state, results showed significant increase in lipid peroxidation in the various organs of toads from the polluted sites compared to that from the control, with the highest in the liver and lowest in the heart. The significant increase in levels of lipid peroxidation observed in this study indicates oxidative stress and may have been the consequence of reactive oxygen species and free radical attack on polyunsaturated fatty acids in cellular membranes to form lipid peroxidation products (Odewabi et al., 2014). This is because in oxidative stress, oxidative reactions generate reactive oxygen species and free radicals in excess of the body's antioxidant system can handle (Christi and Costa, 1984; Yoshikawa and Naito, 2002; Momoh and Oshin, 2015). SOD and CAT are among the chief enzymes that comprise the robust antioxidant system in the body. SOD constitutes the first line of defence against reactive oxygen species, and it is important in the removal of superoxide ion, while catalase is essential for the removal of hydrogen peroxide (H₂O₂) (Scandalios et al., 1997; Alscher et al., 2002). Results from the study showed an increase in the CAT and SOD activity in all the organs of toads from the petroleum-impacted site relative to that of the putative control site. Since both SOD and catalase are linked functionally and occur in tandem, an increase in one is expected to parallel an increase in the other (Halliwall, 1994). Moreover, the increase in the activities of these antioxidant enzymes may be a physiological adaptation for the elimination of ROS generated and protection from chemical toxicants (Saltman, 1989; Gad, 2011). An increase in the activity of these antioxidant defences in response to an increased level of reactive oxygen species has been cited as an indirect measure of oxidative stress (Smirnoff, 1993). The increase in the activities of these enzymes signals a necessity for the toad to cope with recurrent oxidative stress. It is important to note that while this study was conducted on the adult African toad, there is still limited information on the activities of these enzymes at different life stages for this species. Also, no comparative insight into how these activities could differ between different species of amphibians under similar environmental or exposure conditions.

While one study used biotransformation enzymes to understand the fate of these chemicals in vertebrates and invertebrates, as seen with Ibor et al. (2026), the study by Achuba and Ekute (2017) uses enzymatic biomarkers of oxidative stress in African toads to understand the effects of Bufo regularis on chronic petroleum exposure. Although petroleum is a major source of PAHs and TPHs, it is impossible to determine which exact component or hydrocarbons trigger the enzymatic reactions in Bufo regularis. It is also difficult to draw a comparison with the fish vertebrates used in the study by Ibor et al. (2026), as 1) different enzymatic biomarkers were being measured, and 2) there is no information on the PAHs in the study on Bufo. Also, this shows a gap in how biotransformation, distribution, and magnification are understudied in the amphibian species at both the inter- and intra-specific level and different stages of metamorphosis, in the region, as there is limited information on how the Phase I and II transformation enzymes would act when exposed to these xenobiotics compared to fish vertebrates.

6. Knowledge Gaps

One of the most prominent literature gaps observed across this review is the limited research on the biomagnification of PAHs and TPHs across trophic levels. While studies have consistently shown that PAHs and TPHs are usually present in higher concentrations in the sediments compared to the biota, and deposit feeders and organisms in lower trophic levels have higher biota-sediment accumulation factors than those in higher trophic levels (Saunders et al., 2022), limited studies have examined how this bioaccumulation is transferred through the trophic levels and could result in biomagnification in vertebrates and humans.

A significant gap also exists regarding how the physico-chemical parameters of these water bodies might affect biota susceptibility to these contaminants, and how infections that affect host immunity, including parasitic, bacterial and viral infections, could increase susceptibility of the host to bioaccumulation. Other pollutants from agricultural, industrial and domestic wastes could also be present in the water bodies, which might affect sample integrity, and there is limited study on seasonal variability in bioaccumulation of PAHs and TPHs.

Regarding amphibians, since they have an amphibious lifestyle with their early life stage (tadpoles) in the aquatic environment and the adult stages in the moist terrestrial environments, there is little to no study on the effects of these hydrocarbons in the early life stages and how this can translate or manifest in adult stages. There is also very limited study on the effects of PAHs and TPHs in amphibians and biomagnification across the trophic levels and in humans that consume them. Amphibian eggs are laid in sediments and are usually consumed by certain fish vertebrates. It has been previously established that these contaminants are in higher concentrations in sediments, and it is unclear if this could manifest differently in these omnivorous vertebrates when they are ingested through different organisms and not taken directly from the water bodies.

The biotransformation capacity of invertebrates also warrants further investigation. The higher enzymatic levels recorded in periwinkles in the Ibor et al. (2026) study raise interesting questions about whether certain invertebrates develop some biotransformation capacity after prolonged exposure or after reaching certain bioaccumulative thresholds. Research into this could provide valuable insights into the dynamics of biotransformation at lower trophic levels.

Studies show that there is a negative correlation between size and PAH concentration in fish (Saunders et al., 2022), however, there is currently no information on how these increased concentrations of PAH in smaller fish or fish with low tissue weight could affect growth, metabolism or their ability to process these xenobiotics and turn them into harmless compounds. Does this mean there is a higher chance of bioaccumulation in smaller fish than in bigger ones?

Generally, despite the abundance of crude oil and increase in oil exploration in the Niger Delta, there are still very limited reports on PAHs-mediated toxicological responses from low- and middle-income countries like Nigeria due to limited research capacity and infrastructure. There is also a scarcity of data on the carcinogenic and non-carcinogenic risks associated with consuming edible frogs found in contaminated sites, and long-term monitoring studies across taxa remain largely absent from the literature.

7. Implications for Ecosystems and Human Health

The need to study and understand the fate of these hydrocarbons on aquatic ecosystems is not only to protect biodiversity and ecosystem health, but to ultimately determine how these toxicants are going to affect human consumers. A few of these reviews mentioned that bioaccumulation happens more in lower trophic levels, like in periwinkles, which are used in preparing a lot of Niger Delta staple foods and delicacies. The Niger Delta wetlands and mangrove ecosystems are of vital importance to conserve biodiversity and also people because the local communities depend on these ecosystems for food, income and domestic activities.

Due to the carcinogenic properties of the PAHs and TPHs, a few studies use the Benzyl-a-pyrene (BaP) equivalent to measure carcinogenic risks in humans. In the study by Saunders et al. (2022), some of the PAHs detected in the biota (e.g., PHEN, PYR and FLU) were measured at higher concentrations than BaP (the toxicological benchmark PAH), but because of their significantly lower potency, their relative contribution to overall risk was proportionally lower. From the study, the calculated ELCRs associated with human exposure via ingestion over a lifetime ranged between 1.3 and 1.5 × 10⁻⁶ for tiger prawn, tilapia, estuarine shrimp and crab, with the ELCR for periwinkles being higher at 4.1 × 10⁻⁶. This result is, however, expected as the study showed that periwinkles had the highest BSaF value compared to other sample biota, which was attributed to their metabolic capacity and exposure dynamics as deposit feeders. The calculated ELCRs were compared to the target value of 10⁻⁴, which is the target value adopted by the National Institute for Public Health and the Environment (RIVM), and which was subsequently adopted in the Nigerian EGASPIN regulations. The ELCRs associated with dietary exposure to different food sources were 25 to 75-times lower than the threshold risk level, and none exceeded the threshold risk level of 10⁻⁴.

In a recent study by Udofia et al. (2021), which analysed samples of catfish, prawns and periwinkles in another part of the Bodo region, the lifetime cancer risk was greater than values calculated by Saunders et al (2022). Differences in calculated risk between the two studies likely stemmed from the much greater concentrations and differences in PAH detection frequencies. Specifically, in the former, all concentrations of PAHs were approximately 10 to 1000 times greater than those reported in the latter, while the HMW PAHs, which drive increased cancer risk (DBahA, BghiPYR, and I123cdPYR, BaP), were detected more frequently and in greater concentrations across all organisms. Results from the study indicated a very low probability of developing cancer associated with the daily ingestion of mangrove food sources and support the conclusion of the UNEP study (2011) that fish consumption in Ogoniland does not pose a health risk to the community.

While these results show that despite the high concentration and bioaccumulation of these hydrocarbons in the sediments and biota, they do not pose high carcinogenic risks to human consumers, it does not particularly tell the chronic impact of the consumption of the exposed biota on human health. Also, it is important to note that humans are not only in contact with these pollutants by consuming the biota, but children swim in these water bodies, some communities source their drinking water from these sources, and fisherfolk are in constant contact with these sediments and water bodies. The cumulative effect of this constant exposure, through direct consumption or cutaneous exposure, is not accounted for in these studies.

In contrast, in the study by Davies and Oladejo (2025) on hydrocarbon distribution and bioaccumulation in T. fuscatus, results showed that the estimated lifetime cancer risk value warranted more attention. The adult and children cancer risks ranged from 2.04×10⁻⁵ to 2.07×10⁻⁵, while children exhibited higher values between 4.76×10⁻⁵ and 4.84×10⁻⁵. While these values are below the designated threshold by USEPA, there is no doubt that it falls dangerously close to the value, especially in children. This could imply that continuous consumption of biota from these water bodies could increase cancer risk in children due to early life exposure. It is important to note that this study focused on total petroleum hydrocarbons, like benzene, decane, etc. This could account for the difference in calculated cancer risk in adults. However, the calculated hazard quotient (HQ) and Hazard index (HI) values for both adults and children were below the critical thresholds (HQ/HI<1) across all stations. These findings suggest that, although children experience relatively greater exposure, the levels of TPHs in fish remain within tolerable limits for chronic non-cancer effects. Since both studies focused on different hydrocarbons, it is difficult to draw parallels between them.

There is also a scarcity of data on the carcinogenic and non-carcinogenic risks associated with consuming edible frogs found in contaminated sites. These indicate that there is an urgent need for long-term monitoring of the effect of these petroleum activities on aquatic biota across all classes, environmental health and the subsequent effects these have on human consumers. These chemicals are persistent, ubiquitous and have mutagenic and teratogenic properties. Also, since most of the aquatic biota were sampled from the wild, there is a need to account for other factors like co-occurrence of certain chemicals, infectious disease in biota and how these can affect susceptibility or biotransformation in vertebrates, effects on water quality parameters, etc. This is crucial for promoting local sensitisation, government policies, management and mitigative strategies that can help restore and conserve biodiversity and improve the quality of life of those living in this region.

8. Conclusion

TPHs and PAHs enter aquatic environments through multiple sources, with concentrations highest in regions with elevated anthropogenic activities. Their concentration in water bodies is highest in sediments, and they find their way into the biological systems of organisms living in these water bodies. These organisms bioaccumulate these xenobiotics in their tissues, with bioaccumulation being highest in lower invertebrates like periwinkles and lowest in vertebrates like fish. However, there is limited data on bioaccumulation in frogs. Due to their amphibious lifestyles, they do bioaccumulate these PAHs; however, there is little information on how this affects metamorphosis, teratogenicity and the toxicokinetics of these chemicals through the amphibian lifecycle. A few studies concluded that the carcinogenic and non-carcinogenic risks of human consumption of biota from these water bodies pose no real threat, but there is simply too little information available for this to be concluded as not a public health concern. There is an urgent need for research on how these petroleum chemicals that have been leaching into Ogoniland for years, contaminating its water bodies and affecting its biodiversity, have affected the animals in the region and the humans who also depend on these ecosystems for survival.

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