Heavy metal trophic transfer and human health risks in the Northern Benguela current ecosystem: a review with a focus on cape hake species (Merluccius capensis and M. paradoxus)

1 Introduction

1.1 Background and rationale

Trophic interactions play a fundamental role in regulating the transfer of contaminants, including heavy metals (HMs), through marine food webs. Once introduced into aquatic systems, HMs can be taken up by primary producers and subsequently transferred to higher trophic levels via dietary pathways. This process involves bioaccumulation within organisms and biomagnification across trophic levels, often resulting in elevated concentrations in long-lived, high-trophic level predators (1). The efficiency of these transfers is influenced by multiple factors, including species-specific feeding strategies, trophic position, metabolic processes, and environmental conditions such as temperature, productivity, and nutrient dynamics (2, 3). As a result, contaminant burdens can alter predator-prey interactions, affect population dynamics, and disrupt ecosystem structure and functioning. Importantly, because many high-trophic level marine species are also consumed by humans, these processes have direct implications for seafood safety and public health, particularly in regions where fish constitutes a major dietary protein source.

The Benguela Current Large Marine Ecosystem (BCLME), situated off southwestern Africa (Figure 1), ranks among the world’s most productive marine regions, driven by intense upwelling that supports high biological productivity (2, 5). This productivity supports a diverse marine community, including the economically significant Cape hake (Merluccius capensis and M. paradoxus) and a wide range of crustacean species that form key components of regional food webs (3, 6). The BCLME is divided into northern and southern components as indicated in Figure 1 by the intensive persistent upwelling cell adjacent Lüderitz (2). The Northern Benguela subsystem experiences year-round wind-driven upwelling, peaking in late winter and spring, which enhances substantial plankton biomass and energy transfer through the food web (2). As per Hutchings et al. (2) and Blamey et al. (3), these environmental variabilities, driven by fluctuations in physical, chemical, and biological factors, significantly influence trophic interactions and ecosystem dynamics.

However, these naturally productive systems are increasingly affected by anthropogenic pollutants, notably from heavy metals (HMs) such as mercury (Hg), cadmium (Cd), and lead (Pb), which originate from industrial discharge, mining operations, and agricultural runoff (7, 8). Once introduced into the marine environment, these heavy metals accumulate in aquatic organisms through processes of bioaccumulation and biomagnification (1, 9). Cape hakes as mid to high-trophic level demersal predators are particularly vulnerable to HM accumulation, as they consume prey that may already carry elevated levels of these contaminants. This not only poses risks to the health of marine ecologies but correspondingly has significant implications for human wellbeing, especially in communities that rely on hake and other seafood as major protein source (10, 11).

In this study trophic relationships explain how various organisms at different trophic levels interact within an ecosystem, hence providing insights into how ecosystems function. Heavy metals refer to naturally occurring metallic elements with relatively high atomic weight and density that may become toxic at elevated concentrations. Bioaccumulation refers to the gradual uptake and retention of contaminants within an organism from both environmental exposure and dietary intake. Biomagnification describes the progressive increase in contaminant concentrations across successive trophic levels within a food web. Trophic transfer refers to the movement of contaminants between organisms through predator-prey interactions. Human health risk assessment indices including Estimated Daily Intake (EDI), Target Hazard Quotient (THQ), Hazard Index (HI), and Incremental Lifetime Cancer Risk (ILCR) are commonly applied to evaluate risks associated with seafood consumption.

Understanding the trophic interactions and HM transfer mechanisms within the Benguela Current Ecosystem is therefore critical for both ecological conservation and public health. Despite the ecological and economic importance of Cape hakes and their associated crustacean prey, there remains limited integrative understanding of how HMs are transferred across trophic levels within this system and the extent to which these processes influence ecological functioning and seafood safety. Addressing these gaps is essential for improving risk assessment and supporting sustainable fisheries management in the region.

1.2 Systematic review framework

This study was conducted using a structured systematic literature review approach following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Figure 2) to synthesize available evidence on trophic interactions and heavy metal transfer within the Benguela Current ecosystem. Literature searches were conducted using Scopus, Web of Science, Google Scholar, and PubMed databases covering publications from 2015 to 2025 and other relevant studies beyond this time frame. Search terms included combinations of keywords such as “heavy metals or mercury”, “bioaccumulation”, “biomagnification”, “Merluccius capensis” or “shallow water cape hake”, “Merluccius paradoxus” or “deep water cape hake”, “Northern Benguela Current” or “Namibian coastline”, and “trophic transfer” or “trophic interaction”. Studies were included if they: (i) focused on marine ecosystems, (ii) reported heavy metal concentrations in marine organisms, water or sediments, (iii) investigated trophic ecology or contaminant transfer, (iv) were peer-reviewed articles published in English, (v) Studies beyond 2015 but relevant to the review. Whereas (i) studies focusing exclusively on freshwater systems, (ii) non-peer-reviewed and incredible reports, (iii) non-English studies, (iv) duplicate publications, or (v) studies lacking ecological relevance were excluded.

A total of 508 records were identified through database searches. After removal of 96 duplicate records, 412 studies remained for title and abstract screening. Following screening, 276 records were excluded. Studies were screened for relevance based on predefined inclusion and exclusion criteria, with emphasis on marine organisms and trophic transfer processes. Subsequently, 136 full-text articles were assessed for eligibility, of which 28 were excluded. Ultimately, 108 studies were included in the final review, comprising trophic ecology studies (n = 24), heavy metal contamination studies (n = 27), human health risk studies (n = 16) and other additional but relevant studies (n=41). Relevant data were systematically extracted and synthesized into thematic categories (attached as a supplementary file), including mercury sources, bioaccumulation mechanisms, and trophic transfer pathways, to identify key patterns and knowledge gaps.

1.3 Objectives of the review

This review critically synthesizes current knowledge on trophic interactions and the transfer of HMs within the Benguela Current ecosystem, concentrating on Cape hakes (Merluccius capensis and M. paradoxus) and key crustacean species (Aristeus varidens, Funchalia woodwardi, Glyphus marsupialis, Heterocarpus grimaldi, Solenocera africana, Nematocarcinus africanus).

Specifically, the review sought to:

Investigate the feeding relationships between Cape hakes and crustaceans, and their influence on the BCLME food web dynamics and structure. Review mechanisms involved in the HM bioaccumulation and biomagnification in Cape hakes and crustaceans, with a focus on mercury. Provide a critical overview of the methodologies used to study trophic interactions and HM transfers, including stable isotope analysis, stomach content analysis, and the Bayesian mixing isotope model in R (MixSIAR), while identifying limitations and opportunities for future research. Evaluate the human health risks from consuming HM-contaminated seafood, especially for vulnerable groups like pregnant women and children. Highlight areas where current knowledge is lacking and propose specific research directions to address these gaps.

1.4 Scope and limitations

This review focuses on the trophic interactions between Cape hakes and crustaceans, and the subsequent spread of HMs via food webs. Additionally, the study evaluates impacts of HM accumulation on human health. This review explores the theory that Cape hakes, as mid to high-trophic level predators, accumulate HMs from their prey, including crustaceans, and that different hake species may exhibit varying levels of HM accumulation due to differences in feeding habits and habitat preferences. By understanding these processes, we can better predict the possible impacts of HM contamination on both ecosystem health and human health.

However, this review is limited to studies conducted in the Benguela Current ecosystem and other comparable marine environments. While the focus is primarily on mercury, the review also considers other HMs such as cadmium and lead, where data are available. The review does not extensively cover other pollutants (e.g., organic contaminants) or the broader impacts of climate change on HM dynamics, though these are important areas for future research.

Additionally, although there is an increasing concern regarding heavy metal contamination in marine food webs, there remains a scarcity of species-specific contaminant data for Merluccius capensis, M. paradoxus, and their crustacean prey in the Benguela Current ecosystem. Existing studies are fragmented, often lacking standardized sampling designs, tissue-specific measurements, or trophic integration. Therefore, current understanding relies largely on generalized biomagnification principles and evidence derived from broader marine systems (9). This highlights the need for integrated studies combining stable isotope analysis and contaminant quantification to evaluate heavy metals such as mercury transfer dynamics across trophic levels in Namibian marine ecosystems.

1.5 Conceptual framework

The study is based on a trophic contaminant-transfer framework as indicated in Figure 3 where feeding relationships determine mercury movement through the marine food web. Stable isotope analysis is used to define trophic structure and predator-prey interactions, with crustaceans representing lower trophic prey and Cape hake occupying higher trophic positions. Mercury enters organisms through dietary uptake, accumulates over time (bioaccumulation), and increases across trophic levels through biomagnification, such that Cape hake may contain higher THg concentrations than their prey, posing potential human health risks through Cape hake consumption.

1.6 Significance of the review

This review addresses critical issues at the intersection of marine ecology, environmental science, and public health. By synthesizing existing knowledge on trophic interactions and HM transfers in the BCLME, this study contributes to the broader understanding of how anthropogenic pollutants may affect marine ecosystems and human populations. Furthermore, it provides a foundation for forthcoming investigations and policy initiatives intended towards mitigating HM impacts in one of the realm’s most prolific marine ecosystems.

2 Trophic interactions in marine ecosystems

2.1 Overview of trophic interactions in the Northern Benguela ecosystem

The northern Benguela ecosystem, found along the coasts of Namibia and southerly Angola, forms part of the most productive eastern frontier upwelling systems globally, driven by nutrient-rich waters that fuel high primary productivity (2). Phytoplankton form the base of a food web, supporting diverse zooplankton communities, including copepods and euphausiids, which in turn sustain higher trophic levels. Minor pelagic fish including sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus) are key forage types that efficiently transfer energy from plankton to mid- and upper-level predators. Other important consumers include pelagic crustaceans and cephalopods, which serve as prey for mid to high-trophic level predator like hake (Merluccius capensis and M. paradoxus), and for other predators like seabirds, marine mammals, and large pelagic fish (12).

Trophic interactions in this ecosystem are shaped by both natural and anthropogenic drivers, including variable upwelling intensity, climate anomalies (e.g., Benguela Niños), and extensive fishing pressure (2, 3). Overexploitation of sardine plus hake populations have historically disrupted trophic dynamics, leading to altered predator-prey relationships and potential trophic cascades. For instance, Cape hake which are key mid to high-trophic level predators exhibit dietary plasticity, shifting between crustaceans, small fish, and cephalopods depending on availability (13). Seabirds and seals, which rely heavily on pelagic forage fish, are particularly vulnerable to such shifts (13). Overall, the northern Benguela ecosystem food web is characterized by high trophic redundancy and opportunistic feeding, contributing to its resilience but also complicating the implementation of fisheries management that are ecosystem based (14, 15).

Moving towards an Ecosystem Approach to Fisheries Management is significant for several reasons, it enables a wider deliberation of relations between ecologies and fisheries, and it also helps with harmonizing fish production with preservation of biodiversity and habitat safeguarding in our marine ecosystems (16). While the existing fisheries management approaches focus on stock estimation for significant target species, the Ecosystem Approach to Fisheries Management focuses on multi-species and ecosystem assessments through indicators (16). Additionally, the Ecosystem Approach to Fisheries Management utilizes plannings that consider the fishery, environment and anthropological systems and governance.

In this context, trophic ecology plays a critical role, as feeding interactions underpin food web structure and dynamics within systems such as the northern Benguela current ecosystem (14). Importantly, these trophic interactions also mediate the transfer of contaminants, including heavy metals such as mercury, through marine food webs. As a result, bioaccumulation and biomagnification processes can influence not only ecosystem health but also the quality and safety of commercially important fish species.

Consequently, incorporating contaminant dynamics into Ecosystem Approach to Fisheries Management is essential, as heavy metal accumulation may affect fish physiology, growth, and reproductive success, thereby influencing stock productivity and resilience (17). In addition, elevated contaminant levels in harvested species may have implications for seafood safety and marketability, directly linking environmental contamination to fisheries sustainability and management decisions. Therefore, integrating trophic ecology with contaminant pathways provides a more comprehensive framework for understanding ecosystem functioning and supports more effective management of fisheries resources (14, 18).

2.2 Trophic ecology of merluccius capensis and M. paradoxus

Two cape hake species off Namibia (Merluccius capensis and M. paradoxus), as compared in (Table 1) are part of the family Merlucciidae (22). Grounded upon stomach content analyses, the diets of the two hake species have been found to be similar, and they are known to be resourceful mid to high-trophic level predators of minor fish and crustaceans (13). Cape hake feed on numerous preys including myctophids, cephalopods, krill, crustaceans, horse mackerel, goby, and other fish species (13). Bigger hake of both species are noted as cannibalistic (23), with M. capensis exhibiting a noticeable preference for minor hake (24). Moreover, human and some top predators plus seals and sharks feed on hake.

Table 1. A comparison of the two hake species (Merluccius capensis and M. paradoxus) in terms of depth distribution, size, diet, trophic position, and potential differences in heavy metal accumulation.

The two Cape hake species Merluccius capensis and M. paradoxus are said to be similar in morphological appearances (20) including form, structure and color, however, vary in their number of vertebrae (25), coloration on the gill rakers, color of the anal fin, otolith structure and pectoral fin shape and hereditarily (26). These species have been reported to share haulages, hence the fishery can, consequently, be discussed as a mixed fishery (20). Additionally, it is for that reason that lately, Merluccius capensis and M. paradoxus are evaluated and managed as a joint single stock in Namibia (27, 28).

The Cape hakes Merluccius capensis and M. paradoxus have been reported to be dominant and vastly successful mid to high-trophic level predators over the Namibian shelf (13, 29, 30). They are found in a water layer that encompasses myctophids, other mesopelagic fish, euphausiids which are crustaceans and mollusks (31). The geographical distribution of Merluccius capensis and M. paradoxus cover a wide area (12°S - 35°S) from Angola to the east coastline of South Africa (32). In addition, hake are similarly part of the most significant commercially exploited species in Namibia and a significant food source too (30). Thus, understanding their trophic dynamics is vital for their sustainable management and exploitation.

Macpherson and Roel (13) have reported similarities in the diets of Merluccius capensis and M. paradoxus species by employing stomach content studies in their research. Iitembu’s (33) study correspondingly followed the hypothesis that temporally combined nutritional illustrations of the two species diets are comparable, since based on stomach content examinations, they equally forage on minor fishes and shellfish (34). However, their diet is known to change seasonally in relation to prey abundance, hence they are classified as opportunistic mid to high-trophic level predators whereby they feed on available prey (13). Macpherson and Gordoa (24) also noted cannibalism among large Merluccius capensis, attributing this behavior to the dietary preference of large hake for smaller conspecifics. Similarly, Payne et al. (23) had earlier observed that indeed bigger Cape hake of the two species are understood to be cannibalistic. A recent detailed dietary study by Iitembu (35) reported that the feasible nutritional contribution of minor and average size hake signified cannibalism of not more than 6% for Merluccius capensis and not more than for M. paradoxus 4% of prey items.

Importantly, the trophic ecology of Cape hake has direct implications for heavy metal bioaccumulation and trophic transfer within marine food webs. As mid- to high-level predators, hake are exposed to contaminants through their diet, particularly via the consumption of crustaceans and small pelagic fish (13), which are known to accumulate metals such as mercury from sediments, water and lower trophic levels. This dietary pathway facilitates the transfer and potential biomagnification of heavy metals such as mercury, resulting in higher contaminant burdens in predatory species (9). Furthermore, their opportunistic feeding behavior and broad prey spectrum (13) may enhance variability in contaminant exposure across spatial and temporal scales (36). Therefore, understanding the feeding ecology of Merluccius capensis and M. paradoxus is essential for interpreting patterns of metal accumulation and assessing ecological and human health risks within the Benguela current ecosystem.

2.3 Trophic ecology of crustaceans

Marine crustaceans are arthropods of the subphylum Crustacea, and they are a very diverse group of invertebrate animals which include lobsters, crabs, shrimp, copepods, barnacles and krill (37). Several species of crustacean that occur in Namibia waters, like the stripped red shrimp Aristeus varidens Holthuis 1952, Woodward’s pelagic shrimp Funchalia woodwardi Johnson 1867, kangaroo shrimp Glyphus marsupialis Filhol 1884, Grimald’s nylon shrimp Heterocarpus grimaldi (AMike Edwards and Bouvier 1900), African mud shrimp Solenocera africana Stebbing 1917, and Deep-water rose shrimp Parapeneus longirostris Lucas 1846, are known to form part of various fish diets including that of the Cape hake species (13).

According to Iitembu and Dalu (38), these crustaceans constitute a significant trophic link between primary consumers and higher levels, forming a substantial part of Cape hake diets in the northern Benguela ecosystem. Results from this study showed that while these crustaceans’ trophic niche overlapped, there were also some disparities in their carbon sources (δ13C) and feeding positions (δ15N) (38). Specifically, there was evidence of Heterocarpus grimaldi foraging at a higher trophic position, while P. longirostris preyed at the lowest position. It was thus concluded that higher δ15N values of surface sediments from the northern array of the Benguela reported in the past and the impact of the upwelling center along Lüderitz might have had an impact on the disparities in carbon bases and trophic levels of the crustaceans under study Iitembu and Dalu (38).

Marine crustaceans exhibit diverse feeding strategies (carnivorous, omnivorous, detritivorous), preying on zooplankton, polychaetes, mollusks, and mysids (39, 40). Based on Lima et al. (41), the stomach content analyses of most shrimps showed their highly omnivorous tendencies (planktivorous, insectivorous, piscivorous and detritivorous). However, the accuracy of stomach content analysis as an analytic method was questioned by Abed-Navandi and Dworschak (42), as it firstly does not provide adequate feeding evidence for minor crustaceans. Secondly, taxonomic resolution of prey items is challenging since consumed constituents are assorted with fine sediments and ground, by their intestinal mill, making it hard to identify (42). Additionally, larvae of crustacean species are said to feed mainly on planktonic cells by filtration of water (40). Crustaceans of less than 5 g are commonly secured on roots of mangrove in pursuit of prey and waste (43).

The trophic ecology of marine crustaceans also has important implications for heavy metal bioaccumulation and trophic transfer within the northern Benguela current ecosystem. Due to their diverse feeding strategies, including detritivory and benthic foraging (39), many crustaceans are in direct contact with sediments, which act as major reservoirs of heavy metals (44, 45). This exposure facilitates the uptake of contaminants such as mercury, particularly in environments influenced by upwelling and riverine inputs (17). As crustaceans constitute a key dietary component of Cape hake, they serve as critical intermediaries in the trophic transfer of metals from lower to higher trophic levels. Furthermore, variability in feeding strategies and trophic positions, as reflected by δ¹³C and δ¹⁵N signatures (38), may influence differences in contaminant accumulation among species. Consequently, understanding the trophic roles of crustaceans is essential for interpreting patterns of heavy metal bioaccumulation and biomagnification within the Benguela marine food web.

3 Heavy metal contamination in marine organisms

3.1 Overview of heavy metals in marine environments

The term “heavy metals” refers to a group of dense metallic elements with a natural geological presence in the Earth’s crust (1). Although they are known for their naturally occurring nature, most HM contamination and human exposure have been noticed to result from anthropogenic activities (7, 8). As highlighted in Tchounwou et al. (1), these anthropogenic sources include mining and smelting operations, industrial processes, and the widespread use of metal-containing compounds in agriculture and domestic settings as illustrated in Figure 4.

Within the Namibian context, coastal industrial centers such as Walvis Bay and Lüderitz represent important sources of heavy metal inputs into adjacent marine environments. Activities including port operations, mining, and urban expansion contribute to the release of metals such as Pb, Hg, Cr, and Cu into coastal waters (9, 47). In addition, riverine inputs (36) and atmospheric deposition further facilitate the transport of contaminants into the northern Benguela current ecosystem. Once introduced, heavy metals tend to associate with particulate matter and accumulate in sediments, which act as major sinks and potential secondary sources of contamination (18, 44, 48). These characteristics are particularly significant in upwelling-driven systems such as the Benguela, where strong benthic-pelagic coupling (2), enhances the redistribution of contaminants within the water column and across trophic levels. Abdulla et al. (49) highlight that industrial wastewater discharges are often loaded with a variety of toxic heavy metal pollutants, with human activities being the primary drivers of rising environmental contamination. Non-biogenic metals such as Pb, Hg, Se, and Cr, along with their organometallic derivatives like alkylated lead and methylmercury, are highly toxic even at trace levels (9, 50). These metals are environmentally persistent since they are non-biodegradable and resistant to degradation (9). Owing to their hazardous and poisonous properties, they are considered significant environmental threats, necessitating effective handling, monitoring, and control strategies (8) especially in environments like the BCLME.

Key issues contributing to environmental and food contamination by hazardous heavy metals include inadequate monitoring of pollution sources, limited knowledge of safety measures, disregard for regulatory standards, and ineffective management of industrial discharges (51). According to Ruppel (52), the root causes for environmental degradation, which includes water pollution, as experienced worldwide also apply to Namibia. Meanwhile, Namibia has various frameworks governing the environment including our precious marine ecosystem. The primary framework is the Environmental Assessment Policy, which emphasizes the nation’s commitment to safeguarding ecosystems and their ecological functions while ensuring the preservation of maximum biological diversity (53). Despite the existence of these environmental governance frameworks in Namibia, including policies, increasing evidence of anthropogenic pressure suggests that monitoring and management of heavy metal pollution remain challenging. This underscores the need for improved integration of contaminant dynamics into marine environmental management and fisheries sustainability strategies.

3.2 Heavy metals in Namibia’s marine environments

Rising concentrations of metals such as Cd, As, Pb, Cu, Fe, Zn, Mn, and Cr in coastal zones are widely regarded as indicators of human activities, highlighting the need for continuous monitoring. Across Namibia’s coastal and adjacent marine systems, multiple studies (Table 2) reveal consistent spatial patterns of heavy metal distribution and accumulation. In coastal sediments, a recurring trend of metal enrichment has been observed. For instance, Sylvanus et al. (44) reported similar seasonal distributions of metals in shoreline sediments at Walvis Bay and Henties Bay, with iron (Fe) consistently dominating in concentration. Similarly, Vellemu (36), in a comparative assessment along the central Namibian coastline (Walvis Bay, Swakopmund, Henties Bay, and Cape Cross), found that sediments contained significantly higher concentrations of heavy metals (Pb, Cu, Zn, and Fe) than both mussels and water, with iron remaining particularly elevated at Henties Bay and Walvis Bay. Although these concentrations did not exceed United States Environmental Protection Agency thresholds, the observed consistency in distribution suggests the influence of shared controlling factors, likely related to regional hydrodynamics and sediment characteristics.

Table 2. Summary of studies investigating heavy metal contamination in marine organisms and sediments along the Namibian coastline and Benguela ecosystem.

However, evidence from adjacent systems indicates that localized contamination can exceed regulatory limits under intensified anthropogenic pressure. Onjefu et al. (54) reported significantly elevated concentrations of chromium (Cr = 96–107 mg/kg) and arsenic (As = 17–19 mg/kg) in sediments from the Omaruru River delta, exceeding United States Environmental Protection Agency thresholds and reflecting the impact of agricultural, industrial, and commercial activities within the catchment. Similarly, Vellemu (36) associated major coastal rivers, including the Swakopmund, Kuiseb, and Omaruru rivers, with the discharge of effluents into the marine environment, largely driven by mining activities and nearby human settlements. Collectively, these findings suggest that riverine inputs serve as key pathways for the transport of heavy metals into coastal ecosystems.

Building on the role of riverine and land-based inputs, Orani et al. (45) further demonstrated that mining activities and urban development along Namibia’s coastline significantly influence the geochemical composition of trace elements in coastal sediments, reinforcing the importance of anthropogenic sources. In their study, heavy elements, rare earth elements (REEs), and Pb isotope ratios were analyzed from six sediment cores collected along the Namibian coast, with contamination assessed using indices such as the Enrichment Factor, Geo-accumulation Index, and Pollution Load Index. The results revealed notably elevated concentrations of Pb, Cu, As, and Cd in the upper 7 cm of sediment core C5 from Walvis Bay, Namibia’s principal harbor and industrial center, highlighting the impact of recent human activities. Consistent with findings from Vellemu (36) and Onjefu et al. (54), these results emphasize that coastal sediments act as the primary sink and reservoir for heavy metals and therefore represent a key pathway through which metals can enter and transfer within marine food webs in the northern Benguela current ecosystem.

Although many of the cited studies focus on sediments and riverine systems, these compartments play a critical role in facilitating bioaccumulation in marine organisms. Sediments act as long-term sinks for heavy metals, where contaminants can be remobilized into the water column or taken up by benthic and detritivorous organisms, including crustaceans (55). These organisms form an essential component of the diet of mid to high-trophic level predators such as Cape hake (Merluccius capensis and M. paradoxus), thereby providing a direct pathway for the transfer of sediment-associated metals into higher trophic levels (9). In addition, riverine inputs transport contaminants from terrestrial sources into coastal environments, where they become incorporated into local food webs (8). Consequently, the accumulation of heavy metals in sediments and river systems is closely linked to trophic transfer and biomagnification processes, ultimately influencing contaminant burdens in commercially and ecologically important species.

The reviewed studies collectively demonstrate that heavy metal contamination in Namibia is primarily concentrated in coastal sediments and bioindicator organisms, particularly around industrialized regions such as Walvis Bay. Mercury accumulation studies in marine fish indicate tissue-specific differences in bioaccumulation, while sediment investigations consistently report elevated concentrations of Fe, Pb, Cu, As, and Cd linked to anthropogenic activities. However, species-specific studies directly examining heavy metal transfer between Cape hakes (Merluccius capensis and M. paradoxus) and their crustacean prey remain largely absent, highlighting a major knowledge gap in trophic biomagnification research within the Benguela Current ecosystem.

3.3 Mechanisms of heavy metal bioaccumulation and biomagnification

Bioaccumulation refers to the uptake and gradual buildup of heavy metals (HMs) within an organism, whereas biomagnification describes the process by which these metals are transferred along the food chain, resulting in higher concentrations at successive trophic levels (8). Debipersadh et al. (9) compared HM levels in two fish species, to assess their effectiveness as bioindicators of heavy metal (HM) contamination in two distinct marine environments. Their study analyzed HM concentrations in various fish parts of the two species: fleshy tissues, gut and liver. In the present review, emphasis is placed on dorsal muscle tissue, as it is the most analyzed tissue in ecotoxicological studies and is directly relevant for human consumption. Furthermore, Erasmus et al. (18) reported no substantial differences in total mercury concentrations between liver and muscle tissues, supporting the use of muscle tissue as a representative matrix for mercury assessment. However, this observation is primarily applicable to mercury, as other heavy metals may exhibit tissue-specific accumulation patterns, with organs such as the liver and kidneys often showing higher concentrations due to their roles in detoxification and metal regulation (48, 56). Chen et al. (57) highlighted that trophic transfer of heavy metals, particularly mercury, leads to the bioaccumulation and subsequent biomagnification of its toxic form, methylmercury (MeHg), in higher trophic level organisms, including top predator species consumed by humans.

Since heavy metals (HMs) are non-biodegradable, they can be transferred through trophic levels and accumulate within aquatic food webs (9), as indicated in Figure 5. Marine species such as Cape hake and crustaceans are exposed to these metals via water, sediments, and food. Accumulation may occur as contaminants adhere to gill surfaces or bind to the exoskeleton (58). Feeding habits and dietary choices also play a crucial role, since metal concentrations differ across food sources (59). Additionally, trophic position strongly influences metal accumulation, with long-lived predators generally showing higher concentrations due to extended exposure times and the biomagnification of certain metals (58).

A long-term investigation on mussel digestive glands by Ivanković et al. (60) indicated that notable fluctuations in metal concentrations and metallothionein levels could result from nutrient buildup during the pre-spawning period in the dry season, a process attributed to the “biological dilution” effect. Building on this, Tanhan et al. (61) observed that such seasonal dynamics could also lead to increased HM accumulation in tissues during dry periods. Similarly, within the Namibian context, Vellemu (36) reported significant spatial and temporal variations in heavy metal concentrations in mussels along the central coastline, further supporting the influence of environmental conditions and seasonal processes on metal accumulation in marine organisms such as Cape hakes. Collectively, these findings suggest that both seasonal dynamics and local environmental factors play a key role in regulating heavy metal bioavailability and uptake in marine biota. Consequently, seafood consumption during certain periods, particularly the wet season, may pose relatively lower health risks (61). Nevertheless, consistent monitoring of heavy metal levels in seafood remains essential to safeguard consumer health, particularly for populations living in coastal regions.

3.4 Heavy metal pathway and trophic transfers in hake and crustacean species

The movement of metals through the food web culminating in high-trophic level predatory hake species, Merluccius capensis and M. paradoxus can be explained by examining the bottom-up mechanisms of bioaccumulation and biomagnification. The first step from the entry of HMs in marine environments is the uptake by primary producers (phytoplankton) (62). Phytoplankton absorb dissolved metals directly from the water through passive diffusion and active transport across their cell membranes. Heavy metals are then said to bind to organic molecules within phytoplankton cells, becoming part of their biomass (62).

As per González et al. (62), HMs are further transferred to primary consumers in the marine ecosystem, which are zooplanktons. Zooplankton feed on phytoplankton, ingesting the metals accumulated within their tissues. Metals can become concentrated in zooplankton, through a mechanism known as bioaccumulation due to their limited ability to excrete them efficiently (62). As HMs accumulate in zooplankton, they are then transferred to secondary consumers including small crustaceans, such as krill, shrimp and copepods, since they prey on zooplankton.

Various mechanisms of HM accumulation in crustaceans exist. This includes the accumulation of HM to crustaceans through the ingestion of contaminated prey and through direct contact with metal-contaminated water. Heavy metals have been reported to diffuse into tissues from the surrounding water, particularly through gill membranes (63). However, evidence suggests that waterborne exposure may contribute less to overall accumulation compared to dietary pathways. For instance, Vellemu (36) reported relatively low concentrations of heavy metals in seawater compared to significantly higher levels in mussels along the Namibian coast, indicating that bioaccumulation through trophic interactions is likely the dominant pathway. Additionally, crustaceans store heavy metals in various tissues, sometimes sequestering toxic metals in exoskeletal structures or hepatopancreatic cells to minimize toxicity (64).

However, this trophic transfer pathway is not strictly linear. Marine food webs are inherently complex, with multiple interconnected pathways, omnivory, and the presence of intermediate pelagic fish species that can also contribute to the transfer and redistribution of contaminants. For example, crustaceans may feed across multiple trophic levels, while small pelagic fish can act as important intermediary vectors between lower trophic levels and top predators. These interactions create overlapping trophic linkages, which may influence both the rate and variability of heavy metal accumulation within the food web.

Despite this complexity, there is a general trend of increasing HM concentrations at higher trophic levels. For that reason, Cape hake feed on crustaceans and small fish that have already accumulated metals. Biomagnification occurs at this stage, where metal concentrations increase because hake consume large amounts of contaminated prey over their lifetime (63). Metals can bind to proteins in the hake’s muscle tissues and internal organs, where they may persist (63). These HMs then lead to human beings by consuming contaminated hake as food. Thus, while heavy metals typically move from water to phytoplankton, zooplankton, crustaceans, and higher predators, the process is mediated by a dynamic and interconnected food web structure that governs contaminant transfer and accumulation.

Studies have shown that dietary intake is the primary pathway for heavy metal accumulation in marine environments, while food webs serve as the main routes for their transfer (9, 65). The movement of heavy metals through aquatic food webs like that of the Cape hake is crucial for assessing their risks (50), and understanding these transfers is essential for predicting metal concentrations in organisms at mid to high trophic levels. (65). Generally, HM concentrations in aquatic organisms have been extensively studied to assess metal trophic dynamics (9, 56, 66, 67); with, extensive research (18, 36, 48, 68, 69) carried out within the Northern Benguela Ecosystem. However, none of these studies made a predator-prey bioaccumulation comparison in HM concentration. Moreover, different marine organism tissues or organs exhibit different concentrations of HM, thus careful consideration must be taken when choosing the type of tissue or organ to use for a specific HM concentration analysis.

Based on Gao et al. (65), metabolically active organs such as the liver and kidneys, as well as tissues of benthic shellfish, often accumulate high concentrations of heavy metals, making them useful as marker organs for monitoring environmental HM levels. In contrast, muscle tissues are considered particularly suitable for assessing mercury contamination in marine organisms (70). Cape hake and crustaceans off Namibia engage in major trophic interactions that make room for HM trophic transfers between the species. As it is elsewhere in marine ecosystems, these HMs are subjected to bioaccumulation and biomagnification along the food webs through trophic linkages of the two species. No research has been carried out off the Namibian coastline on the hake species (Merluccius capensis and M. paradoxus) and selected crustaceans, pertaining to HM trophic transfers. Yet, trophic transfer of heavy metals likely differs across food webs depending on environmental conditions and community composition (65). Therefore, examining the patterns of heavy metal transfer within the Cape hake food web is essential for evaluating their ecological risks, with further implications for food web dynamics and human health.

4 Implications for food web dynamics

4.1 Impact of heavy metal accumulation on food web structure

According to Landers (58), heavy metals can significantly impact aquatic communities, often leading to marked reductions in both invertebrate abundance and species diversity in heavily contaminated water bodies. Different species experience varying levels of metal exposure depending on their feeding habits, behavior, and life history traits, which can influence the movement of heavy metals through aquatic food webs (58). Furthermore, contaminants such as heavy metals including mercury may alter the strength of trophic interactions by affecting predators, prey populations, or both (71).

In the study by Pardal et al. (71), heavy metal contamination was found to increase barnacle mortality and reduce predation, without altering interaction strength. The authors suggested that this outcome occurred because the higher mortality of prey offset the reduced consumption by predators. Although the interaction strength remained unchanged, these findings indicate a potential reduction in energy transfer through the trophic chain, which could lead to significant shifts in community structure and ecosystem functioning (71). Studies of this nature are valuable for understanding ecological interactions and for clarifying how environmental stressors like mercury as a toxic HM influence the structure and dynamics of marine communities like that of the northern Benguela current.

As per Paine (72) as cited in Pardal et al. (71) the predator-prey interaction is a fundamental ecological process that drives the dynamics of natural communities and depends on the strength of the interaction. Interaction strength quantifies the per capita impact of one species on the abundance of another, and contaminants such as heavy metals can influence this strength by affecting the predator, the prey population, or both (73). However, all these suggestions and reports came as a result of laboratory analyses as compared to field experiments. Based on Mayer-Pinto et al. (74) laboratory studies often demonstrate clear and consistent impacts of metal contamination on organisms, whereas field experiments tend to show little, variable, or weaker effects. Nonetheless, Pardal et al. (71) emphasize that realistic, field-based research is essential for understanding pollution impacts in a more ecologically relevant context.

Moreover, toxic heavy metals can have detrimental effects across all levels of biological organization, influencing ecological interactions such as predation, parasitism, competition, and the overall structure of communities and ecosystems (75). In marine food webs such as the northern Benguela current ecosystem, these effects are particularly relevant, as contamination can alter trophic interactions between key species, including crustaceans and their predators, such as Cape hake (Merluccius capensis and M. paradoxus). Dar et al. (76) reported that, in many invertebrates, negative impacts on metabolism, growth, fecundity, developmental stability, and survival occur once heavy metal accumulation surpasses critical thresholds. While some studies suggest that higher trophic level organisms may exhibit greater contaminant burdens due to biomagnification. Instead, it varies depending on factors such as species-specific physiology, detoxification capacity, and the chemical properties of individual metals (76). Consequently, both predators and prey may be vulnerable to heavy metal exposure, although the pathways and extent of impact may differ across taxa and ecological contexts (65).

5 Implications for human health

5.1 Health risks associated with consuming contaminated seafood

Fish including Cape hake is a dietary staple in many cultures and has long been valued for its high content of essential nutrients, including protein and omega-3 fatty acids (77). Consumption of fish has been linked to numerous health benefits, such as reduced risks of heart disease and stroke, support for embryonic development, and improved cognitive function (78). However, concerns remain regarding whether for instance the Cape hake consumed in Namibia is sufficiently safe to confer these expected health benefits. Growing public awareness about exposure to environmental contaminants like mercury that can accumulate in fish has raised questions about the safety of regular fish consumption (79).

The presence of environmental contaminants in fish introduces potential health risks for human communities such as that of the coastal communities off Namibia’s coastline. Chen and Dong (10) reported that pregnant women, breastfeeding mothers, and young children are particularly vulnerable to exposure through fish consumption. Additionally, Hauptman and Woolf (80) associated the intake of contaminated fish with negative health outcomes, including neurological damage, developmental delays in children, and an increased risk of cancer, particularly from harmful substances like methylmercury. Therefore, Chen and Dong (10) emphasized the importance of using current scientific evidence to inform safe fish consumption decisions.

Fleming et al. (81) stressed that there is growing recognition of the connections between global human health and ocean ecosystems. Humans are primarily exposed to synthetic organic compounds and inorganic substances, such as heavy metals, through the consumption of contaminated seafood like Cape hake. Some heavy metals can cause cytotoxicity as well as kidney and brain damage, leading to neurotoxic effects (81). As in Hauptman and Woolf (80), the consumption of seafood exposed to toxins, has also been linked to the vulnerability of the rapidly growing neurologic tissue of the fetus in pregnant women in Fleming et al. (81).

Despite the significant scale of ocean pollution and increasing awareness of its impacts on human health, substantial gaps persist in understanding the levels of human exposure and the full extent of associated health effects (11), especially within the Benguela current ecosystem. However, there are known issues associated with human exposure to contaminations from eating seafood including Cape hake. In their study, Landrigan et al. (11) reviewed various issues including damage to developing brains of infants in utero, reducing their IQs, hence increasing children’s dangers for autism, attention deficit/hyperactivity disorder (ADHD) and learning difficulties. Moreover, it was also highlighted in Landrigan et al. (11) that adult exposures to such contaminants upsurge dangers for heart illness and dementia.

Table 3 recaps some of the HMs of concern in marine or aquatic environments like the northern Benguela ecosystem, their sources and major associated health issues that could affect the communities that depend on the seafood such as Cape hake from these environments.

Table 3. Major heavy metals, their sources, and the linked health dangers.

5.2 Guidelines for safe seafood consumption

Concerns about food quality and its impact on health have been increasing in recent years. Intake of toxic metals through diet such as the consumption of Cape hake should remain within established safe limits (79). To reduce human exposure, various food safety organizations have set maximum allowable levels of toxic metals in fish and seafood. These bodies include the U.S. Environmental Protection Agency (USEPA), the World Health Organization (WHO), and the Codex General Standard for Contaminants and Toxins in Food and Feed (Codex Standard 193-1995, 1995).

The WHO and Codex Standard 193–1995 recommend a maximum concentration of 0.5 mg/kg for heavy metals in seafood (82, 83), while the EU sets a similar limit of 0.5 mg/kg specifically for methylmercury in fish products (Directive 1881/2006/EU; amended by Regulation 420/2011/EU). Additionally, the EFSA Panel on Contaminants in the Food Chain has established a tolerable weekly intake (TWI) for inorganic mercury of 0.004 mg/kg body weight (84). Differences in regulatory thresholds reflect variations in risk assessment approaches, targeted contaminants, and safety margins, which can have implications for assessing seafood safety across regions.

Debipersadh et al. (9) applied various human health risk assessment metrics recommended by the USEPA, including carcinogenic risk from fish consumption and the target hazard quotient, to evaluate potential health risks for individuals consuming fish. While such assessments are common in other regions, Namibia and many African countries currently lack localized seafood consumption guidelines. Therefore, region-specific studies evaluating human health risks from heavy metal exposure are critical to develop informed regulatory frameworks. Globally, foodborne exposure to mercury is a major health concern. The World Health Organization (WHO) reports that foodborne illnesses account for approximately 600 million cases and 420,000 deaths annually. For populations not exposed occupationally, food is the primary source of mercury, with average dietary intakes ranging from 2x10–⁶ to 2x10–⁵ g/day per person across different countries (85). Mercury is a highly toxic element naturally occurring in the Earth’s crust, with an average abundance of 0.08 mg/kg by mass (67). Table 4 summarizes selected heavy metals of major concern of which they may be present in the Benguela current ecosystem and their maximum permissible limits as defined by various regulatory bodies.

Table 4. Maximum Permissible Limits (MPL) of HMs in seafood as established by different organizations globally.

5.3 Global context on heavy metal contamination

Connelly et al. (86) reported that over half of the seafood samples tested in the United States exceeded the recommended mercury safety limits. In contrast, two studies conducted in the Benguela current ecosystem found that none of the samples exceeded the WHO and EU safe limit of 0.500 mg/kg (18, 48). Erasmus et al. (18) further noted that the mean total mercury concentration was 0.126 ± 0.005 mg/kg in muscle tissues and 0.106 ± 0.005 mg/kg in liver tissues. These findings align with observations from Egypt, where Younis et al. (56) reported that heavy metal concentrations were generally higher in muscle tissues compared to liver and gills. Kapula et al. (48) on the other hand reported that mercury concentration was higher in the liver (0.005 mg/kg) and relatively lower in the gills (0.001 mg/kg). Although none of these studies detected mercury levels above the safe limits, their results do not preclude the presence of mercury in the seafood analyzed.

Only a single study has examined seasonal variations in heavy metal concentrations (Pb, Cu, Zn, and Fe) in the Benguela current ecosystem (36). In contrast, Younis et al. (56) reported a significant increase in metal concentrations (Cd, Pb, Ni, Hg and Sn) during the summer compared to winter in their sampled sites. In Namibia or its Benguela current ecosystem, biomass surveys are normally done only once a year or in one season (January to February for Cape hake) (20), thus it is very difficult to make seasonal comparisons in HM concentrations in that region.

A study in South Africa by Debipersadh et al. (9), reported that cape horse mackerel exhibited heavy metal concentrations of Ni (1.6 ± 0.2 mg/kg), Pb (4.7 ± 0.6 mg/kg), and Zn (52 ± 3.01 mg/kg), exceeding the World Health Organization (WHO) regulatory limits of 0.3, 1.0, and 30 mg/kg, respectively. This is also a significant commercial species and a protein source in the Benguela Current ecosystem, notably caught as bycatch in the cape hake fisheries, however no study has been carried out on its HM concentrations.

6 Methodological approaches in studying trophic interactions and heavy metal transfers

6.1 Stable isotope, stomach content and mixed model analyses

Stable isotope analysis (SIA), stomach content analysis, and Bayesian mixing models are widely used complementary approaches for investigating trophic interactions in marine ecosystems (29, 87). Stable Isotope Analysis provides time-integrated insights into dietary sources by tracing carbon and nitrogen isotope ratios in consumer tissues. However, isotopic approaches may be influenced by baseline variability and trophic discrimination uncertainties. Stomach content analysis offers short-term, species-specific dietary information. Bayesian mixing models, such as Stable Isotope Analysis in R (MixSIAR), integrate isotopic data to estimate the relative contributions of different prey items, thereby reducing biases associated with traditional diet studies (88, 89). These combined approaches are particularly valuable because they allow for more robust interpretations of trophic structure by incorporating both instantaneous and long-term feeding information. However, emerging approaches such as compound-specific isotope analysis of amino acids may further improve trophic resolution and contaminant tracing.

Within the Benguela current ecosystem, integrated approaches combining stable isotope analysis, stomach content data, and Bayesian mixing models have been applied to investigate trophic dynamics (29, 34) as indicated in Table 5; however, their use in assessing the trophic transfer of contaminants such as mercury remains limited. To date, these methods have not been employed to quantify biomagnification pathways or to refine estimates of contaminant transfer to higher trophic levels, including commercially important species such as Merluccius capensis and M. paradoxus. Nevertheless, such an integrative framework has strong potential to enhance ecological and ecotoxicological assessments by reducing uncertainty in dietary reconstructions and providing a more robust basis for evaluating human health risks associated with seafood consumption.

Table 5. Evidence of stable-isotope analysis (SIA), stomach content analysis and mixing model methods previously used in studies within the Benguela current ecosystem.

7 Future research directions and policy recommendations

7.1 Gaps in current knowledge and suggestions for further research

Vast research pertaining to trophic relationships between various species have been carried out by employing stable-isotope, stomach content and mixed model analyses in Namibia (29, 34, 35, 90, 91) and around the world. However, no study focused on heavy metal trophic transfers in Namibia’s marine ecosystem. Additionally, trophic interactions and HMs trophic transfer have mostly been studied in Cape hake species (Merluccius capensis and M. paradoxus) and horse mackerel (Trachurus capensis) (34), however there is a vast number of fish species that could possibly be good candidates for these analyses. Some species form part of bycatches in commercial vessels and these include Jacopevers (Helicolenus dctylopterus), Angle fish (Brama brama) and monk (Lophius vomerinus).

Moreover, it has been stated that several HMs are released in aquatic environments including marine ecosystems that are adjacent to point sources of such metals especially mines and agricultural facilities (9). Apart from the study by Vellemu (36) which focused on HM metals (Pb, Cu, Zn and Fe) contamination in mussels, only Kapula et al. (48) and Erasmus et al. (18) evaluated mercury concentration in mullets and monk species respectively, in the area. However, there were no studies that reported high concentrations of mercury in the region, but its constant monitoring is essential for human health risk assessment reasons. Hence, critical gaps remain in understanding the bioaccumulation and biomagnification of other HMs, like cadmium, lead, and arsenic, which are similarly lethal and prevalent in anthropogenically impacted marine systems. Notably, only one study (36) along the Namibian coastline has assessed seasonal variations (winter and summer) in HM metals (Pb, Cu, Zn and Fe) concentrations, despite evidence from other regions showing significant fluctuations linked to upwelling intensity, riverine inputs, or industrial discharge cycles (56). Future research should thus prioritize multi-metal analyses across trophic levels, coupled with time-series sampling to capture seasonal and interannual variability.

Debipersadh et al. (9) examined the distribution of selected heavy metals in the muscle tissues, gills, and liver of two fish species, and further evaluated the potential human health risks associated with their consumption by applying various health risk assessment indices. However, this review only outlined the utilization of muscle tissues to determine the bioaccumulation along with biomagnification of one heavy metal (mercury) grounded on stable isotope ratios (93). Efforts for future research could be made by using other parts of the fish other than muscle tissues or making use of a combination of parts as in Debipersadh et al. (9). Additionally, no study focused on crustaceans as vital food for human consumption, yet it is mostly sold as seafood at various restaurants in the coastal towns of Namibia. Although, the study by Vellemu (36) highlighted that based on its results mussels grown along the Namibian coastline could be regarded safe for human consumption; however, levels observed in samples from Walvis Bay were of health concerns.

Finally, climate variability and change are poised to significantly alter heavy metal (HM) dynamics in the northern Benguela current ecosystem through multiple pathways. Intensified upwelling, driven by shifting wind patterns, may enhance the remobilization of sediment-bound metals (e.g., Cd, Pb) into the aquatic column, increasing bioavailability for aquatic life. Ocean warming and acidification could further modify metal speciation and toxicity, while altered precipitation regimes may amplify terrestrial runoff, which is a key vector for anthropogenic HM input. Additionally, climate-driven shifts in species distributions and trophic interactions may disrupt existing bioaccumulation pathways or introduce new ones, particularly for mid to high trophic level predators like Cape hake. Despite these potential interactions, the relationship between climate change and HM cycling in the BCE remains insufficiently explored. Addressing this gap will require integrated approaches that couple climate projections with long-term HM monitoring and trophic assessments. Such efforts are essential for improving predictions of future contamination risks and supporting adaptive management strategies in this dynamic and vulnerable marine ecosystem.

7.2 Policy recommendations on heavy metal contamination

Namibia’s Environmental Assessment Policy (1995) along with the Environmental Management Act (2007) provide the legal framework for regulating environmental risks. Yet, with regards to the complex and long-term issue of HM contamination, particularly from mining, smelting, and industrial waste, several critical policy gaps persist. In light of growing scientific evidence on HM bioaccumulation, trophic transfer, and the emerging influence of climate variability in the Benguela Current Ecosystem (BCE), there is a clear need to strengthen policy frameworks to better address both current and future contamination risks.

A key priority is the revision of Environmental Impact Assessment (EIA) guidelines to explicitly require comprehensive HM risk assessments for projects adjacent to marine environments involving mineral extraction, industrial chemicals, or waste disposal. This assessment must include baseline contamination data, pathways of exposure (e.g., through water, air, or food webs), and cumulative impact analysis, especially where multiple pollution sources exist. Importantly, such evaluations should also incorporate climate-related variables such as upwelling intensity, ocean warming, and changing runoff patterns which may influence HM mobility, bioavailability, and toxicity in marine environments like the northern Benguela current ecosystem. Additionally, for these projects, a concise plan on how to prevent or reduce the risk of HM contamination should be established and used as a requirement for obtaining an Environmental Clearance Certificate (ECC).

Moreover, there is limited long-term monitoring of HMs in areas of possible contamination, such as Namibia’s coastal regions underscoring the need for policy reform to mandate continuous surveillance of priority metals (e.g., Hg, Cd, Pb, As) in water, sediments, and biota including ecologically and economically significant Cape hake. (94). These monitoring systems should then be tied to enforcement mechanisms with penalties for non-compliance. Launching an enthusiastic compliance monitoring unit part of the Ministry of Environment, Forestry and Tourism (MEFT), equipped with sufficient technical and financial resources, would help ensure that appropriate moderation measures are employed and maintained.

Based on FAO (95), efforts to tackle heavy metal contamination like mercury in seafood have encompassed a variety of strategies, including strengthening regulations and management practices for industrial and agricultural activities, enhancing monitoring and surveillance of seafood products, and developing innovative technologies to remove heavy metals from contaminated sediments and aquatic environments. These strategies could well be incorporated into the environment-based policies of marine environments of Namibia. Employing bioindicator species in biomonitoring programs offers crucial insights into heavy metal contamination in coastal (36) and estuarine waters, supporting effective environmental management and the safeguarding of public health (96). Therefore, the improved policies should also highlight the need for more fundings towards biomonitoring research projects that targets marine environments pollution including heavy metals.

In some areas of the world monitoring programs aimed at assessing human health risks assessments does not only end in the evaluation of HM concentrations in seafood like fish. However, the extent of these programs goes further to testing for these HMs in human hair and urine. A study by Shaimardanova et al. (97) highlighted that children in Pavlodar, Russia, exhibited hair mercury concentrations of 0.44 ± 0.5 mg/kg, considered high, likely due to emissions from metal processing plants and the chemical industry in the region. Therefore, it could be suggested that biomonitoring programs established in the environment-based policies should also consider projects like these, for instance for Cape hake consuming communities for easier comparisons.

Integrated water administration methods, including aquatic resources management and the water-food-energy nexus, have been recognized as essential for ensuring a sustainable future (79). Based on Zhuzzhassarova et al. (79), this thematic problem explores complex water challenges, providing insights drawn from hydrological research, water quality studies, and ecosystem assessments. Within this framework, it is believed that reviews and case studies offer field-tested strategies for the effective management of the region’s water resources (79). Hence it is very paramount that such integrative approaches are reviewed and incorporated into Namibia’s existing environmental frameworks to enhance our understanding of HM trophic transfers in the northern Benguela current ecosystem. Aligning these strategies with climate-informed risk assessments will be critical for developing adaptive, forward-looking policies capable of managing HM contamination in a rapidly changing Benguela current ecosystem.

8 Conclusion

Studying trophic interactions and HM trophic transfers in aquatic environments is very substantial in the successful and sustainable management of our precious aquatic resources. This review synthesizes current knowledge on trophic interactions and heavy metal (HM) transfer within aquatic ecosystems, with particular emphasis on the Benguela current ecosystem. It highlights the ecological importance of crustaceans as key prey items for commercially important species such as Merluccius capensis and M. paradoxus, reinforcing their central role in structuring trophic pathways and facilitating the movement of contaminants through the food web.

The review further demonstrates that anthropogenic activities are the primary drivers of HM inputs, particularly methylmercury, into marine environments. Due to its strong bioaccumulative and biomagnifying properties, methylmercury poses significant risks at higher trophic levels, including predatory fish such as the Cape hake and ultimately human consumers. These findings underscore the direct linkage between ecosystem processes and public health, particularly in regions where Cape hake constitutes an important dietary protein source like in Namibia. Spatial variability in HM concentrations is influenced by environmental drivers such as upwelling intensity, sediment interactions, and coastal inputs, which affect metal bioavailability. Yet, important gaps remain, including limited data on non-mercury metals, seasonal variability, and multi-trophic level analyses within the region.

Various analytical techniques have been vastly utilized in the efforts to study trophic interactions in aquatic environments. In terms of methodological advances, the integration of stable isotope analysis, stomach content analysis, and stable isotope mixing models provides a more robust framework for quantifying trophic relationships and contaminant transfer. By combining short-term dietary data with time-integrated isotopic signals, these approaches reduce uncertainty in food web reconstructions and improve the accuracy of biomagnification assessments. However, the application of trophic ecology tools, such as stable isotope analysis (SIA) and Bayesian mixing models (MixSIAR), remains underutilized in assessing contaminant pathways. These limitations constrain a comprehensive understanding of HM dynamics in the northern Benguela region.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/frish.2026.1734136/full#supplementary-material

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Footnotes

Edited by: Henry Joseph Oduor Ogola, University of South Africa, South Africa

Reviewed by: Meng Chuan Ong, University of Malaysia Terengganu, Malaysia

Guangyuan Lu, City University of Hong Kong, China