Sediments play a pivotal role in marine ecosystems by functioning as nutrient reservoirs, biological habitats, and biogeochemical regulators that sustain primary productivity and ecological balance [1,2]. They act as natural sinks for organic matter and trace elements, supporting benthic organisms that are essential to nutrient cycling and trophic transfer [3]. However, contamination with heavy metals alters these critical functions by inducing toxicity in sediment-dwelling fauna, disrupting microbial-mediated nutrient processes, and enhancing the bioavailability of toxic elements to higher trophic levels [4,5]. Under fluctuating redox or pH conditions, metals can be remobilized from sediments into the water column, transforming sediments from stable sinks into secondary pollution sources [2,6]. This process elevates metal uptake by benthic flora such as seagrasses and algae, promoting bioaccumulation and biomagnification along the food chain [7,8,28]. Consequently, heavy metal contamination compromises sediment quality, reduces biodiversity, impairs bioturbation and oxygen penetration, and weakens ecosystem services such as nutrient regulation and shoreline protection [1,4]. Collectively, these effects endanger sediment-associated biota and amplify ecological and human health risks through trophic transfer within the marine food web [9,10].

Sea grasses ecological importance is immense despite their relatively low global diversity. Beyond carbon storage, seagrasses provide critical ecosystem services such as acting as natural filters that trap sediments and stabilize the seabed. They provide habitat for commercially valuable fish and invertebrates, shelter juvenile and endangered marine fauna, stabilize shorelines, contribute to primary productivity, and act as long-term carbon sinks [11,12]. In addition, seagrasses are also harvested for commercial and cultural uses, embedding them deeply within the socio-economic fabric of coastal communities [13]. Their extensive root and rhizome systems play a crucial role in sediment stabilization, reducing erosion and resuspension while facilitating the deposition of suspended particles [14] consequently these functions underscore their importance to the health of coastal ecosystems and economies.

Seagrasses have emerged as effective bioindicators of heavy metal pollution due to their sessile nature and ability to integrate environmental exposure over time [7]. They facilitate tropic transfer and absorb trace metals from both sediment and water columns and are easily resampled across seasons, making them ideal for long-term monitoring of coastal contamination [28]. Recent studies indicate that metal accumulation patterns depend on species identity, tissue type (leaf, rhizome, or root), sediment properties, and hydrodynamic conditions [15]. Experimental studies have shown that heavy metals such as lead can be deposited in apoplastic and intercellular spaces, leading to anatomical and physiological impacts, while field surveys reveal that different seagrass tissues can reflect both historical and recent metal deposition [16].

Species-specific variations in accumulation potential are well documented. Zostera capricorni, for example, effectively accumulates Cu, Pb, and Zn, mirroring sediment and water quality [17]. Similarly, Thalassia hemprichii-a long-lived tropical seagrass common in the Indo-Pacific-has been used to monitor Pb, Cd, Cu, and Zn levels [18]. Cymodocea serrulata is known to accumulate Fe, Mn, Cu, Zn, Cd, Cr, Pb, and Ni and exhibits antioxidant and enzymatic responses correlated with metal concentrations [19]. H. ovalis (a small, fast-turnover tropical seagrass) has been widely used in local and regional surveys to profile trace-metal gradients and to compare tissue vs. sediment concentrations [20]. Laboratory exposure experiments further demonstrate measurable sub-lethal effects on photosynthesis, chlorophyll content, and morphology under Cu, Cd, Pb, and Zn exposure [20], reinforcing the utility of seagrasses as biological sentinels of coastal pollution.

In marine ecosystems, prolonged exposure to heavy metals can impair reproductive function, disrupt neurological and respiratory systems, and lead to bioaccumulation within food webs, posing serious health threats to human consumers [5]. While metals such as Fe, Cu, and Zn are essential micronutrients, their concentrations beyond physiological thresholds become toxic [10]. Chronic exposure to high levels of Pb, Cd, and Hg has been linked to degenerative neurological diseases such as Alzheimer’s, Parkinson’s, and multiple sclerosis [9].

The proximity of seagrass meadows to urban and industrial activities makes them particularly susceptible to anthropogenic stressors, including pollution, overharvesting, dredging, and climate change [21]. Physical disturbances such as propeller scarring, anchor damage, and coastal construction can uproot seagrass beds, degrade rhizome networks, and reduce overall productivity [22]. The global decline of seagrass habitats is well documented and primarily attributed to eutrophication, sedimentation, and metal contamination [23]. Physiological responses to heavy metal exposure-such as reduced photosynthetic efficiency, inhibited growth, and altered pigment profiles-serve as reliable indicators of environmental degradation [7]. Variations in biomass, chlorophyll content, and growth rates further provide insights into changing water quality and nutrient enrichment [24]. These characteristics enhance their use as early-warning systems for detecting ecosystem stress and guiding management interventions. Conserving seagrass diversity is thus critical not only for biodiversity maintenance but also for sustaining vital ecological services such as carbon sequestration, nutrient cycling, and sediment stabilization.

In the Chukwani Bay ecosystem, recent evidence indicates declining reproductive success in marine fauna, reduced seagrass coverage, impaired fish mobility, and growing health risks linked to heavy metal bioaccumulation. These impacts are closely tied to intensified anthropogenic pressures-such as coastal development, maritime traffic, and domestic effluent discharge-that collectively threaten the ecological balance of this nearshore environment [25]. Thus, these metals also pose ecological risks by altering biodiversity and disrupting trophic structures within marine ecosystems [8]. Despite increasing concern, comprehensive investigations that integrate sediment contamination, seagrass bioaccumulation, trophic transfer, and human health implications remain scarce in Zanzibar. Therefore, this study aims to; Quantify heavy metal concentrations in sediments, seagrasses, and associated fish species from Chukwani Bay; Evaluate the Comprehensive Pollution Index (CPI) and trophic transfer of metals along the sediment–seagrass–fish continuum; and, Assess potential human health risks associated with seafood consumption. The findings will help to improve understanding of heavy metal dynamics in coastal ecosystems and provide baseline data crucial for evidence-based policy and sustainable management of Zanzibar’s coastal resources.

Location

Chukwani Bay is located in Chukwani Bweni regions, suburbs on the Unguja Island, Zanzibar. It is situated in the west of the island, south of Mbweni region towards the main Zanzibar Harbour. Attached please find the map in figure one below. The encircled shows the Chukwani Bay coverage.

Figure 1: Chukwani Bay.

Sampling of Seagrass Species for Analysis

These seagrasses were sampled in the form of leaves and stems fresh from the ocean, washed to remove sediments, identified and labelled. They were air dried in the open for three days, placed in a laboratory oven at 800C for two hours then pulverized using a clean mortar and pestle to obtain a homogeneous powder [26] finally transported in well dried sealed sample bags to African Minerals and Geosciences Centre (AMGC) laboratory for analysis.

Sampling of Sediment

Three sediment samples were collected in clean polyethylene bags and transported to the Applied Marine Geology and Chemistry (AMGC) laboratory for preparation and analysis. In the laboratory, the samples were air-dried at room temperature to prevent the loss of volatile components. The dried sediments were then thoroughly mixed and homogenized using an agate mortar to ensure uniformity. Subsequently, the homogenized material was sieved through an 80- mesh (180 µm) stainless-steel sieve to remove coarse debris and non-sediment impurities prior to chemical digestion and analysis. All sample pretreatment procedures followed standard sediment preparation protocols recommended by the American Public Health Association [27] and the U.S. Environmental Protection Agency [29].

Sampling of Fish

Fish sampling was conducted at Chukwani Bay and its adjacent landing site at Mbweni Fish Market, situated along the western coast of Unguja Island, Zanzibar. The sampling sites were selected is also influenced by anthropogenic activities and associated seagrass meadows that serve as critical feeding and nursery habitats for various marine organisms. Fresh fish specimens were obtained directly from local artisanal fishermen operating within the Chukwani Bay area to ensure the samples accurately reflected the local catch composition.

Each specimen, identified, then placed in a clean, well labeled polyethylene bag and stored in an icebox containing crushed ice to minimize microbial activity and biochemical degradation during transport. The samples were subsequently transported to the laboratory on the same day and stored at −25 °C until further processing and analysis. The sampling, handling, and storage procedures followed established guidelines for trace metal studies in marine biota [30,31].

Identification of Seagrasses and Fish Samples

Identification procedures was carried out by our marine biologist Dr. Rashid Juma Rashid. He identified the three seagrass species found in the area as Halophila Ovalis, Cymodocea serrulata and Thalassia Hemprichii. They were later abbreviated as HO, CS and TH respectively. The main fishes found and identified from the area were mainly Stegastes nigricans (Changu), Mugil cephalus (Samaki), Siganus sutor (Tasi), Kyphosus vaigiensis (Kumbulimbuli),Octopus cyanea (Octopus), Spratelloides gracilis (Dagaa la kukosha), Penaeia shrimps species and Chlorurus sordidus (Bluish), which inhabits deeper coral areas around Chumbe Island in the Chukwani Bay.

Questionnaire Survey

A questionnaire-based survey was conducted to establish the commonly consumed fish species in Chukwani Bay region of Unguja Island, Zanzibar. Questionnaires were administered to 28 participants randomly selected from kiembe-samaki, Mji mpya, Bweni including Chukwani regions. Apart from the locally consumed fishes, it also inquired about basic information on gender, age, education, income level, frequency and ingestion rates of the commonly consumed fish species

Analysis Using Agilent 5900 Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES)

All metallic analysis was done using the Agilent 5900 Inductive Coupled Plasma – Optical Emission Spectrometry

Quality Assurance Procedures

Appropriate quality assurance procedures and precautions were implemented to ensure the reliability of the results. Procedural blanks, calibration standards, and duplicate samples were utilized for instrument calibration, process control, and quality assurance to evaluate accuracy, precision, limits of quantification and detection, and to validate the Agilent 5900 ICP-OES. Blank digestions were conducted in parallel using the same procedure to correct for any background contamination. Certified reagents, blanks, and certified reference materials (NIST SRM 2702, marine sediment) were employed to assess digestion efficiency, correct for background interference, and ensure the accuracy and precision of the analytical results. All quality control samples were analyzed concurrently with the test samples.

Analysis of Heavy Metals from Sediment

Approximately 0.5 g of each air-dried and homogenized sediment sample was digested using a Milestone ETHOS Easy Microwave Digestion System (Milestone Srl, Italy) equipped with high- pressure Teflon vessels designed for trace metal analysis. The digestion procedure followed the guidelines of U.S. EPA Method 3052, which describes microwave-assisted acid digestion for the total decomposition of siliceous and organically based matrices (U.S. Environmental Protection Agency [32]. Each sample was treated with a mixture of concentrated nitric acid (HNO?, 65%) and hydrofluoric acid (HF, 40%) in a volumetric ratio of approximately 9:3 mL to achieve complete dissolution of silicate and aluminosilicate phases commonly present in sediment matrices.

Digestion was performed under controlled temperature and pressure conditions, with the temperature gradually increased to 200 °C and held for a specific duration as recommended by the manufacturer’s operational protocol for siliceous materials [33]. Upon completion, the vessels were allowed to cool to room temperature before being carefully opened in a fume hood. The resulting clear digests were quantitatively transferred into 50 mL volumetric flasks, treated with 3 mL of boric acid to complex residual fluoride ions and prevent potential interferences, and diluted to volume with deionized water.

The digested solutions were filtered through acid-washed Whatman No. 42 filter paper and stored in pre-cleaned polyethylene bottles prior to elemental determination using the Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES).

Metals Analysis from Seagrasses

Approximately 0.5 g of each dried and ground tissue sample was digested with 7 mL of concentrated nitric acid (65 %) and 1 mL of hydrogen peroxide (30%) in Teflon vessels of an ETHOS Easy Microwave Digestion System (Milestone, Italy). The digestion was performed in high-pressure Teflon vessels designed for trace metal analysis and as per manufactures guidelines. The digestion process involved a gradual temperature ramp to approximately 180 °C, maintained for a specified holding period to achieve complete decomposition of the organic matter. Real-time monitoring of temperature and pressure was conducted using an in-built infrared sensor and pressure control system to ensure safety and reproducibility. After the digestion cycle, the vessels were allowed to cool to room temperature before being carefully opened in a fume hood. The resulting clear digests were quantitatively transferred into 50 mL volumetric flasks and diluted to volume with deionized water. The digested samples were subsequently filtered through acid- washed Whatman No. 42 filter paper and stored in pre-cleaned polyethylene bottles prior to elemental determination by Agilent 5900 Inductively Coupled Plasma–Optical Emission Spectrometry (ICP-OES) [34].

Procedure of Heavy Metal Analysis from the Eight Fish

Fish samples were first allowed to equilibrate to room temperature and then thoroughly rinsed with deionized water to remove adhering salts, debris, and surface contaminants prior to dissection. Dissection was performed using acid-washed stainless-steel scalpels and forceps to prevent trace metal contamination. Approximately 20 g of dorsal muscle tissue was excised from each specimen, rinsed with distilled water, and freeze-dried at −80 °C until a constant weight was achieved. The dried muscle samples were then finely ground and homogenized using a mixer mill to obtain a uniform powder, which was stored in pre-labeled polyethylene containers prior to digestion.

For digestion, accurately weighed 1.0 g aliquots of each powdered muscle sample were transferred into acid-cleaned Teflon digestion vessels and treated with 10 mL of concentrated nitric acid (HNO?, 65%, supra pure grade; Adolf Plinke GmbH). The mixture was left to pre- digest at room temperature for approximately 3 hours before sealing and heating at 80 °C for 7 hours to enhance organic matter decomposition. Subsequently, the vessels were opened and heated at 100 °C for an additional 3 hours to evaporate excess acid until a nearly dry residue remained. This digestion process was repeated until only a minimal white residue was observed. The residue was reconstituted in 80 mL of 2% nitric acid, filtered through Advantec 5C filter paper (110 mm diameter; Advantec MFS, Dublin, CA, USA), and finally diluted to a volume of 100 mL with 2% nitric acid.

Metal concentrations were determined using an Agilent 5900 Inductively Coupled Plasma– Optical Emission Spectrometer (ICP-OES). All results were expressed on a dry weight basis and reported in milligrams per kilogram (mg/kg). Method blanks were processed concurrently following the same procedure to correct for background contamination and ensure analytical quality control. The digestion and analytical protocols followed established guidelines for heavy metal analysis in marine biota [27,30,31].

Potential Risk Assessment using Estimated Daily Intake (EDI), Hazard Index and Comprehensive Pollution Index Methods

Health Risk Assessment

Estimated Daily Intake (EDI) used to assess metal intake in humans, and it is a common method in environmental health and toxicology for evaluating potential exposure to heavy metals or trace elements through food, water, or other sources. It is defined as amount of a substance (like a metal) ingested daily per unit of body weight, typically expressed as:

EDI (mg/kg body weight/day) = (Concentration of metal in food × Daily intake of food) / Body weight. The EDI obtained shall be compared with world statutory health organization values. If it exceeds benchmarks set by World Health Statutory Authorities like the WHO, FAO, USEPA, etc., values, it indicate a potential health risk [31,35,36].

HI: Hazard Index

HI= the Hazard Index which is used to evaluate non-carcinogenic risk from consuming contaminated food (like fish with heavy metals). It is found by summing Hazard Quotient (HQ) for every metallic species, and it is given by; HQ = EDI/RfD, where RFD is Reference Dose (safe daily exposure limit set by USEPA/WHO). HI is the cumulative risk of the multiple contaminants. If it is less than one then it is safe while if it is greater than one then combined metal exposure could pose non-carcinogenic health risk. If also HQ is greater than one, then metal exposure could pose non-carcinogenic health risk. ([31,37,38].

Comprehensive Pollution index (CPI) or Mean Pollution Index (MPI)

To determine pollution index (PI) of the elements, statistical analysis was performed on the elemental concentrations in fish samples. The PI is defined as the ratio of element x concentration in the sample to the element’s maximum allowable level, that is, PI(x) = Metal concentration in the sample/Permissible limit or background value, since we have several metals, the overall or comprehensive pollution index is calculated as CPI

This gives the average pollution level across all analyzed metals. A PI value greater than 1.0 indicates that the concentration of a given metal exceeds its permissible threshold, suggesting potential contamination and ecological risk. Similarly, higher CPI values reflect greater cumulative pollution loads across multiple elements. This approach to pollution index evaluation has been widely adopted in aquatic environmental monitoring and risk assessment studies [28,39,40].

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