Trace metals in the giant tiger prawn Penaeus monodon and mangrove sediments of the Tanzania coast: Is there a risk to marine fauna and public health?

Reviewing the interdecadal dynamics of micropollutants in the Tanzanian coastal zone from 2002 to 2022

The Tanzanian coast is a vastly diversified ecosystem offering ecological, cultural, and economic services. However, anthropogenic and environmental stressors threaten its productivity and sustainability, prompting extensive research to understand the contamination extent, sources, and impacts. This review covers 77 original field research studies published between 2002 and 2022, focusing on the Tanzanian coastal area. A summary of information on the occurrences, levels, distributions, sources, and impacts of organic and inorganic micropollutants in environmental and biological compartments is provided. The studies were further discussed based on their research focus, where seven key areas were identified, which included the types of micropollutants analyzed, the parameters sampled, the locations investigated, and the crucial conclusions reached. The aim was to evaluate the research trends and identify gaps for future investigations. The studies reveal varying contamination levels, from low to severe, encompassing metals, persistent organic pollutants, pesticide residues, nutrients, and microbial contaminants. However, data gaps exist for micropollutant concentrations in ambient air, microplastics, pharmaceuticals, and emerging contaminants of concern. With increasing urbanization and socio-economic activities, the review emphasizes the necessity for solution-based research to remove and control micropollutants in the area. Addressing these challenges is crucial for sustaining the diverse ecological nature of this vital coastal system.

Trophic transfer of heavy metals through aquatic food web in the largest mangrove reserve of China

Understanding the mechanism of trophic transfer of heavy metal through the aquatic food web is critical to ecological exposure risk assessments in mangrove ecosystems. Zhanjiang Mangrove National Nature Reserve (ZMNNR) is the largest and biologically richest mangrove reserve in China, but has been exposed to heavy metal pollutants caused by the progressive industrialization and urbanization. We collected a variety of aquatic consumers, and primary producers, as well as sediments from the ZMNNR and analyzed them for heavy metal (Cd, Cr, Cu, Ni, Pb and Zn) concentrations, and for both δ13C and δ15N values to establish the trophic levels. The trophic magnification factors (TMF) of Cd, Cu and Zn are 0.19 (p < 0.01), 0.07 (p < 0.01) and 0.33 (p < 0.05), respectively, indicating significant biodilution in a simplified food web composed of bivalves, crustaceans and fish. There are also potential tendencies of biodilution for Cr, Ni and Pb. Comparison of heavy metals in representative fish and shrimp in the ZMNNR with those in worldwide mangroves indicate a low risk level for aquatic consumers in our ecosystem. Quantitative source tracking is conducted based on principal component analysis and cluster analysis, which indicate that Cr, Ni and Pb are mainly originated from natural geological processes, Cu and Zn from shrimp farming and agriculture activities, and Cd from the deposition of aerosol released by regional metal smelting industry.

Geospatial modeling and ecological and human health risk assessments of heavy metals in contaminated mangrove soils

Heavy metal-contaminated wastes can threaten mangrove forests, one of the most biodiverse ecosystems in the world. The study evaluated the geospatial distribution of heavy metals concentrations in soils, the ecological and human health risks, and metal contents in soil fractions and mangrove organisms in the Botafogo estuary, Brazil, one of the most environmentally impacted estuaries in the country. The metal concentrations exceeded by up to 2.6-fold the geochemical background; 91%, 59%, 64%, 31%, and 82% of the soils were contaminated with Cr, Zn, Pb, Cu, and Ni, respectively. Adverse effects to the biota may occur due to Cr, Cu, Ni and Pb exposures. Contents of clay and organic matter were the main factors governing the distribution of metals in soil, contributing to up to 63% of the total variability. However, the geospatial modeling showed that the predictive ability of these variables varied spatially with the metal and location. The ecological and human health risks assessments indicated that the metal concentrations in soils are safe for the environment and human beings. There was a low transfer of metals from the soil to the biota, with values of sediment-biota accumulation factor (SBAF) and biological accumulation coefficients (BAC) lower than 1.0, except for Zn (SBAF = 13.1). The high Zn bioaccumulation by Crassostrea rhizophorae may be associated with the concentrations of Zn in the bioavailable fractions.

Distribution and risk of mercury in the sediments of mangroves along South China Coast

The importance of mangrove was widely reported. However, the potential risks of pollutants (e.g., Hg) accumulated in the mangroves are often ignored. Thus, the present study aimed to explore the distribution and risk of mercury (Hg) in the sediments of mangroves along South China Coast. Results showed that concentrations of total Hg ranged from 0.0815 to 0.6377 mg/kg, with an arithmetic mean value of 0.2503 mg/kg. The contamination index (P_i) showed mild pollution toxicity risks in NS, slight toxicity risks in DZG, QZ, SY, ND, GQ, TLG, and free pollutions in BMW, SJ, ZJK and BLHK. NS, DZG and SY scored the highest values of I_geo among the eleven mangrove regions studied, indicating moderate to heavy pollution inputs in these regions. As for the distribution of Hg in the sediments along tidal gradient, concentrations of Hg in the sediments sharply increased from seaward mudflat to landward mangrove, corresponding with the increases of TOC. In summary, the present data indicated that mangrove ecosystem is efficient in Hg reservoir. However, the potential ecological risks of Hg, especially in some mangrove regions easily affected by human activities, should be noted.

The distribution and risk of mercury in Shenzhen mangroves, representative urban mangroves affected by human activities in China

Sediments were collected from urban mangroves in Shenzhen, China, including Shajing (SJ) and Xixiang (XX) featured with industrial district; Futian (FT) and Baguang (BG) featured with central business district and ecological preserve, respectively. Distributions, pollution levels and human health risks of Hg were explored. In both surface and vertical sediments, mean Hg concentration was highest in SJ mangrove, which may be due to its proximity to point-source discharge of Maozhou River in Pearl River Estuary, China. Sediment properties (pH, salinity, TOC and particle size) had limited impact on Hg accumulation due to their non-significant correlations. Geo-accumulation index, pollution load index, potential ecological risk index, and the ecological risk assessment code showed the highest ecological risk in SJ mangrove, followed by XX, FT, and BG mangroves. The assessment of human health risks showed that public experience little adverse health risk due to exposure to Hg polluted sediment in urban mangroves.

A review on mercury biogeochemistry in mangrove sediments: Hotspots of methylmercury production?

Wetlands are highly productive and biologically diverse environments that provide numerous ecosystem services, but can also be sources of methylmercury (MeHg) production and export. Mangrove wetlands contribute up to 15% of the coastal sediment carbon storage and ~10% of the particulate terrestrial carbon exported to the ocean. Thus, mercury (Hg) methylation in mangrove sediments and subsequent MeHg output to adjacent waters could have a great impact on global Hg cycling. In this review, we provide a comprehensive analysis of the literature on worldwide Hg concentrations in mangrove ecosystems, and the results reveal that a large range of total Hg (THg) and MeHg concentrations is detected in mangrove systems. Then, we discuss the potential roles of organic matter (OM) in controlling the Hg biogeochemistry in mangrove sediments. The intense OM decomposition by anoxic reduction (e.g., sulfate reduction) drastically affects sediment chemistries, such as redox potential, pH, and sulfur speciation, all of which may have a great impact on MeHg production. While the outwelling of carbon from mangroves has been extensively examined, little is known about their roles in exporting MeHg to adjacent waters. Our understanding of Hg biogeochemical processes in mangrove systems is constrained by the limited MeHg data and a lack of in-depth studies on the Hg methylation potential in this ecologically important environment. More efforts are needed to gain better insights into the contributions mangrove wetlands to the global Hg cycle.

Occurrence and contamination of heavy metals in urban mangroves: A case study in Shenzhen, China

Urban mangroves are affected by surrounding anthropogenic activities. Heavy metals in urban mangroves lack systematic evaluation, and their relationships with surrounding anthropogenic activities have not been explored with quantified data. In this study, urban mangroves with different urban functional zonings were selected in Shenzhen, South China, including: Shajing mangrove (SJM) and Xixiang mangrove (XXM) featured with industry district, Futian mangrove (FTM) and Baguang mangrove (BGM) featured with central business district and ecological preserve, respectively. The distribution, ecological risk, and pollution sources of heavy metals were determined. Heavy metals' relationships with surrounding functional zonings were also explored. The levels of Cu, Zn, Pb, Ni and Cr in SJM were significantly higher than the other mangrove stands, which may be related to its proximity to point-source discharges of Maozhou River in Pearl River Estuary, China. Potential ecological risk index, pollution load index, mean PEL quotient, mean effect range median quotient, and total toxic unit showed the highest ecological risk in SJM, followed by XXM, FTM, and BGM. With the order of BGM - FTM - XXM - SJM, Zn, Ni, and Cr presented dominantly in the labile fraction, demonstrating increased anthropogenic impacts. Heavy metals in FTM and BGM were mainly from anthropogenic and lithogenic originations, with only anthropogenic origination of heavy metals to be detected in SJM and XXM. Heavy metal pollution was highest in SJM featured with industry district, and metal products industry in surrounding district should attract special attention due to its affinity for heavy metal accumulation in urban mangroves.

Application of enrichment factor, geoaccumulation index, and ecological risk index in assessing the elemental pollution status of surface sediments

Sediment can accumulate trace elements in the environment. This study profiled the magnitude of As, Ba, Cd, Co, Cu, Cr, Ni, Pb, Se, and Zn pollution in surface sediments of the west coast of Peninsular Malaysia. Trace elements were digested using aqua regia and were analyzed using the inductively coupled plasma-mass spectrometry. The extent of elemental pollution was evaluated using with the enrichment factor (EF) and geoaccumulation index (I_geo). This study found that the elemental distribution in the sediment in descending order was Zn > Ba > Cr > Pb > Cu > As > Ni > Co > Se > Cd. Zn concentrations in all samples were below the interim sediment quality guideline (ISQG) (124 mg/kg). In contrast, Cd concentrations (2.34 ± 0.01 mg/kg) at Station 31 (Merlimau) exceeded the ISQG (0.70 mg/kg), and the concentrations of As in the samples from Station 9 (Tanjung Dawai) exceeded the probable effect level (41.60 mg/kg). The I_geo and EF revealed that Station 9 and Station 31 were extremely enriched with Se and Cd, respectively. All stations posed low ecological risk, except Station 31, which had moderate ecological risk. The outputs from this study are expected to provide the background levels of pollutants and help develop regional sediment quality guideline values. This study is also important in aiding relevant authorities to set priorities for resources management and policy implementation.

Effects of mangrove plant species on accumulation of heavy metals in sediment in a heavily polluted mangrove swamp in Pearl River Estuary, China

The present study compared accumulation of heavy metals in a mangrove swamp dominated by Kandelia obovata with that by Sonneratia apetala in Pearl River Estuary, China. The results showed that the concentrations of heavy metals at all sediment depths in the S. apetala site were significantly higher than that in K. obovata. The geo-accumulation index and potential ecological risk index also showed that S. apetala sediment had a higher contamination of heavy metals, especially Cd. S. apetala significantly altered the biogeochemical cycles of Cd, lead (Pb), nickel (Ni) and chromium (Cr). In S. apetala sediment, TOC played an important role in sequestering heavy metals as reflected by its positive correlations with Zn and Pb. This study demonstrated the importance of plant species in altering soil quality and heavy metal accumulation, and S. apetala is more efficiently working as a pollution barrier than K. obovata.

Ingestion of inorganic mercury by juvenile black tiger prawns (Penaeus monodon) alters biochemical markers

There is a lack of information regarding the effects on biochemical markers in invertebrates diet-exposed to inorganic mercury. In the present study, juvenile black tiger prawn (Penaeus monodon) were fed with food dosed with mercuric chloride (low: 0.2 mg kg^-1; medium: 0.77 mg kg^-1; high: 1.41 mg kg^-1; higher: 2.52 mg kg^-1) and control (0.03 mg kg^-1) over 12 days. At the end of exposure periods, ventral nerve cord, compound eyes and muscle were dissected for biochemical marker analyses (acetylcholinesterase - AChE; lipoperoxidation - LPO; glutathione S-transferase - GST; catalase - CAT). Prawn muscle showed an increase in total mercury concentration over time for low and high treatments, but did not show an accumulation in comparison to controls. AChE activity tended to decrease over time in all tissues. CAT activity increased over time in controls and lower dose treatments but was suppressed in the higher treatment relative to controls on day 12; indicating that inorganic mercury is impacting the normal stress response by reducing the capacity to degrade hydrogen peroxide. In contrast, no effect was observed in LPO and GST activity. A depuration treatment was performed and compared to medium treatment; only AChE and GST activities from muscle showed significant difference, with AChE activity from depuration treatment lower than medium treatment, while the opposite was observed in GST activity. Overall, the low mercury accumulation observed over 12 days of exposure may have limited the biochemical stress responses, which could also have limited the detection of differences in the depuration treatment relative to medium treatment.

Human health risks of Hg, As, Mn, and Cr through consumption of fish, Ticto barb (Puntius ticto) from a tropical river, Bangladesh

Metals tend to accumulate in higher organisms, e.g., fish and human through biomagnification effects in food chain. So, their presence in any component of the environment has become a global ecosystem and health concern. Here, we measured four health concerned metals like As, Cr, Mn, and Hg via inductively coupled plasma-mass spectrometry (ICP-MS) and analyzed applying some chemometrics for the assessment of human health risk through consumption of Puntius ticto, a very commonly consumed small fish in Bangladesh. The average concentration (wet weight) of metals was in the following descending order: Hg (0.006 mg/kg) > Cr (0.004 mg/kg) > Mn (0.003 mg/kg) > As (0.002 mg/kg). Hg level exceeded the provisional tolerable weekly intake (PTWI), and all other metals were within the permissible limit. The estimated daily intake (EDI) index of heavy metals showed that all the concentration levels were under the recommended daily intake (RDA) except Hg. Increased level of Hg is of particular concern to human health due to its biomagnification nature and can cause several neurological and physiological disorders including kidney failure. The total target hazard quotients (TTHQs) and carcinogenic risk (CR) matrices revealed that the intakes of Hg and As through fish consumption were higher than the recommended values, indicating consumers' remain non-carcinogenic and carcinogenic (THQ > 1; CR > 10^-5) health risks for lifetime consumption. Multivariate analyses (cluster and principal component) explained the sources of heavy metals in the study area originating from both anthropogenic and geological origin.

Metals and arsenic in sediment and fish from Cau Hai lagoon in Vietnam: Ecological and human health risks

Concentrations of Al, As, Bi, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, U, V and Zn were quantified in surface sediments collected from 13 different sampling sites from Cau Hai lagoon in Central Vietnam, and in 8 wild and farmed-fishes involving both pelagic and benthic species. Multivariate analysis shows that these trace elements are mainly associated with lithogenic matter, and are most likely the result of alteration and erosion processes in the lagoon. Enrichment factors and geo-accumulation indices reveal substantial sediment enrichments for both As and Bi with respect to the mean composition in the upper continental crust. As is enriched in the edible portion of fish tissue with values up to 10 times higher than the allowed limits set up by Health Canada. Target hazard quotient and target carcinogenic risk for As were assessed through fish diet and were greater than 1 and 10^-4, respectively, indicating potential health risks for fish consumers in Cau Hai lagoon.

Bioaccumulation of Heavy Metals in Various Tissues of Some Fish Species and Green Tiger Shrimp (Penaeus semisulcatus) from İskenderun Bay, Turkey, and Risk Assessment for Human Health

In this study, concentrations of heavy metals were determined by ICP-MS in the edible tissues of common sole (Solea solea Linnaeus, 1758), whiting (Merlangius merlangus Linnaeus, 1758), silver sillago (Sillago sihama Forsskål, 1775) and muscle-exoskeleton of green tiger shrimp (Penaeus semisulcatus De Haan, 1844), from the seas of İskenderun Bay, Eastern Mediterranean, Turkey, in January-February, 2016. The lowest and highest mean concentrations of Mn, Cr, Cd, Ni, Hg, As, Pb, and Co in fish and shrimp' muscles were found, respectively, as follows: 0.166-0.382, 0.134-0.336, 0.005-0.008, 0.091-0.140, 0.026-0.228, 1.741-29.254, 0.087-0.110, and <0.0005-0.027 mg kg^-1; in the skin and exoskeleton, the values were found, respectively, as 0.103-15.819, 0.301-0.778, 0.007-0.026, 0.115-0.513, 0.021-0.243, 1.548-17.930, 0.148-0.295, and <0.0005-0.140 mg kg^-1. According to the results, mean concentrations of all metals in the muscles of fish, except for arsenic and chromium, were found to be below certain legal limit values, especially arsenic levels in shrimp that were found to be above all the legal limit values. Also, the hazard quotients (HQ) of individual heavy metals in fish and shrimp, except for As, revealed safe levels for human consumption. However, the HQ values of estimated inorganic As exceeded 1 in the muscles of shrimp (P. semisulcatus), which may constitute a risk to public health.

Genetic diversity and connectivity in the East African giant mud crab Scylla serrata: Implications for fisheries management

The giant mud crab Scylla serrata provides an important source of income and food to coastal communities in East Africa. However, increasing demand and exploitation due to the growing coastal population, export trade, and tourism industry are threatening the sustainability of the wild stock of this species. Because effective management requires a clear understanding of the connectivity among populations, this study was conducted to assess the genetic diversity and connectivity in the East African mangrove crab S. serrata. A section of 535 base pairs of the cytochrome oxidase subunit I (COI) gene and eight microsatellite loci were analysed from 230 tissue samples of giant mud crabs collected from Kenya, Tanzania, Mozambique, Madagascar, and South Africa. Microsatellite genetic diversity (H_e) ranged between 0.56 and 0.6. The COI sequences showed 57 different haplotypes associated with low nucleotide diversity (current nucleotide diversity = 0.29%). In addition, the current nucleotide diversity was lower than the historical nucleotide diversity, indicating overexploitation or historical bottlenecks in the recent history of the studied population. Considering that the coastal population is growing rapidly, East African countries should promote sustainable fishing practices and sustainable use of mangrove resources to protect mud crabs and other marine fauna from the increasing pressure of exploitation. While microsatellite loci did not show significant genetic differentiation (p > 0.05), COI sequences revealed significant genetic divergence between sites on the East coast of Madagascar (ECM) and sites on the West coast of Madagascar, mainland East Africa, as well as the Seychelles. Since East African countries agreed to achieve the Convention on Biological Diversity (CBD) target to protect over 10% of their marine areas by 2020, the observed pattern of connectivity and the measured genetic diversity can serve to provide useful information for designing networks of marine protected areas.

Bioaccumulation and public health implications of trace metals in edible tissues of the crustaceans Scylla serrata and Penaeus monodon from the Tanzanian coast

The coastal population in East Africa is growing rapidly but sewage treatment and recycling facilities in major cities and towns are poorly developed. Since estuarine mangroves are the main hotspots for pollutants, there is a potential for contaminants to accumulate in edible fauna and threaten public health. This study analysed trace metals in muscle tissues of the giant mud crabs (Scylla serrata) and the giant tiger prawns (Penaeus monodon) from the Tanzanian coast, in order to determine the extent of bioaccumulation and public health risks. A total of 180 samples of muscle tissues of S. serrata and 80 of P. monodon were collected from nine sites along the coast. Both species showed high levels of trace metals in the wet season and significant bioaccumulation of As, Cu and Zn. Due to their burrowing and feeding habits, mud crabs were more contaminated compared to tiger prawns sampled from the same sites. Apart from that, the measured levels of Cd, Cr and Pb did not exceed maximum limits for human consumption. Based on the current trend of fish consumption in Tanzania (7.7 kg/person/year), the measured elements (As, Cd, Co, Cu, Mn, Pb and Zn) are not likely to present health risks to shellfish consumers. Nevertheless, potential risks of As and Cu cannot be ruled out if the average per capita consumption is exceeded. This calls for strengthened waste management systems and pollution control measures.

Genetic diversity of the giant tiger prawn Penaeus monodon in relation to trace metal pollution at the Tanzanian coast

The genetic diversity of giant tiger prawns in relation to trace metals (TMs) pollution was analysed using 159 individuals from eight sites at the Tanzanian coast. The seven microsatellites analysed showed high degree of polymorphism (4-44 alleles). The measured genetic diversity (H_o=0.592±0.047) was comparable to that of populations in the Western Indian Ocean. Apart from that, correlation analysis revealed significant negative associations between genetic diversity and TMs pollution (p<0.05), supporting the genetic erosion hypothesis. Limited gene flow was indicated by a significant genetic differentiation (F_ST=0.059, p<0.05). The Mantel test rejected the isolation-by-distance hypothesis, but revealed that gene flow along the Tanzanian coast is limited by TMs pollution. This suggests that TMs affect larvae settlement and it may account for the measured deficiency of heterozygosity. This calls for strengthened pollution control measures in order to conserve this commercially important species.

Influence of introduced Sonneratia apetala on nutrients and heavy metals in intertidal sediments, South China

To investigate the influence of Sonneratia apetala on nutrients and heavy metals in intertidal sediments, sediment cores of S. apetala marsh and mudflat in Shenzhen Bay, China were analyzed. The results showed that S. apetala improved sediment nutrient properties due to increased total carbon (TC), total nitrogen (TN), and total sulfur (TS). The levels of heavy metals were higher in S. apetala site than in mudflat, including chromium (Cr), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As), cadmium (Cd), lead (Pb), and mercury (Hg). In S. apetala site, TC, TN, and TS were not positively correlated with Cr, Ni, As, Cd, and Pb, indicating their less important roles in trapping heavy metals. There were positive correlations among Ni, Cu, Zn, and Cd in both sites, suggesting similar anthropogenic source. Levels of As were higher than the probable effect level at both sites, indicating their toxicological importance. The geo-accumulation index and potential ecological risk index revealed higher metal contaminations in S. apetala site, especially for Cd, Hg, and As. Multivariate analysis implied that S. apetala alter the biogeochemical cycle of Cd and Cr to a certain extent. These findings indicate that S. apetala may improve soil nutrient properties and facilitate heavy metal accumulation in intertidal sediments.