Heavy metals occur naturally in the environment. However, human activities such as industry, agriculture, and mining are responsible for a considerable increase in their concentrations in ecosystems, including waterways and marine environments. As a result, aquatic organisms are directly exposed to these metals, either through water or food [1]. Fish and crustaceans, particularly shrimp, can accumulate these substances to potentially toxic levels. For the general population, the consumption of these aquatic products is a major route of exposure to heavy metals throughout life. The adverse health effects of consuming contaminated food or water vary depending on the dose, duration of exposure, and age of the individual. Mercury (Hg), lead (Pb), cadmium (Cd), and arsenic (As) are among the most toxic metals and are highly bioaccumulative in aquatic organisms and the food chain. In some ecosystems, the presence of these pollutants can cause the disappearance of animal and plant species, leading to the dysfunction of the food chain [2]. The bioaccumulation of heavy metals within the food chain is indeed a threat that can lead to health disasters. Certain organisms, such as shrimp and fish, are powerful bioaccumulators, capable of bioconcentrating these molecules [3]. In humans, accumulation through food is associated with serious diseases and severe physiological disorders, which can even lead to death [4][5]. Work carried out by [6] in Asia revealed very high contamination with heavy metals in aquatic fauna in rural and urban rivers in the Pearl River Delta. In Africa, several studies have also highlighted the bioaccumulation of TMMs in various species of edible fish [7][8]. In Cameroon, research on heavy metal contamination of coastal and littoral ecosystems remains limited. To date, only a few data are available on the study of contamination of aquatic organisms in coastal ecosystems [9][10]. These preliminary studies have already revealed the accumulation of heavy metals in aquatic gastropods and the presence of cadmium in Macrobranchium macrobranchium shrimp. In order to contribute to a better understanding of this issue in Cameroon, the main objective of this work is to assess heavy metal contamination in fish and shrimp from the Wouri, Dibamda, Nkam, Kribi, and Youpwé rivers. Specifically, this study aims to: determine the concentrations of heavy metals in water, sediments, and two species of fish and four species of shrimp; assess the degree of contamination of water, fish, shrimp, and sediments; determine the level of accumulation of heavy metals in fish and shrimp.

The contamination factor (CF), used to determine the degree of environmental contamination by a trace metal element, was calculated using the following

formula: C F = metal content in wet tissue in mg / kg metal content in water in mg / L [11].

The bioconcentration factor (BCF) for each metal was calculated using the following formula:

where BCF is the bioaccumulation factor, C x and C y are the concentrations of heavy metals [9].

The geo-accumulation index (Igéo) was calculated using the formula:

where: C n : concentration measured in the sediment for the element; B n : geochemical background noise for element n; 1.5: constant taking into account natural fluctuations in the content of a given substance in the environment as well as anthropogenic influences [12][13].

Figure 2 shows the variations in the values of the various trace metals (lead, cadmium, nickel, zinc, and mercury) measured in the water during this study. Although occasional peaks were recorded, the concentrations measured are generally higher than the environmental quality standards for surface water (NEQS) applicable to watercourses for all five stations examined (Table 1). It should be noted that zinc levels are below the recommended standards at the Dibamba and Nkam stations. Similarly, mercury concentrations are below the regulatory thresholds at the Kribi and Youpwé stations. Table 1 presents the descriptive statistics for the heavy metals measured. The waters of the Wouri River show the highest average concentrations of lead (86.5 mg/L), cadmium (251.3 mg/L), and nickel (35.3 mg/L). The highest average concentrations of zinc (5.325 mg/L) are observed at Youpwé, while the Nkam station has the highest average mercury concentration (7 mg/L). Analysis of variance (ANOVA) reveals no statistically significant differences (p > 0.05) between the concentrations of heavy metals measured in the waters of the different study locations. With the exception of zinc, all average concentrations of heavy metals exceed the World Health Organization (WHO) guidelines for good ecological status of surface waters. These high concentrations could have negative impacts on aquatic organisms. The significant concentrations of lead and cadmium observed in the Wouri and Dibamba rivers could be attributed to industrial activities in both watersheds. These activities include the manufacture of phosphate fertilizers, plant protection products, and chemicals (SOLEVO), the production of storage batteries (BOCOM and CAMI TOYOTA), the manufacture and coating of construction materials (COMETAL), and cement production. These levels remain higher than those obtained by [13] in the waters of Oued Bouhamra in Algeria.

Figure 2. Average variation in heavy metals values in water from different study areas.

Table 1. Descriptive statistics for heavy metals measured in water and sediments from different study areas.

Figure 3 illustrates the variations in heavy metals concentrations (lead, cadmium, nickel, zinc, and mercury) measured in sediment samples during this study.

Figure 3. Average variation in heavy metals values in sediments from different study areas.

Although there are peaks, the values remain below the environmental standards associated with sediments at the sites studied (Table 1). However, cadmium values exceed the recommended environmental standards for sediments. Sediments from the Wouri, Dibamba, and Nkam rivers have high cadmium values, while mercury values in sediments from all study sites exceed standards. The descriptive statistics for the metals measured in the sediments of the study areas are presented in Table 1. It appears that the sediments of Kribi have high concentrations of lead (10.39 mg/kg), those of Dibamba have high concentrations of cadmium (119.7 mg/kg) and nickel (0.6182 mg/kg), those in Youpwé have high concentrations of zinc (7.7 mg/kg), and those in Wouri have high concentrations of mercury (166.2 mg/kg). However, the ANOVA test shows no significant difference (p > 0.05) between the heavy metal values in the sediments of the study sites. The high heavy metal values in sediments could be explained, on the one hand, by natural inputs, and on the other hand, by industrial effluents or discharges, or indirectly after the leaching of waste deposits along watersheds by rainwater. Moreover, [13][14], in a similar study, show that the main source of heavy metal contamination in sediments is anthropogenic. The cadmium, lead, and nickel levels obtained for this study in sediments are all higher than those obtained in Ghana [15]. This result could be explained by differences in the geological composition of the areas studied [6], differences in the type and extent of anthropogenic pressures on the areas studied, and differences in industrial and household waste management in the two countries. The values obtained remain lower overall than those found in sediments from the Ebrié lagoon in Côte d'Ivoire [13]. Table 2 shows the average values of heavy metals in fish and shrimp from the different study areas in this work. The most abundant traces of heavy metals in the two fish species (Oreochromis niloticus and Chrysichthys nigrodigitatus) were lead and cadmium, particularly at the Wouri and Dibamba sites, with values exceeding the permitted limits. However, the values for nickel, zinc, and mercury remained below the limits overall. All shrimp species harvested in Kribi and Douala had mercury levels above the standard. However, in the city of Kribi, the species Penaeus monodon, Kerathus, and Penaeus notialis had high lead concentrations, while in the city of Douala, Penaeus notialis had levels above the standard.

Table 2. Average values of heavy metals measured in the various organisms collected.

Table 3. Concentration factors for heavy metals in water, sediments, fish, and shrimp in the study areas.

Table 3 shows the factors contributing to heavy metal contamination in fish and shrimp from the different study areas. However, in the Kribi area, there is contamination (CF > 1) of the shrimp species Penaeus notialis by lead and high contamination (CF > 6) of all shrimp species by mercury. In the Youpwé area, high mercury contamination was found in the shrimp species Penaeus monodon and Penaeus notialis. With the exception of lead and cadmium, all trace metal levels found in fish were below WHO guidelines. These high levels in fish could have negative impacts on the health of the population that consumes them.

Lead and cadmium contamination in fish could be explained by the fact that these elements accumulate in the food chain. These heavy metal levels remain higher overall than those found in certain fish in Côte d’Ivoire [16] and those found by [15] in marine fish from the Togolese coast at Agbodrafo (0.24 to 0.38 mg/kg). Lead levels in fish remain lower overall than those in bivalve species (2.9 mg/kg) obtained in the Togolese lagoon system by [17]. Our results could be explained by the fact that demersal and benthopelagic species are closer to sediments, which are reservoirs or sinks for many chemical pollutants [18][19]. Species that are in contact with sediments will be the most contaminated in accordance with their lifestyle, as they feed on benthic organisms and even mud.

Table 4. Bioconcentration of heavy metals in water, fish, and shrimp.

Table 4 shows the level of accumulation of heavy metals in water, fish, and crustaceans in the different study areas. It shows that the fish species Oreochromis niloticus bioaccumulates zinc in the Dibamba River, and the shrimp species Penaeus notialis bioaccumulates lead in Kribi (BCF > 1). The other heavy metals cadmium, nickel, and mercury are not bioaccumulated in any fish or shrimp species. The concentration factor revealed that fish and shrimp in the areas studied are contaminated with lead and cadmium. This contamination could be due to the bioaccumulation, biomagnification, and bioconcentration of trace metals along the food chain. With regard to bioaccumulation in organisms, it appears that the fish species Oreochromis niloticus in Dibamba bioaccumulates zinc, and the shrimp species Penaeus notialis bioaccumulates lead in Kribi.

The objective of this study was to assess the contamination by certain trace metals in the water, sediments, and two fish species (Chrysichthys nigrodigitatus and Oreochromis niloticus) of the Nkam, Wouri, and Dibamba rivers. The study found that the concentrations of trace metals in the water exceeded the WHO guidelines for the ecological status of surface water. The water at urban stations (Wouri and Dibamba) showed very high levels of cadmium, lead, and mercury contamination, which could be due to the presence of various industries. Sediments at all stations were extremely polluted with mercury, while only sediments at the Wouri and Dibamba stations were extremely polluted with cadmium.

This work was supported by the research modernization allowance grants for higher education lecturers offered by His Excellency, the President of the Republic of Cameroon.