The study was conducted in the Aby Lagoon, located in the southeast of Côte d’Ivoire, between latitudes 2˚40' and 3˚20' West and longitudes 5˚20' and 5˚40' North (Figure 1). The Aby Lagoon is the largest lagoon in the country, with an area of 1400 km² and an average depth of 3 m. The Aby Lagoon is fed by two main rivers, the Tanoé and the Bia, which drain a watershed of 19,000 km², covering forest, agricultural, and urban areas. The Aby Lagoon is bordered by a dense mangrove, dominated by the species Rhizophora racemosa and Avicennia germinans, and hosts a diversified fish fauna, including freshwater, brackish, and marine species.

Sediment samples were air-dried, sieved through a 2 millimeters sieve and homogenized. Macrophyte and fish samples were washed in distilled water, dried at 60˚C for 48 hours, and ground into a fine powder.

The sediment was digested using a mixture of nitric acid and hydrogen peroxide (HNO₃/H₂O₂): Approximately 0.25 g of dried and homogenized sediment was precisely weighed in a container. Subsequently, 8 mL of concentrated nitric acid was added to the container, which was hermetically sealed with a reflux column. A plastic gasket has been placed between the backflow column and the container to prevent evaporation. The tube containing the mixture followed the following protocol:

- A first phase of microwave heating at 20 W for 5 minutes.

- Chill for 5 minutes, followed by the addition of 2 ml of hydrogen peroxide (H₂O₂).

- A second phase of microwave heating at 20 W for 5 minutes.

Finally, the mineralized was diluted with Milli-Q water in a volumetric borosilicate glass flask with a capacity of 50 ± 0.06 ml. The resulting mixture was then stored in the refrigerator in a polyethylene tube for further analysis.

Student’s test was used to determine whether differences in selenium concentration (Se) between the matrices studied. The Student’s test is a statistical test used to compare the means of two independent samples. It is based on the null assumption that the two means are equal. If the test yields a p-value below the significance level (usually 0.05), then the null hypothesis is rejected and it is concluded that the two means are significantly different.

Analyses were performed using Perkin Elmer®’s Elan 6000 ICP-MS. The methodology used to optimize the instrument and analytical procedure was detailed by the US EPA (1998a) in Method 6020, specifically for selenium.

The formula for calculating selenium levels in the samples was expressed as follows:

Arsenic   content   ( mg / kg ) : Selenium   content Sample   weight × 1000 (1)

Selenium content in miligrams per kilogram (mg/kg).

Amount of Selenium measured during analysis (micrograms).

Fresh weight test sample mass (kilograms).

Table 1 presented descriptive statistics of selenium concentrations in sediments in the Aby lagoon system. The mean selenium concentration is 0.82 mg/kg with a standard error of 0.09. This indicates that the average is calculated with some uncertainty, which is quantified by the standard error. The 95% confidence interval for the mean is 0.64 to 0.99. This result implies that if the experiment were

repeated many times, 95% of the calculated confidence intervals would contain the true mean. The 5% truncated average is 0.77. This measure implies that the lowest 5% and the highest 5% were removed before the average was calculated. The median is 0.70, which is equivalent to saying that 50% of samples have a selenium concentration of less than 0.70 mg/kg. The standard deviation is 0.69, indicating the variability or dispersion of selenium concentrations around the mean. The 75th percentile is 1.29, which represents the fact that 75% of the samples have a selenium concentration of less than 1.29 mg/kg. The minimum selenium concentration is 0.02 mg/kg and the maximum concentration is 2.96 mg/kg. The interval is 2.94, which is the difference between the maximum and minimum value. The average of 0.82 mg/kg Se in the sediments of Aby Lagoon indicates moderate contamination compared to international threshold values. The high variability in concentrations (standard deviation of 0.69 mg/kg) highlights the influence of local factors such as sources of Se inputs, sediment properties, and biogeochemical processes. The median of 0.70 mg/kg shows that half of the samples have concentrations below this value, indicating an asymmetrical distribution of the data. Se concentrations in Aby Lagoon are comparable to those in other coastal lagoons in West Africa, such as Burullus Lagoon in Egypt [14] and a lagoon off the coast of Jeddah [15] . However, they are lower than those seen in areas heavily contaminated with selenium, such as the San Francisco Bay Area in the United States [16] [17] . The maximum concentration of Se in Aby Lagoon exceeds the EPA’s proposed threshold value of 2 mg/kg for the protection of aquatic life. This suggests that potential ecotoxicological risks exist for some sensitive organisms in certain areas of the lagoon [18] [19] .

Table 2 provided a description of selenium concentrations by site. The mean selenium concentration was lowest in Aby Lagoon north (0.66 mg/kg) and highest in Tendo Lagoon Bays (1.11 mg/kg). The average concentration in the Tanoe River was slightly higher in the rainy season (0.93 mg/kg) than in the dry season (1.13 mg/kg). The Tendo Lagoon channel had an intermediate average concentration (0.76 mg/kg). The standard deviation was highest in the Tendo Lagoon Bays (1.50 mg/kg), reflecting a high variability in selenium concentrations. The standard deviation was lower in the Tanoe River (0.42 mg/kg) and in the Aby Lagoon North (0.63 mg/kg). These results showed a more homogeneous distribution of selenium in these sites. The average selenium concentrations at the different sites in Aby Lagoon were comparable to those of other coastal lagoons in Africa [14] - [21] .

Figure 2 showed the box plots and the scatter plot of selenium concentrations in sediment at the different sites. In North Aby Lagoon, the median selenium concentration was relatively low and the variability of the data was low, as evidenced by the length of the boxplot whiskers. In addition, the individual values were distributed around the median. At the channel, the median selenium

concentration was slightly higher than that of Aby-Nord, but the variability remained similar, with values scattered around the median. As for the bays of Tendo Lagoon, this site had a higher median selenium concentration and the variability of the data was greater, with values spread over a wider range.

In the River Tanoe, the median selenium concentration was the highest among all sites, and the variability was also greater, with more dispersed individual values. Overall, these results highlighted the variations in selenium concentrations between the different sites of the Aby Lagoon North. The box plots showed the distribution of the data, while the scatter plot showed the individual values. The median selenium concentration was significantly higher in the Tanoe River than in the other sites.

The bays of Tendo Lagoon had a higher median concentration than the North Aby Lagoon and the channel. The variability of the data was greater in the Tanoé River and the bays of the Tendo Lagoon than in the North Aby Lagoon and the channel. Indeed, differences in median selenium concentration between sites may be due to several factors, such as sources of selenium inputs, physicochemical properties of sediments, and biogeochemical processes [22] [23] . The high variability of the data in the Tanoe River and the bays of the Tendo Lagoon may be due to the presence of point sources of contamination or to selenium remobilization processes in sediments [24] .

Table 3 has presented data on concentrations and extreme values for two sites, Tendo and Aby North, during the dry (DS) and rainy seasons (RS) during two sampling campaigns.

Table 5 shows median selenium concentrations in Aby lagoon sediments according to rainy season (RS) and dry season (DS). In Aby north Lagoon, the median selenium concentration during the rainy season is 0.39 mg/kg and during the dry season it is slightly higher at 0.50 mg/kg. In the Tanoé River, the median selenium concentration during the rainy season is 0.46 mg/kg and during

**Significant difference, ^§variance equal to the Student test.

the dry season it is significantly higher at 1.13 mg/kg. In the bays of Tendo Lagoon, the median selenium concentration during the rainy season is 0.81 mg/kg and during the dry season it is 1.21 mg/kg. Finally, in the Tendo Lagoon Channel, the median selenium concentration during the rainy season is 0.04 mg/kg and during the dry season it is significantly higher at 1.31 mg/kg. Selenium concentration is generally higher during the dry season than during the rainy season. The only significant difference is observed in the Tendo Lagoon channel, where selenium concentration is significantly higher during the dry season (p < 0.01).

For the other sites, the differences observed between seasons were not statistically significant. The results of this study suggest that the concentration of selenium in and around Aby Lagoon is generally low and does not pose a risk to human health. However, the concentration of selenium in the Tendo Lagoon channel is significantly higher during the dry season and could pose a health risk to local populations. The results indicate significant variations in selenium concentrations between the rainy and dry seasons, as well as differences between sampling sites. These results could be explained by seasonal effects on selenium availability, geographic and hydrological variations, biogeochemical processes, and interactions between seasons and sites.

In terms of seasonal effects on selenium availability, during the rainy season, precipitation could lead to selenium leaching into the sediment, which could lead to relatively lower concentrations in sediment samples. In contrast, during the dry season, with less precipitation, selenium may be less mobile and more likely to accumulate in sediments, resulting in higher concentrations [33] .

In addition, with respect to geographic and hydrological variations, observed differences between sites could be attributed to geographic variations in sediment composition, hydrological conditions, and human practices such as agriculture or industry [34] . For example, sites near agricultural or industrial areas may have higher selenium concentrations due to fertilizer use or surface water contamination.

On the other hand, with regard to biogeochemical processes, selenium concentrations in sediment could also be influenced by phenomena such as sorption, desorption and bioavailability of selenium in sediment [35] . These processes could vary depending on seasonal environmental conditions as well as the specific characteristics of the sediment and vegetation present at each site.

Finally, interactions between seasons and sites revealed significant differences between seasons and sites, suggesting complex interactions between environmental factors and human practices that influenced selenium dynamics in sediment [36] [37] .

Figure 3 showed the means and standard errors of selenium (Se) concentrations in sediments in the North Aby and Tendo lagoons. In the North Aby Lagoon, the average concentration of selenium in sediments is about 0.46 mg/kg. The rainy season samples (shown in orange) have a similar concentration to the

dry season samples (shown in blue). The intensity of selenium enrichment varies from no enrichment to very high enrichment. Sediment pollution is also moderate, ranging from no pollution to heavily polluted. The main pollutants are copper, cadmium and lead. In addition, at the Tendo Lagoon, the average concentration of selenium in the sediments is about 0.46 mg/kg. However, dry season samples have a significantly higher concentration, reaching approximately 1.32 mg/kg. The selenium enrichment intensity is similar to that of Ab north, ranging from no enrichment to very high enrichment. Sediment pollution is moderate, ranging from no pollution to heavily polluted. In sum, these results highlight the need to take into account seasonal and spatial variability as well as interactions between environmental factors for a thorough understanding of selenium dynamics in aquatic ecosystems.