Seasonal functioning of dissolved mercury dynamics in the Ebrié lagoon (Côte d'Ivoire)

Dissolved mercury (Hg) is the most mobile and reactive form of mercury in aquatic ecosystems, yet it is poorly documented in tropical lagoons. This study examines the seasonal dynamics of dissolved total Hg (THg) in the water column and sediment pore waters of the Ebrié lagoon (Côte d’Ivoire), a tropical estuarine system under intense anthropogenic pressure. Sampling occurred at eight stations during the dry (February 2017) and rainy (July 2017) seasons. Total dissolved Hg was measured using thermal decomposition atomic absorption spectrometry and analyzed in relation to physicochemical parameters, microbial activity, and Hg concentrations in Tilapia sp. Dissolved Hg in pore waters (0.07–0.33 µg.L⁻¹) was consistently higher than in the water column (0.03–0.11 µg.L⁻¹), confirming sediments as a labile Hg reservoir. Seasonal differences were limited, whereas spatial variability reflected local anthropogenic inputs and hydrodynamic conditions. Correlations showed distinct seasonal controls: during the rainy season, dissolved organic carbon and oxygen availability were closely linked to Hg solubilization in pore waters; in the dry season, microbial activity, salinity, temperature, and redox conditions more strongly influenced water-column Hg. No direct relationship existed between pore-water and water-column Hg, indicating sediment–water transfer is not systematic. Compared with other lagoonal systems, Ebrié lagoon’s dissolved Hg falls within the upper global range. These results highlight dissolved Hg’s critical role in Hg mobility and bioavailability and underscore the need to integrate dissolved Hg measurements into ecological and human health risk assessments in tropical coastal environments.