KOFFI Épouse COULIBALY Chiayé Larissa^1,, PAN Pekpenin Benjamin Patrick¹, Ouattara Ismaël Ben², MONDE Sylvain²

The Ébrié Lagoon, in the Grand-Bassam area, is exposed to various anthropogenic pressures that may alter is morphological and environnemental charactéristics. This study aims to characterize the morphobathymetric and hydrodynamic structures of this lagoon sector and to assess its physicochemical status. Bathymetric surveys provided 18,474 depth measurements, ranging from 0 to 19 m, over an area of approximately 42 ha. The results reveal two contrasting morphological units : a shallow southern channel characterized by relatively regular bottom topography, and a deeper northern basin reaching 18 to 19 m, with steep slopes and U-shaped and V-shaped cross-sectional profiles. These bathymetric features correspond to areas affected by dredging activities, as observed in the field and reported in previous studies. The distribution of surface currents shows heterogeneous flow directions and a convergence of water masses toward several steep-slope zones, with no dominant outflow toward the southern channel during the survey period. Physicochemical parameters were measured in situ at 33 sampling stations. Temperature, salinity, conductivity, and pH exhibited relatively low spatial variability, with mean values of 29.94 ± 0.30 °C, 1.75 ± 0.14‰, 29.30 ± 2.31 µS/cm, and 7.68 ± 0.10, respectively. In contrast, dissolved oxygen concentrations were very low, ranging from 0.10 to 0.40 mg/L, with a mean value of 0.23 ± 0.07 mg/L, indicating severe hypoxic conditions during the survey. The spatial association between the deep depression, surface circulation patterns, and low dissolved oxygen concentrations suggests that bottom morphological modifications may contribute to reduced water renewal and promote oxygen depletion. However, since physicochemical measurements were collected during a single survey and at a depth of 0.5 m, they do not allow assessment of the persistence or seasonal variability of hypoxia. These findings highlight the importance of considering bottom morphology and hydrodynamic connectivity in the management of lagoon sectors affected by dredging activities.

Ébrié Lagoon, morphobathymetry, hydrodynamics, dredging, dissolved oxygen, hypoxia, tropical lagoon