Spatial environmental variability shapes the diversity of bottom ecosystems on the western Gulf of Guinea shelf

The western Gulf of Guinea is characterised by seasonal upwellings and hosts a large diversity of marine species. Although the assemblages of bottom fishes and benthic organisms are periodically assessed, there has been little research investigating these in tandem. We investigated the shelf bottom assemblages along Côte d’Ivoire and Ghana using data from a 2019 scientific research survey. Standardised species abundances were significantly associated with various environmental bottom conditions recorded on nearby CTD stations. We then used a clustering framework for identifying main assemblages, and compared them with site ordinations fitted with environmental parameters through detrended correspondence analysis. Three groups were determined, structured primarily by depth and distance to shore along an easting gradient, highlighting faunal discontinuities related to different hydrographical conditions and bottom types. These were a shallow coastal assemblage, a deeper sub-thermocline assemblage, and an intermediate assemblage of soft and rocky environments. The steep slope along Côte d’Ivoire was associated with a clear separation between the shallow coastal and deeper assemblages. Species diversity increased longitudinally to the east, commensurate with the broadening and gentler slope of the shelf along Ghana, which was associated with habitats that are more heterogenous. Further study is needed to understand the variability and sensitivity of these ecosystems in relation to upwelling seasonality and environmental conditions, especially as the region experiences intense unmonitored fisheries and warming-induced changes in water stratification.