Wet and dry seasons modulate coastal coccolithophore dynamics off South-western Nigeria (Gulf of Guinea)

Résumé

Coccolithophores are calcifying unicellular phytoplankton at the base of the marine food web, playing a key role in pelagic calcium carbonate production. While their sensitivity to environmental change is well established, their ecological importance in tropical coastal systems remains underexplored, particularly along the African coastline. Here, we present the first multi-seasonal assessment of living coccolithophore communities off Lagos, southwest Nigeria, in the Gulf of Guinea. Periodic sampling was conducted at three coastal stations from December 2018 to April 2021 to evaluate species composition, standing stocks, diversity, and ecological drivers. Coccolithophore abundances showed clear seasonal patterns, with significantly higher ( p < 0.05) standing stocks and diversity during the wet season. Total abundances ranged from 0.3×10 3 cells L −1 in the dry season to 5.5×10 3 cells L −1 in the wet season, with Gephyrocapsa oceanica dominating dry periods and Emiliania huxleyi prevailing during the wet season. Seasonal changes were linked to the migration of the Intertropical Convergence Zone (ITCZ), which modulates precipitation and current direction along the Gulf of Guinea. Interestingly, chlorophyll a concentrations appeared decoupled from coccolithophore abundance, suggesting other phytoplankton groups may drive primary productivity in this region. Despite regional differences in oceanographic settings, the observed standing stocks fall within the global range of coastal coccolithophore assemblages, supporting the hypothesis that these communities are shaped by a set of common ecological constraints. As tropical coastal regions already face multiple pressures from climate change, projected southward shifts of the ITCZ could alter precipitation regimes and current dynamics, with potential implications for coccolithophore community composition and coastal biogeochemical cycling.

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