Phytoplankton Assemblages As Bioindicators of Pollution Stress in the Cotonou Lagoon, Benin

Anthropogenic pollution, particularly the discharge of untreated urban wastewater, increasingly threatens the ecological integrity of coastal lagoons. The study assessed the phytoplankton community of the Cotonou Lagoon (southern Benin) during the major dry season (December-March) where wastewater inputs are most pronounced, to infer water-quality status using phytoplankton bioindicators. Phytoplankton was sampled monthly at six stations using a 25 µm mesh plankton net from December to March at six stations using a 25 µm mesh plankton net, and identified by light microscopy based on standard taxonomic keys. A total of 63 phytoplankton species (50 genera, 8 classes and 5 phyla) were recorded, with diatoms (Bacillariophyceae, Coscinodiscophyceae and Mediophyceae) strongly dominating the assemblage and marine taxa prevailing, indicating marked Atlantic Ocean water intrusion. The widespread occurrence of taxa associated with organic pollution and eutrophic conditions (Anabaena, Aphanizomenon, Microcystis, Phormidium) suggests degraded water quality linked to nutrient enrichment of the lagoon waters. In addition, several potentially harmful taxa (e.g. Dinophysis caudata, Prorocentrum gracile, Microcystis viridis and Pseudo-nitzschia delicatissima) highlight a possible risk of bloom-related ecological impacts and public-health concerns through fisheries consumption. These findings support the need for improved wastewater management and sustained monitoring to guide restoration efforts.