Résumé
Phytoplankton is a reliable bioindicator for assessing water quality in aquatic environments. Research on phytoplankton in African coastal ecosystems, remains limited especially concerning toxic phytoplankton species and the accumulation of their associated toxins in mollusks. The Moulay Bousselham Lagoon is one of Morocco’s most significant wetlands and ecologically protected areas, designated under the Ramsar convention. Its strategic location, connected to the African Atlantic Ocean, supports a rich and diverse assemblage of flora and fauna. A phytoplankton survey conducted from March 2015 to March 2016 at five sampling sites within the lagoon revealed a dominance of diatoms, followed by dinoflagellates. Among the 49 taxa identified, 47% of dinoflagellates and 7% of diatoms were potentially harmful. Dinoflagellates were most abundant during the summer months (June and July 2015), particularly at the lagoon’s mouth (the Passe), with species of the genus Scrippsiella being especially abundant. Their presence declined progressively from downstream (the Passe) to upstream areas (Lâacha). In contrast, diatoms dominated in winter and spring and included Chaetoceros, Coscinodiscus, Rhizosolenia, Pseudo-nitzschia, and Nitzschia. During summer and autumn, dinoflagellates such as Ceratium, Scrippsiella and Prorocentrum became more abundant. It seems that the observed succession between diatoms and dinoflagellates may be influenced by seasonal changes in physicochemical parameters and freshwater inputs considering the differences in downstream and upstream stations. Throughout the study period, analysis of marine biotoxins in shellfish collected from the lagoon revealed no contamination by amnesic or paralytic shellfish toxins. However, lipophilic shellfish toxins were detected on a single occasion in farmed mussels at the Palmier site. The seasonal dynamics and spatial distribution of potentially harmful phytoplankton in the Moulay Bousselham Lagoon highlight the need for integrated coastal management strategies in this lagoon, particularly in monitoring toxic species and mitigating risks related to shellfish contamination.
•Seasonal dominance of diatoms and harmful dinoflagellates was observed across a key Moroccan coastal lagoon.•Nearly half of the dinoflagellate species identified were potentially toxic, peaking during summer at the lagoon’s inlet.•Results underscore the urgent need for continuous phytoplankton and shellfish toxin monitoring in African wetlands.