Tidal Impacts on Zooplankton Dynamics in a Major Ocean-Lagoon Channel: Insights from a 25-Hour Intensive Survey in the Cotonou Channel, Benin

This study investigates the effects of tidal cycles on the zooplankton community within the Cotonou Channel, an important waterway connecting the large Nokoué Lagoon to the Atlantic Ocean in Benin. From the determination of zooplankton composition from 25-hour samples collected in July 2020, alpha diversity indices and abundance were assessed, while relationships between biotic and abiotic parameters were analyzed through Pearson correlation, analysis of variance, and principal component analysis. A total of 66 zooplankton taxa were identified, with rotifers exhibiting the highest species richness (35 taxa), while copepods dominated in abundance (71%). Zooplankton abundance varied significantly, ranging from 2 to 95 ind L−1 depending on the tidal phase. A negative correlation was found between species richness (r = −0.51, p < 0.01) and increasing salinity (3–37), indicating that higher salinity reduced diversity (r = 0.06, p > 0.05). Resilient species like Synchaeta bicornis persisted despite salinity changes. The tidal cycle structurally altered the zooplankton community, with abundance and diversity peaking at different phases, notably higher at high tide (15 ind L−1.) These initial findings underscore the complex interactions between tidal dynamics and estuarine biodiversity, suggesting the need for further research across different tidal and seasonal conditions to inform effective management and conservation efforts.