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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. [ABSTRACT FROM AUTHOR]

This study, based on five years of monthly in situ data collected from 2018 to 2022, examines the seasonal and interannual fluctuations of suspended particulate matter (SPM) concentration in Nokoué Lagoon, Benin. Seasonally, SPM exhibits significant variations primarily influenced by changes in river discharge. During low-flow periods (December to May), SPM concentrations are relatively low (<15 mg L-1) throughout the lagoon. During this time, slight temporal variations are correlated with wind energy and likely associated with wind-induced resuspension of sediments. This is confirmed by slightly higher concentrations of SPM in the bottom layers compared to the surface. Resuspension appears to be lower in the west than in the east, likely due to the increased presence of acadjas (brush parks) in the west, reducing fetch and wind intensity, thus decreasing resuspension. At the onset of the river flood period (July–August), associated with the West African monsoon, increased river flow generates a significant turbid plume extending from the northeast of the lagoon to the Cotonou channel, connecting the lagoon to the Atlantic Ocean. SPM levels then increase considerably (>100 mg L−1), with a pronounced SPM gradient from the western to eastern regions of the lagoon. The less dense freshwater laden with sediment from the rivers flows over the denser saline water of the lagoon, leading to slightly higher SPM concentrations in the surface layers. Between September and November, SPM concentration gradually decreases as river flows reach their peak values. Thus, on a seasonal scale, the relationships between SPM and river discharge show a temporal lag, resulting in a clockwise hysteresis cycle. This is explained by the early mobilization of fine sediments during rising river flows, followed by reduced sediment availability and dilution effects as the flood peaks. On an interannual scale, SPM variations are relatively low with slight temporal shifts observed in the formation and expansion of the turbid plume and peak SPM levels. The total SPM mass in the lagoon ranges from 0.2 to 0.3 × 104 tonnes during low-flow periods to 20-30 × 104 tonnes at the onset of flooding. We also discuss uncertainties associated with SPM determination, estimated at approximately 5–15%. This study leverages a unique database in West Africa and provides valuable insights into the hydro-sedimentary dynamics of Nokoué Lagoon. • Nokoué Lagoon is characterized by strong seasonal variability in SPM. • River flows primarily drive the seasonal variability of SPM. • Wind influences SPM concentration during low-water periods. • A turbid plume of high SPM, with inorganic matter, extends in the eastern lagoon. • SPM in the western lagoon is organic matter-dominated and relatively weaker. [ABSTRACT FROM AUTHOR]

The study concerns the water level (WL) evolution in lagoons under the influence of tides and river fluxes. We derive new approximate analytical solutions of the Stigebrandt (1980) equations and apply them to the Nokoué Lagoon (Benin), a large tropical coastal lagoon fed by substantial river input. We show the solutions accurately predict the mean WL and tide amplitude. In particular the nonlinear combination of the spring-neap tidal cycle and river inflow gives rise to a strong fortnightly variation of the mean WL. The analytical solutions are used to explain this phenomenon. We also calculate the phase shift between the ocean and the lagoon tides and the asymmetry of the ebb and flood tide duration in the lagoon. The asymmetry first increases with the river flux but reduces above a critical flux before becoming symmetric again at river flood peaks. Finally, the analytical solutions are inverted to estimate the net river fluxes entering the Nokoué Lagoon, where no observations are available for rivers but for which we have high frequency observations of the lagoon water level for 2 years. The model is calibrated using few available flux observations in the channel connecting the lagoon to the ocean. Realistic river fluxes are estimated. We subsequently calculate the lagoon WL variations using the simplified model forced with the recalculated river fluxes and tidal forcing from an ocean tide model. The solution accurately represents the observations over the 2 year time period of the study. We conclude that this simple model is able to represent the complex interaction of tides and river fluxes, and its influence on the low frequency WL variations of coastal lagoons. • New analytical solutions for the water level evolution of lagoons. • Interaction of river and tide explains fortnightly variations of the mean level. • New method to estimate river fluxes from water level observations. [ABSTRACT FROM AUTHOR]

This study investigated the main water-level (WL) variability modes of Nokoué Lagoon in Benin (West-Africa). The average WL ranges between 1.3 and 2.3 m between the low- and high-water seasons. Seasonal as well as weak interannual variations between 2018 and 2019 are driven by rainfall regime over the catchment and associated river inflow. At sub-monthly scales, the lagoon is tidally choked: ocean tides can reach 90 cm, whereas in the lagoon semi-diurnal and diurnal tides hardly reach few centimeters. Choking conditions vary with river inflow and ocean tide amplitude, correctly represented by a simple tidal choking model. Diurnal modulation and asymmetry of the tide are stronger (weaker) during high (low) water period. We also observed WL variations of ±5–10 cm at a fortnightly frequency, stronger during wet (high-water) season. Superimposed on the seasonal, fortnightly and tidal WL variations, we further observed short-term high-frequency seiche events. Mostly observed during dry (low-water) conditions, they are characterized by typical standing-wave oscillations of 5–10 cm amplitudes and 3 h periods. They are forced by the passage of fast-moving squall-lines that induce strong wind variations, heavy rainfalls and rapid drop-off of the air temperature. Results obtained in this study provide useful metrics for the validation of flood forecasting models to be implemented in Benin, and elsewhere on the West African coastline. • Nokoué Lagoon is strongly tidally choked due to bottom friction in the Cotonou channel. •WL and tidal choking conditions depend on extreme seasonal hydrological cycle. •Fortnightly WL variations (±5–15 cm) are observed and are stronger during wet season. •High-frequency seiche events have 5–10 cm amplitudes and 3 h periods. •Seiche events are forced by the passage of fast-moving squall-lines. [ABSTRACT FROM AUTHOR]