Search

This study aimed to investigate the seasonal variation of zooplankton diversity and abundance in the Nokoué Lagoon in southern Benin. Through extensive sampling, a total of 109 zooplanktonic taxa were identified and quantified. The average zooplankton abundance was found to be 60 individuals per liter, with copepods and rotifers being the most dominant groups, comprising 68.1% and 29.1% of the total abundance, respectively. The key factor identified as driving the structure of the zooplanktonic assemblages was salinity, which showed significant seasonal variation. The results revealed that during the high water period, when the lagoon was filled with fresh water, rotifers were dominant, zooplanktonic diversity was highest, and abundances were quite high. Conversely, during the low water period, when the lagoon was characterized by brackish water, diversity was minimal, and abundance decreased slightly. The study also found that some areas of the lagoon showed high abundances independent of salinity levels, suggesting that other factors such as riverine inputs or the presence of acadjas (home-made brush parks used as fish traps) may also have notable effects on the zooplankton community. Overall, the findings of this study provide valuable insights into the functioning of one of the most biologically productive lagoons in West Africa. [ABSTRACT FROM AUTHOR]

• Nokoué Lagoon's water circulation and residence times strongly vary with seasons. • They are sensitive to river inflow, tide, salinity gradient and wind forcing. • Mean water residence times are of few days (months) during flood (low-water) periods. • Tides, river inflows and salinity gradients favor evacuation, wind favors retention. • Above a critical water flow of 50-100 m3/s, the lagoon is rapidly flushed (<5 days). Seasonal water circulation and residence times in the large (150 km2) and shallow (1.3 m average dry season depth) Nokoué Lagoon (Benin) are analyzed by means of numerical simulations using the three-dimensional SYMPHONIE model. The average circulation during the four primary hydrological periods throughout the year are studied in detail. Despite the lagoon's shallowness, significant disparities between surface and bottom conditions are observed. During the flood season (September-November), substantial river inflow (∼1200 m3/s) leads to nearly barotropic currents (∼7 cm/s), 'directly' linking rivers to the Atlantic Ocean. Rapid flushing results in short water residence times ranging from 3 to 16 days, with freshwater inflow and winds driving lagoon dynamics. During the salinization period (December-January) the circulation transforms into an estuarine pattern, characterized by surface water exiting and oceanic water entering the lagoon at the bottom. Average currents (∼2 cm/s) and recirculation cells are relatively weak, resulting in a prolonged residence time of approximately 4 months. Circulation during this time is dominated by tides, the ocean-lagoon salinity gradient, wind, and river discharge (∼100 m3/s). During low-water months (February to June), minimal river inflow and low lagoon water-levels prevail. Predominant southwest winds generate a small-scale circulation (∼3 cm/s) with a complex pattern of multiple recirculation and retention cells. Residence times vary from 1 to 4 months, declining from February to June. During the lagoon's desalination season (July-August), increasing river inflows again establish a direct river-ocean connection, and average residence times reduce to ∼20 days. Notably, a critical river discharge threshold (∼50-100 m3/s) is identified, beyond which the lagoon empties within days. This study highlights how wind-driven circulation between December and June can trap water along with potential pollutants, while river inflows, tides, and the ocean-lagoon salinity gradient facilitate water discharge. Additionally, it explores the differences between residence and flushing times, as well as some of the limitations identified in the simulations used. [ABSTRACT FROM AUTHOR]

This study aimed to investigate the seasonal variation of zooplankton diversity and abundance in the Nokoué Lagoon in southern Benin. Through extensive sampling, a total of 109 zooplanktonic taxa were identified and quantified. The average zooplankton abundance was found to be 60 individuals per liter, with copepods and rotifers being the most dominant groups, comprising 68.1% and 29.1% of the total abundance, respectively. The key factor identified as driving the structure of the zooplanktonic assemblages was salinity, which showed significant seasonal variation. The results revealed that during the high water period, when the lagoon was filled with fresh water, rotifers were dominant, zooplanktonic diversity was highest, and abundances were quite high. Conversely, during the low water period, when the lagoon was characterized by brackish water, diversity was minimal, and abundance decreased slightly. The study also found that some areas of the lagoon showed high abundances independent of salinity levels, suggesting that other factors such as riverine inputs or the presence of acadjas (home-made brush parks used as fish traps) may also have notable effects on the zooplankton community. Overall, the findings of this study provide valuable insights into the functioning of one of the most biologically productive lagoons in West Africa.

Nokoué Lagoon in the South of Benin is a large intermittent coastal water body in West Africa, which supports one of the largest inland fisheries of the region. The seasonal and interannual variability of its salinity was studied, based on 3 years of monthly surveys (Dec 2017–Dec 2020). This dataset allows us to identify fine-scale salinity structures and to better understand the salinization/desalinization processes at seasonal scales. During the rainy season from May to November, under the influence of large freshwater inflows from the rivers on its northern shores, the lagoon desalinizes to a salinity of zero in October–November. During the dry season from December to April, under the effect of the ocean tide, Nokoué lagoon becomes progressively saltier, reaching typical salinities of ∼25 in April. On average, the Nokoué lagoon is saltier in its southwestern part and fresher towards the river's mouths. Vertical salinity stratification is largest in December at the beginning of the main dry season. The lagoon displays a very marked interannual variation with mean surface (bottom, respectively) salinity of 25 (25) in April 2018 and 2020, respectively, against 16 (18) in April 2019. In the absence of river inflow data, a box model shows that the average salinity of the lagoon is very sensitive to small changes in river inflow (or discharge), with observed interannual differences in salinity induced by small variations of 10–15 m3 s−1 in inflow during the dry season. During the salinization phase, the model suggests that ∼30% of the seawater entering the lagoon during flood-tides remains trapped and enhances the Nokoué lagoon salinity. This model also indicates that a complete desalinization of the lagoon occurs for river inflow greater than ∼50–60 m3 s−1. The general mixing time scale of the lagoon is of 30–40 days. • Salinity of Nokoué lagoon exhibits large seasonal variations from 0 to ∼25. • Differences of salinity of 10–15 can be observed between the surface and the bottom. • Strong interannual salinity variations (16–25) were observed in dry seasons. • A box model suggests a complete desalinization for river inflows of 50–60 m3 s−1 • In dry season, ∼30% of seawater entering the lagoon by flood-tides remains trapped. [ABSTRACT FROM AUTHOR]