West African Lagoon Research Articles

Seasonal and interannual variations of suspended particulate matter in a West-African lagoon (Nokoué lagoon, Benin): Impact of rivers and wind

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.

Modeling seasonal salinity variations in a large West African lagoon (Nokoué, Benin): Major drivers and mechanisms

Nokoué lagoon is a shallow water body connected to the Gulf of Guinea through the long and narrow Cotonou channel. The salinity dynamics in the lagoon is investigated using the 3D SYMPHONIE numerical model. We first validate the model using salinity and water level data. Simulated and observed salinity and water level variations compare well demonstrating that the reference model simulation correctly reproduces the dynamics of the lagoon. By performing several simulations with varying external forcings and freshwater fluxes, the main drivers of salinity variability in the lagoon are identified. We first focus on the salinization phase of the lagoon at the end of the tropical wet season (between November and February), and we investigate the total change in salinity and the salt fluxes involved in these variations. The high frequency salt fluxes associated with the ocean tide import salt from the ocean via the Cotonou channel to the south of the lagoon. Baroclinic fluxes, associated with the influence of salt on density and associated pressure gradient, increase this local salinity input in the south, but also play a major role in the dispersion of salt throughout the lagoon. Two rivers provide a permanent freshwater inflow in the north/ northeast of the lagoon that limits the salinization of the whole lagoon. Finally, wind-generated high frequency recirculation, partially prevents the salinisation of the North-East of the lagoon. During the desalinization period (between May and September), the lagoon salinity variations are highly sensitive to the magnitude of river inflow. At a constant river flux rate, the lagoon attains an equilibrium state in salinity. As this equilibrium is reached, both the overall salinity level and the time needed to achieve it decrease notably with higher fluxes. Beyond 100 m3 s-1, only a small area near the Cotonou channel retains higher salinity, while surpassing 500 m3 s-1 results in complete desalination of the entire Nokoué lagoon.

Using benthic macroinvertebrates as bioindicators to evaluate the impact of anthropogenic stressors on water quality and sediment properties of a West African lagoon

This study aimed to investigate the impact of anthropogenic stressors (landfilling, navigation for transport of goods, cooling in fossil fuel, urbanization, industrial expansion, agriculture activities, and recreational activities) on environmental variables, microbiological quality, and sediment properties using benthic macroinvertebrates as a bioindicator within Lagos Lagoon, Nigeria. Four (4) sampling stations were established with respect to their importance/anthropogenic activities within the Lagos Lagoon. Surface water, bottom substrates, and benthic macroinvertebrate fauna samples were collected bimonthly from each sampling station for a year and analyzed using appropriate standard methods and procedures. The highest pH range of 7.96–8.01 (7.98 ± 2.35) was recorded at Site IV, while the lowest range of 6.41–7.01 (6.15 ± 1.14) was observed at Site II, and there was a significant difference (p < 0.05) among the pH mean values across the sites. High values of salinity, chloride, sodium, COD, BOD, manganese, nickel, cadmium, and nitrate were recorded among the surface water physicochemical parameters, which were above WHO (2011) permissible limits, while the high concentrations of toxic metals (Pb, Cr, Zn, and Cd) was recorded in sediment. A total of 26 species of benthic macroinvertebrates were recorded during this study, which belongs to eight (8) classes. Gastropoda recorded the highest percentage contribution of 39.12%, followed by polychaeta accounting for 30.34%, while malacostraca contributed 2.63%. The highest abundance of macroinvertebrates was recorded at Site I (256 Indiv/m2), followed by Site IV (252 Indiv/m2), and the least was observed at Site II (195 Indiv/m2). Based on the results of the physico-chemical, heavy metals, microbial quality, and macroinvertebrates assemblage obtained from this study revealed the adverse effect of anthropogenic activities on water quality degradation. It plays a significant role in the distribution and diversity of benthic macroinvertebrates in an aquatic environment.

Physicochemical Drivers of Zooplankton Seasonal Variability in a West African Lagoon (Nokoué Lagoon, Benin)

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.

Diversity of Molluscs in a West African Lagoon, the Ouladine Lagoon in Grand-Bassam (Ivory Coast)

Diversity of Molluscs in a West African Lagoon, the Ouladine Lagoon in Grand-Bassam (Ivory Coast)

Ecological status of a West African lagoon complex under anthropogenic pressure, Toho-Todougba complex, Benin

Management and preservation of aquatic environments are essential for their productivity and the maintenance of aquatic life. This study aims to characterize the trophic state of Toho-Todougba lagoon complex which has been subject for several years, to an intense fish production in cages. For this purpose, physico-chemical parameters were measured from June 2019 to May 2020 as well as chlorophyll-α, phosphorus and nitrogen. The trophic characterization indices were calculated basedo n Carlson (1977), Burns and Bryers (2000), Neverova-Dziopak and Kowalewski (2018), Primpas et al. (2010) and CCME-WQI (2001). It appears that the temperature (29°C), dissolved oxygen (3.19 mg.L -1 to 4.33 mg.L -1 ) and pH (6.66 to 7.31) are those characteristic of tropical lake environments. The production parameters revealed that the chlorophyll-α concentration varies from 0.19 to 37 mg.L -1 (stations and months combined). The concentration of phosphorus ranged from 0.02 to 0.42 mg.L -1 while nitrogen varied from 1.91 to 4.03 mg.L -1 . Only nitrogen is not in critical proportion for the ecosystem. It should be noted that with the exception of the Carlson index, all other indices revealed that the ecosystem is in a state of advanced eutrophication with a tendency to hyper-trophication according to the eutrophication index of Primpas et al. (2010). Consequently, Toho-Todougba lagoon complex is eutrophic and requires adequate measures for its restoration despite its good health revealed by the physico-chemical parameters.

Seasonal and interannual variability of salinity in a large West-African lagoon (Nokoué Lagoon, Benin)

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.

Spatio-temporal variation of environmental variables and aquatic macroinvertebrate assemblages in Lake Nokoué, a RAMSAR site of Benin

Environmental characteristics and macroinvertebrate assemblages of Lake Nokoué (Benin) were investigated from September 2014 to July 2016. Seasonal and hydroclimatic changes, as well as anthropogenic activities were the overriding factors affecting environmental variables investigated. Analysis of macroinvertebrate community structure with several indices revealed a community structure changing across seasons and differing between the sites. Mollusca, Crustacea and Polychaeta were most abundant in the dry seasons, especially at sites close to the ocean, whereas Oligochaeta and Insecta (Diptera, Heteroptera, Coleoptera, Odonata and Ephemeroptera) were abundant in the wet seasons, especially during flood periods (short wet season) at the sites receiving fresh water and on roots of macrophytes. A redundancy analysis placed habitat suitability (temperature and macrophytes) and trophic status variables (NO2−, NO3−, conductivity and pH) as structuring drivers for macroinvertebrate assemblages. The current study demonstrates that spatial heterogeneity of macroinvertebrates of Lake Nokoue is related to the dynamics created by input of fresh water (wet seasons) or salt water (dry seasons), as well as spatial heterogeneity of anthropogenic activities (nutrients). It offers insights into the macroinvertebrate dynamics linked to the limnology of a West African lagoon, which could contribute to a better understanding of management and conservation measures.

Morphological differentiation among populations of Sarotherodon melanotheron melanotheron Rüppell 1852 across the West African Lagoon systems (Benin, Cameroon, Nigeria, Togo)

Morphometric variation among populations of Sarotherodon melanotheron melanotheron across its natural range (Mahin, epe, Lekki, Apese, Kuramo, Lagos, Iyagbe, Ologe, Badagry, Yewa – Nigeria; Be Lagoon – Togo; Idenau – Cameroon, Aheme – Benin; Portonovo – Benin) were studied to assess the level of differentiation of one of the populations that is currently isolated from the rest, the Apese Lagoon population. S. m. melanotheron showed subtle differentiation from some of the other lagoon populations. However, the pattern of variation did not reflect the level of geographical barriers or geographical distance among populations. The characters that form the basis of the differentiation observed were mostly those related to feeding, vision and respiration.

Diversity, biology and exploitation of brackish water crabs in West Africa: A review

Crabs are an important biological component of the West African lagoon fauna and are exploited by fishing communities. This paper presents a synthesis of the current understanding pertaining to the diversity, biology and exploitation of these crabs in order to identify promising research paths for this zoological group. A literature review was done. Thirty-two species belonging to thirteen families and twenty-three genera have been indexed by previous investigators and we plan on making use of this information in defining future research direction. The biology of crabs has been the subject of much research focused on Callinectes amnicola (Rochebrune, 1883) species. C. amnicola is an omnivorous species whose males are known to mature between 63 and 105 mm shell width, while females mature between 83.5 and 116.2 mm shell width. Fertility varies from 0.47 to 4.8 million eggs per female. However, exploitation parameters for this species in various lagoons are not available. A research direction geared towards establishing a new and thorough inventory along with currently nonexistent but relevant exploitation parameters will go a long way in defining sustainable management measures for these crab species. Keywords : Lagoon crabs, diversity, biology and exploitation, West Africa.

Diversity patterns and community characteristics of the fish assemblage of a West African lagoon

Coastal lagoons are important habitats in West Africa, being also irreplaceable for many species of fish. The community structure and the diversity patterns of their fish assemblages have however been poorly studied so far. In order to evaluate the community structure and diversity metrics of fish assemblages of one of these lagoons (Lake Togo, Togo), we surveyed four stations that were different in terms of morphological and ecological characteristics, from July to September 2017. The fishing gears were recorded and catches of small-scale fisheries were analyzed. A total of 40 species in 37 genera, 24 families and 10 orders were recorded, with Cichlidae (six species), followed by Eleotridae (five species) and Gobiidae (three species) being the most diverse families. The most abundant species in the catches were: Sarotherodon melanotheron, Coptodon guineensis, Chrysischthys nigrodigitatus and Ethmalosa fimbriata. Strict estuarine species (Es) were the most represented forms in the catches. Calculated indices of diversity showed that Lake Togo has moderate diversity and a poor organization of individuals within species.

Assessment of the Physico-Chemical Pollution of a Water Body in a Perspective of Integrated Water Resource Management: Case Study of Nokou&amp;#233; Lake

Nokoue Lake is the most important surface of brackish waters of Benin and one of the biggest West African lagoons, in point of view of its surface (about 150 km2), its productivity and exploitation. The aim of this study is to monitor the temporal and spatial variation of physico-chemical parameters of Nokoue lake water for its sustainable management. A total of twenty (20) points of sampling have been followed during a period of one year. The results of the monitoring reveal low dissolved oxygen and high concentration in phosphorus and chlorophyll a, sign of eutrophication. In addition, nutrient concentrations (nitrites, phosphates, nitrates) responsible for the phenomenon compared to water quality assessment grid of the Organisation for Economic Co-operation and Development, (OECD) used to classify the waters in the range of hyper-eutrophic waters characterized by the proliferation of aquatic plants. So some measures have been proposed to remediate to this pollution in order to an integrated management.

English

Although West African lagoons are known to provide significant fishery resources to populations, few studies have been done on processes responsible for their fish composition, abundance and spatial distribution. The aim of this study was to assess fish species distribution along the Aby-Tendo-Ehy lagoon system in C&ocirc;te d&rsquo;Ivoire. Sampling sites were distributed in the lagoon and grouped into three zones from a freshwater zone with high contributions of freshwater (zone 1) to a mixo-eurihaline water zone (zone 3), with a transition site (zone 2) located between the first two zones. Environmental characteristics showed gradual decrease of mean pH from zone 1 to 3, whereas mean conductivity, total dissolved solids and transparency measurements increased from zone 1 to 3. Salinity data were 0 all along the sampling period in zones 1 and 2 and increased up to 3 ppt in zone 3. A total of 67 species belonging to 29 families were caught. Overall fish diversity decreased from zone 1 (n=58) to 3 (n=40), with 45 species caught in zone 2. The number of species in different fish trophic categories (invertivorous, herbivorous, piscivorous and omnivorous species) decreased from the zone 1 to 3. Fish compositions in zones 1 and 2 were dominated by freshwater species, while in the zone 3 it was dominated by estuarine dependent species. The canonical correspondence analysis performed showed a clear-cut influence of some environmental variables on fish ecological categories distribution: the zone under continental freshwater discharges had higher pH and temperature measurements and housed more freshwater, estuarine resident, marine migrant and estuarine dependent freshwater species, while the site with marine influences had higher conductivity, salinity, transparency and dissolved oxygen data and was mainly associated with estuarine dependent marine species. &nbsp; Key words:&nbsp;Ichthyofauna, species distribution, environmental variables, Aby lagoon, C&ocirc;te d&rsquo;Ivoire.

Length-weight Relationships of 30 Fish Species in Aby Lagoon, Southeastern Cote d? Ivoire

Most West African lagoons are very productive ecosystems that support important fishery activities. However, increases in human populations put heavy pressure on their resources. This raised the importance of sustainable management tools for these ecosystems. Length-Weight Relationships (LWRs) which is an important fish stock assessment tool was investigated in the Aby lagoon, southeastern Cote d'Ivoire. Fishes were caught monthly from February 2012 through March 2013 using gillnets. Fishes captured were identified, weighted to the nearest gram and standard length was measured to the nearest millimeter. The 30 fish species selected for this study belong to 18 families. The highest species number (n = 7) was recorded for Cichlidae family. All length-weight regressions were significant with a coefficient of determination (r²) varying from 0.462 (Parailia pellucida) to 0.984 (Elops lacerta). Growth coefficient, b of the LWR ranged from 2.229 (P. pellucida) to 3.811 (Eleotris vittata). Nineteen species exhibited negative allometric growth pattern while 11 had positive allometric growth. These results are useful references for West African coastal lagoon management and particularly for that of the Aby lagoon system.

Distribution of Benthic Insect Fauna in a West African Lagoon: The Porto-Novo Lagoon in Benin

Distribution of Benthic Insect Fauna in a West African Lagoon: The Porto-Novo Lagoon in Benin

The salinity tolerance of Corbula trigona (Bivalvia: Corbulidae) from a West-African lagoon and its variations

Le lac Aheme, localise dans la zone lagunaire du Benin, presente des variations annuelles importantes de salinite. Des lots de Corbula trigona, l'espece de Bivalve la plus abondante de cette lagune, ont ete soumis a differents essais de tolerance a la salinite. Les reponses des individus de deux populations naturelles vivant a des salinites tres differentes sont comparees. L'experience a montre que, si la taille des individus intervient dans les effets temporels de la salinite sur la mortalite, elle n'influence cependant pas les limites de tolerance de l'espece

The Production of Corbula trigona (Bivalvia) in Relation to Its Demographic Strategies in a West African Lagoon

Annual production of Corbula trigona (Hinds) in this freshwater and brackish coastal lagoon was assessed by the removal-summation method and by the size-frequency method. The latter seems to slightly overestimate production, and its application requires a good knowledge of the life history of the species, taking lifetime in the different stations into account. Biomass, production and productivity (P/B) decrease, from the north of the lake where the water is fresh, to the south where the marine influence makes the habitat unstable and stresses the population. The overall high production (300 kg ha-1 yr~ dry weight of organic matter) is due to the great C. trigona biomass, but productivity is rather low. The annual production of shells within the lake is 40000 tons. The filter feeding Corbula trigona thus appears as a major link between the plankton and the sediment.

Detritus and energy consumption and conversion efficiency of Sarotherodon melanotheron (Cichlidae) in a West African lagoon

ummary The growth, standing stocks and production of tilapia Sarotherodon melanotheron (Cichlidae) in the 1 km2 Sakumo Lagoon near Tema, Ghana, are estimated by application of recently developed models to previously published data. The new estimates are used to obtain values of food (i.e. sediment) consumption, some related statistics, and to assess bioturbation as caused by S. melanotheron. Zusammenfassung Detritus, Energieverbrauch und Konversionseffizienz von Sarotherodon melanotheron (Cichlidae) in einer westafrikanischen Lagune Das Wachstum, die Biomasse und die Produktion von Tilapia Sarotherodon melanotheron (Cichlidae) in der 1 km2 Sakumo Lagune bei Tema, Ghana, wurden bestimmt auf der Basis von neueren Modellen und bereits veroffentlichten Daten. Die neuen Modellparameter werden benutzt, um die Nahrungs-, d. h. Sedimentkonsumption und verwandte Grosen zu bestimmen, und um die Bioturbation zu schatzen, die S. melanotheron verursacht. Resume Detritus, depense d'energie et efficience de la conversion de Sarotherodon melanotheron (cichlides) dans une lagune en Afrique de l'ouest La croissance, la biomasse et la production du tilapia Sarotherodon melanotheron (Cichlides) ont ete estimees pour le stock de la lagune de Sakumo (1 km2), pres de Tema, Ghana, en utilisant des modeles recents et des donnees deja publiees. Les nouvelles valeurs sont utilisees pour l'estimation de la consommation en nour re (= sediment) ainsi que quelques autres characteristiques de S. melanotheron, et pour estimer le taux de bioturbation qui resulte de ses activites.

Population Dynamics of Corbula trigona (Mollusca) in Lake Ahémé, a West African Lagoon in Benin

- a continous recruitment in the north of the lake, with a maximum during the rainy season. This is the zone where fresh water flows in and where salinity is lowest. - a recruitment during the rainy season in the south, with a complete interruption and even elimination of the population during the dry season. Salinity then reaches its highest level owing to the influence of sea water and oxygen its lowest level in the lake. The use of the Battacharya method for discriminating cohorts is discussed.

The biology, fishery and potential for aquaculture of Tilapia melanotheron in a small West African lagoon

1. (1) The biology, especially the food and feeding habits of the cichlid fish Tilapia melanotheron Rüppel 1852 in a small West African lagoon is analysed. The food consists of bottom mud, which has an organic content of 30% and a calorific content of 1.2 Kcal/g. The daily food intake per fish weighing 20 g (fresh weight) was estimated to be 1.5 g (dry weight) and the calorific assimilation a minimum of 900 cal/day. Reproduction seemed to occur throughout the whole year, parasitism was low due to varying salinity of the lagoon. 2. (2) The small artisanal lagoon fishery produces about 150 kg fish per ha per year, with a fish catch per unit of effort of up to 1 kg/man-h. The fishing intensity was 70 man-h/km 2. Gill nets, seines and cast nets are used and over 90% of the catch consisted of T. melanotheron. 3. (3) Three alternative methods to increase the quantity of the fish caught in the lagoon are briefly discussed: (a) setting up of an “acadja” fishery; (b) stocking with juvenile T. melanotheron; (c) stocking with Mugil spp. fry ( Tilapia/Mugil polyculture). The availability of Mugil spp. fry along the coast makes the latter alternative look like the most promising one, being the cheapest and easiest to implement on a large scale.