La faune benthique d'une lagune ouest Africaine (le lac Nokoue au Bénin), diversite, abondance, variations temporelles et spatiales, place dans la chaine trophique

Les invertébrés constituent l'un des compartiments souvent ignorés et mal connus mais joue un rôle essentiel dans le fonctionnement des écosystèmes aquatiques. Cette étude concerne la faune d'invertébrés d'une lagune ouest africaine : leur rôle dans le réseau trophique et leur utilisation pour évaluer l'état de santé du lac. Un inventaire de tous les invertébrés du lac a été fait et les résultats de l'étude de la variation spatio temporelle de la diversité, des abondances et des biomasses ont été présentés. La pollution ayant un rôle important dans la réparation des invertébrés après la salinité, une étude plus fine de la relation entre la pollution et les invertébrés a permis d'élaborer un indice biotique de pollution pour la surveillance des lagunes ouest africaines. L'étude du régime alimentaire des poissons dominants a montré le rôle des différents groupes d'invertébrés dans le réseau trophique du lac. Enfin la relation entre les invertébrés et le régime alimentaire des poissons d'une part et la relation entre les invertébrés et la pollution ont été discutés et des propositions ont été faites pour l'aménagement et la gestion du lac. ABSTRACT : Invertebrates are a key component of aquatic ecosystems even if they are quite often unknwon or poorly documented. This study concerns the invertebrate fauna of a West African lagoon, their contribution to the food web, and their possible use in order to assess the the ecological condition of the lake. A sampling of all the invertebrates was carried out during two years and the pattern of spatial and seasonal distribution was obtained in terms of density and biomass. Pollutions and, to a lesser extent, salinity, play a key role in the distribution of this fauna. Therefore, a biological indice could be designed for a quick evaluation and monitoring of the health of any West African brackish water. Finally, the contribution of the invertebrates to the foodweb was evaluated on a quantitative basis. Resulting proposals for sustainable management and exploitation of the living resourcs of the lake were elaborated and discusse

Asymmetry in Chrysichthys Auratus (Geoffroy Saint-Hilaire, 1809) (Siluridae) and Mugil Cephalus Linnaeus, 1857 (Mugilidae) from Lake Ahémé (Bénin, West Africa)

Asymmetry study was performed for four bilateral characters of Chrysichthys auratus (Geoffroy Saint-Hilaire, 1809) (Siluridae) and Mugil cephalus Linnaeus, 1857 (Mugilidae) collected from Guézin and Akodéha-Kpodji localities at Lake Ahémé, Republic of Bénin, West Africa. For both C. auratus and M. cephalus the four morphometric and one meristic characters, bilateral asymmetry was higher in the fish specimens collected from Guézin compared with that of specimens from Akodéha-Kpodji localities within Lake Ahémé. Bilateral asymmetry in C. auratus and M. cephalus increased with the fish size at both localities. The conceivable reasons of bilateral asymmetry in the two fish species are deliberated in relative to diverse pollutants in Lake Ahémé

Elaboration of a biotic index of pollution using macroinvertebrates for the monitoring of Lake Nokoué in Benin

A study on the relationships between the physicochemical parameters and of the macroinvertebrates of the Lake Nokoué was carried out for the purpose of developing a biotic index of pollution. A total of 96 stations were prospected twice during the flooding and the dry seasons from September 2004 to February 2006. At each station, seven physicochemical parameters were measured and macroinvertebrates were collected. Canonical analyses of correspondence were used to show the relations between the parameters of the environment and the macroinvertebrates. A self-organizing map (SOM) of Kohonen allows the categorization of the studied stations into four groups according to different degrees of pollution. Rough scores were allotted to each taxon according to its abundance in the various groups of stations determined by the SOM for each campaign. A synthesis of the data obtained from the four field surveys allows the assignment of a score to each taxon according to its abundance in the different groups of stations. A biotic index was developed and used for the evaluation of the level of pollution of the stations studied. It then emerged that the borders of the lake present a «poor quality» of water; however, during the dry season there is slight improvement.© 2015 International Formulae Group. All rights reserved.Keywords: Biotic index, pollution, invertebrates, lagoons, Benin, West Afric

Analyse Et Illustration De La Valeur Indicatrice Des Espèces Macrobenthiques Du Delta De l’Ouémé Au Bénin

In this work, it was matter of determining the species characteristics of the delta of the Oueme through the illustration of their indicative value. Sampling was carried out in eight heterogeneous stations covering the periods of flood and low water. The various indices including that of biserial correlation IndVal (Indicator Value Index) and the cover sites of groups were used as the basis of analysis. The results showed that the selected significant indicative species are highly correlated and have a preference for stations full of organic and mineralized matter. Also, taxa have a marked preference for the flood season (21 taxa) than low water (5 taxa). Most characteristic taxa highlighted by this procedure should be useful to assess changes in quality of the plants studied. Most taxa characteristics highlighted by this procedure should be useful to assess changes in quality of the plants studied

Effet De L’Assèchement Des Ruisseaux Affluents Des Rivières Alibori Et Sota Sur La Structure Et La Diversité De La Communauté Des Macroinvertébrés Benthiques Au Nord Bénin

La présente étude vise à évaluer l’impact de l’assèchement des rivières sur la structure et la diversité de la communauté benthique de cinq ruisseaux (Bouli; Irané; Tassiné; Sanson et Yassikoga) affluents des rivières Alibori et Sota du bassin cotonnier du nord Bénin. Les données sur les macroinvertébrés benthiques ont été collectées avant et après l’assèchement des lits des stations d’études. Les échantillons des macroinvertébrés benthiques ainsi récoltés ont été identifiés au laboratoire. Les données ont été analysées à l’aide des logiciels Excel (2013) et R3.4.2 grâce au test t student pour déterminer  l’abondance taxonomique, la richesse taxonomique, la fréquence d’observation, l’indice de diversité de Shannon et d’équitabilité de Piélou. Cette étude a permis d’identifier 2926 individus de macroinvertébrés benthiques appartenant à 58 familles  avant assèchement ainsi que 2133 individus appartenant à 49 familles après assèchement des stations d’études. La baisse de l’abondance ; de la richesse taxonomique et les faibles valeurs des indices de diversité de Shannon et de Piélou après assèchement par rapport à l’avant assèchement traduisent les effets de la sècheresse sur la structure et la diversité des communautés benthiques des rivières Alibori et Sota au nord Bénin. La densité élevée des Oligochètes après l’assèchement est caractéristique de leur capacité à recoloniser et à résister à la sécheresse

Evaluation de l’Effet des Eaux Usées Industrielles sur la Qualité de l’Eau de la Rivière Klou au Centre du Bénin

La présente étude a pour objectif d’évaluer l’impact des eaux usées industrielles sur la qualité de l’eau de la rivière Klou. Pour cela, des prélèvements d’eau ont été effectués avec une fréquence mensuelle sur des stations témoins et exposées. Les analyses physico-chimiques et le calcul de l’indice de Pollution Organique (IPO) ont été effectués. En période pluvieuse, les valeurs des paramètres physico-chimiques (pH, conductivité, TDS, ammonium, DBO5, nitrate, nitrite et orthophosphates) etaient faibles. De plus, les fortes valeurs des paramètres étudiés sont observées sur les stations plus exposées. Les résultats de l’analyse en composantes principales (ACP) sur les paramètres physico-chimiques ont révélé deux groupes  de stations. Le premier est constitué des deux stations témoins caractérisées par les faibles valeurs des paramètres étudiés alors que le second est composé des stations plus exposées et celles de l’aval avec les fortes valeurs des paramètres étudiés. L’indice de pollution organique a révélé une pollution modérée au niveau des stations témoins et une pollution organique forte et très forte respectivement au niveau des stations plus exposées et celles de l’aval. Il est nécessaire alors de mettre en place une politique de  restauration de la qualité écologique des eaux de cette rivière et des centres d’épuration opérationnels et efficaces.   The objective of this study was to assess the impact of industrial wastewater on the water quality of the Klou River. For this purpose, water sampling was conducted with a monthly frequency on control and exposed stations. Physico-chemical analyses and the calculation of the Organic Pollution Index (IPO) were carried out. In rainy periods, the values of the physico-chemical parameters (pH, conductivity, TDS, ammonium, BOD5, nitrate, nitrite and orthophosphates) were low. Moreover, the high values of the studied parameters were observed at the more exposed stations. The results of the principal component analysis (PCA) on the physico-chemical parameters revealed two groups of stations. The first consists of the two control stations characterized by low values of the studied parameters, while the second consists of the more exposed stations and those downstream with high values of the studied parameters. The organic pollution index revealed moderate pollution at the control stations and strong and very strong organic pollution respectively at the more exposed stations and those downstream. It is then necessary to put in place a mechanism to restore the ecological quality of the waters of this river and operational and effective purification centres

Etude Préliminaire de la Diversité et de la Structure du Peuplement Zooplanctonique de la Rivière Okpara au Bénin

La présente étude a pour objectif d’évaluer la diversité et la structure du zooplancton dans la rivière Okpara. Le zooplancton de la rivière a été identifié et la structure de sa communauté évaluée dans neuf stations sélectionnées grâce à la richesse taxonomique, à la densité, à l’indice de diversité de Shannon et l’indice d’équitabilité de Piélou. Un total de 68 espèces de zooplancton a été enregistré, réparties en trois groupes : comprenant des rotifères (57), des copépodes (07) et des cladocères (04). Le nombre d’espèces a été plus élevé dans le cours inférieur, que dans le cours supérieur et dans le cours moyen. Les rotifères représentent les groupes les plus abondant avec 78,25 % de l'abondance totale, suivis des copépodes (20,7 %) et des cladocères (1,3 %). Keratella valga et Tropocyclops confinis sont les espèces les plus abondantes. Mytilina sp., Asplanchna girodi, Anuraeopsis sp., Lepadella sp., Asplanchna brightwellii, Asplanchna navicular, Trichocerca sp. et Trichocerca tenuior ont été enregistrés sur toutes les stations. Les valeurs moyennes d’abondance et de densité moyenne du zooplancton sont plus élevées en saison sèche qu’en saison pluvieuse. Les valeurs de la richesse, de l’indice de diversité de Shannon et d’équitabilité de Piélou sont plus élevées en saison pluvieuse qu’en saison sèche. La forte présence de certaines espèces indicatrices de pollution telles que Asplanchna brightwelli, Brachionus angularis, Brachionus falcatus pourrait indiquer la présence de polluants organiques dans la rivière. Il est donc nécessaire de mettre en place des mesures de surveillance efficaces afin de protéger la rivière de toute intrusion anthropogénique.   The objective of this study was to assess the diversity and structure of zooplankton in the Okpara River. The river's zooplankton were identified and their community structure assessed at nine selected stations using taxonomic richness, density, the Shannon diversity index and the Pielou equitability index. A total of 68 species of zooplankton were recorded, divided into three groups: rotifers (57), copepods (07) and cladocerans (04). The number of species was higher in the lower reaches than in the upper and middle reaches. Rotifers were the most abundant group with 78.25% of the total abundance, followed by copepods (20.7%) and cladocerans (1.3%). Keratella valga and Tropocyclops confinis were the most abundant species. Mytilina sp., Asplanchna girodi, Anuraeopsis sp., Lepadella sp., Asplanchna brightwellii, Asplanchna navicular, Trichocerca sp. and Trichocerca tenuior were recorded at all stations. The values for abundance and density of zooplankton are higher in the dry season than in the rainy season. The values for species richness, Shannon diversity index and Pielou equitability are higher in the wet season than in the dry season. The strong presence of certain pollution indicator species such as Asplanchna brightwelli, Brachionus angularis and Brachionus falcatus could indicate the presence of organic pollutants in the river. Effective monitoring measures therefore need to be put in place to protect the river from anthropogenic intrusion

Simulating rewetting events in intermittent rivers and ephemeral streams: a global analysis of leached nutrients and organic matter

Climate change and human pressures are changing the global distribution and extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56‐98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached organic matter. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events

Simulating rewetting events in intermittent rivers and ephemeral streams: A global analysis of leached nutrients and organic matter

Climate change and human pressures are changing the global distribution and the ex‐ tent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (precon‐ ditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experi‐ mentally simulated, under standard laboratory conditions, rewetting of leaves, river‐ bed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative character‐ istics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dis‐ solved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contrib‐ uted most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental vari‐ ables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached sub‐ stances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying event