We investigated spatio-temporal patterns of fish assemblages in four small coastal rivers in SouthEast Ivory Coast. The samples were collected between July 2003 and March 2005 at 8 sampling sites (2 per river: 1 upstream and 1 downstream). A total of 59 fish species belonging to 39 genera, 23 families and 11 orders were captured. Perciforms (30% of the families and 33% of the species), followed by Siluriforms (22% and 22%), Osteoglossiforms (13% and 17%), Characiforms (9% and 10%) and Pleuronectiforms (9% and 3%) were the most abundant orders. Among the families sampled, Cichlidae (20% of the species), Mormyridae (13%), Clariidae (10%), Cyprinidae (10%) and Characidae (8%) were largely represented. Among the 59 fish species captured, we identified fifteen marine/brackish species and two introduced species. To analyse patterns of fish assemblages, we used a non-linear clustering technique, the self-organizing map (SOM). Using SOM, samples were classified into 4 clusters, mainly related to the spatial location of the sampling sites. Except for the distance from the source of the river, environmental variables (width, depth, current velocity) did not show a clear distribution gradient on the SOM map. This pattern was explained by the abundance of estuarine/marine species (i.e. 25% of the species) characterizing the lower course of the rivers studied. Such a distribution of estuarine/marine species colonizing the lower and middle course of the rivers was explained by the absence of dams and contrasts with the patterns observed in more fragmented rivers of Ivory Coast. Therefore, we suggest that these small streams should be preserved in order to permit estuarine/marine species to migrate between streams and the Aby lagoon, thus allowing their large distribution and their reproduction in favourable habitats. We suggest particular attention be given to the Soumié River basin which represents the only preserved stream ecosystem within the Bia River basin. Key words: ichthyofauna / coastal rivers / hierarchical clustering / self-organizing map / Western Africa Résumé-Distribution spatio-temporelle des assemblages de poissons des cours d'eau de la côte ouest africaine : une approche basée sur les cartes de Kohonen. Nous étudions les types de distribution spatio-temporelles d'assemblages de poissons de quatre cours d'eau du sud-est de la Côte d'Ivoire. Les échantillonnages de 8 sites (2 sites par rivière : un amont et un aval) sont effectués entre juillet 2003 et mars 2005. Au total, 59 espèces de poisson appartenant à 39 genres, 23 familles et 11 ordres sont capturées. Les Perciformes sont les plus abondants (30 % des familles et 33 % des espèces), suivis des Siluriformes (22 % des familles), des Ostéoglossiformes (13 % et 17 %), des Characiformes (9 % et 10 %) et des Pleuronectiformes (9 % et 3 %). Les familles les plus largement représentées sont les Cichlidés (20 % des espèces), Mormyridés (13 %), Clariidés (10 %), Cyprinidés (10 %) et les Characidés (8 %). Des 59 espèces de poissons capturés, quinze espèces de milieu ...
La qualité des eaux de puits à usages domestiques dans les quartiers précaires de quatre communes de la ville d'Abidjan a été étudiée à travers l'analyse des paramètres physico-chimiques et microbiologiques couramment utilisés pour la qualité de l'eau de consommation. Les concentrations en nitrates varient entre 0 et 286 mg/L et celles de l'ammonium entre 3,6 et 39,6 mg/L. Plus de 80% des puits ont des teneurs en nitrates supérieures à la norme de 50 mg/L requise, ce qui traduit que les eaux sont d'une mauvaise qualité pour la boisson, mais subissent l'impact d'une insalubrité urbaine. En plus, la qualité des eaux de puits est variable et dépend d'un certain nombre de facteurs tels que l'emplacement des puits par rapport aux sources de contamination urbaines endogènes. La dégradation de la qualité des eaux est fortement impactée par le défaut d'assainissement des quartiers précaires. L'interprétation des données d'analyse, la corrélation existante entre les eaux de puits et la répartition des eaux de puits en groupes sont réalisées en utilisant l'Analyse en Composante Principale Normée (ACPN). Plusieurs facteurs conditionnent la contamination des eaux de puits par les polluants minéraux tels que NO 3-, NH 4 + , SO 4 2et Cl-. Le défaut d'assainissement dans ces quartiers, la mauvaise gestion des déchets urbains, la faible profondeur de la nappe, la nature des sols et la perméabilité de l'aquifère exploité sont les preuves de la vulnérabilité des eaux des puits. En effet, les résultats acquis font ressortir la forte influence de l'activité urbaine sur la qualité de ces eaux qui sont fortement chargées en ions nitrates et ammonium, signe d'une pollution urbaine.
The supply of drinking water is a vital challenge for the people who live on the African continent, as this continent is experiencing strong demographic growth and therefore increasing water demands. To meet these needs, surface water resources are becoming increasingly mobilized because underground resources are not always available or have already been overexploited. This situation is the case in the region of Abidjan in the Ivory Coast, where the drinking water deficit is a growing problem and it is therefore necessary to mobilize new water resources to ensure the supply of drinking water. Among the potential resources, local managers have identified a freshwater lagoon, Lagoon Aghien, That is in close proximity to the city of Abidjan. With the aim of enhancing knowledge on the ecological functioning of the lagoon and contributing to the assessment of its ability to provide drinking water, several physical and chemical parameters of the water and the phytoplankton community of the lagoon were monitored for 17 months (December 2016-April 2018) at six sampling stations. Our findings show that the lagoon is eutrophic, as evidenced by the high concentrations of total phosphorus (>140 μg L-1), nitrogen (1.36 mg L-1) and average chlorophyll-a (26 to 167 μg L-1) concentrations. The phytoplankton community in the lagoon is dominated by genera typical of eutrophic environments including mixotrophic genera such as Peridinium and by cyanobacteria such as Cylindrospermopsis/Raphidiopsis, Microcystis and Dolichospermum that can potentially produce cyanotoxins. The two rainfall peaks that occur in June and October appeared to be major events in terms of nutrient flows entering the lagoon, and the dynamics of these flows are complex. Significant differences were also found in the nutrient concentrations and to a lesser extent in the phytoplankton communities among the different stations, especially during the rainfall peaks. Overall, these results reveal that the quality of the lagoon’s water is already severely degraded, and this degradation could increase in future years due to increasing urbanization in the watershed. These results therefore raise questions about the potential use of the lagoon as a source of drinking water if measures are not taken very quickly to protect this lagoon from increasing eutrophication and other pollution sources.
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