Abstracts
Résumé
Sont considérés dans cette étude, l’impact des rejets polluants d’origines urbaine et industrielle sur la qualité des eaux du Mfoundi, l’abondance, la richesse spécifique et la structure des communautés diatomiques qui peuplent ce cours d’eau. Le calcul du pourcentage de taxons tolérants à la pollution organique a permis d’évaluer la réponse de ces algues à l’enrichissement du milieu en matières organiques. Les analyses physico-chimiques des eaux mettent en évidence l’existence d’un gradient de pollution croissant de l’amont à l’aval de la rivière. Le cours supérieur présente des eaux turbides avec des teneurs élevées de matières en suspension. Les cours moyen et inférieur sont fortement contaminés par les matières organiques qu’apportent les eaux usées domestiques et les effluents industriels qu’ils reçoivent. Au total, 237 taxons de diatomées épilithiques, majoritairement cosmopolites, appartenant à 39 genres répartis dans 25 familles, ont été inventoriés. Le développement dans le cours supérieur du Mfoundi de taxons tels que Gomphonema parvulum (Kützing) Kützing et variétés, Gomphonema gracile Ehrenberg et Navicula radiosa Kützing, est associé essentiellement à la turbidité des eaux de ce tronçon. La distribution de Nitzschia palea (Kützing) W. Smith, Navicula geoppertiana (Bleisch) Grunow, Navicula mutica Kützing et Pinnularia subcapitata Gregory dans les cours moyen et inférieur est étroitement liée à la charge organique et la forte minéralisation des eaux. N. palea est l’espèce la plus abondante dans les stations Mf5 (66 %) et Mf6 (53 %), indiquant une importante pollution organique de ces sites sous l’effet des rejets des industries brassicoles et vinicoles opérant dans leur voisinage. La variation spatiale de la diversité spécifique n’a pas souligné toute l’ampleur de l’altération de la qualité des eaux; toutefois, l’analyse du pourcentage de taxons polluo-tolérants révèle que les apports graduels en nutriments et en matières organiques dans les cours moyen et inférieur contribuent à l’eutrophisation progressive de ces secteurs. Il est montré que les diatomées épilithiques sont sensibles à la variation de la qualité de l’eau du Mfoundi, et peuvent être utilisées comme bioindicateurs de l’état de santé des cours d’eau urbains du Cameroun.
Mots-clés :
- diatomées épilithiques,
- diversité spécifique,
- qualité de l’eau,
- rivière Mfoundi,
- limnologie équatoriale,
- Cameroun
Abstract
The Mfoundi River is used for the disposal of solid wastes and waste waters from domestic and industrial activities, which can alter its water quality and biodiversity. The aim of this study was to assess the structure of diatom communities in the Mfoundi River in relation to some physico-chemical variables, i.e., analyzing the changes in species composition and the dominance profile of the main species along a pollution gradient. The diversity metrics (species richness, Shannon’s diversity index, evenness) and the percent pollution tolerant valves (%PTV) were also tested to characterize the water quality and the response of diatoms to environmental enrichment in organic matter. The physico-chemical analyses of the waters revealed a gradient of increasing pollution from upstream to downstream of the river. The upper reach presented turbid waters with high levels of suspended solids. The middle and the lower reaches were highly contaminated with organic matters originating from domestic sewage and industrial effluents. A total of 237 epilithic diatom taxa, mostly cosmopolitan, belonging to 39 genera distributed among 25 families were inventoried. The development in the upper reach of the Mfoundi of taxa such as Gomphonema parvulum (Kützing) Kützing and varieties, Gomphonema gracile Ehrenberg, and Navicula radiosa Kützing reflected in particular the turbid waters of this section. The distribution of Nitzschia palea (Kützing) W. Smith, Navicula geoppertiana (Bleisch) Grunow, Navicula mutica Kützing, and Pinnularia subcapitata Gregory in the middle and lower reaches was closely related to the organic load and high degree of mineralisation of the waters. N. palea was the most abundant species at the stations Mf5 (66%) and Mf6 (53%), indicating a significant organic contamination of these sites under the effect of discharges from brewery and wine-making industries operating within their vicinity. The spatial variation of diversity metrics did not reflect the full extent of the alteration of water quality, but the analysis of the %PTV revealed that incremental inputs of nutrients and organic matter in the middle and lower reaches of the Mfoundi may be contributing to the start of the eutrophication process in these sectors. This study showed that epilithic diatoms are sensitive to the variations in water quality of the Mfoundi, and can be used as bioindicators of the health status of urban streams in Cameroon.
Keywords:
- epilithic diatoms,
- specific diversity,
- water quality,
- Mfoundi River,
- equatorial limnology,
- Cameroon
Appendices
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