Communautés de diatomées épilithiques du réseau hydrographique de la Mefou: Composition, structure, variations spatio-temporelles et biosurveillance des cours d’eau en zone tropical

Abstract

Diatom communities are known as suitable biological model for the study of the water quality of lotic hydrosystems. In this work, physico-chemical characteristics of the water and the structure of epilithic diatom communities were followed from February 2008 to February 2009 in order to assess the health status of 12 streams draining Yaoundé township and its suburb environment. To allow similar studies in streams where stones are scarce or difficult to access, the colonization patterns of glass slides by these microalgae was evaluated in Mfoundi and Nga, from July to August 2010. The physico-chemical and biological data were collected on 38 stations distributed in these river systems. Characteristics of diatom communities (species richness, diversity index, equitability, growth forms, relative abundance, diatom indices, dry weight and ash free-dry-weight) were compared between urban and rural streams in order to assess their sensitivity to changes in water quality. The physico-chemical results revealed a significant organic pollution of urban streams, particularly in their middle and lower stretches, because of the inputs from domestic sewage and industrial effluents highly loaded by organic matters and dissolved salts. In rainy season, runoff waters accentuate this pollution by the significant supply of pollutants. Compared to the urban streams, the waters of Nga and Mefou presented low to very low values of the most physico-chemical variables analyzed, characteristics of the benchmark streams. The diatom flora collected is diverse, with 435 species and varieties whose existence was hitherto unsuspected. The species composition and community structure varied according to stations and seasons, relative abundance of species was linked with the geological nature of watersheds and to human activities. A biotypology of stations enabled to separate those located in urban areas where proliferate species known to be resistant to organic pollution and high mineralized waters, and those in the rural areas and the upper stretch of some urban streams, dominated by oligosaprobous, N-autotrophic sensitive and polyoxybiontic species. In rainy season, the prostrate taxa were abundant in Nga and Mefou while erect and mobile taxa, more competitive for food resources, showed mass development in Mfoundi and its tributaries. The diversity metrics, although variable from one stream to another, did not reflect the full extent of the impairment of the water quality. However, 4 diatom indices (SPI, BDI, GDI and EPDI) have been successfully tested and revealed a poor to bad water quality (3.6 to 10.9) of Mfoundi and its tributaries, and the high to good water quality (13.5 to 19.8) of Nga and Mefou.The experiment using glass slides showed that they are well suited to study the stream epilithon of Yaoundé region, and also confirmed that the Mfoundi’s waters are more productive than those of Nga. This study suggests that the structure of diatom communities and diatom indices can be used to assess the health status of streams in Cameroon.