Bio-évaluation de la qualité des eaux de surface à Yaoundé : Rôle des Annélides Oligochètes.

Abstract

In Cameroon, the large metropolies that house the majority of the population had inappropriate waste management. This waste generally ends up in the environment without prior treatment and ends up in surface water bodies, where it is the main cause of water quality degradation. Following the desire that is expressed by urban communities to reclaim these waterways, it is a question of providing the institutions in charge of this mission with reliable, permanent, ecological and economic working tools for the quality of these environments. In this respect, aquatic oligochaetes constitute a group of organisms of choice. The objective of this study is to investigate the composition and structure of oligochaete populations in the surface water of the city of Yaounde, in relation to their quality. Eleven sampling stations were selected on two rivers (R1 to R5), three lakes (L1 to L3) and three swamps (M1 to M3) in the city. The water samples were taken at monthly intervals between March 2016 and February 2017. The physico-chemical characterization of the stations were carried out according to standard methods for the examination of Water and Wastewater. Oligochaetes were collected using a 30 cm square, 100 μm mesh aperture haze net, fixed in 10% formalin, and then preserved in 70% ethanol. The megadriles were examined under a binocular loupe and the microdriles were mounted between slide and coverslip for microscopic observation at 40 and 100 objectives. All sampled stations showed signs of organic water pollution. According to the Organic Pollution Index (OPI) values, the pollution was essentially classified between moderate and heavy states. TSS, color and turbidity values were generally moderate to high at all sampling stations. Oxygen saturation rates below 75% and the slight basicity of the water associated with the above observations confirm the state of ecological pollution imbalance of the environments studied. The Discriminant Factor Analysis (DFA) carried out on the basis of abiotic parameters reveals a clear distinction between the three types of ecosystems. The waters of streams are the most mineralized, alkaline and rich in TDS; those of lakes are the warmest, while those of swamps are the most colored, the most turbid and the richest in Total Suspended Solids (TSS) and nitrate. The average cadmium and mercury levels obtained in the swamps do not meet the requirements for fish water quality. Within each ecosystem type, the stations are distinguished mainly by the salinity level of their water. In the streams, R3, R4 and R5 are significantly more saline than the other stations. The same applies to Municipal Lake (L1), Obili Lake (L2) and Tshinga Swamp (M1) respectively. The faunal list ofoligochaetes collected is made up of 81.8% Naididae, 9.1% Lumbriculidae and 9.1% Lumbricidae. From a total of 5,748 individuals, 11 morphotypes were identified at family, genus or species level. The generaxxvi Branchiura, Limnodrilus and Nais are each represented by 1 morphotype. One Lumbriculidae and Eiseniella sp were also identified at station R3 where they had a rare species status. Pristina spp were ubiquitous at all stations. Branchiura sp., Limodrilus sp and Dero sp 1 to 3 changed status between ubiquitous and accessory depending on the station. The subgenus Dero (allodero) was a rare morphotype characteristic of station R4. Nais sp which had never before been collected in surface waters in Cameroon was sampled at stations R1 and R2 of the Nkie stream. Limnodrilus sp. sometimes showed a bought and shrunken tail as a sign of regeneration. In the environments studied, the loss of the caudal region of this oligochaete could be due either to its cutting by predators, or to its self-amputation in order to get rid of the metals that would have accumulated in it, at the risk of releasing them into nature. Mature individuals of Branchiura sp. and Limnodrilus sp were collected throughout the year, a sign of their total bloom in the environment. The same is true for Dero spp, Pristina spp and Nais sp which showed no signs of sexual maturity during the entire study period, their sexual reproduction was being associated with the onset of unfavorable living conditions. Sörensen's similarity coefficient reveals a very high biological similarity between the ecosystem types. The SFM based on biological parameters confirms this result. The Shannon-Weaver diversity and Pielou equitability indices and the Oligochaete Sediment Bio-indication Index (OSBI) reveal almost similar information, i.e. the poor biological quality of all the stations sampled, despite the better organization of the R2 station. Several factors have influenced the distribution of oligochaetes in the aquatic environments studied. The unsatisfactory oxygenation of the water would have favored the preferential installation of Limnodrilus sp., Branchiura sp. and Dero spp. which are generally presenting morphological and physiological adaptations to anoxic conditions. The availability of organic matter in swamps would have induced the dominance of Branchiura sp. in these environments. Similarly, the dominance of Naidinae and Pristinae in the lakes, as well as at station R2 (> 77.5% of the total population) would be determined by the vegetation. Significant positive correlations (p<0.05) were found between parameters indicating water mineralization and species abundances. The oligochaetes collected preferentially colonized muddy substrates. The seasons acting on the hydrographic regime allowed the development of vegetation, which in turn had an impact on the distribution of oligochaetes in the environment. The comparison of the information generated by the IPO and IOBS, shows a greater severity and stability of the IOBS. This tool would therefore provide more reliable information.