Stygofaune en zones industrielles et non industrielles de la ville de Douala (Cameroun) : Impact de quelques facteurs environnementaux

Abstract

A study aimed to determine the influence of urbanization and industrial discharges on the physicochemical quality of groundwater and on the structure of communities of some periurban, urban and industrial groundwater was conducted in the city of Douala during the period from November 2018 to February 2020. Forty-one (41) wells were selected in five (05) sites, including ten (10) in the PK 21 peri-urban site, three (03) in Ndogbong, three (03) in Cité SIC, fifteen (15) in the Bassa industrial zone (ZIBA) and ten (10) others in the Bonabéri industrial zone (ZIBO). The physicochemical parameters of the water considered were analysed according to the classical methods and the fauna was collected using a phreatobiological net. Morphological and biomolecular analyses were performed on stygobitic organisms, dissected and filmed using a coded stereomicroscope with a Leica Microsystems Model M205 C camera. The drawings were made using Adobe Illustrator CC 2019 and a Wacom Intuos Pro Medium graphics tablet. Analysis of the abiotic variables showed that the well water in the peri urban area (PK 21) is of good ecological quality, as it is well oxygenated (51 à 88%), has an average mineralization (333 µS/cm < Conductivity < 666 µS/cm) and a low organic and metallic pollution. In urban and industrialized urban sites, on the other hand, the groundwater is heavily polluted and generally has high levels of heavy metals. Indeed, these urban sites are subjected to domestic, municipal and industrial pollution. Hierarchical (HCA), Principal Component (PCA) and Multivariate Permutational Classification analyses of variances (PERMANOVA) based on the average values of the physicochemical parameters showed that there is a pollution gradient for the different sites studied. The peri urban site of PK 21 is the least polluted, those of Ndogbong, Cité SIC and ZIBA are moderately polluted, while the site of ZIBO is the most polluted. Biologically, 4759 organisms were recorded in the five study sites. These organisms are divided into four (04) phyla (Annelids, Arthropods, Molluscs and Nemathelminthes), six (06) classes (Achaetes, Oligochaetes, Crustacea, Insects, Gastropods and Gordiacea), fourteen (14) orders and thirty (30) families. The fauna of the PK 21 peri urban site is diversified with 196 organisms collected belonging to 15 (fifteen) families, four (04) classes and three (03) phyla. One stygobitic taxon belong to Stenasellidae family, was sampled. At the Ndogbong site, the fauna is not very diversified with 615 organisms collected, belonging to thirteen (13) families, five (05) classes and three (03) phyla. The fauna of Cité SIC is poor and diversified with only 138 organisms collected belonging to sixteen (16) families, three (03) classes and two (02) xxxii phyla. The Bassa Industrial Zone (ZIBA) site has a rich and moderately diversified fauna. A total of 2368 organisms were collected, distributed in seventeen (17) families, five (05) classes and four (04) phyla. The industrial zone of Bonaberi (ZIBO) is richer and more diversified with 1442 organisms recorded and distributed in twenty-five (25) families, six (06) classes and four (04) phyla. In these urban industrialized sites, the fauna is largely dominated by pollutant resistant taxa such as Chironomidae, Lumbriculidae, Cyprididae, Cyclopidae, Culicidae, Psychodidae and Stratiomyidae. Furthermore, the high abundance of Molluscs (Lymnaeidae and Physidae) intermediate hosts of Trematodes and Schistosomes in the industrial sites (ZIBA and ZIBO), constitute a potential risk of helminthiasis emergence. Also, the poor physicochemical quality of groundwater in urban industrialized sites and the high levels of metallic elements constitute a health risk factor for the neighbouring populations. The peri-urban site of PK 21 hosts stygobite organisms of the Stenasellidae family. A total of 35 stygobite organisms (i.e. about 18% of the fauna of this site) were recorded in eight (08) of the ten (10) wells. Morphological and biomolecular investigations showed that these organisms belong to the genus Metastenasellus and are clearly different from all other described and/or sequenced species of the genus. The groundwater of Douala PK 21 therefore hosts a new species, Metastenasellus boutini, the second species of the genus reported in Cameroon after the discovery of the species Metastenasellus camerounensis. The results of the distLM showed that the environmental factors controlling the distribution of the underground aquatic fauna in the city of Douala are mainly Altitude, pH, dissolved CO2, Turbidity and Magnesium. Temperature, alkalinity, oxidability, nutrients (Nitrite, Nitrate and Ammonium) and COD also influence the distribution of this fauna, but to a lesser extent. The distance-base Redundancy Alalysis (dbRDA) reveals that only 51.5% of the total variation in environmental parameters explains 17.7% of the total variation in recorded stygofauna.