Phytoplancton du lac municipal de Yaoundé (Cameroun) : Succession écologique et structure des peuplements

L'évolution spatio-temporelle, qualitative et quantitative du peuplement phytoplanctonique a été suivie au lac municipal de Yaoundé. Pour cela des prélèvements hebdomadaires ont été effectués de novembre 1996 à décembre 1997, au niveau d'une station de la zone centrale du lac.Trois descripteurs (indice de diversité spécifique, régularité et diagrammes rangs-fréquences) ont été utilisés pour l'analyse de la dynamique de la structure du peuplement. L'indice de diversité spécifique obtenu à partir des biomasses spécifiques, par la formule dérivée de celle de SHANNON et WEAVER (1949), est compris entre 0,68 et 4,64 bits/µg. La régularité (PIELOU, 1966) varie de 0,14 à 0,84; les faibles valeurs correspondant dans l'ensemble à la présence des espèces fortement dominantes. Les profils des diagrammes rangs-fréquences, établis en coordonnées log-log, associés aux faibles valeurs de la diversité, sont essentiellement caractéristiques des stades pionniers (1 et 1') de l'évolution d'un lac tel que décrits par MARGALEF (1967) et FRONTIER (1976). Le stade 2 est rare alors que le stade 3 est absent.Une analyse simultanée basée sur les variations spatio-temporelles de la densité cellulaire et de l'indice de diversité spécifique, sur les valeurs de la régularité, et sur les profils des diagrammes rangs-fréquences révèle le caractère immature permanent des populations phytoplanctoniques inféodées à ce biotope, et caractérise ainsi un milieu eutrophe à hypereutrophe où aucun stade d'équilibre n'est atteint. La richesse du milieu en éléments biogènes et sa faible profondeur (Zmax=4,3 m) sont à l'origine de sa vulnérabilité. L'événement susceptible d'interrompre la succession des populations s'est avéré être le brassage des eaux, provoqué aussi bien par les pluies que les vents dont l'énergie cinétique est fréquemment suffisante pour entraîner un brassage complet d'une colonne d'eau de hauteur aussi faible.The Yaounde municipal lake (3°52'N, 11°32'E) is a shallow hypertrophic water body (Z max=4.3 m), situated downstream from Yaounde. It is an artificial lake resulting from a dam constructed in 1951 across a small river named Mingoa. Like many other water bodies in urbanised areas, this ecosystem switched rapidly from being mesotrophic to hypertrophic during the 1980's, due to the expansion of human activity and an inefficient treatment of wastewater in its catchment area. This study was carried out with the objective to analyse the ecological succession and the spatio-temporal fluctuations of the phytoplankton population structure in this shallow equatorial lake. Samples were collected at weekly intervals from November 1996 to December 1997, at a station near its central zone (depth=3.5 m), using an opaque horizontal PVC Van Dorn sampler. Sampling operations were conducted at the following levels: surface; -0.5 m; -1 m; -1.5 m and -2.5 m.Individual phytoplankton from 1% iodine acid preserved subsamples (BOURRELLY, 1990) were enumerated with an inverted microscope (Olympus CK2) at 200X magnification, using a Sedgwick-Rafter counting cell, according to UTHERMÖHL (1958). At least 400 individuals (colonies, bundles or trichomes, depending on existing life forms) were counted on each subsample. Specific biomass was then obtained using the biovolume method. To evaluate the population structure dynamics, the SHANNON and WEAVER (1949) specific diversity index method and the evenness (PIELOU, 1966) method were computerized using specific biomass, the evenness being the ratio between the real and the maximal diversity (log2 S, where S is the number of species). Also, rank-frequency diagrams were carried out in logarithmic co-ordinates according to FRONTIER (1969), using integrated samples obtained from grab samples collected over the entire water column.Cell densities were highest in the uppermost meter of the water column at the beginning of the rainy season, due to the exponential development of species such as Planktothrix mougeotii, Chlamydomonas sp., Eudorina elegans, Euglena gracilis, Gonyostomum semen, Euglena texta, Phacus helicoïdes, Closterium limneticum, Cryptomonas ovata, Peridinium cf. gutwinskii, Trachelomonas hispida var. crenulatocollis and Cyclotella meneghiniana. The specific diversity index varied 0.68 and 4.64 bits/µg, and values gradually decreased from the first uppermost meter to the bottom of the water column. Evenness varied from 0.14 to 0.84 with a somewhat similar spatio-temporal variation as the diversity index. Low values of these descriptors generally correspond to the presence of a highly dominant species. The rank-frequency diagrams were mainly S-shaped, indicating the predominance of one species which in this study was either Chlamydomonas sp or Planktothrix mougeotii.A simultaneous analysis of the ecological succession and the spatio-temporal variations of the phytoplanktonic structure revealed that the functioning of this ecosystem was essentially allogenic in nature and was strongly influenced by the supply of decayed organic matter from the Mingoa stream, and effluents from ministerial buildings and hotels located near the lake. This permanent supply of abundant biogenic elements, coupled with the low depth of the ecosystem, makes it vulnerable. Ecological succession, frequently limited to the pioneer stages (1 and 1'), reflected the briefness of the different phases of phytoplankton growth, and the consequences of disturbances regularly undergone by the phytoplankton community. The second stage was scarcely reached while the third was absent. These successions were mainly controlled by rainfall, as well as wind, which is responsible for the mixing of such a shallow water column. These results can be considered important for the Yaounde municipal lake and other water bodies close to densely populated areas in Cameroon for which there is no management plan. Urgent actions need to be carried out to rehabilitate this ecosystem that fluctuates between the eutrophic and hypertrophic status

Composition et distribution spatio-temporelle des protozoaires ciliés dans un petit lac hypereutrophe du Cameroun (Afrique centrale)

La composition et la distribution spatio-temporelle des ciliés du Lac Municipal de Yaoundé (Cameroun) ont été analysées d’avril à décembre 1997, en relation avec des paramètres physico-chimiques du milieu. Trois stations pélagiques et l’herbier littoral ont été échantillonnés. 58 espèces de protistes ciliés ont pu être identifiées, parmi lesquelles environ 60 % d’espèces littorales et périphytiques et 37 % d’espèces pélagiques. Les espèces les plus courantes sont les espèces pélagiques Uronema nigricans et Coleps hirtus. L’abondance saisonnière des ciliés a atteint une valeur maximale de 21 800 ind•L-1, les valeurs les plus élevées étant enregistrées dans la couche trophogène du lac, particulièrement au cours de la grande saison des pluies. Les variations de la biomasse totale des ciliés sont plus faibles, avec des maxima correspondant également aux fortes pluies. Les espèces dominantes en matière de biomasse sont Prorodon africanum, Coleps hirtus et Uronema nigricans. Enfin, les fluctuations saisonnières de l’abondance et de la biomasse des ciliés sont discutées en relation avec les paramètres environnementaux, parmi lesquels le régime pluvial tient apparemment un rôle central.The Municipal Lake of Yaoundé (3° 51’ 37’’ N and 11° 30’ 40’’ E) is a shallow water body (Zm = 4.3 m), situated in downtown Yaoundé, the political capital of Cameroon (Central Africa). The composition and the spatio-temporal distribution of ciliates in combination with physical and chemical water variables were measured in this lake from April to December 1997. Sampling was carried out weekly, at 5 depths (surface, 0.5 m, 1 m, 1.5 m and 2.5 m) and at 3 pelagic stations, A, B and C, representing the upper, middle and the lower parts of the lake, using a Van Dorn sampling bottle. In the littoral station, the periphytic fauna is collected by agitation of the vegetation in a bucket and filtration of water. The water temperature was measured with a thermometer and the pH was measured in the field with a portable pH-meter. The oxygen concentration was determined according to the Winkler method and the transparency of the water column was determined with a Secchi disk. Light availability was measured with a luxmeter. Water colour, suspended solids and ammonium-nitrogen (NH4-N) were analyzed spectrophotometrically using APHA methods. Ciliate species were identified using appropriate and specific keys and were counted under a stereomicroscope at magnifications of 250X and 500X.Results showed that the lake was hypereutrophic. Throughout the study, the temperature and pH values of the water remained around 25°C and 7, respectively. The dissolved oxygen concentration decreased with depth to almost zero at the lake bottom, while the Secchi disk transparency did not exceed 1 m. Spatio-temporal variations of these variables indicated that the middle zone (0.5 to 1.5 m) was the stratum with a critical role in the functioning of the lake. Indeed, it represented the transitional zone between the trophogenic upper layer where photosynthetic activity occurred and the bottom water layer, formed by the microaerophilic to anaerobic tropholitic layers where reducing processes were important.Fifty-eight ciliate species, belonging to 10 orders and 28 families, were identified among which five (Holophrya sp., Lagynophrya rostrata, Lagynophrya simplex, Pseudoprorodon sp. and Histriculus sp.) were new records for Cameroon fauna. This population contained 62% of littoral species and 37% of pelagic species respectively. Within these taxa, dominant species were Uronema nigricans (present at all pelagic stations) and Coleps hirtus (present at more than 97% of the pelagic stations). Their abundance was greater than 2,000 ind•L-1 during the study period showing variation with many peaks. Prorodon africanum occurred sporadically between October and November when rainfall decreased and their densities reached 5,700 ind•L-1 in November at 0.5 m depth at station A. After November, this species was replaced by Prorodon sp. Most of the species collected were cosmopolite or characteristic of shallow tropical water bodies.The total abundance of the ciliate community was high reaching 21,800 ind•L-1 at station A, at 0.5 m depth. According to several authors, the abundance of ciliates in such tropical water bodies varies from 3.6 to 9.75 x 104 ind•L-1. The highest abundances were observed at depths of 0.5 m to 1.5 m at all 3 stations, especially during the rainy season. This corroborated the importance and the role of this stratum in the functioning of Lake Yaoundé. The lowest abundance of ciliates was observed at station A, at the bottom (2.5 m depth). The total abundance of ciliates showed one or several peaks during the period from July to November. With respect to the total biomass, the variation was more regular with generally one peak during the rainy season. At certain depths at stations B and C, several peaks were observed during the period of April to June and from September to November. The total biomass values obtained were higher than those reported by other authors. This can be explained by the fact that reported values in these latter studies were expressed as dry weight while values reported in this study were expressed as wet weight. With respect to biomass, the dominant species was Prorodon africanum (5,300 µg•L-1), followed by Coleps hirtus (3,800 µg•L-1) and Uronema nigricans (3,100 µg•L-1). The highest biomass was 130 x 105 µg•L-1, and was observed at station A, at 0.5 m depth.Generally large size ciliates were more abundant during the dry season whereas small size species were abundant during rainy season. Chilodonella uncinata and Prorodon africanum grew in surface layers (between the surface and 1 m depth) while Paradileptus conicus and Uronema nigricans preferred deep water (1 to 2.5 m depth). There was a great development of ciliates during the transitional period between the rainy and dry seasons. Moreover there was a close relationship between environmental parameters and the ciliate community. In fact, 2 to 3 physico-chemical predictors (dissolved oxygen, temperature and conductivity or pH) explained spatio-temporal distributions of different species. The variation of the total biomass in station A was explained by the dissolved oxygen concentration (r2 = 0.366; p<0.001) and by the pH (r2 = 0.274; p<0.001); at stations B and C, the variation was explained by ammonium-nitrogen (r2 = 0.178 and r2 = 0.294 for p<0.01). The most important abiotic factor that influenced the density and biomass variation of ciliate communities of Lake Yaoundé was the precipitation rate. Throughout the study, station B was the most characteristic of the structure of the ciliate community of this lake

Variations spatiales et temporelles de la richesse et de l'abondance des rotifères (Brachionidae et Trichocercidae) et des cladocères dans un petit lac artificiel eutrophe situé en zone tropicale

Les communautés de rotifères (Brachionidae et Trichocercidae) et de cladocères peuplant les eaux du lac Municipal de Yaoundé situé en zone tropicale (Cameroun), ont été examinés en relation avec quelques variables physico-chimiques du milieu. Des 26 espèces et sous-espèces de rotifères (Brachionidae et Trichocercidae) et 15 espèces de cladocères inventoriées, environ 16 sont identifiées pour la première fois au Cameroun. Parmi les espèces quantitativement dominantes, les espèces fréquemment rencontrées sont Brachionus angularis angularis, B. falcatus falcatus, et B. calyciflorus chez les Brachionidae, Trichocerca elongata elongata, et T. bicristata bicristata chez les Trichocercidae, et Ceriodaphnia cornuta et Chydorus eurynotus chez les cladocères. Plus de 75% des espèces répertoriées sont des espèces périphytiques communément considérées comme littorales, dont le développement important a été observé dans la zone pélagique du lac qui, sans doute, offre de nombreuses ressources et constitue un refuge vis-à-vis notamment de la prédation. L'absence de corrélations entre les communautés zooplanctoniques étudiés et les quelques variables physico-chimiques suivies, nous a amenés à considérer que le développement de ces communautés est sous la dépendance d'autres facteurs. D'ailleurs, une analyse canonique de correspondance indique que les stations, profondeurs et mois de prélèvement expliqueraient respectivement 3%, 5% et 36% de la variance totale associée à l'abondance des espèces prises en compte. Ce qui nous a permis d'émettre l'hypothèse selon laquelle les principales conditions environnementales qui affectent le développement potentiel des espèces analysées et le déroulement de leurs cycles biologiques, seraient dépendantes des deux saisons (la saison des pluies et la saison sèche) caractérisant les climats tropicaux.The rotifer and cladoceran communities of a small tropical eutrophic lake (the municipal lake of Yaoundé, Cameroon) were examined from November 1996 to December 1997, in relation to some physical and chemical variables. Using a Van Dorn bottle, water samples were collected weekly from 5 depths (0 m, 0.5 m, 1 m, 1.5 m and 2.5 m) at three stations (I, II and III) representing the upper, middle and lower parts of the lake, respectively. Water temperature was measured with a thermometer placed in the sampling bottle and pH was measured in the field with a portable pH-meter. Oxygen concentrations were determined according to the Winkler method. Water colour, suspended solids, and ammonium-nitrogen (NH4 -N) were analyzed spectrophotometrically using standard methods. The transparency of the water column was determined with a Secchi disk. Zooplankton species were identified alive under a phase contrast microscope using several manuals, and rotifers and cladocerans were counted in a Dolfuss chamberThroughout the study, temperature and pH values of the water remained around 25°C and 7, respectively. The Secchi disk transparency did not exceed 1 m and the dissolved oxygen concentration decreased with depth to almost zero in the bottom waters. Spatial and temporal variations of these variables indicated that the middle zone (0.5 and 1.5 m) was the stratum with a critical role in the functioning of the lake. Indeed, it represented the transition zone between the trophogenic upper water layer (0 to 0.5 m) where photosynthetic activity occurred and the bottom water layer (1.5 to 2.5 m) formed by the microaerophilic to anaerobic tropholitic layer where reducing processes were important.The rotifer and cladoceran communities were relatively diverse, with 41 species collected during the study year (i.e., 26 species of Brachionidae and Trichocercidae and 15 species of Cladocera), of which 16 are reported for the first time in Cameroon. These organisms filter small organic particles (fresh algal detritus and bacteria) and their presence in the lake could be related to the relatively high algal biomass found there. In terms of relative abundance, rotifers appear to be more important than cladocerans. Among the quantitatively dominant species, the most frequent ones were the Brachionidae (Brachionus angularis angularis, B. falcatus falcatus, B. calyciflorus), the Trichocercidae (Trichocerca elongata elongata and T. bicristata bicristata) and the Cladocera (Ceriodaphnia cornuta and Chydorus eurynotus).Brachionus angularis was the most abundant and frequent rotifer observed during this study. According to several authors, a high abundance of Brachionus can be considered as a biological indicator of more eutrophic waters. This corroborates the hypereutrophic status assigned to the lake through previous studies dealing with bacterioplankton and phytoplankton in this system. Among Cladocera, Daphnia, an important bacterivore, was absent despite the high density of bacteria (~108 cell mL-1) in the lake. Previous studies in other Cameroonian aquatic systems reported the scarcity of Daphnia and these earlier authors argued that it was probably replaced by smaller sized species, which in the case of the present study would be Ceriodaphnia cornuta and Moina micrura, the only representatives of Daphniidae encountered. Up to 75% of the number of species identified were periphytic species that generally develop in the littoral zone. The massive occurrence of these species in the plankton sample of the lake was related to the high particle content. Abundance generally peaked in the middle zone of the water column, likely due to the availability of resources and the general potential for such zones to provide a refuge from predation.However, there did not appear to be any correlation between the rotifer and cladoceran communities and physico-chemical variables when the data set was examined as whole, or grouped by seasons. This suggests that the heterogeneity in species abundances was controlled by other factors, among them sampling points, depths and time of sampling. A multivariate test using canonical correspondence analysis suggested that about 3% and 5% of the variance associated with the species abundance heterogeneity could be explained by sampling point and depth, respectively. Up to 36% of this variance was attributed to the sampling months. We therefore hypothesize that the environmental conditions affecting the development and biological cycles of the studied species are highly dependent on the prevalent seasons found in a tropical zone, i.e. the rainy and the dry seasons

Short-term variations in the abundance and cell volume of bacterioplankton in an artificial tropical lake

International audienceCircadian and spatial fluctuations in bacterioplankton abundance and cell volume were examined, for the first time, in the Municipal Lake located in the down town area of Yaoundé (Capital of Cameroon, Central Africa, ca 3 520 N, 11 310 E). Bacterial cell volumes (range, 0.05 to 0.2 m3) were consistent with those reported for other aquatic systems while bacterial densities (0.8 to 2 108 cells m

Freshwater Microcosms-Based Assessment of Eco-toxicological Effects of a Chemical Effluent from the Pilcam Industry in Cameroon

We studied the acute toxicity of a raw effluent from a battery manufacturing plant (Pilcam) in Douala, Cameroon, to a freshwater fish (Oreochromis niloticus), and subsequently evaluated its sub-acute effects on water quality and the biota in freshwater microscosms. The acute toxicity test was based on 96 hrs static renewal bioassays that resulted in 96-h LC50 and LC90 values of 16 and 20.7% (v/v), respectively. The sub-acute experiments were conducted by exposing several species of aquatic organisms (plankton, macro-invertebrates and mollusks) to lower effluent concentrations [1.6%, 8.0%, 16% (v/v)] for six weeks, and monitoring their survival rates, as well as the physical and chemical characteristics of water. These concentrations were based on 10%, 50%, and 100% of the 96 h - median lethal concentrations (LC50) of the effluent to the freshwater fish, Oreochromis niloticus. Significant effects on functional parameters, such as, chlorophyll-a and total protein could not be demonstrated. However, the activity of alkaline phosphatase was significantly inhibited at all concentrations tested. Phytoplankton, zooplankton, macro-invertebrate communities and snails were negatively affected by the effluent application at concentrations ≥ 8% (v/v), with chlorophyta, ciliates, ostracoda, annelida, planaria and snails being the most sensitive groups. The snails were eliminated after 24 h exposure from microcosms treated with effluent at concentration ≥ 8% (v/v). Effluent exposure also caused significant effects on water quality parameters (DO, pH, hardness, conductivity, color, turbidity, ammonia) in general at concentrations ≥ 8% (v/v). Temperature and alkalinity were not significantly affected. Overall, data from this research indicate that a dilution of the Pilcam effluent down to 1.6% does not provide protection against chronic toxicity to aquatic organisms. Further studies are needed to determine the no observable adverse effect level (NOAEL), as well as a chronic reference concentration for this effluent

Biodiversity and spatial distribution of Rotifera in a shallow hyperuetrophic tropical Lake (Cameroon)

No Abstract.Journal of the Cameroon Academy of Sciences Vol. 6 (3) 2006: pp. 149-16

Transfert des bactéries fécales vers une nappe phréatique à travers une colonne de sol en région équatoriale :influence de la charge en eau appliquée en surface.

L’alimentation des eaux souterraines procède entre autres par des apports d’eau d’infiltration à travers les différents horizons du sol qui séparent la surface du sol du toit de la nappe phréatique. Une étude a été réalisée au laboratoire sur le rôle de la charge d’eau introduite dans une colonne de sol de 1 m de hauteur située au toit de la nappe, dans le transfert à l’eau souterraine, des bactéries indicatrices de pollution de l’eau de boisson. Les charges d’eaux usées de 50 mL, 100 mL et 250 mL ont été appliquées. Les analyses des eaux avant et après percolation ont concerné les coliformes thermotolérants et streptocoques fécaux, pour les paramètres bactériologiques, et le pH, NH4+ et la conductivité électrique, pour les paramètres chimiques. Les résultats révèlent une réduction du nombre de microorganismes dans les eaux qui ont percolé à travers la colonne du sol. Cette réduction est imputable à la rétention de ces cellules par la colonne de sol. À la charge de 50 mL d’eau appliquée au-dessus de la colonne du sol, cette réduction a été de l’ordre de 7 unités logarithmiques pour les coliformes thermotolérants, et de 6 pour les streptocoques fécaux. En appliquant la charge de 250 mL, la réduction a plutôt été de l’ordre de 6 unités logarithmiques pour les coliformes thermotolérants, et de 7 pour les streptocoques fécaux. Cette réduction de la concentration microbienne circulante observée dans les eaux percolées a été de l’ordre de 7 unités logarithmiques pour les deux groupes de bactéries lorsque la charge de 100 mL a été appliquée. À charge d’eau usée élevée, la colonne de sol semble ainsi retenir plus de streptocoques fécaux que de coliformes thermotolérants. Ce comportement de la colonne de sol semble s’inverser lorsque la charge d’eau appliquée est relativement faible. Les éléments chimiques sont également retenus par le sol. Les caractéristiques des eaux qui percolent évoluent dans le temps, montrant que la rétention des polluants des eaux d’infiltration par une colonne de sol est un processus dynamique. Summary Faecal bacteria transfer through a soil column to the groundwater in an equatorial region: influence of the applied surface water load Water percolation through different soil horizons is one of the main mechanisms contributing to the improvement of the microbial quality of ground water. These soil horizons separate the soil surface from the groundwater table. Wastewater often contains chemicals and microbial pollutants, generally at high concentrations. On the other hand, ground water constitutes a major natural resource in most regions of the world. The present study was carried to examine the transfer of bacterial pollutants to the ground water, with the objective of evaluating the influence of the rate at which wastewater percolates through a soil column overlying the groundwater table. The soil column was 25 cm in diameter and one meter high. It was composed of two horizons of different heights with pH values that varied from 4.43 to 4.56. Wastewater percolation tests were carried out with volumes of 50 mL, 100 mL and 250 mL, which were introduced every 30 minutes for each experiment. Chemical analysis was performed for pH, NH4+ and electrical conductivity. Bacteriological analysis was also carried out for thermo-tolerant coliforms and faecal streptococci quantification. These analyses were first carried out on each wastewater sample before introduction into the soil column, and then again after their percolation through the soil column. Results showed that the lapse of time needed to observe the first percolated water drop was longer for low water loads than for the higher water loading rates. On the other hand, the time necessary to collect an adequate volume of percolated water for analysis was shorter for low water loads than for the high water loads. The average volume of percolated water per hour was thus high at low water loads, and relatively low at high water loading rates. A comparison of the microbial characteristics of the introduced wastewater and those of the percolated water showed that the bacterial load in the percolated water was lower. This reduction was due to bacterial retention by the soil column. At an applied load of 50 mL, this reduction was of the order of 7 log units for thermo-tolerant coliforms, and 6 log units for faecal streptococci. When water load of 250 mL was applied, the reduction was of the order of 6 log units for thermo-tolerant coliforms, and 7 log units for faecal streptococci. The reduction was in order of 7 log units for both bacterial groups at an applied wastewater loading of 100 mL. It thus appears that, at high applied wastewater loadings, the soil column retained faecal streptococci better than thermo-tolerant coliforms. The soil column behaviour was reversed when low wastewater loads were applied. It was also noted that for electrical conductivity, a reduction varying from 6,240 to 6,550 µS/cm was obtained in the water leaving the soil column. The concentration of ammonia decreased from 44‑50 mg/L at the entrance to the column to around 1 mg/L at the exit of the column. Average pH values of water percolated through the two soil horizons varied from 5.70 to 7.32, whereas pH values of water introduced into the soil column varied from 7.43 to 8.02. It thus appears that the pH of these two soil horizons strongly influenced the pH of the percolating water that would enter the underlying ground water. The chemical and bacteriological characteristics of percolating water exhibited temporal variations, showing that the retention of pollutants by the soil column was a dynamic process.info:eu-repo/semantics/publishe

Évaluation de l'adsorption de deux bactéries fécales par l'horizon argilo-sableux et l'horizon quartzo-feldspathique d'un sol ferralitique au Cameroun (Afrique Centrale)

Une étude conduite au laboratoire visait à évaluer les capacités de rétention de Streptococcus faecalis et Escherichia coli par deux horizons d'un sol ferralitique :l'horizon superficiel argilo-sableux (HA), en relation avec la surface de sol, et l'horizon profond quartzo-feldspathique (HQ), qui est en relation avec le toit de la nappe phréatique. L'adsorption bactérienne à leurs particules a été rapide au cours des premières 30 min suivant le mélange des cellules bactériennes. Le nombre de cellules adsorbées a subi parfois au cours du temps d'importantes fluctuations. Il ressort des isothermes de Freundlich que le coefficient de linéarité, variant de 0,5139 à 0,8932 pour HA et de 0,0634 à 0,5849 pour HQ, est plus faible lorsque E. coli et S. faecalis sont simultanément présents, et est plus élevé lorsque S. faecalis est présent seul. Le coefficient d'adsorption a varié de 174 à 49,55 x 106 bactéries/mg de sol pour HA, et de 4026 à 13,74 x 104 bactéries/mg de sol pour HQ. Ce coefficient d'adsorption était faible pour HA, lorsque S. faecalis était seul présent, et faible pour HQ, lorsque E. coli était seul présent. En présence des deux espèces bactériennes, les coefficients d'adsorption des deux horizons de sol ont augmenté. ___________________________________________________ A laboratory study has been carried out to assess the retention capacity of Streptococcus faecalis and Escherichia coli by two ferralitic soil horizons: the upper soil horizon (HA), linked to the soil surface and of sandy-clayed texture, and the deeper soil horizon (HQ), related to the groundwater table and of quartzo-feldspathic texture. Bacterial sorption on soil particles swiftly took place during the first 30 min that followed mixture of bacteria to the aqueous medium containing soil. The number of adsorbed bacteria sometimes undergoes temporal fluctuations. From Freundlich isotherms used, the linear coefficient of HA led from 0.5139 to 0.8932 and that of HQ varied from 0.0634 to 0.5849. It was lower when E. coli and S. faecalis were present at the same time, and greater when S. faecalis only was present. The adsorption coefficient of HA varied from 174 to 49.55 x 106 bacteria/mg of soil, and that of HQ varied from 4026 to 13.74 x 104 bacteria/mg of soil. A low adsorption coefficient of HA is seen when S. faecalis only is present, and that of HQ is also low when E. coli only is present. In the presence of the two bacterial species, adsorption coefficients of the two soil horizons rises.info:eu-repo/semantics/publishe

Assessment of Streptococcus faecalis retention capacity by equatorial soil horizons in Cameroon (Central Africa)

L'épuration bactériologique des eaux d'infiltration résulte principalement de l'adsorption des cellules sur les particules du sol. Une étude visant à évaluer les potentialités d'adsorption de S. feacalis surles particules des 8 horizons (H1-H8) de sol, séparant la surface de sol du toit de la nappe phréatique en région équatoriale au Cameroun, a été réalisée au laboratoire. Il en ressort que l'adsorption bactérienne sur les particules de sol, en milieu aqueux, est très rapide au cours des 30 premières minutes d'incubation. Il résulte des isothermes de Freundlich que le coefficient d'adsorption, qui mesure la capacité d'adsorption d'un sol, varie de 3 à 27632 cellules adsorbées.par mg de sol. Aucun gradient de capacité de rétention n'a été observé entre les différents horizons. La rétention est relativement faible sur les horizons H4 et H5 situés au milieu de la colonne de sol, elle est au contraire relativement importante sur l'horizon supérieur de sol H3 ainsi que sur l'horizon profond H7. Le coefficient de linéarité, inversement proportionnel à l'intensité d'adsorption, varie de 0,4483 à 2,2123, les valeurs les plus élevées étant liées aux horizons H2 à H5, et les plus faibles étant liées aux horizons H7 et H8. La variabilité des potentialités de rétention des divers horizons du sol serait imputable aux différences entre les propriétés des divers sites de fixation bactérienne, localisés à la surface des particules de sol.info:eu-repo/semantics/publishe