Note relative aux premières observations sur le phytoplancton d'un paysage lagunaire de l'estuaire du fleuve Sénégal (lagune de Dielenbad ou Leybar)

La lagune de Dielenbab, située en première ligne et donc toujours en eau, est l'un des principaux éléments du complexe lagunaire disposé longitudinalement et parallèlement au fleuve Sénégal. L'intensité probable des échanges eau-sédiments liée aux faibles profondeurs et à un régime de turbulances fluctuant, tend à suggérer que ces lagunes seront le siège de remaniements des nutriments importants et de cycles de productivité différents de ceux rencontrés dans l'estuaire. Un suivi en un point fixe proche de l'exutoire de la lagune a été entrepris sur une journée afin de se faire une idée des échanges/estuaires lagune tant au niveau des caractéristiques physico-chimiques que biologiques (biomasse et peuplements phytoplantoniques

Characterization of small reservoirs in Burkina Faso

Small reservoirs (SRs) are used for multiple purposes in Burkina Faso and West African agroecological landscapes (Venot et al. 2012). These SRs are used for domestic (drinking, bathing and washing) and agricultural (crop, livestock and fishery) purposes, among others (McCartney et al. 2012). However, natural processes and human-induced activities in the surrounding landscapes can be a threat to the water quality and sustainable use of SRs. In order to reduce negative effects, several projects have been designed to study different aspects of how SRs are used and impacted by natural and human-induced activities. But the number of SRs in Burkina Faso (about 1,450) (Cecchi et al. 2009) makes it practically impossible to study every single one of them. A possible solution to this is to categorize the SRs into groups in terms of their responses to the aforementioned natural and anthropogenic influences. Such categorization can provide the basis for site selection in subsequent research programs and further permit generalization (upscaling) of results obtained for selected SRs (at local scale) to many other SRs (at national scale)

Valeurs d'usage ou performances techniques : comment apprécier le rôle des petits barrages en Afrique subsaharienne ?

Use-value or performance: Towards a better understanding of small reservoirs in sub- Saharan Africa Small reservoirs are a reality of rural sub-Saharan Africa. They trigger technical and institutional innovations, appear to be in high demand among local communities, and remain popular on the agendas of national policy-makers and international development partners in spite of recurrent analyses highlighting that these systems function well below the expectations of their promoters. This paper proposes an analytical framework to understand this apparent contradiction. Local communities do make use of small reservoirs in many ways but not always as implied by policy discourses and development strategies. Social, eco-technical and managerial analyses would then not disclose the real use-value of these innovations at either the local or the regional (watershed) scales. Understanding the opportunities and risks linked to an intensification of the multiple uses of small reservoirs requires considering them as rural development and planning interventions. They induce changes in the relations that societies nurture with their environment and catalyze new and multiple claims and uses that sometimes appear conflictual and irreconcilable

Acute toxicity tests of two herbicides diuron and atrazine on the beetle Crenitis sp in Volta Basin, Burkina Faso

Acute toxicity tests were performed on Crenitis spp (Coleoptera; Hydrophilidae) using two herbicides, atrazine and diuron in the laboratory. The experiment was to investigate the effect of high doses of these pollutants in individually and together for 12 h each on Crenitis spp a species that abounds during the dry period in the shallow hydro-agricultural waters reservoirs of the Volta Basin. Individual macroinvertebrates were collected from puddle areas of the shoreline of Bama Reservoir in the Volta basin. The dry period is the period of rest for agricultural activities at the reservoir. Tests have shown that the toxic effects of the two herbicides on the species of beetles can be enhanced when the both products act synergistically. For diuron, the effective concentration that immobilizes 50% (EC50) of the insects is 44.96 g/l only, but drops to 11.72 g/l in the mixture; while in the same order, atrazine shows 11.75 g/l only and then drops to 7.33 g/l in synergy. It is concluded from this study that works on ecotoxicology should consider the additive or synergistic effects of herbicides to define the bio-ecological traits of macroinvertebrate species living in frequently polluted hydro-agricultural systems

Spatio-temporal dynamics of population structure for macro invertebrates families in a continuum dam - effluent - river in irrigated system. Volta Basin (Burkina Faso)

A monthly sampling of benthic macro invertebrates was carried out at the hydro-agricultural dam of Boura in the Volta watershed basin (Burkina) during the period of February through July 2012 in order to describe the structuring of insect succession along with changes occurring in habitats of this irrigated dam farming system. The samples of insects were collected from 6 stations located inside the littoral (Station I), the sublittoral (Station II), the sewage channel or effluent (Stations III, IV, V) and the Mouhoun River (Station VI). The sampling method employed is a conventional method by the European Union named ” Directive Cadre sur l’Eau (DCE) ” recommended for the survey of benthic macro invertebrates. The survey reveals a community composed majorly of insects (more than 75%) variously distributed. On one hand at the shoreline and the coastal- adjoining zone in the dam, 23 families of macro invertebrates were identified; mostly belonging to the shoreline except for 10 families identified as endemic to the adjoining zone of the coastline. These two zones of the dam shelter the same malacological fauna consisting of the Unionidae, Lymneidae, Ampullariidae, Planorbidae, Valvatidae and Bulinidae families. The survey reveals otherwise that the differences between the Shannon biological diversity indices for these two zones were more pronounced during the month of July, the rainy period in the basin. On the other hand, concerning the dam-effluent-river continuum, a total of 35 families are sampled: 27 from dam water and stations near the irrigated zones; and 32 from the station of the sewage channel far from the irrigated zone and the river. The identification of individuals belonging to the family of the Baetidae and the Ephemerellidae (order of Ephemeroptera) in the river water highlights a subsequent reconstruction of the biodiversity in the river as the presence of both families is an indicator of fertile water. Further analysis on the spatial and temporal distribution involved 17 families out of the 35 sampled not common to all habitats. This lead to the conclusion that changes in natural habitats dictate the clustering patterns of macro invertebrates populations during the year long

Multi-metal contamination of a calcic cambisol by fallout from a lead-recycling plant

The present study deals with the impact of a lead-recycling plant on metal accumulation in soils, evaluated by a global pedological analysis. This general approach can be used on various contaminated sites to evaluate impact of an anthropogenic activity and inform on metal origin and behavior. A soil profile collected in the vicinity of a lead-recycling plant in operation for 40 years was studied. Correlations between major and trace elements highlighted different patterns of metals according to their origins. Two groups of metals were identified: (i) Pb, Sb, Sn, As, Cu and Zn of anthropogenic origin and (ii) Ni and Cr of natural origin. The results showed that Pb, Sb and Sn presented the highest relative contamination followed by Cu, As and Zn. Moreover, Pb and Sb migrated most along the profile at an estimated rate of 1.5 cm y−1, followed by Sn, then Zn, Cu and finally As. Sequential extractions showed that all metals were mainly solubilized by reduction and therefore estimated to be bound to iron oxides, except lead which was rather in the acid-soluble fraction in the contaminated horizons. Furthermore, high levels of lead were found in water-soluble and exchangeable fractions (4.2 mg kg−1) suggesting the occurrence of lead transfer towards the trophic chain

Phytoplancton et conditions de milieu dans l'estuaire du fleuve Sénégal : effets du barrage de Diama

La dynamique phytoplanctonique dans l'estuaire du fleuve Sénégal est fortement affectée par les lâchers d'eau douce récurrents en provenance du barrage de Diama. La spatialisation des peuplements d'algues est finement déterminée par la spatialisation des divers régimes hydrodynamiques qui animent les masses d'eau. La partie amont de l'estuaire demeure durablement marquée par l'influence continentale et il s'y développe des assemblages spécifiques, caractérisés en particulier par la petite taille des algues. A l'aval, l'influence marine est toujours dominante, notamment au travers des pulsations de l'upwelling côtier qui contribuent à enrichir sensiblement les eaux estuariennes en stimulant plus ou moins directement et rapidement la production phytoplanctonique. La superposition et les interactions à court terme des processus physiques horizontaux (apports continentaux, circulations liées à la marée) et verticaux (diffusion verticale turbulente, stratification) sont décrite