Freshwater bivalve shell oxygen and carbon stable isotope ratios (δ 18 O, δ 13 C) may act as recorders of hydroclimate (e.g., precipitation−evaporation balance, discharge) and aquatic biogeochemistry. We investigate the potential of these hydroclimate proxies measured along the growth axis of shells collected from the Oubangui River (Bangui, Central African Republic) and the Niger River (Niamey, Niger). Biweekly water samples and in−situ measurements collected over several years, along with daily discharge data from both sites allowed a direct comparison with proxies recorded in the shells. Data from a total of 14 unionid shells, including three species (Chambardia wissmanni, Aspatharia dahomeyensis, and A. chaiziana), confirmed that shells precipitate carbonate in oxygen isotope equilibrium with ambient water. Because water temperature variations were small, shell δ 18 O values (δ 18 O shell ) also accurately record the seasonality and the range observed in water δ 18 O (δ 18 O w ) values when calculated using an average temperature. Calculated δ 18 O w values were in good agreement over the entire record of measured δ 18 O w values, thus δ 18 O shell records can be reliably used to reconstruct past δ 18 O w values. Discharge and δ 18 O w values from both rivers fit a logarithmic relationship, which was used to attempt reconstruction of past hydrological conditions, after calculating δ 18 O w values from δ 18 O shell values.A comparison with measured discharge data suggests that for the two rivers considered, δ 18 O shell data are good proxies for recording discharge conditions during low(er) discharge levels, but that high discharge values cannot be accurately reconstructed due to the large scatter in the discharge−δ 18 O w relationship. Moreover, periods of bivalve shell growth cessation due to high turbidity or air exposure should be taken into account. While δ 13 C values of dissolved inorganic carbon in both rivers showed clear seasonality and correlated well with discharge, most of the shells analyzed did not record these variations adequately, likely due to the complication of vital effects including the variable contribution of metabolic CO 2 . Thus, tropical African unionid δ 18 O shell values can be used to reconstruct δ 18 O w values with high confidence to provide insight on past hydroclimate such as precipitation−evaporation balance and periods of low discharge.

The characteristics of colored dissolved organic matter (CDOM) as well as the concentrations and stable isotope composition (d 13 C) of dissolved organic carbon (DOC) were characterized in several large rivers of Africa including the Congo, Niger, Zambezi, and Ogooué basins. We compared the spatial and temporal patterns of dissolved organic matter (DOM) quantity and quality along with various environmental gradients, including hydrology, river size, catchment vegetation, and connectivity to land. The optical proxies used include the absorption coefficient at 350 nm, the specific ultra-violet absorbance, and the spectral slope ratio (S R = 275-295-nm slope divided by 350-400-nm slope). Our results show that land cover plays a primary role in controlling both DOC concentration and optical properties of DOM in tropical freshwaters. A higher cover of dense forest in the catchment leads to a higher quantity of highly aromatic DOM in the river network, whereas an increasing savannah cover results in lower DOC concentrations and less absorptive DOM. In addition to land cover, the watershed morphology (expressed by the average slope) exerts a strong control on DOC and CDOM in tropical rivers. Our results also show that the percentage of C3 and C4 vegetation cover is not an accurate predictor for DOM and CDOM quality in rivers due to the importance of the spatial distribution of land cover within the drainage network. The comparison of our results with previously published CDOM data in temperate and high-latitude rivers highlights that DOM in tropical freshwaters is generally more aromatic, and shows a higher capacity for absorbing sunlight irradiance.

Objective: Fingerlings of Oreochromis niloticus and tadpoles were exposed to the acute concentrations of aqueous extract of the bark of Balanites aegyptiaca (date palm desert) to determine the 96h-LC50 (lethal concentration 50, concentration that will kill half of the test animals exposed). Methodology and results: The O. niloticus (Nile tilapia) was further subjected to sub-lethal concentrations of the plant extract to determine the effect on growth and haematological indices. The time for toxicity disappearance (TTD) was also estimated. The experiments were conducted using the static renewal bioassay technique at UNAAB Fish Laboratory. The 96h-LC50 for O. niloticus was estimated to be 26.22 mg l-1 while that of Tadpole (Rana species) was13.77 mg l-1. The TTD of B. aegyptiaca was estimated to be 48 hours. During sub-lethal exposure, there was a slight decrease in the mean body weight and all the haematological parameters as the concentration of the toxicant increased except MCV, MCH, MHC that were more or less equal. However, statistical analysis on these values of haematological indices did not show any significant difference at 5%. The use of Balanites aegyptiaca is recommended in control of predators e.g. tadpoles in pond fish culture system because the product is non-toxic to fish at lower concentration, biodegradable and very cheap. The uncontrolled use in open water body for fishing should be prohibited, as the resultant deleterious effects will subsequently lead to death of not only target fish but also other aquatic organism. Hence, contribute to reduction in the biodiversity. Conclusion and application of results: The results of this study show the toxic effect of B. aegyptiaca on O. niloticus and Tadpoles. The lethal doses (96h-LC50) obtained are 26.22 mg l-1 and 13.80 mg l-1 respectively. Therefore, B. aegyptiaca powder can be use in selective eradication of aquatic organism to control unwanted predatory species i.e. tadpoles. B. aegyptiaca bark powder can be recommended because it is biodegradable and leave no adverse effect on environment.

RESUMELe fleuve Niger est le récepteur principal des rejets d'eaux usées de la ville de Niamey, capital du Niger, provenant des activités industrielles, domestiques, hospitalières et agricoles. La quantité des déchets solides produits chaque année est estimée à 273 750 tonnes. Or, les eaux du fleuve sont utilisées entre autres pour l'alimentation en eau potable de Niamey et pour l'agriculture irriguée. Les eaux souterraines qui alimentent le fleuve Niger, se dégradent au fil des années au voisinage de Niamey à cause de l'infiltration des eaux issues de la pollution urbaine ainsi que des latrines et fosses sceptiques. La présente étude vise à évaluer la qualité des eaux du fleuve Niger en se basant sur la physico-chimique et les communautés de macroinvertébrés. 36 taxons de macroinvertébrés ont été récoltés le long du fleuve. La description de ces taxons montre une variation de la structure des communautés caractérisée par une baisse de la richesse taxonomique en aval des points de rejet. La présence des taxons comme, Melania sp., les familles de Syrphidae et Culicidae indique une détérioration de la qualité des eaux due à la matière organique dans certains sites proches des points de rejet. En revanche, la présence de Thraulus sp., Elassoneuria sp., Afronurus sp., Centroptiloides sp., Adenophlebia sp., Dipseudopsis sp. et Neoperla sp., reflète une eau de meilleure qualité dans les sites de référence situés en amont de tous les points de rejet. L'analyse canonique des correspondances révèle que, l'oxygène dissous, le phosphore total, les orthophosphates, le pH et la Demande Chimique en Oxygène (DCO) sont les principaux facteurs qui expliquent de manière significative (P<0,05) la distribution des taxons le long du fleuve Niger à Niamey.

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