RESUMELes écosystèmes aquatiques constituent une source importante d'émission des gaz à effet de serre très souvent négligés dans l'établissement des bilans globaux dont les principaux gaz sont le CO 2 , le CH 4 Flow of greenhouse gases (CH 4 and N 2 O) in the River Niger upstream from Niamey (Niger) ABSTRACTAquatic ecosystems constitute an important source of greenhouse gases, including CO 2 , CH 4 and N 2 O, emission that is often neglected globally. This study aimed at determining the source of the flux of CH 4 and N 2 O and the assimilation capacity of this by River Niger in Niamey area. From April 2011 to March 2013, samples were taken every two weeks to measure the concentrations of methane (CH 4 ) and nitrogen protoxide (N 2 O) dissolved in the River Niger waters in Tondibia, a small town upstream Niamey city. These concentrations were determined by gas chromatography. Additional measures of physicochemical parameters (temperature, pH, conductivity, dissolved oxygen and alkalinity) were performed. Temperature, conductivity, dissolved oxygen and pH were measured in situ using HANNA HI 9828 Multiparameter. The alkalinity is determined by titration, with 0.1 M hydrochloric acid. Results showed that the River Niger constituted a B. ALHOU et I. BOUKARI / Int. J. Biol. Chem. Sci. 10(5): 2369-2378 2370 potential source of the flux of these gases toward the atmosphere. These fluxes represented 470 µmol/m 2 /day for the protoxide of nitrogen and 3.59 µmol/m 2 /day for methane. They emanated from the degradation of organic matter inputs into the River from the terrestrial environment.

The Niger River is the third more important river in Africa and drains a surface of about 2,120,000 km². It includes six hydrographic regions representing West African ecosystems. Despite the importance of this river at the regional and continental scale, little information has been collected on its biogeochemical characteristics and particularly on its role in the transportation and the transformation of matter (carbon, nitrogen and phosphorus). This study present the results of two years investigation, April 2011 to March 2013 in the middle Niger, upstream Niamey (Niger) city [2.01° E, 13.57° N], according to a bi-weekly observation frequency. The variables measured are temperature, conductivity, dissolved oxygen, pH, suspended matter, dissolved inorganic carbon (C) and nitrogen (N) and particulars organics C and N as well as isotopic composition of these elements. Daily discharges of the river come from hydrologic station measurement of the Niger authority Basin (NBA) at Niamey city. Hydrologic situation was strongly contrasted between the 2 years of survey. The mean discharge of the first year was only 673 m3 s-1 (one of the weakest discharge recorded on the river Niger at Niamey since 1940), while the mean discharge of the second year was 1,096 m3 s-1. Our results show that suspended matter, particular organic carbon, dissolved organic carbon and dissolved inorganic carbon are transported mainly during the local flood, induced by precipitations in July and August. The second flood (Guinean flood) which occurred, during November to January, is characterized by low temperatures and clean waters.