Apports En Carbone Et Azote Dans Le Fleuve Niger À Tondibia (Niamey) : Résultats De Deux (2) Ans D’observations

Abstract

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.