Trophic state of reservoirs in the cascade of a river in the Brazilian semiarid

Abstract

The present study evaluated the influence of the river flow on the trophic state of the reservoir cascade located between the medium and lower São Francisco River, through quaterly samplings between December 2007 and September 2008, at the reservoirs of Sobradinho, Itaparica, Moxotó, Paulo Afonso-I, II, III, Paulo Afonso-IV and Xingó. The variables analysed were inflow and outflow, and water transparency, chlorophyll-a, total phosphorous and reactive soluble phosphorous used to calculate the trophic state indexes. Sobradinho differed from the other reservoirs for all variables and indexes analysed, and was regarded as eutrophic and mesotrophic. Itaparica, Moxotó, Paulo Afonso-I,I I, III and Paulo Afonso-IV reservoirs presented a similar seasonal variation and trophic state varying from mesotrophic to oligotrophic. A water oligotrophication along the reservoir cascade was detected, whose effect was attenuated by river contributions (Pajeú and Moxotó rivers), and effluents from the cities located along the system, mainly Paulo Afonso, in the state of Bahia. Sobradinho reservoir presented the typical trophic condition of a system charger (eutrophic - mesotrophic), while those between Itaparica and PA-IV are subject to the modifications associated with the presence of tributaries and bordering cities, whereas Xingó presents a prevailing oligotrophic condition. Seasonally, only the trophic state of Sobradinho differed during the study period, and the other reservoirs presented a similar pattern between them, and little trophic variation between months. The reservoirs located on the medium and submedium stretches showed a downstream tendency of oligotrophication, depending on their relative position along the cascade, being subject to a seasonal variation of their trophic status according to Sobradinho reservoir outflow, and the operational use of water for energy generation at each power plant along the cascade.