The threatened aquatic oasis of the Brazilian semi–arid

Background/Question/Methods In tropical semiarid regions, environmental conditions, including irregular rainfall and high evaporation rates affects substantially the availability of surface water. Therefore, many reservoirs are constructed in these regions with the main purpose of storing water for multiple uses, including human supply, irrigation, recreation and agricultural applications.  Since the drainage basins of the reservoirs must occupy an area much larger than the area of the reservoir itself, this important morphometric characteristic affects several ecological processes in these ecosystems, including concentrations of nutrients, suspended solids and salts,  and eutrophication. In order to investigate the trophic status of some man-made lakes in Northeast Brazil, we conducted a survey (from 2000 to 2006) in six reservoirs ((Armando Ribeiro Gonçalves, Itans, Boqueirão de Parelhas, Gargalheiras, Sabugi, and Passagem das Traíras reservoirs) in the semi-arid region of Rio Grande do Norte State (Northeast Brazil).  

Results/Conclusions The results obtained have shown that an increasing condition of eutrophication occurs in all six studied reservoirs. Algal biomass ranged from 15±12.4 µg.L^-1 to 450±5.4 µg.L^-1, in association with high levels of total N (from 5.0±4.5 mg.L^-1 to 18.0±9.2 mg.L^-1) and total P (from 0.04±0.03 mg.L^-1 to 0.35±0.25 mg.L^-1).  All the reservoirs showed high turbidity (averaging < 30 cm Secchi Disc) and occurrence of cyanobacterial blooms, mainly composed by Cylindrospermopsis raciborskii, Microcystis sp., Aphanizomenon sp., Anabaena circinalis e Planktothrix agardii. The presence of cianotoxins was confirmed in the water of three of the six reservoirs studied (Itans, Passagem das Traíras and Sabugi reservoirs). Ecotoxicological tests revealed high levels of cianotoxins (> 30 mg. L^-1) in the drinking water. The high load of nutrients, elevated period of water retention and high temperatures in the reservoirs influence the water quality and consequently, the multiple uses of water. Increasing water demands from a growing population, economic expansion and rising use is actually not supported by the actual ecological status of these “aquatic oasis”. We need urgently to emphasize the wise use, proper management and protection of the water in the semi-arid region of Northeast Brazil. Sustainable management of the drainage basin must be encouraged to protect and to guarantee water quality in the man-made lakes of Brazilian semi-arid.