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The objective of this study was to characterize the limnological, microystin and phytoplankton community of five tropical eutrophic reservoirs located in the Brazilian northeastern semi-arid region, used for domestic use at the time of extreme drought and reduction of water volume. The study was conducted in July and August 2015, and an integrated sample of the water column was collected at three points near the dam in each reservoir. Analysis of limnological parameters, identification and quantification of phytoplankton, with emphasis on cyanobacteria were performed, as well as detection of microcystin by means of immunoassay (ELISA). The reservoirs presented ~ 90% water volume reduction. High turbidity and concentrations of nitrogen and phosphorus, as well as high cyanobacterial densities, revealed an increase in the eutrophic state for hypereutrophy. The total biovolume of phytoplankton and cyanobacterial density is high, plus an average increase in relation to previous studies of 350% and 150%, respectively. The density of cyanobacteria and microcystin concentration presented values ​​above acceptable levels for drinking water according to Brazilian legislation. A phytoplankton community was represented by 17 functional groups, including potentially toxic cyanobacteria species such as Planktothrix agardhii (S1), Microcystis aeruginosa (M), Anabaena planktonica e Anabaena spp. (H1), Cylindrospermopsis raciborskii (Sn). Our results confirm that conditions of extreme drought and reduction of the volume of the reservoirs influence the composition, biovolume of phytoplankton and water quality, but not the increase of total microcystin in the analysed, although above 1μg-1 registered a significant decrease of water quality in used for human consumption.

Phytoplankton scenario and microcystin in water during extreme drought in semiarid tropical water supplies, Northeastern Brazil