Abstract
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.
Highlights
The Brazilian semi-arid region is characterized by a peculiar hydrological regime, prolonged droughts, alternated by short periods of rain, with low annual volume (~400mm), high irradiation and evaporation rate, long residence time (Arfi, 2003; Burford and O’Donohue, 2006; Romo et al, 2013)
Studies on the ecology of shallow tropical lakes in the semi-arid region have clearly shown that such climatic conditions favor the increase of turbidity, nutrient concentration and increase of algal biomass, which associated with anthropic pollution causes a decrease in water quality (Naselli-Flores, 2003) and the establishment of a permanent scenario of eutrophication in reservoirs, which may promote a dominance of bloom-forming potentially toxic cyanobacteria (Chorus and Bartram, 1999; Romo et al, 2013)
Semi-arid regions are characterized by prolonged drought and a short period of rainfall, which, together with multiple uses, leads to a reduction in water volume and an increase in water residence time, in reservoirs, due to the need to maintain a minimum flow to guarantee uses in periods of extreme scarcity
Summary
The Brazilian semi-arid region is characterized by a peculiar hydrological regime, prolonged droughts, alternated by short periods of rain, with low annual volume (~400mm), high irradiation and evaporation rate, long residence time (Arfi, 2003; Burford and O’Donohue, 2006; Romo et al, 2013). The description and analysis of the limnological scenario, as well as the availability of phytoplankton nutrients, such as phosphorus and nitrogen, and attributes contribute to a better environmental ecological diagnosis of the ecosystem, besides identifying the conditions that determine water quality (Pinto and Becker, 2014). From this knowledge it is possible to draw a profile of an optimal environment for the blooms in an aquatic environment
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