Dinâmica de cianobactérias durante período de estiagem prolongada no semiárido brasileiro e suas implicações para a qualidade da água

Abstract

The distribution and occurrence of cyanobacteria blooms are regulated by environmental and biotic factors, so that the proliferation of different populations is favored by the metabolic versatility and by the structural and enzymatic adaptations that these populations use in order to survive in adverse and limiting conditions. The species belonging to this group differ widely in morphological and functional aspects that affect the production of toxins and the interaction with other organisms. In this sense, the objective of this study was to understand the environmental factors that directly influence the ecological functioning of the reservoirs, as well as the dynamics of cyanobacteria and toxin production during prolonged drought, and to test the adsorption efficiency of natural clay with and without association with poly aluminum chloride and aluminum sulphate in the removal of cyanobacteria cells and the reduction of total phosphorus and chlorophyll a concentrations in eutrophic reservoir waters of the Brazilian Northeast semi-arid region. Thus, in the first chapter, the effect of the dry period on the composition, biomass and dynamics of phytoplankton functional groups was evaluated in order to understand the driving factors of the community during this extreme event. The results indicate that with prolonged drought the factors physical and chemical characteristics of water and the structure of the phytoplankton community undergo changes, reflecting changes in biomass and composition of phytoplankton functional groups. The main guiding factor of the phytoplankton community in the studied environments was the reduction of the accumulated water level, reflecting a change in the resources for phytoplankton - light and nutrients with permanence of the cyanobacteria throughout the sampling period in all systems. In the second chapter, the factors that could influence the proliferation of cyanobacteria and the production of their toxins in tropical reservoirs in the semi-arid region of Brazil were identified. The results indicated that the high availability of nutrients (total P) associated with the reduction of the accumulated volumes in the reservoirs favored the dominance of toxic cyanobacteria, as well as they were correlated with the concentrations of toxins registered in the environments. In the 3rd chapter the efficiency of the application of poly aluminum chloride and aluminum sulfate, with and without association with natural clay, in the removal of concentrations of total phosphorus and chlorophyll a and cyanobacteria cells was tested. The combined application of aluminum sulphate and Poly Aluminum Chloride with natural clay (bentonite) functioned as an efficient technique in the removal of concentrations of total phosphorus and chlorophyll a, in addition to reducing the density of cyanobacteria. The reduction in the number of cyanobacteria cells with application of bentonite and associated coagulants was promising. The results of this study allowed to increase the perception of the effect of prolonged drought on the dynamics of cyanobacteria and toxin production, besides proposing an alternative for the removal of cyanobacteria cells in waters used for public supply. The high retention time of water related to the reduction of the stored volume, favored the growth of cyanobacter ia. More turbid and eutrophic environmental conditions have become ideal for the emergence of potentially toxic cyanobacteria. Thus, extreme drought phenomena, through volume reduction, negatively affect the water quality of reservoirs in semi-arid regions.