Avaliação dos efeitos da aplicação de técnicas para mitigação de florações de cianobactérias na liberação e destino de cianotoxinas em dois sistemas eutróficos tropicais

Abstract

As a result of intense eutrophication, frequent blooms of potentially toxic cyanobacteria affect the water quality in continental and coastal aquatic systems. In this study, using a deep eutrophic reservoir and a hypertrophic shallow coastal lagoon, both tropical aquatic systems, and scenarios of recurrent cyanobacterial bloom events as models, we investigated the occurrence and diversity of microcystins, other cyanopeptides, and their potential producers as well as flocculation and sedimentation techniques aiming for efficient and safe removal of cyanobacterial biomass. Through LC-MS analyses, we identified compounds belonging to microcystins, aeruginosins, microginins, cianopeptolins, and microviridins classes in both environments, many of which exhibit seasonality between dry and rainy seasons. During the study, Microcystis aeruginosa, Dolichospermum circinalis, and Planktothrix isothrix were the most representative potential microcystins producers. In biomass removal tests using the Floc & Sink technique, we employed combinations of polyaluminum chloride (PAC) as a flocculant, lanthanum Phoslock® (LMB) alone or combined with local red soil (LRS) as ballast, and the algaecide hydrogen peroxide (H2O2). In the reservoir, testing the PAC+LMB and PAC+LMB+LRS combinations, we obtained similar results for cyanobacteria removal between microcosms and mesocosms, with removal rates of 92-100% of biomass and 94-100% of intracellular microcystins. No signs of cell lysis were observed in any of these experiments. In tests where extracellular microcystins (MCs) were measured, a significant reduction in toxin concentrations was recorded due to the ballast used. In the lagoon, the results of biomass removal were less pronounced in the microcosm, ranging from 78-85%. The integrated Kill, Floc & Sink method achieved the best removal result, where H2O2 was applied prior to PAC and LMB. In this experiment, we did not identify dissolved MCs in the water, either before or after treatments. Furthermore, the tested treatments reduced intracellular MC concentrations below the detection limit. In mesocosm tests, both variations of treatments removed similar amounts of biomass, ranging from 92-97%, and intracellular MCs with a 95-100% reduction, with no MCs release into the water column. At the end of the experiments, we concluded that flocculation and sedimentation techniques effectively remove cyanobacterial biomass and MCs, with a low risk of cell lysis. However, their effectiveness depends on the appropriate combination of flocculant and ballast, their respective dosages, intrinsic characteristics of the environment (e.g., depth, pH), and, finally, the morphophysiological characteristics of the dominant cyanobacterial species.