Impactos de técnicas da geo-engenharia sobre as comunidades fitoplanctônica e bentônica

Abstract

Geo-engineering techniques and materials are recognized as promising tools in mitigating and controlling eutrophication in eutrophicated waters. The Floc & Sink (F&S)/Lock (F&L) technique promotes, through the application of coagulants and clays, the removal of phosphorus and algal biomass from the water column, mainly toxic cyanobacteria, through flocculation and sedimentation. However, little is known about the impacts on other phytoplankton species that make up the aquatic biota and are essential for maintaining the ecological health of the aquatic ecosystem; as well as benthic macroinvertebrates, organisms that live in the sediment, which are strongly impacted by the products, enabling changes that can compromise the functional dynamics of the aquatic body. Based on this, this thesis aimed to evaluate the potential impacts of geo-engineering materials on phytoplankton and benthic communities in eutrophic waters from a eutrophic reservoir in the tropical semi-arid region. Based on this, two specific objectives were outlined that structure the two chapters of this thesis. In the first chapter, the impacts of applying the F&S technique were investigated, through a bench-scale experiment with water from the Acauã Reservoir, through the addition of coagulants aluminum sulfate (SUL), polyaluminum chloride (PAC) and chitosan (CHI), isolated and combined with lanthanum-modified bentonite (LMB) and natural bentonite (BEN) on the phytoplankton community. The results showed that the technique caused significant effects on the structure of the phytoplankton community. SUL and PAC alone and combined with LMB effectively sedimented cells regardless of the presence of buoyancy mechanisms, such as aerotopes and flagella. On the other hand, the natural coagulant and clay, CHI and BEN, isolated and combined were not effective in removing species with such mechanisms, and also caused the formation of cell aggregates suspended in the water column, which may represent concern regarding to the possible reestablishment of the remaining species in the water column. Furthermore, species essential to the ecological health of the system were removed from the water column, potentially causing important ecological implications for the functional dynamics of the aquatic ecosystem. In the second chapter, using the F&L technique, addition of LMB and PAC, isolated and combined, the potential ecotoxicological effects of cyanotoxins and changes in the enzymatic activity of Melanoides tuberculata snails (Müller, 1774) and Chironomus sancticaroli larvae (Strixino and Strixino, 1981), two benthic species found abundantly in eutrophic reservoirs in the semi-arid region. Our findings revealed that PAC alone and combined with LMB, regardless of the species present, induced an increase in cylindrospermopsin concentrations in the water column, while the presence of snails favored the reduction of saxitoxins in the water. High concentrations of saxitoxins and cylindrospermopsins were found in both species in treatments with LMB, while the other products provided lower concentrations in snails. The products reduced cholinesterase activity (in vivo tests) in animals. PAC inhibited (in vitro tests) the activity of this enzyme in snails by more than half, and on the other hand, also combined with clay, it caused activation of the BChE and trypsin enzymes. These enzymatic changes and the bioaccumulative effects of cyanotoxins in benthic species reflect more accurately the potential ecotoxicological impacts of the products of the technique on aquatic communities essential for maintaining the ecological health of the aquatic ecosystem. This allows the development, in the field of recovery of aquatic ecosystems, plans to control and mitigate eutrophication in water bodies in the semiarid region.