Avaliação da longevidade da remoção de fósforo e biomassa de cianobactérias pela técnica de coagulação e precipitação (“Floc & Lock”) em dois sistemas eutróficos

Abstract

Cyanobacterial blooms is one of the symptoms of eutrophication. The Floc & Lock (F&L) technique has shown to be a promising tool in controlling the internal phosphorus (P) stock, and mitigating the occurrence of cyanobacterial blooms. Although many studies have reported good efficiency from the F&L technique, others reported the necessity of reapplications. This study aimed to evaluate the longevity of F&L treatments in sediment cores at the end of mesocosms experiments, in two tropical eutrophic systems. Mesocosms were performed in a deep reservoir (Funil Reservoir) in the presence and absence of cyanobacterial blooms and in a shallow coastal lagoon (Jacarepaguá Lagoon) in cyanobacterial bloom absence. In addition, the potential of LMB penetration was evaluated in “low density” sediments and with a lot of organic matter, such as the Jacarepaguá Lagoon. At the end of the mesocosm experiments, sediment cores were collected in each mesocosm unit. The sediment cores were incubated in the laboratory for four months and a half, and weekly, pH, dissolved oxygen (DO), chlorophyll-a, RSP (reactive soluble phosphorus) and total phosphorus (TP) were measured. Based on the longevity experiment results of Jacarepaguá Lagoon, the LMB percolation experiment was performed. In a transect in the Jacarepaguá lagoon, sediment cores were collected in order to realize the translocation experiment and granulometric analyzes. The experiment last 35 days and were measured RSP, PT, RSP and pH. In addition, the LMB penetration was visual measured in the sediment, as the lanthanum (La) present at six sediment depths was quantified. F&L treatments were efficient over time in retain P in the sediment regardless of variations in DO and pH in the water for the Funil Reservoir. Therefore, for this system, treatments should focus on the cyanobacterial biomass removal from the water column and can be applied at any time. On the other hand, the F&L treatments for the Jacarepaguá Lagoon lost efficiency over time in retaining the phosphorus in the sediment and preventing the recruitment of cyanobacteria cells. Conditions of anoxia, unconsolidated sediment, with a lot of organic matter, and high decomposition rate, probably influenced the longevity. In the percolation of LMB experiment the clay penetrated quickly into the sediment until 20 cm, regardless of the sediment granulometric composition. However, percolation was more pronounced in the core collected in the area where the mesocosm experiment was performed. The fast penetration of the LMB can promote the release of RSP from the sediment. Considering systems with unconsolidated sediments and with a lot of organic matter like the Jacarepaguá Lagoon, the reapplication of treatments can be considered.