Effects of the Floc & Sink technique on the biomass and composition of phytoplankton morpho-functional groups using natural ballasts

The different characteristics of phytoplankton species, such as morphology and the mechanism of resistance to sedimentation, can impact the effectiveness of the Floc & Sink (F&S) technique. In this study, we aimed to analyze the effects of the F&S on biomass and phytoplankton composition, in eutrophic waters from Brazilian semiarid. We applied a chemical coagulant (polyaluminum chloride—PAC), isolated and in combination with natural ballasts (planosol, beige lime and white lime). To assess adaptive strategies, such as positive buoyancy, we applied the morphology-based functional groups (MBFG) approach. The technique effects on the phytoplankton biomass and composition varied according to the sedimentation resistance mechanism. Group IV, lacking specialized characteristics, sedimented in all treatments. The presence of a mucilage sheath and aerotopes prevented its sedimentation in all treatments, and when Microcystis aeruginosa was not present, Group VII sedimented after all treatments. Neither Group V (flagellates) and VI (small diatoms) exhibited sedimentation. Also, filamentous cyanobacteria (Group VIII) demonstrated enhanced resistance to sedimentation and only sedimented when there was a combination of PAC + natural ballasts. Furthermore, employing natural ballast materials provides a cost-effective alternative for removing algal biomass, being observed a drop of 70–80%. Thus, the combination of PAC + natural ballasts was more suitable to remove biomass than PAC alone.