Cyanobacteria dynamics and phytoplankton species richness as a measure of waterbody recovery: Response to phosphorus removal treatment in a tropical eutrophic reservoir

Abstract

External nutrient inputs account for the main cause of eutrophication of aquatic systems and intensification of toxic cyanobacteria blooms. But internal loads from sediments significantly contribute to environmental degradation. In-lake treatments may therefore be combined to accelerate the restoration process. A tropical, eutrophic reservoir with persistent cyanobacteria blooms was subjected to applications of a lanthanum-enriched clay (Phoslock®) designed to control internal phosphorus loading. This study aimed to evaluate if the treatment could lead to cyanobacteria reduction and improvement of the phytoplankton community and environmental status. Data were obtained during the application and post-application months, and compared with pre-treatment datasets. Significant decrease in phosphorus concentration occurred, but only in the second treatment year. Cyanobacteria responded after five months of applications, reducing biomass with a 4-fold increase in water transparency, but blooms were able to re-establish. However, by the end of the second year and until the interruption of the treatment, cyanobacteria dominance was significantly reduced and replaced by a more diverse phytoplankton community rarely seen in this environment. At this time, the number of species belonging to groups other than cyanobacteria increased, characterizing more than 80% of the total of species. While pointing to the system resilience and the treatment limitations, this study also indicated that under a long-term control of phosphorus load, lakes could be restored and their aquatic communities recover. Decrease of cyanobacteria biomass and increase in non-cyanobacteria biodiversity were key factors to evaluate the treatment success. The study reinforces the importance of species and communities as sentinels of a healthier ecosystem.