Experience

Abstract

Freshwater Macrophytes Can Suppress Harmful Algal Blooms Caused by Microcystis

Nicolas Vergara

Harmful Algal Blooms (HABs) are excessive algal growth, such as cyanobacteria, that threaten aquatic ecosystems and public health. Biological controls such as aquatic macrophytes offer long-term and environmentally friendly HAB treatment. The macrophytes Oedogonium foveolatum, Egeria densa, Elodea canadensis, Ludwigia sedoides, and Salvinia minima were tested in this study as potential species to mitigate harmful cyanobacteria. Chl-a concentrations, a proxy for cyanobacterial growth, were measured using a bbe Moldaenke FluoroProbe throughout the experiments to determine how macrophytes affected cyanobacterial growth. A CAAS Cube Automated ELISA System was used to measure microcystin concentrations. All five macrophyte species showed lower final Chl-a concentrations than the control, with O. foveolatum, E. densa, E. canadensis, L. sedoides, and S. minima ending at Chl-a concentrations that were 94.7%, 94.9%, 32.7%, 88.8%, and 74.4% lower than the control, respectively. However, O. foveolatum did not restrict cyanobacterial growth in future experiments. This suggests that these macrophytes used biological mechanisms to outcompete M. flos-aquae effectively. E. densa and E. canadensis treatments tested for microcystins both exhibited decreased total microcystin concentrations, but only E. canadensis decreased the microcystin per cyanobacterial biomass, suggesting it may be more effective at decreasing microcystin production. To test whether allelopathy was the mechanism, aqueous extracts of E. densa and S. minima were prepared and added to M. flos-aquae cultures. The E. densa extract treatment reduced Chl-a concentration by 80.5% compared with the control. This suggests that E. densa contained allelochemicals that strongly inhibited M. flos-aquae. Either extracts or live plants could be applied to a harmful algal bloom, depending on how the plant affects the ecosystem and whether it is invasive. These findings provide future researchers with target species to study further and fully understand the consequences of adding these macrophyte treatments to ecosystems before testing them in field-scale experiments to mitigate harmful algal blooms.