Synergistic Oxidative Stress of Co‐Occurring Cyanobacterial Bloom and Invasive Fish on an Endangered Macrophyte

ABSTRACT In freshwater ecosystems, aquatic macrophytes often have to cope with multiple stressors, in particular those caused by human activities. Cyanobacterial blooms spurred by eutrophication and the introduction of non‐native species can cause significant changes in macrophyte health and stability. Here we examined how the bloom‐forming cyanobacteria Microcystis aeruginosa and the invasive fish Pseudorasbora parva impact the growth and survival of Ottelia acuminata and how these stressors may interact to impair ongoing restoration efforts of this endemic macrophyte. Macrophytes were exposed to each species alone or in combination, and their effects on growth, antioxidant mechanisms, anti‐grazing deterrents, and osmotic regulation were assessed. Exposure to either M. aeruginosa or P. parva increased oxidative damage, stimulated the production of antioxidant enzymes catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD), and increased concentrations of protective compounds tannins and total phenols in O. acuminata leaves. The significantly elevated oxidative damage observed with combined exposure indicated a potential synergistic effect on lipid peroxidation. Glutathione reductase and total flavonoids were also significantly increased in this treatment, with the combined cyanobacteria‐fish exposure producing an exacerbated effect compared to the single treatments. Coinciding stressors of cyanobacterial blooms and invasions by P. parva have the potential to interact synergistically by exacerbating oxidative stress and stimulating both enzymatic (e.g., GR) and nonenzymatic (e.g., flavonoids) stress response components while also stimulating other protective systems, including grazing deterrence and osmotic solute production. Interactions between invasive species and harmful algal blooms, two common stressors across freshwater ecosystems, represent a critical area of study for future restoration efforts for both O. acuminata and other imperilled aquatic macrophytes, allowing stakeholders and researchers to optimise the effectiveness of management plans.