Harmful algal blooms, produced by cyanobacteria, not only impact water quality, but they also have the potential to produce toxins that can harm humans, pets and wildlife. The blooms are caused by a combination of factors, one major factor being warmer temperatures. 2015 was the warmest year on record. Before that it was 2014. The long-term trend of rising temperatures is likely to further promote the global expansion of cyanobacteria. As copper is commonly used as an algaecide to eradicate cyanobacterial blooms, we investigated the combined effects of copper and the toxic cyanobacterium Microcystis aeruginosa and how these were affected by temperature and food concentration. Our study shows that the interactive effects between copper and harmful algae blooms are not overly dependent on temperature and food concentration. Nevertheless the present study warns against the use of copper-based algaecides and suggests that environmental risk assessment of copper should consider specific situations where harmful M. aeruginosa blooms can co-occur.