Eutrophication and climatic changes lead to unprecedented cyanobacterial blooms in a Canadian sub-Arctic landscape

•Cyanobacterial blooms were observed in a sub-Arctic lake in 2013.•Multi-proxy paleolimnological assessment was used to provide long-term context.•Multi-trophic shifts in biota in response to mining, urbanization, and climate.•Eutrophication and climatic changes act concertedly and favor bloom forming conditions.•Eutrophic northern lakes may be vulnerable to cyanobacterial blooms in a warming world.