Marine heatwaves and invasive tunicate fouling increasingly co-occur in mussel aquaculture, yet their combined effects on rope-level performance and plankton dynamics remain unclear. A 9-day field-based mesocosm experiment in Georgetown Harbour (Prince Edward Island, Canada) examined the independent and interactive effects of heatwaves (~4.5 °C above ambient) and tunicates on 50 cm sections of Mytilus edulis culture rope. Oxyg...