Cyanobacteria bloom suppression by quagga mussels disappears with global warming

Climate warming and biological invasions are global stressors that jointly influence harmful algal blooms (HABs) in freshwaters. Quagga mussels (Dreissena rostriformis bugensis), a major invasive species in the Northern Hemisphere, may suppress HABs, but this effect disappears above critical temperatures. The combined effects of these stressors remain poorly quantified. We used 10 years of monitoring data from a temperate lake, before and after quagga mussel invasion, to calibrate, parametrize and modify the ecosystem model PCLake+ and simulate combined effects of mussel invasions and warming on HABs. We tested whether (1) incorporating mussels into PCLake+ improves model performance and captures mussel-mediated HAB suppression, (2) suppression is lost under future climate projections, and (3) strong nutrient reduction is required to compensate. Including mussels substantially improved post-invasion model performance, reducing RMSE by 25% for chlorophyll a and 67% for cyanobacteria biomass. After invasion, summer cyanochlorophyll concentrations declined by 40% under ambient conditions. Under moderate warming (RCP 4.5 and 2.6), HAB suppression persisted. However, it was lost when epilimnion temperature increased by ∼2.6 °C in the unmitigated scenario (RCP 8.5) by 2100, causing a fivefold increase in cyanochlorophyll and 50% of summer days exceeding WHO bathing water thresholds. Mussel filtration collapse increasingly drove HAB resurgence with rising temperature under RCP 8.5. External nutrient load reductions of 90% were required to prevent HABs, securing recreational and drinking water safety. Positive ambient effects of quagga mussel invasions on HABs will be lost with future warming, highlighting the need for integrated local nutrient reduction and global climate mitigation.