Understanding the drivers of ecological stability is a central goal of ecology given current global environmental change. Field experiments tackling these questions by manipulating temperature have largely not matched climate change projections, clouding our inference of global change impacts on ecosystems. Here, we examine the temporal and spatial stability of algal function (biomass) and composition in response to artificially induced heatwaves through a novel aquatic mesocosm experiment in Horonai stream, Hokkaido, Japan. We expose naturally assembled communities of algae and macroinvertebrates to combinations of predator removal (extinction) and aquatic heatwaves at two levels (current [+2.8C] versus projected future heatwave [+6.1C]) using statistically downscaled values to produce locally realistic temperature increases. We found that algal dynamics were resistant to both heatwave conditions and that the effect of predators on algae and macroinvertebrates remained constant across heatwave conditions. However, when considering time-varying interaction strengths between algal groups and the contribution of predators to algal dynamics, our heatwaves decoupled the spatial and temporal dimensions of functional and compositional stability. Our results highlight the complexity of climate change impacts on aquatic systems and the need to consider community dynamics over simple measures of biomass or composition when assessing global change impacts on ecological communities.