Cyanobacterial blooms increasingly threaten vital freshwater ecosystems, with harmful impacts exacerbated by climate change and eutrophication. Despite extensive research on temperature and nutrient effects, our explanatory and predictive capacity remains limited. We propose that this limitation stems from insufficient understanding of how biotic and abiotic factors interact to modify cyanobacterial net growth. Using five years of daily monitoring data from a eutrophic lake and causal inference via state-space reconstruction modelling, we show that interactions with co-occurring plankton taxa fundamentally reshape the realised niche of bloom-forming cyanobacteria over environmental gradients. Biotic interactions shift temperature boundaries for net growth by up to 13°C and phosphorus requirements by over 20 μg/L in toxic *Microcystaceae* and *Dolichospermum spp*. Grazing inhibits bloom formation across cyanobacterial taxa, while facilitation by other phytoplankton may allow blooms at unexpectedly low temperatures and phosphate concentrations. These findings address a fundamental research gap—how biotic interactions shape realised niches in natural ecosystems—while offering practical insights for bloom management. By integrating biotic interactions into monitoring programs and predictive models, we can improve forecasting accuracy and develop targeted interventions that complement traditional nutrient control approaches. These findings parallel recent advances in ecology suggesting the fundamental role of biotic interactions in mediating species' responses to environmental change.