<p>Cyanobacterial blooms increasingly threaten vital freshwater ecosystems, with harmful impacts exacerbated by climate change and eutrophication. Cyanobacteria produce toxic metabolites whose concentrations increase with biomass accumulation, posing direct risks to drinking water, aquatic ecosystems and recreational use while generating substantial economic costs. 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 and, consequently, biomass accumulation. Here, 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 realized 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⁻¹ in toxic <i>Microcystaceae</i> and <i>Dolichospermum</i> 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 realized niches in natural ecosystems—while offering practical insights for harmful algal bloom management.</p>