<p>Coral bleaching is primarily driven by heat stress, yet additional environmental factors, such as nutrient availability and light attenuation, can affect bleaching severity. In this study, we assessed the interactive effects of chlorophyll <i>a</i> concentrations with heat stress (Degree Heating Weeks, DHW) on bleaching severity across inshore and offshore reefs of the Great Barrier Reef (<i>n</i> = 1397). Using Generalised Additive Models, we tested multiple chlorophyll <i>a</i> metrics (mean, maximum, minimum, and cumulative sum) across different time frames (3–12 weeks) to determine their predictive value. Results revealed that the relationship between chlorophyll <i>a</i> and bleaching severity varied by reef location. At inshore reefs, where chlorophyll <i>a</i> served as a proxy for nutrient enrichment, exposure to elevated concentrations lowered bleaching thresholds, increasing bleaching sensitivity. In contrast, at offshore reefs, where oligotrophic conditions dominate, chlorophyll <i>a</i> fluctuations likely reflected upwelling-induced cooling and moderate nutrient enrichment, initially buffering corals from bleaching but increasing severity when excessive accumulation occurred. Long-term chlorophyll <i>a</i> metrics (9–12-week mean and cumulative sum) improved predicted bleaching (adj. R² = 0.55–0.57) compared to heat stress alone (adj. R² = 0.49), lowering DHW thresholds inshore but raising them offshore. These findings suggest the value of integrating chlorophyll <i>a</i> with DHW in forecasting frameworks to improve coral bleaching risk assessments, supporting policies focused on nutrient mitigation at inshore sites and monitoring of upwelling-modulated dynamics offshore. As global warming intensifies, increasing DHW and shifting nutrient dynamics will likely alter these relationships, necessitating further refinement of bleaching prediction to support conservation strategies.</p>