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Thermal sensitivity in ectothermic organisms is often contingent upon environmental factors. Nutrient availability in particular is believed to influence the physiological responses of primary producers to global warming and is thus relevant to consider when forecasting the structure and functioning of future marine ecosystems. This study measured the effect of nutrient enrichment on the thermal sensitivity of 4 genera of Galápagos seaweeds (Ulva,Caulerpa,Padina, andOchtodes), estimated as the thermal optimum (T_opt), performance maximum (P_max), activation energy, and deactivation energy. These parameters were quantified by modeling thermal performance curves for net photosynthesis under ambient and nutrient-enriched conditions. Our findings revealed variation inT_optamong genera, ranging from 27.6° to 36.0°C. Nutrient additions enhancedT_optby ~2°C for 2 (PadinaandCaulerpa) of the 4 taxa and also significantly increasedP_maxinPadina, suggesting the ability for warming-induced ocean stratification and associated effects (i.e. decreasing nutrient availability) to reduce the capacities of these populations to maintain and support new growth. No significant differences inT_optorP_maxwere observed for eitherUlvaorOchtodeswith enrichment. Ambient net photosynthesis and respiration rates were also compared across genera;P_maxrates for net photosynthesis were consistently higher than those for respiration (i.e. until just beyondT_opt); however, photosyntheticT_optvalues were lower. Thus, this study suggests that further warming could reduce overall net primary productivity, with potentially far-reaching implications for marine food webs.