In this study, juvenile sporophytes were cultured using parental material from five geographically and genetically distinct populations of the giant kelp Macrocystis pyrifera from the northern and southern edges of the species' range in Aotearoa New Zealand. These sporophytes were exposed to simulated 22-day marine heatwaves at either 18°C or 22°C, or a 14°C control. The sporophytes were then kept at 14°C for a further 21 days to observe whether they showed signs of recovery. The experiment also assessed whether the long-term storage of gametophytes under red light could affect sporophyte performance; this was achieved by comparing two cultures from the same population (Wellington) that were cultured either with or without long-term storage under red light. The experiment was carried out in Wellington, New Zealand between June and July 2024.Survival was assessed visually at the end of the heatwave and recovery phases. Length was measured with a ruler, and wet weight was measured with a balance, before and after the heatwave and after the recovery phase, and relative growth rates were calculated based on both metrics. Maximum quantum yield was measured using a Walz Diving-PAM blue light fluorometer before and after the heatwave and after the recovery phase. Blade tissue samples were taken at the end of the heatwave and recovery phases; these were oven dried, ground, and analysed using a mass spectrometer to determine total carbon and nitrogen content and δ13C and δ15N stable isotope ratios.It was found that the 22°C heatwave treatment caused 64% mortality and a significant reduction in linear growth rates across all kelp cultures, as well as a non-significant but noticeable reduction in maximum quantum yield. However, all cultures were comparatively resistant to the 18°C treatment, with just 23% mortality during the heatwave. Chemical composition of sporophytes was altered in both heatwave treatments, with total carbon content, carbon:nitrogen ratio, and δ15N values increasing significantly. There was little evidence that long-term storage of kelp gametophytes under red light impacts sporophyte survival or growth. Although survival rates and chemical composition differed between some populations, there was no consistent evidence overall for significant differences in thermal tolerance between northern and southern M. pyrifera cultures.