Estimates of Critical Upper Maxima (CTmax) of Kelp Species Are Strongly Influenced by Biogeography and Thermal History.

Experimentally derived critical thermal maxima values (CTmax) are widely used to estimate species' thermal tolerances and, increasingly, to predict their vulnerability to chronic warming and heatwaves. However, accumulating evidence from model taxa such as fish and insects suggests that CTmax is dynamic and sensitive to multiple environmental and methodological factors, potentially limiting its application as a fixed species-level metric. Here, we quantified CTmax in three habitat-forming kelp species (Laminaria digitata, Saccharina latissima and Laminaria ochroleuca) that occupy contrasting positions of their biogeographic range in the Western English Channel (northeast Atlantic). We assessed how methodological approach (whole plants vs. excised meristematic discs) and thermal history across multiple timescales (seasonal climatology and acute marine heatwaves, MHWs) shape upper thermal limits. Meristematic discs provided CTmax estimates comparable to whole plants in the two cool-water species (L. digitata and S. latissima), but significantly underestimated whole-plant tolerance in the warm-affinity species (L. ochroleuca). This divergence most likely reflected species-specific biology with L. ochroleuca relying more strongly on whole-organism integration and internal storage to maintain high thermal performance. Seasonal climatology influenced CTmax in both cool-water species, with pronounced declines in autumn relative to spring and summer, consistent with narrower thermal safety margins and cumulative stress accrued over summer. Exposure to an extreme simulated MHW caused substantial reductions in CTmax across all species, with the largest decline (> 7°C) observed in L. ochroleuca. Together, these results show that CTmax is highly plastic, shaped by species' biological traits, methodological approach and thermal history. Treating CTmax as a fixed trait risks misrepresenting population-level vulnerability of habitat-forming kelp species and, most likely, other marine ectotherms. Incorporating thermal history and organismal biology into thermal-tolerance assessments will improve predictions of the resilience of kelp species and the ecosystems they underpin, under future ocean warming.