Cool-edge populations of the kelp under global ocean change scenarios: strong sensitivity to ocean warming but little effect of ocean acidification.

Kelp forests are threatened by ocean warming, yet effects of co-occurring drivers such as CO are rarely considered when predicting their performance in the future. In Australia, the kelp forms extensive forests across seawater temperatures of approximately 7-26°C. Cool-edge populations are typically considered more thermally tolerant than their warm-edge counterparts but this ignores the possibility of local adaptation. Moreover, it is unknown whether elevated CO can mitigate negative effects of warming. To identify whether elevated CO could improve thermal performance of a cool-edge population of , we constructed thermal performance curves for growth and photosynthesis, under both current and elevated CO (approx. 400 and 1000 µatm). We then modelled annual performance under warming scenarios to highlight thermal susceptibility. Elevated CO had minimal effect on growth but increased photosynthesis around the thermal optimum. Thermal optima were approximately 16°C for growth and approximately 18°C for photosynthesis, and modelled performance indicated cool-edge populations may be vulnerable in the future. Our findings demonstrate that elevated CO is unlikely to offset negative effects of ocean warming on the kelp and highlight the potential susceptibility of cool-edge populations to ocean warming.