Identifying thermally tolerant bull kelp (Nereocystis luetkeana) source populations for restoration under climate change.

Kelp forests are declining in many parts of the world due to climate change and other stressors. Restoration is one solution to return kelp forests to places where they have been lost, yet continually warming oceans may necessitate the identification of thermally tolerant strains to facilitate the long-term success of kelp restoration. The Salish Sea is a waterway in British Columbia, Canada, and Washington State, United States, where ocean temperatures increasingly exceed the thermal limit for bull kelp (Nereocystis luetkeana), causing substantial losses along parts of its range. To identify thermally tolerant source material, we cultured bull kelp sourced from five populations along a thermal gradient in the northern Salish Sea and outplanted them in a common garden experiment at a warm site where bull kelp has been extirpated: Maude Reef on Hornby Island. We tracked stipe length, blade elongation, blade condition, sorus formation, and persistence. Kelp from the nearest cool site reached the largest sizes overall and produced the most sori. In the warm summer, kelp from the nearest warm site had significantly greater blade elongation rates and persisted longer than kelp from distant sites. Blade opacity did not show any distinct pattern, suggesting that loss of photosynthetic pigmentation is likely not the mechanism or consequence of observed differences in growth rate. Significant population-level variation in growth and blade condition may hint at possible adaptive strategies for heat avoidance in bull kelp. From these findings, we suggest population sourcing or mixture strategies that may increase restoration success.