Intraspecific plasticity of different kelp populations along distributional gradients to thermal and irradiance stress has largely been understudied. Eliminating the effect of environmental histories by raising a generation under common garden conditions allows unbiased comparison between population specific adaptive potentials to different environmental factors. Following this approach, this study aims to test the hypothesis of adaptive differentiation between populations of L. ochroleuca from locations with distinct temperature and depth ranges across life-stages (microscopic gametophytes and macroscopic sporophytes). Gametophytes of four geographically distinct populations were subjected to different temperature treatments and gametophyte survival during thermal stress conditions as well as resilience, reproductive success and photosynthetic responses during recovery were investigated. Young sporophytes of two populations with contrasting natural habitats were subjected to different temperature/irradiance treatments and growth and photosynthetic performance investigated. Intraspecific variation in adaptive traits to thermal and irradiance stress was found in L. ochroleuca. Gametophytes exhibited differential thermal tolerances. The most northern distributed population was the most thermally sensitive and populations from Spain and Morocco exhibited overall very low reproductive success. Sporophytes from Morocco exhibited overall higher growth rates. Given the common garden conditions, likely genotypic differentiation based on adaptive traits towards latitudinal differences of temperature and irradiance levels.
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