Kelp forests are susceptible to climate change as their sessile nature and low dispersal capacity hinders tracking of suitable conditions. The emergence of a wide array of approaches to increasing thermal tolerance seeks to change the outlook of biodiversity in a changing climate but lacks clear targets of impactful thermal resilience. Here, we utilize species distribution models (SDMs) to evaluate the potential of enhanced thermal tolerance to buffer the effects of climate change on cold-adapted kelp species: Saccharina latissima, Alaria esculenta, Laminaria hyperborea, and Laminaria digitata. For each species, we compared a baseline model - where the thermal niche remained unchanged - to models where the simulated maximum sea surface temperature tolerance was increased by 1-5°C. These models were projected into three climate change scenarios: sustainability (Shared Socioeconomic Pathway (SSP) 1-1.9, Paris Agreement), regional rivalry (SSP3-7.0) and fossil-fuel development (SSP 5-8.5). Our SDMs demonstrate that an increase of 1-2°C in thermal tolerance could recover over 50% of predicted losses of suitable habitat for cold-adapted kelps. However, A. esculenta and L. digitata still faced persistent habitat reductions (up to 20%) under SSP1-1.9 and SSP3-7.0, even with a simulated 5°C increase in thermal tolerance. Furthermore, A. esculenta, a species of growing commercial interest, showed disproportionate habitat contraction across all climate change scenarios and simulated tolerance increases. Our findings highlight the need for a two-pronged approach to conserve cold-adapted kelp forests: stringent reductions in greenhouse gas emission reductions in line with the SSP1-1.9 scenario, and strategies to boost kelp thermal tolerance by at least 1-2°C. This dual approach is crucial to maintain 90% of the current suitable habitat of S. latissima and L. digitata, and 70% for A esculenta and L. hyperborea. Relying on mitigation or adaptation alone will likely be insufficient to maintain their historic range under projected climate change.

Seagrasses play a vital role in structuring coastal marine ecosystems, but their distributional range and genetic diversity have declined rapidly over the past decades. In order to improve conservation of seagrass species, it is important to predict how climate change may impact their ranges. Such predictions are typically made with correlative species distribution models (SDMs), which can estimate a species’ potential distribution under present and future climatic scenarios given species’ presence data and climatic predictor variables. However, these models are typically constructed with species-level data, and thus ignore intraspecific genetic variability of populations that potentially have adaptations to heterogeneous climatic conditions. Here, we explore the link between intraspecific adaptation and niche differentiation in Thalassia hemprichii, a seagrass broadly distributed in the tropical Indo-Pacific Ocean and a crucial provider of habitat for numerous marine species. Using microsatellite-based genotyping, we identified two distinct phylogeographical lineages within the nominal species and found an intermediate level of differentiation in their multidimensional environmental niches, suggesting the possibility for local adaptation. We then compared projections of the species’ habitat suitability under climate change scenarios using species-level and lineage-level SDMs. In the Central Tropical Indo-Pacific region, both models predicted considerable range contraction in the future, but the lineage-level model predicted more severe habitat loss. The two modelling approaches predicted opposite pattern in habitat change in the Western Tropical Indo-Pacific region. Our results highlight the necessity of conserving distinct populations and genetic pools under climate change and have important implications for guiding future management of seagrasses.