Many marine foundation species are now at high risk of widespread, climate-mediated collapse. Understanding how species with contrasting life history and dispersal traits differ in genetic diversity, adaptive capacity, and patterns of gene flow is crucial for predicting their responses and designing effective management. Temperate seaweed forests have experienced accelerating losses over the past 50 years, with 60% of global long-term records showing decline. Using cutting-edge genomics, biophysical and climate models, we investigated patterns of genetic diversity and gene flow in two foundational seaweeds, Cystophora brownii and Cystophora racemosa, and explored the role of genetic diversity and dispersal in species-specific responses to marine heatwaves and ocean warming. We calculated local genomic offset (maladaptation) and forward genomic offset (which accounts for migration) using climate-associated loci, comparing these metrics with ecological data on historical population declines. Losses of C. brownii, which has limited dispersal, were best predicted by local genomic offset, while declines in the widely-dispersing C. racemosa were better explained by forward offset. Our results suggest that high dispersal potential, such as with C. racemosa, may improve population trajectories under future warming, while species with low dispersal, such as C. brownii, will likely require assisted migration to persist in coming decades.