Oceanographic connectivity strongly restricts future range expansions of critical marine forest species.

Climate change is driving a global redistribution of marine biodiversity. As habitats shift, oceanographic connectivity (the transport of dispersive stages via ocean currents) becomes a critical, yet poorly understood, factor that can either facilitate or hinder species' expansions into new areas. To quantify this influence, we developed a framework linking species distribution models with biophysical connectivity models to examine the redistribution of 467 marine forest species (seagrasses and brown macroalgae) under end-of-century climate change scenarios. Our projections show substantial habitat loss for both groups, reaching up to 50% (seagrasses) and 58% (brown macroalgae) of current habitats under higher emissions. Despite potential poleward expansions, oceanographic connectivity emerges as a major limiting factor. Accounting for average dispersal duration, range expansions are reduced by up to 38% in area (seagrasses) and 48% (macroalgae), and by up to 64% and 72% in distance, respectively. This reduction significantly increases the percentage of species facing a net loss of suitable habitat. Notably, well-defined dispersal barriers restrict expansions into highly suitable regions (e.g., the Okhotsk Sea, New Zealand, and the Arctic). Our findings underscore the need to explicitly integrate both habitat suitability and oceanographic connectivity to accurately predict marine biodiversity redistribution and inform effective conservation strategies.