Projected 21st-century distribution of canopy-forming seaweeds in the Northwest Atlantic with climate change

Aim Climate change is predicted to alter the distribution and abundance of marine species, including canopy-forming seaweeds which provide important ecosystem functions and services. We asked whether continued warming will affect the distribution of six common canopy-forming species: mid-intertidal fucoids (Ascophyllum nodosum, Fucus vesiculosus), low-intertidal Irish moss (Chondrus crispus), subtidal laminarian kelps (Saccharina latissima, Laminaria digitata) and the invasive Codium fragile. Location Northwest Atlantic. Methods We used occurrence records and the correlative presence-only species distribution model Maxent to determine present-day distribution. This distribution was compared to each species' warm-water physiological thresholds indicating areas of stable or reduced growth and mortality. Present-day models were then projected to mid-century (2040-2050) and end-century (2090-2100) using two contrasting carbon emission scenarios (RCP2.6 and 8.5) and two global climate models from CMIP5 based on changes in ocean temperatures. Results Projected range shifts were minimal under low emissions (RCP2.6), but substantial species-specific range shifts were projected under high emissions (RCP8.5), with all species except C. fragile predicted to experience a northward shift in their southern (warm) edge of <= 406 km by the year 2100. Northward expansions outweighed southern extirpations for fucoids and C. crispus leading to overall range expansions, while range contractions were projected for kelps and C. fragile. Model projections generally agreed with physiological thresholds but were more conservative suggesting that range shifts for kelps may be underpredicted. Main conclusions Our results highlight the benefits to be gained from strong climate change mitigation (RCP2.6), which would limit changes in rocky shore community distribution and composition. The business-as-usual RCP8.5 scenario projected major range shifts, seaweed community reorganization and transitions in dominant species south of Newfoundland by 2100 (similar to 47 degrees N). As canopy-forming seaweeds provide essential habitat, carbon storage, nutrient cycling and commercial value, understanding their response to continued climate warming is critical to inform coastal management and conservation planning.