Climate-driven Community Shifts Reshape Productivity in Marine ForestsBiogeochemical Cycling

Bianca Reis^1,2, Francisco Arenas¹, Hugo Sainz-Meyer¹, Jonas de Azevedo¹, Oscar Babé-Gomez¹, Pieter van der Linden³, Álvaro Sanchez-Gallego², Isabel Sousa-Pinto¹, João N. Franco²

¹University of Porto, Portugal - ²MARE-Marine and Environmental Sciences Centre & ARNET-Aquatic Research Network Associated Laboratory - CETEMARES, Portugal - ³University of Coimbra, Portugal

Tropicalisation is causing significant shifts in coastal marine ecosystems worldwide. Canopy-forming macroalgae, crucial for ecosystem productivity and carbon sequestration, are replaced by less productive warm-affinity species. Despite the growing concern, there is a critical lack of in situ measurements quantifying the direct impact of these community shifts on ecosystem productivity and carbon uptake in transitional zones. Here, in situ incubations comparing different habitats (Canopy (cold-affinity) and Low-canopy (warm-affinity)) and different macroalgal species were performed along a tropicalised geographical transition zone in the subtropics. Net Community Productivity (NCP) and carbon-based NCP (NCP_C) were calculated to quantify differences in productivity and carbon uptake potential between habitats and species. Canopy habitats showed significantly higher productivity compared to low canopy ones, with NCP being 104.46% higher and NCP_C 86.37% higher. Within habitats, productivity was species-specific, with Laminaria hyperborea forests showing the highest values.
Our findings provide empirical evidence that climate-driven shifts from kelp-dominated to warm-affinity-dominated communities significantly reduce coastal productivity and carbon uptake potential. This transition could have far-reaching consequences for ecosystem functioning and services. Conservation strategies must adapt to address these changes, potentially focusing on maintaining cold-affinity species or enhancing the resilience of transitional ecosystems.