Trait-based functional shifts in seaweed community across a temperature gradient

Climate change is driving widespread shifts in species distributions and community compositions, with important consequences for ecosystem processes. Among the most vulnerable ecosystems, temperate seaweed forests underpin critical ecosystem services but are increasingly being reorganised. Yet, it remains unclear whether climate-driven shifts in community composition translate into changes in community-level traits, and how these changes cascade to affect ecosystem functioning. Here, we investigate temperate seaweed community trait variation along a latitudinal gradient following extensive regime shift at the warmer range edge, providing insight into how future temperate seaweed forests may function under continued environmental changes.

We used a space-for-time substitution and applied a trait-based framework to quantify temperature-driven functional shifts of seaweed communities. We compiled information on traits underpinning three key ecosystem functions (carbon cycling, habitat structure, and sediment dynamics) for 43 dominant seaweed taxa and defined functional entities (FEs) based on uniquely shared trait combinations. We then examined the distribution of FEs across the gradient, studied the shift in functionality depending on sea surface temperature, and derived functional indices.

Our results revealed that carbon cycling and sediment-related processes underpinned by seaweed communities shift along the temperature gradient. Warmer sites were characterized by more seasonal carbon dynamics, slower decomposition, and greater potential for sediment accumulation. Warm-affinity seaweeds shared similar habitat-structure traits with cool-water taxa, suggesting a degree of functional redundancy across the latitudinal gradient. Finally, cool-water seaweeds exhibited a high degree of functional uniqueness across all three ecosystem functions.

Overall, our findings suggest that rising temperatures will restructure the functional composition of seaweed forests, leading to reduced functional diversity and altered carbon cycling and sediment dynamics, with implications for the stability of temperate reef ecosystems.