Using functional traits and photophysiology to understand the facilitative role of canopy-forming seaweedsMacroalgal Traits

Hannah Hall¹, Robin Fales², Carmen Ott¹, Meili Soriano¹, Matthew Bracken¹

¹University of California, USA - ²Soka University of America, USA

Foundation species sustain biodiversity and ecosystem functioning by providing habitat for associated organisms. In rocky intertidal environments, canopy-forming seaweeds can serve as foundation species, mitigating physical stressors and facilitating the broader community, including other seaweeds. This study quantified the facilitative role of canopy-forming seaweeds on their understory counterparts during low tide. We identified understory-obligate species through community and canopy-layering surveys in southern California (USA). Focusing on Pterocladiella capillacea (Rhodophyta, Florideophyceae), an obligate understory species, we measured photosynthetic capacity in-situ with a pulse-amplitude-modulated (PAM) fluorometer before and after removal of a canopy-forming rockweed, Silvetia compressa (Heterokontophyta, Phaeophyceae). Additionally, we performed a functional trait analysis on both S. compressa and canopy obligate and stand-alone P. capillacea individuals to test for intraspecific trait variation associated with canopy presence. Across multiple sites, the cover of obligate understory species increased with the cover of S. compressa, whereas non-understory species showed no correlation. Our PAM fluorometry results revealed an 86% reduction in photosynthetic yield of P. capillacea when the canopy was removed, likely due to the role of the canopy in buffering heat, desiccation, and other physical stresses associated with low tides. Finally, trait measurements demonstrated that the rockweed, S. compressa, was thicker and less finely branched than the species it facilitates, suggesting that morphological attributes can predict the roles of seaweeds. Our findings highlight the critical facilitative roles of canopy-forming seaweeds such as rockweeds on temperate rocky reefs. These foundation species support a diverse understory community and mitigate the impacts of climate change, highlighting the importance of their preservation.