Macrofaunal diversity and secondary production in kelp holdfasts (Phaeophyceae) within a climatic transition zone

Despite substantial research on kelp holdfast biodiversity, the drivers of macrofaunal diversity, abundance, and biomass across holdfast types and different climatic regions remain unresolved. This uncertainty extends to ecosystem functioning, where empirical estimates of secondary production are rare. We quantified the relative influence of regional environmental context and holdfast morphology on macrofaunal assemblages inhabiting holdfasts of Laminaria ochroleuca and Saccorhiza polyschides in two contrasting regions along the western Iberian coast climatic gradient: the northern A Coruña, Spain (COR; cooler, lower wave exposure) and the southern Peniche, Portugal (PEN; warmer, higher wave exposure). We also investigated ecological functioning by estimating secondary production. Macrofauna diversity, abundance, biomass (ash-free dry mass; AFDM), and secondary production were assessed at the holdfast level in samples collected from both regions in summer 2021. Holdfast structural complexity was quantified as surface-to-volume ratio (SA/V). Underwater Visual Census (UVC) surveys were conducted to provide reef-scale context for regional differences. Secondary production (consumer incorporation of organic matter per time) was estimated with Edgar's empirical model to infer energy-transfer dynamics. The cooler region, COR, exhibited significantly higher overall diversity and production than PEN. While S. polyschides supported lower taxonomic diversity than L. ochroleuca, it exhibited higher secondary production. These findings highlight the importance of regional climatic context in shaping kelp holdfast biodiversity and functioning. Differences between cooler and warmer regions suggest that variation in thermal regimes can be associated with changes in holdfast community structure and secondary production.