Undaria pinnatifida Creates Novel Habitats and Drives Benthic Invertebrate Community Shifts in North Patagonia

Abstract

Kelps, as habitat-forming species, play a crucial role in structuring ecosystems by providing habitat and refuge. They influenceabiotic factors that affect resource availability, impacting the abundance and diversity of sub-canopy epifauna. Sincethe introduced kelp Undaria pinnatifida (hereafter Undaria) is the only kelp in the intertidal zones of San Matías Gulf(Argentina), we evaluated the abundance and composition of invertebrate assemblages, considering separately its holdfast andfrond to assess the role of Undaria as a habitat-forming species. In two intertidal sites, we seasonally sampled invertebratesassociated with three microhabitats: Undaria’s frond, Undaria’s holdfast and Undaria-free substrate. In both study sites,Baliza San Matías (BSM) and Punta Verde (PV), invertebrate richness and abundance showed significant interactions between‘microhabitat’ and ‘season’. Species richness was consistently lower in fronds, differing significantly from holdfasts andsubstrates. Differences between holdfasts and substrates occurred only in winter and spring in BSM and in winter in PV. InBSM, invertebrate abundance always differed between fronds (83.4 ± 5.25; mean ± SE) and holdfasts (570 ± 47.82), but frondsdiffered from substrates (773.5 ± 243.85) only in autumn and winter. In PV, invertebrate abundance in fronds (83.4 ± 11.63)always differed from the other microhabitats, while holdfasts (2170.16 ± 159.4) and substrates (1642.08 ± 311.04) never did.Beta diversity revealed compositional differences between sites, with larger dissimilarities in fronds (due to both turnoverand nestedness), while dissimilarities in holdfasts and substrates were mainly driven by turnover. Our results suggest thatUndaria shelters a distinct, richer and more even invertebrate assemblage than the surrounding substrate, supporting the ideathat it can function as a habitat-forming species. Though its structural effects are seasonal, Undaria provides new habitatand structure for invertebrate assemblages in San Matías Gulf coastal ecosystems.