The giant kelp holdfasts macroinvertebrate assemblages: Towards benthic management and conservation using alpha and beta diversity in Northern Patagonia

The Inner Sea of Chiloé (ISC), northern Patagonia is a topographically and hydrographically heterogeneous area of critical interest to the Chilean blue economy due to the intensive artisanal benthic fisheries and aquaculture practices. Using benthic macroinvertebrate assemblages from Macrocystis pyrifera holdfasts, this study exposes how environmental heterogeneity inside the ISC can also be expressed in biological terms. Four zones were considered in the ISC from north to south (Reloncaví, Chacao, Quinchao, and Quellón), with five sampling sites within them. The spatial variability of macroinvertebrate density, composition, species/taxa richness (S), Shannon´s diversity (H´), and overall beta diversity (β_sor) in 200 kelp holdfasts (n =50 per zone) between zones and sites were analyzed. ANOVA results showed that the holdfast size, zones and sites within zones explained the variability of density, S and H´. Tukey HSD test also indicated significant (p < 0.01) differences between sampling zones and sites. Distribution patterns included: i) a latitudinal increase in density from the north to the centre of the ISC (max. of 67 ind 10 cm⁻², Quinchao); ii) higher S values in the north (max. of 37, Reloncaví); iii) a latitudinal decrease in H´ (max. of 3.2, Reloncaví); iv) lower values for all the ecological attributes in the south (Quellón). Overall beta diversity analysis (β_sor) showed high variability inside each zone, mainly driven by species turnover (β_sim) rather than species nestedness (β_sne). nMDs analysis suggested that the northern zones of ISC can be considered the same and differentiated from the others, but they are highly variable within them. PERMANOVA analysis showed that the macroinvertebrates' composition patterns were significantly (p < 0.01) explained by the holdfast size, and sampling sites/zones. SIMPER analysis revealed the contribution of annelids, arthropods, and molluscs to dissimilarities between zones. The results highlighted the biological heterogeneity of the ISC, suggesting that site-specific environmental conditions affect M. pyrifera populations and, therefore, macroinvertebrates inhabiting their holdfasts. These findings contribute to a better understanding of the ecological complexity of the region, providing a baseline for future studies on the status and composition of benthic communities, and therefore their management and conservation.