The kelp forests of southern Patagonia have a large diversity of habitats, with remote islands, archipelagos, peninsulas, gulfs, channels, and fjords, which are comprised of a mixture of species with temperate and sub-Antarctic distributions, creating a unique ecosystem that is among the least impacted on Earth. We investigated the distribution, diversity, and abundance of marine macroinvertebrate assemblages from the kelp forests of southern Patagonia over a large spatial scale and examined the environmental drivers contributing to the observed patterns in assemblage composition. We analyzed data from 120 quantitative underwater transects (25 x 2 m) conducted within kelp forests in the southern Patagonian fjords in the Kawésqar National Reserve (KNR), the remote Cape Horn (CH) and Diego Ramírez (DR) archipelagos of southern Chile, and the Mitre Peninsula (MP) and Isla de los Estados (IE) in the southern tip of Argentina. We observed rich assemblages of macroinvertebrates among these kelp forests, with a total of 185 unique taxa from 10 phyla and 23 classes/infraorders across the five regions. The number of taxa per transect was highest at IE, followed by MP, CH, and KNR, with the lowest number recorded at DR. The trophic structure of the macroinvertebrate assemblages was explained mostly by wave exposure (28% of the variation), followed by salinity (12%) and the KNR region (11%). KNR was most distinct from the other regions with a greater abundance of deposit feeders, likely driven by low salinity along with high turbidity and nutrients from terrigenous sources and glacial melt. Our study provides the first broad-scale description of the benthic assemblages associated with kelp forests in this vast and little-studied region and helps to establish baselines for an area that is currently lightly influenced by local anthropogenic factors and less impacted by climate change compared with other kelp forests globally.

Characterization of the macrobenthos was conducted by scuba divers along two 25-m long transects at each sampling location. Transects were run parallel to the shoreline, with a target depth of 10 m, depending on location of the lower edge of the kelp forest. Since sampling sites targeted kelp forests only, the underlying habitats were biased towards hard substrate.

For sessile and mobile invertebrates, the number of individuals was estimated on 1-m of either side of the transect line (50 m²). For colonial organisms (sponges, some cnidarians, bryozoans, and some tunicates) colonies, rather than individuals, were counted. When a species was extremely abundant (i.e., > 500) along the transect, abundance was estimated considering the number of individuals/colonies m^-2 and scaled to the total area of the transect (50 m²). Only non-cryptic invertebrates ≥1 cm were enumerated. A second diver counted the number of kelp (Macrocystis pyrifera and Lessonia spp.) stipes within 1-m on either side of the transect and recorded the bottom type for each transect. Salinity and temperature measures were recorded at each site using a YSI model 556 handheld multiparameter instrument or RBR concerto multi-channel logger. Salinity was recorded in parts per thousand (ppt) ± 0.1 and temperature was recorded in ^oC ± 0.1.

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