DOI: 10.5061/dryad.qz612jmd6
title: Disentangling the links between habitat complexity and biodiversity in a kelp‐dominated subantarctic community

description: Habitat complexity is one of the most important factors modulating species
 diversity. This feature comprises several interrelated attributes, such as
 number, size, and spatial arrangement of complexity-forming elements.
 However, the separate and joint effects of these attributes on diversity
 and community structure are still not well understood. Here, we assess the
 relationships between several structural-complexity attributes of the
 subantarctic kelp Lessonia flavicans and species richness, total
 abundance, and structure of kelp-associated macrobenthic communities. We
 predicted that longer thalli and larger holdfasts favour greater species
 richness and total abundance of invertebrate organisms. To test the
 prediction, an observational sampling program was established in two sites
 of the Strait of Magellan. Uni- and multivariate analyses revealed both
 positive and negative effects of kelp structural-complexity attributes on
 diversity. Holdfast diameter and maximum frond length, followed by thallus
 wet weight, had the strongest positive fits to species richness and total
 abundance; the number of stipes, on the other hand, was negatively
 associated with both response variables. Longer fronds were associated
 with greater abundances of spirorbid polychaetes. Larger holdfasts
 supported larger abundances of Nereididae and Terebelidae polychaetes and
 the limpet Nacella mytilina. Contrarily, kelps with longer fronds and more
 stipes supported fewer amphipods. In this way, we demonstrate that
 different dimensions of habitat complexity can have contrasting effects on
 diversity and community structure, highlighting the fundamental role of
 multiple dimensions of kelp habitat complexity for local biodiversity.
description: <i>Study
 sites and sampling design</i> The Magellan region is located in
 the South East Pacific, encompassing a diversity of habitats that include
 fjords, inland seas, glaciers, gulfs, and channels—a complex landscape
 resulting from the combined effect of tectonic processes and glaciation.
 The oceanographic features and diverse environmental conditions determine
 a particular marine biogeographic unit (Camus 2001) dominated by benthic
 invertebrates and extensive kelp forests (Friedlander et al.
 2018). To analyse the
 potential spatial variation in the effects of kelp’s structural
 complexity, the study was conducted at two sites (4 km apart) in the
 Strait of Magellan (Fig. 2), named “Bahía Buzos” (-53.627°S, -70.919°W)
 and “Carrera” (-53.587°S, -70.921°W). Both sites harbour <i>L.
 flavicans</i> forests of around 400 m<sup>2</sup>,
 located between 0.5 and 4 m depth (C. Velasco-Charpentier pers. obs.).
 Both sites are moderately wave-exposed, with a hard substratum consisting
 of boulders and large rocks covered by crustose coralline algae. The
 patchy rocky reefs extend from the intertidal to ~ 8 m depth.
   An observational sampling program of
 kelp habitat complexity and associated communities was established in both
 sites between November 2018 and August 2019. Observations were conducted
 every three months in both sites. Punta Carrera was sampled only during
 spring and summer due to logistics limitations and harsh weather. At each
 sampling time and site, SCUBA divers deployed a 100 m transect parallel to
 the coastline. The transects were placed on substrata of similar rock
 composition (boulders), depth (0.5 – 2 meters), and inclination (~ 0°). In
 each transect, ten 0.25 m<sup>2</sup> quadrants separated by ~
 3 meters were photographed with a Nikon D7200 camera equipped with an
 Ikelite housing and a rectilinear Tokina 11-16 lens. We selected quadrants
 with at least one <i>L. flavicans </i>specimen. So, the unit
 of replication was each kelp specimen (N = 10 per site and season). Each
 thallus was wrapped with a mesh bag (1 mm<sup>2</sup> pore
 size). Then, the holdfast was detached from the rock with a knife and the
 bag was swiftly closed to minimise the escape of mobile macrobenthic
 organisms. Within one hour after collection, samples were transported to
 the Laboratorio de Ecofisiología y Biotecnología de Algas, Universidad de
 Magallanes to measure the thallus morphology attributes and species
 sorting. <i>Estimation of
 kelp structural complexity and associated
 biodiversity</i> Once in the
 laboratory, we measured five kelp structural complexity attributes. The
 number of stipes, maximum length (i.e. thallus length, from holdfast base
 to the apex in cm), holdfast diameter (i.e. maximum width of the holdfast
 base in cm), and total wet weight in g (0.01 g precision) were directly
 measured from the collected kelps. Foliage cover was estimated from each
 digital photography. This variable was categorized as 1 (fronds cover
 between &lt; 10 - 30% of the quadrant), 2 (fronds cover between
 31-50%), 3 (fronds cover between 51 - 80%), and 4 (frond cover between
 81-100%). For each <i>L.
 flavicans</i> thallus, macrobenthic (&gt; 1 mm) mobile and
 semi-sessile organisms (<i>e.g</i>. mussels), in addition to
 tube dwelling sessile organisms (e.g. spirorbid polychaetes) were
 collected from thallus surface with aid of forceps and scalpels. In
 addition, the stipes and discs were dissected to collect organisms living
 in crevices and galleries. We used specialized literature and field guides
 to identify each individual to the lowest taxonomic level possible,
 usually species (Osorio et al. 1979; McLean 1984; Pastorino 2005;
 Häussermann and Försterra 2009; González-Wevar et al. 2018).
 Species-specific abundance was estimated as counts of individuals. These
 data were used to estimate species richness, defined as the number of
 taxonomic identities; total abundance, defined as the total number of
 individuals per taxonomic identities; and community structure, defined as
 the combination of species identities and abundances. Before the analyses,
 Pearson-product moment correlations were calculated among the explanatory
 variables in order to account for collinearity.