Among anthropogenic habitats built in the marine environment, floating and non-floating structures can be colonized by distinct assemblages. However, there is little knowledge whether these differences are also reflected in the functional structure. This study compared the functional diversity of sessile and mobile invertebrate assemblages that settle over 3 months on floating vs. non-floating artificial habitats, in two Chilean ports. Using morphological, trophic, behavioral, and life history traits, we found differences between mobile and sessile assemblages regarding the effect of the type of habitat on the functional diversity. Compared to sessile assemblages, a greater functional similarity was observed for mobile assemblages, which suggests that their dispersal capacity enables them to balance the reduced connectivity between settlement structures. No traits, prevailing or selected in one or the other habitat type, were however clearly identified; a result warranting further studies focusing on more advanced stages of community development.

cf. Figueroa NN, Brante A, Viard F, Leclerc J-C (2021) Greater functional similarity in mobile compared to sessile assemblages colonizing artificial coastal habitats. Marine Pollution Bulletin 172:112844; DOI: 10.1016/j.marpolbul.2021.112844

Study area

The study area comprised two ports (separated by 30 linear km) of the Biobío Region, Chile: San Vicente (36.7591ºS, 73.1551ºW) and Coliumo (36.5377ºS, 72.9571ºW). As a sidenote, San Vicente is open to international trade, while Coliumo is only open to regional traffic (see Leclerc et al. 2018, 2020b), although maritime traffic was not the scope of our study. These two ports are located in two different bays (San Vicente and Coliumo bays) characterized by distinct biotic compositions in either natural (e.g. for intertidal mollusks; Aldea and Valdovinos, 2005) and artificial habitats, although little is known regarding soft sediment habitats in ports (but see Leclerc et al. 2018 for rapid assessment surveys of conspicuous taxa). Both ports present various types of artificial habitats that are part of the coastal infrastructure, and which were categorized for this study as: (1) non-floating habitats: those attached to the docks by rigid steel foundations/pillars and in direct contact with the benthos; and (2) floating habitats: comprising longlines or buoys that remain in the water column with a weaker, less prominent link to the benthos.

Dataset overview, study design, sampling and species identification

Our study capitalized on the sampling carried out between March and June, 2017, by Leclerc et al. (2020b), who focused on sessile assemblages. In brief, the sampling consisted of the deployment of a series of 15 cm x 15 cm black polypropylene settlement plates, arranged in structures (experimental units) held by PVC tubes (for details, see Leclerc et al. 2020b). The plates were used to have a standardized substrate (surface and material) and new available surface area (which is a main limiting resource, e.g. Sellheim et al. 2010) to be colonized by sessile and mobile taxa (flora and invertebrates).

In each of the study ports and on the two types of habitats (floating vs. non-floating), two experimental units bearing plates were placed vertically, separated by 20 m to 50 m, and at approximately 3 m to 4 m depth. In the non-floating habitats of both ports, these were placed on the pillars of the pier, while in floating habitats they were attached to buoys (San Vicente) or longline (Coliumo) by ropes. Four plates per experimental unit were removed three months after installation, which is sufficient time for the settlement and growth of the sessile and mobile assemblages to take place on the plates (see Leclerc and Viard 2018, Sellheim et al. 2010). The plates were individually transferred underwater in polypropylene rubble bags (mesh size < 0.5 mm) to minimize the loss of the mobile fauna and were kept for 4 h in a tank with seawater until they were processed in the laboratory.
In the laboratory, sessile (Leclerc et al. 2020b) and mobile taxa (this study) were identified under a dissecting microscope at the lowest possible taxonomic level. The abundance of the sessile taxa was estimated as the coverage at 100 intersection points in an area of 120 cm × 120 cm, as detailed in Leclerc et al. (2020b), while the abundance of the mobile taxa was estimated by counting the number of individuals per plate. The mobile fauna was sampled, after the identification of the sessile taxa, by washing the plate using a 500 µm mesh sieve. The samples were preserved in 95 % EtOH for later identification. Most mobile taxa were identified morphologically to the genus or family level due to a lack of taxonomic literature for the South East Pacific taxa of many abundant groups (e.g., amphipods).

Functional traits categorization

After identification, the sessile and mobile taxa were classified according to several functional traits related to their life history, behavior, and feeding habits. In this work, sessile taxa were considered to be those species with no or low mobility, those considered as habitat formers, and those which, after their settlement, do not migrate between plates. Their functional traits were subdivided into different modalities as proposed by Bremner et al. (2003) and Beauchard et al. (2017). The information on life history traits of the individual taxa was extracted from different online sources, such as GBIF, NEMESIS, NIMPIS, MarLIN, NAS, MSIP, BIOTIC and Polytraits, as well as bibliographic sources. Each trait was categorized into three to five modalities (e.g., the trait “Larval development” was given three modalities: pelagic planktotrophic, pelagic lecithotrophic or direct benthic) obtaining a total of 11 traits and 46 modalities of traits, as detailed in Table 1. The affinity of each taxon with the modalities of each trait was assigned so that the "total affinity" of each trait equaled 1 (Chevenet et al. 1994). This fuzzy coding allows a taxon to display modalities of a trait to different degrees (Chevenet et al. 1994). When information about a particular trait in a taxon could not be obtained, the affinity value of a similar taxon within its taxonomic group (genus or family, whichever is closest available) was used as a proxy, however only whenever variations of the trait within the taxonomic group chosen had not been reported. Otherwise, an equal affinity value was assigned for all modalities of that trait for the taxon.

Functional trait data were analyzed separately for each assemblage (mobile vs. sessile). From the trait matrix (“Traits-taxon matrix”) of the sessile and mobile assemblages, respectively, an analysis of biological traits was performed (BTA; Bremner et al. 2003, 2006). This was based on combining this matrix with the taxa abundance matrix ("Taxon-plate matrix") by means of a canonical scalar product that transforms and weights the scores (between 0 and 1 following the fuzzy coding) of each trait modality by the abundance of each taxon. This procedure allowed for the generation of a functional trait abundance matrix ("Traits-plate matrix"), on which the subsequent functional structure analyses were based.

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