Patterns of Spatial Variability of Mobile Macro-Invertebrate Assemblages Within a Posidonia Oceanica Meadow R

Journal of Natural History, 2015 Vol. 49, Nos. 41–42, 2559–2581, http://dx.doi.org/10.1080/00222933.2015.1021872 Patterns of spatial variability of mobile macro-invertebrate assemblages within a Posidonia oceanica meadow R. Bedini*, M. Bedini, L. Bonechi and L. Piazzi Istituto di Biologia ed Ecologia Marina, Piombino, Italy (Received 22 April 2013; accepted 22 January 2015; first published online 10 April 2015) The study evaluated patterns of spatial variability of mobile macro-invertebrate assemblages associated with Posidonia oceanica leaves within a meadow at three different depths and three different coastal wave-exposures. A total of 171 taxa was found, among them two Nemertea, 65 Mollusca, 91 Arthropoda, eight Annelida and five Echinodermata. The total number of taxa per sample was higher in intermediate and deep stands than in shallow ones, while no significant differences among coastal wave-exposures were observed. Multivariate analyses detected significant differences in the structure of assemblages in relation to depth and coastal wave-exposure. Mollusca were more abundant in shallow stands, while Crustacea and Echinodermata increased in intermediate and deep stands. Moreover, patterns of spatial variability changed with depth: shallow and intermediate assemblages showed high small-scale (tens of metres apart) variability, while deep assemblages showed high variability at intermediate scale (hundreds of metres apart). Keywords: depth; mobile macro-invertebrates; orientation; Posidonia oceanica; spatial variability Introduction Natural systems may vary in space and time, following patterns of organismal distribution (Menge and Olson 1990; Schneider 1994). Moreover, the variability of natural assemblages is scale dependent (Underwood and Chapman 1996; Benedetti- Cecchi 2001; Terlizzi et al. 2007), making it crucial for ecologists to identify the main scales of variability and the processes generating these patterns (Levin 1992). Downloaded by [University of Cambridge] at 05:12 08 April 2016 Knowledge of spatial patterns of assemblages may allow discrimination between natural variability and human-induced effects in environmental monitoring surveys and impact evaluation studies (Hewitt et al. 2001; Bishop et al. 2002; Fraschetti et al. 2005) and sampling design pertinent to ecological questions (Underwood 1993; Benedetti-Cecchi et al. 2003). Seagrasses and their associated assemblages represent one of the main coastal ecosystems and they are the object of a lot of studies because of their ecological relevance in coastal waters and their role as bioindicators (Kirkman 1996;Kuo et al. 1996). Seagrass meadows host complex macro-invertebrate assemblages both sessile, as epiphytes of leaves and rhizomes, and mobile (Mazzella et al. 1992). Mobile macro-invertebrate assemblages associated with seagrasses are particularly *Corresponding author. Email: [email protected] © 2015 Taylor & Francis 2560 R. Bedini et al. abundant and diverse thanks to the habitat complexity of meadows, and their role as refuges from predators and as a food supply (Heck and Orth 1980). In fact, the structure of seagrasses can create many microhabitats characterised by different environmental conditions, allowing the coexistence of species with different ecological requirements (Stoner 1980;Orthetal.1984). Moreover, plants and their epiphytes represent an important food source for both grazers and detritivorous organisms (Van Montfrans et al. 1984;KlumppandVanderValk1984; Edgar 1999;Duffyetal.2003). Although widely studied (Mattila et al. 1999; Atrill et al. 2000; Bostrom et al. 2006), patterns of spatial variability of macro- invertebrate assemblages of seagrass meadows and factors influencing these patterns are not completely known. In the Mediterranean Sea, Posidonia oceanica (L.) Delile represents the most important seagrass in terms of meadow extent and their role in coastal systems (Mazzella et al. 1992). The structure of macro-invertebrate assemblages associated with P. oceanica has been widely studied (Russo et al. 1991; Scipione et al. 1996; Bedini et al. 1997; Zakhama-Sraieb et al. 2011; Albano and Sabelli 2012; Urra et al. 2013) and spatial variability of assemblages has been evaluated at a large scale (among meadows and habitats) and in relation to depth gradients (Russo et al. 1984; Scipione and Fresi 1984; Mazzella et al. 1989; Gambi et al. 1992, 1995; Borg and Schembri 2000; Borg et al. 2010; Bedini et al. 2011; Belgacem et al. 2011, 2013). However, little is known about spatial patterns of variability within meadows, and about mechanisms determining these patterns. The present study aimed to evaluate patterns of spatial variability of mobile macro-invertebrate assemblages associated with the leaves within a Posidonia oceanica meadow, in relation to depth and coastal wave-exposure. For this purpose, a hierarchical sampling design was used to study a P. oceanica meadow at multiple spatial scales in relation to three different depths and three different coastal wave- exposures. Material and methods The study was carried out around the Island of Pianosa, in the Tuscan Archipelago National Park (northwestern Mediterranean Sea), in July 2010 (Figure 1). The sampling period was chosen because it corresponds to the maximum vegetative development of Posidonia oceanica. The island has very Downloaded by [University of Cambridge] at 05:12 08 April 2016 low anthropogenic pressure and all activities are forbidden for about 2 km from the shore. The seafloor slopes gently, allowing the development, between 5 and 40 m depth, of a Posidonia oceanica meadow about 16 km2 wide (Cinelli et al. 1995). The meadow is installed on matte (structures constituted of several layers of dead rhizomes and trapped sediment); rocky and sand bottoms are present above and below the meadow all around the island. The ecological quality of the site allows the elimination of variation due to anthropogenic disturbance; also, the structure of the meadow may be considered similar within a given depth range. Three depth ranges were sampled: 8–10 m (shallow = S), 18–20 m (intermediate = I) and 30–35 m (deep = D). Three sides of the island corresponding to different coastal wave-exposures (east = e, south = s, west = w) were considered, and four areas 100s of m distant from each other were sampled Journal of Natural History 2561 Downloaded by [University of Cambridge] at 05:12 08 April 2016 Figure 1. Map of Pianosa Island. Stars indicate the study areas. at each depth and side. Western and southwestern winds are dominant in the area, making the western side the most exposed to water movements (Region of Tuscany 1993). At each area, 10 replicate measures of density were made using a 40 × 40 cm quadrat and 10 shoots were collected to assess the meadow canopy (Pergent et al. 1995). A hand-towed net (40 × 20 cm, 0.4-mm mesh size) was used to sample mobile organisms living in the leaf stratum. Two samples tens of metres distant from each other were collected at each area. Each sample consisted of a series of 60 strokes over the seagrass canopy (Russo et al. 1985; Buia et al. 2003). 2562 R. Bedini et al. In the laboratory, taxa were identified and the abundance was expressed as number of individuals per sample. Species composition and abundance, were analysed by permutational analysis of variance (PRIMER 6 + PERMANOVA, Anderson 2001). A three-way model was used with depth (S vs I vs D) and side (e vs s vs w) as fixed and crossed factors, area (four levels) as random factor nested in the interaction depth × side. Species- abundance data were log(x + 1) transformed before calculation of Bray–Curtis index of dissimilarity. A two-dimensional non-metric multidimensional scaling (nMDS), based on centroids for replicated areas, was used for a graphical repre- sentation of the data. Pseudo-variance components in species composition and abundance were calculated for each spatial scale at each depth. The SIMPER (similarity percentages) routine was performed on untransformed data to establish which taxa contributed most to the dissimilarity. Shoot density, leaf length, the number of taxa, the abundance of organisms per sample and the abundance of the main phyla were analysed by analyses of variance (ANOVA), with the same factors and levels used for multivariate analyses. Cochran’s C-test was utilised before each analysis to check for homogeneity of variance, and data were transformed when necessary. The Student–Newman–Keuls (SNK) test was used for a posteriori multiple comparison of means (Underwood 1997). Results Meadow structure Shoot density ranged from 665.3 ± 33.5 to 181.4 ± 21.1, decreasing from shallow to deep stands (Figure 2b), while values of mean leaf length were higher in intermediate stands (Figure 2b). ANOVA and SNK test detected significant differences among depths (even if non-homogeneity of data has to be considered for shoot density) but not among coast sides (Table 1). Macro-invertebrate assemblages A total of 171 taxa was determined, among them two Nemertea, 65 Mollusca, 91 Arthropoda, eight Annelida, and five Echinodermata (Table 2). The total number of taxa per sample was higher in intermediate and deep Downloaded by [University of Cambridge] at 05:12 08 April 2016 stands than in shallow ones, while no significant differences among sides were observed (Table 3; Figure 3a). Differences among depths and sides were not significant for the mean number of organisms per sample (Table 3)evenifthe abundance of the main taxa changed along the depth gradient: Mollusca were more abundant in shallow stands, while Crustacea and Echinodermata increased in intermediate and deep stands (Table 3; Figure 3b). Crustacea also differed between east and west sides (Table 3). PERMANOVA showed significant differences related to both depth and side for species composition and abundance of macro-invertebrate assemblages (Table 4). Pair-wise tests showed that depths were significantly different to each other. Differences were also significant between the east and west sides (Table 4). The nMDS showed that shallow stands were well separated from intermediate and deep ones, and, within the shallow stands, the west side was separated from the others (Figure 4).

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