Abstract
Arborescent macro-algae forests covering temperate rocky reefs are a known habitat for juvenile fishes. However, in the Mediterranean, these forests are undergoing severe transformations due to pressures from global change. In our study, juvenile fish assemblages differed between pristine arborescent forests (Cystoseira brachycarpa var. balearica) versus an alternate state: bushland (Dictyotales – Sphacelariales). Forests hosted richer and three-fold more abundant juvenile assemblages. This was consistent through space, whatever the local environmental conditions, along 40 km of NW Mediterranean subtidal rocky shores (Corsica, France). Among Cystoseira forests, juvenile assemblages varied through space (i.e. between localities, zones or sites) in terms of total abundance, composition, richness and taxa-specific patterns. More than half of this variability was explained by forest descriptors, namely small variations in canopy structure and/or depth. Our results provide essential cues for understanding and managing coastal habitats and fish populations. Further studies are needed to explain the residual part of the spatial variability of juvenile fish assemblages and to help focus conservation efforts.
Highlights
In community ecology, whether terrestrial or marine, environmental drivers may act at multiple and nested spatio-temporal scales to shape communities
The aim of the present study was to test two hypotheses regarding spatial variations of juvenile fish assemblage structure, which was analyzed using both multivariate and univariate descriptors: (1) structure of juvenile fish assemblages differs between arborescent forests (C. balearica) and bushland (Dictyotales – Sphacelariales) consistently through space; (2) focusing on juvenile fish assemblages in arborescent forests, juvenile fish assemblage structure is affected by the depth and the three-dimensional structure of the forest
The present study, through a large spatial scale sampling effort, (1) has demonstrated that fish juvenile assemblage differed between arborescent C. balearica forests and Dictyotales – Sphacelariales bushland, and that this pattern was consistent through space
Summary
Whether terrestrial or marine, environmental drivers may act at multiple and nested spatio-temporal scales to shape communities. It has been observed that seagrass meadows and ‘macro-algae’ (i.e. Multicellular Photosynthetic Organisms belonging to the Chlorobionta, Rhodobionta and Phaeophyceae) forests are being replaced by less complex habitats[10] Such habitat shifts are due to various factors and their possible synergetic effects, such as water pollution[11], invasive species[12], overfishing and resulting trophic cascades[13], or physical disturbances such as trampling[14,15]. This study was restricted to only one site in Corsica (NW Mediterranean) and few fish species The consistency of these results had to be confirmed through time and space (another year, and at a greater number of sites), and considering the full necto-benthic juvenile assemblage of the habitat. The aim of the present study was to test two hypotheses regarding spatial variations of juvenile fish assemblage structure, which was analyzed using both multivariate (taxonomic composition and density) and univariate descriptors (total juvenile fish density and taxonomic richness): (1) structure of juvenile fish assemblages differs between arborescent forests (C. balearica) and bushland (Dictyotales – Sphacelariales) consistently through space (among coastal sites scattered over 40 km, Fig. 1); (2) focusing on juvenile fish assemblages in arborescent forests, juvenile fish assemblage structure is affected by the depth and the three-dimensional structure of the forest (canopy cover and height)
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