Size-spectra across geographical and bathymetric gradients reveal contrasting resilient mechanisms of recovery between Atlantic and Mediterranean fish communities

Aim Marine conservation often prioritizes measures based on ecosystem diversity, missing important information on the size and structure of communities such as that provided by body size-spectra (SS). While most studies have focussed on temporal or spatial patterns of SS, there is no research investigating their spatial heterogeneity, which is a key community trait revealing mechanisms of resilience. We aimed to comparatively determine the main patterns of variation of fish SS and their spatial heterogeneity across geographic and bathymetric gradients. Location Shelf and slope Atlantic and Mediterranean demersal ecosystems. Methods Using the information obtained from the experimental surveys, we calculated fish SS at four bathymetric strata and investigated the influence of the biomass of small-sized and large-sized fish on their temporal and spatial variation. We also assessed the functional relationship between the mean pattern and the spatial heterogeneity of SS as a characteristic structural trait of a community. Results The variation in SS demonstrates that Atlantic communities were geographically and bathymetrically more connected, while those in the Mediterranean were more fragmented. The relationship between mean SS and its spatial heterogeneity was a community fingerprint differentiating two contrasting pathways of community recovery between Atlantic and Mediterranean ecosystems. Atlantic communities were recovering because of the temporal increasing contribution of large fish and a ‘rescue effect’ of neighbouring areas, which have spatially homogenized size structure. Mediterranean communities were, by contrast, spatially clustered and highly dependent on small-sized fish variability that hampers the capacity of ecosystems to recover after a chronic degradation. Main conclusions Patterns of SS variation depend on the spatial complexity of communities, which contrasts between Atlantic and Mediterranean systems. Integrative analyses of size-based indicators (mean and variance) provide a complete assessment of the structural properties of communities and characterize the resilience mechanisms of recovery against perturbations under contrasting ecological contexts.