The biodiversity and metabolic functioning of alternative macroalgal habitats on Mediterranean rocky reefs

Human pressures are leading to the replacement of macroalgal forests by alternative opportunistic species on shallow temperate reefs. Nonetheless, the ecological consequences of habitat reconfiguration for coastal biodiversity and ecosystem functioning remain poorly quantified. By means of a field study, we compared the metabolic functioning and biodiversity of macroalgal forests dominated by the fucoid Ericaria brachycarpa at pristine sites with that of assemblages formed by a shrub-like rhodophyte (i.e., Halopithys incurva) at urban sites. Dominant macroalgae at pristine and urban sites supported similar abundance and species richness of vagile invertebrates, but macroalgal forests supported higher invertebrate biomass. Shrub-like assemblages at urban sites sustained an autotrophic metabolism with a net diel O₂ production throughout the year, whereas macroalgal forests tended to be heterotrophic during warmer months. Diel fluxes of total dissolved inorganic carbon, as well as the contribution of production/respiration, were consistent with O₂ fluxes, with E. brachycarpa forests functioning as a heterotrophic carbon source in summer. This could be the result of reduced photosynthetic performance of the dominant brown macroalga and/or increased community respiration in warmer seawater. Our findings suggest that benthic assemblages in urban areas, formed by large and architecturally complex macroalgae, do not markedly differ from those found in pristine areas in terms of supported biodiversity and may sustain a more stable autotrophic balance under varying environmental conditions. Avoiding further degradation of these urban habitats (i.e., shift from shrub-like to mat-like turfs) could be a viable strategy for sustaining ecosystem functioning along peri-urban and urban Mediterranean coasts.