The Structure of Mediterranean Rocky Reef Ecosystems across Environmental and Human Gradients, and Conservation Implications

TLDR

Historical exploitation and lack of rigorous baselines hinder assessment of Mediterranean marine ecosystem health and conservation efficacy. The study establishes the first contemporary baseline and gradient of nearshore rocky reef ecosystem structure across the Mediterranean. Underwater surveys were performed at 14 marine protected areas and 18 open access sites, covering a 31‑fold range of fish biomass (3.8–118 g m⁻²). The data revealed marked variation and three distinct community states, with the healthiest reefs combining high fish and algal biomass; fish biomass was higher in well‑enforced no‑take reserves, while protection level and primary production were the only variables significantly linked to community structure, providing a degradation trajectory useful for assessing reef health and MPA effectiveness.

Abstract

Historical exploitation of the Mediterranean Sea and the absence of rigorous baselines makes it difficult to evaluate the current health of the marine ecosystems and the efficacy of conservation actions at the ecosystem level. Here we establish the first current baseline and gradient of ecosystem structure of nearshore rocky reefs at the Mediterranean scale. We conducted underwater surveys in 14 marine protected areas and 18 open access sites across the Mediterranean, and across a 31-fold range of fish biomass (from 3.8 to 118 g m(-2)). Our data showed remarkable variation in the structure of rocky reef ecosystems. Multivariate analysis showed three alternative community states: (1) large fish biomass and reefs dominated by non-canopy algae, (2) lower fish biomass but abundant native algal canopies and suspension feeders, and (3) low fish biomass and extensive barrens, with areas covered by turf algae. Our results suggest that the healthiest shallow rocky reef ecosystems in the Mediterranean have both large fish and algal biomass. Protection level and primary production were the only variables significantly correlated to community biomass structure. Fish biomass was significantly larger in well-enforced no-take marine reserves, but there were no significant differences between multi-use marine protected areas (which allow some fishing) and open access areas at the regional scale. The gradients reported here represent a trajectory of degradation that can be used to assess the health of any similar habitat in the Mediterranean, and to evaluate the efficacy of marine protected areas.