Invasive Fish and Sea Urchins Drive the Status of Canopy Forming Macroalgae in the Eastern Mediterranean

SIMPLE SUMMARY: The Mediterranean marine forests of canopy brown algae are important habitats that have been in decline in recent decades. This study examines the distribution and status of macroalgal communities in relation to the populations of the main herbivores (fish and sea urchins) in the eastern Mediterranean. In the warmer South Aegean and Levantine Sea, invasive herbivorous fish seem to drive canopy algae towards shallower waters, while native sea urchins have become rare, indicating population collapses. In the North Aegean, macroalgal forests were observed in intermediate depths, and native sea urchins still thrive and appear to exert grazing pressure on macroalgal forests in shallower waters. Our results provide useful information for policymakers and help guide future research and conservation efforts. ABSTRACT: Canopy-forming macroalgae, such as Cystoseira sensu lato, increase the three-dimensional complexity and spatial heterogeneity of rocky reefs, enhancing biodiversity and productivity in coastal areas. Extensive loss of canopy algae has been recorded in recent decades throughout the Mediterranean Sea due to various anthropogenic pressures. In this study, we assessed the biomass of fish assemblages, sea urchin density, and the vertical distribution of macroalgal communities in the Aegean and Levantine Seas. The herbivore fish biomass was significantly higher in the South Aegean and Levantine compared to the North Aegean. Very low sea urchin densities suggest local collapses in the South Aegean and the Levantine. In most sites in the South Aegean and the Levantine, the ecological status of macroalgal communities was low or very low at depths deeper than 2 m, with limited or no canopy algae. In many sites, canopy algae were restricted to a very narrow, shallow zone, where grazing pressure may be limited due to harsh hydrodynamic conditions. Using Generalized Linear Mixed Models, we demonstrated that the presence of canopy algae is negatively correlated with the biomass of the invasive Siganus spp. and sea urchins. The loss of Cystoseira s.l. forests is alarming, and urgent conservation actions are needed.