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

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.

Distribution, health and threats to Mediterranean macroalgal forests: defining the baselines for their conservation and restoration

“…Finally, stressors related to climate change and overgrazing by invasive species showed a strong zonation pattern, especially the latter, which was restricted to a small area of the Cyclades Islands (Aegean Sea) and the northern Israeli coast. KDE values of overgrazing are not necessarily representative of the true extent of grazing pressure and are associated with the high availability of data in the Cyclades Islands (Sala et al, 2011;Nikolaou et al, 2023). KDE values show that climate change has higher density values in the Levantine part of the Mediterranean Sea, but also in the northern part of the Adriatic Sea, coinciding with the northernmost and coldest part of the Mediterranean Sea (Bianchi and Morri, 2000) and of the distribution of Cystoseira s.l.…”

“…Nevertheless, as they have been extensively documented across the entire Mediterranean basin, they pose a significant challenge to the basin-wide conservation of these habitats. More recently, overgrazing by invasive species (e.g., Sala et al, 2011;Giakoumi, 2014;Vergeś et al, 2014;Nikolaou et al, 2023) has been added to this list, along with the direct effects of climate-driven impacts (e.g., marine heatwaves; Verdura et al, 2021;Mulas et al, 2022). Specifically, the effects of invasive alien species are concentrated in the eastern Mediterranean Sea, where Cystoseira s.l.…”

Cumulative Negative Impacts of Invasive Alien Species on Marine Ecosystems of the Aegean Sea

“…Several studies in the Aegean have linked the presence of Siganus spp. with adverse grazing effects on macroalgal communities [15,[75][76][77][78] and a degraded rocky reef ecological state [79,80]. Three macroalgal ICAS follow in the list of the most impactful species on shallow hard substrates: the green alga C. cylindracea, and the red algae L. lallemandii and Asparagopsis spp.…”