Macroalgal restoration to reverse degraded rocky-shallow habitats: the case of Gongolaria elegans forestation

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Alba Medrano, Bernat Hereu, Maria Cleminson, Marta Pagès-Escolà, Graciel·la Rovira, Jordi Solà, Cristina Linares

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Macroalgal beds dominate the shallow benthic Mediterranean habitats where they play a pivotal ecological role. Among them, the canopy-forming Cystoseira sensu lato species represent the highest structural complexity level and provide unique habitats with ecological services comparable to terrestrial forests. Canopy-forming algae are in decline in many coastal areas where, among other impacts, overgrazing by herbivorous can lead to the loss of these diverse habitats shifting towards degraded barren grounds. Once established, low productive barren grounds are considered stable states maintained by several positive feedback mechanisms that prevent the recovery of marine forests. To revert this global decline, local ecosystem conservation tools such as the No-take zones (NTZs) and active restoration strategies are widely proposed to speed up the recovery of impacted ecosystems. Here, we tested the success of forestation techniques to promote functional and productive Cystoseira s.l. forests in degraded sea urchin barren grounds by combining different restoration strategies (active, passive, and combined actives with passive strategies) inside and outside the Mediterranean NTZ of the Medes Islands. Forestation success was assessed in 6 barren grounds (3 inside and 3 outside the NTZ) one year after the following three-step protocol: 1) sea urchin population eradication, 2) seeding with Gongolaria elegans, and 3) enhancement of G. elegans recruitment. Successful forestation was achieved after combining active with passive restoration strategies, which improved the associated benthic community and the cover of G. elegans. Our results encourage forestation of barren grounds to shift from less productive habitats to complex macroalgal forests, highlights the effectiveness of combine active with passive restoration strategies and the important role of marine reserves.

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