Addressing reproductive stochasticity and grazing impacts in the restoration of a canopy‐forming brown alga by implementing mitigation solutions

Abstract

1. The worldwide decline of marine forests, due to human impacts and climate change, emphasizes the need to develop and implement effective and sustainable solutions to restore these endangered habitats and to re-establish the services they provide.2. In this study, the ex situ restoration of Gongolaria barbata (=Treptacantha barbata), a Mediterranean subtidal habitat-forming species of brown seaweed, was for the first time implemented in a marine protected area in the Adriatic Sea. Two restoration efforts were performed in 2019. The first was started in winter, after a marine heatwave that triggered early fertility, and the second in spring, when the species usually reproduces.3. This study aimed to evaluate: (i) the disruptive effects of a thermal anomaly on the reproductive biology and performance in culture of G. barbata; and (ii) the impact of the grazing pressure on juveniles after the outplanting. 4. The first cultivation was more productive than the second, in terms of zygote release and germling growth. To mitigate the low efficiency of the second culture and to avoid prolonged highly demanding maintenance in the mesocosms, the cultivation period was extended outdoors using a structure suspended in the water column.5. The modular frames conceived for outplanting G. barbata proved to be effective because of their easy operability and low cost. Controlling for herbivorous fish had significant positive effects on both juvenile survival and growth.6. The outcomes highlighted that an unpredictable climatic event and fish grazing were major threats that impaired the restoration process of G. barbata. These stressors should be considered when developing plans to implement effective large-scale restoration of canopy-forming macroalgae.

A Roadmap for the Restoration of Mediterranean Macroalgal Forests

“…Techniques can be selected according to different factors such as the availability of culture facilities, dispersal capacity of the target species, the hydrodynamic regime of the selected site to restore, and the grazing pressure. For the ex situ technique, production is hampered by the availability of facilities (e.g., the dimension and number of environmentally controlled rooms, the number of aquaria, labor; Savonitto et al, 2021) and the proximity of the restoration site, considering that transport to the field is a critical step for the success of the intervention (Tamburello et al, 2019). The in situ technique and the ex situ technique appear to be especially suitable for species with a higher and a lower dispersal capacity, respectively (Verdura et al, 2018;De La Fuente et al, 2019).…”

“…Contrarily, the ex situ cultivation of zygotes should be prioritized (Verdura et al, 2018) in areas with high hydrodynamic conditions (e.g., on exposed intertidal and shallow subtidal rocky reefs), where zygotes can be swiped out to the water column. Other factors apart from hydrodynamism, such as herbivory, may lead to opting for the ex situ technique, thus avoiding the grazing of the recruits (Savonitto et al, 2021).…”

“…As demonstrated by Marion and Orth (2010), seagrass seed production in donor forests can vary dramatically from year to year, also due to climate change (Bevilacqua et al, 2019). Likewise, abnormal winter temperatures and exceptional wind periods (Bevilacqua et al, 2019;Orlando-Bonaca et al, 2021;Savonitto et al, 2021) lead to serious biological anomalies and the loss of the reproductive potential of Gongolaria barbata. Therefore, it is crucial to act as fast as possible during the reproductive season of the selected species to collect an appropriate number of mature apexes.…”

“…Furthermore, the potential effects of protection on the effectiveness of macroalgae restoration is still controversial, according to the available data. In this line, while examples of macroalgae restoration can be found in both MPAs and nonprotected areas (e.g., Verdura et al, 2018;Medrano et al, 2020), studies commonly point to herbivory as the major threat to restoration processes (Carney et al, 2005;Campbell et al, 2014;Tamburello et al, 2019;Savonitto et al, 2021). Some MPAs have worked to restore kelp populations, especially where cascading effects have facilitated grazer control and where no other stressors are present (e.g., increases in the populations of urchin predators such as sea otters, fish, or lobsters) (e.g., Shears and Babcock, 2003;Watson and Estes, 2011;Caselle et al, 2018;Eger and Baum, 2020).…”

Mediterranean rocky reefs in the Anthropocene: Present status and future concerns

“…Yet, significant research is emerging, aiming at identifying both the feasibility and self-sustainability of restoration efforts as well as adequate techniques and spatial layouts (Claudet and Fraschetti, 2010). Concerning rocky habitats, most of these research efforts have focused on the recovery of macroalgal stands (Verdura et al, 2018;Guarnieri et al, 2020;Orlando-Bonaca et al, 2021;Savonitto et al, 2021). Recent efforts have also focused on the restoration of populations of invertebrate species, such as the scleractinian coral Astroides calycularis (Musco et al, 2017), the bryozoan Pentapora fascialis (Pagès-Escolà et al, 2020), and the limpet Patella ferruginea (Ferranti et al, 2021), with many novel technologies and approaches emerging, such as or the use of electro-mineral accretion for enhancing the settlement of the red coral Corallium rubrum (Benedetti et al, 2011).…”