The synergic pressures of human activities and climate change threaten the structure and functioning of marine ecosystems, leading to habitats depletion and endangering all life in the worldwide seas. Cumulative impact assessment studies on key taxa or ecosystems have dramatically updated information for the Mediterranean Sea, where up to the 99% of the territorial waters of EU member states are subjected to high impact, and less than 1% of the surface is relatively unaffected by human activities (Micheli et al., 2013; Gerovasileiou et al., 2019). The effects of the multiple drivers that lead to the degradation of habitats within the European seas will increase in the next few decades without a sustainable approach to using natural resources, large-scale actions to mitigate human impacts through ecosystem-based management, and effective restoration programs (Danovaro et al., 2021).

The restoration ecology encompasses a wide range of concepts driving different actions to improve the recovery of habitats and ecosystems after natural or anthropogenic changes (Elliott et al., 2007). Within the restoration ecology, the mitigation aims to reduce and offset the environmental impacts deriving from development projects through conservation or active measures. It refers to in situ actions to make an impact less severe and represents a way of reconciling economic development and biodiversity conservation (Jacob et al., 2018). On the other hand, when the rehabilitation occurs in another system rather than the impacted one (i.e., ex situ creation of habitats) or after the habitat or ecosystem loss due to disturbances we should refer to compensation (Elliott et al., 2007). Following the Impact Assessment Directive (2014/52/EU) and national laws of EU members, marine development projects are subject to Environmental Impact Assessment (EIA) or Strategic Environmental Assessment (SEA). When the works are expected to cause environmental impacts on priority habitats, EIA often requests mitigation measures that permit or consent for any projects carried out at sea (Elliott and Cutts, 2004; Wood, 2014). The current Italian legislative framework imposes mitigation and compensation measures within certain limits of authorized impact, following the cost-benefit analysis of a given project. Under this scenario, transplantations of Posidonia oceanica (L.) Delile, 1813 shoots have been carried out in Italian waters as a suitable measure to mitigate or compensate for the impacts caused by coastal maritime works (Mancini et al., 2021; Zenone et al., 2021).

The Mediterranean Sea is relatively poorly affected by the oil and gas industry’s impacts if compared to other basins (Micheli et al., 2013; Fraschetti et al., 2016), although the explorations and exploitations of the deep sea are increasing concerns worldwide (Cordes et al., 2016). If the major threats are related to episodic and unpredictable oil spills, the installation of structures on the seafloor (i.e., oil rigs, pipelines, and anchors) is harmful to marine benthic communities due to physical disturbance and sediment resuspension. Along the Italian coasts, the installation of methane pipelines was reported as impacting the shallow water habitats by removing seagrass meadows and modifying the substrate features and sedimentary processes (Di Carlo et al., 2005; Badalamenti et al., 2006). Consequently, several interventions to recover the meadows impacted by these coastal infrastructures have been carried out (Alagna et al., 2019; Alagna et al., 2020; Calvo et al., 2020). Subsea pipelines are a crucial component of oil and gas installations, being the pathways used for natural gas and liquid petroleum transportation. Pipelines are made of steel tube sections - generally 12 m long - with concrete coatings joined together to cover long distances; they can be installed directly on the seafloor or within trenches and tunnels. Several studies highlighted how submerged oil and gas installations act as artificial reefs, providing hard substrates for benthic organisms colonization and attracting fish fauna (Jensen, 2002; Consoli et al., 2013; Rouse et al., 2018; Rouse et al., 2019; Redford et al., 2021). Such evidence offers alternatives for the decommissioning process of subsea platforms and pipelines that should be carefully evaluated, considering opportunities given by in situ decommissioning and the potential impacts of removal activities (Macreadie et al., 2011; Burdon et al., 2018).

The Trans Adriatic Pipeline (TAP), the Trans-Anatolian Natural Gas Pipeline (TANAP), and SCP (South Caucasus Pipeline) are the three segments of the Southern Gas Corridor, a 3,500‐kilometer-long gas value chain to transport natural gas from the Caspian basin to Europe (Figure 1; https://www.tap-ag.com/). It has been named a Project of Common Interest for Europe’s energy future, and it has several implications in the geopolitical scenario involving the EU, USA, and Russia. TAP crosses Greece, Albania, and the Adriatic Sea: natural hazards (Marinos et al., 2019; Slejko et al., 2021), local conflicts (Mocavini, 2019), archaeological findings (Bejko, 2020), and environmental matters arose during planning (2008-2016) and construction phase (2016-2020). As for the latter issue, the presence of protected and vulnerable marine ecosystems along the Apulian coasts, where the final section of the pipeline was located, imposed the development of biodiversity conservation measures to avoid or mitigate impacts on natural habitats. In particular, mesophotic reefs structured by hexacorals, bivalves, serpulids, and bryozoans create a mosaic of bioconstructions in the area at depths between 30 and 70 m (Bracchi et al., 2017; Corriero et al., 2019; Piazzi et al., 2019; Cardone et al., 2020; Giampaoletti et al., 2020). These invertebrates can build structurally complex and taxonomically rich bioconstructions by playing the role of primary builders, such as calcareous red algae belonging to the Corallinales and Peyssonneliales in the coralligenous reefs. The heterotrophic dominance shown by these reefs contributes to diversifying the panorama of coastal Mediterranean bioconstructions (Ingrosso et al., 2018), attracting increasing interest from scientists, and stressing the need for their protection and conservation. Following the EIA emitted for TAP, the Italian Ministry of the Environment imposed ad hoc mitigation measures, also based on improvement proposals directly made by TAP, encompassing both monitoring and active actions to reduce the impact on mesophotic reefs,.

The present work examines the approach, decision-making, tools, and technologies applied to mitigate the interference between TAP pipeline and mesophotic reefs built by invertebrates, stressing the policy relevance of mitigation interventions following human underwater constructions/installations. Through a detailed description, this unique case study outlines the planning, intervention, and monitoring phases that constitute a paradigmatic example of oil and gas industry involvement, together with scientific researchers, in mitigating the loss of Mediterranean benthic habitats. The scope of the intervention was to avoid the loss of organisms relocating living portions of the bioconstructions, certainly affected during the installation works, on the concrete coating of the pipeline. We expected that the proposed approach will reduce the impact on the benthic habitats (mitigating the impact of man-made structure without considering a donor site except for those affected by the pipeline installation), reflect the ecological complexity of the mesophotic reefs, and facilitate the presence of slow-growing species of high ecological/conservation concern as fouling on the pipeline.

The present study sets up a stepwise approach for the mitigation of the impacts on mesophotic reefs deriving from the laying of the TAP pipeline, based on the accurate investigation of the area, the choice of the taxa on which to focus the interventions, and the following monitoring activities, by means of the employment of technologies and tools of the oil and gas industry. Each phase is propaedeutic for the next one and outlines the workflow of decision-making and operational phases that represent the first intervention of relocation of portions from the mesophotic reefs. Such workflow is summarized in Figure 9.

As highlighted by Fraschetti et al. (2021) in their recent analysis, the knowledge of the structure and state of ecosystems affected by anthropogenic disturbances, and the understanding of the disturbances, play a key role in the development (and effectiveness) of actions aiming at the recovery of biological communities. We accurately mapped and characterized the benthic assemblages within the area affected by the pipeline installation through the preliminary surveys carried out in 2017. The assemblages showed different structures and compositions according to depth. The reefs dominated by encrusting red algae belonging to the coralligenous reefs were located between 30 - 50 m depth. The assemblages of the biogenic outcrops between 50 and 80 m depth share the features of the mesophotic reefs described in studies carried out along the Apulian coasts at similar bathymetric ranges (Longo et al., 2018; Corriero et al., 2019; Piazzi et al., 2019; Cardone et al., 2020; Giampaoletti et al., 2020), forming a unique environment in the Mediterranean basin. In these temperate reefs, the dominance of encrusting red algae typical of the coralligenous is shifted in favor of invertebrate taxa (mainly scleractinians), showing similar growth and functioning patterns if compared to the deep cold-water coral habitats (Roberts et al., 2009). Despite the importance of the development project due to the geopolitical and energy context, and the small number of interfered outcrops, these findings highlighted the need to plan an intervention to mitigate the impacts related to the laying of the pipeline. TAP pipeline represented a physical disturbance for the outcrops located within its footprint (i.e., the area with direct impact on the seafloor), as well as the placement of these infrastructures on the seabed may cause a transient increase in local sedimentation (Cordes et al., 2016). Mitigation measures have been implemented in situ to avoid the loss of organisms and facilitate the presence of species of high ecological/conservation concern as fouling on the pipeline. We adopted a hybrid method that reflects two different approaches:

- The use of artificial structures (namely the cause of the disturbance, TAP pipeline), as in cold-water corals or deep-sea restoration interventions due to operational issues (O’Connor et al., 2020; Montseny et al., 2021);

The relocated nuclei have the function of recreating the morphology and complexity of the biogenic outcrops which, together with the functional traits of the dominant species and the duration of the immersion of the substrate, have been recognized as the main factors accounting for the similarities (or dissimilarities) between artificial and natural reefs (Perkol-Finkel et al., 2005; Perkol-Finkel et al., 2006). The nuclei were designed as expansion patches that will colonize the pipeline themselves (Rouse et al., 2019). Furthermore, the presence of several taxa on the nuclei reflected the complexity of the ecological interaction within the mesophotic reefs, overcoming the limitations of an intervention conducted on a single species (Fraschetti et al., 2021) and transferring to the pipeline, as less altered as possible, the assemblages found on the interfered outcrops.

To preserve the richness and complexity of the assemblages found on the outcrops, we focused the efforts on those taxa of most significant ecological (or functional) and conservation interest. On the one hand, this reflects the general features of the restoration actions, but on the other, it constitutes one of the novelties of the study. The application of species functional trait-based approaches is receiving growing attention in the restoration ecology and can represent significant progress for maintaining or recovering ecosystem services (Laughlin, 2014; Laughlin et al., 2018; Carlucci et al., 2020). Using these approaches, some species are selected instead of others based on their functional traits and the expected outcomes. While this is widely recognized for terrestrial habitats, one of the main issues in applying these metrics in the marine environment is the lack of taxonomic and functional information. The lack of knowledge is exacerbated in poorly explored and recently defined habitats, such as mesophotic reefs, in which it is urgent to define or update the information associated with the taxa structuring the assemblages. In this context, the relocation score was conceived as a synthetic tool, simple but effective in selecting the key species whose loss had to be minimized without altering the original assemblages. Therefore, we selected the set of the most abundant taxa responsible for the growth and the three-dimensional structure of the reef. Due to the impossibility of entirely relocating or moving the outcrops, this approach encompassed the need to mitigate the loss of species and habitats. Furthermore, the relocation score contributed to selecting the taxa whose size and consistency took into account operational limitations and the possibility of handling the organisms for relocation purposes.

The relocation of the nuclei allowed the presence of taxa on the pipeline that would otherwise have taken decades to colonize it (i.e., A. cannabina, A. polypoides, A. verrucosa, and scleractinians) due to their low dynamics and slow growth features (Teixidó et al., 2011; Montero-Serra et al., 2018). On the other hand, the monitoring of the nuclei provided information on fast-growing species that rapidly colonized both the nuclei and the pipeline concrete coating after one year of immersion (i.e., encrusting corallinales, encrusting sponges, Reteporella sp., and serpulids). These taxa have been reported as opportunistic or characteristic of the first stage of community development on coralligenous reefs (Fava et al., 2016; Sartoretto et al., 2017; Casoli et al., 2020b; Turicchia et al., 2021). Albeit not analyzed in the present work, fast-growing species settled on the pipeline sections not interested by nuclei relocation, so we can argue that their settlement was not influenced by the relocation activities. Ad-hoc sampling methods will be conceived in the future to identify and quantify the occurrence of competition phenomena among fast and slow-growing taxa and assess the influence of the relocated nuclei on the colonization of the TAP pipeline.

Effectiveness and success are key concepts in restoration ecology, but difficult to quantify because they are strictly related to the period of monitoring, which often has a limited duration, and due to the absence of univocal indicators to assess what was expected (Bayraktarov et al., 2016; Abelson et al., 2020; Basconi et al., 2020). The results gained through the monitoring activities are extremely limited in time; they must be evaluated with caution, assessing the survival rate of the nuclei on the pipeline and the structure of the assemblages after one year from the intervention exclusively. The high survival rate of the nuclei, as well as the settlement of new taxa, are encouraging data for future measures. However, these results represent the first efforts of a long-term and annual monitoring that will last 10 years. At the end of this period, the same measurement will be acquired, with particular attention to assessing if the nuclei will increase in size and will influence the biological colonization along the traits of the pipeline interested by the relocation activities.

One of the main novelties of this study is the involvement and support (economic, technological, and operational) of the private oil and gas sector, which made the intervention possible. This study represents a paradigmatic case of involvement of the oil and gas industry in the Mediterranean to mitigate habitat loss utilizing vessels, ROV’s, and saturation commercial diving technologies. The activities followed the Ministry for the Environment requirements, and were continuously monitored and shared with the Italian Institute for Environmental Protection and Research (ISPRA) and the Regional Environmental Protection Agency of the Apulia Region (ARPA Puglia). After the accurate preliminary surveys along the pipeline route, the activities required extensive seafloor monitoring and mitigation measures for the negative impacts that could not be avoided. The recovery of degraded or damaged habitats represents a crucial aid for management strategies to provide biodiversity conservation and improving services for human health and well-being (Possingham et al., 2015). However, active marine habitat restoration has extremely high costs (Bayraktarov et al., 2016) that in deep habitats are estimated to rise by two or three orders of magnitude per hectare if compared to the costs of activities carried out in coastal marine ecosystems (Van Dover et al., 2014; Danovaro et al., 2021; Montseny et al., 2021). The potential of recovery actions, proven by positive outcomes of worldwide efforts carried out both on land and underwater, together with the need to reverse the trend of ecosystems degradation, are the pillars of the UN Decade on Ecosystem Restoration 2021–2030: a global rallying cry to heal our planet (https://www.decadeonrestoration.org/). Both costs and operational issues are not fully manageable by the sole scientific community, especially when dealing with deep-sea environments. The sole restoration actions, albeit carried out over large spatial scales, have a low chance of success without the support of different spheres of our society (Waltham et al., 2020): a further step should be adopted to achieve the goals we hope, involving governments, industries, and finances.

Despite the scope of the study, we stress the importance that the mitigation actions, such as those described in this study, should not be used to tolerate impacts created by development projects and not be taken as a “license to trash “. Furthermore, among the criteria that guide the choice of the route of a pipeline in the projecting phase, environmental issues must have the same weight as economic (including the length of the pipeline) and operational factors (Hamid-Mosaku et al., 2020). In the specific case of TAP, the number of affected outcrops was relatively low to allow the development of actions to mitigate the impacts; such an approach should be carefully evaluated case by case. In parallel, it is necessary and imperative to create a policy framework that drives decisions on the exploitation of marine resources and take into consideration i) to avoid irreversible impacts, ii) impose mitigation interventions, iii) assess costs and iv) identifies who takes responsibility for financing (Da Ros et al., 2019).

According to FAO’s guidelines, mesophotic reefs represent vulnerable marine ecosystems: their knowledge and distribution are far from being wholly achieved. However, they are receiving growing attention worldwide as they may offer refuge to shallow species exposed to unstable environmental conditions and climate change effects (Cerrano et al., 2019). In the light of their role in the functioning of the marine ecosystems, as well as the provision of essential goods and services for human populations, conservation and restoration measures should be developed in the future to decrease human impacts and guarantee the sustainable use of natural resources (Chimienti et al., 2019; Bevilacqua et al., 2021).

This study represents the first intervention in deep Mediterranean coastal habitats to actively mitigate impacts linked to human economic activities, such as pipeline laying, using oil and gas industry technologies and tools. The support of the private sector can be fundamental for recovery actions to operate in environments that would otherwise be difficult (due to depth or spatial constraints) and cover expensive costs. However, a correct evaluation during the project phase (i.e., ante operam) should drive the choice of the site in light of its environmental features, economic and political reasons.

Policymakers, industry, and researchers should work side by side in the next future, developing practical projects and interventions to mitigate the effects of the exploitation of marine resources. This is particularly relevant with the upcoming development of offshore wind farms in the Mediterranean Sea. Integrated management involving all the stakeholders is urgently needed to identify feasible solutions or avoid irreversible degradation.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

EC: conceptualization, data curation, investigation, methodology, formal analysis, writing—original draft, writing—review and editing; DV: data curation, investigation, formal analysis, writing—review and editing; GM: data curation, formal analysis, writing—review and editing; AB: investigation, methodology, writing—review and editing; GA: supervision, conceptualization, writing—review and editing. All authors contributed to the article and approved the submitted version.

All the authors, as scientists of the Sapienza University of Rome and CIBM, were commissioned by TAP to assess the environmental risks of the pipeline laying operations, to monitor the environmental conditions during the works, and to follow the relocation activities to mitigate the impacts. As mentioned in the text, the authors’ work was controlled and shared with the Italian Institute for Environmental Protection and Research (ISPRA) and the Regional Environmental Protection Agency of the Apulia Region (ARPA Puglia).

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.