A review of water-energy-food-ecosystems Nexus research in the Mediterranean: evolution, gaps and applications

The water-energy-food-ecosystems (WEFE) Nexus has gained recognition as an innovative approach for analysing the interconnectedness of global resource systems and achieving sustainability goals. In the Mediterranean, where water scarcity, climate change, and ecosystem degradation pose significant challenges, implementing an integrated WEFE Nexus approach is crucial. We conducted a comprehensive review of scientific literature through the lenses of Nexus ‘ideas’, ‘relationship’ and ‘practices’. A total of 142 research articles were selected and characterized in terms of WEFE interlinkages being investigated, explored topics, methods and scales of analysis, and contexts of operationalization. We found that water-energy interdependences dominate WEFE Nexus research in the Mediterranean, driven by the large presence of energy-intensive water abstraction and distribution systems to meet drinking and irrigation water demands. At the same time, the expansion of the Nexus approach to additional components is only partial, mostly focusing on assessing impacts on the physical environment and climate, without capturing feedback dynamics. Geographically, Nexus research in the Mediterranean is primarily conducted in isolated case studies, with few large scale assessments developed at the entire Mediterranean scale, and with some countries not yet represented, in particular the Western Balkans. Although WEFE Nexus research in the Mediterranean is recognizing the importance of transdisciplinarity moving beyond biophysical assessments to encompass societal and governance dynamics, further research is needed on understanding the economic implications of WEFE Nexus interactions. To advance Nexus implementation in the region, sustainable technology, and natural resources management, which are key fields of WEFE Nexus research operationalization, would benefit from harmonization in their design objectives. This would enable a more comprehensive and coherent approach towards achieving water, food, and energy security while preserving the environment in the Mediterranean region.


Introduction
While the concept of integrated resources management acknowledges the prevailing dynamics between the several resources examined (Florin and Gabriel 1991, Pahl-Wostl 2007, Rockström et al 2009, the emergence of the Nexus 8 approach (Hoff 2011) during the last decade highlighted the importance of accounting for these dynamics, with a specific emphasis on the interconnectedness present when dealing with the management of water, energy, and food (Ringler et al 2013). One of the earliest explicit introductions of the Nexus approach sprung in parallel with the debate on the sustainability of biofuel production as an energy source promoting reduction in carbon emissions (Bauen et al 2015). Nexus based studies pointed out how the land use change impact induced by the cultivation of crops used in biofuels could be in conflict with availability of land for food production, could increase water consumption and determine an expansion of agricultural activities contradicting the desired reduction in emissions (Moioli et al 2018, Viccaro et al 2022. Hence, the Nexus approach acknowledges the mutual interlinkages between water, energy and food, and calls for the creation of systems that maximise synergies and minimise trade-offs among sectoral needs to optimise the sustainable management of natural resources and to improve resource use efficiency (Bazilian et al 2011). Embracing such a holistic approach in lieu of a silo sectorial thinking could help create integrated solutions for the achievement of the Sustainable Development Goals (Carmona-Moreno et al 2021). Even more so in the Mediterranean, a region of the world characterised by severe threats of water scarcity, increasing food and energy demands and degradation of natural ecosystems. Indeed, climate projections for the Mediterranean clearly point to this region as a hotspot of change, with rates of warming and reduction in precipitation above the global average, especially in the southern and eastern Mediterranean countries (Lionello and Scarascia 2018). The anticipated changes in temperature, precipitation, sea level and in the frequency of droughts and floods will combine with existing environmental and socio-economic pressures, i.e., population growth, conflicts, water pollution, potentially leading to severe impacts for water security, food security, ecosystems health, and ultimately human security (Cramer et al 2018). The Union for the Mediterranean (UfM) placed the waterenergy-food-ecosystems (WEFE) Nexus at the heart of their Water Policy Framework for Actions 2030 to increase the resilience against such compound risks and to promote social stability, suggesting interventions at local and regional scales, including Nexus assessments, multi-stakeholder policy dialogues, testing of Nexus pilot applications and leveraging finance for scaling up Nexus solutions (Union for the Mediterranean 2020). Research and innovation are key enablers of such a transformation, since they can help understand the complexity of Nexus interactions, demonstrate the benefits of adopting a Nexus approach and provide evidence for informed decision making. In such a context, the Partnership for Research & Innovation in the Mediterranean (PRIMA) Foundation introduced in 2019 the WEFE Nexus as a new thematic stream of funding to mobilise researchers, stakeholders and innovators from the Euro-Mediterranean region and advance Nexus implementation (Riccaboni et al 2022). However, a comprehensive review of whether current research across the Mediterranean is aligned with the pressing need of adopting such an integrated approach in managing and governing natural resources is yet to be produced. In fact, assessments of Nexus implementation across the region have to date focused on reviewing projects' case studies (Hoff et al 2019, Malagó et al 2021), analysing policies and legislations (Bazzana et al 2023) and gathering regional experts' opinions (Markantonis et al 2019). No studies have investigated outputs of scientific research specific to the region.
Numerous literature reviews on Nexus research at the global scale exist and they have all emphasised the emergence of a large diversity of definitions, design objectives, modelling approaches and applications (Albrecht et al 2018, Galaitsi et al 2018, Purwanto et al 2021, Taguta et al 2022. However, only by adopting a geographical focus in the review process, main drivers, key traits, and limitations of Nexus research in a specific region can be identified. The goal of this review article is, therefore, to assess Nexus research in the Mediterranean in terms of critical interlinkages being investigated, explored topics, methods and scales of analysis, and context of application. In doing so, research gaps and possible future directions for research in the Mediterranean are revealed and, secondly, compared against other regional and global findings. Moreover, we deliberately expand the review to the fourth 'Ecosystem' component, the WEFE Nexus, to align with the recent research priority given to integrating natural ecosystems and their services into the Nexus approach (Hülsmann et al 2019). Nexus researchers and practitioners interested in understanding the methodological approaches that address the WEFE Nexus in the Mediterranean are considered the major audience of this review. Figure 1 illustrates the procedure adopted in this study for the review of scientific literature on the WEFE Nexus in the Mediterranean. It consisted of three steps: (i) identification of articles from research databases, (ii) screening of the identified articles based on exclusion criteria and (iii) analysis of the selected articles.

Methodology
Step 1 'identification' and Step 2 'screening' followed the PRISMA guidelines for conducting meta-analyses (Page et al 2021), while in Step 3 the integrative review approach presented by Snyder (2019) was adopted to complement the aim of understanding the evolution of Nexus research in the Mediterranean in terms of ideas, relationships, and practices.

Step 1: Identification of articles At
Step 1, peer-reviewed articles were identified from SCOPUS and Web of Science database by performing multiple search queries in the title, abstract and keywords of the database articles. The terms of each query consisted of keywords representing the multiple possible combinations that can be obtained from the components of the WEFE Nexus, e.g., waterenergy Nexus, energy-food-water Nexus, waterenergy-food Nexus, and the list of the Mediterranean countries, or the word 'Mediterranean' . All countries surrounding the Mediterranean Sea and Portugal constituted the list of Mediterranean countries: Spain, France, Monaco, Italy, Slovenia, Croatia, Bosnia-Herzegovina, Montenegro, Albania, Greece, Turkey, Syria, Lebanon, Palestine, Israel, Egypt, Libya, Algeria, Tunisia, Morocco, Cyprus, Malta, and Portugal. The term 'resource Nexus' was also used in combination with the geographical restriction to identify articles that studied the Nexus among natural resources without explicitly mentioning the WEFE Nexus components. The search was limited to peer-reviewed articles published in the English language from 2011 onwards. The use of an application programming interface and the pybliometrics 3.4.0 Python package (Rose and Kitchin 2019) allowed direct access to the SCOPUS database and the automated retrieval of articles based on the set search queries. Retrieval of articles from Web of Science was performed manually. The last search query was carried out on 31/10/2022 and produced a total of 421 articles.

Step 2: Screening of identified articles
The articles identified in Step 1 were screened based on four exclusion criteria to ensure that the review focused only on articles pertinent to understanding the emergence and operationalization of the WEFE Nexus approach in the Mediterranean region. Firstly, we included only peer-reviewed and original research, therefore excluding review articles and those book chapters and conference articles whose research content was replicated in other articles. Secondly, we focused on articles explicitly analysing interconnections among water, energy, food, and ecosystems. In general, most studies that address synergies and trade-offs between any two components, hold the potential to be labelled as a Nexus study. Therefore, we avoided any ambiguity by selecting the articles that explicitly addressed interconnections between at least two of the four WEFE Nexus components. Thirdly, the assessment of the practicality of academic research on the WEFE Nexus was ensured by excluding articles without a defined methodology and application in a case study. Lastly, articles whose case study fell outside the defined Mediterranean region were not considered in the review. The application of the exclusion criteria described above led to the selection of 142 articles.

Definition of Mediterranean region
There is no single definition of the Mediterranean region (Merheb et al 2016) and the delimitation of Mediterranean boundaries in WEFE Nexus applications is even more difficult given the spatial heterogeneity of the WEFE Nexus components. While hydrological applications adopt a catchment area definition, ecological studies expand such limits to entail a bigger bio-climatic zone (UNESCO and FAO 1963). From an economic and political perspective, the region may also be defined through the administrative boundaries of countries surrounding the Mediterranean Sea (PNUE/PAM-Plan Bleu 2009). Adopting a single definition of the Mediterranean region in the context of WEFE Nexus research is also challenged by the various spatial scales addressed by Nexus assessments, ranging from local to national and international. Thus, in the absence of a systematic definition of Mediterranean and of a WEFE Nexus study scale, we adopted a two-faceted definition of the Mediterranean region. For all studies addressing the WEFE Nexus at the sub-national scale, we considered the union between the Mediterranean bioclimate limit and the Mediterranean catchment basin, with the exclusion of the Nile River. The Nile River basin was excluded in our analysis since most of the discharge of the Nile in Egypt comes from runoff generated in the equatorial lake regions and Ethiopia (Elsayed et al 2022), areas which we do not deem Mediterranean from a bioclimatic, hydrological, and administrative perspective. Thus, for the country of Egypt, we considered Mediterranean those case studies that fall within the area draining into the Mediterranean Sea or that are hydrologically disconnected from the Nile River. For studies addressing the WEFE Nexus at the national or international level, we considered as Mediterranean all the countries surrounding the Mediterranean Sea with the addition of Portugal. By doing so, a bigger geographical extent is adopted that allows to identify whether the WEFE Nexus is dealt with as a water-related problem (hence focused on water catchments areas) or crosses these boundaries with the integration of elements like ecosystems, and trade-offs with exogenous factors emerging at a larger scale. The definition of both bioclimatic limit and catchment area is adopted from the Plan Bleu (PNUE/PAM-Plan Bleu 2009).

Step 3: Analysis of screened articles
In Step 3, the 142 selected articles were reviewed based on three different levels of analysis and six classification criteria to provide a qualitative and quantitative representation of the results. This is in accordance with the integrative approach stated by Snyder (2019), suitable for emerging cross-cutting topics, such as the water-energy-food Nexus. The levels of analysis were 'ideas' , 'relationships' , and 'practices' , and these were revealed by classifying each article based on six criteria, described in table 1. The initial indication of 'ideas' in WEFE Nexus research emerges from the keywords used by the author to describe their article and by the specific Nexus components being addressed, e.g., water-food, energyecosystems, WEFE. 'Relationships' instead depict in this review the various geographic scales of analysis adopted in each article, spanning from the micro scale (i.e., building, field) to the entire Mediterranean region. Within 'relationships' , the focus is also put on the interactions between authors and the local social Table 1. The analysis framework adopted for the review of selected articles on the water-energy-food-ecosystems Nexus. For each classification criteria, the reader is directed to the corresponding section of the review article.

Classification criteria Description
Level of analysis

Data visualization
The results of the analysis were presented using a suite of visualization tools including bar plots, pie charts, network diagram and Sankey diagrams. Bar plots and pie charts were created with ggplot2 package in R (Wickham 2016, R Core Team 2022), while VOSViewer software (van Eck and Waltman 2010) was used to produce the network diagram visualizing the co-occurrence of the articles' keywords. The network diagram is composed of vertices, representing the single keywords, and of links representing the co-occurrence of two keywords in the same article. To limit the hairball effect, keywords occurring in less than four articles were excluded from the diagram. Finally, Sankey diagrams were created using ggsankey package in R and used to visualize the distribution of the reviewed literature across the classification criteria. In fact, the 142 reviewed articles were published in 76 different journals, 70% of which contained only one article (figure 3(a)). Only Water and Sustainability counts more than eight articles each, followed by two journals in the field of Environmental Science, i.e., Science of the Total Environment and Journal of Cleaner Production, and two journals in the field of Energy, i.e., Applied Energy and Renewable  Energy, all with five articles each (figure 3(b)). Agronomy comes after, with 4 articles.

Keywords
Whilst 501 different keywords were used by the authors of the reviewed articles to describe their research on the Nexus in the Mediterranean, we focused on the authors' keywords that re-occurred at least four times, i.e., a group of 19 terms. Figure 4 shows the frequency of occurrence of the 19 keywords and the intensity of their co-occurrence. The most frequently used keywords were water-energy Nexus, water-energy-food Nexus, sustainability, and Nexus. There is a clear bipolarization around the concepts of water-energy Nexus and water-energy-food Nexus, creating two distinct, but interconnected, groups of keywords. Both water-energy-food and water-energy Nexus are linked with water scarcity and climate change, highlighted in the reviewed literature as main drivers for an integrated management of natural resources in the Mediterranean, and with sustainability, the ultimate goal of adopting a Nexus approach. Water efficiency and energy efficiency arise in the keywords' cloud as concrete outcomes from the promotion of a Nexus approach to achieve sustainability. Desalination is the only process within the water, energy and food supply chains that appears in the authors' keywords map, highlighting the centrality of this source of water for the Mediterranean region. Desalination is strongly linked with water-energy Nexus and renewable energy, as innovative solutions are being sought to make this process more environmentally and economically feasible (Eltawil et al 2009). The keyword sustainability acts as a bridge between the three Nexus groups and a cluster of keywords representing the pressure of human activities on the environment, i.e., life cycle assessment (LCA), environmental footprints. Yet, the ecosystem component of the WEFE Nexus does not emerge in the map of the 19 most frequently used keywords, nor do other terms frequently associated with it, such as biodiversity, environment, and natural resources.
Research methods commonly used to analyse the Nexus in the Mediterranean stand out as recurring keywords in the reviewed articles and these include footprinting, LCA, system dynamics and optimization techniques. Table 2 presents the combinations of WEFE Nexus components identified in the reviewed literature. As shown by the keywords network diagram, waterenergy and water-energy-food Nexus are the most recurring arrangements accounting for 69% of the reviewed articles and followed by WEFE, energyfood, and water-energy-food-land-climate, with 4% each.

Water-energy
The bidirectional relations among water and energy are extensively studied by Nexus research in the Mediterranean, with the intent of identifying hotspot of resource use and testing options for improved resource efficiency. On the one hand, consumption of energy associated with supplying water for both domestic and agricultural use was quantified. Arfelli et al (2022)

Water-energy-food
Agriculture stands out as a major sectorial focus of water-energy-food Nexus assessments in the Mediterranean. A detailed quantification of water and energy consumption in agricultural production was conducted at different scales, from the farm to the irrigation district and river basin scales. When accounting for energy inputs into food production, some studies limited their calculation to the energy required for extracting and distributing water, e.g., de Vito et al (2017) . Although all these studies allowed the identification of trade-offs and synergies among crop yield, energy use and water use, their findings were limited to the local scale, failing thus to reveal interlinkages among Nexus sectors at higher organisational levels and beyond agriculture. A more comprehensive overview of the multiple interconnections that exist among activities in the water, energy and food sectors was provided by Mayor et al (2015) for the Duero River basin in Spain, while González-Rosell et al (2020) integrated water, energy, and food/ land modules with economic variables into a unified model to perform a joint analysis of the three economic sectors in Andalusia. To support the governance of water-energy-food Nexus interactions, a number of studies assessed the cross-sectoral impacts resulting from the implementation of policies in a Nexus sector. Examples include subsidising energy for irrigation in Morocco (Doukkali and Lejars 2015), changing water allocation/deallocation rules in Israel (Teitelbaum et al 2020) and increasing irrigation water use efficiency in Spain (Willaarts et al 2020). Institutions and actor groups can play a role in mediating trade-offs among sectors and in maximising synergies along the supply chains, as illustrated by Villamayor-Tomas et al (2015) for the Spanish region of Aragon.

Energy-food
Both energy for food and food for energy interactions are captured by Nexus research in the Mediterranean. On one hand, the use of renewable energy to produce food in rooftop greenhouses (Nadal et al 2017) and plant factories (Xydis et al 2021), and to guarantee preservation of fresh food in humanitarian context (Aste et al 2017) was investigated. On the other hand, Mantziaris et al (2017) performed an economic analysis of different energy crops as source of renewable energy for Greece, while Valenti et al (2020) demonstrated through a case study in southern Italy that anaerobic digestion of agro-industrial by-products can play a relevant role in renewable energy production.

Water-energy-food and climate
Climate was predominantly added to Nexus analysis as a criterion to evaluate the environmental sustainability of activities and processes within the water, energy and food sectors including: wastewater treatment (Marinelli et (Tobarra et al 2018). Through a more cross-sectorial approach, Ioannou and Laspidou (2022) quantified green-house gases emissions coming from Nexus sectors and integrated them in a system dynamic model to simulate interlinkages and feedback loops in the Greek water-energy-food system and to assess the resilience of the system against external shocks caused by climate change.
Finally, some studies pointed out that policies for climate change adaptation and mitigation could lead to adverse impacts in other sectors. Zografos (2017) highlighted the trade-offs between adapting to climate change, preserving natural ecosystems and sustaining agricultural production in the Ebro River Delta in Spain, while Kati et al (2021) warned against the increasing competition over land use between policies on wind energy and biodiversity conservation.

Water-energy-food and ecosystems
Ecosystems is the least represented component of the WEFE Nexus in the Mediterranean and, similarly to climate, its integration has been mostly limited to assessing the impact of the water, energy, and food sectors on the environment. Through a life cycle analysis approach, a large range of environmental impacts, including eutrophication, ecotoxicity and acidification, were evaluated for activities related to the water, energy, and food supply chains: agro-industrial waste disposals (Valenti et al 2020), olive cultivation (Fotia et al 2021), biogas production (Pacetti et al 2015) and groundwater pumping (Pradeleix et al 2015). At the national level, relationships between natural resources, energy production (Rahmane et al 2021), and food consumption (Lacirignola et al 2014), were investigated through the concept of ecological footprinting. Yet, all these studies fall short in identifying the specific ecosystem within the study area that mutually interacts with the water, energy, and food sectors. With this objective in mind, AbdelHady et al (2017) put the Wadi El Rayan Protected Area in Egypt at the centre of a water-energy-food Nexus analysis to quantify trade-offs among ecosystem health, food production and energy production under different water management scenarios. With a stronger emphasis on the benefits derived from ecosystems, Ioannidou et al (2022) qualitatively identified the links between water-energy-food Nexus parameters and ecosystems services when assessing potential trade-offs between yield, soil fertility and nutrient cycling in organic orchards, in Cyprus. However, no studies were found that quantified the flows of ecosystem services to the WEF Nexus sectors in the Mediterranean. This lack of evidence may in part explains the finding of Karabulut et al (2019), who revealed, through consultation with experts across the Mediterranean, that restoration of ecosystems and their services would theoretically have large benefits across all Nexus sectors, however its implementation is hindered by a lack of awareness among policy makers. Figure 5 shows the distribution of the case studies examined in the reviewed literature across the Mediterranean region and represented according to the geographical scale of analysis. Northern Mediterranean countries showed the highest density of Nexus applications both at sub-national scale (i.e., micro, local and river basin) and at national level, with the largest occurrence being in Spain, followed by Greece and Italy. Among the Middle East and North Africa countries, Egypt and Morocco showed the largest implementation of Nexus research with a total of 8 and 5 case studies, respectively. Western Balkan countries, Slovenia, Libya, Syria, and Malta did not contribute with any case study to the reviewed literature. Nexus research in France is also quite limited, with only two case studies at local and river basin scale. It is worth noting that other sub-national scale case studies were identified for Morocco, Egypt, and Spain, but excluded because they do not fall within the defined Mediterranean region.

Geographical scale and location of case studies
Interconnections among WEFE Nexus components in the Mediterranean were captured in the reviewed literature at different spatial scales, from the energy saved in household water conservation measures (Barberán et al 2019) to the virtual water flows embedded in international food trade (López et al 2022). In figure 6 the number of case studies for each geographical scale are reported.
Generally, studies working at micro and local scales focused on quantifying input-output relationships in coupled water, energy, and food production chains, while articles addressing larger scales assessed the implications of these interlinkages in the wider national and international context. As an example, the micro scale adopted by Bertsiou and Baltas (2022) enabled a detailed quantification and optimization of energy use in a desalination plant, but only a study, performed at the national level, could assess the implications of introducing such alternative water sources in meeting national greenhouse gases (GHG) emissions targets (Martin-Gorriz et al 2014). Thus, choosing the scale of Nexus analysis requires acknowledging the compromise between the level of analytical detail and the complexity of interactions that can be captured. Few studies attempted to address such shortcomings by working across scales. Examples include the optimization of water and energy supply chains from the technology to  Only 6 out of 142 reviewed articles analysed the Mediterranean scale in its entirety. Water and energy footprint of irrigated agriculture was calculated by Daccache et al (2014) for the full Mediterranean region, while Lacirignola et al (2014) broadened the analysis to include ecological footprint and to consider food consumption in addition to agricultural production. These early studies highlighted the need of intersectoral policies addressing trade-offs between water use, intensifying food production and reducing the pressures on the scarce resources of the Mediterranean region. Following the delineation of the Agenda 2030, the importance of addressing such water, energy and food interdependencies to coherently achieve the Sustainable Development Goals in all Mediterranean countries was again highlighted by Malagó et al (2021) and Saladini et al (2018), who proposed indicator-based frameworks to monitor progress towards sustainable development in the Mediterranean.

Approaches and tools for Nexus analysis
Forty-two different methods were employed to study the Nexus in the Mediterranean. Table 3 lists all methods used in the reviewed articles, by grouping them according to the research field they originate from, and by providing examples of their applications in the Mediterranean. The categorization in research fields is based on Albrecht et al (2018).
Overall, most of the reviewed articles (75%) used quantitative methods, followed by 9% using qualitative methods alone and 16% using mixed methods. Data analysis and statistical techniques (e.g., cluster analysis and regression analysis) were the most frequently used methods to quantify variables in Nexus sectors and to assess their correlations, therefore providing an important insight into interlinkages among Nexus components (Emir and Karlilar 2022, Sesma-Martín and Puente-Ajovín 2022). Water-related methods were deployed in 20% of the reviewed articles, with a clear preference for water footprint, followed by hydrological modelling and water distribution modelling. Footprinting, more in general, dominated the methodological landscape of Nexus research in the Mediterranean, with nearly 30% of the reviewed articles using at least one among water, energy, carbon and ecological footprints. By linking two sectors in one metric, footprints were employed to quantify resource use along the water, energy, and food supply chains. Similarly, LCA was the most used method from the environmental field to assess the impact of products, processes and activities in Nexus sectors across a wide range of environmental factors, including physical environment and ecosystems. The focus on the waterenergy Nexus in the Mediterranean is reflected in a significant use of energy-related methods, mostly targeted to the analysis of coupled water-energy technologies. Few studies, 13%, utilised methods from the field of economics and their use was limited to conducting economic appraisals of technological solutions. Economic methods that instead support the analysis of interactions between economy and the use of natural resources, such as computable general equilibrium model and multiregional input-output model, were barely used by Nexus research in the Mediterranean. The uptake of social science methods for Nexus research in the Mediterranean is relevant. Twenty percent of the reviewed literature deployed at least one social science method to investigate sociocultural dynamics in governing water, energy, and food sectors, and to account for them in Nexus modelling. Although 40% of the reviewed articles were found to use a combination of methods, integration usually took place across similar disciplines, i.e., energy, environment, water. LCAs offered the largest opportunity for integration, being used with water footprinting for assessing biogas production from energy crops in Italy (Pacetti et al 2015) and cultivation of medicinal plants (Litskas et al 2019) in Cyprus, and in combination with optimization modelling for identifying solutions that minimise impacts of food processing (Leivas et al 2020) and food waste management (Laso et al 2018) on natural resources. Integration between natural and social sciences was achieved in the reviewed literature through the use of participatory research techniques (e.g., focus groups, Delphi survey, fuzzy cognitive mapping) to inform the definition and development of system dynamics models simulating the complexity of WEFE Nexus systems (Martinez et al 2018, Sušnik et al 2018.
Choosing appropriate methods for analysis depends on the type and complexity of Nexus interdependencies being studied and on the geographical scale being addressed. Figure 7 illustrates the scales at which the methods identified in the reviewed literature were applied. At the micro and local scale, LCA, energy modelling and technical-economic analysis tended to prevail, whereas at national level data analysis, statistics and water footprint were more frequently used. Optimization modelling and index/ indicators, instead, were more ubiquitous across scale.

Stakeholder engagement
Stakeholders were engaged in 39 studies, 30% of the reviewed articles. Among these, 17 studies approached farmers, local authorities, citizens, and managers of the private sectors to conduct data collection through questionnaires and surveys. In 14 studies, engagement went a step further by co-defining with stakeholders local Nexus challenges and interlinkages among sectors. In-depth semi-structured interviews were conducted to further delineate the local problem and, in some cases, integrated with the construction of casual loop diagrams representing the causes and consequences of the problem from the interviewee's perspective (Halbe et al 2015, de Vito et al 2019). The highest level of engagement was achieved in 8 studies (out of the total 142 reviewed) Table 3. List of methods used to analyse the Nexus in the reviewed articles. Methods are grouped by research field. The percentage of articles using methods from the listed discipline is provided in parentheses. For each method, representative examples of their application in the reviewed literature are given.

Economics (12%)
Computable General Equilibrium Model 1 Assessing the economic impacts of sectoral and regional competition for water resources under a climate change-driven future scenario (   Representing and simulating various water supplies (surface, groundwater, treated wastewater) and demands (municipal, irrigation) (Almulla et al 2022) Crop water modelling 1 Calculate crop water requirements to evaluate water and energy savings through irrigation planning (Brik et al 2022) Other (1%) Laboratory Experiment 1 Testing the application of fertilizer drawn forward osmosis (Amin et al 2021) Water-energy audit 1 Quantifying water flows and energy consumption in a building to assess the impact of implementing water efficiency measures (Rodrigues et al 2020) by including stakeholders in the co-creation of methods of analysis and in the co-development of solutions. González-Rosell et al (2020) built on the results of a fuzzy cognitive mapping performed with stakeholders in Andalusia to create a model of interconnected water, energy and food sectors that was subsequently validated and tested by the same stakeholders through an iterative process. A constant flow of communication among researchers and stakeholders, and their regular consultation throughout the research project, were central to the co-development of a serious game to explore the water-energyfood-land-climate Nexus in Sardinia (Sušnik et al 2018) and to the co-creation of Nexus decision metrics for managing the agriculture-water-energy Nexus in the Souss-Mass basin (Almulla et al 2022).

Nexus research operationalization
The classification of the reviewed articles against the criteria revealed a clustering of Nexus research in the Mediterranean around two main contexts of operationalization, defined in this review as 'natural resources management' and 'sustainable technology' . The natural resources management group gathers articles that employ Nexus thinking to advance knowledge on the interconnectedness of water, energy and food sectors and their relationships with society and economy, while the sustainable technology cluster encompasses articles that examine the implementation of specific technological solutions to support the efficient use of Nexus resources. Figure 8 shows the relative contribution of the two clusters to the reviewed literature and their predominant attributes for each classification criteria. The sustainable technology group is dominated by research on the Nexus between two components, mostly waterenergy Nexus and energy-food Nexus, and it focuses on technological systems, such as renewable energies for desalination, hydraulic pump reservoirs for energy storage, agricultural waste for bioenergy production and micro hydropower plants in irrigation networks. These studies aimed at supporting the implementation of technologies by assessing their best siting and feasibility, and by optimising their design and operations. Technical-economic analysis was often used in the planning stage to assess their feasibility alongside the use of LCA for assessing their environmental impact, whereas optimization techniques and detailed water or energy modelling were deployed to inform their design. The operationalization of the Nexus approach in the context of natural resources management is mostly characterized by studies addressing Nexus combinations of three or more components, i.e. water-energy-food, WEFE and water-energy-food-climate. These studies deployed a wide range of quantitative and qualitative methods at different spatial scales to understand the complexity of the resource system, to quantify the relationships among Nexus sectors and to assess future scenarios for adequate policy responses and good governance. Although the reviewed literature offered a rather sharp delineation of the two contexts in which Nexus research in the Mediterranean has been operationalized, few articles were found to be placed at the interface between the natural resources management and sustainable technology groups. In most cases, such articles evaluated constraints and benefits of adopting a type of technological or infrastructural intervention in terms of natural resources availability and security at the local or national scale. Mancini et al (2021) assessed the advantages of implementing an integrated wastewater, solid waste and energy generation system in a metropolitan area of southern Italy to increase circularity in the use of resources, to support local energy needs and to reduce emissions. For a neighbourhood in Barcelona, Toboso-Chavero et al (2019) analysed the technical feasibility and environmental implications of introducing urban agriculture, rainwater harvesting and photovoltaics panels on roofs of residential buildings to meet the local water, energy, food demands, optimise land use and reduce emissions. The aim of such studies was therefore not to design specific technologies but rather to assess their contribution to a sustainable use of natural resources.

From water-energy-food to water-energy and WEFE Nexus
The strong focus on the water-energy Nexus is a key trait of the Mediterranean region that has not been reported in previous evaluations of Nexus research at the global scale (Fernandes Torres et al 2019) or regionally (Bian and Liu 2021). While Botai et al (2021) shows that for Africa, food security is central to Nexus research and that water resources are seen as one of the enabling factors to achieve such security, in the Mediterranean region the attention is placed on ensuring availability of water for domestic and irrigation purposes, and on its dependency on energy use. In fact, to cope with water scarcity Mediterranean countries need to tap into energy-intensive water resources, while maintaining water and energy services affordable. At the same time, the scope of Nexus assessment has expanded to additional components, with land being the first one to be added, followed by climate and ecosystems. Yet, in the Mediterranean, the integration of ecosystems and climate to the WEF Nexus was shown to be only partial and predominantly focused on assessing unilateral impact relationships, rather than identifying the mutual interlinkages and feedback dynamics. Thus, we call for new studies that comprehensively analyse the impacts on Mediterranean ecosystems originated from the water, energy and food resource sectors and assess how these impacts may inhibit the capacity of ecosystems to provide services to the same sectors. These studies shall account for the intrinsic interlinkages between climate and ecosystems, which are likely to amplify the risks posed to the water, energy, and food security of the region, through the consequences of climatedriven issues such as wildfires, spread of non-native species, water pollution and biodiversity loss (Doblas-Miranda et al 2017). The WEFE-climate Nexus therefore represents a promising research perspective to untangle and explore interconnected risks for ecosystems and society of the Mediterranean. To this end, the First Mediterranean Assessment Report by the Mediterranean Experts on Environmental and Climate Change represents a strong reference for any Nexus studies in the region, since it provides a comprehensive synthesis of knowledge on the major drivers of change and impacts across resource systems (MedECC 2020).

Multidimensionality of Nexus research
Nexus research in the Mediterranean has also evolved in terms of dimensions being investigated. Interlinkages among water, energy and food were not only assessed from a biophysical modelling perspective but have also been considered under socioeconomic and governance dimensions. Such a need for Nexus research has been extensively pointed out, but not fully addressed yet (Galaitsi et al 2018, van Gevelt 2020. The reviewed literature contributes to the debate by offering insights on how economic instruments (e.g., energy market, water pricing, subsidies) can influence trends in Nexus interlinkages and on how different social groups relate to the local WEF Nexus systems. In particular, the social security of local communities may be compromised by trade-offs arising among the Nexus sectors, (Zografos 2017, Yoon et al 2018, while, at the same time, citizens affect the development of WEF systems through their consumption patterns (Lombardi et al 2021). The consideration of the societal dimension is also manifested by the inclusion of sectorial perspectives expressed by local stakeholders in the codefinition of Nexus issues, knowledge, and solutions. Such an attitude to the science-stakeholder interface, defined by Alamanos et al (2022) as a 'System Innovation Approach' , supports the achievement of a coordinated and socially acceptable management of natural resources , helps improving the governance impacts of Nexus assessment and, we argue, shall be extended to those applications of Nexus research that to date have seen less stakeholder involvement. This is the case for the reviewed articles operationalizing Nexus research in the context of sustainable technologies. The involvement of stakeholders and users in assessing the needs for specific technologies and in evaluating their local socioeconomic feasibility shall become more prominent in Nexus research and could build upon the methodological frameworks presented by Aste et al (2017) for energy-food technology planning in humanitarian context and by Emec et al (2015) for designing production systems with hybrid water and energy generation. Overall, the governance, socio-economic and biophysical dimensions of the Nexus were often investigated separately, with only few studies trying to holistically analyse Nexus interactions across these dimensions. Considering the role of institutions, stakeholders and citizen in shaping WEF Nexus systems in combination with a bio-physical assessment of WEF Nexus resources and an analysis of economic dynamics, would provide a comprehensive overview of Nexus systems for evidence-based decision-making (Akinsete et al 2022).

Variety of methods and scales of analysis
A multidimensional examination of Nexus interlinkages implies embracing multidisciplinary, gathering data from different scientific fields and it calls for an integration of methods and geographical scales, an aspect that was rarely found in the reviewed literature. In fact, the methodological landscape emerging from the reviewed literature is faced by a disparity of discipline-focused methods with few being recurrently deployed: data analysis and statistics, footprint, index/indicators, LCA and optimisation modelling. At the global level, Albrecht et al (2018) provides a comprehensive synthesis of methods for Nexus assessments. Although their results cannot be directly compared to our findings, since they disregarded Nexus types of two components from the analysis (e.g., water-energy, water-food), some observations can be made. Firstly, while economic methods are significantly used for Nexus assessments globally, their deployment in the Mediterranean is very limited, especially at the national and international scale. This results in a lack of quantified evidence on how economic factors and policies impact Nexus systems and how the WEF Nexus influences the economic growth in the Mediterranean region. Economic assessments of Nexus systems are therefore needed to support what Markantonis et al (2019) highlight as a key recommendation for Nexus operationalization in the region, i.e., implementing water pricing and other economic instruments. Secondly, and in contrast with Albrecht et al (2018), we found that a relevant number of studies used data and statistical analysis as their main method to explore the interactions between Nexus sectors. Although they represent a first step in understanding, quantifying and correlating resource consumption in water, energy and food supply chains, their ability to inform policy making remains limited (Zhang et al 2018). Lastly, energy-related methods are more frequently used in the Mediterranean then at the global scale, mostly driven by the regional interest on the water-energy interdependencies.
The departmental work throughout the different scales often ignored exogenous variables and drivers, resulting in 'isolated spaces' studies. For example, LCA-based assessments mainly focused on micro (farm or industry) and local scales, providing a strong numerical output, but ignoring the Mediterranean and contextual setting. Hence, new studies addressing the entire Mediterranean region, which were rare in our review, could provide a common background to connect these "isolated spaces". We recommend building on existing methodologies and expand them to larger geographical scale without developing specific methods or theories. For instance, a two-component water-energy study in a specific country can be expanded to the Mediterranean basin by considering energy imports/exports, transboundary condition, and ecological constraints. The large divide in frequency of Nexus case studies among Mediterranean countries observed in this review calls for increased cooperation among researchers and highlights the need of proving the benefits of the Nexus approach in those countries to date not yet covered by demonstration sites, particularly the Western Balkan countries surrounding the Mediterranean Sea. In doing so, however, it is important to recognise and harness the plurality of knowledge, perspectives and challenges present across the Mediterranean region to more holistically and equitably advance Nexus implementation, avoiding therefore a large divide between places of Nexus research production and its operationalization (Wiegleb and Bruns 2018).

Applications of Nexus research
The reviewed articles highlight sustainable technology as a key field of Nexus operationalization. Integrated technologies in the water, energy and food supply chains are crucial for simultaneously achieving water, energy, food security, and promoting resource use efficiency. Such technologies are also helping to mainstream the Nexus approach as they constitute tangible demonstrators of the benefits gained by adopting a multi-sectoral thinking. Yet, to date, the Nexus has mostly provided a perspective under which the role of existing and emerging technologies in achieving sustainable development can be further promoted, rather than it being the initiator of new technological solutions. For example, the use of renewable energies for desalination, now often referred to as Nexus technology, has been widely studied even before the conceptualization of the WEF security Nexus. This may change in the future with international initiatives such as the Water and Energy for Food Grand Challenge 9 , which harness the interdependencies among these sectors to stimulate the development and upscaling of technological innovation. However, Hoff et al (2019) argue, through a review of 4 project case studies in the Mediterranean, that for a successful implementation of technological advancements, these need to be framed in a policy and regulatory environment so that synergies are maximised and potential negative consequences for the environment and society are controlled. We expand by affirming that to do so, technology-oriented studies under a Nexus perspective should go beyond pure engineering and modelling exercise, which is the case for the majority of the reviewed articles, and provide a systematic evaluation of synergies and trade-offs across sectors and scales. We therefore call for further Nexus studies that harmonize research objectives and methodologies from the field of natural resources management and sustainable technologies, to provide policy makers with actionable information for a coherent achievement of sustainable development.

Conclusions
A novel literature review was presented investigating the status-quo of the WEFE Nexus scholarship, in terms of academic research outputs supporting the operationalization of the WEFE Nexus in the Mediterranean. The large variety of reviewed topics, methods and applications indicates a lack of harmony in the 'ideas' , 'relationships' and 'practices' associated with the Nexus. On the other hand, this expected heterogeneity unveils the potential for Nexus thinking to becoming a leading transdisciplinary framework to address the diverse pillars of sustainability. Moreover, by focusing on the interlinkages between resources rather than on the resources themselves, the Nexus approach demonstrates its applicability across different research fields and questions, whether concerning the management of natural resource, or the development of innovative technological solutions. Such a holistic and transversal perspective therefore offers an opportunity for the Mediterranean region to coherently mitigate nuisance effects of climate extremes and natural disasters and adapting to future climate and environmental changes, while accelerating progress towards the achievement of the Sustainable Development Goals. However, our review reveals that current research still falls short in providing multifaceted and integrative Nexus assessments. Thus, in summarizing outcomes of the presented WEFE Nexus review, we highlight future directions for the Nexus research community in the Mediterranean, that might catalyse further advancements: 1-The level of knowledge and research operationalization reached in the water-energy Nexus, first, and, to a less extent, in the water-energy-food Nexus, shall be firmly pursued in more complex systems. Within the WEF Nexus, more evidence is needed on the energy-to-food and foodto-water interlinkages, prompting additional data collection and analysis. Further research is also needed in the identification and quantification of the mutual relationships between climate, ecosystems and the WEF systems. 2-The assimilation of socio-economic variables and processes in biophysical-driven Nexus assessment shall be further promoted to account for external drivers determining resources use and for understanding the socio-economic impacts of actions taken in WEFE Nexus sectors. This would also help in making findings of Nexus research more relevant to governance. 3-The Mediterranean scale should receive further attention. Besides having biophysical similarities and sharing common risks associated with climate and environmental change, Mediterranean countries also represent a unique socio-economic region, where interconnected societal and biophysical spheres are impacted by a variety of administrative, historical, and cultural barriers. WEFE Nexus assessments performed at the scale of the Mediterranean region could therefore help identify hotspots in the use of water, energy and food resources, their drivers, and it would provide a strong regional background for the implementation of local Nexus solutions.
Our proposed research gaps aim at providing a pathway to develop the representation of such complex systems and allowing the dissemination of the Nexus approach, while increasing its validity and significance across multiple geographic and socioeconomic scales.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).