Challenges of the Blue Economy: Evidence and Research Trends

Background The Blue Economy is a recent eld of study that encompasses economic activities that depend on the sea, often associated with other economic sectors, including tourism, maritime transport, energy and shing. Blue growth supports the sustainable growth of the maritime and marine sectors as the oceans and seas are engines of the global economy and have great potential for growth and innovation. This article undertakes a bibliometric analysis in the terms of blue economy, maritime economy, ocean economy, marine economy, and Blue Growth to analysis the scientic production of this eld of study. The main objective is to nd out if there is a link between the blue economy and the circular economy. Results The results show an evolving and growing trend during the last decade regarding the number of published articles and citations. Nascent and future research trends include terms such as small-scale sheries, blue degrowth, aquatic species, biofuel, growth of the coastal blue economy, and internationalization. New researchers, experts, public institutions, and private companies who wish to understand the roots of the Blue Economy and its evolution over time may nd this article useful to design and develop strategies that lead to its ecient management, preservation, and sustainability.


Background
Throughout history, the sea has always been present in the economic activities of all civilizations as a food resource, a means of transportation and commercial trade. In recent years the term Blue Economy (BE) has become a concept closely related to maritime resources and developed economies in the oceans. The BE aims to promote economic growth, improve life and social inclusion without compromising the oceans' environmental sustainability and coastal areas since the sea's resources are limited and their physical conditions have been harmed by human actions (EC, 2020).
The rst appearance of the term BE dates back to 2009, at the congress of the Senate Committee on Commerce, Science and Transportation of the United States. The importance of the BE for the USA's overall economy, the excellent business opportunities it provides, and the concerns about climate change are excellent opportunities for new blue jobs in renewable energy (Cantwell, 2009). In that same year, the International Symposium on Blue Economy Initiative for Green Growth in Korea took place, where "the concept of using ocean resources in a way that respects the environment can evaluate how both business activity models and new technologies satisfy economic and environmental conditions, contributing to the sustainability of these resources" (Joroff, 2009). At the United Nations Conference on Sustainable Development held in Rio de Janeiro in 2012, the oceans were deemed to be priority areas, with some initial objectives being proposed such as the "sustainable consumption and production patterns", food security, sustainable energy for all and disaster risk reduction and resilience (UNCTAD, 2014).
Undoubtedly, ocean resources generate numerous bene ts to the world economy and offer essential opportunities for transportation, food production, energy, mineral extraction, biotechnology, human settlement in coastal areas, tourism and recreation, and scienti c research (Kaczynski, 2011).
In academic research, a literature review on the BE also needs to include similar concepts. Lee et al.
(2020) state that "the term BE has been used in different ways and similar terms such as "ocean economy" or "marine economy" are used without clear de nitions." At the same time, when analyzing other articles that address BE, it was observed that ocean economy (OE), marine economy (ME), and blue growth (BG) were also used as synonyms (Kaczynski, 2011;Spamer, 2015). Table 1 shows some of the de nitions of these terms. The sustainable use of ocean resources for economic growth, the improvement of livelihoods and jobs, and the health of ocean ecosystems.

Costa et al., 2019
The concept of rethinking industrial processes and searching for a viable biological solution that reduces contamination.

Phelan et al., 2020
It has become synonymous with generating wealth from activities related to the oceans while protecting and supporting marine ecosystems.

Graziano et al., 2019
It arises from the growing worldwide interest in the growth of waterbased activities. It has emerged in the last two decades from various forums, but above all from within the policy and practice of environmental development.

BG Caswell et al., 2020
Emerging from national and international marine policy, it aims to promote the growth of ocean economies and at the same time holistically manage marine socio-ecological systems.

Dalton et al., 2019
The creation of economic activity and jobs at sea, while the multiple uses of spaces make e cient use of the available sea surface by combining industries.

Boonstra et al., 2018
It is relatively new and aims to achieve economic growth based on the exploitation of marine resources while avoiding their degradation, excessive use, and pollution.

Burgess et al., 2018
It is the most recent term to refer to the most holistic management of complex marine socio-ecological systems. It is a dynamic and complex system that covers all industries and regions.

ME Qi, & Xiao, 2019
It is a dynamic and complex system that covers all industries and regions.

Wenwen et al., 2016
It is a new economic form that emphasizes a new development concept, a new operating mechanism, and a management model.

Caban et al., 2017
It is particularly exposed to dangers due to the environment of its operations. Potgieter, 2018 It is considered a crucial factor for global economic growth and development, offering excellent opportunities, challenges, and risks. Colgan, 2013 They are marine construction, resource, shipping, and tourism and recreation industries whose establishments are located near ocean shorelines or large lakes.
It is important to highlight that the concept of BE gives rise to two con icts of interest. On the one hand, those linked to economic growth and development, and on the other, those linked with safeguarding and protecting the ocean's resources. Kathijotes (2013) states that the objective of BE models is to transfer resources from scarcity to abundance and address the issues that cause environmental problems.
For this reason, it is necessary to propose solutions that take advantage of all available opportunities and analyze the threats to the OE. Lee et al. (2020) link the BE and the UN Sustainable Development Goals (SDGs) and conclude that the objectives that are linked to the BE are: underwater life (14); land ecosystems (15); peace, justice, and stable institutions (16) and alliances to achieve the objectives (17).
Linking BE with SDGs is challenging, and con ict areas exist primarily around divergent views on the legitimacy of different sectors as components of BE, particularly high carbon-intensive industries such as oil and gas and the emerging seabed mining industry . Furthermore, the increase in human activity, in the form of new or intensi ed uses, such as the generation of renewable marine energy, exert greater in uence and cause con icts between BE sectors (Hoerterer et al., 2020). Through bibliometric analysis, this study's main objective is the quantitative study between 1979 and 2020 of the evolution of the scienti c production on the BE. It aims to evaluate the importance of those articles focused on this line of research. Its main contribution is the assimilation of the BE into the Circular Economy (CE) since both are key to supporting BG and guaranteeing the sustainability of economic activities developed within the blue context.
The results point to 2010 being a turning point that marks a continuous growing trend. Subsequently, the keywords, their trends and thematic evolution are examined. It is followed by a discussion of the link between themes, the most relevant ndings, and future lines of research.

Methodology
Bibliometry offers useful results from the authors' production in a eld of research, trends, the most cited articles, and the concentration of documents in impact journals (Junquera & Miter, 2007).
The rst step was to select the terms through a prior review of those economies linked to the seas and oceans, with BE, BG, ME, and OE being the most representative terms. The next step involved locating and extracting data on all the documents in the WoS (Web of Science) Core Collection that contain the terms established in the search criteria in order to visualize the behavior of scienti c production over time, providing high-quality data and a complete description that facilitates data processing and for the broad recognition it has obtained in the scienti c community ( The data was then processed to analyze the number of articles published per year, the number of citations, and their h-index. Bibliometric analysis, the study of the scienti c activity of authors, has been used to prepare this article and has been used in various areas. Figure 1 depicts the descriptive statistical analysis of the main variables: cites per article, total cites, hindex, year, and articles. The matrix indicates the correlation between them (the closer to 1, the stronger the correlation; if the values are closer to -1, there is an inverse correlation between variables, while if there is a zero value, there is no correlation). In the present case, the articles variable is highly correlated with the total of citations, h-index, and years. In contrast, there is an inverse correlation with the variable cites per article and year.
In this work, the number of articles, citations, citations per article, and h-index, the most relevant countries, institutions, and authors are studied. The keywords are also taken into account in order to discover new lines of research. A database search has been carried out, ltering by topic in the title, the abstract, author keywords, and Keywords Plus. The search for the keywords was carried out using the terms: "blue economy" or "ocean economy" or "blue growth" or "maritime economy" or "marine economy." In the next stage, 780 results were obtained, from which articles were ltered, leaving 499 articles to be processed and analyzed in the fourth stage. The results were then ltered to include only articles as many of them have been published in journals with an impact factor in Journal Citation Reports (JCR). This indicator is Lastly, once the document search was ltered to include only articles, the data was exported and processed using two tools: Vosviewer and Biblioshiny. In this way, clusters can be created by downloading information from the Web of Science database.
The VOSviewer program offers the basic functionality necessary to visualize bibliometric networks, citation links between publications, collaboration between researchers, and co-occurrence links between scienti c terms (Van Eck & Waltman, 2011). According to a logical bibliometric work ow, the Bibliometrix tool, developed in the statistical and graphical language R, is also used to add weight to the study. R is highly extensible as it uses a functional, object-oriented programming language, and therefore it is relatively easy to automate parsing and create new functions. It is utilized to create graphs for three metrics at different levels: sources, authors, and documents, and analyzing knowledge structures at the conceptual, intellectual and social level (Aria & Cuccurullo, 2017) (Fig. 2). Figure 3 represents a descriptive analysis of the terms ME, OE, BE, and BG, making a note of the chronological order of the articles when these concepts rst appeared.

Descriptive Analysis
The rst mention of ME appeared in 1979 in the article entitled "Marine Economy of Poland 1945-1975". It addressed the growth of two port complexes located on the Polish Baltic Sea coast, Gdansk-Gdynia and Szczecin-Swinoujscie, pointing to the important economic activities in the area such as the export of coal and coke, ships, minerals, cereals, gypsum, rolled steel products, wood, cement, and food. It also pointed to tourism development along the coastal regions that required environmental protection (Leszczycki, 1979). Later, in 1992, the article "The Intercolonial Railway, Freight Rates and The Maritime Economy in Canada" stated that this infrastructure was a crucial piece in the history of maritime economic development and a transport link between the maritime islands and the center from Canada (Cruikshank, 1992).
In 2004 the article "Employment and Wages for the U.S. Ocean and Coastal Economy" was published on the subject of OE and performed a preliminary analysis of the United States' coastal and ocean economy between 1990 and 2001 to prepare coherent national estimates of economic values, based on economic and other measures related to the coasts and the oceans (Colgan, 2004).
The article "The Future of Blue Economy: Lessons for European Union" marked the beginning of research on BE and made some preliminary considerations about the growing convergence of economic, social, technical, and environmental factors that contributed to generating new opportunities in the world's oceans. Furthermore, thanks to the cooperation between European ocean industries and government institutions, together with the training of various experts, they became the epicenter of applying the European BE at sea (Kaczynski, 2011).
In 2013 the BG Strategy, "Scenarios for Selected Maritime Economic Functions Union," appeared and examined the usages of the scenarios of the BG project. It aimed to develop the maritime dimension of the Europe 2020 strategy, with a 15-year horizon (2025-2030). In this regard, the scenarios were understood and developed in two ways: the micro-future scenarios and the general scenarios (Wolters, 2013). Table 2 shows the evolution of scienti c production in the period (2020-1979) by the number of articles, citations, citations per article (average), and the annual h-index. In 2010, there was an increase in the number of articles and citations resulting from the article, "The importance of estimating the contribution of the oceans to national economies" by Kildom & McIlgorm (2010). The authors stated that the oceans were in trouble and experienced changes that could compromise life on both the sea and the land, affecting the economy and the environment.

Scienti c production analysis
The year 2018 stands out for the number of citations, reaching its maximum value of 531 ( Figure 4). The most cited article, "Blue growth: savior or ocean grabbing?" questions political proposals and places them within the framework of the broader debates on the neo-liberalization of nature (Barbesgaard, 2018). Other authors with a high number of citations address BG, tracing its roots to the conceptualization of sustainable development under the title "What is blue growth? The semantics of "Sustainable Development" of marine environments" (Eikeset et al., 2018). The authors further manifest the complexity of ocean systems, combined with data and capacity constraints, demanding a pragmatic management approach (Burgess et al., 2018).
The most cited articles closely related to the terms "BE, BG, ME, and OE," in addition to the authors, journal, date of publication, and total citations, are listed in Table 3. Of note is the article by Silver et al.  The remaining articles address the marine sector's role in the national economy, the concept of BG in the marine environment, the integrated maritime policy, and platforms for harvesting marine renewable energy.

Analysis of keywords
The keywords used in the article titles and abstracts are then analyzed according to their relevance and co-occurrence to create a co-occurrence map of all the terms used in the 499 selected articles ( gure 6), using Vosviewer software. The minimum number of occurrences of a selected term is set at 25. Of the 12,858 terms found, 133 met the threshold and were included in the nal analysis. From these results, a relevance score was calculated. The title and abstract elds were used to extract data. To extract the highest number of terms from the publications, the labels of the structured abstracts and the copyright statements are included.
An analysis of the results reveals that the most productive period, between 2016-2020, has produced the most relevant terms. The ten most relevant terms are linked to maritime spatial planning, China, marine economy, ocean economy, economic development, e ciency, and coastal areas. The most current terms have to do with the marine industry and access. In this sense, due to the boom in emerging marine industries and the support of nation states for marine technology, many Chinese universities have added specialties related to marine technology (Li,  An analysis of the keywords identi es the most used terms and the most current trends related to the new areas of the concepts studied. The trend analysis depicted in Figure 7 uses a color scale that goes from blue to yellow and categorizes the terms used in this eld of study from the least to the most innovative in the period studied. Trends linked to concepts such as small-scale shing, degrowth, aquatic species, biofuel, growth of the coastal BE, and internationalization are observed. These recent trends arising from the BE address the need to connect human and industrial activities that obtain their inputs from the sea by creating cooperative alliances at an international level, promoting sectors such as shing, tourism, and energy. In addition, BE favors environmental sustainability since it uses renewable energy. The growth of the coastal BE can be linked to the government of Taiwan, which proposes a growth program for the coastal BE at the national level to promote ocean-related industries based on sustainable development. In general, farming the sea is based on arti cial technologies and it is argued that by developing marine sh farming, it is possible to contribute to the transformation of capture sheries by integrating the concept of BE (Chen et al. 2020). As for the term "blue degrowth," this calls for the retreat of speci c activities currently in the hands of large companies. In this sense, the decrease is intended to criticize the traditional ideas of growth and sustainability by promoting an equitable reduction in production and consumption, along with a socially transformative vision (Carver, 2020).
The role of biofuels in the BE is gaining momentum. The research of Kaşdoğan (2020) examines algaebased biofuel production systems designed on the high seas and integrated with wastewater treatment and carbon dioxide absorption processes to revitalize faith in biofuels in the BE.

Discussion
Past research has addressed aspects of OE and ME from the perspective of the economic activities derived from the sea, speci cally food catches, commercial transport, and the maritime industry (Keefer et al., 1998;Benevolo, 1999). Subsequently, the sustainability approach within the maritime economy was sectors, which can and should be sustainable. In this context, the development of integrated maritime policies is based on the belief that maritime zones can achieve higher growth rates, pointing out that the European Atlantic Arc could contribute to this BG.
Most activities related to the economic exploitation of the maritime environment carried out by humans do not conform to the notions of a "BE" since this economic exploitation does not often focus on a sustainable maritime environment (Rayner et al., 2019). Similarly, it has been found that the synergies, con icts between sectors, and political decisions could in uence the sustainable growth of the BE in highly contested regions such as the North Sea basin. This could result in sustainable growth having a stronger in uence than the effects of climate change, making it a more exible and adaptable approach to policymaking that considers changing economic, social, and environmental realities (Hoerterer et al., 2020).
Coastal communities are directly affected by the BE and the effective management of ocean resources for BG. Although the term ocean economics is often promoted as something new, there are historical analogies that can provide insights for contemporary planning and implementation of BG (Potgieter, 2018). In this sense, thanks to the use and treatment of raw materials of marine origin, such as macroalgae, their multiple uses are essential for the e cient recovery of marine biomass (Prabhu et al., 2020).
While the protection of marine areas is considered a fundamental part of mitigating climate change, on a practical level, its success is overshadowed by the current expansion of offshore drilling for oil and gas (Brent et al., 2020). The prospects for growth in the ocean economy are promising because ocean industries address issues such as food security, energy security, and climate change (Zghyer et  Changes in the balance of surface mass, relative to changes in solid ice discharge, are vitally important across the Arctic areas and will continue in the future (Moon et al., 2018). For example, the Netherlands, a region crisscrossed by large rivers such as the Rhine, Meuse, and Scheldt, is protected by a network of levees. Approximately 59% of its territory is at risk of ooding, 26% is under the sea level and 29% can ood if the rivers over ow. According to the commission that manages the Delta Plan, which addresses the threat of water, a temperature increase of two degrees in the North Sea could mean a rise of between one and two meters.
Scienti c studies about New York City reveal that the sea level could rise by two meters by 2100, endangering the survival of Manhattan, a threat for which the city is already taking measures (Oppenheimer et al., 2019). In Florida, the effects of climate change are likely to include ooding associated with rising sea levels, increased invasive species, damage to coral reefs, and increasing frequency of damaging hurricanes. Tide levels along the US eastern seaboard of the United States during the past century were spatially variable, with the relative sea-level rising more rapidly along the Mid-Atlantic Bay than along the Bay of the South Atlantic and the Gulf of Maine (Piecuch et al., 2018). Other authors (Kong et al., 2016) state that rising sea levels, tides, extreme weather conditions, high temperatures, and ocean acidi cation present serious problems that could affect shipping, shipbuilding, the shing industry, and coastal tourism and even compromise human health and labor-intensive production activities, such as the sea salt industry, sea shing and the use of seawater.

Conclusions
The BE presents signi cant challenges at an economic, social, and environmental level, which is why the BG strategy is presented as the key piece of the puzzle to guarantee environmental sustainability and e cient management of the seas and oceans' resources. In this context, the SDGs imply that economic development is both inclusive and respectful of the environment, and it is necessary to nd a balance between economic, social and environmental spaces. For example, one cannot consider eradicating poverty without guaranteeing the health of ocean ecosystems that are fundamental to food security, livelihoods, and economic development. Therefore, it is urgent to set goals with objectives and indicators that demand productive, healthy, and resilient oceans.
As analyzed, BE is a recent term rooted in sustainable development, so it needs more time for it to be adopted by all economic agents, politicians, and society in general. Thanks to the BG Strategy, it is possible to continue with economic activities arising from the seas and oceans in a more sustainable way that reduces the direct and indirect effects of its execution and minimize the negative impact on the ecosystem.
Regarding scienti c production related to these concepts, there is a noticeable growing trend in the number of articles published in journals with high impact factors, especially in the last decade, which is evidence of a growing interest in investigating these terms and this novel eld. Although in practice, the BE has always been present in the economic activity and the political agenda of all the countries of the world.
Analysis of keyword trends shows the need to protect coastal areas and traditional activities against the marine industry. These include the urgent transformation of large farms, waste treatment, and a commitment to cleaner energy that respects maritime ecosystems. The oceans are recognized as being essential to sustaining life on Earth, and the overexploitation of their resources jeopardizes their ability to continue to provide food, economic bene ts, and environmental services to society. Another critical issue is the role that community ecotourism plays within the dynamics of the BE since marine and coastal tourism constitutes one of the largest and fastest-growing segments of tourism. There are sustainability problems related to the marine tourism sector, especially in protected areas, which could be reduced if the BG strategy is further promoted.
The main conclusion of this research is that BE poses some fundamental con icts of interest. On the one hand, some studies support growth and development, while others prioritize the protection of ocean resources. Thus, it is essential to harness resources and promoting renewable energies with the resources offered by the oceans and seas, create alliances with different interest groups, unite efforts, and nd common elements to continue with the BG, taking into account each community's problems and constituting a signi cant global challenge.
One of the limitations of this study is the di culty in measuring the impacts of economic activity and therefore quantifying the environmental impacts. Therefore, it would be interesting to carry out studies that can provide solid arguments to support it (Surís-Regueiro, 2013; Moore et al., 2016).
Possible future lines of research on the BE could focus on incorporating this model of the CE since few articles have addressed this aspect jointly. The relationship between BE and CE should go beyond addressing the issue of global marine waste, but rather be an integrated part of the BG strategy, the circular BG strategy in a broader sense: new components and more respectful treatments, less polluting marine minerals, sustainable management and the equitable distribution of marine resources. Regarding the political agenda, there should be speci c lines of nancing that support research into the blue economy and sectors of the CE. Together, the two must be integrated to achieve more e cient and sustainable results. The datasets used and/or analysed during the current study are available from the corresponding author on request.
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Competing interests
The authors declare that they have no competing interests.