A Graphic Review of Studies on Ocean and Mediterranean Sea Environment Quality

Abstract

With so many studies today on the water quality of the sea, one can hardly comprehend the multitude of topics that arise all over the world. This study provides a few graphic syntheses related to the most frequent words (including their clustering and links), trend topics, the spatial distribution of the researched areas, and the thematic evolution of the research directions over the decades. The most frequent authors’ keywords have a 50% similitude between the ocean studies and the studies related to the Mediterranean Sea; these keywords are part of a causal chain that dominates the marine studies on water quality: nutrients → eutrophication → phytoplankton → chlorophyll → seagrass. The most frequent words in the titles and abstracts of the selected papers from the Web of Science are “concentration” and “species”; in the Mediterranean studies, “chlorophyll” and “temperature” are the most frequent. In close connection with water quality, Zostera marina (eelgrass) and Crassotrea virginica (eastern oyster) prevail at the global scale, while Posidonia oceanica (Neptune grass) is relevant in the Mediterranean space. Some of the most studied water bodies are the South China Sea, San Francisco Bay, Chesapeake Bay, and, in the Mediterranean Sea, the Adriatic, Ionian, Aegean, and Marmara seas. “Climate change” and “remote sensing” are trend topics that shape the current studies on water quality; the increasing sea surface temperature enhances algal blooms—these need to be monitored using satellite imagery for the sustainable evolution of human activities, including aquaculture.

1. Introduction

The water quality of the ocean has been of high interest in the past decades because of the increasing impact of human society on the environment [1,2]. Various wastewaters originating mainly from the mainland have impurified/contaminated, if not polluted, in some places, the coastal waters with various substances that destabilize the ecosystems [3,4]. The microplastic issue ends up in the ocean due to the flow in the water cycle and adds to the already existing problems of the aquatic biota [5,6]. Due to the ocean currents, part of the thermohaline circulation, some man-made wastes are pushed away from the shores to the high seas where important dilution may occur or, on the contrary, higher concentrations build up—as is the case of microplastics in the subtropical gyres [7,8]. The parts of the world ocean that are mostly enclosed by land have limited exchanges with the outer oceans and are prone to being highly contaminated by humans [9,10]. A Mediterranean sea is a type of sea that can be almost completely surrounded by land; the best example is the Mediterranean Sea itself: placed between three continents (Europe, Africa, and Asia) and sustaining numerous civilizations over time, today, it has a high human density on its coasts (mostly due to urbanization and tourism) and highly contaminated waters [11,12].

Numerous studies on seawater quality have been carried out and some of them were synthesized in review articles [13,14]. Among the efforts to summarize the general scientific research that reaches higher and higher production each year, the bibliometric studies bring new tools that aid in understanding the general research characteristics and trends in one field or another, especially in the past three decades, after the spread of the World Wide Web and digital documents, which favor the analysis of linkages [15]. There are similar recent approaches in the field of hydrology too [16,17], and some of them deal with water quality [17,18,19]. These studies are mostly scientometric analyses (e.g., interested in who published in what journal, which are the impact factors, etc.) and secondly, classic reviews of the most prominent ideas in the analyzed field. Although a few studies that use bibliometric tools to comprehend some parameters related to the water quality of the world ocean have already been published [20,21], to our knowledge, there is no paper that has, as its main objective, the bibliometric study of the ocean’s water quality as a whole.

The main scope of this study is to provide a hybrid approach to ocean water quality by merging a bibliometric analysis and a classic review into a graphic review that only takes into account the bibliometric graphic outputs that provide details about research ideas and areas, and not, for example, about authors and institutions; next, the graphics are discussed using the structure of an original research paper type. The secondary scope is to compare the seawater quality research studies of the Mediterranean Sea with those of the world ocean in order to identify some differences between regional and global, as a result of the secluded position of the sea.

2. Materials and Methods

A bibliometric analysis has been conducted using scientific documents found in the Web of Science Core Collection. The search for the proper scientific literature was split into two parts: a. searching for water quality studies regarding the whole world ocean and b. searching for water quality studies about the Mediterranean Sea. The analyzed papers were found by searching for the following words (using Boolean operators; filtered by topic, which includes the paper’s title, abstract, and keywords):

• For world ocean studies: “water quality” AND (“ocean *” OR “sea” OR “gulf” OR “bay” OR “bight” OR “sound” OR “strait”) NOT (“river” OR “stream *” OR “brook” OR “creek” OR “tributary” OR “groundwater” OR “ground water” OR “aquifer” OR “runoff” OR “lake” OR “pond” OR “reservoir”);
• For Mediterranean Sea studies: “water quality” AND (“Mediterranean Sea” OR “Libyan Sea” OR “Levantine Sea” OR “Aegean Sea” OR “Ionian Sea” OR “Adriatic Sea” OR “Tyrrhenian Sea” OR “Sea of Sardinia” OR “Ligurian Sea” OR “Alboran Sea” OR “Sea of Marmara” OR “Gulf of Gabes” OR “Gulf of Sidra” OR “Gulf of Lion”) AND (“sea” OR “gulf” OR “bay” OR “bight” OR “sound” OR “strait”) NOT (“river” OR “stream*” OR “brook” OR “creek” OR “tributary” OR “groundwater” OR “ground water” OR “aquifer” OR “runoff” OR “lake” OR “pond” OR “reservoir”); one can observe that we included in this search the name of the main water bodies in the Mediterranean Sea in order to increase the number of the relevant papers.

The search results, as of 7 October 2023, included a few thousand papers starting from the year 1976 (a paper allocated to 2024 was excluded), and these documents were then filtered to include the following major standard types only: article, proceeding paper, review article, and book chapter. Over 80% of the remaining documents were of the article type (percentage variable depending on the search branch). Another filter was applied concurrently—the papers were selected from the following indices: Science Citation Index Expanded, Conference Proceedings Citation Index—Science, Emerging Sources Citation Index, and Book Citation Index—Science (these sources contained 94.2% of the papers in the world ocean search results; 97.3% in the Mediterranean Sea search results).

Ultimately, the studies related to water quality were limited to the top 10 Web of Science categories (top based on the number of papers). In the ocean search branch, these categories were Environmental Sciences (2718 papers), Marine Freshwater Biology (1645), Oceanography (874), Water Resources (863), Ecology (615), Fisheries (465), Engineering Environmental (454), Geosciences Multidisciplinary (446), Remote Sensing (393), and Imaging Science Photographic Technology (253). This filtering was performed in order to restrict the database to articles directly related to the seawater quality and to avoid papers in potentially improper categories, such as Robotics or Metallurgy Metallurgical Engineering. After applying the filter, 81.4% (5170 documents) of the initial papers remained.

Most Mediterranean Sea water quality studies fall into some of the main categories of ocean studies, but new categories also exist, indicating preferred themes in this area, such as Biodiversity Conservation. The top 10 categories of this search branch, based on the number of papers, from the original pool of papers spanning 1991–2023, were Environmental Sciences (179 papers), Marine Freshwater Biology (118), Oceanography (51), Ecology (38), Water Resources (37), Geosciences Multidisciplinary (29), Fisheries (24), Biodiversity Conservation (21), Remote Sensing (19), and Engineering Environmental (17). Example of an avoided category: Engineering Aerospace. Remaining documents after filtering: 88.4% (321 documents).

The graphs in this study were made using two pieces of software: biblioshiny 4.1.3 (a web interface for bibliometrix R package) [22] and VOSviewer 1.6.18 [23]. Both tools are frequently used in the bibliometric analysis of the hydrological literature [24,25].

Using the results of the two search branches, we analyzed the annual scientific production and statistically processed the words using tree maps. Using biblioshiny, additional useful outputs were also created, as follows:

• Three-field plots (Sankey diagrams), by using the top 10 countries (middle column) that generated relevant scientific content; the citing/cited authors’ countries are linked to the authors’ keywords (left column) and to the KeyWords Plus field (KeyWords Plus are generated by a custom Clarivate algorithm from the cited titles of the selected papers by using the words or phrases that frequently appear in the mentioned titles, but not in the title of the citing papers);
• Trend topics plots, by using the authors’ keywords that had a minimum frequency of at least 80 occurrences in total (for the world ocean studies; 10 occurrences for the smaller list of Mediterranean studies—the numbers were empirically chosen to ensure similar outputs) and that occurred at least twice per year;
• Countries’ production and collaboration world maps, by using a social structure analysis that involved the country of the corresponding author’s institution and the national scientific productivity;
• Thematic evolution graphs, resulting from the conceptual structure of the selected articles—this analysis took into account the abstracts and composed trigrams (groups of three connected words) if the words to be grouped had a minimum of 100 occurrences; the clustering used the edge betweenness algorithm (this algorithm estimates the number of the shortest paths that go through an edge in a network [26]); the inclusion index was weighted by word occurrences (minimum weight of 0.1); the minimum cluster frequency per thousand documents was set as the default 5; and the number of labels/trigrams per cluster is 1 (these numbers were chosen empirically to avoid overcrowding the graphic representation with secondary words).

Co-occurrence network analyses were produced in VOSviewer. The abstracts and titles were considered, and the contained words were used in the analyses if their minimum frequency was 160 (world ocean) or 10 (Mediterranean Sea) occurrences (the difference in numbers originated in the fact that there are ~16 times more papers about the world ocean than about Mediterranean Sea papers). The items’ counting was binary (presence or absence per article). The other settings were left to be the software’s default ones: only the first 60% most frequent terms were drawn in the co-occurrence maps; the terms may generate a cluster if they are in a group of at least 10 items.

Semantic filtering, different for each branch of the search results, was conducted using both software to remove the irrelevant words where possible. A filtration to exclude non-representative items is needed in bibliometrics [27]; examples of removed items are vague or contextless comparison terms (“total”, for example, when it has no adjacent word), measurement units (e.g., degree C), narrative terms (e.g., the present study, study area, the literature), simple default terms (e.g., work, data, station, month, evidence, need, research), terms that were implicitly necessary for generating search results (e.g., water quality, Mediterranean Sea—only for its specific analysis), etc. Biblioshiny allows the use of replacement lists in order to indicate the synonym words and replace them with a single word/group of words (examples of equivalences: medium resolution imaging, moderate resolution imaging; seawater reverse osmosis, reverse osmosis SWRO). Some analyses have not allowed for deletions or using synonyms (software limitations), such as with regard to the production and collaboration maps and the three-field plot and thematic evolution analyses.

3. Results and Discussion

3.1. Temporal, Spatial, and Thematic Overview

The scientific production of ocean studies about seawater quality was scarce before 1990 and some of the scientific articles of that epoch were not digitized for inclusion in modern databases. After 1990, a long-term ascending trend can be observed in the annual scientific production (Figure 1; the production of 2023 was not included because it was not yet completed at the moment of paper sampling). The ascending trend observed in the scientific production of seawater quality is a general behavior of many scientific fields in the past decades, including hydrology [19,24].

As of October 2023, the studies about the water quality of the Mediterranean Sea (counted beginning with 1991, according to our results) represented ~1/16 of the studies regarding the water quality of the world ocean. This is a better representation than when taking into account only the studies published before the year 2000—the past ratio was ~1/25. By analyzing the ratio of the surfaces of the Mediterranean Sea and the ocean (~1/144), one can observe that the representation of the Mediterranean Sea in the total number of marine studies is very high. This overrepresentation is due, directly and indirectly, to the fact that it is surrounded by consistently inhabited lands: the civilizations around the sea are old enough to facilitate marine studies starting from many years ago, especially in those countries that evolved to a well-developed status, and the populations are numerous and dense enough to generate environmental issues, such as sporadic oil spills [28] or persistent untreated wastewater discharges [29,30,31], that need to be inspected in order to understand our impact and find solutions.

The analysis of the authors’ most frequent (top 10) keywords of the papers in the two selected groups indicates that 50% of these keywords are common in the two selected groups (Figure 2). This overlap is caused by widespread environmental problems caused by common phenomena, such as the warming and pollution of seawaters. The common keywords are part of a single bigger picture/causal chain: nutrients → eutrophication → phytoplankton → chlorophyll → seagrass; the excessive influx of nutrients in coastal areas leads to algal (phytoplankton and macroalgae) blooms [32], which increase the chlorophyll level in the water column [33] and endanger the seagrass ecosystems by reducing light availability [34].

Climate change causes warmer waters at higher latitudes [35] and invasions of some sub-tropical seaweed species that outcompete the seagrass [36]—seagrass meadows are important sources of food and habitat for marine organisms [37]. The Mediterranean Sea contains large underwater meadows of Posidonia oceanica (Neptune grass), a seagrass species that is endemic to that sea and endangered by algal blooms that reduce biodiversity [38]. The Water Framework Directive (WFD) is a piece of European Union (EU) legislation that relates to the health of coastal and marine environments in the EU waters by setting objectives for the protection and sustainable management of water resources and, starting from the year 2000, by establishing measures to achieve and maintain a good ecological status [39], contributed to increasing the water quality of the European seas [40]. Under WFD, Posidonia oceanica meadows are recognized as essential habitats because this species serves as an indicator of the ecological status of coastal ecosystems [41,42].

At the global scale, the aquaculture sustains a dynamic research field because of its economic, social, and environmental impacts [43,44], and numerous studies are focused on improving feed efficiency, avoiding the impact of contaminated water or finding sustainable practices [45,46]. Remote sensing has been intensively used in the past decades to measure the water quality over large surfaces [13] or to detect the pollution of the marine environment [47]. The remote sensing capabilities include measuring the chlorophyll levels [48] or nitrogen concentrations by using the color of seawater.

In the Mediterranean Sea, the most studied water bodies are the Adriatic Sea and the Aegean Sea, which both sustain significant fishing and aquaculture activities; they are rich in biodiversity (including the seagrass) and face challenges related to climate change and pollution, partly due to maritime transport, coastal development, and urbanization [49,50].

Among the authors’ top keywords in seawater quality studies, “eutrophication” is a persistent topic over decades, while “climate change” is the newest persistent topic in marine studies (Figure 3) that tends to become dominant for a long time due to the aggravating status of the global marine environment [51]. Climate change determines episodes of increased runoff from agricultural or urban areas, ultimately leading to increased contaminant concentrations that cause eutrophication in the receiving seawaters [52,53]. “Nutrients” and “nitrogen” seem to be less used by scientists today than “chlorophyll”, even if they were constantly involved in numerous studies until recently (2018).

Due to the fact that the contemporary scientific research is still highly constrained by the distance between the researcher and its potential study area and by the limited access to proper research infrastructure in the field, a significant part of the studies with author’s affiliation in a country will have the study area in the same country. In this study, the mentioned relationship indirectly indicates the most studied parts of the world ocean, adjacent to the national territories (Figure 4). Most studies (3834) about the ocean water quality were written by scientists from USA institutions; China is the second source of papers (1545), while other top countries have contributions of only a few hundred papers each one (Australia, placed the third, produced 889 papers). The water quality of the Mediterranean Sea is studied mostly by the researchers based in countries around it, such as Italy (220 papers), France (71), Greece (71), Turkey (65), and Spain (63). The scientific production of the mentioned Mediterranean countries along with the spatial limitations of the scientific research explain why the Adriatic and Aegean seas were among popular topics in Figure 2 and Figure 3.

The Sankey diagrams that show both the authors’ keywords and the KeyWord Plus items with the countries as the common denominator (Figure 5) indicate a few more frequent words, such as “management”, “dynamics”, or “variability”. Case studies on seawater quality underscore the importance of understanding the complex dynamics of the bays and seas that are part of greater water bodies [54]; in some such embayments, most of the anthropogenic imbalances occur, a few examples on which include the Gulf of Mexico [55], Black Sea [56], Baltic Sea [57], and Yellow Sea [58]. The entire Mediterranean Sea is one of the most polluted seas in the world [59]. The “bay” term is a frequently used term in the KeyWords Plus field and common to both the world ocean and the Mediterranean Sea studies, indicating, on the one hand, that our society can easily impact this type of water body and, on the other hand, that the research on the sea-water is concentrated here as a consequence of the proximity of both contamination sources and research facilities.

Numerous studies related to the water quality of marine water bodies are suggesting various strategies to mitigate eutrophication, such as nutrient reduction management strategies [60] or the optimized ways for preserving natural ecosystems like seagrass beds [61].

Because of the growing impact of both anthropogenic and climatic perturbations on coastal waters, an adapted nutrient management is needed for estuarine and coastal ecosystems [62]. Researchers try to understand nitrogen versus phosphorus limitation in some case studies about seawater [63]; Liebig’s Law of the Minimum, stating that the growth of various organisms in an ecosystem is limited by the nutrient in the shortest supply, is translated into the fact that the balance between nitrogen and phosphorus availability can determine the dominant type of algae. Changes in nutrient inputs caused by human activities may lead to harmful algal blooms [33], and this is why monitoring nutrients is essential to prevent eutrophication, maintain water quality, and preserve marine ecosystems.

3.2. The Relationship and Evolution of the Research Themes

The co-occurrence network analysis grouped the most relevant items from the titles and abstracts of the selected documents into 3 and 4 clusters for the studies related to the world ocean and, respectively, the Mediterranean Sea (Figure 6).

World ocean clusters highlight two important items: “concentration” (1521 occurrences) and “species” (997)—the other top items have almost similar occurrences (~800); the clusters, sorted by descending importance, are the following:

• The first/red cluster (23 items)—it contains ecosystem, process, assessment, management, development, use, aquaculture, and climate change; part of this group is also the spatial reference item “China”;
• The second/green cluster (21 items): concentration, temperature, salinity, variation, chlorophyll, nitrogen, coastal water, organic matter, and phosphorus; it also contains time descriptors such as a month or a season;
• The third/blue cluster (12 items): species, abundance, habitat, growth, and reduction; spatial reference items: USA and Chesapeake Bay;

Mediterranean Sea clusters highlight “temperature” (57 occurrences) and “chlorophyll” (56) as the most frequent items; the sorted clusters are:

• The first/red cluster (24 items): tool, management, development, evaluation, mussel, and WFD; spatial reference items: Ionian Sea, Italy, Greece, and France;
• The second/green cluster (20 items): chlorophyll, temperature, nutrient, bay, salinity, satellite, seasonal variation, and TRIX (trophic index, an indicator of trophic status with a formula using the deviation from saturation of the dissolved oxygen and the concentrations of chlorophyll a, nitrogen, and phosphorus [64]);
• The third/blue cluster (18 items): abundance, structure, relation, diversity, coastal ecosystem, phytoplankton, phosphate, and diatom (this type of microalga plays a major role in seawaters by consuming both CO₂ and HCO₃⁻ [65]); spatial reference items: Aegean Sea, Marmara Sea, and Turkey;
• The fourth/yellow cluster (13 items): biomass, density, growth, habitat improvement, seagrass, and Posidonia oceanica; spatial reference item: northern Adriatic Sea.

The clusters are made of items of heterogeneous ages (Figure 7); some of them might be uncommon/unused today but may have generated premises/scientific grounds for the appearance of today’s research directions. The first studies about the water quality of the ocean were scarce and less diverse than today; they approached some basic themes, such as the water quality of the coastal waters in terms of the concentration of fecal coliform bacteria [66], sometimes in areas used for recreational bathing [67], in order to indicate the main measures needed to improve the seawater quality [68].

The water quality parameters have diversified vastly in the first decade of this millennium, generating useful results for the actual study of seawater reverse osmosis, including, for example, preventing the adverse effect of algal blooms on the desalination process [69]. Sea surface temperature began being a significant concern starting from ~20 years ago as it has a complex relationship with climate changes [70] and impacts aquatic ecosystems [71].

Research comparing the spatial and temporal distribution of previous studies is less common but provides useful insights [20]. Satellite remote sensing was used intensively after 2010 to detect submerged aquatic vegetation [72], harmful algal blooms [73], and sea level rise [74], each one having its implications on seawater quality. In the same latter period, scientists were more interested than those earlier in terms of understanding the aquatic vegetation, in general, and particularly, Zostera marina (eelgrass; composes declining seagrass meadows that provide essential habitat for diverse marine life and depends on water quality parameters, such as temperature [75]) and Crassostrea virginica (eastern oyster; it improves water quality by removing particulate matter and nutrients and has significant commercial value (for human consumption from aquaculture and fishing) that leads to its habitat loss, a loss that is also caused by pollution [76]).

After 2010, the most studied areas of the world ocean were San Francisco Bay and the South China Sea. At the same time, the Mediterranean Sea, the northern Adriatic Sea, and the western Mediterranean have raised the greatest interest.

WFD marked the scientific landscape of the Mediterranean for two decades (Figure 7b). After 2010, the sea surface temperature became a persistent theme in the Mediterranean studies because of the growing impact of climate change, which adds to the already old problem of water contamination.

Understanding the marine environment poses great challenges that are yet to be overcome because of the huge surface area and volume of seawater and because of the constant diversification of research topics that continuously change the focus and expand the scope of marine research. There are still limitations in both remote sensing and in situ technologies [77] that need to be overcome for a comprehensive understanding of the marine environment.

4. Conclusions

The scientific production about seawater quality increased consistently and quite steadily in the past 30 years. This has led to the establishment of well-diversified research themes since 2000 and even in the short time interval of 2021–2023 alone. Not only have the themes been diversified, but the study areas as well, which tend to be located mostly near the territories of USA or China (or the EU in the case of the Mediterranean Sea); also, the coastal waters are some of the most polluted and, at the same time, the most studied waters.

The most frequent words in the studied literature are eutrophication, nutrients, remote sensing, chlorophyll, phytoplankton, and seagrass. Climate change and remote sensing are trend topics and are related to eutrophication in marine studies: sea temperature increase worsens the algal blooms and satellite imagery is ideal for assessing the dynamics of the human impact on the nutrient loads by using, for example, the detection of chlorophyll a.

Preserving the biodiversity of marine ecosystems is very important for a sustainable future. Seagrass species will continue to be an indicator of habitat health (as Posidonia oceanica is under the rules of WFD). The proper management of the seas has immediate consequences too—the example of aquaculture shows that this practice needs to be environmentally friendly in order to prevent the degradation of the environment used for efficient production.

Microplastic pollution in the marine environment has emerged as a critical ecological and human health concern. Microplastics have permeated marine ecosystems at all levels due to their persistent nature, which leads to harmful effects throughout the food chain. Through a better understanding of the pathways and impacts of microplastics, governmental action can drastically reduce microplastic inputs into marine environments.