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International Journal
Sustainable Future for Human Security
J-SustaiN Vol. 8 (2021) 20-32
http://www.j-sustain.com
ENVIRONMENT
Visualized Quantitative
Research of Marine
Pollution in the South
China Sea
Wei Yeab *
aGuangdong Ocean University, Zhanjiang, China
bChiang Mai University, Chiang Mai, Thailand
Received: May 6, 2021/ Accepted: August 30, 2021
Abstract
The rapid social-economic development of the countries
bordering the South China Sea has resulted in increased
marine pollution of the region, yet little study has been
carried out on marine pollution in the region. This article
is one of the first few attempts among existing literature to
understand marine pollution in the South China Sea as a
large marine ecosystem and its implications for
sustainability. We have conducted systematic literature
reviews for marine pollution in the South China Sea and
three other regional seas/large marine ecosystems, namely,
the Gulf of California, the Mediterranean Sea, and the
North Sea with the assistance of CiteSpace. By comparison
of studies in the three regional seas, the research of marine
pollution in the South China Sea only touches on the
impact of microplastics, and existing literature in the area
is less efficient in the study of impacts of modern pollution
on marine organisms and the uncertain bioaccumulation
effect on humanity. Therefore, it is recommended that
researchers in the region cooperate to conduct a holistic
study of ocean pollution, and study of biological impacts of
pollution on marine organisms should be strengthened.
Keywords: marine pollution; South China Sea; regional sea;
CiteSpace; visualized quantitative research
1. Introduction
The South China Sea is the world’s most diverse shallow-
water marine area. Bordered by nine countries, it is home to
over 270 million people that have had some of the fastest
development and most vibrant economies on the globe
(Morton and Blackmore, 2001). The past half a century has
witnessed a rapid socio-economic development of countries
bordering the South China Sea. Over-exploitation of coastal
environment and marine resources, such as land
reclamation, waste water discharge, aquaculture and
overfishing in the South China Sea, has resulted in severe
marine pollution of the region (Han et al., 2012). On one
hand, marine pollution leads to environmental degradation
and depreciation of the value of marine resources; on the
other hand, it poses an imminent threat to human health as
well as the sustainability of economic development by
increasing the costs for environmental conservation and
social stability.
Little existing literature has studied the South China Sea
as a whole largescale marine ecosystem. Given the
geographic and meteorologic complexity of the South China
Sea, and the fact that many island groups of this marginal
sea are disputed with sovereignty claimed by a number of
littoral states, the whole South China Sea is poorly
understood regarding its marine biota, ecology and the
human impacts upon it. Anthropogenic activities, such as
over-exploitation of marine resources and marine pollution,
are anticipated to be huge although, in reality, relatively
little is known about them. What is known is most often
contained in reports, workshop and conference documents
that are not available to the wider scientific community.
Conflicts arising in recent decades have interrupted
research on marine pollution in the South China Sea.
Therefore, it is essential to study marine pollution in the
South China Sea from a holistic perspective as it is required
by the transboundary nature of marine pollution and the
interconnectedness of things in and between ecosystems.
This study is one of the first few attempts among
existing literature to understand marine pollution in the
South China Sea as a large marine ecosystem. Given the
emerging economies around the coast of the South China
Sea and people’s increasing awareness of marine
conservation, it is pressing to understand the status quo of
marine pollution in the South China Sea, and how pollution
impacts on marine ecosystem as well as the human society.
Only from knowing the damage that has been caused and
how bad the damage is and will be, can humans start to
make amendments, for the sustainability of marine
ecosystem, of economy development, and for human beings’
sustainability on earth.
The objective of this study is to provide a systematic
literature review for marine pollution in the South China
Sea, as a large marine ecosystem; and via learning from the
research of marine pollution in other regional seas/large
marine ecosystems, it can help researchers to devise more
targeted studies to understand the status of marine
pollution in the South China Sea and to build a common
governance mechanism among the neighboring states for
the sustainable use of the ocean.
*
Corresponding Author
Tel.: +86 13828224831; E-mail: alwaysbeakid@hotmail.com
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
21
Based on the relevant literature in the core database of the
Web of Science, this article reviews the current research
status of marine pollution in the South China sea, along
with three regional seas, namely, the Gulf of California, the
Mediterranean Sea, and the North Sea. With the
information visualization analysis software CiteSpace, a
scientometric of the literature (from the year 1996 to 2020)
relating to marine pollution in the above-mentioned
regional seas are conducted. Through literature co-citation
analysis, keyword co-occurrence analysis, and keyword
burst detection, the knowledge structure, research hotspots
and frontiers of marine pollution in the regional seas are
displayed in a holistic manner in the map of knowledge.
2. Methodology
2.1 Research methods
This study conducts a scientometric analysis of marine
pollution in the South China Sea along with that in three
comparable regional seas to draw a comparison. CiteSpace
is employed in the research to do a comprehensive
literature review of the study of marine pollution in the
context of regional seas. Based on co-citation analysis
theory and network pathfinding algorithm, CiteSpace can
explore the research themes, hotspots and evolution history
of a certain knowledge domain, and help researchers to
form an overall understanding of the knowledge structure
and frontier dynamics in the field.
In order to investigate the research theme in the field of
marine pollution in the three chosen regional seas of the
world, literature co-citation analysis is first conducted on
related research groups. Key literature is identified through
citation frequency, centrality and burst index provided in
the literature co-citation analysis results (Zhang et al., 2019).
The LLR (Likelihood ratio test) algorithm is selected to
extract cluster names from keywords to add labels since the
keywords extracted with this method emphasize the
characteristics of research (Chen, 2017). Then cluster
analysis is carried out on the literature co-citation network.
Combined with the core literature, the cluster view is
analyzed to understand the distribution of research topics
of marine pollution in the three selected regional seas.
Furthermore, the cluster view is transformed into a timeline
view to study the evolution of these research topics. As to
research hotspots in the field of marine pollution in the
three sampled regional seas, keyword co-occurrence
analysis of the related research groups was conducted, then
a cluster analysis and keyword burst detection of the
visualized network was undertaken (Luo et al., 2020).
Learning from the cluster view and burst keywords,
research hotspots of marine pollution in the three
prominent regional seas are located. Additionally, the
cluster view is transformed into a timeline view, and the
evolution history of the research hotspots is looked into
along with the burst keywords. Lastly, through a keyword
co-occurrence analysis of the marine pollution related
literature in the South China Sea, the study in the South
China Sea is compared with that in the sampled regional
seas to understand the current research progress of marine
pollution in the South China Sea.
2.2 Sample selection
The sampling strategy to select the comparable regional
seas used in this study followed the stated logic: (1) Both the
Gulf of California and the South China Sea are regional seas
of the Pacific on a similar latitude, so they are border the
same ocean with some geographical similarity. They are
both bordered by a large dominant economy (the Gulf of
California - the United States, the South China Sea - China)
and other small developing states. This influences how the
states interact within the area, usually the dominant
economy takes the lead in handling regional marine
problems. (2) The Mediterranean and South China seas are
both famous for biodiversity. Most of the ecosystems in the
South China Sea can find replicates in the Mediterranean,
such as coral reefs and seagrass; therefore, the impacts of
marine pollution on both large marine ecosystems are
comparable. (3) A common feature that the North Sea and
the South China Sea share is that they both have a large
fishing industry and a rising aquaculture industry, the
economic value of the fishing industry is important for the
bordering countries of the two regional seas, and they suffer
from the same pollution from aquaculture. States in both
these regional seas have to consider the environmental
impact from economic development for a sustainable
future. (4) The majority of cutting-edge marine science and
advanced ocean governance research institutes are situated
on the coast of the three regional seas selected in the study,
so it is essential for the South China Sea, a less developed
area, to learn from them.
2.3 Data acquisition
The data in this study are sourced from the core database of
the Web of Science. The retrieval method is as follows:
Topic = ［ (marine pollution) And ("Gulf of California" or
"the Mediterranean" or "North Sea”) ］. The time span is
all the years (1996-2020), and the update time is January 14,
2021. A total of 1076 literature items were retrieved.
It should be noted that there is often a trade-off between
recall and precision in data retrieval. In the case of this
study, the above-mentioned keyword retrieval scheme can
achieve a relatively high precision rate on account of low
recall ratio. Some papers that have substantial studies of
marine pollution in the three regional seas might not be
included in our sampled literature due to the search
keywords adopted in this research. For the purpose of
supplementing the undesirable effects caused, it is
necessary to expand the existing retrieval results. Chen et al.
(2012) indicate that citing articles of the initial retrieval
results can be obtained through “Create Citation Report” in
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
22
the database, and provided that such citing articles have
referred to the results of the initial search, it is reasonable to
assume that these citations are also relevant in the research
field. In consideration of this approach, this study has
enriched its search results by creating a citation report in
the core database of the Web of Science, and 24191 articles
without self-citations were retrieved.
3. Results and discussion
3.1 Result overview
To begin with, a preliminary descriptive summary on the
distribution of publication time, country/area, and research
themes of the 1076 keywords retrieval articles and 24191
citing articles without self-citations is looked into to foster a
general understanding of the research situation in the field
of marine pollution in the three chosen regional seas.
As shown in Figure 1, there has been a trend of
fluctuating growth of the research on marine pollution in
the designated regional seas during the 25-year span
between 1996 and 2020. 1998, 2009, 2013, 2016~2020 are the
four periods that have witnessed a surge of academic
outputs on the subject. As for the citing articles, there has
also been a steady increase over the two decades, as
illustrated in Figure 2. The results indicate that marine
pollution has gradually come to the attention of academia,
and nowadays it is one of the research hotspots in the three
sampled regional seas.
Figure 1 Distribution of publication time of the 1076 initial
retrieved articles
Sources: Web of Science search result analysis
Figure 2 Distribution of publication time of the 24191 citing articles
without self-citations
Sources: Web of Science search result analysis
In regards to country/area, Italy is the first in record
count (231 articles) for keyword retrieved articles and
second in the record count (3034 articles) for citing articles
without self-citations (see Table 1). Other leading nations in
the study of marine pollution in the regional seas include
Spain, Germany, France, England and the United States,
each taking up 14.03%, 13.85%, 11.25%, 10.04% and 7.62% of
the total publications respectively. For citing articles, the
United States has a record of 3700 articles, which accounts
for 15.39% of the sum. Spain takes the third place with a
ratio of 11.67%, while China follows closely behind with a
percentage of 11.05%. The distributions of publication
location are aligned with the key coastal states that border
the three more-developed regional seas. In general, Europe
and the United States are the research frontiers of marine
pollution. Though a major developing country on the South
China Sea, China pays quite a lot of attention to marine
pollution research in the three selected regional seas, and
the lessons learned are critical research themes in China.
Table 1 Top countries with publications of marine pollution
research in the three selected regional seas
Countries
Record Count
of 1076
% of 1076
% of 24191
Italy
231
21.47%
12.54%
Spain
151
14.03%
11.67%
Germany
149
13.85%
8.04%
France
121
11.25%
9.42%
England
108
10.04%
8.00%
USA
82
7.62%
15.39%
China
--
--
11.05%
Sources: organized from the Web of Science search result analysis
When it comes to research areas, for either keyword
retrieved articles or citing articles, Environmental Sciences
Ecology has an absolute advantage in terms of the number
of articles, accounting for 64.13% (638 articles) and 56.18%
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
23
(12138 articles) (See Treemap 1 and Treemap 2 below). This
indicates that marine pollution related research in the three
regional seas has received special attention from the field of
ecological and environmental science. Other than that, the
number of papers published in the research direction of
Marine Freshwater Biology, Oceanography, Toxicology and
Engineering are also quite prominent, which prove that
marine pollution has been the common concern of many
research fields, and the studies on marine freshwater
organisms, toxicology of marine pollution, and engineering
means for their treatment are relatively sufficient.
Figure 3 Distribution of research topics in Web of Science
Categories of the 1076 keywords retrieval articles
Sources: Web of Science search result analysis
Figure 4 Distribution of research topics in Web of Science
Categories of the 24191 citing articles without self-citations
Sources: Web of Science search result analysis
3.1 Quantitative analysis
3.2.1 Literature co-citation analysis
With the use of CiteSpace, we can visualize a literature co-
citation network with 959 nodes and 3177 links. When we
continue to do a cluster analysis on the literature co-citation
network, the visualized result is shown in Figure 5 (a); then
the cluster view was converted to the timeline view, with
the visualized result is displayed as Figure 5 (b). With the
modularity being 0.7 and the mean silhouette being 0.5, the
cluster structure in the visualization is considered to be
significant and reasonable.
(a)
(b)
Figure 5 Results of the co-citation analysis of literature concerning
marine pollution in the three selected regional seas
In the visualization of the co-citation network, nodes are
the analysis subject (article). The node size represents the
citation frequency of the article, and the connection
between nodes represents the co-citation relation. The color
and width of the layered rings in the nodes represent the
citation frequency of different periods, and the change of
color from the cold color band (purple and blue etc.) to the
warm color band (orange and yellow etc.) represents the
timeline from early to recent. The meaning of the line color
is the same as that of the node color. From the visualization
in Figure 5, we can see that most of the research concerning
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
24
marine pollution in the three regional seas are quite recent
since most of the nodes and links in the visualized results
are either orange or yellow.
As displayed in Figure 5 (a), literature concerning marine
pollution in the three regional seas can be sorted into four
main categories, i.e., No. 0 trophic transfer, No. 1
Mediterranean Sea, No. 2 microplastic, and No. 7 riverine
litter. As CiteSpace automatically filters cluster groups that
have less than 10 members, so the numbering of clusters is
not sequential in this case.
3.2.1.1 Key literature
Key literature in a research domain is a fundamental
breakthrough in the field which plays an important role in
the whole literature group, and is a good representation of
the research topic. The criteria that CiteSpace provide for
identifying key literature include citation Frequency,
Centrality, and Burst. Literature with higher citation
frequency and centrality has higher research value and can
be regarded as the foundation literature in the field; while
burst literature means that its citation frequency increases
rapidly in a certain period, and it has the potential to
become the central literature in the later stage (Zhang et al.,
2019).
As the study of marine pollution in the three regional
seas are quite scattered, we can only derive centrality of
0.01. Along with the consideration of citation frequency,
there are 12 articles that can be seen as essential literature in
the study of marine pollution in the three regional seas.
Information concerning the 12 key articles is illustrated in
Table 2.
Overall, the research topics of the 12 articles can be
categorized into two classes; one concerns (micro)plastic
pollution in the oceans, and the other focuses on
microplastic contamination in marine organisms. As early
as 2014, Eriksen et al. (2014) made estimation of worldwide
plastic pollution in the oceans, and hinted that since a
tremendous loss of microplastics was observed from the
ocean compared to expected rates of fragmentation, there
were mechanisms at play that removed plastic particles
from the sea surface. Hidalgo-Ruz et al. (2012) reviewed the
methods used for identification and quantification of
microplastics in the marine environment. Lusher et al.
(2013) perceived that plastic particles were ingested by most
fishes from the English Channel regardless of their feeding
habitat; Foekema et al. (2013) came to the same conclusion
for most North Sea fishes irrespective of their size-weight
relationship. Cole et al. (2011) conducted a comprehensive
review of microplastic contaminants; as an illustration, their
properties, pollution routes, the detection methods, spatial
and temporal trends of microplastic abundance and their
environmental impact. The study of Devriese et al. (2015)
furthered the understanding of microplastic intake by
brown shrimp in the coastal waters of the Southern North
Sea and Channel area. Farrell and Nelson (2013) investigated
the trophic transfer of microplastic from mussels to crabs,
which was the first to show 'natural' trophic transfer of
microplastic. Later on, Romeo et al. (2015) looked into the
presence of plastic debris in large pelagic fish in the
Mediterranean, and shed light on the potential transfer of
contaminants on human health. Rochman et al. (2015) also
reported plastic debris in fishes directly sold for human
consumption which raised concerns for human health. The
survey carried out by Guven et al. (2017), which was believed
to have the largest sample size analyzed at the time being,
indicated that the higher microplastic particle count in the
seawater and sediment a site has, the higher number of
ingested particles that would be found in the fish
originating from that site. The findings of a study by
Lebreton et al. (2017) contended that a large number of
plastic wastes entered the ocean every year from rivers, and
67% of polluting rivers of the globe were located in Asia;
their work provided baseline data for ocean plastic mass
balance exercises, and assisted in prioritizing future plastic
debris monitoring and mitigation strategies. Lastly, Free et
al. (2014) evaluated microplastic pollution in freshwater
systems.
It can be noted in Table 3 that Eriksen and Cole’s articles
have burst values of 4.3 and 3.91 respectively, meaning that
their work was a research hotspot at the time which aroused
the attention of peer researchers. Along with them, other
literature that has a burst value ≥6 is summarized in Table
3. Barnes (Barnes et al., 2009) in 2009 have raised concern
of plastic debris in the open ocean, on shorelines of even the
most remote islands and in the deep sea. The study of
Browne et al. (2011) suggested a large proportion of
microplastic fibres found in the marine environment might
be derived from sewage as a consequence of washing of
clothes. Law et al. (2010) gave a quantitative description of
the scope of plastic marine pollution in the North Atlantic
Ocean, and concluded that the majority of surface plankton
net tows collected plastic debris, and the concentration of
plastic pieces in subtropical latitudes was quite severe due
to sea current dynamics.
From Table 3, it may be inferred that the period from
2008 to 2010 was significant for the evolution of the study of
marine pollution. And Browne has acted as a vital facilitator
for the development of the field since some of his work,
Accumulation of Microplastic on Shorelines Worldwide:
Sources and Sinks with a burst value of 7.91 and Ingested
Microscopic Plastic Translocates to the Circulatory System
of the Mussel, Mytilus edulis (L.) with a burst value of 6.63,
have pushed for transition of the domain in different times.
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
25
Table 2 Information table of key literature
No.
Centrali
ty
Count
Author
Publicat
ion year
Title
1
0.01
64
Eriksen
M
2014
Plastic
Pollution in
the World's
Oceans: More
than 5 trillion
Plastic Pieces
Weighing
over 250,000
Tons Afloat at
Sea
2
0.01
53
Hidalgo-
Ruz V
2012
Microplastics
in the Marine
Environment:
A Review of
the Methods
Used for
Identification
and
Quantificatio
n
3
0.01
52
Lusher
AL
2013
Occurrence of
microplastics
in the
gastrointestin
al tract of
pelagic and
demersal fish
from the
English
Channel
4
0.01
51
Foekema
EM
2013
Plastic in
North Sea
Fish
5
0.01
41
Cole M
2011
Microplastics
as
contaminants
in the marine
environment:
A review
6
0.01
36
Devriese
LI
2015
Microplastic
contaminatio
n in brown
shrimp
(Crangon
crangon,
Linnaeus
1758) from
coastal waters
of the
Southern
North Sea and
Channel area
7
0.01
31
Farrell P
2013
Trophic level
transfer of
microplastic:
Mytilus edulis
(L.) to
Carcinus
maenas (L.)
8
0.01
31
Romeo T
2015
First evidence
of presence of
plastic debris
in stomach of
large pelagic
fish in the
Mediterranea
n Sea
9
0.01
26
Rochma
n CM
2015
Anthropogeni
c debris in
seafood:
Plastic debris
and fibers
from textiles
in fish and
bivalves sold
for human
consumption
10
0.01
22
Guven O
2017
Microplastic
litter
composition
of the Turkish
territorial
waters of the
Mediterranea
n Sea, and its
occurrence in
the
gastrointestin
al tract of fish
11
0.01
17
Lebreton
LCM
2017
River plastic
emissions to
the world’s
oceans
12
0.01
14
Free CM
2014
High-levels of
microplastic
pollution in a
large, remote,
mountain
lake
Table 3 Literature with a high burst value
No.
Burs
t
Author
Publica
tion
year
Title
1
14.66
Barnes
DKA
2009
Accumulation and
fragmentation of plastic
debris in global
environments
2
7.91
Browne
MA
2011
Accumulation of
Microplastic on Shorelines
Worldwide: Sources and
Sinks
3
6.72
Law KL
2010
Plastic Accumulation in the
North Atlantic Subtropical
Gyre
4
6.63
Browne
MA
2008
Ingested microscopic
plastic translocates to the
circulatory system of the
mussel, Mytilus edulis (L.)
5
6.54
Gregory
MR
2009
Environmental
implications of plastic
debris in marine settings—
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26
entanglement, ingestion,
smothering, hangers-on,
hitch-hiking and alien
invasions
6
6.42
Boerger
CM
2010
Plastic ingestion by
planktivorous fishes in the
North Pacific Central Gyre
7
4.3
Eriksen
M
2014
Plastic Pollution in the
World's Oceans: More than
5 trillion Plastic Pieces
Weighing over 250,000
Tons Afloat at Sea
8
3.91
Cole M
2011
Microplastics as
contaminants in the
marine environment: A
review
3.2.1.2 Cluster analysis
Cluster analysis of the literature co-citation network enables
grouping of literature with similar research content.
CiteSpace will automatically sum up the name for each
cluster, which grasps the common topic of the class of
literature. The view of results of cluster analysis is a good
representation of the evolution of the research topics. In
order to better understand the theme of each cluster, based
on citation frequency and centrality, classical literature of
each cluster is summarized in Table 4.
Table 4 Information table of highly cited classical literature in each
cluster
No.
Label
Title of classical literatures
0
trophic
transfer
(1) Occurrence of microplastics in the
gastrointestinal tract of pelagic and
demersal fish from the English Channel;
(2) Plastic in North Sea Fish; (3) The
physical impacts of microplastics on
marine organisms: A review; (4) Plastic
ingestion by pelagic and demersal fish
from the North Sea and Baltic Sea; (5)
Microplastic contamination in brown
shrimp (Crangon crangon, Linnaeus 1758)
from coastal waters of the Southern
North Sea and Channel area
1
Mediterranean
Sea
(1) Plastic waste inputs from land into the
ocean; (2) Plastic Pollution in the World's
Oceans: More than 5 trillion Plastic
Pieces Weighing over 250,000 Tons
Afloat at Sea; (3) Plastic debris in the
open ocean; (4) Plastic Accumulation in
the Mediterranean Sea; (5) Accumulation
and fragmentation of plastic debris in
global environments
2
microplastic
(1) Microplastics in the Marine
Environment: A Review of the Methods
Used for Identification and
Quantification; (2) Microplastics in the
marine environment; (3) Microplastics as
contaminants in the marine
environment: A review; (4) Accumulation
of Microplastic on Shorelines Worldwide:
Sources and Sinks; (5) Spatial Patterns of
Plastic Debris along Estuarine Shorelines
7
riverine litter
(1) River plastic emissions to the world's
oceans; (2) The Danube so colorful: A
potpourri of plastic litter outnumbers
fish larvae in Europe's second largest
river; (3) Microplastic particles in
sediments of Lagoon of Venice, Italy:
First observations on occurrence, spatial
patterns and identification; (4) The
Arctic Ocean as a dead end for floating
plastics in the North Atlantic branch of
the Thermohaline Circulation; (5)
Microplastics in freshwater systems: A
review of the emerging threats,
identification of knowledge gaps and
prioritization of research needs
The classical literature in Cluster 0 (trophic transfer)
focuses on the harmful effects of microplastic pollution in
marine organisms. Wright et al. (2013) discussed the
physical impacts of microplastics on marine organisms in
terms of sources, susceptibility, and trophic transfer, and
presented the findings that there was not only microplastic
transfer though pelagic food chains, but that microplastics
can also modify marine organism population structure. The
study of Rummel et al. (2016) found higher occurrence of
plastic ingestion in pelagic fish than demersal fish; and it
was very likely that fish have adapted to microplastic
ingestion in their body.
In the case of Cluster 1 (Mediterranean Sea), it confirms
the selection of this regional sea as a study sample for a
great deal of research was centred in the Mediterranean.
Within this group of literature, the article of Jambeck et al.
(2015) entitled Plastic Waste Inputs from Land into the
Ocean was the most frequently cited within the 1076
selected research articles, with 76 counts. The focus of
Jambeck’s work was on land-based pollution of the ocean.
He maintained that population size as well as the quality of
waste management system of a country determined its
plastic debris discharge into the sea. Hence, they urged for
waste management infrastructure improvement to alleviate
the problem.
Cluster 2 consists of literature with the theme of
microplastic. Discussion of classical literature in this regard
refers to the sources, trajectory, sinks, identification and
quantification etc. of microplastics resulted from mass
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
27
production half a century ago. Andrady (2011) again
reviewed the mechanisms of generation and potential
impacts of microplastics on the marine environment, and
found that as the relevant distribution coefficients for
common persistent organic pollutants (POPs) favoured
plastic media, when marine organisms ingested
microparticles, they were likely to be exposed to high levels
of POPs; consequently, they called for more study to
understand the influence of microplastics exerting on the
ocean food web and the unforeseen damage posed by plastic
pollution on the marine ecosystem.
Lastly, Cluster 7 attributes marine pollution to riverine
litter. Plastic debris in freshwater systems which would flow
into the three regional seas was analysed in the light of
occurrence, circulation, spatial patterns and identification.
Interestingly, Eerkes-Medrano et al. (2015) compared
microplastic presence and interactions in freshwater
systems to marine systems and pointed out the similarities
shared by the two systems. They put forward the urgency to
understand how microplastics transfer from marine to
terrestrial ecosystems and eventually affect the health of
humans.
The distribution of research topics reflected by the 4
cluster groups of literature is basically consistent with that
of the 12 key articles identified in Table 2. To be more exact,
the research topics primarily fall into two aspects: (1) the
sources, spatial distribution, influencing factors, and
prevention and remediation of plastic pollutants in the
marine environment; (2) the occurrence, identification and
quantification of microplastics in marine organisms and the
biomagnification effect they might have on human beings.
In accordance with the analysed results of key literature
identified in the literature co-citation analysis and cluster
analysis, we may reach the conclusion that research themes
of marine pollution in the three chosen regional seas are
centred around (1) the sources, spatial distribution,
influencing factors, and prevention and remediation of
plastic pollutants in the marine environment; (2) the
occurrence, identification and quantification of
microplastics in marine organisms and the biomagnification
effect they might have on human beings.
To further investigate the evolution of the research topics,
the timeline view of the literature co-citation network is
analysed (Figure 5 (b)). In the timeline view of the co-
citation network, the literature nodes with the same first
citation time are placed in the same timeline, and the
connection between nodes in the timeline represents the
progress of research, thus showing the evolution of the
research topics from the time dimension (Chen, 2012).
For the first three clusters, it can be seen intuitively that
the nodes in each timeline are dense and evenly distributed
from 2006 to 2019, indicating that research interest in
trophic transfer and microplastic in the three selected
regional seas was relatively high during this period, and a
large number of influential research results emerged, such
as the literature identified in the above section. In this
stage, the links between nodes in the timeline are also
highly intensive, which means a strong inheritance
relationship among literature in the period. The important
literature achievements in the early stage have a certain
degree of theoretical innovation, which provide a
foundation for the later research.
As for Cluster 7 riverine litter, critical literature of this
topic is scattered around the timeline of 2012-2019.
Comparatively speaking, the number of nodes and links in
this timeline is relatively small; that is, there are few
influential research results, and they are not closely related
to the previous publication. All in all, analysis of the
timeline view of the literature co-citation network indicates
that research interest in the field of marine pollution in the
three selected regional seas have substantially increased
since 2006, and the research topics presented a trend of
diversification.
3.2.1 Keywords co-occurrence analysis
Keywords are the highly condensed contents of literature.
Through keyword co-occurrence analysis, we can identify
the scientific questions that are jointly discussed by much
literature, that is, the research hotspots.
3.2.2.1 Cluster analysis
Running CiteSpace, we obtain a keyword co-occurrence
network with 942 nodes and 2926 links. The cluster view of
keyword co-occurrence analysis is visualized in Figure 6 (a),
while the timeline view is displayed as Figure 6 (b).
(a)
(b)
Figure 6 Results of keyword co-occurrence analysis of literature
concerning marine pollution in the three selected regional seas
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
28
Cluster analysis was performed on the keyword co-
occurrence network to observe the distribution of research
hotspots (Figure 6 (a)). There are altogether 16 significant
clusters of marine pollution in the three selected regional
seas. They include Cluster 0 microplastics, Cluster 3 North
Sea, Cluster 4 species diversity, Cluster 5 heavy metals,
Cluster 6 shipping, Cluster 7 chlorinated compound, Cluster
8 surface ozone, Cluster 9 macroplastic, Cluster 10 mullus
barbatus, Cluster 11 parasite community, Cluster 12 marine
eutrophication, Cluster 13 macrobenthos, Cluster 14
nitrogen compounds, Cluster 16 conservation, Cluster 17
metabolization, Cluster 19 mollusca.
Along with the conclusion from the literature co-citation
analysis, we can see that for the first research theme of “the
sources, spatial distribution, influencing factors, and
prevention and remediation of plastic pollutants in the
marine environment”, keyword Cluster 0 microplastics,
Cluster 5 heavy metals, Cluster 6 shipping, Cluster 7
chlorinated compound, Cluster 8 surface ozone, Cluster 9
macroplastic, Cluster 12 marine eutrophication, Cluster 14
nitrogen compounds may be deemed as research hotspots.
Microplastics are the most popular research subject, and are
often related to the study of heavy metal, chlorinated
compound, surface ozone, macroplastic and nitrogen
compounds contamination. In the field of marine pollution
prevention and remediation, marine eutrophication is
constantly a topic of discussion.
As to “the occurrence, identification and quantification
of microplastics in marine organisms and the
biomagnification effect they might have on human beings”,
keyword Cluster 0 microplastics, Cluster 4 species diversity,
Cluster 10 mullus barbatus, Cluster 11 parasite community,
Cluster 13 macrobenthos, Cluster 16 conservation, Cluster 17
metabolization, Cluster 19 mollusca are viewed as the
research hotspots, to be more exact, marine biological
association, biomarkers for evaluation and the conservation
of marine species.
The timeline view of the keyword co-occurrence
network is looked into (Figure 6 (b).) to form a visual
understanding of the evolution of the research hotspots. For
over two decades, the study of microplastics has been the
main line of marine pollution research. It provides the basis
for other research progress, and the knowledge inheritance
in the period is strong. Other than that, the research topics
on marine pollution in the three regional seas have been
quite diverse and scattered.
3.2.2.2 Keywords burst detection
Aside from cluster analysis, keyword burst detection is also
a good representation of research hotspots, and it can
provide quantitative description of the research hotspots
and their evolution (see Chen et al., 2012). Figure 7 shows
the top 25 keywords with the strongest citation bursts in a
chronological order. Aligned with the two key research
themes identified above, the 25 high burst keywords can be
sorted into two categories: (1) pollutants: pollution, PCB
(polychlorinated biphenyl), cadmium, trace metal, PAH,
mercury, heavy metal, persistent organic pollutant; (2)
marine organisms: mussel, biomarker, fish, mytilus eduli,
benthic community. As this result is basically consistent
with the analysis above, it further confirms the conclusions
about the research themes and research hotspots.
Figure 7 Results of keywords burst detection of literature
concerning marine pollution in the three selected regional seas
Note: the color red represents the duration of the active period of
the keyword, while the color blue represents the inactive period of
the keyword
Emergence and duration of burst keywords can reflect
the evolutionary process of research hotspots. The top 25
high-intensity burst keywords selected in this study
appeared in a concentrated time respectively, in the time
span of 1996-2012, which shows that this period is an active
period of marine pollution research in the chosen regional
seas. There were no high burst keywords appearance after
2012, meaning that no new research hotspots and frontiers
emerged in the field of marine pollution in the regional seas
selected thereafter. This phenomenon is largely attributed
to the fact that researchers have endeavored to deepen their
understandings on the impacts of microplastics as it has
become mainstream research for marine pollution (see
Figure 6 (b)).
At an early stage, “North Sea” obtained a high burst
strength of 10.72, which indicates that the study of marine
pollution has been carried out in the North Sea for a long
time. Since 2005, as seen from a high burst value of 8.57,
“trace metal” has been closely studied for marine pollution,
and this induced a shift of research direction in the field.
Overall, PCB has sustained a lasting period of interest in the
time frame and been thoroughly studied by scholars in the
three selected regional seas. Again, the variance in strongest
cited keywords along the timeline explains the research
focus of marine pollution over the years in the three
selected regional seas as being quite diversified.
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
29
3.3 Comparison of the study of marine pollution
in the three selected regional seas and in the
South China Sea
Learning from the research of marine pollution in other
regional seas can help to develop more specific research in
the South China Sea to understand the status of marine
pollution and to devise a common governance mechanism
among the neighboring states for the sustainable use of the
ocean. By comparing the research of marine pollution in the
three regional seas, the study of marine pollution in the
South China Sea can be improved and the pressure of
marine pollution can be alleviated in the area.
3.3.1 Literature review of marine pollution studies
in the South China Sea
Song (2008) examined the potential threat of marine
pollution caused by offshore oil and gas development in the
disputed areas of the South China Sea, and discussed the
legal obligations and political commitment of the littoral
states in regard to marine environment protection in the
area. Xu & Li (2013) further emphasized the importance of
strengthening cross-strait cooperation for environmental
preservation in the South China Sea. In terms of ship-source
pollution in waters surrounding the disputed parts of the
South China Sea, Beckman & Bernard (2013) proposed
designation of Particularly Sensitive Sea Areas (PSSAs) to
prevent, reduce and control marine pollution from shipping
activities. Recently, Deng et al. (2021) have reviewed the
current knowledge of microplastics pollution in mangrove
ecosystems in the South China Sea. In addition, the forty-
year pollution history of microplastics in the northern South
China Sea was studied to get insights of how microplastics
have contaminated this marginal sea (Chen et al., 2020).
In short, the current literature on marine pollution in
the South China Sea either focuses on a specific kind of
marine pollution in the area, or the research work merely
covers parts of the South China Sea. Particularly, there have
been quite active academic reflections on the environmental
pollution in the coastal waters off south China, i.e., the
northern South China Sea. Han et al. (2012) provided a
comprehensive overview of the environmental problems as
well as existing policies in the south China coastal sea, and
pointed out the challenges going forward, as well as
recommendations for the sake of sustainable use of the
ocean. Peng et al. (2020) analyzed the relationship between
fisheries economic growth and marine pollution in China’s
coastal regions, and found that this relationship in the
southern coast of China was an inverted U-shape.
3.3.2 Quantitative analysis of literature of marine
pollution in the South China Sea
This study further conducts a scientometric analysis of
literature concerning marine pollution in the South China
Sea with CiteSpace to grasp the map of knowledge of the
subject. Again, the data used for the study of marine
pollution in the South China Sea are sourced from the core
database of the Web of Science. The retrieval method is as
follows: Topic = ［ (marine pollution) And ("South China
Sea”) ］. The time span is all the years (1996-2020), and the
update time is February 16, 2021. A total of 183 articles were
retrieved.
A keyword co-occurrence analysis was undertaken for
the 183 retrieved literature regarding marine pollution in the
South China Sea, with the visualization of the cluster
analysis illustrated in Figure 8 (a). It can be seen that most
of the relevant literature has selected China (Cluster 2) and
Japan (Cluster 7) as the research areas, and they focus on
microplastic pollution (Cluster 1). Biomonitoring (Cluster 0)
as well as canonical correspondence analysis (Cluster 4) are
the typical research methods. Researchers’ attention is
directed towards “butyltin residue” (Cluster 5), “sediment
quality criteria” (Cluster 8), “enrichment factor” (Cluster 10)
and “depositional history” (Cluster 11).
Compared with the study of marine pollution in the
South China Sea, academic discussion of the subject in the
three selected regional seas is more active and the research
topics are more diverse. At present, the research of marine
pollution in the South China Sea simply touches on the
impact of microplastics. Although “fish” (Cluster 3) is
included in the critical keywords, literature in the South
China Sea is less efficient in the study of the impacts on
modern pollution on marine organisms, let alone the
uncertain bioaccumulation effect of ocean pollution on
humanity.
From the timeline analysis (Figure 8 (b)), we can further
infer the below problems in the existing study of marine
pollution in the South China Sea, (1) From the time
dimension, the earliest literature of marine pollution in the
three selected regional seas dates back to 1989, while
literature of the kind in the South China Sea didn’t begin
until 1998, which is relatively late. (2) From the research
area dimension, most of the existing literature of marine
pollution in the South China Sea focuses on the northern
South China Sea (southeast China); there are few studies on
the west and south coasts of the South China Sea, which are
bordered by a variety of smaller developing nations where,
due to the social-economic development pressure,
anthropogenic impacts, such as over-exploitation of
resources and pollution, are anticipated to be huge
although, in reality, relatively little is known about them.
Additionally, as conflicts have in recent decades arisen over
the island groups of the South China Sea because of
perceived national rights, there is a lack of coordinated
research on the overall planning for regional and marine
pollution prevention and control. (3) From the perspective
of the distribution of research topics, the research on
marine pollution in the South China Sea is relatively hot on
microplastic pollutants and chemical pollutants; in
comparison, the research on the impacts of pollution on
marine organisms is relatively minimal and primitive, and
there are relatively few central problems that have received
the common concern of all the neighboring states, despite
the transboundary nature of marine pollution.
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
30
(a)
(b)
Figure 8 Results of keyword co-occurrence analysis of literature
concerning marine pollution in the South China Sea
The coastal fringes of the South China Sea have some of
the fastest developing economies in the world, most of
which are dependent on low-order manufacturing.
Moreover, as it provides shelter for more than 270 million
people, the contradiction between socio-economic
development and environmental protection has been
highlighted, yet little research has been undertaken with
regards to marine pollution in the South China Sea (Morton
and Blackmore, 2001). As marine pollution is transboundary
and sensitive to land-based pollution, it has been proved
that pollutants will affect adjacent waters and marine
organisms therein through migration and transformation.
Given the populated demographic profile on the coast of the
South China Sea, ocean pollution will not only threaten
marine biodiversity in the region, but also bring non-
negligible influence on the health of the coastal residents.
Consequently, when dealing with marine pollution in the
South China Sea, it is essential to consider the matter in a
holistic manner, to devise governance policy from the
perspective of the entire marine ecosystem, and to take
marine pollution prevention and control as a breakthrough
to promote and protect marine biodiversity. It has been
concluded in the previous analysis that the study of marine
pollution in the other three regional seas attaches
considerable efforts to the impacts of ocean pollution on
marine organisms, and has enriched the academic debate in
the regard. It is necessary to supplement and improve and
the study of the association between marine pollution and
marine organisms in the South China Sea.
4. Conclusion
Through literature co-citation analysis, keyword co-
occurrence analysis, and keyword burst detection of
CiteSpace, the study finds that research theme of marine
pollution in the regional seas of the Gulf of California, the
Mediterranean Sea, and the North Sea mainly focuses on
two aspects: (1) the sources, spatial distribution, influencing
factors, and prevention and remediation of plastic
pollutants in the marine environment; (2) the occurrence,
identification and quantification of microplastics in marine
organisms and the biomagnification effect they might have
on human beings.
The study of marine pollution in the South China Sea
took-off relatively late, and most of the existing literature is
centered in the northern South China Sea (southeast
China), and there are few studies on the west and south
coasts of the South China Sea. Compared to the three
selected regional seas, the research of marine pollution in
the South China Sea simply touches on the impact of
microplastics. Although “fish” is included in the critical
keywords, literature in the South China Sea is less efficient
in the study of the impacts of modern pollution on marine
organisms, let alone the uncertain bioaccumulation effect of
ocean pollution on humanity.
By analyzing the research themes and hotspots of
marine pollution related literature in the world's three
major regional seas and drawing on the experience of
cutting-edge marine pollution studies, the following
suggestions are put forward for further research on marine
pollution in the South China Sea: (1) Study of marine
pollution on the west and south coasts of the South China
Sea should be conducted, enhanced and integrated along
with the study of the situation on the north coast.
Academics of the bordering states of the South China
Sea should cooperate in terms of information sharing and
knowledge structure perfecting, to carry out joint studies for
ocean pollution in the South China Sea so as to improve the
present unbalanced distribution of research areas. (2)
Marine pollution in the South China should be studied as a
whole marine ecosystem. Study on the impacts of pollution
on marine organisms should be strengthened, further
research on the land-based source pollution should be
conducted to reexamine the association of anthropogenic
activities and the marine ecosystem. Marine pollution risk
early warning systems and comprehensive treatment plans
should be set up to prevent and control ocean pollution in
the South China Sea for the sake of all the residents in the
area.
Ye Wei/ J- SustaiN Vol. 8 (2021) 20-32
31
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