3.2.1. Co-Authorship Network Analysis of Countries

Figure 2 illustrates the collaboration network of countries conducting MPs research from 2004–2019. The number of publications threshold was set at 30, and of the 104 countries considered, 29 met this threshold. The whole network consisted of 29 nodes (referred to as countries) and 306 links (total link strength = 1815). England and the USA were the most affiliated countries; their close international cooperation was indicated by 28 links and a total link strength of 353 and 369, respectively. They were followed by Germany (links = 27, total link strength = 274), France (links = 26, total link strength = 218), and the Netherlands (links = 25, total link strength = 239). Academic collaboration between China and the USA was far more frequent than that of any other two countries (link strength = 52), which may be attributed to the high number of Chinese postgraduates/visiting scholars studying or working on MPs research in the USA. Other countries had fewer academic exchanges, such as Turkey (links = 9, total link strength = 12), possibly due to the consequence of language and finance barriers.

**Figure 2.** Co-authorship network diagram showing cooperation between countries (with a threshold of 30).

3.2.2. Co-CitationNetwork Analysis of Cited References

Of the 57,834 cited references from MPs articles published between 2004 and 2019, 713 references that were cited at least 30 times were used to create the co-citation network diagram (five clusters with different colors, Figure 3). Each cluster contained some core literatures with high citation rates and academic relationships, which revealed a knowledge base in the MPs research field.

**Figure 3.** Co-citation network diagram of cited references from MPs articles cited a minimum of 30 times.

In cluster red, the references with the largest nodes were the articles by Browne et al. [34] and Hidalgo-Ruz et al. [35], published in *Environ Sci Technol*, both with 712 co-citations and total link strengths of 25,800 and 24,418, respectively. Browne et al. [34] was the first study to explore the global distribution of MPs, which formed the knowledge base for MPs spatial distribution research. Hidalgo-Ruz et al. [35] reviewed 68 studies, to compare the methodologies used for MPs identification

and quantification from seawater and sediment samples, and they called for standardized sampling programs to develop a more comprehensive understanding ofMPs distribution. This study undoubtedly formed the knowledge base for MPs analytical methods. In cluster green, the documents with the largest nodes were authored by Andray [36] (published in *Mar. Pollut. Bull.*) and Thompson et al. [25] (published in *Science*). These articles were co-cited 712 times and had total link strengths of 29,817 and 26,743, respectively, indicating that they played a crucial role in the MPs co-citation network structure. Thompson et al. [25] clearly defined the term "MPs" and initiated global research on them. Andray [36] discussed the mechanism by which MPs are derived from marine debris, forming the knowledge base for MPs sources. In cluster blue, the document with the largest node (712 co-citations, 23,574 total link strength) was the article authored by Wright et al. [8] and published in *Environ Pollut.* Additionally, the laboratory experiments conducted by Setälä et al. [37] and Mattsson et al. [38] confirmed that MPs could transfer through food chains, and that lower trophic organisms could be the vector. In cluster yellow, the document with the largest node was written by Teuten et al. [39], who examined the uptake and subsequent release of hydrophobic organic contaminants present on plastic debris. This study formed a knowledge base about the interaction of MPs with contaminants. In cluster purple, Zettler et al. [40] first described a microbial community as a "plasticphere", and called for research on the interaction between MPs and microorganisms.
