(1) In Publications Extracted from WoSCC

There were 4957 unique author keywords recorded in extracted publications from WoSCC. Among them, 3785 words (76.4%) were only used once, 566 (11.4%) were used twice, and 178 (3.6%) were used three times. These author keywords emphasized the breadth of MPs-related research, but also indicated a lack of continuity in research focuses. Some author keywords had different forms, but the same meaning (e.g., "FT-IR Spectroscopy" and "FT-IR" manually standardized as "FT-IR"), so we manually standardized 299 author keywords (with a minimum of 5 occurrences) to 230 keywords, and used them for co-occurrence network analysis (Figure 4). "Microplastic" (the biggest dot, Occurrence = 1146) was the most frequently used author keyword (and was used as our search term). The keywords "marine environment pollution" (occurrence = 366), "marine debris" (occurrence = 296), "ingestion" (occurrence = 105), "nanoplastic" (occurrence = 89), "sediments" (occurrence = 88), "polystyrene" (occurrence = 69), "FT-IR" (occurrence = 66), "freshwater" (occurrence = 60), and "polyethylene" (occurrence = 57) ranked second to tenth in the author keywords analysis, during the period from 2004 to 2019. These keywords were used in a large number of articles dealing with the distribution of MPs in different environments (e.g., marine environment, sediments, and freshwater), the ingestion of MPs by organisms, analytical techniques and quantification of these particles, and the biological effects of exposure to polystyrene or polyethylene nanoplastic. Figure 4 shows that the author keywords provided for MPs articles formed different clusters (by color), which represented global hotspots in MPs-related field (see Section 3.3).

**Figure 4.** Co-occurrence network diagram of author keywords, appearing in a minimum of five publications between 2004 and 2019.
