Plastic accounts for 80% of material waste in the ocean. The field of marine microplastic research is relatively new and is growing rapidly, in terms of published papers as well as institutions and countries conducting research. To combat plastic pollution, there is sufficient evidence that policies can lead to reduced plastic production and consumption both locally and globally. We aim to understand how marine plastics research and policies have grown and spread. Specifically, we used scientometric and spatial diffusion methods to best explain how ideas (in this case science and policy) clustered and spread geographically through time. We performed systematic literature searches to determine the spatial and temporal growth of marine microplastic publications and national plastic policies from 1900-2019. We found that more countries adopted national plastic policies than those that have conducted marine plastic research. Doubling times of each temporal growth rate analyzed (research paper, institution, country, and national policy) ranged from 1.1 – 4.05 years. Further, each temporal growth rate had a break point where doubling time changed significantly. Marine microplastic research has grown exponentially since 2006, and the topics of inquiry have increased steadily. Marine microplastic publication spread at the institution level is best explained by a hybrid of expansion and relocation diffusion while national plastic policy spread is best explained by expansion diffusion. Marine microplastic research activity was not a good indicator of a country’s resources or motivation toward national plastic policies.

Marine plastic peer reviewed paper selection: Growth of marine microplastic (MP) publications was compared to other types of plastic research by performing a systematic literature search of peer-reviewed papers from Scopus, Elsevier’s abstract and citation database, in April 2020. The search used five sets of keywords: marine AND plastic*, marine AND “plastic bag*”, marine AND “single use plastic*,” marine AND microbead*, and marine AND microplastic*. The asterisk at the end of a word ensured both the singular and plural forms were considered. Within each of these sets of keywords the “analyze search results” feature was used in Scopus to record the quantity of papers published annually and cumulative number of papers published by country for 1900-2019. We note that many early papers studying mussel feeding physiology used poly-microbeads since the 1980s but were not included in any of the keyword searches. Papers were randomly spot-checked to ensure they fit within the keywords, if they did not, they were removed from our selection.

Metadata from marine MP papers were collected from a systematic literature search of peer-reviewed papers from Web of Science in April 2020. The search criteria used were the keywords marine AND microplastic* and all years (1900-2019), the same as the Scopus search. Publishing date, institution of lead author (including latitude and longitude), country of lead author, journal, and title were collected. Papers addressing non-marine MP topics (e.g. table salt or freshwater), highlights, commentary, news features, correspondences, opinion, and review papers were removed. Each marine MP paper was categorized based on focus topic: chemistry, environment, organism, policy, or review. If a paper studied multiple focus topics, only the predominate one was recorded. Organism papers were further categorized into functional groups: bacteria, fungus, invertebrate, small vertebrate, large vertebrate, macroalgae, phytoplankton, and zooplankton (includes fish larvae). If a paper studied multiple organisms, all were categorized by functional group and included.

National plastic policy selection: To evaluate plastic policy growth and diffusion, a systematic literature search for national plastic policies implemented through 2019 was conducted. Policy data was collected from Xanthos and Walker (2017), Schnurr et al. (2018), Lam et al. (2018), Plastic Policy Inventory from Duke’s Nicholas Institute for Environmental Policy Solutions (2020), and news articles from Wikipedia’s “phase-out of lightweight plastic bags” page (April 2020). Country, implementation year, type (plastic bag, microbead, single use plastic; SUP), and level (levy, ban) were recorded. All policies were cross-validated with an internet news search and policies that failed cross-validation were not included. Voluntary national plastic levies and bans were not included. Policies were evaluated at a national level, where countries with multiple levels or types of policies were only counted once in analyses.