Microplastic pollution in marine environments is an emerging global issue revealing the importance of microplastic monitoring. Although numerous papers have been published in recent years in relation to microplastics, the methods and techniques for identifying and quantifying them remain underdeveloped, and many researchers are working to develop optimal protocols. Some of the methods currently used include: (1) extraction from seawater, sediment, and organisms; (2) isolation and purification of microplastics from environmental matrices by removing non-plastic materials, both inorganic and organic; and (3) identification of microplastics using a combination of microscopy and chemical analyses. However, these methods are too diverse, often complicated, and they have not been standardized. In this review, we summarize the methods and techniques for sampling, processing, identifying and quantifying microplastics in marine environments and discuss the advantages and limitations of these techniques. We also comment on some future directions for analytical methods of microplastics.

Seawater certified reference materials (CRMs) are indispensable for the accurate quantification of long-term changes in the marine environment. Nutrient concentrations, for instance, have been measured since the earliest days of scientific oceanographic observations in the late 19th century. This long history of measurements implies that numerous nutrient concentration datasets exist from many parts of oceans. Such datasets could greatly facilitate studies of environmental changes in oceans. However, there have been few studies of environmental changes based on nutrient concentrations in oceans because historical nutrient concentration datasets have poor internal and external comparability; thus, it is difficult to compare data analyzed by one laboratory with data from a similar part of the ocean analyzed by another laboratory. Nutrient CRMs developed in Japan and distributed from the country are beginning to be widely used by the global oceanographic community. This development allows for the detection of long-term changes in the marine environment based on nutrient concentration measurements. In Chapter 2 of this review, we describe the scientific background of nutrient CRMs and the history of their development. We then describe the methods by which nutrient CRMs are produced and distributed for use in oceanographic observations. We also describe the development of nutrient CRMs traceable to SI units. In addition, we describe the activity of the Scientific Committee on Oceanic Research (SCOR) Working Group #147, the purpose of which is to facilitate the international distribution of nutrient CRMs. In Chapter 3, we describe the application of nutrient CRMs for quality control and assurance. The use of nutrient CRMs in monitoring climate change in oceansʼ and fisheriesʼ research is illustrated. We also describe the merits of using nutrient CRMs for observations, which require high accuracy, such as the Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP). In Chapter 4, we describe the development of CRMs for dissolved organic matter, which is likely to be needed in the future. Furthermore, we discuss the development of multiparametric standard seawater in a single bottle in which multiple properties (carbonate system parameters, concentrations of nutrients, dissolved oxygen, fluorescent dissolved organic matter, and density (absolute salinity)) are certified. One purpose of distributing nutrient CRMs is to facilitate the elucidation of global-scale temporal changes in the oceanic environment using nutrient data. Global-scale variations of oceanic inorganic carbonate system properties have already been detected owing to the availability of the Dickson CRM, which has been available and used worldwide since the 1990s. We discuss the Dickson CRM because its availability and use provides motivation for the worldwide distribution of nutrient CRMs. In the last chapter, we summarize issues related to seawater environmental standards. The most important issue is how to maintain a system for distributing CRMs. The maintenance of that system must address issues including the collection of seawater as raw material for CRMs, the identification of certified distribution centers, and the use of CRMs for commercial and research purposes.