**2. Microplastics**

Microplastics are pieces of plastic that are between a millimeter and a nanometer in size and are invisible to the human eye. The term "microplastics" has been defined differently by various researchers (see [13–16]), including a workshop on the topic. Microplastics are defined as being in the size range <5 mm [17] (recognizing 333

*Over 1000 rivers are accountable for 80% of worldwide aquatic plastic pollution in the ocean.*

nanometers as a practical lower limit when neuston nets are used for sampling). Particles of plastics of sizes ranging from a few nanometers to 500 nanometers (5 mm) are commonly present in marine waters [18, 19]. For better understanding, the size range stated above is referred to as "microplastics" here. Other larger particles such as virgin resin pellets are referred to as "mesoplastics" [14].

Also, microplastics are tiny plastic granules used as scrubbers in cosmetics and airblasting, and small plastic fragments are derived from the breakdown of macroplastics [20–22]. The presence of small plastic fragments in the open ocean was first highlighted in the 1970s [23], and a renewed scientific interest in microplastics over the past decade has revealed that these contaminants are widespread and ubiquitous within the marine environment, with the potential to cause harm to aquatic lives [24, 25]. Typically, these are the smaller pieces of bigger plastic objects, which are introduced into the marine ecosystem by a variety of mechanisms, such as industrial processes, human clothes (microfibers), and cosmetics (small beaded plastic). Due to their microscopic nature, they gradually make their way through the water systems where they are not cleaned out before being pumped back into the drainage channels. Persistent organic pollutants (POPs) that occur globally in marine waters at very low concentrations are picked up by these meso-microplastics via partitioning. The hydrophobicity of POPs facilitates their concentration in the meso-microplastic litters at a level higher than that in the marine ecosystem [26]. These contaminated plastics when ingested by aquatic organisms pose a serious problem by which the POPs can enter the marine food web. Unlike macroplastics, microplastics are not readily visible to the naked eye; even resin-pellets (mesoplastics) mixed with sand are not easily discernible. Net sampling does not of course collect the smaller microplastics and no acceptable standard procedure is presently available for their enumeration in water or sand [26].
