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As reported in previous studies, the abundance of MPs in water samples is lower than that of sediment samples (Ta and Babel 2020b). Thus, a large quantity of water is usually collected to acquire a representative sample. Volume reduced samples were commonly applied for the sampling of surface water. Until now, most studies used manta trawls or plankton nets to collect water samples. Different mesh sizes of these nets are used, ranging from 50 to 3000 μm (Tan et al. 2019; Zhang et al. 2015), while 333 μm is the most commonly used in all studies (Anderson et al. 2017; Free et al. 2014; Su et al. 2016; Ta and Babel 2020b; Wong et al. 2020). Flow meters are attached to plankton nets or manta trawls to determine the filtered water volume. This helps to normalize the number of MPs to the volume of filtered water. Another technique is the measurement of the sampling area by the Global Positioning System (GPS). Thus, results are presented as the number of MPs per surface area. Trawling speed depends on the water velocity or wave action, but usually ranges from one to five knots (Ta and Babel 2020a; Tan et al. 2019; Wong et al. 2020). Another sampling method is direct filtration of water through sieves or by the collection of batch samples (Crew et al. 2020; Yan et al. 2019). In this method, a smaller water volume can be collected, thus the sample representability is reduced (Wang et al. 2017).

Figure 1.5 Techniques reported in the literature for identifying MPs in sediment and water samples.