Beach combing is a sampling technique that is typically used by researchers and environmental awareness groups, and involves the collection and identification of plastic litter items, in a systematic manner along a specified stretch of coastline. Beach combing at regular intervals allows the monitoring of plastic debris accumulation over time [6,25]. However, microplastics invisible to the naked eye tend to go unnoticed by using such a technique . Further, plastic debris along a coastline consists of a mix of terrestrial litter left by recreational beach activities and sea-deposited debris, and may not be an accurate indicator of plastic debris in the marine environment .
Sediment sampling involves the assessment of benthic material from estuaries, beaches and the seafloor for the presence of microplastics. Several methods that are used for sediment sampling include the use of saline water or mineral sediments for low-density microplastic flotation, application of lipophilic dye to stain the plastic particles and assist in microscopic techniques, and the use of Fourier-transform infrared (FTIR) spectra to compare with known polymers [23,30,31].
The microplastic samples within a water column can be collected by conducting marine trawls using fine meshes. Different types of trawls that can be used for sampling include mantra trawls for sampling surface water, bongo nets for mid-water level sampling and benthic trawls for assessing seabed [25,26,31]. The presence of microplastics can then be determined by examination of the collected samples under a microscope or investigating the residue left behind after allowing the seawater to evaporate [6,30].
Marine Observational Surveys
The marine surveys involve the assessment of the type, size and location of visible plastic debris by observers on boats or other sea vessels . While the surveys are helpful in determining macroplastic debris over larger areas, the detection of microplastics using this technique is difficult. Further, since the debris is not collected, no further assessment of litter can be done. The subjective nature of observations during these surveys increases the susceptibility to bias [25,32].
Several marine animals can mistake plastic fragments for prey and consume them. The dissection of marine animals washed ashore or prompting regurgitation in seabirds allows the analysis of their gut contents for the presence of plastic, which can then be identified and quantified [33,34,35].
The microplastics sampled from marine environments are typically extracted by density separation, filtered, and are purified by rinsing, chemical or enzymatic digestion. Smaller plastic particles require visualization via microscopy and formal identification using pyrolysis gas chromatography (pyrolysis-GC) or FTIR/Raman spectroscopy, which require expensive capital investments and are often time-consuming. Further, the characterization of plastic particles into different shapes, sizes and colors and their subsequent quantification play important roles in the accurate estimation of microplastics abundance.