Standardized protocols for microplastics determinations in environmental samples from the Gulf and marginal seas

Highlights

• Standardized protocols for sampling, sorting, and identification of MP in the region are recommended.

• A two-tier identification approach using microscopic identification and polymeric characterization should be followed.

• Sampling strategies for quantifying MP in biota are evaluated and recommended.

• To overcome data gaps in the Gulf region, MP analyses in bottom sediments, aerosols, and sewage sludge should be undertaken.

Abstract

Microplastics are a group of ubiquitous persistent pollutants that have rapidly attracted much attention from the scientific community as well as the general public due to the growing awareness of the environmental risks they pose. However, due to limitations and variations in sampling, analytical measurement methods, and the different units used for reporting data, reliable comparisons between studies in the Gulf region and internationally are not straightforward. This study proposes standardized protocols for marine sediment, seawater, marine biota and aerosol (1) sampling, (2) sample processing, (3) sample identification and (4) reporting units to be used. An attempt has been made to highlight the limitations of the widely employed strategies for sampling microplastics in seawater, where a large portion of the microplastics is not sampled due to the mesh sizes used. The issues with the processing of biota samples and aerosols are likewise addressed, and recommendations are also made for standardization of units for reporting microplastic quantification. Protocols for collection of bottom sediments and aerosols are also proposed. These are the environmental matrixes for which there are no harmonized protocols in the Gulf region; hence if a standardized approach is adopted, it will enable and improve comparisons between the studies within this region and can be useful for similar studies in other marine areas as well.

Introduction

Ever since the terms ‘microlitter’ (Gregory and Andrady, 2003) and ‘microplastic’ (MP) were introduced by Thompson (Thompson et al., 2004), microplastic research has attracted enormous interest due to its abundance, omnipresence and potential impact on marine biota. Microplastics have been reported from everywhere including Antarctica, mountain-tops and deep ocean basins (Allen et al., 2019; Bergmann et al., 2019; Oliveira and Almeida, 2019). A search of the keyword ‘microplastic’ on Scopus identified 2106 published documents from January 2004 until 31 March 2020 (Fig. 1) highlighting the importance the topic has received. Micro and nanoplastics are recognized as emerging persistent contaminants and their ability to adsorb hydrophobic contaminants further underscores the importance their assessment commands (Ferreira et al., 2019; Oliveira and Almeida, 2019; Oliveira et al., 2019; Pan et al., 2019; Zhang et al., 2019). The scale of the microplastics' issue can be understood from the fact that 4.85 trillion particles of a size between 0.33 and 4.75 mm are floating in the world ocean with an approximate mass of 35,540 tons (Eriksen et al., 2014). The marginal seas and densely populated coastal areas have even higher microplastic densities (Eriksen et al., 2014; Esmaili and Naji, 2018; Seltenrich, 2015; Uddin et al., 2020).

With an influx of research articles, the need to have standardized sampling and analytical protocols has been felt. Several authors from different regions have proposed adopting standardized methodologies for microplastic sampling, extraction and identification (Bessa et al., 2019; Frias et al., 2018, Frias et al., 2019; Hartmann et al., 2019; Masura et al., 2015; OSPAR, 2010; Shim et al., 2017; Van Cauwenberghe et al., 2015; Zhang et al., 2019). Our paper is an attempt to propose a standardized methodology for the Gulf region (and similar marginal shallow marine areas) through critical comparison and evaluation of different approaches in various environmental matrixes including seawater, sediments and marine biota proposed by different researchers. Adopting such an approach will enable comparison between the studies carried out within the Persian/Arabian Gulf region and in other marine areas.

Several issues emanate from the published papers on MPs in different environmental matrixes, i.e., the validation and harmonization of sampling and analytical methods and the identification and quantification of microplastics. The more specific issues are (i) the proposed upper and lower cut off size for microplastics, (ii) sample type, (iii) sampling procedure, (iv) laboratory sample processing (segregation), (v) identification techniques and (vi) reporting units.

There is a general consensus on the lower size limit of 1 μm for MP (Gigault et al., 2018; Van Cauwenberghe et al., 2015), but the lower size limit in MP assessment studies is highly dependent on the selection of the sampling and extraction techniques applied. Often, the technical constraints associated with the extraction of small microplastics (SMPs), and the identification techniques used result in the omission of this lower size limit that can lead to a gross underestimation of MP concentrations. Several workers have demonstrated that this missing fraction of SMPs represents 35–90% of all microplastics present in the marine environment (Eriksen et al., 2014; Van Cauwenberghe et al., 2015).

The proposed protocols cover various aspects of sample collection, sample preparation, and analysis of MPs in different environmental matrixes. It also focuses on best practices, reporting units with the aim of collection quality and reliable quantification while also allowing comparison among different studies in the Gulf and globally.