Microplastic inventory in sediment profile: A case study of Golden Horn Estuary, Sea of Marmara
Graphical abstract
Introduction
The pollution of the marine environment by plastics is one of the major environmental issues and is estimated to accelerate in the future (Jambeck et al., 2015; Do Sul and Costa, 2014), since the plastic industry is more rapidly growing than the mitigation efforts of plastic pollution such as bioplastic usage, remediation, multiuse and recycling of plastics. Most plastics settle into sediment when they are discharged into the marine environment due to their greater density compared to seawater. Besides, the densities of some plastic particles are analoguos to seawater and remain suspended in the water column or float on the surface. Epifauna, including biofilm, increases the density of plastic-biogenic matrix complex and facilitates their moving to deep-sea, and sediment (Zettler et al., 2013; Rummel et al., 2017). Most of the microplastics deposit in the benthic zone over time and thus can accumulate in the bottom sediment. However, the pollution history of microplastics in the sediment is still not fully understood in most of marine environments including open seas and coasts by virtue of the fact that the paramount significance of plastic pollution has prevailed in the last two decades. There is limited information available so far about when and how microplastics accumulated in marine sediments over the course of the last decades, creating a gap in the assessment, prevention and control of plastic pollution (Chen et al., 2020). The low MP concentrations in the water column may be due to their substantial sequestration in sediment, creating a sink for long-term deposition (Martin et al., 2020). Only a few attempts have been made to unravel the chronologic accumulation of MP in marine sediment although most of the MP ends up in the sediment (Martin et al., 2020; Li et al., 2020; Matsuguma et al., 2017; Lin et al., 2021). For instance, Chen et al. (2020) and Uddin et al. (2021) determined the chronology of MP pollution by using 210Pb dating in South China and Kuwait, respectively. They emphasized that the microplastic abundance in the sediment profile has reached its highest level at the uppermost layer in the last decade, progressively increasing over several decades.
The fragmentation of MPs increases as the time of progress they spend in water increases. Biological, physical and chemical processes degrade plastics into smaller pieces (micro and nanoplastics) (Andrady, 2011; Dawson et al., 2018). For instance, UV radiation degrades plastic particles. However, seawater reduces the degradative capacity of UV. The three types of degradation processes (biological, physical and chemical) occur in the plastic particles that incorporated in the sediment column. For instance, some microplastics can be decomposed by the microorganisms in the water body and sediment (Harrison et al., 2014; Syranidou et al., 2017).
Various techniques have been used for the determination of microplastics in marine sediment samples. Nile red staining is a relatively inexpensive and convenient method for the visual determination of microplastics. This technique has been used in many studies so far (Uddin et al., 2021; Erni-Cassola et al., 2017; Shim et al., 2016; Maes et al., 2017). The main advantage of the technique is that it does not require ATR-FTIR or RAMAN spectroscopes unless polymer characterization is aimed. It is simply based on the detection of microplastics stained with Nile red under a fluorescent microscope. Although it is presented as a useful method due to the aforementioned advantages, its limitations should be taken into account. First; it does not provide information regarding the polymer types of plastics. Second; synthetic and textile-based coloured polymers cannot be detected since they are poorly stained by the Nile red (Stanton et al., 2019). Therefore, especially coloured fibers are not properly stained with Nile red (Wiggin and Holland, 2019; Stanton et al., 2019). However, it is very suitable for detecting plastic particles that have discoloured (or white), and that degraded by various biological/chemical/physical processes.
Golden Horn Estuary is located south of the Bosphorus, Sea of Marmara. It has been moderately/heavily polluted with untreated municipal and industrial discharges over the second half of the 20th century (Coleman et al., 2009; Kalaycı et al., 2021; Kılıç and Çotuk, 2011). Large-scale restoration studies have been conducted in the estuary since 1990 (Coleman et al., 2009; Erdik et al., 2019). Trace element and radionuclide levels and the microbial quality of the estuary have been widely reported after both the intense pollution and the restoration period (Kalaycı et al., 2021; Kılıç and Çotuk, 2011; Taş et al., 2006). However, the chronology of microplastic pollution, including preindustrialization (until to mid 20th century), pollution (from mid to the end of the 20th century) and restoration (1990-today) periods have not been reported so far. Indeed, the pollution of the Golden Horn Estuary is in line with the historical trend of global plastic production. Thus, this study aims to determine the chronological accumulation of MPs in radio-dated (210Pb and 137Cs radiochronology) sediment.
Section snippets
Sampling
One sediment core was taken at the Golden Horn Estuary, Sea of Marmara in 2014 (Fig. 1). The overall width of the estuary is 460 m in which sediment core was taken. The depth at the sampling location was 32 m. The core was taken by using a gravity corer (150 cm in length, 10 cm in diameter) and immediately transported to the laboratory and frozen. Cs-137 and 210Pb dating was applied for every 2 cm-slices of the core while MPs concentrations were determined in 10 slices of the entire core (Table
Results
We observed MPs in each layer of the sediment profile (Table 1). Some of MPs images are given in Fig. 2. This study allowed us to determine MPs up to 105 cm deep in the sediment profile of Golden Horn Estuary and MP was observed even in the deepest layer. The abundance and area of >100 μm MPs per kg of sediment across the core are given in Fig. 3 and Fig. 4, respectively. MPs number per kg ranged between 700 and 4100 by the fluorescent microscope (Nile red staining) while it was ranged between
Nile red staining
Nile red (9-diethylamino-5H-benzo[α]phenoxazine-5-one) is a fluorescent dye and is especially used for examining lipids by fluorescent microscopy (Shim et al., 2016; Erni-Cassola et al., 2017; Maes et al., 2017). Since Nile red also stains polymers such as plastics, it provides a useful method for examining many plastic polymers (Prata et al., 2019). Polyethylene, polypropylene, polystyrene, polycarbonate, polyurethane and poly(ethylene-vinyl acetate) can be dyed effectively with the Nile red,
Conclusion
This study demonstrated that Golden Horn Estuary has been polluted with MPs, at a greater extent relative to the most marine environments. There is a huge variability of the MPs concentration in marine sediment reported in the literature owing to the spatial variabilities of the pollution, select size range, analysis methods and oceanographic features. Relatively higher MPs concentrations indicated that intense human activities in big cities could result in MPs pollution on their shores. Nile
Funding
This study was supported by the Scientific Research Projects Coordination Unit of Istanbul University with project number: 30590 and The Scientific and Technological Research Council of Turkey (TÜBİTAK) with project number: 112Y060.
CRediT authorship contribution statement
Murat Belivermiş: Conceptualization, Methodology, Investigation, Visualization, Writing – original draft, Writing – review & editing. Önder Kılıç: Conceptualization, Methodology, Investigation, Writing – review & editing. Narin Sezer: Conceptualization, Methodology, Investigation, Writing – review & editing. Ercan Sıkdokur: Methodology, Investigation, Writing – review & editing. Nihal Doğruöz Güngör: Conceptualization, Project administration. Gülşen Altuğ: Conceptualization, Project
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We would like to thank to Meral Yurtsever for her advices on Nile red staining.
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