Macro problem of microplastic: Assessment of microplastic pollution along the strandline of Kilnsea beach

Development of the first modern plastic took place in 1907 when Leo H. Baekeland patented the first synthetic plastic (Crespy et al., 2008), heralding the beginning of the “Plastic Age” (Cozara et al., 2014). With further optimization of manufacturing techniques, mass production of lightweight and durable plastics began in 1940 and has continued to increase rapidly to this day (Cole et al., 2011). Nowadays, almost all consumer goods contain or are contained by plastic (Moore, 2008), whichis further aggravated by the ample use of “throw-away” plastics, making plastic the most rapidly growing component of waste (Cole et al., 2011). In fact, it comprises approximately 10% of the globally produced municipal waste and accounts for 50-80% of the waste found on beaches, seabed and ocean surface (Barnes et al., 2009). The latter is of particular concern because distribution of plastic debris around the ocean is very patchy and it is driven by winds, currents, large-scale oceanic circulations, coastline geography and the points of entry into the marine system (ibid.).

Recently, environmental concern arose around the new area of research focusing on plastics of a much smaller dimension, almost too small to be easily detected by eye (Law and Thompson, 2014). The presence of small plastic pellets in the Atlantic Ocean was first pointed out in academic literature by E. J. Carpenter and K. L. Smith in 1972. They discovered that

“The increasing production of plastic, combined with present waste-disposal practices, will probably lead to greater concentrations on the sea surface. At present, the only known biological effect of these particles is that they act as a surface for the growth of hydroids, diatoms, and probably bacteria” (Carpenter and Smith, 1972, p.1241).

However, it was not until 2004 when the term “microplastic” was first used in scientific literature and presented by R.C. Thompson et al. (2004) in the paper “Lost at sea: Where is all the plastic?”. Their publication marked the beginning of the global efforts to document the presence of microplastic and the impacts it has on marine ecosystems (Barboza and Gimenez, 2015), and also inspired my own undergraduate dissertation. With the advancement of research on microplastic, it was soon realised that environmental concern about microplastic pollution is just the tip of the iceberg. Emerging research indicated not only toxicological implications of microplastic ingestion by marine organisms, but also uncovered the presence of microplastic in food consumed by people, drinking water and air samples. It is now hard to turn a blind eye to the fact, that this widespread issue is essentially the result of our everyday lives and activities.

During my dissertation project as part of the BSc Environmental Science programme I have conducted a study that investigated the presence, abundance, size and types of microplastic found along the strandline of Kilnsea beach. The collected data allowed to explore the possible connection between abundance of microplastic and presence of macroplastic observed along the strandline, evaluate oceanic and terrestrial contribution to the observed microplastic pollution and identify its possible sources.

To begin with, I will present a few remarks on the definition of microplastic, since it remains variable and study-specific in current absence of the universally accepted definition and sampling protocol. Generally, microplastics constitute plastics smaller than 5 mm (Arthur et al., 2009), although lower size cut off for this definition is not yet formally established (Cole et al., 2011). Ongoing debate on the minimum size of microplastic is essentially tied to the fact that lower limit chosen for each individual study depends on its sampling and processing methods (Hidalgo-Ruz et al., 2012).

Microplastics occur in an array of shapes, colours, specific densities, chemical compositions and sizes and are classified as either primary or secondary microplastics, depending on their origin (Hidalgo-Ruz et al., 2012). Primary microplastic is manufactured to be £5 mm in size and is usually found in facial-cleansers and cosmetics, or as abrasives in industrial and domestic applications (Ivar do Sul and Costa, 2014). Additionally, virgin plastic production pellets (i.e. nurdles or mermaid tears) are also classified as primary microplastic in the majority of studies (Cole et al., 2011). Secondary microplastic is derived from degradation and subsequent fragmentation of larger plastic debris (macroplastics) both at sea and on land (Cole et al., 2011). Exposure to sunlight, temperature, and oxygen are the factors that drive the degradation rate, making this process largely location dependent (Andrady, 2011). In general, plastic fragmentation occurs most readily on beaches, since they provide optimal conditions for chemical and mechanical weathering, further induced by availability of sunlight and oxygen (Cole et al., 2011).

The location for the case study of my dissertation was chosen to be the sandy beach of Kilnsea, since it was identified to be under exposure of several possible microplastic pathways, such as: litter disposal, industrial activities along the Holderness coast and within the Humber estuary, as well as transport by marine and estuarine waters. Moreover, Kilnsea beach is part of the Spurn National Nature Reserve, that is known for its wildlife, rich history and geological features. This area is highly valued by conservationists, since it provides landfall for many migrant birds and habitat for marine mammals, such as harbor porpoises, grey and common seals (Yorkshire Wildlife Trust).

The methodology of this dissertation followed similar studies, such as Thompson et al. (2004), with further adjustments in respect to limitations, availability of equipment and aims of this study. Despite being a widely addressed issue, the universally accepted protocol for microplastic assessment studies is still not yet established, thus sampling, preparation, detection and reporting of results remain optimized based on each researcher’s goals. Sampling was conducted along a 1 km transect with samples being collected at a 100 m interval. A total of 10 samples were obtained along the strandline and brought into a laboratory to be separated into size fractions. In the laboratory sediment was wet-sieved through a series of sieves sized 5mm, 2mm, 1mm, 710 μmand 300 μm. Upper size limit of 5 mm for studies on microplastic is widely agreed (Arthur et al., 2009), however the lower limit is highly variable and underreported in peer-reviewed studies. It is largely dependent on the objectives of each individual study and the available equipment. For this particular study, microplastics smaller than 300 μm were not considered, since identification below that size is highly unreliable without spectroscopic analysis. Therefore, a sieve cascade eliminated all the materials bigger and smaller than the defined size range, reducing the overall volume of sand for further procedures. Following that, a density separation method, as described in study by Thompson et al. (2004), was employed for extraction of microplastic. Isolation of microplastic from sediment using this method is achieved by introducing a sample to a solution of an intermediate density, where less dense material floats and separates out from the denser sinking material. Since sand has higher density (>1.44 g mL^-1) than polymers, microplastic detaches from the sand matrix and floats. To quantify and identify microplastic, the floating material was examined using a stereo microscope. Microplastic was identified by its physical properties, such as texture, flexibility, colour and structure, adapting identification criteria from Horton et al. (2017). Since there are no official and universal definitions for microplastic types, categorization into types is very inconsistent among the studies. However, major and most distinct types, such as fragments, fibres and pellets, are used in the majority of studies. Additional type definitions are used for foam, film, sheet and expanded polystyrene. For this particular project, types used for microplastic categorization were based largely on the study by Free et al. (2014), since it provides the most detailed explanation on the definitions used. Therefore, microplastics were categorized into: fragments, lines/fibres, pellets/beads and films. Due to further discussed reasons, paint particles were also added as a separate type of microplastic.

As a result, presence of secondary microplastics was identified within a Site of Special Scientific Interest along the Holderness coast resulting in significant risk to coastal and marine life. Overall 254 microplastics were recorded, becoming the first scientific evidence of microplastic occurrence along the Holderness coast. However, in contrast to previous publications and studies with similar aims, this study highlighted that almost half (49%) of the microplastic was paint fragments. This unexpected finding opened a new direction to the project since usually paint particles are either not recorded or not included in microplastic counts, limiting research on marine particle pollution to either microplastic or paint particles separately. However, not only these pollutants pose similar threats to the marine environment, but they are also alike and interdependent in their chemical composition. To begin with, plastics are formed by reactions of polymerization that create polymeric molecules from large number of monomer units (Rosato, 2004).These molecules have high molecular weight and strong bonds that give plastics their resistance to degradation (ibid.). In order to attain specific performance qualities to the plastics, a wide variety of chemicals can be introduced to a polymer (Cole et al., 2011). For instance, pigments can be added to the polymer blend to achieve coloration of a plastic product. On the other hand, polymers themselves serve as binders to the pigments in order to produce colourful paints and coatings (Song et al., 2014; Imhof et al., 2016; Horton et al., 2017). Furthermore, both microplastics and paint particles act essentially the same way in the marine environment. In the same manner as plastic particles, paint coatings undergo weathering and abrasion, leading to formation of smaller paint particles. Likewise, they can be ingested by an array of marine organisms, exposing them to toxic and harmful additives that are mixed into the paints, as well as POPs and toxic metals that adsorb onto them (Turner, 2010; Imhof et al., 2016). Often paints are grouped together with fragments, however in this dissertation paints and fragments were separated by visual sorting into separate categories. Both types have distinct physical differences allowing their effective separation, namely: paint particles are extremely brittle, whereas fragments are thick, sturdy and angular in their appearance.

Similarly, as in studies conducted by Ivar do Sul and Costa (2014) or Song et al. (2015), paint chips recorded in this study most likely originate from ship repair, maintenance and cleaning activities. Assessment of the Holderness coast north of Kilnsea did not locate any shipyards, however upstream of Humber estuary, just 30 km away from Kilnsea, a wide variety of ship maintenance works take place in Hull, where high pressure blasting works are conducted in addition to a variety of painting services. Paint chips, which are an inevitable result of such abrasive cleaning, are then likely to enter the watercourse and be transported towards the ocean, where they deposit on Kilnsea beach sediments. However, determining the transport of paint particles and other suspended particulate matter through the Humber estuary mouth into the ocean is very complex, owing to the variable nature of currents and changes in ebb and flood dominated net transport of material across the mouth of the estuary (Tappin et al., 2003). Without further detailed assessment of the microplastic transport within waters of the Humber estuary and evaluation of paint presence in sediments of the Spurn Point and areas of the Humber estuary, both upstream and downstream of the source, this assumption remains hypothetical, but yet the most conceivable for observed abundance of paint particles. Nevertheless, the fact that paint particles are present not only on a beach surface, but also within the water column of the sea and estuary, highlights previously discussed biological and environmental implications that are likely to be observed when such a prominent contamination is recorded even 30 km downstream of the potential source.

The rest of the recorded particles were identified as secondary microplastics, majority of which were classified as films. Microplastic films are strongly associated with plastic bags and wrappers that fragment under exposure to UV radiation and oxygen on a beach surface. Recent research has indicated that 35% of anthropogenic pollution on British beaches is attributed to “public litter” with 66% of that accounting solely for plastic (Nelms et al., 2017). Similarly, Kilnsea beach had prominent accumulation of plastic debris at the strandline. Throughout the study, it became clear that there was a strong association between macroplastic presence and microplastic abundance, meaning that areas of litter accumulation had higher microplastic counts. In addition to films, fibres were found among the recorded secondary microplastics as well, however in a significantly smaller numbers. Studies previously conducted across the globe indicated fibres, that originate from the synthetic clothing, as the predominant microplastic type (Lusher et al., 2013; Browne et al., 2011; Claessens et al., 2011; Thompson et al., 2004). However, this dissertation did not take potential synthetic fibres into account for various reasons. Firstly, these microplastic fibres are highly variable in size, with usual dimensions ranging between 15 and 30 μm(Thompson et al., 2004; Cole, 2016). Therefore, they fall largely outside of the size limits set for this research. Secondly, synthetic (e.g. polyester) and natural (e.g. coloured cotton) fibres often appear identical under the microscope and reliable identification requires subsequent spectroscopic analysis (Shim et al., 2017).  Thus, a total of 25 fibres recorded in this study can be entirely attributed to the fishing and shipping industries. To further support this assumption, fibres were compared against the macroplastics found within the samples and in most of the cases there was a clear match in colour and physical properties between the microplastic fibre and the fishing line or net it potentially derived from. Nowadays, most of the fishing equipment is made from nylon, polyvinylidene fluoride or polyethylene, all of which are polymers that have been frequently detected in microplastic assessments (Ivar do Sul and Costa, 2014). Once fishing equipment made of these materials is left unattended and subject to the subtidal conditions over a prolonged period of time, it exhibits a reduction in weight of about 1% per month, suggesting that this process is essentially attributed to the release of microplastics (Welden, 2015).

The principal finding of this study is that the observed pollution is predominantly attributed to secondary microplastics with terrestrial sources. Thereby, this dissertation endorses the widely acknowledged finding brought up by Andrady (2011) that in 80% of the time plastics that are found in the marine environment are in fact coming from terrestrial sources. This finding is confirmed by numerous studies looking both into macro- and microplastic pollution, and the current research further contributes to the wider literature by agreeing on this matter. Regarding paints, findings of this dissertation offer a novel insight addressing the previously unobserved pollution source of a high concern. There are strict guidelines on collection and safe disposal of residual paint particles from boat maintenance activities (Takahashi et al., 2012), however, as the results of this study indicate these measures are either not effective, or not followed. In the UK pollution potentially derived from the commercial or naval ship maintenance falls under the European Comission’s Integrated Pollution Prevention and Control Directive (Turner, 2010). Although numerous toxic metals associated with paints have already been identified and listed by the EU as priority pollutants within the Water Framework Directive, some toxic substances, including metals like Cr and Pb remain in elevated concentrations near sites of boat maintenance activities (Takahashi et al., 2012). Therefore, estimating the quantities of paint particles entering the aquatic environment, establishing the persistence of the chemical components associated with them and identifying impacted organisms, will enable to tailor control measures more effectively to address this issue on site-specific and larger scales.

Overall, despite the fact that this pilot study is only a “snapshot” of microplastic pollution along the Holderness coast, it provides significant evidence that the overall observed microplastic pollution is attributed to secondary microplastic with terrestrial sources, rather than primary or marine-sourced microplastic.

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