MICROLITTER
1. Plastics are distributed on beaches three-dimensionally
2. Composition of microplastic litter on the beaches
3. Microplastics are abundant in exposed beaches
CORNER: Microplastics on the beaches of the Baltic Sea
References
1. Plastics are distributed three-dimensionally
Although surveys and coastal clean-ups have traditionally focused on macro-sized debris on the shores, recently also microplastics have gained more attention since it has been noticed that in many beaches microplastics outnumber meso- and macro-sized litter. For example in the estuarine shorelines of UK microplastics smaller than 1 mm accounted for 65% of all found litter. A study that compared the abundance of micro-, meso- and macroplastics on South Korean beaches found out that there is a strong positive correlation between microplastics and mesoplastics, and between meso- and macroplastics on the study area. If a similar correlation between different sized plastics is found worldwide, the surveys of mesolitter might serve as a cost-effective proxy for microplastic concentrations on the beaches, since sampling microplastics from the shores require significantly more effort and equipment than collecting meso- or macro-sized litter. For example, in a recent study carried out on the Baltic Sea beaches of the isle of Rügen (Germany), the beach sediment samples were collected in 50 or 100 m sequences, where sampling transects were made 25 m apart from each other. The study was carried out on 4 beaches and this kind of sampling protocol produces altogether 57 samples covering areas of low and high tide.
Usually microplastics on the beaches are studied by collecting the top 5 cm of sand from the chosen sampling sites, which can be single squares on random locations on the beach or be arranged to form transects. This sand is then further processed in laboratory to extract and collect microplastics that are present among the sand grains. Even though many studies have so far sampled microplastics from the surface sediments of beaches along the axis from the sea to the back-beach, a recent study indicates that microplastics should be sampled also along their vertical gradient in the beach sediments. In the Kamilo Beach, Hawaii, microplastics were sampled until the depth of 25 cm and the results showed that more than half of all plastics were found in the top 5 cm and nearly 95% from the top 15 cm. Microplastics are sometimes found even deeper: plastic resin pellets were discovered as deep as two meters below the beach surface in the Santos Beach, Brazil. In this study 81.9% of all pellets occurred below the 0.2 m of the beach surface. Even though the proportion of microplastics deeper in the sand is likely to vary between beaches, these results highlight the need to understand the three-dimensional distribution of microplastics to properly evaluate their concentration on the beaches.
2. Composition of microplastic litter
A majority of beached microplastics are usually fragments from larger plastic items. In Brazilian beaches 96.7% of small plastics (0.5–20 mm) were fragments and virgin plastic pellets comprised only 3.3% of the total amount of litter in this size category. Similarly, in Hawaii 87% of the collected small plastic pieces (1–15 mm) were fragments and 11% plastic pellets, and the rest constituted of plastic line, film and foam. When considering strictly only microplastics, in New Zealand 86.3% of found microplastics were fragments and pellets made up 11%. Sometimes the most abundant microplastic material is Styrofoam, which for example accounted for 96–99% of microplastics in the beaches of South Korea. On the other hand, on the Belgian and England coastlines fibres formed the majority of microplastics.
In general, smaller microplastics seem to be more abundant in the environment than larger ones. In many studies small-sized microplastics less than 1 mm are relatively more abundant than particles ranging in size from 1 mm to 5 mm. For example 35.8% of fragments found from Brazilian beaches were less than 1 mm in their size and in Alabama, USA, microplastics in size fraction from 0.2 to 1 mm made up 38.6 % of all microplastics (0.2–5 mm). On the other hand, some studies report results that are quite the contrary; in New Zealand, microplastics smaller than 1 mm accounted for only 16.4% of all microplastics.
The dominant types of microplastics vary between different study locations. For example in Alabaman shorelines the most abundant category was hard plastics (47.8%) followed by strands (22.3%), foam (18.3%) and film (12.2%). These consisted of polyethylene (PE), polypropylene (PP), polystyrene (PS), polyester and nylon (PA). In New Zealand, the dominant type was polystyrene (54.8%), whereas polyethylene accounted for 20.5% and polypropylene 11%. The large proportion of low-density plastics may reflect the true patterns of their distribution in the environment or be partly a consequence of the research methods that are more effective on discovering low-density plastics. However, in estuarine shorelines of UK the majority of microplastics consisted of denser plastic types including polyester (35%), polyvinyl chloride (26%) and polyamide (18%).
3. Microplastics are abundant in exposed beaches
Similar to macro-sized litter, higher concentrations of microplastics are also typically found on areas with close proximity to densely populated, urban areas, but also other factors influence microplastic abundance and distribution. In a study made on the estuary shorelines of Alabama, USA, the mean concentration of microplastics in beach sediments was higher in more marine-influenced locations compared to freshwater-dominated sites. A similar trend was observed in New Zealand, where microplastic concentrations in exposed beaches were greater than in harbour and estuarine environments.
Microplastics can sometimes be very abundant in beach sediments; a study conducted in a heavily polluted Kamilo Beach, Hawaii, observed that the topmost 5 cm of the beach sediment contained on average 3.3% microplastics by weight. The maximum observed value in the samples was 30.2%, which is considerably high especially when considering the low density of plastics compared to sand grains. On a less polluted control beach microplastics accounted for 0.1% of the weight of the sediment. In a study carried out on German Baltic coasts intensive beach use (tourism) was found to increase the content of fibres by up to one magnitude. However, another recent work on four German beaches in the Baltic Sea did not reveal any statistical differences between beaches which different characteristics, but showed the tendency of microplastics to accumulate into topographic depressions.
Average concentrations of small plastic particles on beaches in different locations around the world (range in parentheses). Note different units in the reviewed studies.
| Concentration | Size fraction (mm) | Shore type | Area | Author |
| 0–45.4 pieces kg-1 (d.w.) | 0.032–5.6 | – | Canterbury, New Zealand | (Clunies-Ross et al., 2016) |
|
(1–14 fibres 10 g-1) (0–621 granules 10 g-1) |
> 0.0012 | sandy beach | Katchelotplate, East Frisian Islands, Germany | (Liebezeit & Dubaish, 2012) |
| 20–80 fibres 10 g-1 | > 0.0008 | sandy beach, mudflat | Nova Scotia, Canada | (Mathalon & Hill, 2014) |
| (0–4 pieces 250 g-1 d.w.) | > 0.0016 | – | Singapore | (Ng & Obbard, 2006) |
| 8205–27 606 pieces m-2 | 1–5 | sandy beach | South Korea | (Lee et al., 2013) |
| 5–117 pieces m-2 | 0.2–5 | sandy beach | Mobile Bay, Alabama (USA) | (Wessel et al., 2016) |
| 558 pieces m-2 | > 2 | sandy beach | Geoje Island, South Korea | (Heo et al., 2013) |
| 1 000 virgin pellets m-2 | 0.5–1 | dissipative sandy beach | Recife, NE Brazil | (Costa et al., 2010) |
| (8–99 pieces 500 ml-1) | > 0.02 | – | KwaZulu-Natal, South Africa | (Naidoo, Glassom, & Smit, 2015) |
| Stolte 2015 | ||||
|
Median abundances: 76.27 particles kg -1
93.45 particles kg -1 |
(0.063 – 5mm) |
sandy beach intertidal zone
sandy beach high tide line
|
(Hengstmann et al., 2018) |
A South Korean study has demonstrated that on the same beach concentration of small plastic particles (2–50 mm) can vary considerably according to the chosen sampling method. The concentration of small plastics was twofold when sampled from the high strandline compared to cross-sectional line on the beach. This observation highlights the need of harmonized methodology for sampling microplastics on beaches to obtain comparable results. In this South Korean study, the highest concentration of small plastic particles was found from the upper littoral zone, which was explained to be due to the high amount of small pieces of Styrofoam which can be redistributed further inland due to winds. In Hawaii, microplastic fragments have been reported being more abundant in the high tide line rather than in the backbeach berm.
In a study made in Nova Scotia, Canada, the microplastic concentration peaked on the high tide line in an exposed, high-energy beach, whereas in protected beaches with reduced wave action the highest microplastic concentration was found at the low tide line. It is possible that in high-energy environments waves transport microplastics farther to the shore compared to environments where deposition might be enhanced due to lack of strong waves. However, different types and sizes of plastic may be distributed on the beaches in a different manner. A study conducted in the East Frisian Islands, Germany, observed that only granular microplastics were most abundant in the high water line. Fibres, on the contrary, seemed to be more evenly distributed between the water and high water line, which might indicate a different settling behaviour of distinct shapes of microplastics.
Also direction of the wind may affect microplastic distribution. In Tamar estuary, UK, more microplastics were found at the leeward shorelines of the estuary. Especially denser microplastics made of acrylic, polyester, and polyvinyl chloride were more abundant on leeward study sites, whereas the less dense microplastics, such as particles made of polypropylene and polyethylene, were evenly distributed in leeward and windward sites.
KNOWLEDGE CORNER
MICROPLASTICS ON THE BEACHES OF
THE BALTIC SEA
Most of the information on microplastics on the beaches and beach sediments of the Baltic Sea comes from the coast of Germany. The areas studied include the isle of Rugen and several beaches along the German Baltic coast in the greater area of Rostock. Also beaches in Lithuania and Poland have been sampled for microplastics. All the studies have used different approached for sampling and sample analyses thus it is difficult to compare their results. It has been proposed that the Baltic Sea should be could act as sink for microplastics since the limited water exchange with the North Sea and high discharges from the rivers and cities in the area. Microplastics from rivers and other waterways would accumulate also on the beaches. To study this, in the study of Stolte et al. (2015) samples were collected from the topmost 2 cm of beach sediment in several locations including areas of extensive tourism, city and harbour discharge and low anthropogenic influence, but failed to find clear correlation between human activities and microplastic concentrations in the beach sediments. The study found microplastic concentrations of 0–7 particles and 2–11 fibres kg-1 (dw) of sediments were found, but report that the amounts were likely underestimations since only intensely coloured pieces were counted. In opposite to that, Graca et al. (2017) found that the concentration of microplastics along the Polish coast of the Baltic Sea varied from 25 particles kg−1 dw.at the open sea beach to 53 particles kg−1 dw at beaches of strongly urbanized bay. In bottom sediments, microplastics concentration was visibly lower compared to beach sediments (0–27 particles kg−1 dw.) and decreased from the shore to the open, deep-sea regions.
