1. Visual surveys are used to study floating litter
2. Floating litter is abundant next to populated areas
3. Plastic dominates the litter
4. Modelling reveals the accumulation zones
5. The fate of floating plastic
1. Visual surveys are used to study floating litter
Most of floating marine litter is plastic. Litter on the sea surface can be observed visually from ships and smaller vessels, although working conditions, such as rough sea conditions, ship speed, elevation of the observer and observation bias for example towards larger items, affect the quantity and types of litter that is detected. Sometimes floating litter is monitored with surface trawls or even surveyed from a low-flying helicopter or an aircraft. These aerial surveys are used usually to spot relatively large items, such as derelict fishing nets.
The densities of floating plastics are usually reported per transect length or per surface area. Interpretation of results is often compromised by the varying litter categories used in different surveys. Floating litter can be reported as identified items, if they are recognized, as well as according to material (e.g. plastics, wood, glass) or their possible sources (e.g. fisheries, aquaculture, packaging).
Densities of floating marine litter are usually reported varying from 0 to beyond 600 items per km2 when considering items larger than 2 cm. As with beached macrolitter, the densities of floating macrolitter are high in regions close to highly populated areas and areas with large human activity, which indicates a large input of litter from these areas. For example the Mediterranean Sea is observed to have high concentrations of marine litter of all sizes due to strong human pressures and slow water exchange rate.
On the other hand, floating litter is easily transported and hence can accumulate also far from the source. This was been the case for example after the 2011 tsunami in Japan, when approximately 3.5 million tonnes of items such as cars, pieces of buildings, all possible plastic items and pieces were washed into the sea. One year after items were found washed ashore e.g. in Oregon, southern Alaska and British Columbia. A study made in the Mediterranean Sea and in the Straits of Malacca reported that floating anthropogenic litter is often accumulating on the same areas as natural floating material (e.g. driftwood, and macroalgae). This suggests that the same processes, such as surface currents, fronts, gyres and convergence zones, are responsible for the distribution of both natural and anthropogenic litter afloat at the sea. Due to these processes that transport and pile floating material to certain areas, the distribution of marine litter is rather patchy than uniform on the sea surface.
In addition to the enclosed sea areas with high population densities, large concentrations of floating litter have also been found to accumulate to five subtropical gyres. These gyres are located in the northern and southern Atlantic Ocean, northern and southern Pacific Ocean and in the Indian Ocean, where sea currents transport floating litter. In these gyres litter densities are multiple orders of magnitude higher than in the surrounding open sea areas. Despite high densities of floating litter in subtropical gyres and the Mediterranean Sea, there seems to be a difference in the litter composition on these areas. In the Mediterranean Sea larger proportion of litter consists of larger plastic items rather than fragments, which was suggested to reflect the nearby sources of macro-sized litter.
Abundances of floating litter (number km-1) surveyed around the world (range in parentheses). Note different units in the reviewed studies.
|Size (cm)||% plastic||Area||Author|
0.001 items km-1
(0–0.216 items km-1)
|> 20||100||Fam Strait and Barents Sea||(Bergmann et al., 2016)|
|(1–250 items km-2)||> 2||~80*||Fjords, gulfs and channels of southern Chile||(Hinojosa & Thiel, 2009)|
|(<1–36 items km-2)||–||86.9||Coastal waters of Chile||(M. Thiel et al., 2003)|
|24.9 ± 2.4 items km-2 (0–162.1 items km-2)||> 2||95.6||Central and western Mediterranean Sea||(Suaria & Aliani, 2014)|
|1.5–25 items km-2||–||–||Ligurian Sea, NW Mediterranean||(Aliani, Griffa, & Molcard, 2003)|
|13 614 items km-2 (4 241–31 811 items km-2)||> 1.5||100**||Central Mediterranean Sea||(Cózar et al., 2015)|
1.48 items km-2
(0.91–2.27 items km-2)
|–||> 85||Coastal waters of British Columbia, Canada||(Williams, Ashe, & O’Hara, 2011)|
4.95 items km-2
(0.28–16.89 items km-2)
|<2.5-10||64.3||Northern South China Sea||(Zhou et al., 2011)|
|0.032–6 items km-2||> 1||96||African sector of the Southern Ocean||(Ryan, Musker, & Rink, 2014)|
32.4 items km-2
(25.1–38.6 items km-2)
|> 2||> 70||German Bight, North Sea||(Martin Thiel, Hinojosa, Joschko, & Gutow, 2011)|
|578 ± 219 items km-2||> 1||98.8||The Straits of Malacca||(Ryan, 2013)|
|8.8 ± 1.4 items km-2||> 1||95.5||The Bay of Bengal||(Ryan, 2013)|
*including only Styrofoam, plastic bags and plastic fragments
**observed only plastics
A vast majority of all floating litter consists of plastics – their proportion ranges from 64.3% to even 100% in some areas. The dominance of plastic among floating litter can be explained by their low density and high floatability. For example in a survey made in the central and western Mediterranean Sea only 4.4% of all floating litter consisted of non-plastic man-made items. These items included for instance aluminium cans, manufactured wood, glass bottles, paper and cardboard boxes. Also in the Straits of Malacca and the Bay of Bengal items other than plastic formed only 1.2–4.5% of all litter and included mainly wood, light bulbs and glass bottles.
The composition of floating litter varies in different areas, but in many places expanded polystyrene (EPS), known also as Styrofoam, is the single most prominent material. For example it accounted for 63.2% of the floating litter in the straits of Malacca, 48.8% in coastal waters of British Columbia and 19.4% in the northern South China Sea when considering floating litter that is smaller than 10 cm. When combining the results of 24 visual surveys conducted in world’s subtropical gyres, the most abundant macroplastic items were made of foamed polystyrene and accounted for 26% of all the observed items. Despite being highly floatable, the large proportion of expanded polystyrene among floating litter can also partly be due to the fact that it is easily recognized from other plastic materials even from relatively long distance.
Another common category of floating litter are the various kinds of unidentifiable plastic fragments, which in the Bay of Bengal accounted for 30.5% of litter. They are also abundant in the subtropical gyres (21.5%). Almost as abundant in the subtropical gyres are fishing related litter, such as buoys, lines and nets, which has been reported to form 20% of the litter. In the Bay of Bengal and British Columbia they form 6.3% and in the coastal waters of Chile they accounted for only 3.6%.
Plastic bottles and plastic bags are also frequently reported floating on the sea surface. Plastic bottles accounted for 18.4% of all the floating litter in subtropical gyres, 15.1% in the Bay of Bengal and 14.7% in the British Columbia, Canada. The proportion of plastic bags and food wrappers was 47.6% in the coastal waters of Chile, 16.5% in the Straits of Malacca, 10.5% in the British Columbia and 9.8% in the subtropical gyres. Also it has been reported from the Mediterranean, that majority of the floating plastics were various fragments, bottles, containers, plastic film and plastic bags.
4. ModelLing reveals the accumulation zones
Modelling is an efficient way to predict where marine debris converges and it has been used both globally and regionally to estimate plastic abundance and mass in the ocean. To build a model, knowledge about the sources and amounts of debris as well as their transport and accumulation processes are needed. Many models have utilized the data gathered by surface drifters, which are equipment floating in the currents with satellite transmitters. In addition, visual surveys or surface tows telling about plastic abundance in different parts of the sea are used.
Different studies have localized the accumulation zones for floating plastic debris in five subtropical gyres, that are located in in the northern and southern Atlantic Ocean, northern and southern Pacific Ocean and in the Indian Ocean. In these areas plastic concentrations are observed to be high. A study modelling all sizes of floating plastic (0.333-1 mm, 1.01-4.75 mm, 4.76-200 mm and > 200 mm) in the world’s oceans estimated that at least 5.2 trillion plastic particles weighting 269 940 tons are floating at sea. According to the estimations northern hemisphere contains approximately a bit more than a half of all plastic mass and items compared to the southern hemisphere. From the five subtropical gyres where plastic is accumulating, North Pacific in the northern hemisphere and Indian Ocean in the southern hemisphere contain the highest concentration and mass of plastic litter.
When distinct size categories of floating plastic debris are compared, two smallest categories containing microplastics between 0.333 and 4.75 mm are estimated to account for 92.4% of the total amount of plastic items. However, when comparing plastic mass instead of particles, 75.4% of the mass of plastic is composed of large macroplastic (> 200 mm). The numerical dominance of small plastic particles over larger items has been observed also in samples taken from the environment. For example in the western North Atlantic Ocean 88% of plastic was smaller than 10 mm and in the Mediterranean Sea, 83% of floating plastics collected from the sea surface were in the size range of microplastics (< 5 mm).
5. The fate of floating plastic
Even though plastic production and disposal has constantly increased, similar increase is not observed in the concentrations of floating macroplastics since the late 1980s which indicates that the surface waters are not the final destination for plastics. Floating plastic litter is thus possibly removed from the surface by several mechanisms including sedimentation, shore deposition, ingestion and fragmentation into non-detectable particles. Breakdown into smaller particles is likely, since the studies so far have showed that small fragments are numerically more abundant compared to larger plastic litter.
Sedimentation of floating litter is also possible but so far there is a lack of studies that would assess marine litter in different compartments in the same area. The studies made in the Fram Strait have enabled the comparison of floating and benthic litter concentrations, and shown that litter quantities on the sea surface are 1–2 orders of magnitude lower compared with the litter on the seafloor.