Our results highlight the presence of floating plastic litter items in the Barents Sea and in the Arctic. However, since the distance between the objects and the ship was not estimated and as no calibration was undertaken prior to helicopter flights, we can only provide linear densities (km−1) rather than litter densities per area (km−2). Therefore, our data are not comparable with those from many other studies (e.g. Ryan 1988; Lecke-Mitchell and Mullin 1997; Barnes and Milner 2005; Pichel et al. 2007, 2012). Still, conversion of figures given in Ryan (2013, 2014), Ryan et al. (2014) and Miranda-Urbina et al. (2015) to items km−1 enables a comparison with our ship-based data. This suggests that floating debris in the Barents Sea/Fram Strait area (0.0039 items km−1) is slightly higher than that in the Antarctic (0.0013 items km−1) and sub-Antarctic Southern Ocean (0.0015 items km−1), but much lower than that at lower latitudes such as the temperate Southern Ocean (0.0217 items km−1), South Atlantic (0.1030 items km−1), South Pacific (0.0768 items km−1), Bay of Bengal (0.2484 items km−1) or even the Straits of Malacca (15.9389 items km−1). It could be concluded that sea ice still hinders the spread of floating litter to polar regions to some extent and/or that the distance to more populated areas currently still limits the spread of litter to polar regions.
In annual Barents Sea fisheries surveys, plastic also dominates floating litter and tends to drift along the main currents (Prokhorova 2014), with most counts located between 69° and 74°N and 25° and 45°E—an area influenced by the North Cape and Murman Currents. However, the area surveyed is located further to the east than ours, north of Murmansk, and cannot be compared as no reference to distance or area covered is provided. Their highest litter counts coincide with areas of intensive fishery and shipping. Indeed, Sswat et al. (2015) reported evidence of trawling activities at all stations >300 m depth on the seabed northwest of Svalbard.
Our linear litter densities can also be compared with those from the seafloor of the HAUSGARTEN observatory, which is located below the parts of the present study area: analysis of images taken by a towed camera system yielded 2.24–18.47 items km−1 at 2500 m water depth (data from Bergmann and Klages (2012), converted to linear transect length for comparison). Surface litter quantities recorded in the HAUSGARTEN area (this study) were between 0 and 0.22 items km−1 and were thus 1–2 orders of magnitude lower compared with benthic litter. From this, it could be inferred that the seafloor may act as a sink of litter, as proposed by Woodall et al. (2014). Contrary to a common notion that most plastics are characterised by a low density and will only sink after fouling organisms and sediments have added weight, it has been estimated that 50 % of the plastics from municipal waste exceed the density of sea water such that it readily sinks to the seafloor (Engler 2012), which is enhanced by strong winds and storms (Kukulka et al. 2012). However, litter quantities on the seafloor were probably also higher because the camera was towed at lower altitude (1.5 m) compared with the distance between helicopter- or ship-based observers and the sea surface, resulting in higher counts of (smaller-sized) litter on the seafloor.
Despite the advantage of the large geographic ranges covered, few studies rely on aircraft for the assessment of floating litter. Ryan (1988) reported mean densities of large plastic items on aerial transects 10 and 50 km off Cape Columbine and Cape Point of 1.64 and 19.64 items km−2, respectively, at a flight altitude of 130 m. Lecke-Mitchell and Mullin (1997) reported densities of 1 litter item km−2 from the Gulf of Mexico (229 m altitude). Although Pichel et al. (2007) flew at lower speed (100 m s−1) but at a higher altitude (300 m), their surveys in the North Pacific yielded seemingly higher litter counts (279–875 litter items). However, strictly speaking no comparison can be made as no area estimate was provided. Pichel et al. (2012) reported 102 items of anthropogenic or terrestrial origin in the Gulf of Alaska, but no transect lengths were given. Sighting data from aerial surveys conducted for a variety of purposes (e.g. fishery patrols, coast guard) could be used in an ‘aircraft-of-opportunity’ approach to increase our knowledge on the global distribution of litter, especially in poorly known remote areas. However, standard operational protocols are needed to ensure the comparability of data. To aid comparability, future surveys should provide both linear and area density estimates or at least survey distances as ‘ship/aircraft-of-opportunity’ type of surveys may not always be able to do the calibrations required to derive area density estimates, whereas platform positions are often recorded by default. From this, survey distance and linear density can be calculated, which would increase our knowledge base, especially for poorly sampled regions.
Since the global plastic production grows ~4 % per year and demand reached 299 million t in 2013 (PlasticsEurope 2015), the contamination of the ocean with litter is likely to rise (Jambeck et al. 2015). Our report highlights once again that even remote and thus presumably pristine environments such as the Arctic are not exempt from plastic pollution. Indeed, litter pollution in the Arctic is likely to increase as anthropogenic pressure will grow due to easier access to this region caused by the decreased sea ice cover. In addition, the long-term mean net volume transport in the West Spitsbergen Current, which is derived from the North Atlantic, was estimated at 6.6 Sverdrup (Beszczynska-Möller et al. 2012). This implies that there will be a constant supply of litter transported to the north with water masses of Atlantic origin.
This notion is corroborated by models projecting the formation of a sixth garbage patch in the Barents Sea region (van Sebille et al. 2012), which is probably due to highly populated coasts of the North Atlantic and may leak to the north. Assuming that significant litter inputs began in the 1970s and that the formation of this garbage patch is projected for 50 years, it seems reasonable to assume that there are already significant quantities in this region to be detected. Unfortunately, we cannot compare our data with these projections due to low spatial model resolution and because no true litter density units are provided.
One of the threats posed by floating litter is the risk of alien invasion through long-distance transport and ingestion. Indeed, plastic debris from Svalbard harboured xenobiota such as the barnacle Semibalanus balanoides and the bryozoan Membranipora membranacea (Barnes and Milner 2005). The risk of alien invasion in the Arctic may be ever higher when sea ice shrinkage reduces an effective barrier to both litter and exotics (Barnes 2002). In terms of the risk of ingestion, 8 % of Greenland sharks (Somniosus microcephalus) caught off South Greenland and 3 % of conspecifics from Kongsfjord, close to our study area, ingested litter as did 88 % northern fulmars (Fulmarus glacialis) from the nearby Isfjord (Leclerc et al. 2012; Nielsen et al. 2014; Trevail et al. 2015).
The global increase in marine litter, even at remote locations such as the poles, highlights the fact that the implementation of the current legislation does not suffice to tackle the problem of poor practices of solid waste management. Unless effective action is taken, it will only continue to worsen in years to come (Jambeck et al. 2015).