Marine Pollution Bulletin 79 (2014) 220–224
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Litter survey detects the South Atlantic ‘garbage patch’ Peter G. Ryan ⇑ Percy FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa
a r t i c l e
i n f o
a b s t r a c t
Keywords: Marine debris Plastic litter South Atlantic gyre Seaweed Survey Windage
A distance-based technique was used to assess the distribution and abundance of floating marine debris (>1 cm) in the southeast Atlantic Ocean between Cape Town and Tristan da Cunha, crossing the southern edge of the South Atlantic ‘garbage patch’ predicted by surface drift models. Most litter was made of plastic (97%). Detection distances were influenced by the size and buoyancy of litter items. Litter density decreased from coastal waters off Cape Town (>100 items km2) to oceanic waters (100 m. The size of each debris item was allocated to one of five size classes based on its longest dimension: a < 5 cm, b = 5–15 cm, c = 15–30 cm, d = 30–60 cm, and e > 60 cm. Minimum item size was approximately 1–2 cm. Litter items were placed into one of the following categories based on the type of material and likely use of the item. Plastic items were divided into packaging (bottles, tubs/cups, lids and lid-rings, bags, food
wrapping, polystyrene, and other packaging such as packing strips, etc.), fishery-related plastic articles (ropes and nets, floats, and other fishing gear such as fish trays), other plastic user items (designed for repeated use, unlike packaging, divided into three categories: buckets, shoes/gloves/hats, and other user items), and finally, other plastic pieces (mostly fragments of items that could not be identified, but some items too deep to see clearly also were placed into this category). Non-plastic items were divided into glass jars/bottles, light bulbs, tins/aerosols, cardboard/paper, and wood (worked timber). The incidence of encrusting biota on litter items was recorded. The effect of item size on detection distance was determined from the frequency of encounters in relation to distance from the ship (Ryan, 2013). v2 goodness-of-fit tests were used to compare the effect of distance on detection rate within each size category between this study and Ryan (2013), as well as the effect of buoyancy on detection distance. A simple correction factor for items within 50 m of the ship was calculated by assuming that all litter items were detected within the 10-m wide distance band with the largest number of encounters. Other zones within 50 m of the ship were scaled to compensate for ‘missed’ items by standardizing relative to the maximum count (Ryan, 2013). Correction factors were then applied weighted by the size composition of litter items. This size-based counting technique allows estimates of the densities of different litter size classes at sea. Densities of all litter items as well as items >5 cm were estimated to facilitate comparison with estimates from studies with different minimum item sizes. 3. Results Just under 79 h of litter counts were conducted covering 1963 km of transects (38 50-km transects), counting 281 litter items (Table 1) and 90 seaweed clumps. Almost a third of litter items (30%) and 8% of seaweeds were found in coastal waters over the African continental shelf, despite comprising only 2.6% of transect distance (51 km). Average wind speed during transects was 8.8 ± 2.4 knots (SD, range 5–15 knots), with slightly greater wind speeds in Region C (10.7 ± 2.0 knots) than in the other three regions (8.3 ± 2.1 knots; F3,73 = 6.2, P < 0.001; see Fig. 1 for region Table 1 The abundance and composition of litter in coastal (51 km) and oceanic waters (1911 km) in the southeast Atlantic Ocean during September–October 2013. Type of litter
All plastic items Packaging Bottles Polystyrene Bags Food wraps Tubs/cups Lids and lid-rings Other packaging Fishing/boating Ropes/nets Floats Other fishing gear User items Buckets Shoes/gloves/hats Other user items Pieces All non-plastic items Glass bottles Light bulbs Tins/aerosols Cardboard/paper Total (all artefacts)
Coastal waters
Oceanic waters
Total
n
%
n
%
n
%
82 63 2 0 34 27 0 0 0 6 6 0 0 0 0 0 0 13 2 0 0 0 2 84
98 75 2 0 40 32 0 0 0 7 7 0 0 0 0 0 0 15 2 0 0 0 2
191 80 25 5 30 3 4 9 4 44 10 6 28 14 10 2 2 53 6 3 2 1 0 197
97 41 13 3 15 2 2 5 2 22 5 3 14 7 5 1 1 27 3 2 1 1 0
273 143 27 5 64 30 4 9 4 50 16 6 28 14 10 2 2 66 8 3 2 1 2 281
97 51 10 2 23 11 1 3 1 18 6 2 10 5 4 1 1 23 3 1 1 0 1
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boundaries). Sea surface temperature decreased from east to west (Regions A and B 16.1 ± 0.7 °C, greater than C 15.1 ± 0.4 °C and D 14.1 ± 0.7 °C; F3,73 = 22.8, P < 0.001), with a corresponding decrease in salinity (Regions A and B 35.40 ± 0.25‰; Regions C and D 35.22 ± 0.11; F3,73 = 5.3, P < 0.01). 3.1. Composition of debris and encrusting biota Overall, 97% of litter items were made of plastic (Table 1). Glass was the most common non-plastic litter material, followed by metal. No wooden items were observed, and cardboard was only found in coastal waters close to Cape Town. Among plastic litter, packaging was the most common application, but the proportion of packaging was greater in coastal waters than in oceanic waters (Table 1). Fishery-related items (mainly pieces of fish trays) made up a greater proportion of litter offshore, as did unidentified pieces of plastic (Table 1). Encrusting biota were observed on very few litter items in coastal waters (1%), but were common in oceanic waters, where goose barnacles Lepas spp. and other large organisms were seen on at least 27% of debris items, and more than half of all litter items (58%) had a yellow-brown biofilm of algae. These are minimum estimates, because it was not possible to check more distant items for encrusting biota. 3.2. Buoyancy, colour and density correction factors Most litter items (64%) were floating at the surface without significant windage; only 17% of items protruded above the surface, with the remaining 19% completely submerged (estimated 0.2– 5 m below the sea surface). However, these proportions were biased by the greater detection range of protruding items; 41% of these items were observed >50 m from the ship, compared to only 6% of items at the surface and 0% of submerged items (v2 = 40.6, df = 2, P < 0.001). It is thus likely that submerged items were under-represented relative to surface and protruding items. Litter types differed greatly in their buoyancy; bags and to a lesser extent lids were most commonly submerged, whereas polystyrene and other foamed plastics, bottles and floats were the most common items with significant windage (Table 2). Most litter items were white (46%) or clear (18%), although both these categories usually were stained by an algal biofilm in oceanic waters. Other common colours were light blue (10%), yellow (8%), orange/red/pink (6%) and green (5%). Few black (1%) or grey (100 items km2, decreasing to levels similar to adjacent oceanic waters >20 km from land. Within oceanic waters there were consistent spatial differences in the density of floating litter. Dividing the oceanic transects into four regions with similar sampling effort (Fig. 1), litter densities estimated per 50 km transect block differed among regions (F3,32 = 10.4, P < 0.001). Region B (3–8°E) had significantly greater litter densities (6.2 ± 1.3 items km2) than adjacent Regions A and C (2.7 ± 0.3 items km2), and the density in these waters was greater than that around Tristan da Cunha and Gough Island (Region D, 1.0 ± 0.4 items km2). Limiting data to larger litter items (>5 cm longest dimension) reduced the average density to 28.5 items km2 in coastal waters and 2.0 ± 0.3 items km2 in oceanic waters (4.2 ± 0.9 items km2 in Region B, 1.8 ± 0.9 in Regions A and C, and 0.7 ± 0.3 in Region D). Almost half as many seaweed clumps (42%, n = 83) as litter items were encountered in oceanic waters. Ecklonia was observed up to 400 km west of the Cape coast, and Macrocystis occurred sporadically throughout, but was most common up to 1150 km east of Tristan. At the 50-km transect scale there was a negative relationship between seaweeds and litter (litter density = 1.99–0.079 seaweed density), but the correlation was not significant (r34 = 0.088, NS). Drifting biota were recorded throughout, but densities were greatest in Region B, where more than 90% of Physalia and Velella, and 75% of Dosima were encountered. 4. Discussion
Table 2 The proportions of different types of litter that protruded above the sea surface, were at the surface, or were submerged in oceanic waters of the southeast Atlantic Ocean.
a
Litter type (n)
Protruding (%)
Surface (%)
Submerged (%)
Polystyrene and foamed plastics (7) Floats (4) Bottles (36) Ropes and nets (11) Fish trays (26) Pieces (61) Lids (9) Bags and food wraps (34)
100 75 53 0 0 2 0 0
0 25 36 100 96 88 67 32
0 0 11 0 4 10a 33 68
Includes unidentified items.
There are few published estimates of the densities of floating litter in the South Atlantic Ocean. Litter densities from net samples have been reported up to 7°E off the Cape (Morris, 1980; Ryan, 1988), but sampling was limited to small litter items (10 cm) in the Atlantic Ocean from 60°N to 60°S, but their cruise track for 20–40°S ran along the South American coast. The densities of large litter items (>20 cm) counted during aerial surveys off the Cape west coast decreased with distance from shore: 19.6 items km2 10 km off the coast, and 1.6 items km2 50 km offshore (Ryan, 1988). This is the same order of magnitude as the densities
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1.0
1-5 cm
5-15 cm
15-30 cm
>30 cm
Proportion
0.8
0.6
0.4
0.2
0.0
1.0
Proportion
0.8
0.6
0.4
0.2
0.0 0
10
20
30
40
50
100
0
10
20
30
40
50
100
Perpendicular distance from the vessel (m) Fig. 2. The effect of litter size on detection distance (proportion relative to peak count zone) based on all observations in the southeast Atlantic Ocean.
estimated from the limited sampling in coastal waters in 2013, and supports the finding of a marked offshore gradient west of Cape Town. My data thus appear to be the first published estimates of macro-litter densities from oceanic waters in the temperate southeast Atlantic Ocean. The densities are much greater than crude estimates for the Agulhas Retroflection south of Africa in the 1980s (0.04–0.09 large items km2; Ryan, 1990). Despite the high spatial heterogeneity and generally low densities of floating litter at sea (Ryan et al., 2009), consistent regional differences in litter density were detected at the 50-km sample scale on both west- and east-bound survey legs conducted one month apart. The greatest density was found in Region B from 3 to 8°E. Litter counts in Region C, around the Greenwich Meridian, may have been depressed slightly by the higher wind speeds (and associated rougher seas) experienced in this region, but the difference was minor (50°S). Encrusting biota are common at lower latitudes, and most items stranding on the Tristan archipelago are heavily encrusted with goose barnacles and other biota (pers. obs.). The low density of seaweeds in Region B further supports the inference that litter is accumulating in this region. Large seaweeds (kelps) are commonly encountered drifting at higher latitudes in the South Atlantic (Barnes and Milner, 2005), providing an index of natural drifting debris from land-based sources. However, they are less persistent than plastics, so their scarcity in Region B suggests that the litter found there has been adrift for a long period. The density of litter in Region B was comparable to the density estimates in oceanic waters of the Bay of Bengal (Ryan, 2013), and enclosed seas with high levels of litter input (e.g. South China Sea, Zhou et al., 2011; Mediterranean, Aliani et al., 2000), but was much lower than densities recorded in the North Pacific gyre (Titmus and Hyrenbach, 2011). However, it is likely that litter densities farther north and west in the South Atlantic are higher, because the sample area was south and east of the core area where floating litter is predicted to accumulate (Lebreton et al., 2012; Maximenko et al., 2012; van Sebille et al., 2012). The low densities of litter in Region D might appear surprising, given that the core of the predicted accumulation zone lies north of this region. However, Tristan da Cunha lies beyond the southern edge of the accumulation zone, in an area of higher surface currents (Maximenko et al., 2009, 2012), and so might be expected to have lower litter densities than
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adjacent waters slightly farther north. Additional surveys should be conducted across the South Atlantic at 25–35°S to obtain a better idea of the densities of litter in the core of the gyre. Ryan (2013) highlighted the need to standardize at-sea surveys of floating large debris by (1) reporting the minimum size of litter items scored, (2) correcting for differential detection with distance from the vessel (unless the transect width is very narrow), and (3) calculating different detection functions for items of different sizes. The results from this study yielded very similar size-dependent detection rates to those reported from the Bay of Bengal (Ryan, 2013), suggesting that they may be consistent at least for a given observer. Other factors such as sea state, item type, colour and buoyancy, cloud cover and angle of the sun, and observer height and fatigue probably also play a role in detection rate (Dahlberg and Day, 1985). The results confirm that buoyancy is important, so observers should record item colour and buoyancy when conducting at-sea surveys. Provided these steps are followed, ship-based observations from vessels of opportunity can yield comparable, cost-effective litter density estimates at large spatial scales suitable to test the predictions of the surface drifter models. By gathering large volumes of such data we can begin to assess how (and perhaps better understand why) the densities of floating litter vary in both space and time (e.g. Aliani et al., 2003). Acknowledgements I thank Bruce Dyer, Mara Nydegger and Chris Bell for assistance with litter surveys, and the officers of the S.A. Agulhas II for permission to use the bridge during inclement weather. The project was part of the South African National Antarctic Programme, with financial support from the National Research Foundation and the University of Cape Town. References Aliani, S., Griffa, A., Molcard, A., 2003. Floating debris in the Ligurian Sea, northwestern Mediterranean. Mar. Pollut. Bull. 46, 1142–1149. Barnes, D.K.A., Milner, P., 2005. Drifting plastic and its consequences for sessile organism dispersal in the Atlantic Ocean. Mar. Biol. 146, 815–825. Dahlberg, M.L., Day, R.H., 1985. Observations of man-made objects on the surface of the North Pacific Ocean. In: Shomura, R.S., Yoshida, H.O. (Eds.), Proc. Workshop on the Fate and Impact of Marine Debris. US Dept Commerce: NOAA Tech. Mem., NOAA-TM-NMFS-SWFSC-54, pp. 198–211.
Eriksen, M., Maximenko, N., Thiel, M., Cummins, A., Lattin, G., Wilson, S., Hafner, J., Zellers, A., Rifman, S., 2013. Plastic marine pollution in the South Pacific subtropical gyre. Mar. Pollut. Bull. 68, 71–76. Kaiser, J., 2010. The dirt on ocean garbage patches. Science 328, 1506. Law, K.L., Morét-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E.E., Hafner, J., Reddy, C.M., 2010. Plastic accumulation in the North Atlantic subtropical gyre. Science 329, 1185–1188. Lebreton, L.C.-M., Greer, S.D., Borrero, J.C., 2012. Numerical modelling of floating debris in the world’s oceans. Mar. Pollut. Bull. 64, 653–661. Lewison, R.L., Freeman, S.A., Crowder, L.B., 2004. Quantifying the effects of fisheries on protected species: the impact of pelagic longlines on loggerhead and leatherback sea turtles. Ecol. Lett. 7, 221–231. Maximenko, N., Niiler, P., Rio, M.-H., Melnichenko, O., Centurioni, L., Chambers, D., Zlotnicki, V., Galperin, B., 2009. Mean dynamic topography of the ocean derived from satellite and drifting buiy data using three different techniques. J. Atmos. Ocean Technol. 26, 1910–1919. Maximenko, N., Hafner, J., Niiler, P., 2012. Pathways of marine debris derived from trajectories of Lagrangian drifters. Mar. Pollut. Bull. 65, 51–62. Morris, R.J., 1980. Plastic debris in the surface waters of the South Atlantic. Mar. Pollut. Bull. 11, 164–166. Moore, C.J., Moore, S.L., Leecaster, M.K., Weisberg, S.B., 2001. A comparison of plastic and plankton in the North Pacific central gyre. Mar. Pollut. Bull. 42, 1297–1300. Ryan, P.G., 1987. The origin and fate of artefacts stranded on islands in the African sector of the Southern Ocean. Environ. Conserv. 14, 341–346. Ryan, P.G., 1988. The characteristics and distribution of plastic particles at the seasurface off the southwestern Cape Province, South Africa. Mar. Environ. Res. 25, 249–273. Ryan, P.G., 1990. The marine plastic debris problem off southern Africa: types of debris, their environmental effects, and control measures. In: Shomura, R.S., Godfrey, M.L. (Eds.), Proc. Second International Conference on Marine Debris. US Dept Commerce: NOAA Tech. Mem., NOAA-TM-NMFS- SWFSC-154, pp. 85–102. Ryan, P.G., 2013. A simple technique for counting marine debris at sea reveals steep litter gradients between the Straits of Malacca and the Bay of Bengal. Mar. Pollut. Bull. 69, 128–136. Ryan, P.G., Watkins, B.P., 1988. Accumulation of stranded plastic objects and other artefacts at inaccessible Island, central South Atlantic Ocean. S. Afr. J. Antarct. Res. 18, 11–13. Ryan, P.G., Moore, C.J., van Franeker, J.A., Moloney, C.L., 2009. Monitoring the abundance of plastic debris in the marine environment. Philos. Trans. R. Soc. B 364, 1999–2012. Thompson, R.C., Moore, C.J., vom Saal, F.S., Swan, S.H., 2009. Plastics, the environment and human health: current consensus and future trends. Philos. Trans. R. Soc. B 364, 2153–2166. Titmus, A.J., Hyrenbach, K.D., 2011. Habitat associations of floating debris and marine birds in the North East Pacific Ocean at coarse and mesospatial scales. Mar. Pollut. Bull. 62, 2496–2506. van Sebille, E., England, M.H., Froyland, G., 2012. Origin, dynamics and evolution of ocean garbage patches from observed surface drifters. Environ. Res. Lett. 7, 044040. Zhou, P., Huang, C., Fang, H., Cai, W., Li, D., Li, X., Yu, H., 2011. The abundance, composition and sources of marine debris in coastal seawaters or beaches around the northern South China Sea (China). Mar. Pollut. Bull. 62, 1998–2007.
Contents lists available at ScienceDirect
Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul
Litter survey detects the South Atlantic ‘garbage patch’ Peter G. Ryan ⇑ Percy FitzPatrick Institute of African Ornithology, DST/NRF Centre of Excellence, University of Cape Town, Rondebosch 7701, South Africa
a r t i c l e
i n f o
a b s t r a c t
Keywords: Marine debris Plastic litter South Atlantic gyre Seaweed Survey Windage
A distance-based technique was used to assess the distribution and abundance of floating marine debris (>1 cm) in the southeast Atlantic Ocean between Cape Town and Tristan da Cunha, crossing the southern edge of the South Atlantic ‘garbage patch’ predicted by surface drift models. Most litter was made of plastic (97%). Detection distances were influenced by the size and buoyancy of litter items. Litter density decreased from coastal waters off Cape Town (>100 items km2) to oceanic waters (100 m. The size of each debris item was allocated to one of five size classes based on its longest dimension: a < 5 cm, b = 5–15 cm, c = 15–30 cm, d = 30–60 cm, and e > 60 cm. Minimum item size was approximately 1–2 cm. Litter items were placed into one of the following categories based on the type of material and likely use of the item. Plastic items were divided into packaging (bottles, tubs/cups, lids and lid-rings, bags, food
wrapping, polystyrene, and other packaging such as packing strips, etc.), fishery-related plastic articles (ropes and nets, floats, and other fishing gear such as fish trays), other plastic user items (designed for repeated use, unlike packaging, divided into three categories: buckets, shoes/gloves/hats, and other user items), and finally, other plastic pieces (mostly fragments of items that could not be identified, but some items too deep to see clearly also were placed into this category). Non-plastic items were divided into glass jars/bottles, light bulbs, tins/aerosols, cardboard/paper, and wood (worked timber). The incidence of encrusting biota on litter items was recorded. The effect of item size on detection distance was determined from the frequency of encounters in relation to distance from the ship (Ryan, 2013). v2 goodness-of-fit tests were used to compare the effect of distance on detection rate within each size category between this study and Ryan (2013), as well as the effect of buoyancy on detection distance. A simple correction factor for items within 50 m of the ship was calculated by assuming that all litter items were detected within the 10-m wide distance band with the largest number of encounters. Other zones within 50 m of the ship were scaled to compensate for ‘missed’ items by standardizing relative to the maximum count (Ryan, 2013). Correction factors were then applied weighted by the size composition of litter items. This size-based counting technique allows estimates of the densities of different litter size classes at sea. Densities of all litter items as well as items >5 cm were estimated to facilitate comparison with estimates from studies with different minimum item sizes. 3. Results Just under 79 h of litter counts were conducted covering 1963 km of transects (38 50-km transects), counting 281 litter items (Table 1) and 90 seaweed clumps. Almost a third of litter items (30%) and 8% of seaweeds were found in coastal waters over the African continental shelf, despite comprising only 2.6% of transect distance (51 km). Average wind speed during transects was 8.8 ± 2.4 knots (SD, range 5–15 knots), with slightly greater wind speeds in Region C (10.7 ± 2.0 knots) than in the other three regions (8.3 ± 2.1 knots; F3,73 = 6.2, P < 0.001; see Fig. 1 for region Table 1 The abundance and composition of litter in coastal (51 km) and oceanic waters (1911 km) in the southeast Atlantic Ocean during September–October 2013. Type of litter
All plastic items Packaging Bottles Polystyrene Bags Food wraps Tubs/cups Lids and lid-rings Other packaging Fishing/boating Ropes/nets Floats Other fishing gear User items Buckets Shoes/gloves/hats Other user items Pieces All non-plastic items Glass bottles Light bulbs Tins/aerosols Cardboard/paper Total (all artefacts)
Coastal waters
Oceanic waters
Total
n
%
n
%
n
%
82 63 2 0 34 27 0 0 0 6 6 0 0 0 0 0 0 13 2 0 0 0 2 84
98 75 2 0 40 32 0 0 0 7 7 0 0 0 0 0 0 15 2 0 0 0 2
191 80 25 5 30 3 4 9 4 44 10 6 28 14 10 2 2 53 6 3 2 1 0 197
97 41 13 3 15 2 2 5 2 22 5 3 14 7 5 1 1 27 3 2 1 1 0
273 143 27 5 64 30 4 9 4 50 16 6 28 14 10 2 2 66 8 3 2 1 2 281
97 51 10 2 23 11 1 3 1 18 6 2 10 5 4 1 1 23 3 1 1 0 1
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boundaries). Sea surface temperature decreased from east to west (Regions A and B 16.1 ± 0.7 °C, greater than C 15.1 ± 0.4 °C and D 14.1 ± 0.7 °C; F3,73 = 22.8, P < 0.001), with a corresponding decrease in salinity (Regions A and B 35.40 ± 0.25‰; Regions C and D 35.22 ± 0.11; F3,73 = 5.3, P < 0.01). 3.1. Composition of debris and encrusting biota Overall, 97% of litter items were made of plastic (Table 1). Glass was the most common non-plastic litter material, followed by metal. No wooden items were observed, and cardboard was only found in coastal waters close to Cape Town. Among plastic litter, packaging was the most common application, but the proportion of packaging was greater in coastal waters than in oceanic waters (Table 1). Fishery-related items (mainly pieces of fish trays) made up a greater proportion of litter offshore, as did unidentified pieces of plastic (Table 1). Encrusting biota were observed on very few litter items in coastal waters (1%), but were common in oceanic waters, where goose barnacles Lepas spp. and other large organisms were seen on at least 27% of debris items, and more than half of all litter items (58%) had a yellow-brown biofilm of algae. These are minimum estimates, because it was not possible to check more distant items for encrusting biota. 3.2. Buoyancy, colour and density correction factors Most litter items (64%) were floating at the surface without significant windage; only 17% of items protruded above the surface, with the remaining 19% completely submerged (estimated 0.2– 5 m below the sea surface). However, these proportions were biased by the greater detection range of protruding items; 41% of these items were observed >50 m from the ship, compared to only 6% of items at the surface and 0% of submerged items (v2 = 40.6, df = 2, P < 0.001). It is thus likely that submerged items were under-represented relative to surface and protruding items. Litter types differed greatly in their buoyancy; bags and to a lesser extent lids were most commonly submerged, whereas polystyrene and other foamed plastics, bottles and floats were the most common items with significant windage (Table 2). Most litter items were white (46%) or clear (18%), although both these categories usually were stained by an algal biofilm in oceanic waters. Other common colours were light blue (10%), yellow (8%), orange/red/pink (6%) and green (5%). Few black (1%) or grey (100 items km2, decreasing to levels similar to adjacent oceanic waters >20 km from land. Within oceanic waters there were consistent spatial differences in the density of floating litter. Dividing the oceanic transects into four regions with similar sampling effort (Fig. 1), litter densities estimated per 50 km transect block differed among regions (F3,32 = 10.4, P < 0.001). Region B (3–8°E) had significantly greater litter densities (6.2 ± 1.3 items km2) than adjacent Regions A and C (2.7 ± 0.3 items km2), and the density in these waters was greater than that around Tristan da Cunha and Gough Island (Region D, 1.0 ± 0.4 items km2). Limiting data to larger litter items (>5 cm longest dimension) reduced the average density to 28.5 items km2 in coastal waters and 2.0 ± 0.3 items km2 in oceanic waters (4.2 ± 0.9 items km2 in Region B, 1.8 ± 0.9 in Regions A and C, and 0.7 ± 0.3 in Region D). Almost half as many seaweed clumps (42%, n = 83) as litter items were encountered in oceanic waters. Ecklonia was observed up to 400 km west of the Cape coast, and Macrocystis occurred sporadically throughout, but was most common up to 1150 km east of Tristan. At the 50-km transect scale there was a negative relationship between seaweeds and litter (litter density = 1.99–0.079 seaweed density), but the correlation was not significant (r34 = 0.088, NS). Drifting biota were recorded throughout, but densities were greatest in Region B, where more than 90% of Physalia and Velella, and 75% of Dosima were encountered. 4. Discussion
Table 2 The proportions of different types of litter that protruded above the sea surface, were at the surface, or were submerged in oceanic waters of the southeast Atlantic Ocean.
a
Litter type (n)
Protruding (%)
Surface (%)
Submerged (%)
Polystyrene and foamed plastics (7) Floats (4) Bottles (36) Ropes and nets (11) Fish trays (26) Pieces (61) Lids (9) Bags and food wraps (34)
100 75 53 0 0 2 0 0
0 25 36 100 96 88 67 32
0 0 11 0 4 10a 33 68
Includes unidentified items.
There are few published estimates of the densities of floating litter in the South Atlantic Ocean. Litter densities from net samples have been reported up to 7°E off the Cape (Morris, 1980; Ryan, 1988), but sampling was limited to small litter items (10 cm) in the Atlantic Ocean from 60°N to 60°S, but their cruise track for 20–40°S ran along the South American coast. The densities of large litter items (>20 cm) counted during aerial surveys off the Cape west coast decreased with distance from shore: 19.6 items km2 10 km off the coast, and 1.6 items km2 50 km offshore (Ryan, 1988). This is the same order of magnitude as the densities
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1.0
1-5 cm
5-15 cm
15-30 cm
>30 cm
Proportion
0.8
0.6
0.4
0.2
0.0
1.0
Proportion
0.8
0.6
0.4
0.2
0.0 0
10
20
30
40
50
100
0
10
20
30
40
50
100
Perpendicular distance from the vessel (m) Fig. 2. The effect of litter size on detection distance (proportion relative to peak count zone) based on all observations in the southeast Atlantic Ocean.
estimated from the limited sampling in coastal waters in 2013, and supports the finding of a marked offshore gradient west of Cape Town. My data thus appear to be the first published estimates of macro-litter densities from oceanic waters in the temperate southeast Atlantic Ocean. The densities are much greater than crude estimates for the Agulhas Retroflection south of Africa in the 1980s (0.04–0.09 large items km2; Ryan, 1990). Despite the high spatial heterogeneity and generally low densities of floating litter at sea (Ryan et al., 2009), consistent regional differences in litter density were detected at the 50-km sample scale on both west- and east-bound survey legs conducted one month apart. The greatest density was found in Region B from 3 to 8°E. Litter counts in Region C, around the Greenwich Meridian, may have been depressed slightly by the higher wind speeds (and associated rougher seas) experienced in this region, but the difference was minor (50°S). Encrusting biota are common at lower latitudes, and most items stranding on the Tristan archipelago are heavily encrusted with goose barnacles and other biota (pers. obs.). The low density of seaweeds in Region B further supports the inference that litter is accumulating in this region. Large seaweeds (kelps) are commonly encountered drifting at higher latitudes in the South Atlantic (Barnes and Milner, 2005), providing an index of natural drifting debris from land-based sources. However, they are less persistent than plastics, so their scarcity in Region B suggests that the litter found there has been adrift for a long period. The density of litter in Region B was comparable to the density estimates in oceanic waters of the Bay of Bengal (Ryan, 2013), and enclosed seas with high levels of litter input (e.g. South China Sea, Zhou et al., 2011; Mediterranean, Aliani et al., 2000), but was much lower than densities recorded in the North Pacific gyre (Titmus and Hyrenbach, 2011). However, it is likely that litter densities farther north and west in the South Atlantic are higher, because the sample area was south and east of the core area where floating litter is predicted to accumulate (Lebreton et al., 2012; Maximenko et al., 2012; van Sebille et al., 2012). The low densities of litter in Region D might appear surprising, given that the core of the predicted accumulation zone lies north of this region. However, Tristan da Cunha lies beyond the southern edge of the accumulation zone, in an area of higher surface currents (Maximenko et al., 2009, 2012), and so might be expected to have lower litter densities than
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adjacent waters slightly farther north. Additional surveys should be conducted across the South Atlantic at 25–35°S to obtain a better idea of the densities of litter in the core of the gyre. Ryan (2013) highlighted the need to standardize at-sea surveys of floating large debris by (1) reporting the minimum size of litter items scored, (2) correcting for differential detection with distance from the vessel (unless the transect width is very narrow), and (3) calculating different detection functions for items of different sizes. The results from this study yielded very similar size-dependent detection rates to those reported from the Bay of Bengal (Ryan, 2013), suggesting that they may be consistent at least for a given observer. Other factors such as sea state, item type, colour and buoyancy, cloud cover and angle of the sun, and observer height and fatigue probably also play a role in detection rate (Dahlberg and Day, 1985). The results confirm that buoyancy is important, so observers should record item colour and buoyancy when conducting at-sea surveys. Provided these steps are followed, ship-based observations from vessels of opportunity can yield comparable, cost-effective litter density estimates at large spatial scales suitable to test the predictions of the surface drifter models. By gathering large volumes of such data we can begin to assess how (and perhaps better understand why) the densities of floating litter vary in both space and time (e.g. Aliani et al., 2003). Acknowledgements I thank Bruce Dyer, Mara Nydegger and Chris Bell for assistance with litter surveys, and the officers of the S.A. Agulhas II for permission to use the bridge during inclement weather. The project was part of the South African National Antarctic Programme, with financial support from the National Research Foundation and the University of Cape Town. References Aliani, S., Griffa, A., Molcard, A., 2003. Floating debris in the Ligurian Sea, northwestern Mediterranean. Mar. Pollut. Bull. 46, 1142–1149. Barnes, D.K.A., Milner, P., 2005. Drifting plastic and its consequences for sessile organism dispersal in the Atlantic Ocean. Mar. Biol. 146, 815–825. Dahlberg, M.L., Day, R.H., 1985. Observations of man-made objects on the surface of the North Pacific Ocean. In: Shomura, R.S., Yoshida, H.O. (Eds.), Proc. Workshop on the Fate and Impact of Marine Debris. US Dept Commerce: NOAA Tech. Mem., NOAA-TM-NMFS-SWFSC-54, pp. 198–211.
Eriksen, M., Maximenko, N., Thiel, M., Cummins, A., Lattin, G., Wilson, S., Hafner, J., Zellers, A., Rifman, S., 2013. Plastic marine pollution in the South Pacific subtropical gyre. Mar. Pollut. Bull. 68, 71–76. Kaiser, J., 2010. The dirt on ocean garbage patches. Science 328, 1506. Law, K.L., Morét-Ferguson, S., Maximenko, N.A., Proskurowski, G., Peacock, E.E., Hafner, J., Reddy, C.M., 2010. Plastic accumulation in the North Atlantic subtropical gyre. Science 329, 1185–1188. Lebreton, L.C.-M., Greer, S.D., Borrero, J.C., 2012. Numerical modelling of floating debris in the world’s oceans. Mar. Pollut. Bull. 64, 653–661. Lewison, R.L., Freeman, S.A., Crowder, L.B., 2004. Quantifying the effects of fisheries on protected species: the impact of pelagic longlines on loggerhead and leatherback sea turtles. Ecol. Lett. 7, 221–231. Maximenko, N., Niiler, P., Rio, M.-H., Melnichenko, O., Centurioni, L., Chambers, D., Zlotnicki, V., Galperin, B., 2009. Mean dynamic topography of the ocean derived from satellite and drifting buiy data using three different techniques. J. Atmos. Ocean Technol. 26, 1910–1919. Maximenko, N., Hafner, J., Niiler, P., 2012. Pathways of marine debris derived from trajectories of Lagrangian drifters. Mar. Pollut. Bull. 65, 51–62. Morris, R.J., 1980. Plastic debris in the surface waters of the South Atlantic. Mar. Pollut. Bull. 11, 164–166. Moore, C.J., Moore, S.L., Leecaster, M.K., Weisberg, S.B., 2001. A comparison of plastic and plankton in the North Pacific central gyre. Mar. Pollut. Bull. 42, 1297–1300. Ryan, P.G., 1987. The origin and fate of artefacts stranded on islands in the African sector of the Southern Ocean. Environ. Conserv. 14, 341–346. Ryan, P.G., 1988. The characteristics and distribution of plastic particles at the seasurface off the southwestern Cape Province, South Africa. Mar. Environ. Res. 25, 249–273. Ryan, P.G., 1990. The marine plastic debris problem off southern Africa: types of debris, their environmental effects, and control measures. In: Shomura, R.S., Godfrey, M.L. (Eds.), Proc. Second International Conference on Marine Debris. US Dept Commerce: NOAA Tech. Mem., NOAA-TM-NMFS- SWFSC-154, pp. 85–102. Ryan, P.G., 2013. A simple technique for counting marine debris at sea reveals steep litter gradients between the Straits of Malacca and the Bay of Bengal. Mar. Pollut. Bull. 69, 128–136. Ryan, P.G., Watkins, B.P., 1988. Accumulation of stranded plastic objects and other artefacts at inaccessible Island, central South Atlantic Ocean. S. Afr. J. Antarct. Res. 18, 11–13. Ryan, P.G., Moore, C.J., van Franeker, J.A., Moloney, C.L., 2009. Monitoring the abundance of plastic debris in the marine environment. Philos. Trans. R. Soc. B 364, 1999–2012. Thompson, R.C., Moore, C.J., vom Saal, F.S., Swan, S.H., 2009. Plastics, the environment and human health: current consensus and future trends. Philos. Trans. R. Soc. B 364, 2153–2166. Titmus, A.J., Hyrenbach, K.D., 2011. Habitat associations of floating debris and marine birds in the North East Pacific Ocean at coarse and mesospatial scales. Mar. Pollut. Bull. 62, 2496–2506. van Sebille, E., England, M.H., Froyland, G., 2012. Origin, dynamics and evolution of ocean garbage patches from observed surface drifters. Environ. Res. Lett. 7, 044040. Zhou, P., Huang, C., Fang, H., Cai, W., Li, D., Li, X., Yu, H., 2011. The abundance, composition and sources of marine debris in coastal seawaters or beaches around the northern South China Sea (China). Mar. Pollut. Bull. 62, 1998–2007.
