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Floating persistent debris, primarily made from plastic, disperses long distances from source areas and accumulates in oceanic gyres. However, biofouling can increase the density of debris items to the point where they sink. Buoyancy is related to item volume, whereas fouling is related to surface area, so small items (which have high surface area to volume ratios) should start to sink sooner than large items. Empirical observations off South Africa support this prediction: moving offshore from coastal source areas there is an increase in the size of floating debris, an increase in the proportion of highly buoyant items (e.g. sealed bottles, floats and foamed plastics), and a decrease in the proportion of thin items such as plastic bags and flexible packaging which have high surface area to volume ratios. Size-specific sedimentation rates may be one reason for the apparent paucity of small plastic items floating in the world's oceans.

South Africa is thought to be one of the worst contributors of plastic into the sea globally. Although some plastic items derive from offshore sources (mainly fishing and other maritime activities, but also long-distance transport), the importance of local, land-based sources is indicated by the composition of beach debris and the concentration of macro-, meso- and microplastics close to urban source areas. Some 60-90% of plastic from land-based sources is expected to strand on beaches, but plastic standing stocks on beaches are much lower than global model predictions of land-based pollution. Burial in beaches and transport into backshore vegetation are significant sinks, although this plastic is likely to be released as the climate crisis leads to rising sea levels and more extreme storms. Most buried items are fairly small, while many larger items, which account for most of the mass of plastic, are removed from beaches by cleaning efforts. However, even daily accumulation rate estimates--which exclude the effects of cleaning--fall well short of model predictions of plastic leakage from land-based sources. Oceanographic models predict that plastics entering the sea from South Africa are exported to the South Atlantic and Indian Oceans, with the proportion depending on source location and item density. At sea, floating macroplastic is concentrated close to urban centres. Farther offshore, plastic items tend to be large and buoyant because biofouling causes small, low buoyancy items to sink. Size-selective removal of plastics by biota might also contribute to the paucity of floating microplastics (<1 mm). The seabed is likely to be the main long-term sink for waste plastics, but the limited data available indicate low levels of plastics on the seabed off South Africa. Only a small proportion of plastic predicted to leak into the sea from South Africa can be accounted for. However, this should not delay the implementation of effective mitigation measures to limit plastic leakage. Significance: * High densities of waste plastic around urban centres indicate that most macro- and microplastics come from local, land-based sources and do not disperse far at sea. * Beach clean-ups remove up to 90% of the mass of stranded plastic, largely found in macroplastic items (>25 mm). * The seabed is a long-term sink for marine plastics, but densities of plastic on the seabed around South Africa are still modest. * The global model prediction of plastic leakage from South Africa into the sea probably is a gross overestimate. KEYWORDS: beach debris, floating debris, seabed debris, biofouling, sedimentation, ocean transport, burial, exhumation

Plastic pollution is ubiquitous throughout the marine environment, yet estimates of the global abundance and weight of floating plastics have lacked data, particularly from the Southern Hemisphere and remote regions. Here we report an estimate of the total number of plastic particles and their weight floating in the world's oceans from 24 expeditions (2007-2013) across all five sub-tropical gyres, costal Australia, Bay of Bengal and the Mediterranean Sea conducting surface net tows (N = 680) and visual survey transects of large plastic debris (N = 891). Using an oceanographic model of floating debris dispersal calibrated by our data, and correcting for wind-driven vertical mixing, we estimate a minimum of 5.25 trillion particles weighing 268,940 tons. When comparing between four size classes, two microplastic <4.75 mm and meso- and macroplastic >4.75 mm, a tremendous loss of microplastics is observed from the sea surface compared to expected rates of fragmentation, suggesting there are mechanisms at play that remove <4.75 mm plastic particles from the ocean surface.

Most plastic debris floating at sea is thought to come from land-based sources, but there is little direct evidence to support this assumption. Since 1984, stranded debris has been recorded along the west coast of Inaccessible Island, a remote, uninhabited island in the central South Atlantic Ocean that has a very high macrodebris load (∼5 kg·m−1). Plastic drink bottles show the fastest growth rate, increasing at 15% per year compared with 7% per year for other debris types. In 2018, we examined 2,580 plastic bottles and other containers (one-third of all debris items) that had accumulated on the coast, and a further 174 bottles that washed ashore during regular monitoring over the course of 72 d (equivalent to 800 bottles·km−1·y−1). The oldest container was a high-density polyethylene canister made in 1971, but most were polyethylene terephthalate drink bottles of recent manufacture. Of the bottles that washed up during our survey, 90% were date-stamped within 2 y of stranding. In the 1980s, two-thirds of bottles derived from South America, carried 3,000 km by the west wind drift. By 2009, Asia had surpassed South America as the major source of bottles, and by 2018, Asian bottles comprised 73% of accumulated and 83% of newly arrived bottles, with most made in China. The rapid growth in Asian debris, mainly from China, coupled with the recent manufacture of these items, indicates that ships are responsible for most of the bottles floating in the central South Atlantic Ocean, in contravention of International Convention for the Prevention of Pollution from Ships regulations.

Plastic debris has significant environmental and economic impacts in marine systems. Monitoring is crucial to assess the efficacy of measures implemented to reduce the abundance of plastic debris, but it is complicated by large spatial and temporal heterogeneity in the amounts of plastic debris and by our limited understanding of the pathways followed by plastic debris and its long-term fate. To date, most monitoring has focused on beach surveys of stranded plastics and other litter. Infrequent surveys of the standing stock of litter on beaches provide crude estimates of debris types and abundance, but are biased by differential removal of litter items by beachcombing, cleanups and beach dynamics. Monitoring the accumulation of stranded debris provides an index of debris trends in adjacent waters, but is costly to undertake. At-sea sampling requires large sample sizes for statistical power to detect changes in abundance, given the high spatial and temporal heterogeneity. Another approach is to monitor the impacts of plastics. Seabirds and other marine organisms that accumulate plastics in their stomachs offer a cost-effective way to monitor the abundance and composition of small plastic litter. Changes in entanglement rates are harder to interpret, as they are sensitive to changes in population sizes of affected species. Monitoring waste disposal on ships and plastic debris levels in rivers and storm-water runoff is useful because it identifies the main sources of plastic debris entering the sea and can direct mitigation efforts. Different monitoring approaches are required to answer different questions, but attempts should be made to standardize approaches internationally.

In the context of marine anthropogenic debris management, monitoring is essential to assess whether mitigation measures to reduce the amounts of waste plastic entering the environment are being effective. In South Africa, baselines against which changes can be assessed include data from the 1970s to the 1990s on microplastics floating at sea, on macro- and microplastic beach debris, and interactions with biota. However, detecting changes in the abundance of microplastics at sea is complicated by high spatial and temporal heterogeneity in net samples. Beach debris data are easier to gather, but their interpretation is complicated by the dynamic nature of debris fluxes on beaches and the increase in beach cleaning effort over time. Sampling plastic ingested by biota is a powerful approach, because animals that retain ingested plastic for protracted periods integrate plastics over space and time, but there are ethical issues to using biota as bioindicators, particularly for species that require destructive sampling (e.g. turtles, seabirds). Bioindicators could be established among fish and invertebrates, but there are technical challenges with sampling microplastics smaller than 1 mm. Fine-scale debris accumulation on beaches provides an index of macroplastic abundance in coastal waters, and offers a practical way to track changes in the amounts and composition of debris in coastal waters. However, upstream flux measures (i.e. in catchments, rivers and storm-water run-off) provide a more direct assessment of mitigation measures for land-based sources. Similarly, monitoring refuse returned to port by vessels is the best way to ensure compliance with legislation prohibiting the dumping of plastics at sea. Significance: * Monitoring is required to assess whether mitigation measures to reduce waste plastics at sea are making a difference. * Monitoring the leakage of plastic from land-based sources is best addressed on land (e.g. in storm drains and river run-off) before the plastic reaches the sea. * Illegal dumping from ships is best addressed by monitoring the use of port waste reception facilities. * Sampling plastic ingested by biota is a powerful approach, using fish and invertebrates as bioindicators for larger microplastic fragments. KEYWORDS: adaptive management, marine debris surveys, bioindicators, turnover, upstream monitoring

The cost of moult is substantial, and the timing and intensity of flight feather moult can influence survival and fitness, especially in large, long‐winged species such as many seabirds. We explore variation in wing and tail moult in > 2400 white‐chinned petrels Procellaria aequinoctialis killed in fisheries off southern Africa to assess how they integrate moult into their annual cycle and whether wing moult impacts their behaviour at sea. All petrels showed a simple descendent primary moult and one active moult centre, although moult of P2–3 sometimes started before P1. The Underhill–Zucchini moult model estimated that adult primary moult started after breeding on 7 May (± 8 days SD) and lasted 103 days (mean end date 20 August ± 10 days). Adult males started and finished moult 10 days before females. Immature petrels started primary moult earlier than adults, and their moult was probably more protracted as they moulted fewer primaries at once (1.9 ± 1.2) when compared to adults (2.3 ± 1.1), independent of sex. Adult moult was particularly intense in the inner primaries, growing up to six feathers at once, slowing to at most 3–4 outer primaries. The secondary moult started two weeks after the primary moult, once 3–4 primaries had been dropped. Secondary moult typically started with the innermost secondaries, plus inward waves from S1 and S5 in 2.7 ± 1.3 active moult centres (range 1–6), replacing 4.6 ± 2.7 (1–13) secondaries at once. Adults had more intense secondary moult (4.7 ± 2.8 growing feathers) than immatures (3.6 ± 2.3), with no difference between the sexes. However, photographs of non‐moulting birds at sea show that 27% of birds do not replace all secondaries each year. The tail moult usually commenced at the start of the secondary moult and was highly variable, with 1–12 rectrices growing at once. Adults had more active centres (3.0 ± 1.4) than immatures (2.3 ± 1.0). Moult symmetry was greater among the primaries (84%) than either the secondaries (46%) or rectrices (68%). Although adult wing moult was intense, there was no marked reduction in flight activity among breeding adults fitted with leg‐mounted activity loggers during the moult period. Our findings are largely in accord with previous studies of moult in petrels, but our large sample size reveals considerable variation among individuals, which is surprising given the high cost of moult. Future studies should attempt to investigate the factors determining this variation.

We question whether the rapid growth in research on the impacts of environmental plastics over the last decade has substantially improved our understanding of these impacts. By the mid-1990s, the major environmental and economic impacts of plastics were sufficiently well known to conclude that they posed a significant environmental threat. Accordingly, the focus of the Third International Marine Debris Conference shifted from researching impacts to devising solutions. We should re-embrace this message, and study how best to change the inappropriate human behaviours that lie at the heart of the plastics crisis. The main role of natural scientists should be to provide robust monitoring data to assess the success of the various mitigation efforts.

Marine predators, such as seabirds, are useful indicators of marine ecosystem functioning. In particular, seabird diet may reflect variability in food-web composition due to natural or human-induced environmental change. Diet monitoring programmes, which sample diet non-invasively, are valuable aids to conservation and management decision-making. We investigated the diet of an increasing population of greater crested terns Thalasseus bergii in the Western Cape, South Africa, during three successive breeding seasons (2013 to 2015), when populations of other seabirds feeding on small pelagic schooling fish in the region were decreasing. Breeding greater crested terns carry prey in their bills, so we used an intensive photo-sampling method to record their diet with little disturbance. We identified 24,607 prey items from at least 47 different families, with 34 new prey species recorded. Fish dominated the diet, constituting 94% of prey by number, followed by cephalopods (3%), crustaceans (2%) and insects (1%). The terns mainly targeted surface-schooling Clu-peiformes, with anchovy Engraulis encrasicolus the most abundant prey in all three breeding seasons (65% overall). Prey composition differed significantly between breeding stages and years, with anchovy most abundant at the start of the breeding season, becoming less frequent as the season progressed. The proportion of anchovy in the diet also was influenced by environmental factors; anchovy occurred more frequently with increasing wind speeds and was scarce on foggy days, presumably because terns rely in part on social facilitation to locate anchovy schools. The application of this intensive and non-invasive photo-sampling method revealed an important degree of foraging plasticity for this seabird within a context of locally reduced food availability, suggesting that, unlike species that specialise on a few high-quality prey, opportunistic seabirds may be better able to cope with reductions in the abundance of their preferred prey.

Our view of the Antarctic Polar Front (APF) as a circumpolar biogeographic barrier is changing (Chown et al. 2015). The APF marks the convergent boundary between cold Antarctic water and warmer sub-Antarctic water, and has long been considered to prevent north-south dispersal in the Southern Ocean (reviewed by Clarke et al. 2005, Fraser et al. 2012). Our multi-year survey data provides evidence that rafting organisms readily cross the APF.