The abundance, distribution and accumulation of plastic debris in Table Bay, Cape Town, South Africa

Master Thesis


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University of Cape Town

Stranded debris and beach litter were examined at two sites in Table Bay, South Africa, repeating a survey made at the same two beaches in 1994. One beach (Milnerton) is a popular recreational beach 12 km from the city centre, whereas the other (Koeberg) is situated in a nature reserve, with limited access by the general public, and is 27 km farther from the city. Daily accumulation rates of manufactured items (>1 cm diameter) were measured at both beaches for ten days in October, November and December 2012. Of the 124,646 items collected, 93% were made of plastic, but these items comprised only 59 % of the total weight. There was generally consistent but large within-site variability in accumulation rate; the within-site coefficients of variation (CVs), which range from 23.7 % to 101.5 %, respond in the same way across months. There was also considerable daily variation (CVs range from 13.6 % to 92.8 %). The mean density of items decreased with distance from Cape Town. Since 1994, the composition, abundance and accumulation rate of debris has changed on these two beaches. The mean (s.e) accumulation rate of plastic articles at Milnerton increased 257 %, from 378 (72.3) plastic m-1 of beach to 1350 (126.7) 1.100m-1. The increase at Koeberg was from 44 (2.7) m-1 to 100 (17.3) m-1. Evidence of increased input during the peak holiday season (December) was recorded at both beaches. The mean accumulation rates of most materials had increased at Milnerton since 1994 and the composition of the materials had also changed. The non-plastics were numerically dominated by cloth, paper and wood in 1994 but cigarette butts dominated in 2012. In contrast, at Koeberg the accumulation rates of most non-plastic materials decreased since 1994 and there were small differences in composition. No correlation was found between total weights and total counts of plastic items on the beaches. Daily variability (accumulation rate and accumulating weight) was generally not correlated with weather conditions. Since 1994, the accumulation rate of small, unidentified plastic fragments increased by more than 200- fold at Milnerton and by a factor of 80 at Koeberg. To improve our understanding of the vertical distribution, abundance and composition of microplastics (articles < 10 mm), samples were taken at 5 cm depth intervals (0 to 25 cm) on Milnerton. The number of microplastics, sized 2 mm- 10 mm, found in each layer decreased with depth. Smallest plastic items (0.5 - 2 mm) were randomly distributed in the surface layers (top 10 cm) but had low densities in the bottom layers. Plastic pellets had the same decreasing trend with depth. Amounts of plastic litter have increased by two orders of magnitude over an 18-year period, reflecting both accumulation of plastic debris in coastal environments and increased use of plastics during the past decades.