Characteristics and distribution of litter and microplastics in Table Bay, Cape Town, South Africa

Abstract

The increasing amount of waste such as litter and microplastics has drawn society’s attention because of their ubiquitous and complex nature and associated damage to the economy, human, and ecological well-being. This study investigated the characteristics and distribution of litter (> 25 mm) and microplastics (<5mm) in Table Bay, Cape Town. The study went further to also investigate meso-sized items (5–25 mm). The two sites, Woodbridge Island and Derdesteen beaches were selected based on their similar exposure to wind and current as well as their difference in the frequency of visitors, major land use, and proximity to residential areas (suburban or rural) and rivers. Each sampling site was 100 meters long, where litter, mesolitter, water and sediment samples for the extraction of microplastics were collected in 2021. The collection of litter was done for 10 consecutive days on both sites in summer and autumn, where all visible discarded objects were collected. Mesolitter was collected in summer alone for 10 consecutive days using a 0.5 m x 0.5 m sieve with a 2 mm metallic mesh. Water and sediment samples were collected on both sites in summer and autumn for microplastics extraction. Polymer identification was done through spectroscopy, using a Perkin Elmer-Two Fourier transform infrared attenuated total reflectance (FTIR-ATR) Spectrometer. Non-parametric analyses were conducted on the data using a Mann–Whitney (MW) test for two groups, as the test for normality were not met. Data variances for statistical analysis were presented using the standard error of the mean (SE) and parameter significance was set at p < 0.05. All analyses were done using the SPSS v28 . A total of 11 179 litter items weighing 20 028.07 g (20 kg) were collected during this study. The occurrence of litter varied seasonally with autumn accounting for more items than summer by count (52.63% and 47.37% respectively) and less by weight (24.45% and 75.55% respectively). In addition, the litter count showed no significant between the two seasons (U = 741, p = 0.570) while the litter weight showed a significant one (U = 595, p = 0.049). Litter also varied by site, with 89.49% of the total count found in Woodbridge Island compared to 10.51% at Derdesteen. The same trend was observed for the total weight of litter where 61.62% was found at Woodbridge Island compared to 38.38% at Derdesteen. The statistical tests showed significant differences in litter count (U = 67, p < 0.001) and weight (U = 500, p = 0.004) between the two sites. The daily accumulation rate of litter varied over the 10 consecutive days of sampling in summer and autumn. The brand audit showed that most of the identified brands in Table Bay were of local origin, mostly made of snack (78.95%) and beverages packaging (21.05%). Plastic was the most abundant litter type recorded, accounting for 90.27% by count and 43.45% by weight. Hygiene objects, metal, wood, rubber, marine, clothing, paper, glass, unclassified items were encountered alongside plastic. Foam, fragments, cigarette butts, wrapping, bottles, lollipop sticks, straws, utensils, and shopping bags were the types of plastic litter found during the survey. Of these plastic litter types, foam (Styrofoam) was the most abundant both by count and weight, with 41.74% and 48.32% of all the plastic litter collected. Furthermore, Polyethylene (PE) was the dominant polymer type found (69.23%) and was followed by polypropylene (PP), cellulose acetate (CL), and polystyrene (PS). The Clean Coast Index was calculated and both beaches could be classified as very clean. For meso-sized litter items, only collected in summer, 1428 items were counted and weighed 12.20 g. Of all mesolitter collected, 98.46% were recorded at Woodbridge Island and 1.54% at Derdesteen while all litter weight (100%) was found at Woodbridge Island. In addition, there were significant differences in mesolitter count (U = 5172, p = 0) and weight (U = 11 460, p = 0.02) between the two sites. Inter and intra-beach variabilities were observed in this study. The daily accumulation rate of mesolitter varied over the 10 consecutive days of sampling. In terms of mesolitter characteristics, plastic items (mesoplastics) were the most abundant type found in this study, making up 99.05% by count and 78.69% by weight, with metals accounting for 21.31% of the remaining weight. Of the mesoplastics, pellets represented 69.75 % of all meso-plastics by count, followed by foam (18.49 %) and fragment (10.78 %). Polyethylene (PET) was the most dominant polymer type found in mesolitter with 59.52 % of all polymers found. Of the 688 microplastics (MPs) extracted from water and sediment samples, 57.81 % were recorded in summer and 40.19% in the autumn however, no significant difference was found between the two seasons (U = 525, p = 0.861). 75.66 % of all microplastics were recorded on Woodbridge Island, compared to 24.34 % in Derdesteen. Statistical tests showed a significant difference in microplastics between the Woodbridge Island and Derdesteen (U = 258.5, p = 0.002). Fibre was the most abundant MPs type found, while blue was the most abundant colour, and the size category of 0.1 to 0.5 mm was dominant. This study provided evidence of the occurrence of litter, mesolitter, and microplastics and highlighted their seasonal and site variabilities, and their characteristics in the coastal environment of Table Bay in Cape Town. In addition, factors such as wind, land use, proximity to metropolitan areas, and public access contributed to litter, mesolitter, and microplastics occurrence in Table Bay. The findings of this study laid the groundwork for a better understanding of the spatial and temporal variations and characteristics of litter, mesolitter and microplastics. They contributed towards historical data of litter, mesolitter, and microplastics in Table Bay and serve as baseline for future research.