Microplastics associated with shoreline marine organisms at Nature's Valley in South Africa

Abstract

Plastic is an essential product of modern life, with people relying on it daily. A variety of hazards are related to plastic, from being a choking- and strangling hazard to releasing harmful toxicants into the environment. Plastics are broken down into small particles known as microplastics (MPs) when smaller than 5 mm. Plastic is globally omnipresent and is abundant in water, soil, air, and even biota. Current literature presents conflicting viewpoints on trophic transfer of MPs in marine ecosystems. This dissertation aimed to i) determine the extent to which MPs transfer between different trophic levels in a shoreline marine environment at Nature’s Valley, and ii) assess the toxic risks associated with different plastic polymers. Seawater, sand, seaweed, and invertebrates were sampled from rocky tide pools from Nature’s Valley, South Africa and the MPs therein were quantified and classified. Primary producers (seaweed: Ulva fasciata, Sargassum incisifolium, and Plocamium corallorhiza), primary consumers (Siphonaria concinna, Oxystele tigrina, Afrolittorina knysnaensis, Parvulastra exigua, Parechinus angulosus), and tertiary consumers (Thalessa savignyi, Pseudactinia flagellifera, and Roweia stephensoni) were the analysed biota. Notably, secondary consumers were not included in this study. Vacuum filtering and density separation were used to isolate putative MPs from seawaterand sand samples, respectively. Chemical- and enzymatic digestion was used to dissociate the molecular integrity of invertebrate organisms and the seaweed samples, hereby, isolating its MP content. Microplastics were manually quantified and categorised into different morphotypes (fibres, fragments, pellets, and film), colours (white, transparent, blue, and othercoloured), and sizes (10–100 µm, 100–250 µm, 250–500 µm, 500–1000 µm, and 1000–5000 µm). The type of plastic polymer was determined by Fourier Transform Infrared Spectroscopy (FT-IR) on a selected subset of samples (5% of the total samples) as a representative sample. Pearson’s chi-squared analysis and adjusted standardised residuals (z-scores) were used to determine associations between the different trophic levels based on the MP profiles (colours, morphotypes, sizes, and polymer composition). Principal component analysis was also used to identify associations and patterns based on the proportional compositions of the MP profiles. From 239 samples collected, 7053 putative MPs were counted and classified. Only in five samples (2%) no putative MPs were found. The water and sand samples contained 3.42 MPs/L and 9 MPs/kg, respectively. The dominant MP morphotype was fibres, followed by fragments, and 22 films and pellets. White MPs were the most common in terms of colour, followed by transparent and blue. A wide range of different sizes of MP was recorded, with the dominant size categories in the different trophic levels being 250–500 and 500–1000 µm. The use of FT-IR analysis revealed that polyamide (PA), polypropylene (PP), and polyethylene (PE) were among the most common polymers in biotic and abiotic samples.

This is the first study to report MP data for U. fasciata, S. incisifolium, P. corallorhiza, S. concinna, A. knysnaensis, P. flagellifera, R. stephensoni, and T. savignyi

Different feeding strategies, interactions with MPs, and the retention time within organisms influence the consumption and characteristics of MPs which can determine the characteristic profiles of MPs available for trophic transfer. The overall Pearson chisquare analysis rejected the null hypothesis of no association meaning that associations were present between the MP profiles and trophic levels (Table 1). The post-hoc tests and contingency analyses suggest patrial trophic transfer may have occurred. Blue- and other-coloured MPs were the only two colours associated across the entire food web. Fibres and fragments had associations between some of the trophic levels; specifically, primary producers, primary consumers, and water. Only two size categories—250–500 and 1000–5000 µm—were associated across the entire food web. Polyamide (PA) was the only polymer that had an association across the entire food web. Principal component analyses (PCA) ordinated the relative proportional compositions of the MPs according to colours, morphotypes, size categories, and polymer composition, across different trophic levels (Table 1). The PCA bi-plots indicated partial trophic transfer based on the relative profiles of MP colours, morphotypes, and size compositions. In contrast, the PCA bi-plot for MP proportional polymer composition did not support the notion of trophic transfer. Microplastics pose a toxic risk to biota based on polymer toxicity. The polymer hazard index (PHI) indicated that polymers such as polyurethane (PU) found at Nature’s Valley posed a greater hazard than the number/concentration of MPs. The PHI indicated that MPs in the tide pools in the nearshore marine environment at Nature’s Valley pose a significant ecological threat, reflected by a high PHI score