Environmental concentrations and risk assessment of microplastics in selected echinoderms in rocky shores of the Western Cape, South Africa

Abstract

Plastic debris is accumulating in all environments globally and South Africa’s poor waste management plan has led to an increase in plastic contamination throughout the country. Microplastics (MPs) are defined as plastic particles smaller than 5 mm . Information about MPs in coastal environments and biota in South Africa is poor. The aim of this study was to determine coastal MP concentrations in water (MPs/L) and sediment (MPs/kg) and ingested MP in echinoderm species (MPs/g and MPs/I). Sampling took place during in summer 2020 during low at 14 sites along the coast of the Western Cape, South Africa. Water (n= 5 per site), sediment (n= 5 per site) and echinoderms (n= 20 per site) were sampled at each site. Sampling efforts for echinoderms were subject to availability, but at least two echinoderm species were analysed. Environmental and biological samples were digested in 10% KOH at 60 oC (24 hours). MPs were extracted and analysed based on visual type, colour, size and polymer type (using an FTIR-ATR). A risk assessment was done to assess the risks posed by MPs in all sample types. The results showed a higher mean concentration in sediment (185.07 MPs/kg; ± 15.25 SE) samples followed by echinoderms (1.44 ± 0.12 MPs/g) and water (1.33 ± 0.15 MPs/L) samples, suggesting sediment is a MP sink. Gordan’s Bay (site 12) had the highest concentration in sediment samples (360 ± 36.74 MPs.kg), identifying harbours as a source for MP contamination. Kalk Bay (site 9) displayed the highest concentration in both water and echinoderm samples (4.97 ± 0.18 MPs/L and 2.90 ± 0.38 MPs/g respectively), suggesting the source of MPs are from stormwater outfall pipes. In addition MPs present in the water column are ingested directly by echinoderms based on feeding strategy. Filaments were the most dominant MP type (89.33%) with black/grey being the most dominant colour (41.12%). PET was the most dominant polymer type (41.33%). Based on the risk assessment, MPs recorded at Mouille Point (site 6) poses the greatest risk associated with polymers. MP concentrations reported in this study provide a baseline for future studies, with a need for investigations to focus on the effects of MPs on echinoderms in rocky shores environments along the Western Cape coastline, South Africa.