Towards Characterising Microplastic Abundance, Typology and Retention in Mangrove-Dominated Estuaries

Joelene Govender,Trishan Naidoo,Astika Bhugeloo,Senzo Cebekhulu,Sershen Sershen,Anusha Rajkaran

doi:10.3390/w12102802

Joelene Govender, Trishan Naidoo + Show 4 more

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https://doi.org/10.3390/w12102802

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Abstract

Plastic and, particularly, microplastic (MP) pollution is a growing research theme, dedicated largely to marine systems. Occurring at the land–sea interface, estuarine habitats such as mangroves are at risk of plastic pollution. This study compared MP pollution (level, morphotype, polymer composition, size and colour) across four South African estuaries, in relation to the built and natural environment. Mouth status, surrounding human population densities and land-use practices influenced the level and type of MP pollution. Systems that were most at risk were predominantly open estuaries surrounded by high population densities and diverse land use types. Microplastic levels and the diversity of types detected increased with increasing levels of anthropogenic disturbance. Overall, microfibres dominated in estuarine water (69%) and mangrove sediment (51%). Polyethylene (43%) and polypropylene (23%) were the dominant polymers overall. Weathered fishing gear, weathered packaging items and run-off from urban/industrial centres are probable sources of MP pollution. Increased run-off and river input during the wet/rainy season may explain the markedly higher MP loads in estuarine waters relative to the dry season. By contrast, MP deposition in mangrove sediment was higher during the dry season. Sediment MP abundance was significantly positively correlated with both pneumatophore density and sediment size (500–2000 µm). This study highlights the role of mangroves as MP sinks, which may limit movement of MPs into adjacent environments. However, under conditions such as flooding and extreme wave action, mangroves may shift from sinks to sources of plastic pollution.

Highlights

Global plastic production has increased exponentially from 0.35 million tons in the 1950s [1] to 359 million tons in 2018 [2], with the majority being single-use products [3]
This study showed that, as in other estuaries subject to anthropogenic pressures [47,48], the systems studied here are burdened by high levels of macroplastic (Figures 2 and 3) and microplastic pollution (Figure 5)
The degree of MP pollution appears to be dependent on natural factors such as mouth status, rainfall, sediment size, root density and anthropogenic factors such as surrounding land use types

Summary

Introduction

Global plastic production has increased exponentially from 0.35 million tons in the 1950s [1] to 359 million tons in 2018 [2], with the majority being single-use products [3]. Whilst global microplastic (MP) research surged in the mid-2000s [7], studies in Africa only constitute 2% of this (from 2010–2019) [8], the majority of which comes from South. In the face of unemployment, inadequate provision of water and sanitation, and high HIV/AIDs infection rates, plastic pollution had received limited attention in South Africa [12]. This began to change over the last decade, initially with an increased focus on recycling and environmental behaviour during the country’s hosting of the 2010 FIFA Soccer World Cup and, more recently, due to growing research interest in MP pollution in the country. Studies along the country’s coastline (e.g., [13,14])

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