Estimating the presence and diversity of microplastics in South African seagrass meadows
Date
2024-12
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Stellenbosch University
Abstract
Microplastics, plastic particles <5 mm in size, are a widespread phenomenon in marine ecosystems that threaten biota and human wellbeing. Seagrasses have been hypothesised to act as natural filters of microplastics due to their particle trapping abilities, yet little is known about the extent of microplastics within the sediment of seagrass beds. The aim of this study was to evaluate and compare microplastic accumulation in the sediments of Zostera capensis meadows with adjacent bare sediments in four South African estuaries. Sediment samples were collected from two locations containing Zostera capensis meadows in the middle reaches of each estuary and density flotation was used to separate microplastics from sediments. The mean number of microplastics ranged between 23.5 ± 24.9 and 30.1 ± 22.1 microplastics per Kg sediment, with up to 70% of all identified particles being fibre microplastics. In three of the four estuaries, significantly more microplastics were found in areas of dense seagrass coverage compared to areas of bare sediment (p<0.05), with fibres and fragments found to be the dominant microplastic type, while blue was revealed to be the dominant microplastic colour throughout the estuaries. This study confirmed the trapping ability of seagrass meadows for microplastics and highlights the need for research into the negative effects of microplastics on seagrass health. Furthermore, since accurately assessing microplastics in marine environments is challenging due to the lack of standardized protocols for sample collection and analysis, a review was conducted which compared methodologies from 36 studies on microplastics in sediment, water, and biota from South Africa's marine environments, including the sea, coastline, estuaries, and harbours. Sample collection methods varied with bulk sampling being most common for sediment while mesh nets and sieves were used for water, however, variations in the sample collection devices and mesh net pore sizes were observed between studies. Laboratory procedures consistently involved density separation, digestion, filtration, and microscope identification, but varied in specifics like separation solutions, digestion methods, and filter pore sizes. These variations could affect microplastic abundance estimates. Standardizing sampling methods and laboratory protocols is essential for accurate comparisons of microplastic levels across marine environments and for implementing effective management plans.
Description
Thesis (MSc)--Stellenbosch University, 2024.