The interaction between seagrass meadow density and microplastic retention in four cool-temperate estuaries

Microplastics are widespread pollutants of estuarine ecosystems. Seagrasses have been hypothesized to filter microplastics through their dense meadows, yet the mechanisms governing their interaction with microplastics are not well understood, particularly within a South African context. Here we compared how microplastics might accumulate in the sediments associated with Zostera capensis meadows across dense and patchy meadows and unvegetated sediment. In addition, estuarine surface waters were sampled and analysed. The number of microplastics ranged between 23.5 ± 24.9 and 30.1 ± 22.1 microplastics per Kg sediment, with up to 70 % identified as fibres. In three of the four estuaries, a greater abundance of microplastics were found in areas of dense seagrass coverage compared to bare sediment, with fibres and fragments found to be the dominant microplastic. Estuarine surface waters revealed microplastic concentrations ranging from 1.7 ± 1.6 to 2.5 ± 1.4 MPs per m³, with 88 % of samples containing microplastics. This study confirms the trapping ability of Z. capensis meadows for microplastics and highlights paucity of regional knowledge into the effects that microplastics may have on seagrass health and persistence.