Abstract
A growing body of research is documenting the accumulation of microplastics within marine sediments around the world. The hydrodynamic influences of seagrasses in coastal environments are shown to increase sedimentation of finer particles and as a result there has been speculation that this attribute will lead to seagrass meadows acting as a site of elevated microplastic contamination. To date a range of localised studies have provided conflicting answers to this hypothesis. Seagrass meadows provide multiple ecosystem services including vital support roles for a range of fisheries; therefore, there are considerable human health implications for understanding their role as sinks of microplastics. This research investigated the abundance and diversity of microplastics present in temperate North Atlantic seagrass meadow sediments relative to unvegetated sediments and examined how they correlate with the meadow structure and the sediment type. We also placed this data in the context of the current knowledge of microplastics in seagrass sediments through a global meta-analysis of published data. Eight seagrass meadows and adjacent unvegetated sites around the UK were sampled to test for the abundance of microplastic particles in the sediment. Microplastics were found in 98% of the samples, with fibres making up 91.8% of all microplastics identified. Abundance was recorded to overall be 215 ± 163 microplastic particles (MP) kg−1 Dry Weight (DW) of sediment in seagrass and 221 ± 236 MP kg−1 DW of sediment in unvegetated habitats. There were no significant differences found between the number of MP with respect to vegetation. We report evidence of the almost ubiquitous contamination of seagrass sediments with microplastics both in the UK and globally but find that the contamination reflects a general build-up of microplastics in the wider environment rather than becoming concentrated within seagrass as an enhanced sink. Microplastic build up in sediments is hypothesised to be the result of local hydrodynamics and plastic sources rather than the result of elevated habitat level concentration. Although not of a higher abundance in seagrass, such contamination in seagrass is of cause for concern given the high dependency of many species of fish on these habitat types and the potential for plastics to move up the food chain.
Highlights
Seagrass meadows provide multiple ecosystem services, including creation of spawning grounds for a variety of fish species, uptake and storage of carbon and coastal protection [1,2]
While some studies have highlighted the presence of microplastics at elevated concentrations within seagrass ecosystems relative to unvegetated habitats, all previous studies have been spatially limited and presented conflicting information as to whether the accumulation of plastic particles is heightened in the sediments of seagrass meadows, especially in the context of non-vegetated sea floor [25,26,27]
We found microplastic items in UK seagrass sediments (215 ± 163 microplastic particles (MP) kg−1 Dry Weight (DW)) to be an order of magnitude higher than those recorded within a less spatially expansive surveys conducted in Portugal and the US, but at a similar level to those within a Scottish survey and at sites in Portugal [23]
Summary
Seagrass meadows provide multiple ecosystem services, including creation of spawning grounds for a variety of fish species, uptake and storage of carbon and coastal protection [1,2]. A significant basis for many of these services is the creation of a three-dimensional environment that reduces the velocity of water whilst increasing sedimentation and aiding particle trapping [3,4]. These highly productive ecosystems are vulnerable to a Oceans 2021, 2, 162–176. There has been increased recognition and concern of plastic pollution in the marine environment and concerns that seagrasses may act as plastic traps [7]. Plastic pollution has risen with increasing production, with up to 90% of marine litter and an estimated 4.8–12.7 million tonnes of plastic waste entering the oceans every year, with river systems being identified as the primary vector for transport [10,11,12]
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