Particle size segregation of turbidity current deposits in vegetated canopies

Microplastic shape influences fate in vegetated wetlands

Coastal areas are prone to plastic accumulation due to their proximity to land based sources. Coastal vegetated habitats (e.g., seagrasses, saltmarshes, mangroves) provide a myriad of ecosystem functions, such as erosion protection, habitat refuge, and carbon storage. The biological and physical factors that underlie these functions may provide an additional benefit: trapping of marine microplastics. While microplastics occurrence in coastal vegetated sediments is well documented, there is conflicting evidence on whether the presence of vegetation enhances microplastics trapping relative to bare sites and the factors that influence microplastic trapping remain understudied. We investigated how vegetation structure and microplastic type influences trapping in a simulated coastal wetland. Through a flume experiment, we measured the efficiency of microplastic trapping in the presence of branched and grassy vegetation and tested an array of microplastics that differ in shape, size, and polymer. We observed that the presence of vegetation did not affect the number of microplastics trapped but did affect location of deposition. Microplastic shape, rather than polymer, was the dominant factor in determining whether microplastics were retained in the sediment or adhered to the vegetation canopy. Across the canopy, microfibre concentrations decreased from the leading edge to the interior which suggests that even on a small-scale, vegetation has a filtering effect. The outcome of this study enriches our understanding of coastal vegetation as a microplastics sink and that differences among microplastics informs where they are most likely to accumulate within a biogenic canopy.

Role of mangrove forest in interception of microplastics (MPs): Challenges, progress, and prospects

Mangroves receive microplastics (MPs) from terrestrial, marine and atmospheric sources, acting as a huge filter for environmental MPs between land and sea. Due to the high primary production and complex hydrodynamic conditions in mangroves, MPs are extensively intercepted in various ways while flowing through mangroves, leading to a long-standing but fiercely increasing MPs accumulation. However, current researches mainly focused on the occurrence, source and fate of MPs pollution in mangroves, ignoring the role of mangrove forests in the interception of MPs. Our study firstly demonstrates that mangrove ecosystems have significantly greater MPs interception capacity than their surrounding environments. Then, the current status of studies related to the interception of MPs in mangrove ecosystems is comprehensively reviewed, with the main focus on the interception process and mechanisms. At last, the most pressing shortcomings of current research are highlighted regarding the intercepted flux, interception mechanisms, retention time and ecological risks of MPs in mangrove ecosystems and the relevant future perspectives are provided. This review is expected to emphasize the critical role of mangrove forests in the interception of MPs and provide the foundational knowledge for evaluating the MPs interception effect of mangrove forests globally.

Macroalgal morphology mediates microplastic accumulation on thallus and in sediments

The accumulation process of microplastics (MPs) is a key to understanding their fate in the environment. However, there is limited information about the short-term accumulation of MPs on macrophytes. The ability of macrophyte to attenuate wave and reduce current velocity is potentially facilitating MPs deposition. We hypothesize that the macroalgae retain MPs with their morphologies (filamentous and non-filamentous) being one of the factors to govern retention. Our hypothesis was tested by field observation during the dry season in Hong Kong when the macroalgae communities were the most diverse. MPs per biomass, surface area, or interstitial volume were used to represent the abundances on macroalgae. We found that filamentous algae retained a 2.35 times higher number of MPs when compared with non-filamentous algae if unit per biomass was considered. Other units, however, showed insignificant differences in MPs abundances between algal morphologies. Fibre was the most dominant shape of MPs with no significant difference in their abundances between filamentous and non-filamentous algae, suggesting fibres were retained regardless of the algal morphologies. To further evaluate the potential accumulation in the environment, sediment samples were also collected under the algal mat and immediate vicinity (~50 cm) of the algal mat. We found that sediment collected under the vegetated area contained significantly higher MPs. This was 3.39 times higher than the unvegetated area. Sediment collected under/near filamentous algae retained much higher abundances of MPs than those of non-filamentous algae. Provided that the observed retention of MPs on macroalgae, we speculate macrophyte system is one of the short-term MPs accumulation hotspots where the temporal increase of MPs depends on the seasonality of macrophyte in a given region.

How mangrove plants affect microplastic distribution in sediments of coastal wetlands: Case study in Shenzhen Bay, South China

Microplastic pollution is common in marine and coastal ecosystems, especially in mangrove wetlands. However, factors affecting the distribution of microplastics, such as plants, have not been sufficiently studied. We investigated the effect of different plant species on the distribution of sediment microplastics in two Nature Reserves in South China, viz. Futian Mangrove and Mai Po Mangrove. In Futian Mangrove, the abundance of total microplastics among three monospecific mangrove stands dominated by Sonneratia caseolaris, Kandelia obovata, and Sonneratia apetala was similar. The abundance of microplastics in the mudflat was similar to that in the forest interior, except for the fact that more fiber was found in the mudflat than in the interior of Sonneratia apetala. This suggested that the dense pneumatophores at the fringe prevented fibers from entering the mangrove forest. The significant positive dependence (p < 0.05) between the density of Sonneratia pneumatophores and the abundance of fibers highlighted the importance of pneumatophores. The abundance of total microplastics, predominantly in the form of fibers, in sediments at the forest fringe (2835 ± 713 items/kg d.w. and 2070 ± 224 items/kg d.w. in Futian and Mai Po, respectively) was higher than that in the forest interior and mudflat. There was no difference between the two latter locations in both mangroves, which demonstrated the significance of the fringe effect. This paper reports for the first time that the spatial distribution of microplastics in mangrove sediments was affected by plant species, which provides useful information for environmental processes of microplastics in coastal wetlands.