Coastal guardians: How mangroves filter microplastics, trap pollutants, and store carbon

 

Microplastics, defined as plastic particles smaller than 5 mm, have become ubiquitous in coastal and marine environments worldwide. They originate from a complex web of sources: degraded plastic waste, synthetic clothing fibres, industrial runoff, and aquaculture operations, among others. For mangrove ecosystems, which occupy the interface between land and sea, microplastic contamination is a growing concern.

However, the same physical characteristics that make mangroves vulnerable to microplastic accumulation also make them remarkably effective at intercepting these particles before they reach the open ocean. The dense root systems of mangrove trees reduce water velocity, promote sedimentation, and physically trap suspended particles, including microplastics (Martin et al., 2019; Horstman et al., 2014). Research in the Xixi Estuary of China found that microplastic abundance in mangrove sediments was significantly higher than in adjacent non-mangrove areas, with particles detected at every depth layer of the sediment column, demonstrating that mangroves trap and bury microplastics throughout their root zone (Zhang et al., 2024). One study estimated that mangrove forests bordering a river intercepted between 11% and 56% of the microplastics flowing downstream before they could reach the ocean (as reviewed in de Vries et al., 2022).

A landmark 2020 study by Martí and colleagues in Science Advances provided some of the most compelling evidence yet for mangroves as long-term plastic sinks. By extracting and dating microplastics from sediment cores collected from Avicennia marina forests along the Saudi Arabian coast, the researchers demonstrated an exponential increase in plastic burial rates (approximately 8.5% per year) since the 1950s, mirroring the global increase in plastic production. They estimated that 50 and 110 metric tons of plastic had been buried since the 1930s in mangrove sediments of the Red Sea and Arabian Gulf, respectively (Martí et al., 2020). Importantly, the study found that microplastics smaller than 0.5 mm dominated in mangrove sediments, helping to explain why these particles appear to be scarce in surface waters of the region.

A 2025 field study in the Biscayne Bay Coastal Wetlands of Florida further confirmed that mangrove wetlands function as a natural filtration system for microplastics, identifying this as a novel ecosystem service worthy of integration into coastal management strategies (MDPI, 2025). The researchers noted that the physical processes of gravitational settling under low water velocity and interception by vegetation are the primary mechanisms driving microplastic removal from the water column.

While this post has focused primarily on the pollutant removal capabilities of mangroves, no discussion of mangrove ecosystem services would be complete without acknowledging their extraordinary capacity for carbon storage. Mangroves are among the most carbon-rich ecosystems on the planet. They contain an average of approximately 937 tonnes of carbon per hectare, and their carbon burial rate of roughly 174 gC m⁻² year⁻¹ exceeds that of virtually every other habitat except salt marshes (Alongi, 2012). Despite occupying only about 0.36% of global forest area, mangroves store up to five times more organic carbon per unit area than tropical upland forests (Donato et al., 2011) and account for approximately 14% of carbon sequestration by the global ocean (Alongi, 2012).

 

Current studies suggest that mangroves and coastal wetlands sequester carbon at a rate roughly ten times greater than mature tropical forests on an annual basis (NOAA, n.d.). Most of this carbon is stored belowground, locked away in deep, waterlogged soils where oxygen-poor conditions dramatically slow decomposition. Mangrove soils can extend to depths of three metres or more, and the carbon they contain can persist for centuries or even millennia. This makes the loss of mangrove habitat particularly damaging from a climate perspective: when mangrove soils are disturbed, they can release massive quantities of stored carbon back into the atmosphere. Estimates suggest that emissions from mangrove degradation may account for up to 10% of global deforestation emissions, despite mangroves covering less than 1% of tropical forest area (The Blue Carbon Initiative, n.d.).

 

This dual function of carbon storage and pollutant filtration is what makes mangrove conservation such a high-value proposition. When we protect or restore a mangrove forest, we are not simply preserving a single ecosystem service. We are maintaining an integrated system that simultaneously sequesters atmospheric carbon, traps microplastics, filters heavy metals and chemical pollutants, protects coastlines from erosion and storm surge, and supports the biodiversity and fisheries that coastal communities depend upon.