Microplastics in the mix: types and retention of microplastics within a mixed species seagrass meadow in the Tamar Estuary

Microplastic pollution in coastal systems is becoming increasingly prevalent and a commonly discussed topic in science and the media. Seagrass beds have been identified as highly vulnerable environments to microplastic pollution due to their nature as traps for particulate matter in the water column, and their role in habitat production and blue carbon storage makes them of high conservation concern. This study examined a mixed species seagrass bed in the Tamar Estuary in Plymouth Sound, composed of Nanozostera noltei and a morphology of Zostera marina, to identify how seagrass shoot density and sediment particle size impacted the number and types of microplastic particles retained within the sediment matrix. Samples were added to a microplastic separation column and individual microplastic particles were counted with FTIR analyses to identify plastic polymer types. Particle size analysis was also conducted, and SEM photographs of different plastic particles were taken. Microplastics were found in all samples, and their abundance ranged from 176.4 to 1,354.4 particles per kg-1 of dry sediment. Fibres dominated the samples, and the most frequent colours were black, blue, and white. As expected, ANCOVA results showed that both seagrass shoot density and sediment particle size collectively led to an increased presence of microplastic particles (p < 0.001) within the sediment profile. Interestingly, FTIR analyses revealed that nearly all plastic particles originated from land sources rather than marine, with a particular prevalence of clothing fibres. This study indicated that in understudied estuarine systems, land-based plastic sources represent most microplastic pollution, and richer and denser seagrass habitats may be more vulnerable to the effects of microplastic pollution, with the possibility of having cascading detrimental effects on several marine food webs through biomagnification.