Organic matter, microplastics and macrofaunal communities associated with Posidonia oceanica meadows

Coastal ecosystems are increasingly affected by anthropogenic pressures such as organic enrichment and microplastic contamination, which threaten benthic habitats and biodiversity. Seagrass meadows, such as those formed by Posidonia oceanica, are particularly vulnerable in urbanized areas, making it essential to understand the combined effects of multiple stressors on their functioning and associated communities. Here we study the benthic macrofaunal community structure at two coastal sites characterised by the presence of Posidonia oceanica meadows affected by microplastic contamination and organic enrichment. The results showed that faunal abundance and taxa richness were lower in sediments characterised by higher organic matter and MPs amount, and lower P. oceanica density, coinciding with the area subjected to higher urbanisation, as revealed by the biopolymeric carbon and total phyto-pigments analysis. Therefore, these results suggest that the high input of organic matter of human origin can represent a stressor for P. oceanica meadows, leading to a reduction in shoot density. This condition, associated with a greater accumulation of microplastics in the underlying sediments, appeared to have negative effects on the associated biodiversity. Our findings contribute to advancing current knowledge on the effect of multiple stressors (i.e. the organic enrichment and the MPs contamination) on marine ecosystems and on the species diversity they support, and highlight the urgent need to implement the investigations on potential cumulative effects among pollutants and actions mitigating pressures on key coastal habitats such as seagrass meadows.