Beyond a simple sink: Spatial and seasonal drivers of microplastic accumulation in a tropical seagrass meadows

Seagrass meadows are increasingly recognized as important components of microplastic (MP) dynamics in coastal environments, yet their role as sinks remains debated, particularly in tropical systems dominated by small-bodied species. We investigated how seasonal variability, spatial position, meadow structure, and sediment grain size interact to shape MP accumulation in sediments of a Halodule wrightii meadow in the southwestern Atlantic. Sediment samples were collected across a spatial gradient (outside, edge, and inside the meadow) during dry and rainy seasons, alongside measurements of seagrass structural traits and sediment granulometry. MPs occurred in 85% of samples, with abundance and size varying across seasons and spatial positions. MP abundance was higher outside the meadow and during the dry season, while smaller particles dominated during the rainy season. Seagrass structural traits, particularly leaf length, were positively associated with MP accumulation in meadow sediments, although their effects depended on the interaction between season and spatial position. Blue fibers were the most common MPs, and five plastic polymers were identified. Our results showed that H. wrightii meadows are not uniform sinks for MPs. Instead, MP accumulation appeared to arise from the interaction between spatial gradients and plant structural traits, which jointly influenced accumulation patterns. In this context, adjacent unvegetated areas may function as zones of greater accumulation, while seagrass structure modulates local retention processes. These findings provide new insights into the mechanisms governing MP distribution in tropical seagrass meadows and highlight the need to consider interacting environmental and biological drivers when assessing MP fate in coastal environments.