Microplastics are turning up almost everywhere - from oceans and rivers to soils and sediments - yet far less is known about what happens when they enter freshwater wetlands. Often overlooked in favor of marine and coastal environments, wetlands can act as both pathways and traps for microplastics, potentially capturing and retaining pollution.
That knowledge gap is particularly important in small, urban-adjacent wetlands throughout Florida, where stormwater runoff, recreational activity and connections to rivers and other waterways can introduce microplastics. Yet little is known about how these particles move and accumulate within these ecosystems.
To help fill these gaps, Florida Atlantic University researchers examined microplastics at Spruce Bluff Preserve, a 97-acre freshwater wetland along the St. Lucie River in Port St. Lucie. The study provides the first detailed look at how microplastics are distributed across the preserve and how vegetation, sediment composition and proximity to water influence where particles accumulate.
Researchers collected 40 surface sediment samples from eight transects spanning different ecological zones. Samples were collected every 5 meters in December 2024 during the dry season, and researchers recorded each site's distance from water. They also used infrared spectroscopy to confirm a subset of particles, providing a more accurate estimate of microplastic abundance.
Results of the study, published in the journal Evolving Earth, reveal that microplastics were found throughout Spruce Bluff Preserve, and detected in 38 of 40 sediment samples (95%) across every ecological zone. Most of the confirmed particles were tiny - about 88% were smaller than 2.5 millimeters - and nearly half were fibers, suggesting that synthetic textiles and other everyday materials may be contributing to plastic pollution in the preserve.
But the particles were not distributed randomly. The highest concentrations were found along the St. Lucie River and an outlet canal used for flood control, suggesting that water movement is helping transport and concentrate microplastics within the wetland.
"Perhaps the most striking finding is that the microplastics weren't simply scattered randomly across the preserve - they appeared to follow the pathways of water," said Erik N. Johanson, Ph.D., senior author and an associate professor in the Department of Geosciences within FAU's Charles E. Schmidt College of Science. "The river edge and flood-control canal had some of the highest concentrations, suggesting that these areas can act as conduits and collection points for plastic moving through the landscape. It shows that even in a protected wetland, what happens upstream can ultimately show up in the sediment."
The findings indicate that water-connected areas may act as pathways and collection points for microplastics entering the preserve through runoff and other waterborne sources, while interior areas farther from active water flow generally had lower concentrations.
"The way water moves through the landscape may be more important in determining where microplastics accumulate than simply how close a location is to water," Johanson said.
The types of particles also offer clues about their origins. Fibers were the most common type, consistent with inputs from synthetic textiles and urban runoff, while the predominance of small particles suggests that plastics are breaking down and weathering over time. The mix of particle types and colors points to multiple, diffuse sources rather than a single source of contamination.
The findings underscore that protecting a wetland from development does not necessarily protect it from pollution. Spruce Bluff Preserve reflects a broader Florida landscape in which natural wetlands are interconnected with rivers, stormwater systems, flood-control canals and other engineered waterways.
These systems can act as both filters and conduits, trapping microplastics in sediments while also transporting them downstream.
The researchers say the findings point to the need to consider plastic pollution alongside the nutrients, sediment and other contaminants already monitored in Florida's wetlands and stormwater systems. Incorporating microplastic monitoring into existing environmental management programs could help identify where plastics are entering waterways and where they are accumulating.
"Wetlands are often viewed as places that protect us by filtering what moves through the landscape, but they can also capture and redistribute pollutants," said Johanson. "If we want to protect these ecosystems for the long term, we need to understand not only what is entering them, but how water moves that pollution through the system. A protected wetland is not an isolated wetland - it is connected to everything upstream and downstream through the movement of water."