Even A Protected Florida Wetland Isn’t Safe From Microplastic Pollution

Spruce Bluff Preserve sits along the St. Lucie River in Port St. Lucie, Florida, known for hiking trails and birdwatching, the kind of protected natural area people think of as untouched by pollution. But when scientists dug into its soggy sediment, they found plastic almost everywhere they looked, carried in not by nearby litter but by the same water that feeds the preserve.

Those particles weren’t scattered at random. They clustered along the river’s edge and a flood-control canal, the exact points where water enters and exits the wetland. That pattern, the study’s authors say, is the real finding: a protected wetland is still connected to everything upstream and downstream through the movement of water, and pollution moves with it.

That finding, published in the journal Evolving Earth, comes with a caveat about how the plastic was counted. Not every particle that looked like plastic under a microscope actually was. When a batch of the suspicious specks was tested with a more precise tool, most turned out to be something else entirely: cotton fuzz, cellulose fibers, pine material, or plain mineral grains that only resembled plastic on first glance. That doesn’t undercut the core finding; it just means the true scale of the contamination needed a closer look to pin down.

Researchers Tested 40 Sediment Samples Across Five Habitat Types

Researchers from Florida Atlantic University collected 40 sediment samples from Spruce Bluff Preserve in December 2024, during the dry season, so water levels would stay relatively steady. They laid out eight transects cutting through five habitat types: marsh, pine woods, scrub, a hydric hammock (a shaded forest on wet ground), and open water near the river and a flood-control canal, scooping up soil every 5 meters along each path.

Back in the lab, the team treated each sample with a chemical solution to break down organic material, then used a salt-water bath to separate lighter plastic particles from heavier soil. Whatever floated was filtered, dried, and examined under a microscope, sorted by shape, size, and color. Because a microscope alone can’t prove a speck is really plastic, researchers tested a subset using infrared spectroscopy, which identifies materials by how they absorb light.

That step delivered the surprise. Of 197 particles tested this way, only 82 were confirmed as actual plastic. Researchers applied those confirmation rates to the full visual tally, producing an FTIR-corrected estimate of 97 plastic particles, a conservative lower bound rather than a count of individually tested particles.

Fibers Dominated the Confirmed Plastic, Mostly Small and Worn Down

Plastic appeared in some form across 38 of the 40 samples, or 95%, scattered throughout the preserve rather than clustered in one spot. As the paper puts it, “visual identification alone substantially overestimates MP abundance.”

Fibers, the kind of tiny threads that shed from clothing, ropes, and fabric, made up close to half of the corrected plastic estimate, the most common type found. Most confirmed pieces were smaller than a tenth of an inch, pointing to older, worn-down debris rather than freshly dumped trash, likely the slow breakdown of plastic circulating in the environment for a while.

Hydrology, More Than Vegetation, Shaped Where Microplastic Collected

One finding runs against a common assumption: that plant-heavy areas trap more plastic, since thick vegetation and rich organic soil have been shown in other wetland studies to catch and hold floating debris. That’s not the pattern researchers saw at Spruce Bluff.

Instead, the highest concentrations sat closest to open water, along the outlet canal and the river’s edge. The outlet canal transect alone averaged 117 microplastic particles per kilogram of dried sediment, by far the highest sampled, while interior marsh and wooded areas showed lower amounts. A model accounting for organic matter and habitat type found the open-water zone had notably higher plastic levels than the shaded hammock zone, with soil richness playing a smaller role. As the study concludes, “Hydrological position, rather than vegetation or organic content, was the dominant control on MP distribution.”

“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,” said Erik N. Johanson, the study’s senior author and an FAU associate professor, in a university statement.

Plastic levels dropped somewhat as organic matter increased, but that pattern was driven mostly by pellet-shaped particles, many turned out to be mineral grains rather than plastic. Fibers, the particles most reliably confirmed as plastic, showed little connection to organic matter at all.

A Protected Wetland Is Still Connected to Everything Upstream

These findings underscore something the authors say matters well beyond Spruce Bluff: keeping land undeveloped does not seal it off from pollution carried by water. “Wetlands are often viewed as places that protect us by filtering what moves through the landscape, but they can also capture and redistribute pollutants,” Johanson said in a university statement.

Visual counting alone put the plastic estimate at more than double the FTIR-corrected figure, a gap worth keeping in mind for any microplastic survey that skips chemical confirmation. That correction doesn’t change the bigger picture: confirmed plastic still turned up in nearly every habitat type tested, concentrated most near the canal and river edge, the kind of engineered drainage feature common across Florida’s wetlands. Being protected from development is not the same as being sealed off from what flows in upstream.

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