Wetlands are among the most valuable yet vulnerable ecosystems on Earth. They retain nutrients, buffer floods, shelter wildlife, and soften the urban heat island effect, but their position at the bottom of urban catchments makes them final sinks for contaminants ranging from hydrocarbons and heavy metals to plastic debris. Microplastics—plastic particles ranging from 1 to 5,000 micrometers—have been found everywhere from Antarctic sea ice to deep ocean trenches. They come in two flavors: primary microplastics, intentionally manufactured for products like cosmetics and exfoliants, and secondary microplastics, which form when larger plastic items are broken apart by ultraviolet radiation, wind, surface runoff, and chemical weathering. Only about 9 percent of the world’s plastic waste is adequately recycled, meaning the vast majority disperses into the environment, fragmenting over time into ever-smaller particles that organisms mistake for food and that can travel along food chains.

The Laguna del Camarón wetland, covering roughly 5.64 hectares within Mazatlán’s Central Park Modernization Project, maintains a permanent underground connection to the sea and continues to provide flood regulation, refuge for local and migratory species, and cultural and tourist benefits. But the surrounding city is booming. A surge in hotel demand has triggered large-scale coastal development, and the park itself draws crowds to dining areas, an outdoor event stage, and cycling, skating, and athletics paths. Following the park’s modernization, the wetland was divided into two water bodies: a main lagoon and a smaller inner lagoon situated in the zone of highest visitor concentration. The research team selected seven sampling sites—five in the main lagoon and two in the inner lagoon—and collected water during two climatic seasons, October 2022 for the rainy season and March 2023 for the dry season.

The sampling protocol was rigorous. From a kayak, the researchers gathered approximately thirty liters of water per replicate at each site, from a depth of 15 to 20 centimeters, using pre-cleaned glass containers. The samples were sieved through a 63-micrometer stainless-steel mesh, capturing particles between 63 and 5,000 micrometers, then transported at 4 degrees Celsius to the laboratory. There, organic matter was digested overnight with filtered hydrogen peroxide at 60 degrees Celsius—a treatment shown to release microplastics without damaging their polymeric structure—before filtration onto glass fiber filters and microscopic examination. Stringent quality controls, including cotton laboratory clothing, flow-hood processing, filtered reagents, and ten laboratory blanks, established a detection limit of 6.5 particles per sample, ensuring the counts reflected genuine contamination rather than airborne or procedural contamination.

The results were striking. A total of 740 microplastic particles were recovered during the rainy season, averaging 1.17 particles per liter, but concentrations rose significantly during the dry season to 2.05 particles per liter. Statistical analysis using a two-way factorial ANOVA on square-root-transformed data, verified with Cochran’s C test, revealed significant differences among sites, between seasons, and in their interaction. The Tukey post hoc test pinpointed the source of the pattern: two sites in the small inner lagoon, LC6 and LC7, which during the dry season reached maximum concentrations of 3.30 and 7.63 particles per liter respectively—levels significantly higher than nearly every other site-season combination. Intriguingly, during the rainy season these inner lagoon sites were statistically indistinguishable from the rest of the wetland, suggesting the enrichment is seasonal and tied to dry-season dynamics.

Two mechanisms may explain this hotspot. First, the inner lagoon sits closest to the park’s most intensively used recreational zone, making it a likely recipient of plastic waste from food packaging, littering, and airborne deposition of textile fibers. Second, the researchers observed a large biomass of the aquatic plant Najas marina in the main lagoon during the dry season. Aquatic vegetation can act as a physical barrier that traps microplastics and reduces their transport across a water body, potentially isolating the inner lagoon and allowing particles to accumulate there. Conversely, slightly elevated concentrations at two main-lagoon sites during the rainy season reflected runoff from nearby streets funneled by a local slope, illustrating how the same wetland can receive microplastics through entirely different pathways depending on the weather.

Beyond counting particles, the team characterized their morphology and chemistry. Fibers dominated, accounting for more than 60 percent of all particles, followed by foam at roughly 20 percent, fragments at about 15 percent, and film at under 5 percent. At the inner lagoon sites, fibers exceeded 75 percent of the total. Black and blue colors predominated—a pattern the authors link to textile microparticles shed from visitors’ clothing, since the indigo dyes used in denim and other fabrics produce exactly this color profile. Foam and fragments, meanwhile, point to single-use food packaging and the intense hotel and condominium construction along the wetland’s margins. Micro-Fourier-transform infrared spectroscopy, performed on a random 10 percent subsample of 200 particles with match rates above 80 percent, identified polyester as the dominant polymer—48 percent in the dry season and 62 percent in the rainy season—followed by polypropylene, polyethylene, and polystyrene, with smaller amounts of polyacrylonitrile, PVC, and various resins. These polymers map neatly onto local sources: polyester from sportswear and everyday clothing, polyethylene from bags and bottles, and polypropylene and polystyrene from utensils, packaging, and construction materials.

To translate these measurements into a pollution assessment, the researchers calculated the Pollution Load Index for each site. In the rainy season, values ranged from 1.12 to 2.30, classifying LC6 as moderately polluted. In the dry season, the index climbed to 4.01 at LC7—firmly in the highly polluted category—while the wetland as a whole remained unpolluted to moderately polluted. These levels sit above those reported for some Colombian wetlands, where concentrations between 0 and 0.31 particles per liter have been recorded, but below the 3.1 to 14.9 particles per liter documented in urban wetlands in Australia and China. The comparison underscores a broader truth: microplastic pollution in urban waters scales with the intensity of nearby human activity, population density, and the proximity of roads and commercial development.

The study’s authors are careful to note its limitations—only surface waters were sampled, sediments and organisms were excluded, and quantitative source apportionment was not performed—so the findings should be read as a baseline. Even so, the implications are actionable. The research identified clear hotspots linked to visitor use and construction, and the team recommends targeted interventions: improved onsite waste collection, greater visitor surveillance, and the strategic placement of bins and informative signs throughout the park. Given that direct release of polyester microfibers from wearing garments may be as significant as release through wastewater, simply managing trash is not enough; the wetland’s pollution load is woven, quite literally, into the fabric of urban life.

What makes this study resonate far beyond Mazatlán is its demonstration that a wetland’s microplastic profile can serve as a forensic record of how people use the space around it. The dominance of polyester fibers in the quiet inner lagoon tells the story of crowds gathered at an outdoor stage; foam and fragments near the main lagoon tell of construction cranes and takeout containers; black particles washing in with the rains tell of city streets upstream. As urban wetlands worldwide face mounting development pressure, this Mexican lagoon offers both a warning and a template—proof that with careful seasonal monitoring, chemical fingerprinting, and spatially targeted management, cities can identify exactly where their plastic footprint lands and begin, site by site, to erase it.

Subject of Research: Microplastic pollution and its anthropogenic sources in an urbanized Mexican wetland

Article Title: Impact of anthropogenic activities on the microplastic pollution status of an urbanized wetland

Article References: Rivera-Hernández, J. R., Hernández-Angeles, J., Green-Ruiz, C. R., Amezcua-Martínez, F., & Alvarado-Zambrano, D. (2026). Impact of anthropogenic activities on the microplastic pollution status of an urbanized wetland. Environmental Monitoring and Assessment, 198(9), Article 1008. https://doi.org/10.1007/s10661-026-15857-2

Image Credits: AI Generated

DOI: 10.1007/s10661-026-15857-2

Keywords: microplastics, urban wetland, Mazatlán, tourism pollution, FTIR spectroscopy, polyester fibers, pollution load index, construction activities, seasonal variation, surface water, Environmental Monitoring and Assessment, Mexico