Freshwater Ecosystems Face a Double Threat from Microplastics and Toxic Chemicals

Freshwater pollution is becoming harder to manage because its major threats rarely occur in isolation. Plastic waste, agricultural runoff, industrial chemicals, pharmaceutical residues and weak wastewater treatment increasingly converge in the same rivers and lakes, especially in rapidly urbanizing regions with limited monitoring capacity.

The review "Microplastics as Carriers of Co-Occurring Pollutants in Freshwater Ecosystems: Mechanisms, Environmental Fate, and Ecotoxicological Risks," published in Microplastics, examines how these pressures interact. Authors Raissa Okwuosa, Thendo Mutshekwa and Jeffrey Lebepe reviewed 89 studies on microplastics and co-occurring pollutants. They find that plastic particles can alter the movement and biological availability of heavy metals, pesticides, pharmaceuticals, PFAS and persistent chemicals.

For developing countries, the findings raise a difficult policy question: how can authorities regulate complex pollution mixtures when analytical capacity, wastewater infrastructure and freshwater data remain limited?

Microplastics are becoming mobile chemical platforms

Microplastics enter freshwaters through wastewater effluent, urban runoff, textile shedding, road transport, agricultural drainage, industrial activity and the breakdown of larger plastic waste. Once in a river or lake, they do not remain chemically inert. Their surfaces can attract pollutants through hydrophobic partitioning, electrostatic forces, hydrogen bonding, pore filling and other mechanisms. Polymer type matters: polyethylene, polypropylene, PVC and polystyrene have different surface chemistry, density and affinity for contaminants. Particle size and shape also influence how easily plastics move, settle or are ingested.

Environmental aging makes the picture more complex. Sunlight, abrasion, oxidation and microbial colonization can create cracks, roughness and new chemical groups on plastic surfaces. These changes may increase the binding of some metals and pharmaceuticals. Yet aging can also reduce the attraction of certain hydrophobic chemicals by making the surface more polar. The review therefore rejects the idea that weathered plastic is always more dangerous in the same way.

Biofilms add another layer. Microplastics can become "plastisphere" habitats for microorganisms, changing buoyancy and pollutant adsorption while potentially transporting pathogens and antibiotic-resistance genes. Rivers may then function as both highways and storage zones: particles are deposited in sediments, resuspended during storms and moved downstream in pulses.

This is crucial for regulators because a particle count alone says little about chemical risk. Two water bodies with similar microplastic concentrations may face very different hazards depending on polymer composition, particle age, surrounding pollutants and local hydrology.

The vector effect is real, but not automatically dominant

The review confirms that microplastics can bind cadmium, lead, copper, zinc, pesticides, pharmaceuticals, polycyclic aromatic hydrocarbons, PFAS and persistent organic pollutants. But it also highlights a critical distinction: the ability to adsorb a contaminant does not prove that plastics are the most important carrier in nature.

Freshwater contains suspended sediment, organic matter, colloids and biological particles that also bind chemicals—and often exist in far greater quantities. In one comparison reviewed by the authors, microplastics showed higher sorption per unit mass for some pollutants, but natural sediment remained the more important transport phase because it was much more abundant.

Water chemistry can further weaken laboratory findings. pH, salinity, dissolved organic matter and competing ions can reduce or redirect pollutant binding. A pesticide that adheres strongly to plastic in purified water may behave very differently in a river carrying minerals, organic debris and multiple chemicals.

The same caution applies to biological exposure. A contaminant detected on a plastic particle is not necessarily absorbed by an organism. It must first desorb in the digestive tract, cross a biological membrane and reach tissue at a toxic concentration. In some cases, acidic gastrointestinal conditions can release metals from ingested plastics. In others, strongly bound pollutants may remain attached and pass through the organism.

Microplastics can therefore raise, reduce or leave exposure unchanged. Their role depends on the pollutant's chemical activity, the plastic's surface properties, ingestion rates and alternative exposure through water, sediment and food. This uncertainty is not a reason for inaction; it is a reason to improve risk assessment.

Mixture toxicity could reshape food webs, but the evidence remains uneven

The review finds credible evidence that combined exposure to microplastics and pollutants can cause oxidative stress, inflammation, tissue damage, altered metabolism and reproductive impairment in freshwater organisms. Fish, zooplankton and benthic invertebrates are especially relevant because they ingest suspended or sedimented particles and occupy key positions in aquatic food webs.

Some experiments reported synergistic effects, meaning the combined harm exceeded the effect expected from each contaminant alone. Other studies found additive, antagonistic or endpoint-specific responses. Particle size, polymer type, aging, pollutant concentration and species biology all influenced the outcome.

The variability shows why microplastics should not be regulated as a single substance with a uniform toxicity value. A weathered fragment carrying cadmium poses a different risk from a pristine fiber exposed to a pharmaceutical, even when both are counted as one microplastic particle.

The review also raises concerns about trophic transfer. Contaminated particles can move from primary producers and invertebrates to fish and higher consumers. This creates a plausible pathway toward aquaculture species, wildlife and potentially humans. But the authors stress that evidence of transfer is not the same as proof of large-scale biomagnification. Natural prey and dissolved contaminants may still dominate exposure in many systems.

A major weakness in the evidence base is the gap between laboratory design and environmental reality. Many experiments use pristine spherical particles, short exposure periods and concentrations well above those recorded in freshwater ecosystems. Such studies identify potential hazards, but they cannot by themselves establish the probability or scale of real-world damage.

Freshwater policy must move from counting particles to managing mixtures

The policy implications are broader than plastic-waste control. Governments need to treat microplastics as part of a combined pollution problem involving wastewater, agricultural chemicals, industrial discharges, urban runoff and contaminated sediments.

In the short term, source reduction remains the most defensible intervention. Measures targeting textile fibers, tire wear, plastic pellets, untreated wastewater and unmanaged waste can reduce both particle loads and the surfaces available to carry other pollutants. Wastewater and stormwater investments are especially important in rapidly urbanizing regions where plastic leakage overlaps with pharmaceutical, industrial and agricultural contamination.

Monitoring systems must also improve. Programs should identify polymer types, particle sizes and shapes, not just total counts. Where possible, they should measure pollutants attached to particles and compare them with burdens in sediments, water and natural suspended matter. Without this comparison, authorities cannot determine whether microplastics add a substantial pathway or merely a visible one.

The review calls for standardized methods, including common sampling procedures, contamination controls, polymer verification and reporting in both particle-number and mass-based units. This is particularly important for developing countries, where limited access to advanced spectroscopy can leave major freshwater systems outside the global evidence base. Regional laboratories, shared protocols and lower-cost analytical tools could help close that gap.

Future research should prioritize long-term field and mesocosm studies using naturally weathered particles, realistic concentrations, mixed pollutants and natural food webs. Researchers should measure internal contaminant burdens and ecological outcomes rather than infer risk from adsorption alone.