Beneath the surface of tidal flats and shallow bays, lugworms spend their lives swallowing sediment, digesting organic matter, and pushing the leftovers back to the surface in coiled casts. That relentless conveyor belt oxygenates buried layers, recycles nitrogen, and helps anchor the sediment that protects shorelines. Now, a convergence of experimental findings shows that microplastics are undermining these animals and the broader biological machinery they belong to, with measurable consequences for nutrient cycling, seagrass productivity, and carbon storage.
“We are only beginning to appreciate how much coastal ecosystem functioning depends on organisms most people never see,” said Mark Mayfield, a coastal sediment ecologist at the University of Plymouth, in an April 2026 interview. “When you compromise the worms and microbes that turn over the seafloor, you compromise everything built on top of that process.”
The worms at the center of the problem
Lugworms (Arenicola marina) are classified as ecosystem engineers because their burrowing and feeding physically restructure the seafloor. Field sampling along coastal sites compared microplastic concentrations in lugworm casts against surface sediment and subsurface layers at roughly 20 cm depth, the zone where the worms take in food. The results confirmed that lugworms actively pull microplastics deeper into the sediment column through their feeding and egestion cycles, redistributing contamination to layers where it persists longer and exposes a wider range of buried organisms.
The physiological cost is well documented. In a 2013 study by Mark Anthony Browne and colleagues, published in Current Biology, lugworms exposed to microplastic-laden sediment accumulated chemical concentrations in body-wall tissue up to roughly 950% higher than unexposed animals. Separate work indexed in PubMed linked microplastic ingestion to depleted energy reserves, tracing the mechanism through reduced feeding activity, longer gut residence time, and inflammation. When worms eat less and process food more slowly, they also mix less sediment. That creates a feedback loop: the very organisms responsible for turning over the seafloor become less capable of doing so as plastic loads increase.
Bioturbation losses ripple through the system
Bioturbation, the mixing and aerating of sediment by burrowing animals, is not a background process that waves and tides can replace. Research published in PNAS Nexus demonstrated that deposit-feeding worms control subsurface transport of organic carbon and shape microbial communities even in intertidal zones where physical reworking is intense. If microplastics weaken the worms, wave action alone cannot compensate for the lost oxygenation and redistribution of organic material in deeper layers.
Researchers have begun quantifying how that weakness scales up. Using luminophore-tracer experiments to measure how different species mix sediment, one team fed those measurements into a transport-reaction model that projects broad changes in seafloor ecosystem functioning. The results were species-specific and functional-group-specific: some communities lose their mixing capacity faster than others when microplastics are present, meaning the damage depends on which organisms dominate a given stretch of coast.
Polymer type matters too. Mesocosm experiments testing PVC and polyethylene found that different plastics produced different degrees of disruption to both organism health and sediment biogeochemistry, including nutrient cycling and productivity, according to a study in Environmental Pollution. The composition of plastic pollution, not just its quantity, influences how severely benthic communities are affected.
How much microplastic is already in coastal sediments
Quantifying the scale of contamination helps frame the biological findings. Peer-reviewed surveys of intertidal and subtidal sediments in Europe and Asia have reported concentrations ranging from tens to thousands of microplastic particles per kilogram of dry sediment, with heavily urbanized estuaries and harbors at the upper end of that range. A synthesis drawing on data from nearly 2,000 ocean sampling stations collected between 2014 and 2024 confirmed that microplastics occur throughout all ocean depths, according to a U.S. National Science Foundation report. As particles settle, resuspend, and resettle, they repeatedly pass through the guts of deposit feeders and interact with microbial biofilms, making microplastics a persistent structural element of modern marine sediments rather than a passing contaminant.
“The numbers vary enormously from site to site, but the direction is the same everywhere: concentrations are rising,” said Helena Carvalho, a marine biogeochemist at GEOMAR Helmholtz Centre for Ocean Research Kiel, in a May 2026 interview. “What the biological experiments tell us is that even the lower end of observed concentrations can alter how sediment communities function.”
Seagrass and deep-sea sediments are not spared
The effects extend well beyond worms. A 28-day mesocosm experiment with eelgrass (Zostera marina) found that microplastics in coastal sediments shifted microbial communities in ways that cut leaf growth rate by 39% and net production by 5%, according to research by Zhengyi Cao and colleagues published in Marine Pollution Bulletin (2025). Seagrass beds rank among the most effective coastal carbon sinks, so even modest productivity declines carry outsized consequences for blue-carbon storage and the fisheries that depend on seagrass habitat.
Deeper waters face their own version of the problem. Experiments with sediments collected from upper bathyal zones at approximately 530 meters depth showed that a single pulse of naturally weathered microplastics in the 70 to 210 micron size range altered benthic biogeochemistry and organic matter cycling, according to work in Environmental Pollution. Remote, low-light environments are not insulated from microplastic-driven changes to nutrient and carbon processing.
Where the science still has gaps
Nearly all of the evidence connecting microplastics to disrupted bioturbation comes from laboratory mesocosms, short-duration field comparisons, or modeled projections. No long-term field study has tracked wild lugworm populations across varying coastal conditions to measure how their ecosystem services change as microplastic loads accumulate over years or decades. The eelgrass mesocosm ran for just 28 days; whether the 39% drop in leaf growth persists, worsens, or partially recovers over a full growing season remains unknown. The deep-sea biogeochemistry experiment used a single pulse of weathered microplastics rather than the chronic, mixed-source inputs typical of real-world settings.
The modeling work that projects broad-scale shifts relies on bioturbation coefficients measured under controlled conditions. How well those coefficients translate to complex, multi-species communities with variable sediment types and hydrodynamic regimes has not been validated in the field, where storms, temperature swings, and fluctuating organic inputs could amplify or dampen the effects seen in simplified experiments.
Interaction with other stressors is another blind spot. Coastal sediments are simultaneously dealing with warming, deoxygenation, nutrient enrichment, and chemical contamination from metals and hydrocarbons. Experiments to date rarely combine these pressures, making it difficult to predict whether microplastics will mainly add incremental harm or trigger non-linear shifts such as sudden losses of seagrass cover. Species-specific sensitivity compounds the uncertainty: the experimental record focuses heavily on Arenicola marina and Zostera marina, while many other infaunal species, including smaller deposit feeders and burrowing crustaceans, have not been tested with comparable rigor.
Scaling from organism-level effects to landscape-scale outcomes is perhaps the largest remaining challenge. Demonstrating a change in nitrogen flux or carbon burial in a mesocosm is not the same as quantifying how much shoreline protection or blue-carbon storage a bay will lose over 20 or 30 years. Answering that will require coordinated monitoring of sediment chemistry, organism health, and microplastic loads across multiple sites, paired with models that explicitly link bioturbation to services like coastal erosion buffering and carbon sequestration.
Why microplastic policy belongs in coastal resilience planning
Despite these gaps, the direction of the evidence as of April 2026 is consistent: microplastics are not inert grains of sand. They change how worms feed, how microbes process organic matter, and how seagrass grows. Because those processes underpin sediment stability and carbon storage, even moderate disruptions could ripple outward to affect fisheries, water quality, and the capacity of coastal ecosystems to buffer climate change.
For coastal managers, the findings argue for treating microplastic reduction as part of climate and habitat strategies rather than framing it solely as a litter or wildlife concern. Cutting plastic inputs to rivers and coasts will not remove the particles already buried in sediments, but it can limit additional stress on the organisms that keep those sediments functioning. At the same time, filling the research gaps on long-term, field-scale impacts will be essential for designing realistic restoration and adaptation plans in an era when plastics have become a permanent feature of the seafloor.
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*This article was researched with the help of AI, with human editors creating the final content.
