Yuxi You, CC BY-NC-NDWalk across a mudflat at low tide and you might notice small, neat mounds of sediment scattered across the surface.These so-called “chimneys” are the calling card of the humble bamboo worm (Macroclymenella stewartensis) which inhabits sandy sediments within New Zealand’s sheltered bays and estuaries.Despite their hidden lives and small size – most measure just a few centimetres long – these worms have an outsized influence on the health of our marine environment.But now there are troubling signs that microplastics – tiny but pervasive fragments of broken-down plastic – are disrupting the vital role the worms play, with potentially wider effects we are only just beginning to understand.Hidden heroes of the seafloorOver time, scientists have come to recognise the role bamboo worms and other tiny creatures have in bioturbation: a process essential to the functioning of coastal ecosystems.When healthy, the worms burrow in the seafloor, enabling oxygenated water to enter deeper into the sediment. This, in turn, breathes life into the seabed. The humble bamboo worm plays an outsized role in keeping coastal ecosystems healthy, through a process called bioturbation. Yuxi You, CC BY-NC-ND The worms also feed on organic matter, helping regulate carbon and nitrogen in the sediment and surrounding waters. As they deposit small piles of waste, they provide nutrients for microscopic plants, supporting coastal food webs.When these processes are disrupted, the impacts can ripple outward.Nutrients can build up, increasing the risk of algal blooms that strip oxygen from the water. This can worsen conditions to the point where fish and other marine life can no longer survive. This image shows surface signatures of bioturbation, in which tiny creatures such as bamboo worms burrow into muddy sediments, enabling the oxygenation and nutrient cycling that keeps coastal ecosystems healthy. Yuxi You, CC BY-NC-ND Healthy marine sediments also act as a buffer against climate change by locking away carbon. When that balance is lost, sediments can instead release greenhouse gases such as nitrous oxide and methane.How microplastics mess with marine lifeMarine microplastics – fragments smaller than 5 millimetres from sources such as vehicle tyres, synthetic clothing fibres and degraded plastic waste – are now found from the tropics to Antarctica. Some estimates suggest there may be more than 170 trillion pieces in the world’s oceans today.In New Zealand, scientists have been surprised to find them building up even in seemingly pristine marine environments, far from towns and major sources of pollution.Their impacts are wide-ranging and still being uncovered.Their small size makes them easy for marine organisms to ingest, often by mistake, where they can cause physical damage and leave animals malnourished. Microplastics can also carry toxic chemicals that interfere with reproduction and development, with these effects building up through the food chain.When we look at how microplastics affect life on the seafloor, the picture becomes yet more complex.In a recent study carried out at the University of Auckland’s Leigh Marine Laboratory, we found bamboo worms became less active when exposed to them.It’s still not clear why. The worms may be ingesting plastic, absorbing chemicals from contaminated sediments, or simply finding less food if microplastics reduce algal growth. Marine microplastics are small fragments of plastic debris that measure less than 5 millimetres long. Milos Bicanski/Getty Images What matters is that their behaviour shifts as microplastic levels increase – with potentially important implications for bioturbation and ecosystem health.It might also be causing knock-on impacts for wider food chains, as seabirds and eagle rays feed on worms and other tiny creatures in the seabed.A micro pollutant, a macro problemWhile plastic continues to accumulate in the marine environment, some microplastics may break down in sediments over time. Even so, this is unlikely to offset the growing volume, meaning the overall burden continues to rise.People can help tackle the microplastic problem by reducing the amount of plastic they buy, picking up plastic rubbish on the beach, supporting harbour clean up groups and buying clothing made of natural fibres.Presently, there are no limits set for safe levels of microplastic pollution in New Zealand – and policies will be needed to manage the problem.Clean coasts are highly valued by New Zealand communities, but the health of these environments depends as much on what lies beneath the surface as what is visible above it.While attention often focuses on those “charismatic” species such as dolphins and penguins, the small organisms living in the seabed play an equally important role in keeping ecosystems functioning.Simon Francis Thrush receives funding from The Auckland Foundation.Yuxi You receives funding from Oceans of Change II project.