Beneath the waves of the Wadden Sea, something is changing. The mud is disappearing.

Grains are coarsening. Sediments once fine and rich are turning sandy, gritty, hostile. For the worms, clams, and countless other creatures that spend their lives burrowing through this underwater soil, the shift could prove catastrophic.

This isn't an isolated quirk of one ecosystem. Across the globe, coastlines are undergoing what scientists are calling "sandification"—a creeping transformation driven by rising seas, fiercer storms, and decades of human meddling with sediment flows. New research from the Netherlands reveals just how vulnerable marine communities are to this change, and the findings carry a warning that extends far beyond the Dutch coast.

The Vanishing Mud

The Wadden Sea is a UNESCO World Heritage site. Stretching along the coasts of the Netherlands, Germany, and Denmark, it's one of the world's most productive tidal ecosystems. Mudflats here support extraordinary densities of worms, snails, and shellfish—organisms that in turn feed millions of migrating shorebirds each year.

But the sediment beneath these waters is in flux.

Between 1950 and 2010, the global flow of sediment from land to sea dropped by nearly a quarter. Dams trap fine particles upstream. Mining operations strip riverbeds. Meanwhile, coastal managers pump massive volumes of sand onto eroding shores to hold back the ocean. In the Netherlands alone, sand nourishment programs have grown sharply over recent decades as sea levels climb and storms intensify.

All this sand has to go somewhere. And when it does, it can fundamentally alter the character of coastal sediments—making them coarser, less muddy, less suitable for the species that evolved to thrive in fine-grained environments.

Mapping Preference in a Shifting Landscape

To understand how marine communities might respond to sandification, researchers turned to the Borndiep, a tidal basin near the island of Ameland. Over six years, they collected nearly 3,000 samples from intertidal flats, subtidal channels, and the delta just beyond the inlet. Each sample revealed what lived there and what kind of sediment it called home.

The dataset was immense. Grain sizes ranged from silt-like particles barely 20 micrometers across to coarse sands more than 400 micrometers wide. Mud content varied from zero to over 80 percent. Across this gradient, the team identified 22 of the most common species and asked: where does each one actually thrive?

The approach they used—quantile regression—focuses not on average abundances but on peak abundances. It identifies the conditions under which a species reaches its highest densities, filtering out the noise of all the other environmental factors that might limit it elsewhere. Think of it as finding where an organism is at its best, not just where it manages to survive.

Generalists, Not Specialists

The results defied expectation. Most species proved to be sediment generalists, capable of tolerating a surprisingly wide range of grain sizes and mud content. Only a handful showed narrow preferences.

Some patterns did emerge. Certain worms—Nephtys cirrosa, Magelona mirabilis, Scoloplos armiger—favored coarser sediments. Others, like the ragworm Hediste diversicolor and the Baltic clam Macoma balthica, preferred muddier habitats.

But here's the crucial finding: species that preferred coarse sediments tended to have narrower tolerance ranges. They were more specialized. Sand lovers, in other words, are picky. Mud lovers are flexible.

Why? Living in mud requires adaptations. Organisms must tolerate low oxygen, separate food from fine particles, burrow through cohesive sediment. These challenges breed versatility. Sandy habitats, by contrast, offer fewer obstacles but also fewer niches. Species there are streamlined for a narrower set of conditions.

Living in the Wrong Neighborhood

Then came the surprise.

When researchers calculated the community-wide sediment preferences—essentially, the average optimum of all species present at each site, weighted by their abundances—they discovered a mismatch. At many locations, the actual sediment was coarser and sandier than the community's collective preference.

The discrepancy was most pronounced on intertidal flats. Here, species were often living in conditions that fell outside their ideal range. Not uninhabitable—these organisms were still present—but suboptimal. They weren't reaching the densities or biomasses they could achieve in finer sediments.

In the subtidal channels, the fit was better. Communities there inhabited sediments closer to their preferences. But even in deeper waters, certain areas showed the telltale pattern: too much sand, not enough mud.

Hotspots of Vulnerability

Spatial patterns emerged with stark clarity. Intertidal flats beneath the western end of Ameland island and the eastern stretch of Terschelling island showed the greatest mismatch. At these sites, grain sizes exceeded the tolerance ranges of significant fractions of the resident community—in some cases, more than 75 percent of species present.

These are sensitivity hotspots. If sediments coarsen further—if more sand arrives, if mud continues to disappear—these locations will feel the impact first.

The researchers saw no evidence that the 2018 sand nourishment placed on the ebb-tidal delta had reached these vulnerable areas. Not yet. Sediment modeling suggests that nourishment sand follows pathways unlikely to deliver it deep into the basin. But the broader message remains: this ecosystem is already on edge. Many communities inhabit sediments coarser than they prefer. Further sandification would push them over.

Why This Matters Beyond the Wadden Sea

Benthic organisms are ecosystem engineers. Worms aerate sediments. Clams filter water. These tiny architects structure the seafloor, cycling nutrients, stabilizing substrates, supporting food webs that extend to fish, seabirds, and marine mammals.

When sediment composition shifts, these functions change. Some species decline. Others expand. The balance tips. Productivity may drop. Birds find less food. Fisheries suffer.

And the Wadden Sea is hardly unique. Coastlines worldwide face similar pressures. Sea level rise accelerates erosion. Storms grow more intense. Rivers deliver less sediment as upstream dams multiply. Coastal nourishment projects—essential for protecting infrastructure and communities—redistribute massive quantities of sand.

In some regions, the process is already underway. The eastern Wadden Sea may be mud-limited—demand for fine sediment exceeds supply—and this deficit is expected to worsen as seas continue to rise. Elsewhere, similar imbalances loom.

Decisions Informed by Data

The tools demonstrated in this study offer a path forward. By quantifying sediment preferences and tolerance ranges for key species, managers can map vulnerability. They can identify where proposed nourishment projects might do the most ecological harm. They can select finer-grained materials when appropriate. They can monitor hotspots more closely.

Some experimental nourishments have already tested this principle. Pilot projects using finer sediments showed promise in creating more suitable habitat for native benthic communities, though recovery times varied. The key is matching the intervention to the ecology.

This is not an argument against sand nourishment. In many places, it's the only viable option for protecting coastlines and the communities that depend on them. But it's a call for precision. For understanding that sediment is not a commodity but a habitat. That grain size matters. That mud matters.

The Future Written in Grains

The sediments beneath our coasts are changing. Not everywhere, not uniformly, but persistently. The organisms that inhabit these environments are adapted to specific conditions, and many are already living near the edge of what they can tolerate.

What happens when the last of the mud washes away? When tidal flats harden into sandy deserts? The Wadden Sea offers a preview: communities compressed into shrinking refuges, species declining, functions lost.

But it also offers a blueprint. Sensitivity maps can guide where we place sand and where we don't. Grain-size matching can minimize harm. Monitoring can track whether ecosystems are adapting or collapsing.

The question is whether we choose to use these tools. Whether we recognize that the sand we pump onto beaches doesn't simply sit there, inert and harmless. It moves. It spreads. It transforms entire seafloors.

Beneath the waves, life is reading the sediment like a text. And right now, the story it tells is one of narrowing options and mounting stress. We have the means to write a different ending. The grains are still shifting. There's time.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1016/j.ecss.2025.109303