Definition
A tidal flat is a broad, flat stretch of coast that appears when seawater goes out and disappears when it comes back in, twice a day. It forms when fine mud and sand washed down from land settle over a long time in calm, shallow water. It is home to countless creatures and acts as a natural filter for organic pollution.
The Mysterious Land That Appears When the Sea Exhales
The coast isn't always crashing waves and sandy beaches. Where waves are strong, light muddy particles get swept away. But where the waves are gentle and the water is shallow, very fine, soft mud particles quietly sink to the bottom.
The tides lend a hand. The pull of the moon and sun makes the sea rise and fall on a regular cycle. When the tide comes in, fine sediment carried by the water slowly settles on the seafloor. When it goes out, a wide, gently sloping mud surface is left exposed.
Over thousands of years, tiny bits of sediment pile up layer by layer and form land that is underwater at high tide and exposed at low tide. This boundary between water and earth becomes one of the most life-rich environments on the planet.
The Earth's Kidneys, Filtering Dirty Water Clean
What if all the waste from rivers and towns flowed straight into the deep sea? The ocean would soon turn foul, and fish couldn't survive. Tidal flats sit between land and sea and work as a huge natural filter, catching organic matter and nutrients.
The mud is packed with countless bacteria and tiny algae such as diatoms. These microbes busily break down organic waste from land and turn it into nutrients. On top of that, shellfish like clams and cockles, along with marine worms, constantly filter plankton and impurities out of the water, cleaning it as they feed.
A well-preserved tidal flat is said to have cleaning power comparable to a sewage treatment plant. Like the kidneys that filter waste from our bodies, it breaks down organic pollutants. There are limits, though. Some pollutants, such as heavy metals, don't break down and can build up in the mud and in shellfish. And if pollution goes beyond what the flat can handle, the flat itself gets damaged.
A Closer Look: A Giant Carbon Vault
In the past, tidal flats were seen as useless, squishy mud. People built dikes and filled them in to make farmland or factory sites, a practice called land reclamation. But modern ecology and climate science show that tidal flats are a precious asset for protecting the planet.
Tidal flats are drawing attention as a blue carbon (carbon stored by ocean ecosystems) reservoir that traps carbon dioxide, a main driver of global warming. The blue carbon ecosystems officially recognized in international guidelines (the IPCC) are mangroves, salt marshes, and seagrass meadows. Whether to include unvegetated tidal flats is still under discussion. Tiny algae on the surface take in carbon through photosynthesis, and when living things die, they get buried deep in the mud.
Deep in a tidal flat, there is almost no oxygen. So the buried carbon doesn't break down easily and can stay locked underground for thousands of years. But if a flat is filled in or dug up, that trapped carbon can escape again.
Common misconceptions
Tidal flats are useless mud where hardly anything can live.
Tidal flats are highly productive for their size. Everything from microbes to clams, crabs, marine worms, and shorebirds lives there, which makes them a treasure house of life.
Where you meet it
The west coast tidal flats of Korea, where you can watch mudskippers and small mud crabs crawl across the flat at low tide.
A rest stop for tens of thousands of sandpipers flying from Siberia to Australia, who refuel there along the way.
In one sentence
Tidal flats are coastal landforms shaped by the tides. They filter organic pollution, lock away large amounts of carbon, and shelter countless living things, making them one of Earth's natural treasures.