Wetlands: Nature's Water Filters

What is a wetland?

A wetland is exactly what it sounds like: land that is wet for all or part of the year. The water may sit on the surface, like a marsh, or simply keep the soil soaked, like a bog. This constant water creates waterlogged soil and special plants that can survive with their roots underwater.

There are several kinds of wetland:

• Marshes are full of soft plants like reeds, rushes and grasses.
• Swamps are wetlands with trees and shrubs, such as the mangrove swamps along tropical coasts.
• Bogs and fens are waterlogged areas where dead plants pile up over time instead of fully rotting, forming a spongy material called peat.

Wetlands form where rivers spill over, where the land is low and flat, and along the edges of lakes and coasts. To see how they fit into the bigger water picture, explore the water cycle.

Nature's water filter

One of the most valuable jobs a wetland does is clean water. As water flows slowly through the maze of stems and roots, several things happen:

• Sediment settles out. The slow flow lets mud and dirt sink to the bottom instead of being carried downstream.
• Pollutants are absorbed. Wetland plants and soil take up harmful chemicals and excess fertiliser that would otherwise feed harmful algae in rivers and lakes.
• Bacteria break down waste. Tiny organisms in the wet soil break down pollution into harmless substances.

The result is that water leaving a wetland is often far cleaner than the water that entered. Some towns even build constructed wetlands on purpose to treat their wastewater naturally.

A sponge against floods

Wetlands also protect us from flooding. When heavy rain falls or a river bursts its banks, a wetland soaks up the extra water like a giant sponge. It then releases that water slowly over the following days and weeks. This evens out the flow, so there is less flooding when it is wet and more water available when it is dry. Coastal wetlands, especially mangroves and salt marshes, also break the force of storm waves, shielding the land behind them.

Bursting with life

Wetlands are among the richest habitats on Earth — true biodiversity hotspots. They are breeding grounds and feeding stops for huge numbers of:

• Birds such as herons, ducks and migrating waders.
• Amphibians like frogs and newts, which lay their eggs in the water.
• Fish, including young fish that grow up sheltered among the reeds before heading to open water.
• Insects, from dragonflies to the larvae that feed countless other animals.

This makes wetlands important links in many food chains and ecosystems, and a key part of protecting biodiversity and conservation.

Climate champions

Peatlands deserve special mention. Although they cover only a small part of the planet, the waterlogged peat locks away an enormous amount of carbon — more than all the world's forests combined. As long as a peatland stays wet, that carbon stays trapped. But if it is drained, the peat dries out and rots, releasing carbon dioxide and worsening climate change. Keeping peatlands wet is one of the simplest ways to fight global warming.

Wetlands in danger

Despite all they do, wetlands have been disappearing faster than almost any other ecosystem. For centuries people saw them as useless "wasteland" — places full of mosquitoes that were better drained for farms and cities. As a result, a large share of the world's wetlands has already been lost. Today, conservationists are working to protect and restore them, sometimes by re-flooding land that was once drained.

Try it yourself: build a wetland in a tray

You can show how a wetland filters and slows water.

1. Take a baking tray and prop one end up to make a gentle slope.
2. At the lower end, build a "wetland" out of a thick layer of moss, grass clippings or a sponge.
3. Mix some soil and a little food colouring into a cup of water to make "dirty floodwater."
4. Pour it in at the top of the slope and watch what reaches the bottom.

Notice how the wetland layer slows the water down and traps much of the soil, so cleaner water comes through. Now try pouring water down the bare tray with no wetland — it rushes straight off, dirty and fast. That is exactly why real wetlands are worth protecting.