Estuaries are created where a river meets the sea. The environment is constantly changing as freshwater draining from the land is mixed with salty water from the incoming tide. Plants and animals have adapted to tolerate these changes in salinity and make use of the calmer waters and nutrient-rich waters.
Estuaries are especially important for young fish in the first years of their lives. The estuaries are shelters from the open seas, creating hiding places for the small fish from predators and storms. The estuaries also offer a range of smaller prey that young fish can feed on. Research has proven that estuaries are so important to the growth and development of young fish, that they have been named ‘nursery areas’ and need special protection.
Find out more about the estuarine habitats below and why they are important nursery areas.
Although they appear lifeless on the surface, mudflats are full of life and play a very important role in sea defenses and nutrient cycling.
The majority of life on mudflats is underneath the surface, where worms and shellfish hide from the waters above. The burrowing animals are vital to the mud ecosystem because of their roles in the circulation of oxygen and nutrients, very much like earthworms in soils.
The physical movement of their burrowing circulates oxygen deep into the mud. When they burrow down, fresh seawater is drawn down into the hole, replenishing the oxygen in the surrounding mud. Many remain under the surface for their entire lives, poking their mouths above the surface to feed from the overlying water whenever the tide comes in. This creates a pathway for nutrients to reach deeper down into the mud. Through this circulation of oxygen and nutrients, the mud is more fertile which allows more life to settle or burrow further down.
On the surface, animals with high movement such as crabs and shrimp survive by moving along the surface, feeding on the worms and shellfish that have emerged to feed. But if the environment is left undisturbed for long enough, small stones or shells can provide opportunities for things to attach to it, like seaweeds or mussels. This in turn invites other invertebrates or small fishes can hide or feed, creating new ‘micro-habitats’.
If mudflats are disturbed, then the worms and shellfish that live close to the surface can become dislodged or injured under the surface, reducing the environment’s ability to cycle nutrients. This will have a knock-on effect for the wider food web, including the young fish as they rely on the mudflats as a source of food.
Saltmarshes are the bridge between land and sea. They are formed at the upper reaches of the incoming tide, where the tidal energy is low enough for roots to hold into the mud. Few plants can survive the constant flooding of salty water over them, but the unique grasses are adapted to thrive in the salty conditions and create a unique habitat.
The dense grasses and deeper creeks create a natural barrier to storms, that absorb much of the wave energy before it reaches the shore. Saltmarsh grasses also remove carbon dioxide (a harmful greenhouse gas) from the air through photosynthesis and store it underground which can help reduce the rate of climate change. Saltmarshes are more effective at this than other habitats because of the wet soils and low activity underneath the surface help to keep the carbon stored below the surface.
Scattered throughout the saltmarsh are creeks, which fill up with water first and take most of the force from the tide. Once full, the creeks overflow and flood the saltmarsh, which changes the habitat entirely. The tall grasses rise up to create a three-dimensional environment where young fishes can hide among the stems to avoid predators and hunt the small invertebrates that have also come out to feed.
Saltmarshes are vulnerable to sediment settling on top of the grasses and smothering them. This reduces the carbon storage, but also reduces the habitat for fishes. If enough saltmarsh is lost, it won’t act as an effective barrier to storms, which could mean flooding of coastal communities and the further destruction of the habitat.