Estuary environment fact sheet | Field of Mars EEC

What are estuary environments?

Estuary environments form where freshwater from rivers or creeks meets salt water from the sea. Their water is often brackish, which means it is a mix of fresh and salty water. Some estuaries stay open to the ocean, while others open and close naturally.

Estuary environments change with tides, rainfall, waves, currents and storms. They may include open water, tidal channels, mudflats, sandflats, seagrass meadows, mangroves, salt marshes and sheltered shorelines. These features provide habitat for fish, crabs, shellfish, birds and many tiny living things.

In NSW, estuary environments occur along the coast wherever catchment water can meet the sea. They are useful places for fieldwork because students can observe water quality, living and non-living features, food webs, human impacts and environmental change.

An estuary environment.

Estuaries are shaped by freshwater, salt water, tides and coastal landforms.

Where are estuary environments found?

Estuary environments are found along coastlines where rivers, creeks, lakes or lagoons connect with the sea. In NSW, estuaries include drowned river valleys, coastal lakes, lagoons, bays, tidal creeks and river mouths.

Some estuaries stay open to the ocean all the time. Others may naturally close and reopen as waves move sand across the entrance, or after floods push water out to sea. Their water can become fresher after heavy rain and saltier during dry weather or strong tides.

Some estuaries include wide channels, wetlands and sheltered shorelines.

What living and non-living features make up estuary environments?

Living features in estuary environments include plants, algae, plankton, fish, crabs, molluscs, worms, birds and decomposers. Non-living features include water, salinity, tides, temperature, sunlight, oxygen, sediments, rocks, nutrients and currents.

These features change often. Incoming tides bring seawater, while rivers bring freshwater, sediment and nutrients from the catchment. Mudflats may be covered at high tide and exposed at low tide. Plants and animals in estuaries must cope with water that changes in depth, movement, oxygen and saltiness.

Salt marsh and mangroves grow where tides, mud and salt water shape the shore.

What plants and animals live in estuary environments?

Estuary environments can support mangroves, saltmarsh plants, seagrasses, algae and shoreline plants. Animals may include fish, prawns, crabs, shellfish, marine worms, snails, stingrays and waterbirds. Larger marine animals may also enter open estuary channels.

Many young fish and prawns use sheltered mangroves, seagrass meadows and shallow edges as nursery areas. Shorebirds feed on small animals in mudflats and shallow water. Crabs, worms and bacteria help break down plant matter and recycle nutrients through the estuary food web.

Estuary wetlands provide feeding places for birds, fish and other animals.

How do living things depend on estuary environments?

Energy in estuary food webs begins with algae, seagrasses, mangroves, saltmarsh plants and plankton. Dead leaves and plant fragments become detritus, which is broken down by bacteria and eaten by worms, crabs, shellfish and other small animals. These animals then become food for fish, birds and larger predators.

Living things also depend on the structure of estuary environments. Mangrove roots shelter small fish and crabs. Seagrass slows water and provides cover. Salt marshes provide feeding and roosting places. Mudflats hold many small burrowing animals that are important food for shorebirds.

Small crabs help recycle nutrients in muddy estuary habitats.

How are estuary environments connected to Aboriginal Peoples and Country?

Estuary environments are part of Country. Aboriginal Peoples have continuing connections to coastal and estuarine places through knowledge, responsibilities, cultural practice and caring for Country. In the Sydney region, estuaries such as Sydney Harbour, Parramatta River and Lane Cove River connect salt water, fresh water, mangroves, salt marshes, mudflats, plants, animals and people.

For many coastal Aboriginal communities, estuaries have long provided food and materials, including fish, shellfish, crustaceans and useful plants. Shell middens found around coastal and estuary places can show long connections between Aboriginal Peoples and these environments. Local stories, language names, cultural practices and site details belong to the Aboriginal Peoples connected to that Country and should be shared with care.

Estuary environments are connected places of water, plants, animals and people.

Why are estuary environments important?

Estuary environments are important because freshwater, salt water, sediments and nutrients meet there. This mix supports rich food webs and provides nursery areas, feeding places, shelter and migration stopovers for many animals. Estuaries also connect rivers, wetlands and marine environments.

Healthy estuaries help filter sediments and nutrients before water reaches the ocean. Mangroves, salt marshes and seagrasses can reduce erosion, store carbon and help protect shorelines from waves and storms. Estuaries also support recreation, fishing, tourism, learning and cultural values.

Mangroves can create sheltered estuary habitat along urban waterways.

What threatens estuary environments?

Estuary environments can be harmed by coastal development, habitat clearing, stormwater pollution, litter, sewage, excess nutrients, sediment, acid sulfate soils, altered freshwater flows and overfishing. Loss of mangroves, salt marshes and seagrass reduces shelter, feeding places and nursery habitat.

Climate change adds more pressure through sea-level rise, warmer water, ocean acidification, stronger storms and changing rainfall. Because estuaries are low-lying and respond to both catchment water and ocean conditions, changes in one part of the system can affect water quality, habitats and food webs.

Litter and stormwater pollution can affect rivers, estuaries and coastal waters.

How can you help protect estuary environments?

You can help estuary environments by keeping litter out of drains, creeks and waterways, reducing plastic use, staying on tracks and boardwalks, and not trampling mangrove roots, salt marsh or seagrass. Schools can support clean-ups, water-quality investigations, citizen science, careful wildlife observation, planting and weed control led by councils or local groups.

Boardwalks help people visit mangroves without trampling soft mud and roots.

Related fact sheets

Environments and ecosystems

• Ecosystem and environment fact sheets⁠ – Explore more fact sheets about ecosystems and environments.
• River and creek environment fact sheet⁠ – Compare freshwater flows with estuary environments where rivers meet the sea.
• Marine environment fact sheet⁠ – Learn about the saltwater environments connected beyond estuary mouths.
• Mangrove forest ecosystem fact sheet⁠ – Explore a tidal ecosystem that grows along sheltered estuary edges.
• Coastal salt marsh ecosystem fact sheet⁠ – Learn about low salt-tolerant plants in tidal estuary wetlands.
• Beach, dune and rocky shore environment fact sheet⁠ – Compare sheltered estuary shores with more exposed coastal environments.

Habitats

• Habitat fact sheet⁠ – Learn how habitats provide food, shelter and breeding places.
• Water habitats fact sheet⁠ – Explore water habitats used by frogs, insects, birds and other animals.

Plants found in this environment

• Grey mangrove fact sheet⁠ – Learn about a salt-tolerant tree with breathing roots.

Animals in this environment

• Semaphore crab fact sheet⁠ – Discover a crab that lives on mangrove mudflats and tidal flats.
• Invertebrate fact sheets⁠ – Browse invertebrates including crabs and other animals linked to damp habitats.

Field of Mars Reserve and local environments

• Field of Mars Reserve environment fact sheet⁠ – Explore a local reserve connected with creeks, wetlands, mangroves and salt marsh.

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