Sandy soft sediments and seagrass

What are sandy soft sediments? 

Soft sediments are the sandy, silty and muddy habitats on the estuary floor. They may look bare, but they are home to worms, shellfish, tiny crustaceans and other animals that are important food for fish and shorebirds. In shallow areas, seagrass can grow across these sediments and create a more complex habitat.

What is seagrass? 

Seagrass can be described as a flowering plant adapted to live in seawater, with leaves, roots, rhizomes, flowers, fruits and seeds. These grass-like plants adapted for life underwater in coastal environments. Through their root system (rhizomes) they can form extensive seagrass beds or meadows. These highly productive plants provide food and shelter for many fish, birds and other animals and are a critical link in the marine food web. Seagrasses are usually found in shallow water, as like land-plants they need to photosynthesise and are very sensitive to water pollution by sediments and nutrients.

Seagrass meadow (Zostera tasmanica) - image by Olivia Johnson

Why are they important? 

Seagrass and soft sediment habitats in the Derwent Estuary support many animals, including fish, shellfish, worms, crustaceans, black swans and native ducks. They stabilise soft sediments, help trap fine particles, and slow water movement, which can improve water clarity and support a healthier estuary. These habitats may look quiet from the surface, but they are busy feeding and nursery areas.

Where are seagrass meadows found in the Derwent? 

There are a few small seagrass beds in the bays and coves of the middle and lower Derwent Estuary, but the largest areas are found in the region between Cadburys Point and the Granton/Bridgewater area. The Habitat Atlas indicates that there are 6.6 square kilometres of brackish seagrass beds (macrophytes) and 0.2 square kilometres of marine seagrasses remaining in the Derwent Estuary. In some areas, where nutrient levels are high, seagrass beds may become overgrown by thick mats of slimy, green algae.

Aerial image of macrophyte beds in the upper estuary - Google Earth 2026

Did you know?

• Seagrasses are not seaweeds. They are flowering plants with roots, leaves and underground stems.
• You can often see hundreds of black swans and native ducks feeding on seagrass on both sides of the Bridgewater Causeway, particularly during dry summer months.
• Seagrasses can be seen from the air and monitoring seagrass in the Derwent Estuary involved collecting aerial imagery and photo-quadrats in the field. See the report for more details.

In-situ seagrass bed photoquadrat from DEP routine monitoring

Why seagrass matters

• Provide nursery habitat for fish and invertebrates
• Feed birds such as black swans and ducks 
• Stabilise sediments and reduce resuspension
• Improve habitat complexity in otherwise sandy or muddy areas
• Support the estuary food web 
• Can indicate changes in water clarity and nutrient levels

What affects seagrass health?

Seagrass needs sunlight, so anything that makes the water cloudy can reduce its growth.

Key pressures include:

• Sediment washed in from stormwater, sewage, erosion or construction
• Excess nutrients, which can encourage algal growth, smothering the seagrass
• Propeller scarring, anchoring or mooring damage
• Dredging or disturbance of the seabed
• Changes in freshwater flow, salinity or temperature

Things to explore

Learn more about seagrass at Seagrass Watch and consider how seagrasses in Tasmania may differ from tropical areas.