A Delicate Balance

When you see black swans gathering across Moreton Bay’s shallow tidal flats, they are often following a food source hidden beneath the surface: seagrass.

The sight of black swans swimming and feeding may seem simple, but the conditions that sustain the seagrass beneath them are anything but. Seagrass health is shaped by a shifting balance between water depth, sunlight, nutrients and sediment—and that balance can produce very different outcomes across Moreton Bay.

Moreton Bay supports seven seagrass species, each with its own growth strategy and capacity to cope with disturbance. Zostera muelleri, known to many as eelgrass, is one of the most widespread and abundant. It commonly grows across intertidal flats and shallow subtidal areas, particularly in the western and southern Bay, where water quality is often poorer and seagrass diversity is lower.

For all seagrasses, light is fundamental. Like plants on land, they need sunlight to photosynthesise. Suspended sediment makes the water turbid, reducing the amount of light that reaches the seabed. The deeper the meadow, the farther that light must travel through the water—and the greater the effect of murky conditions.

A shallow-water advantage

In shallow intertidal areas, Zostera has an important advantage. Even at high tide, the water above it may be relatively shallow, giving sunlight a shorter path through the water column. At low tide, when the meadow is exposed, its leaves can continue photosynthesising while they remain moist.

This does not make the plant immune to stress. Prolonged air exposure, drying, heat and intense sunlight can reduce photosynthetic activity. But its ability to keep photosynthesising at low tide may help explain why Zostera meadows remain productive around the Bay’s shallow edges, while deeper seagrass meadows can decline as turbidity reduces the light reaching the seabed.

Nutrients add another layer to the story

Seagrass meadows can absorb dissolved nitrogen from the water. Following the 2011 floods, nitrogen uptake in Moreton Bay was highest in seagrass meadows closer to the source of the floodwater. Where sufficient light remains, shallow Zostera may be able to make use of some of that additional nitrogen, potentially supporting meadow growth across nearshore tidal flats.

But more is not always better.

Excess nutrients can stimulate phytoplankton and algae, which compete with seagrass for light. Fine sediment can settle directly on the leaves, smothering and shading them. It can also accumulate around the roots and rhizomes, altering oxygen levels and the chemistry of the seabed.

These pressures rarely operate alone. Nutrients, sediment, turbidity, water depth, tides and wave action all interact—sometimes helping a meadow persist and sometimes pushing it beyond its capacity to recover.

When seagrass disappears the effects ripple through the entire food web. These meadows shelter juvenile fish, prawns, crabs, seahorses and countless other small animals, providing food, refuge and nursery habitat among their leaves, roots, rhizomes and surrounding sediments. They also sustain larger animals, including green turtles, dugongs, fish and black swans, which either feed directly on the plants or depend on the life seagrass supports.

Losing these underwater meadows would not mean losing one habitat alone. It could trigger cascading declines across the Bay, stripping away feeding grounds, nursery areas and shelter for species throughout the ecosystem.

This is why runoff cannot be described simply as “good” or “bad” for seagrass. A shallow intertidal meadow may cope with turbid water and absorb additional nitrogen, even as deeper seagrass nearby struggles to receive enough light. Different species, locations, seasons and flood histories can produce very different responses.

There are limits, however. Recent monitoring shows limited recovery and ongoing decline over 25 years in some key seagrass meadows, with catchment-derived sediment remaining one of the most significant pressures on Moreton Bay. Continued increases in sediment and nutrient loads risk shrinking the area in which healthy, diverse meadows can survive.

Black swans are a visible reminder that Moreton Bay’s seagrass meadows are shaped by forces far beyond the shoreline—from tides and rainfall to treated urban wastewater, runoff and sediment flowing down from the catchments.

Keeping the balance in nature’s favour

Protecting these meadows means committing to firm environmental standards for development, controlling sediment and nutrient pollution, improving wastewater treatment and restoring damaged waterways. As A Blueprint for a Sustainable Moreton Bay makes clear, the Bay cannot absorb unlimited pressure.

Where the Zostera grows, black swans follow. Their presence points to a much larger story: a healthy Moreton Bay depends on all its seagrass meadows—shallow and deep, intertidal and subtidal—having the conditions they need to thrive. Human pressures continue to mount; protecting the Bay means working together to restore the balance back in nature’s favour.