Introduction

Seagrasses are marine flowering plants that have adapted to growing in muddy, near-shore environments. They form meadows wherever the ocean is shallow enough to allow the sunlight to reach the sea floor. Seagrass meadows are found in river estuaries, along the coast and, if the water is clear enough, way out to sea in waters up to 60 m deep. Recent mapping of the seagrass distribution along the eastern half of the Northern Territory coastline found over 70,000 hectares of intertidal and shallow sub-tidal seagrass beds.

Just like the grasses you may have in your lawn, seagrasses can sprout from spreading underground stems, but they are not true grasses. Of the 60 or so species of seagrass in the world, ten species have been recorded growing in Northern Territory waters.

As long as they remain healthy, seagrass beds filter nutrients and stabilise sediments, helping to keep the water clear. They also support a vast array of marine life. Seagrasses are key habitat for a range of marine species, providing refuges and feeding areas for several species of prawns and fish. Seagrasses are the major food source for the dugong and green turtle. They also support species further up the food chain. The nationally threatened Grey Nurse Shark is one of many species that feed on the schools of fish that live in seagrass meadows.

Dugong trails in seagrass meadows

Photo: © Anthony Roelofs

Issues and threats

When seagrasses are shaded, their growth slows and many plants die. Algal blooms as a result of high levels of nutrients in agricultural or urban runoff are the most significant source of seagrass shading. Seagrasses are also easily smothered by sediment in the water, such as from the outwash from a recently cleared catchment in a heavy downpour. Though seagrass beds are dynamic systems that are often in recovery from the most recent storm or cyclone, they may have trouble persisting if these events become more frequent. Recovery from dredging is even more difficult, as this removes both the seed bank and the plant's entire root system. Other disturbances that can cause long-term damage include trawling and the churning of boat propellers in shallow waters. Rising sea levels may increase the area of available habitat, but seagrass establishment is likely to be prevented by the associated sediment disturbance.

Any loss of seagrass condition may lead to a reduction in populations of fish, turtles, dugong and dolphins, and thus flow on to impact the health of other marine life.

Australian Snubnose Dolphin survive in seagrass beds

Photo: © Marguerite Tarzia

Management

Seagrass beds need protection from over-nutrification, excess sediment flow and physical disturbance. Management on land for seagrass protection should include practices that minimise loss of nutrients, pesticides and sediment into waterways. If land clearing is necessary, leave adequate buffers around streams and ensure land is stabilised before the first wet season storms. When boating, avoid shallow waters where the boat's hull or propeller may disturb the sea bottom, and do not anchor in seagrass beds. Trawl nets should not be used in seagrass beds.

Seagrass Watch provides ranger groups and volunteers with the opportunity to contribute to our understanding of the condition and management needs of seagrass beds.

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