South East Queensland Seagrasses

What are seagrasses?

Seagrasses are flowering plants that have adapted to living in the ocean in temperate and tropical regions. There are about 60 species of seagrasses worldwide. They often grow in large ‘meadows’ that can, at first glance, look like the grass in your backyard. Seagrasses grow in sheltered coastal waters and bays. They anchor their roots in sand or mud in areas with good light.

Seagrasses complete their entire life cycle, including flowering and pollination, under water. Seagrasses need sunlight to produce energy and oxygen (photosynthesis). Seagrass distribution – or how many hectares of seagrass that grow from year to year – is highly dependent on the amount of light that can reach the seagrasses. Three things affect the amount of light available to seagrasses:

  1. Sediment, or soil and dirt particles in the water can make the water murky or ‘turbid’. Sunlight cannot easily filter through turbid water.
  2. The amount of phytoplankton in the water can also reduce the amount of sunlight that reaches seagrasses. Phytoplankton are microscopic floating plants.
  3. Epiphytes or algae that cling to seagrass leaves can also restrict the amount of sunlight that actually reaches the leaves of the seagrasses.

Why are seagrasses important?

Seagrasses play an important ecological role as habitats and nursery areas for young fish. Dugongs and sea turtles also feed on seagrasses. Seagrasses trap and stabilise sediments, preventing erosion in estuaries and providing shelter for fish and invertebrate fauna such as snails and sea urchins.

Seagrass habitat in South East Queensland

Seagrasses are widespread in Moreton Bay and where they grow, or their patterns of distribution, depend on the disturbance caused by water movement, wind, light, overall water quality, and grazing by dugongs and turtles. Boat moorings, anchorages, and damage from boat propellers can also affect patterns of seagrass distribution. Slow growing seagrass species are found in areas with relatively low disturbance, while species that grow rapidly and produce large numbers of seed are more often found in highly disturbed areas. Moreton Bay contains hundreds of dugongs and thousands of green sea turtles. Turtles eat the seagrass leaves, whereas dugongs remove the whole plant. Dugongs prefer the succulent root stalks (rhizomes) of the species known as Halophila and Halodule, which are found growing primarily over the eastern banks of Moreton Bay.

Dugongs and seagrasses

As dugongs graze on seagrasses, organic matter is stirred up, aerating the sediment. This creates good conditions for the fast growing, pioneer seagrass species, which dugongs prefer. The pioneer species are the first species to grow back after a disturbance and they are the fastest growing species. The more dugongs graze, the more the disturbance they create and the more the seagrasses grow. This pattern is called ‘cultivation grazing’.

Protecting seagrasses in Moreton Bay

As turbidity increases and light penetration is reduced, the depth at which seagrass can grow declines. Measuring the depths where seagrass are growing over time provides a good indicator of water quality. Changes in seagrass distribution can be used to map and track changes in environmental characteristics and conditions.

In recent decades there have been substantial changes in the distribution of seagrasses in central and southern Moreton Bay. Since mapping started, there are about 2300 fewer hectares of seagrasses in South East Queensland. This is the size of over 300 Suncorp Stadiums! In some areas, such as around the mouth of the Logan River, the distribution of seagrass meadows is highly variable from year to year, but the overall amount of seagrass does not really change in size. However, there are many areas where seagrasses have declined significantly. These areas include Peel Island; Banana Banks, close to Victoria Point; Pelican Banks at Scarness; the north-west of Coochiemudlo Island; in the southern Broadwater on the Gold Coast; and in the Nerang River.

Research shows that if the amount of sediment entering South East Queensland waterways is not reduced, excessive turbidity will continue to cause loss of seagrass habitat in large areas of Moreton Bay. This will have a huge impact on the commercial fisheries in the region and will also impact marine species, such as dugongs and sea turtles, which rely on seagrasses for their survival.

HLW-8-Dugong Fact

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