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Seagrass

Seagrass is often first noticed as piles of decaying plant material washed up on foreshores within the marine park. But where does it come from and what is it?

Seagrasses are sometimes mistaken for grass or seaweed, but they are really aquatic flowering plants. In fact, they are more closely related to lilies and gingers than grasses. They have tiny flowers and strap-like or oval leaves and form meadows in estuaries and shallow coastal waters. Unlike the majority of flowering land plants which have male and female parts on the one plant, most seagrasses have separate male and female plants. They can reproduce by seed and they grow by sending new leaves up from a rhizome buried in the sand or mud. Their roots and vascular tissue allow them to absorb nutrients from the soft sediments on the sea floor. 1 This sets them apart from seaweeds which lack this vascular root system.

Essential to marine life

Seagrass is central to the marine web of life. Only a few animals - dugong, green turtles, sea urchins and some fish - have the ability to digest cellulose and feed directly on the leaves themselves. However, seagrass leaves also support an array of attached seaweeds and tiny filter-feeding animals. These organisms provide food for small fish, which are then eaten by larger fish.

Dead seagrass also provides food for worms, sea cucumbers, crabs and filter feeders such as anemones. As seagrasses decompose, the release of dissolved nutrients supports the growth of phytoplankton and other marine plants which, in turn provides food for fish and other species.

Seagrass meadows provide important habitat for many commercially and recreationally important species of fish and prawns. They also recycle nutrients, trap sediments and stabilise the sea floor. Seagrasses are very sensitive to changes in water quality and are used to measure ecosystem health. All seagrasses, dead and alive, are protected under the Fisheries Act 1994.

How does seagrass grow?

Seagrasses need light and nutrients to grow. Nutrients are obtained from sediment and water. They have the highest light requirements of all the submerged marine plants and cannot easily grow in murky water. They thrive in shallow coastal waters sheltered from wave action and strong currents which can erode their roots. Although they are normally found in shallow water they can grow at depths of 32 metres and have been found in clear water at 68 metres.

Seagrass in Moreton Bay Marine Park

Seagrass meadows form one of the most widespread marine habitats. They occur in the coastal waters around all of the world's continents, except Antarctica. 2 There are approximately 60 species of seagrass worldwide, of which seven are found in Moreton Bay Marine Park. These species represent all three of the global seagrass taxonomic families. Some seagrass species in the marine park are not found any further south on the east coast. 3

The distribution of seagrass in Moreton Bay Marine Park is primarily determined by the amount of sunlight that reaches the sea floor which, in turn, depends on the turbidity (or murkiness) of the water. In the western bay, where the water is more turbid, seagrass is restricted to shallower areas. In Bramble Bay, virtually no light reaches the seafloor so no seagrass grows. 4 On the eastern side of the bay two seagrass species, Halophila spinulosa and H. ovalis, have been recorded in water depths up to 12 metres. At this depth, only six percent of the surface light reaches the sea floor, which is just enough for this species to survive. 5

Seagrass plays a critical role in the health of the Moreton Bay Marine Park. 6 It provides habitat for many of the marine park's species and is the primary food source of dugong and some marine turtle species.

Pressures

The distribution and health of seagrass in Moreton Bay Marine Park is under threat. Since the late 1980s there have been several documented losses of seagrass in Moreton Bay. The most notable area of loss is in southern Deception Bay where some 2000ha of seagrass habitat has been lost, however there have also been losses in Bramble Bay and near the mouth of the Logan River.

Turbid water caused by sediment runoff from the land and waves and tidal action is threatening the marine park's seagrass in the western bay. Blooms of the cyanobacteria Lyngbya majuscule, which are occurring more frequently in the marine park, can also lead to seagrass loss. During these blooms, the lyngbya forms large mats which smother the seagrass, cutting off its essential light source and eventually killing it. Seagrasses are also threatened by direct physical disturbance such as dredging, bait gathering and vessel grounding.

More information

Further reading

1 Waycott, M., McMahon, K., Mellors, J., Calladine, A. and Kleine, D. (2004) A guide to tropical seagrasses of the Indo-West Pacific, James Cook University, Townsville

2 Green, E.P. and Short, F.T. (2003) World atlas of seagrasses, University of California Press

3 Waycott, M., McMahon, K., Mellors, J., Calladine, A. and Kleine, D. (2004) A guide to tropical seagrasses of the Indo-West Pacific, James Cook University, Townsville

4 Ibid.

5 Udy, J. and Levy, D. (2002) Deep seagrass and coral habitats found in Eastern Moreton Bay, University of Queensland, Brisbane

6 Dennison, W.C. and Abal, E.G. (1999) "Moreton Bay study: A scientific basis for the Healthy Waterways campaign", SEQ Regional Water Quality Management Strategy, Brisbane

7 Ibid.

8 Ibid.

Related information



Moreton Bay Marine Park, our bay our future

Seagrass is central to the marine web of life, providing essential habitat and food for many of the marine park's species.

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Moreton Bay Marine Park seagrass

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Last reviewed
20 March 2015
Last updated
11 May 2007