Western Australia has the highest diversity of seagrass species in the world, with 25 species represented, of which 12 are found in the Kimberley (Thalassia hemprichii, Thalassodendron ciliatum, Enhalus acoroides, Halophila ovalis, Halodule uninervis, Halophila minor, Cymodocea angustata, Syringodium isoetifolium, Cyrnodocea serrulata, Halophila spinulosa, Halodule pinifolia and Halophila decipiens). Seagrass are marine flowering plants, which like terrestrial plants, are photosynthetic and need sunlight to survive. The four habitat requirements of seagrasses are a marine environment, adequate substrate to lay down roots, sufficient immersion in seawater and light, to maintain growth.
Seagrass also contributes significantly to carbon capture, trapping approximately 12% of the total marine carbon captured. A new Rapid Assessment Report released in October 2009 by the UN and other scientists, found that of all the biological carbon, or green carbon captured in the world, over half (55%) is captured by marine-living organisms. The scientists coined a new term, “Blue Carbon”. Marine-living organisms range from plankton and bacteria to seagrasses, saltmarsh plants and mangrove forests.
The ocean’s vegetative habitats, in particular, mangroves, salt marshes and seagrasses, cover less than 1% of the seabed. These form the planets blue carbon sinks and account for over half of all carbon storage in ocean sediment and perhaps as much as over 70%. They comprise only 0.05% of the plant biomass on land, but store a comparable amount of carbon per year, and thus rank among the most intense carbon sinks on the planet.
Seagrass meadows grow in estuaries, in the lee of islands and in sheltered bays such as Roebuck Bay, which has some of the most extensive seagrass meadows in the world. The Bay supports an exceptionally high biomass and diversity of benthic invertebrates (estimated to be between 300 500 species), placing it amongst the most diverse mudflats known in the world (de Goeij et al. 2003). At low tide, approximately 160km2 of mudflats are exposed. The most vigourous stands of seagrass occur in areas that are exposed for less than 2 hours at low tide. Seagrass meadows also occur on the Dampier Peninsular, on the reef platforms of One Arm Point and Sunday Island.
Seagrass plants cannot withstand much wave action. The shallower edge of seagrass meadows is limited by water depth and the ability of the plants to withstand breaking waves, and the deeper edge is limited by the availability of light. In the tropics the seagrass may extend into the mangrove systems.
Seagrasses are sometimes known as ‘ecosystem engineers’ because of their ability to create their own habitat. Seagrasses usually grow in soft sediment habitats such as sandy or muddy substrates, depending on their rhizomes or root systems for anchorage. This in turn stablizes the substrate, and provides colonization for macroalgae and invertebrates, which use the leaves and stems as anchors. Seagrass leaf canopies can increase the substrata tenfold, increasing microhabitats comparted to non vegetated areas.
Seagrass meadows act as nurseries for many fish and crustacean species. The seagrass affords protection for the juveniles who also feed on epiphytes and the detritus which seagrass traps. Green Turtles (Chelonia mydas) and Dugong (Dugong Dugon) both feed exclusively on seagrass, preferring Halophila and Holodule genera. These two genera have relatively high species diversity with more than 10 species per genera. Green turtles eat only the seagrass leaves and the dugong eat leaves and roots. At low tide dugong foraging trails can be observed in the shallows.
The loss of seagrass meadows is often caused by human interference such as dredging, and increased nutrient load. Smaller seagrass recover more rapidly from disturbance, but larger species may take decades if recovery is possible.