GREAT BARRIER REEF
// Outlook Report 2014
nursery and breeding sites39; depositional areas for suspended sediments from the water; and physical barriers to storms and weather events.35,40,41 Mangrove habitats are dynamic, with some localised declines and some expansions.34 In contrast to international trends, the overall condition of mangrove forests in and adjacent to the Region is relatively stable and abundance is being maintained.34,35,40,41
2.3.4 Seagrass meadows
Seagrass meadows are an important component of the Reef ecosystem. They are the main food source for dugongs and green turtles; provide nursery habitat for many commercial fisheries species42,43; are a major source of primary production44,45,46 and sequester significant amounts of carbon47. Seagrass meadows also contribute to trapping and stabilising large amounts of sediment48,49 and nutrient cycling50. Seagrass meadows grow in estuaries, shallow coastal waters, and in the lagoon — sometimes in association with coral reefs.51,52,53 Intertidal and shallow subtidal seagrasses (less than 15 metres deep) are estimated to cover approximately 5700 square kilometres.54 Deep-water seagrasses (deeper than 15 metres) are estimated to cover 40,000 square kilometres, although at these depths seagrass generally becomes very sparse (less than five per cent cover).53,55,56,57,58 The earliest ‘baseline’ for the condition and distribution of seagrass is from 1984 to 1988.42,59,60 However this baseline may be shifted as hindcast estimates of dugong populations prior to historical commercial harvesting (Section 2.2.1) suggest far more seagrass would have been needed to support larger dugong populations.3 The Outlook Report 2009 noted the overall area of seagrass meadows was considered to have been relatively stable over the preceding 20 year period. Since then, monitoring of about 30 intertidal seagrass meadows along the central and southern coast indicates that their overall abundance has declined (Figure 2.4). Other indicators of the condition of seagrass meadows such as reproductive effort and nutrient status have also deteriorated. Shallow subtidal seagrass meadows are less extensively monitored, but many sites also show declines in abundance. Examples of intertidal and subtidal meadows declining include Mourilyan Harbour where seagrass meadows had been consistently present since 1993 but have now almost all been lost61, as well as substantial reductions in the meadows adjacent to Cairns52, Townsville62,63 and Gladstone64. Remaining seagrasses are highly vulnerable to further impacts as they have been reduced to small remnant patches and have few seed banks.65 These broadscale losses of seagrass abundance are thought to be mainly due to a combination of acute disturbances (for example significant losses occurred between Cairns and Townsville in 2011 due to physical damage by cyclone Yasi63) and ongoing chronic impacts such as poor water quality65 and extended periods of cloud cover in the wet season (which limits growth through a reduction in light). Intertidal and subtidal seagrass meadows that have been relatively unaffected by disturbances since 2012 are showing early signs of recovery beginning with the return of fast-growing pioneer species.65,66 A more diverse seagrass habitat generally takes a number of years to re-establish.51,63,67,68 The abundance and condition of deep-water seagrass meadows is less studied, and few are routinely monitored. In a number of central and northern parts of the Region these meadows are dominated by pioneer species and can be highly seasonal or annual.69,70 A number of deep-water seagrass meadows affected by floods and cyclones are showing early signs of recovery.71
Figure 2.4 Seagrass abundance score for intertidal seagrass meadows, 1999–2013
Many inshore seagrass meadows have declined since 2009.