Halimeda banks comprise large areas of the northern Great Barrier Reef, inshore of the Ribbon Reefs, and are also found further south (Figure 2.6).13 They have a thin top layer of living macroalgae — predominantly calcareous green algae (Halimeda species) which forms banks when it dies108, typically up to 20 metres thick109,110. They are usually in waters deeper than 40 metres.111 The active calcification and accretion of Halimeda banks over thousands of years is recognised as one of the Reef’s attributes that contributes to its outstanding universal value. As reported in Outlook Report 2009, there remains limited information on the condition and trend of this habitat. Halimeda banks are isolated from landbased impacts and have a high level of protection from trawling through zoning.9 The future condition of Halimeda banks is likely to be affected by declining rates of calcification from changes in ocean chemistry112 and any changes in nutrient upwellings113 because of changes in ocean circulation.
2.3.9 Continental slope
The continental slope is a complex area composed of relic reefs, landslides, canyons and plateaux that extends down to more than 1000 metres.117,118 It comprises approximately 15 per cent of the Region or about 51,900 square kilometres.8 The continental slope supports many species, including some at-risk skates and rays.9,119 There has been little investigation of this remote habitat or the deep-water seabed habitats beyond and no ongoing monitoring.8
Figure 2.6 Locations of Halimeda banks
Areas in the Region containing known Halimeda banks are indicated. The banks are dominated by Halimeda algae, and provide habitat for a range of other species. Those in the north are particularly well developed. Source: Australian Institute of Marine Science 1988114,
Drew and Abel 1988115, Orme and Salama 1988110, Pitcher et al. 200713, Hurrey et al. 2013116
Much of the continental slope remains undisturbed.
A deep-water trawl fishery (from 90 to over 200 metres deep) has been operating in continental slope habitat in the south-eastern part of the Region for several decades, with high levels of fishing effort.9,120 Based on the amount of fishing effort and a lack of knowledge about habitats and associated species, trawling has been graded as presenting a precautionary high ecological risk in the area that includes these fishing grounds.9
2.3.10 Open waters
The Region has a total water volume of around 7200 cubic kilometres.121 This open water habitat is critical to the healthy functioning of the whole Great Barrier Reef ecosystem. It provides connectivity between other habitats, from the coast to beyond the continental slope. Open water is dominated by microorganisms (plankton) and supports a range of other plants and animals such as invertebrates, fishes, reptiles and marine mammals. Inshore areas of open water have been degraded, particularly in the southern two-thirds of the Region. Elevated concentrations of nitrogen and suspended sediment are affecting the overall quality of this habitat122, especially inshore, for a range of dependent species. Offshore and northern open water areas are considered to be in better condition (Figure 6.12). More information on the condition and trends in water quality of the open water habitat is provided in Chapters 3 and 6.
Inshore open water habitats are degraded in the southern two-thirds of the Region.
Open waters connect the Reef’s habitats
© Chris Jones