Big Bank Shoals of the Timor Sea
An environmental resource atlas

FILTER-FEEDING ECOSYSTEMS

Filter-feeding banks of the Big Bank Shoals

Previous work on filter-feeding communities in the region

Little scientific research has been undertaken on the gorgonians of Indo-Pacific waters. Sampling of these assemblages is difficult, since most epi-benthic communities dominated by these filter-feeders are in deeper water. Furthermore, the paucity of specialists in these taxa has tended to limit the collection effort for gorgonians by the Northern Territory (Russell and Hanley, 1992) and the Western Australian Museums (Berry, 1986, 1993) in north-western Australia.

Collections of sponges have been relatively extensive in the Timor Sea region (compared to gorgonia), (van Soest, 1989; Hooper, 1994). Large collections have been made in both shallow coastal waters and areas on the Continental Shelf and Slope (see Hooper, 1994). Most results of these studies have remained unpublished. Sponge collections, documenting 138 species, have been undertaken on the emergent reefs (Ashmore, Cartier and Hibernia) in the Timor Sea (Hooper, 1994). The assemblages varied remarkably between reefs. For example, Hooper (1994) found that only 13 percent of species were common between Hibernia and Ashmore Reefs (35 km apart).

Figure 36. Map of the Big Bank Shoals showing the distribution of Halimeda, encrusting sponges, hard and soft corals, and gorgonians.

General aspects of these filter-feeding communities

The benthos of Sneezy, Wicked and South Banks are likely to be light-limited to varying degrees. Reef building Halimeda and hard corals that have been found to dominate most banks of the Shoals are rare or absent on these banks (Figure 38, p67). Given the depth of Sneezy and Wicked Banks, and the lack of reef-building taxa, it seems that these banks are drowned reefs that have been unable to keep pace with rising sea levels since the last ice age.

South Bank has a distinctly larger and more complex benthic ecosystem than Wicked or Sneezy, but is linked to the other two banks by a benthic fauna dominated by a diverse assemblage of filter-feeders. These fauna are heterotrophic and extract their food from the surrounding waters. In particular, a mixture of sponge types (Figure 36, p65) and soft corals (Figure 37, p66), such as gorgonians, were common features on these three banks.

Whereas most hard corals require light to produce energy, gorgonians and many soft corals filter plankton from the water column. Consequently these taxa can flourish in deeper, darker waters such as that found at Sneezy and Wicked Banks. Gorgonians are highly prevalent on these banks while hermatypic corals and macro-algae are rare (Figure 38, p67). For example, Sneezy Bank, a submerged pinnacle that plateaus to 100 metres beneath the sea surface, is covered in gorgonians (up to 30 percent of the substratum), (see film strips FF01 to FF04, p66) while scleractinian corals are absent. Filter-feeders dominate the deep water, light-limited habitats on Sneezy and Wicked. Overall cover of these deep water banks is relatively low compared to the shallower banks of the area. Wicked Bank has an overall live cover of 20 percent, while the Halimeda banks generally have an epi-benthic coverage greater than 50 percent (see Halimeda ecosystems, p20). The dominant invertebrates are sea fans and whips, the soft coral Dendronephthea sp. and a range of sponges. The gorgonian fans can reach heights of up to two metres on these banks. Many have filter-feeding feather stars or crinoids living upon them. Interspersed amongst these fans are areas of numerous sea whips (Junceella sp.) and small colourful sponges (see film strips FF05 and FF06, p68). Some very large vase-shaped sponges were also observed at both banks.

Figure 37. The percent coverage of the major benthic categories of the filter-feeding banks.

While overall cover is relatively low at Sneezy and Wicked, the sampling method is likely to underestimate the true cover of gorgonians. The ROV videos the substratum (and the epi-benthic community growing upon it). However, gorgonians grow as fans or whips perpendicular to this substratum. For example, Sneezy Bank has an overall coverage of gorgonians of 25 percent, however large fans (greater than 1 metre in diameter) were observed at approximately 1 metre intervals, indicating that these communities are very rich in terms of overall abundance.

Figure 38. Map of the Big Bank Shoals showing the distribution of all major benthic groups.

Gorgonians, being sessile filter-feeders, require the currents to bring plankton to them and are consequently found in sites which have higher water movement. Since the fans orientate perpendicular to the prevailing current, the direction of the current at Sneezy and Wicked Banks can be implied. Fans growing upon these banks face towards the south-southwest, indicating that the water is from this direction. Interestingly, shallow water currents recorded at other locations around the Big Bank Shoals seem to run in an east-northeast direction (see Currents, p13). However, complex localised water eddies may cause currents to be different compared to the region as a whole.

Individual banks

Among the banks surveyed, Sneezy, South and Wicked Banks were dominated by filter-feeders. Both sponges and gorgonians made significant contributions to the epi-benthic communities found on these plateaux. They are described in more detail below.

Sneezy Bank
Wicked Bank
South Bank

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Last updated - 1 September 98

Copyright ©1996-1998 Australian Institute of Marine Science

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