Mesophotic Coral Ecosystems

3.7. Gulf of Carpentaria, Australia Peter T. Harris , GRID-Arendal, Norway

The Gulf of Carpentaria (Figure 1) is known to contain fringing reefs and isolated coral colonies, but no shallow-water patch or barrier reefs (Veron 1993, 2000). This has, however, not always been the case: expeditions carried out in 2003 and 2005 revealed the presence of a new MCE province covering at least 300 km 2 in the southern Gulf (Harris et al. 2004, 2008). The upper surfaces of the patch reefs are at a mean water depth of 28.6 ± 0.5 m, were undetected by satellites or aerial photographs, and were only recognized using multibeam swath sonar surveys supplemented with seabed sampling and video. Their existence points to an earlier, late Quaternary phase of reef growth under cooler-climate and lower sea level conditions than those of today (Harris et al. 2008). Submerged bank features identified on nautical charts were selected for detailed multibeam sonar surveys in the Gulf (Reefs R1–R7; Figure 1). They exhibited evidence of coral reef geomorphology. drilling and underwater video surveys have confirmed all seven reefs (R1–R7) to be composed of Holocene and Pleistocene coral limestone, which currently supports varying degrees of live coral (Harris et al. 2004, 2008; Figure 2). The reef complex includes several patch reefs (R1–R3 and R6–R7), as well as a submerged barrier reef (R4–R5) extending westwards from Mornington Island (Figure 1). In tropical northern Australia, west of Torres Strait, geomorphic banks on the continental shelf are estimated to cover 44,290 km 2 (Heap and Harris 2008), much of which is potentially submerged coral reef habitat.

Underwater video footage showed that generally the Gulf of Carpentaria MCEs could be classified as either: i) predominantly bare limestone substrate with a consistent, scattered coverage of sessile macrofauna, including single soft corals (alcyonaceans), gorgonians, hard plate corals (e.g. Turbinaria sp.) and sponges; or ii) dominated by a diverse, complex, coral reef-like coverage of macrofauna, consisting of these same species groups, and often interspersed with small patches of bare substrate. Luxuriant framework coral reef growth of Acropora sp., Turbinaria sp., and plate, brain and staghorn corals was observed in a few locations (e.g. Reefs R1, R2 and R6; Harris et al. 2008). Age determinations from coral drill-core samples indicate that reef growth commenced shortly after the Pleistocene pedestals were submerged by rising sea level during the early Holocene (Harris et al. 2008). Coral growth commenced by around 9.9 to 9.5 kyr before present (BP) on all seven of the reefs and persisted for approximately 2,000 years, but had ceased at most locations by circa 8 kyr BP. Based on three measured intervals, reef growth (accretion) rates ranged from 0.95 to 4.0 m kyr -1 . The coral ages and the thickness of Holocene-aged coral limestone deposits encountered in drill cores show that reef growth was widespread in the region during the early Holocene. Underwater video footage indicates that present- day luxuriant framework reef growth is observed only on Reef

Gulf of Carpentaria

Groote Eylandt

Submerged Reefs?

R1

Sir Edward Pellew Group

R7

R2

R6

R5

R3

Submerged reefs?

Depth Key

R4

5 m 10 20 30 40 50 60

Study Area

Mornington Island

Australia

R7

Mesophotic Reefs

0 km

250

100

Karumba

Figure 1. Location map of MCEs in the southern Gulf of Carpentaria, covering over 300 km 2 (although their full extent is unknown). Locations cited in the text are indicated.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 37