Mesophotic Coral Ecosystems

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?

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

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

(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

3.7.

Gulf of Carpentaria, Australia

Peter T. Harris

, GRID-Arendal, Norway

Figure 1.

Location map of MCEs in the southern Gulf of Carpentaria, covering over 300 km

(although their full extent is unknown).

Locations cited in the text are indicated.

Karumba

Mornington

Island

Groote

Eylandt

0 km

250

Mesophotic Reefs

5 m

Depth

Key

Australia

Study Area

Submerged reefs?

Submerged

Reefs?

100

Gulf of Carpentaria

Sir Edward

Pellew

Group