Mesophotic Coral Ecosystems

Talus Slope

Raised Rim

27 m

Spur and groove

50 m

Figure 2. Three-dimensional view of submerged coral reef R1 in the Gulf of Carpentaria. The reef is approximately 10 km wide and covers 72 km 2 . Water depths on the reef platform average 27 m, compared with 50 m in the surrounding areas. The reef exhibits classical reef geomorphology, including a raised rim, flat lagoon and spur and groove edges. A talus slope on the southern edge of the reef is attributed to the transport of cyclone sediment (Harris and Heap 2009).

R1, R3 and R6. Late Holocene reef growth has been measured only on Reef R6, which could indicate that part of this reef has regenerated, while the other reefs have not. Since circa 8 kyr BP, coral growth has not resulted in the deposition of significant amounts of framework reef limestone, and the reefs have remained submerged 14 to 30 m below present sea level. The relatively thin veneer of Holocene limestone has been bored by benthic animals and the reef limestone has become cemented to form local hard ground deposits. Elsewhere, there is no evidence of any Holocene reef deposition, which implies either local absence of reef growth (non-deposition) and/or erosion. Interestingly, Holocene reef growth was not always located on the reef crests (e.g. Reefs R4 and R5). The typical pattern of growth is initiation on the slightly raisedmarginal rims and then, soon after, growthwithin the central platform and crests. The underlying Pleistocene limestone is exposed at the surface at many locations, and hence the morphology of the reefs is mostly relict, and is the product of several episodes of reef growth during the Pleistocene.

A number of factors could have limited regional reef development, including the rate of reef growth relative to sea level rise, water temperature variations, changes in water turbidity, and/or a reduction in larval dispersion due to changes in water circulation (e.g. Montaggioni 2005). Tropical cyclones regularly impact the Gulf of Carpentaria region and substantial talus sediment deposits have accumulated against the down-drift side of the patch reefs (Harris and Heap 2009). Cyclones and their associated sediment transport events (and turbidity) may also limit regional reef growth. The Gulf of Carpentaria mesophotic reefs R1 to R5 are programmed to receive protection by the Australian government, by being included within a Commonwealth Marine Reserve (Department of Environment 2015). However, until the current review of the marine reserves by the Australian government is completed, the MCEs are not protected. Moreover, the MCEs around the Sir Edward Pellew Group (i.e., Reefs R6 and R7) are not included in the proposed Commonwealth Marine Reserve.

MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS? 38

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