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MESOPHOTIC CORAL ECOSYSTEMS – A LIFEBOAT FOR CORAL REEFS?
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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.
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).
Talus
Slope
Spur and groove
Raised Rim
50 m
27 m