New constraints on the spatial distribution and morphology of the Halimeda bioherms of the Great Barrier Reef, Australia

Abstract

Halimeda bioherms occur as extensive geological structures on the northern Great Barrier Reef (GBR), Australia. We present the most complete, high-resolution spatial mapping of the northern GBR Halimeda bioherms, based on new airborne lidar and multibeam echosounder bathymetry data. Our analysis reveals that bioherm morphology does not conform to the previous model of parallel ridges and troughs, but is far more complex than previously thought. We define and describe three morphological sub-types: reticulate, annulate, and undulate, which are distributed in a cross-shelf pattern of reduced complexity from east to west. The northern GBR bioherms cover an area of 6095 km 2 , three times larger than the original estimate, exceeding the area and volume of calcium carbonate in the adjacent modern shelf-edge barrier reefs. We have mapped a 1740 km 2 bioherm complex north of Raine Island in the Cape York region not previously recorded, extending the northern limit by more than 1° of latitude. Bioherm formation and distribution are controlled by a complex interaction of outer-shelf geometry, regional and local currents, coupled with the morphology and depth of continental slope submarine canyons determining the delivery of cool, nutrientrich water upwelling through inter-reef passages. Distribution and mapping of Halimeda bioherms in relation to Great Barrier Reef Marine Park Authority bioregion classifications and management zones are inconsistent and currently poorly defined due to a lack of high-resolution data not available until now. These new estimates of bioherm spatial distribution and morphology have implications for understanding the role these geological features play as structurally complex and productive inter-reef habitats, and as calcium carbonate sinks which record a complete history of the Holocene post-glacial marine transgression in the northern GBR.

The evolution of the Great Barrier Reef during the Last Interglacial Period

“…A distinct unit boundary identified in RBR_5 at 24.9 m, below PMSL, represents the antecedent surface of the pre-LIG reef, presumed to be MIS-7 in age (Webster andDavies, 2003, Braithwaite andMontaggioni, 2009). Immediately above this boundary This transition from shallow to deep coralgal assemblages coincides with an increase in Halimeda-rich grainstone, similar in character to Holocene deposits found behind the northern Ribbon reefs of the modern GBR below 20 m depth (Orme et al, 1978, Marshall and Davies, 1988, McNeil et al, 2016. These results are in good agreement with Braga and Aguirre (2004) and Webster and Davies (2003) suggesting the reef was at or close to sea level.…”

Successive phases of Holocene reef flat development: Evidence from the mid- to outer Great Barrier Reef

“…Increased upwelling of cool, nutrient-rich water via shelf break upwelling or tidal jetting in the northern and central regions of the GBR at ~11 ka has been considered responsible for supporting the construction of large Halimeda bioherms behind reef platforms on the mid-outer shelf (Searle and Flood, 1988, Wolanski et al, 1988, Hopley et al, 2007, McNeil et al, 2016. However, only small accumulations of Halimeda have been identified within the Holocene platform reefs themselves with no regional variations in Halimeda abundance in the northern, central or southern reefs Hopley, 1983, Hopley et al, 2007).…”

Some Implications of High Biodiversity for Management of Tropical Marine Ecosystems—An Australian Perspective

“…Coral reefs account for approximately 35,000 km 2 or 10% of the area of the Great Barrier Reef Marine Park [40]. Similar comments can be made about the less studied inter-reefal soft sediment habitats, and the 6000 km 2 of Halimeda meadows and bioherms that have received very little scientific attention in tropical coastal and shallow shelf waters [41]. While cross shelf transects have been carried out along the Great Barrier Reef, to study the impact of trawling on these inter-reefal habitats infauna was not identified and only gross classifications of epifaunal communities were made [42].…”