The hydrodynamic and sedimentary setting of nearshore coral reefs, centralGreat Barrier Reef shelf, Australia: Paluma Shoals, a case study

The Great Barrier Reef (GBR) shelf contains a range of coral reefs on the h ighly turbid shallow inner shelf, where interaction occurs with terrigenous sediments. The modern hydrodynamic and sedimentation regimes at Paluma Sho als, a shore-attached 'turbid-zone' coral reef, and at Phillips Reef, a fri nging reef located 20 km offshore, have been studied to document the mechan isms controlling turbidity. At each reef, waves, currents and near-bed turb idity were measured for a period of approximate to1 month. Bed sediments we re sampled at 135 sites. On the inner shelf, muddy sands axe widespread, wi th-admixed terrigenous and carbonate gravel components close to the reefs a nd islands, except on their relatively sheltered SW side, where sandy silty clays occur. At Paluma Shoals, the coral assemblage is characteristic of i nner-shelf or sheltered habitats on the GBR shelf (dominated by Galaxea fas cicularis, up to >50% coral cover) and is broadly similar to that at Philli ps Reef, further offshore and in deeper water. The sediments of the Paluma Shoals reef Rats consist of mixed terrigenous and calcareous gravels and sa nds, with intermixed silts and clays, whereas the reef slope is dominated b y gravelly quartz sands. The main turbidity-generating process is wave-driv en resuspension, and turbidity ranges up to 175 nephelometric turbidity uni ts (NTU). In contrast, at Phillips Reef, turbidity is <15 NTU and varies li ttle. At Paluma Shoals, turbidity of >40 NTU probably occurs for a total of >40 days each year, and relatively little time is spent at intermediate tu rbidities (15-50 NTU). The extended time spent at either low or high turbid ities is consistent with the biological response of some species of corals to adopt two alternative mechanisms of functioning (autotrophy and heterotr ophy) in response to different levels of turbidity. Sedimentation rates ove r periods of hours may reach the equivalent of 10 000 times the mean global background terrigenous flux (BTF) of sediment to the sea floor, i.e. 10 00 0 BTF, over three orders of magnitude greater than the Holocene average for Halifax Bay of <3 BTF. As elsewhere along the nearshore zone of the centra l GBR, dry-season hydrodynamic conditions form a primary control upon turbi dity and the distribution of bed sediments. The location of modern nearshor e coral reefs is controlled by the presence of suitable substrates, which i n Halifax Bay are Pleistocene and early Holocene coarse-grained (and relati vely stable) alluvial deposits.