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Life in the Turbid Zone: Can Corals Cope with Conditions Long Thought to be Deadly to Them?
Browne, N.K., Smithers, S.G. and Perry, C.T. 2012. Coral reefs of the turbid inner-shelf of the Great Barrier Reef, Australia: An environmental and geomorphic perspective on their occurrence, composition and growth. Earth-Science Reviews 115: 1-20.

Turbid zone reefs, in the words of the authors, "are typically situated in near-shore coastal settings where they may be directly or indirectly exposed to terrigenoclastic sediments through sediment deposition and accumulation, or by sediment resuspension and elevated turbidity." In the first of these instances, they note that high levels of sedimentation can reportedly increase coral mortality by smothering and burial, by reducing larval settlement, and by increasing the prevalence of tissue infections; while in the second instance they indicate that high levels of turbidity can reduce light availability for photosynthesis and energy production. In addition, they state that "nutrient concentrations may also be elevated near the coast due to increased land and river runoff," which phenomena may "threaten reefs by causing macro-algal proliferation, increasing the abundance of bio-eroding filter feeders, and raising the frequency and severity of coral disease."

What was done
In an attempt to gain a better understanding of this host of challenges to coral well-being, Browne et al. reviewed numerous compilations of geological, palaeoecological and ecological data from Australia's Great Barrier Reef (GBR) - which they augmented with data from the Caribbean, Asia and Africa - in order to determine and assess key environmental controls on turbid zone reef occurrence, coral community composition and reef growth over a range of timescales, providing thereby "a synthesis of current knowledge on coral composition and distribution on turbid zone reefs."

What was learned
The three researchers report that "many turbid zone coral reefs actually have live coral cover (>30%) and diversity (>100 species; Veron, 1995; DeVantier et al., 2006), contain temporally stable community assemblages over decadal to centennial timescales (Riegl et al., 1995; McClanahan and Oburu, 1997; Ayling and Ayling, 1999; Perry et al., 2008, 2009), have actively and rapidly accreted (Smithers and Larcombe, 2003; Perry and Smithers, 2006; Palmer et al., 2010; Perry et al., 2012) and are able to recover quickly from periodic setbacks such as flood events and cyclones to which they are regularly exposed (Ayling and Ayling, 2005; Browne et al., 2010)."

What it means
Although sediment deposition, accumulation and resuspension are anathema to many species of coral, they do not pose significant threats to others. This knowledge, however, is by no means an invitation to neglect prudent measures to protect those corals that cannot tolerate such conditions. It merely demonstrates that there are, in point of fact, species of coral that are adapted to a vast array of disparate coastal environments, analogous to the homily that what is one person's trash is another person's treasure.

Ayling, A.M. and Ayling, A.L. 1999. The Dynamics of Cairns Section Fringing Reefs. Unpublished report to the Great Barrier Reef Marine Park Authority. Townsville, Australia.

Ayling, A.M. and Ayling, A.L. 2005. The Dynamics of Cairns and Central Section Fringing Reefs. Unpublished Report to the Great Barrier Reef Marine Park Authrity. Townsville, Australia.

Browne, N.K., Smithers, S.G. and Perry, C.T. 2010. Geomorphology and community structure of Middle Reef, central Great Barrier Reef, Australia: an inner-shelf turbid zone reef subjected to episodic mortality events. Coral Reefs 29: 683-689.

DeVantier, L.M., De'ath, G., Turak, E., Done, T. and Fabricius, K.E. 2006. Species richness and community structure of reef building corals on the near shore reefs of the Great Barrier Reef. Coral Reefs 25: 329-340.

McClanahan, T.R. and Oburu, D. 1997. Sedimentation effects on shallow coral communities in Kenya. Journal of Experimental Marine Biology and Ecology 209: 103-122.

Palmer, S.E., Perry, C.T., Smithers, S.G. and Gulliver, P. 2010. Internal structure and accretionary history of a near-shore, turbid-zone coral reef: Paluma Shoals, central Great Barrier Reef, Australia. Marine Geology 276: 14-29.

Perry, C.T. and Smithers, S.G. 2006. Taphonomic signatures of turbid-zone reef development: examples from Paluma Shoals and Lugger Shoal, inshore central Great Barrier Reef, Australia. Palaeogeography, Palaeoclimatology, Palaeoecology 242: 1-20.

Perry, C.T., Smithers, S.G., Palmer, S.E., Larcombe, P. and Johnson, K.G. 2008. 1200 year paleoecological record of coral community development from the terrigenous inner shelf of the Great Barrier Reef. Geology 36: 691-694.

Perry, C.T., Smithers, S.G. and Johnson, K.G. 2009. Long-term coral community records from Lugger Shoal on the terrigenous inner shelf of the central Great Barrier Reef, Australia. Coral Reefs 28: 941-948.

Perry, C.T., Smithers, S.G., Gulliver, P. and Browne, N.K. 2012. Evidence of rapid coral reef growth under high turbidity and terrigenous sedimentation. Geology 40: 719-722.

Riegl, B., Schleyer, M.H., Cook, P.J. and Branch, G.M. 1995. Structure of Africa's southernmost coral communities. Bulletin of Marine Science 56: 676-691.

Smithers, S.G. and Larcombe, P. 2003. Late Holocene initiation and growth of a near shore turbid-zone coral reef: Paluma Soals, central Great Barrier Reef, Australia. Coral Reefs 22: 499-505.

Veron, J.E.N. 1995. Corals in Space and Time: The Biogeography and Evolution of the Scleractinia. Unniversity of New South Wales Press, Sydney, Australia.

Reviewed 6 March 2013