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Effects of sediment on growth and survival of various juvenile morphologies of the scleractinian coral, Oculina arbuscula (verrill)
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2011
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Description:Sedimentation can play an important role in shaping tropical and temperate benthic marine communities. Here, I quantified sedimentation rates on a hard-bottom reef in the South Atlantic Bight (SAB) off the coast of Georgia, USA and assessed its effects on a conspicuous member of the benthic community: the scleractinian coral, Oculina arbuscula. I addressed the predictions that: 1) inorganic and organic sedimentation rates vary in relation to wind speed and wave height in the SAB, 2) the ability of juvenile O. arbuscula to shed sediment depends on skeletal morphology, and 3) inorganic sedimentation reduces growth and survival of encrusting O. arbuscula juveniles more than those with a branching morphology. The first prediction was tested by quantifying sedimentation rates from July 2008-October 2010 at J-Y Reef, a hard bottom reef at 18-20 m depth ~32 km off the coast of Sapelo Island, GA. Collected sediments were divided into inorganic and organic fractions and compared with wave height and wind speed from the region. Results showed that organic material comprised ~3% of total sediment. Inorganic sediments consistently exceeded 50 mg cm-2 day-1 and were positively correlated with wave height and wind speed. The second prediction was investigated by documenting passive and active sediment shedding abilities in juvenile O. arbuscula with morphologies ranging from encrusting to branching. Laboratory experiments showed that juveniles with greater branching morphology exhibited better passive sediment shedding ability under both low (30 mg cm-2) and high (100 mg cm-2) sediment regimes. However, this morphological advantage was nullified when active sediment shedding mechanisms were included. The third prediction was tested by investigating the relationship between inorganic sedimentation and growth and survival of O. arbuscula juveniles settled on artificial recruitment tiles. Mortality of coral recruits was positively correlated with sedimentation. Juveniles with a branching morphology survived best under sediment stress. Collectively, these results provide evidence that sedimentation can affect the population structure of O. arbuscula via mortality at the juvenile stage and that developing a branched morphology early in life provides a selective advantage for countering the negative impacts imposed by the high sediment loads present on Georgia offshore reefs.
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