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Effects of marine snow and muddy terrigenous sediments on the survival of hard coral recruits

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Fabricius, K. E. , Wild, C. , Wolanski, E. and Abele, D. (2003): Effects of marine snow and muddy terrigenous sediments on the survival of hard coral recruits , Estuarine coastal and shelf science , 56 , pp. 1-9 .
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Sedimentation is a major cause of mortality in scleractinian coralrecruits. In this study, we compared the effects of muddy coastalsediments, with and without enrichment by "marine snow" on thesurvivorship of recruits of the hard coral Acropora willisae. Transparentexopolymer particles (TEP) were measured as characteristiccomponents of marine snow using a staining method (Passow&Alldredge1995). Four-week old recruits were exposed to (i) muddy coastal sediments, (ii) TEP, (iii) TEP-enriched muddy coastal sediments, and (iv) unfiltered sea water, for 43 h in aerated flow chambers. Thirty-three percent (± 5 SE) of coral recruits died after 43-h exposure to TEP-enriched muddy coastal sediments (~14 mg cm-2 sediments enriched with 3.8 ± 0.2 mg cm-2 gum xanthan equivalents [GX] TEP). In contrast, no or minimal mortality was observed in the other three treatments. Mortality increased to >80% when the amount of deposited TEP was almost tripled (10.9 ± 1.3 mg cm-2 GX) and sediment increased by 50%. Thus, coral recruits survived short-term exposure to low levels of TEP and low levels of muddy sediments, but sediments enriched with TEP at concentrations recorded at some of the inshore stations proved to be detrimental. Concentrations of TEP were measured around and away from reefs in inshore and shelf regions ???of the central Great Barrier Reef (latitude 16 ? 18° S) in summer, the season of coral spawning and recruitment. Within <10 km off the coast, TEP concentrationswere high (mean = 291 ± 49 SE mg GX L-1, range = 152 - 791 mgGX L-1). Concentrations declined with increasing distance from thecoast, and averaged 83 (± 26 SE) mg GX L-1 around oceanicreefs >40 km off the coast. Our study suggests that both sedimentcomposition and short-term (43 h) sediment deposition affect survivalof coral juveniles, which has implications for the capacity of inshorereefs to be recolonised by corals to recover from large-scaledisturbance events.

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