Effects of transparent exopolymer particles and muddy terrigenous sediments on the survival of hard coral recruits

Abstract

Sedimentation is a major cause of mortality in scleractinian coral recruits. In this study, we compared the effects of muddy coastal sediments, with and without enrichment by ‘marine snow’, on the survivorship of recruits of the hard coral Acropora willisae. Transparent exopolymer particles (TEP) were measured as characteristic components of marine snow using a staining method (Passow & Alldredge, Limnol. Oceangr. 40 (1995) 1326). Four-week-old recruits were exposed to: (1) muddy coastal sediments; (2) TEP; (3) TEP-enriched muddy coastal sediments; and (4) 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 μg 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 μg 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 in 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 concentrations were high (mean = 291±49 SE μg GX l −1, range = 152–791 μg GX l −1). Concentrations declined with increasing distance from the coast, and averaged 83 (±26 SE) μg GX l −1 around oceanic reefs >40 km off the coast. Our study suggests that both sediment composition and short-term (43 h) sediment deposition affect survival of coral juveniles, which has implications for the capacity of inshore reefs to be recolonised by corals to recover from acute disturbance events.