Rapid smothering of coral reef organisms by muddy marine snow

Estuarine mud, when resuspended in nutrient-rich near-shore water, aggregates to marine snow, and within minutes to hours can exert detrimental or even lethal effects on small coral reef organisms. In a pilot study, estuarine mud was suspended in near-shore and off-shore waters of the Great Barrier Reef to a final concentration of 170 mg 1(-1). The short-term responses of a coral (Acropora sp.) and coral-inhabiting barnacles (subfamily Pyrgomatidae), exposed to either near-shore or off-shore water, were microscopically observed and video recorded. In the off-shore water treatment, flocculation was minor, and aggregate sizes were c. 50 mu m. The organisms were able to clean themselves from these small settling aggregates at low siltation (<0.5 mg cm(-2)), and struggled and produced mucus only at high siltation (4-5 mg cm (-2)). In contrast, in near-shore, nutrient-enriched waters, the suspended mud aggregated into large sticky flocs of marine snow (200-2000 ym diameter). The organisms responded to a thin coat of deposited flocs with vigorous cleaning by cirri and tentacle beating. After 5 min struggle, the barnacle stopped moving, calanoid copepods were entangled in the aggregates, and thick layers of mucus were exuded by the coral polyps. Both barnacle and copepods died after < 1 h exposure; a short time compared with natural occurrences of marine snow deposition on coral reefs. Enhanced nutrient concentrations are known to contribute to enhance biologically mediated flocculation. This pilot study suggests that the concentration of suspended mud, and extent of stickiness and flocculation, can synergistically affect reef benthos organisms after short exposure. The enclosed macro video recordings clearly visualize these effects, and help convey the important implications for managers: that inshore reefs of the Great Barrier Reef cannot be sustainably managed without managing the adjacent land. (C) 2000 Academic Press.