Rapid smothering of coral reef organisms by muddy marine snow

Estuarine mud, when resuspended in nutrient-rich near-shore water, aggregat es to marine snow, and within minutes to hours can exert detrimental or eve n 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 Pyrgomatid ae), 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 sus pended mud aggregated into large sticky flocs of marine snow (200-2000 ym d iameter). The organisms responded to a thin coat of deposited flocs with vi gorous cleaning by cirri and tentacle beating. After 5 min struggle, the ba rnacle stopped moving, calanoid copepods were entangled in the aggregates, and thick layers of mucus were exuded by the coral polyps. Both barnacle an d copepods died after < 1 h exposure; a short time compared with natural oc currences of marine snow deposition on coral reefs. Enhanced nutrient conce ntrations are known to contribute to enhance biologically mediated floccula tion. This pilot study suggests that the concentration of suspended mud, an d extent of stickiness and flocculation, can synergistically affect reef be nthos organisms after short exposure. The enclosed macro video recordings c learly visualize these effects, and help convey the important implications for managers: that inshore reefs of the Great Barrier Reef cannot be sustai nably managed without managing the adjacent land. (C) 2000 Academic Press.