Responses of corals to chronic turbidity.

Dredging increases suspended sediment concentrations (SSCs), causing elevated water turbidity (cloudiness) and light attenuation. Close to dredging, low light periods can extend over many days, affecting phototrophic epibenthic organisms like corals. To improve the ability to predict and manage dredging impacts, we tested the response of corals to an extended period of elevated turbidity using an automated sediment dosing system that precisely controlled SSCs and adjusted light availability accordingly. Replicates of four common species of corals encompassing different morphologies were exposed to turbidity treatments of 0-100 mg L-1 SSC, corresponding to daily light integrals of 12.6 to 0 mol quanta m-2 d-1, over a period of ∼7 weeks. Symbiotic dinoflagellate density and algal pigment concentration, photosynthetic yields, lipid concentrations and ratios and growth varied among the turbidity treatments, with corals exhibiting photoacclimation within low turbidity treatments. A range of physiological responses were observed within the high turbidity treatments (low light), including bleaching and changes in lipid levels and ratios. Most corals, except P. damicornis, were capable of adjusting to a turbidity treatment involving a mean light level of 2.3 mol photons m-2 d-1 in conjunction with a SSC of 10 mg L-1 over the 7 week period.

Sediment characteristics influence the fertilisation success of the corals Acropora tenuis and Acropora millepora.

Elevated suspended sediment concentrations (SSCs) often impact coral fertilisation success, but sediment composition can influence effect thresholds, which is problematic for accurately predicting risk. Here, we derived concentration-response thresholds and cause-effect pathways for SSCs comprising a range of realistic mineral and organic compositions on coral fertilisation success. Effect concentration thresholds (EC10: 10% fertilisation inhibition) varied markedly, with fertilisation highly sensitive to inshore organic-clay rich sediments and bentonite clay at <5 mg L-1. Mineral clays and organic matter within these sediments likely promoted flocculation of the coral sperm, which in turn reduced fertilisation. In contrast, sediments lacking these properties bound less sperm, leading to higher SSC thresholds for coral fertilisation (EC10 > 40 mg L-1). The effect thresholds for relevant sediment types were combined with in situ turbidity data from locations near dredging operations to assess the risks posed by dredging to coral fertilisation at these locations.

Coral morphology and sedimentation.

The sediment rejection ability of 8 coral species of 5 families and 3 morphologies were assessed in a series of short term exposure tests over a sedimentation range of 0.5-40mgcm-2d-1 and one longer term exposure test of 235mgcm-2. Sediment accumulation rates on live corals and dead (enamel-covered) skeletons varied between morphologies, with branching species often more adept at self-cleaning. Flow rates (0-17cms-1) significantly affected sediment-shedding ability as did differences in particle sizes, with coarse silt rejected faster than fine silt, but only at very high (235mgcm-2) deposition rates. Siliciclastic sediment was rejected faster than carbonate sediments and smothering for many days by mms of low organic content carbonate sediment resulted in bleaching, but no mortality. The findings are discussed with respect to turbidity generated in natural and dredging-related resuspension events and in the context for impact prediction for dredging projects.