Impacts of turbidity on corals: The relative importance of light limitation and suspended sediments.

As part of an investigation of the effects of water quality from dredging/natural resuspension on reefs, the effects of suspended sediment concentrations (SSCs) (0, 30, 100mgL-1) and light (~0, 1.1, 8.6molphotonsm-2d-1) were examined alone and in combination, on the corals Acropora millepora, Montipora capricornis and Porites spp. over an extended (28d) period. No effects were observed at any sediment concentrations when applied alone. All corals in the lowest light treatments lost chlorophyll a and discoloured (bleached) after a week. Coral mortality only occurred in the two lowest light treatments and was higher when simultaneously exposed to elevated SSCs. Compared to water quality data collected during large dredging programs and natural resuspension events (and in the absence of sediment deposition as a cause-effect pathway) these data suggest the light reduction associated with turbidity poses a proportionally greater risk than effects of elevated SSCs alone.