Dredging to maintain safe/navigable shipping channels creates plumes of suspended sediment that can migrate over nearby sensitive habitats, such as coral reefs. Under such circumstances, proactive management may be employed (e.g. altering dredging practices) to minimize any potential impacts. This relies on suitable guidelines or threshold values which can relate changes in environmental quality associated with the plumes with biological response(s) of the underlying communities. For photoautotrophs a well-known cause-effect pathway for guideline development is the attenuation of light by scattering and absorption. The aim of this work is to better understand the relationship between changes in light and coral health for guideline development using an exposure characterisation from a field-based study, and a hazard characterization from a series of laboratory-based studies.
Above/below water light was measured over a 3-year period in coral reef communities of the inner, central Great Barrier Reef. The time series was analysed over multiple running mean time periods (days to weeks) and percentile values calculated to characterise the light reduction levels the corals experienced naturally during sediment resuspension events, from clouds, and from nearby dredging activities. Multi- and hyper-spectral light sensors were used to quantify changes in the spectrum (quality) of underwater light. A multiple component empirical optical model was developed and used to define the exposure conditions of a series of aquarium-based experiments where different coral species were subjected to a range of light attenuation levels over time. Lethal and sublethal endpoints (density of algal symbionts, growth, lipid concentrations and ratios) were used to define the pressure-response relationship. The results are discussed in terms of accurately measuring light underwater, identifying light reduction by sediments as opposed to clouds, and the development of light-based thresholds for reactive monitoring and management plan when dredging near reefs.