Light limitation affects coral health more than sediment
Researchers have compared the impact low light and suspended sediment particles have on coral and found that, of the two events associated with dredging, several coral species are more likely to be affected by the loss in light intensity.
The results, undertaken as part of the Western Australian Marine Science Institution’s Dredging Science Node at the AIMS National Sea Simulator in specially developed tank systems, have been published in the Marine Pollution Bulletin.
Lead researcher, Pia Bessell-Browne from The University of Western Australia Oceans Institute, Centre for Microscopy, Characterisation and Analysis and Australian Institute of Marine Science, said corals face increasing pressures from coastal development, particularly through dredging for ports.
“In order to appropriately manage these pressures, we need to improve our understanding of the key impacts of dredging on corals,” Ms Bessell-Browne said. “Sediments released from dredging activities can reduce or block light, clog feeding and smother coral. The sediments can also affect many aspects of coral reproduction and recruitment processes.
“High light levels are considered key to the health of corals as it allows for photosynthesis by algae that live within the coral. The products of this photosynthesis provide the coral with a food source.”
A range of light and suspended sediment concentrations were tested to determine their impacts on coral health.
Representative coral colonies from experimental manipulations that were exposed to no, low and high light, showing decreased colour in no and low light treatments (AIMS)
Exposure to low light conditions was found to result in coral bleaching, where algae that contribute to the nutrition of their partners, leave their coral host. Mortality of corals was only observed in low light levels, regardless of the amount of suspended sediment in the water column.
“When comparing experimental treatments with conditions experienced during dredging and natural sediment re-suspension events, such as storms and tidal currents, the results suggest that light reduction resulting from increased suspended sediments poses more of a risk to corals than suspended sediments alone,” Ms Bessell-Browne said.
Further work will determine the low light thresholds for corals.
Bessell-Browne P, Negri A.P., Fisher R, Clode P.L., Duckworth A, Jones R, Impacts of turbidity on corals: The relative importance of light limitation and suspended sediments Marine Pollution Bulletin 2017 http://dx.doi.org/10.1016/j.marpolbul.2017.01.050
The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.