Fine sediment budget on an inner shelf island of the Great Barrier Reef

Full description

The deposition and removal budget of fine sediments on the windward and leeward sides of an inner-shelf coral-fringed island (High Island) of the central Great Barrier Reef was examined.Oceanographic instruments were deployed at 3, 7, and 12 m depths during 3-15 January 2005, along two transects (windward=seaward, leeward=landward side). At each of the 6 stations, 2 sediment traps and an Analite nephelometer were mounted 0.8 m above the reef substratum. A current meter was deployed at the two 7 m depths (windward and leeward sides), with the sensors located 0.5 m above the substratum. The current meters measured 1 min-averaged currents and tides every 10 min, as well as a 20-min long burst of wave height data every hour. The nephelometers recorded 10 s averaged data (sampled every 0.5 s) every 5 min and had wipers to prevent biofouling. The sediment traps were replaced daily 3-7 January, then left for one week during a storm.The amount of sediment resuspended was measured using an Analite nephelometer using the upper 90 percentile of the nephelometer readings fromeach 3 min run. The resuspender was run at 3 replicate patches of bare substratum near each mooring site in the week before the storm.
To determine how much riverine sediment is deposited on the windward and leeward sides reef and whether this sediment is flushed out during resuspension events.
The data show that total sedimentation rates were about 2000 mg cm-2 yr-1 at all depths, with 30% to 60% imported from the reef-surrounding waters during calm periods, and the remaining material locally resuspended during storms. Storms resuspended fine sediment at depths less than ~ 5.5 m on the leeward reef side and ~12 m on the windward side. In these shallow waters there appeared to be a net annual sediment balance between import and export by resuspension events. Below these depths, there was no resuspension during storms; further there was a ten-fold increase of the sedimentation rate during storms and most of this additional mud originated from resuspended material from shallower waters on the island slopes. The mud had accumulated on these deeper coral reefs to levels of about 9000 mg DW cm-2, equivalent to ~ 4 years of sediment deposition that is presumably flushed out only during tropical cyclones. The data show that increased rates of advection of fine suspended sediments from soil erosion on land can result in increased sediment accumulation on inshore coral reefs below the depth of storm resuspension.

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Maintenance and Update Frequency: asNeeded

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Credit
Fabricius, Katharina E, Dr (Custodian)

Credit
Wolanski, Eric J, Dr (Principal Investigator)