M.T. McCulloch and T. Wyndham
Australia is in the midst of a major drought
with many areas having less than 10% to 30% of normal seasonal
rainfall (Bureau of Meteorology rainfall data to August 2002).
Although the main focus of public attention has been on the loss
of agricultural related GNP, equally devastating impacts may
soon occur in the Great Barrier Reef (GBR). Depending on the
character of drought-breaking floods, major increases in suspended
sediment loads can be anticipated in river runoff from the two
largest river systems that drain into the GBR, the Fitzroy and
Burdekin Rivers. These rivers have large semi-arid catchments
and hence are particularly susceptible to large-scale erosion
due to loss of groundcovers and chronic overstocking that inevitably
occurs during periods of drought. What is the likely impact of
drought-breaking floods on the GBR?
High-resolution Ba/Ca studies of inshore corals
now provide insights into this question. Barium is desorbed from
fine grained particles in the estuarine mixing zone, and then
advected in river flood plumes and finally partitioned into the
coral skeleton. Coral |
records from Havannah Island reef study site
thus show that suspended sediment loads in drought-breaking floods
can be 2 to 3 times greater than in floods that occur during
non-drought periods. Prominent recent examples include the drought-breaking
floods of 1927, 1936, 1968, 1970 and 1988. In contrast, prior
to European settlement, the coral record indicates that floods
generally carry 0.1 to 0.2 lower concentrations of suspended
sediment. The only significant events which are registered in
the coral are the drought breaking floods of 1761, 1765, and
smaller events in 1795 and 1801.
The five to tenfold increase in suspended
sediment entering the inner GBR since European settlement is
likely to have major ecological consequences. The Burdekin River
typically carries ~107 tonnes of sediment of which ~106 tonnes
of fine grained clays will remain entrained in the flood plume
and be dispersed to the inner/mid shelfs. A portion of this is
likely to be carried directly to mid-reef sites such as Rib and
Britomart.. In the inner shelf, supply of sediment available
for later resuspension by wind/wave action will also be replenished.
Fluxes of total P (90% particle bound) and to a lesser extend
N (~40% particle bound) will increase, although particulate P
is not released directly into solution, requiring a more protracted
cycle of anoxic reduction. Terrestrial runoff into coral reefs
needs to be reduced, especially following droughts, if corals
are to survive the dual impacts of direct anthropogenic disturbance
and coral bleaching due to unusually warm ocean temperatures.
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