Impact of drought on sediment fluxes entering the Inner Great Barrier Reef

Impact of drought on sediment fluxes entering the Inner Great Barrier Reef

 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.

Figure 17: Plot Showing the Ba/Ca ratios (ie suspended sediment loads) versus Burdekin River flow. Floods that follow droughts generally have much higher Ba/Ca ratios consistent with greater erosion for larger magnitude drought breaking floods.