| Abstract | This report discusses the results of an investigation to establish a potential cause of the mangrove dieback that has occurred within the Pioneer River estuary and Bassett Basin, Mackay, Central Queensland from 1998 to 2001. Only one mangrove species, Avicennia marina, is apparently affected by dieback within this area. The principal objective of this study was to assess the hypothesis that a flood event and subsequent deposition of mud and silt in the mangrove communities was a likely cause of the dieback. This report refutes the argument that the most likely cause of the dieback is herbicide (diuron and ametryn) use within the Pioneer River catchment. Observations of mangrove communities in the Pioneer River estuary made by one of the authors in 2001 indicated that large amounts of mud and silt had been deposited in the estuary and had buried Avicennia marina pneumatophores in dieback areas. Burial of pneumatophores is a commonly reported cause of Avicennia spp. dieback in other parts of Australia and overseas. Recent and historical aerial photographs of the Pioneer River estuary were examined to determine the temporal and spatial extent of the current mangrove dieback and any past dieback events within the estuary. The current dieback event appears to have commenced no earlier than June 1998 and was clearly visible in November 1999 photographs. Other historical cases of mangrove dieback in the estuary have been documented. Flood records from 1884 to 2000 at Mackay were examined and found to correlate with the onset of the present dieback event. As a result of this analysis, an unseasonal flood event in August 1998 was deemed to be a likely cause of the current dieback. Rainfall and stream height data for 1998, especially for August of that year, were analysed to determine the source of these floodwaters. Intense rainfall had occurred across the whole catchment over a three day period in late August 1998, resulting in high river flows across the catchment and within the Pioneer River estuary. Examination of aerial photographs of the Brisbane River, Queensland, revealed that a major flood in 1974 had caused similar mangrove dieback in that river, most likely by depositing mud and silt in the mangrove communities. |
