By Shearer, Heather
Ecos , No. 119
The intensive agriculture of north Queensland has recently become known for its downstream environmental impacts. Now a new threat is looming. Researchers have recently discovered that a common agricultural herbicide, diuron, is correlated with the severe dieback of common grey mangroves (Avicennia marina), a widespread species integral to coastal ecosystems of the north.
Back in 1993, concerned Mackay fishermen contacted Sunfish Queensland (Queensland's peak recreational fishing body) about the mangrove deaths they were seeing in the region's estuaries. Initially, a major cause was thought to be pollution from a nearby sewage treatment plant. However, Noel Whitehead, a Sunfish representative and member of the Mackay Marine Advisory Committee says, 'articles on the internet about using mangroves to treat sewage in artificial wetlands made me wonder if this was the only cause.'
When, in recent years the situation worsened, Sunfish Mackay and other local conservationists--including the Bird Observers Club--contacted Dr Norm Duke, the Leader of the Marine Botany Group, Centre for Marine Studies at the University of Queensland. Dr Duke, a mangrove specialist, took a research team to Mackay in 2002 to begin investigating the unusual dieback.
After assessing the scale of the problem, Dr Duke concluded 'The dieback is serious. It's the worst case I've seen in 27 years of researching mangroves'.
His research team investigated a number of possible causes including sedimentation, insect damage, disease, herbicides and excess nutrients. They set up transects and plots in the forest, surveyed the affected areas, and used aerial photography and satellite imagery to accurately map the extent of the dieback.
The researchers discovered that the sediments and core water had high levels of agricultural herbicides, especially diuron. Furthermore, levels of diuron correlated with the amount of dieback and poor health of remaining trees and seedlings. No such correlations were found with other factors.
The team then conducted greenhouse trials using high concentration levels of the chemicals, and found, of the four species tested, the grey mangrove was most affected by the herbicides. Of the herbicides tested--diuron, atrazine and ametryn--diuron proved the most lethal. Diuron is also toxic to seagrasses.
Commenting on the study's results, Norm Duke said 'The implications of this are enormous. This is a common chemical and if it causes this reaction to one of the hardiest types of mangrove, then what is it doing to the more sensitive seagrasses and reef systems of the Great Barrier Reef?'
The research project has been controversial. Some organisations have disputed the outcome, arguing that the mangrove deaths arose from excess sedimentation after an unseasonal flood event in 1998. However, Noel Whitehead from Sunfish Queensland says, 'Mangrove deaths are observable on aerial photographs dating as far back as 1991.'
If the mangrove deaths are from diuron--and the evidence seems to indicate this--it is unlikely to be caused by only one industry. Diuron is used in many industries and on boats as an anti-fouling agent. However, landowners in the Mackay region are noted as some of the largest users of diuron in Queensland.
Noel Whitehead says, 'Our major concern is what can we do to alleviate the problem and bring life back into the mangrove forest. We don't know all the answers and neither does anyone' else. …