Corals already under pressure from global climate change are facing an additional threat in the form of pesticides running off from the land, shows a new scientific study published in the journal Marine Ecology Progress Series.
Corals can be harmed by agricultural chemicals at levels so low as to be practically undetectable, a ground-breaking study by scientists at the Australian Institute of Marine Science (AIMS), the ARC Centre of Excellence for Reef Studies (ARC CoE) and James Cook University (JCU) concludes.
Reefs on a global scale are under threat from many sources; one of the most insidious is land based pollutants from agriculture – and the new research indicates that this threat may have been underestimated.
The study measured the sensitivity of the eggs, larvae and adults of the broadcast spawning coral, Acropora millepora, to a number of common pollutants including four classes of agricultural insecticides and a fungicide commonly used in Great Barrier Reef river catchments.
According to AIMS scientist, Dr Andrew Negri the novelty of the study is that it explored the effects of insecticides on many different life stages of the coral.
Normally developing coral embryos at the "prawn chip" stage, 12 hours after fertilisation. Image: Andrew Negri
"Previous studies have focused only on the adults, which seem more robust to insecticides. Our study looked at fertilization, larval development, survival and metamorphosis and we found that some of these stages were very vulnerable to these chemicals at very low concentrations".
Coral embryos exposed to 1 µg/L of the fungicide MEMC. The embryos have not developed normally and will not make it to the larval stage. Image: Andrew Negri
Kathryn Markey, of JCU, continues "Neither fertilization rates nor adult branches were affected by any of the insecticides. In contrast, coral settlement was reduced by between 50 and 100% following 18 hours exposure to very low concentrations of each insecticide".
Dr Andrew Baird of the ARC CoE for Reef Studies says "These developmental stages and events are critical points in the life history of corals. The failure of any one of these events could seriously reduce the ability of coral populations to replenish themselves".
Close up image of a coral branch exposed to 1 µg/L of the fungicide MEMC. The brown tissue is normal but the MEMC has caused tissue death, exposing the white skeleton underneath.
Image: Andrew Negri
In addition, the study found that coral at all life stages was particularly sensitive to the agricultural fungicide MEMC, which caused bleaching in adult corals at levels so low as to be scarcely measurable.
These are some of the most sensitive biological responses to pesticide contamination in the marine environment yet demonstrated. The researchers say the real worry is that the effects of these chemicalswere found at such low levels. In addition, the high sensitivity of coral settlement also suggests current water quality guidelines may not adequately protect all coral life stages.
The team says that both state and federal governments have recognised the pesticide threat to the Great Barrier Reef where up to 80 per cent of the catchment contains some form of agriculture.
The Great Barrier Reef Marine Park Authority, with other government agencies, is currently overseeing the implementation of the "Reef Water Quality Protection Plan", a ten-year $40 million program to halt and reverse the declining quality of water entering the GBR Marine Park by improving land management practices.
However such measures do not necessarily apply elsewhere in the world.
Dr Negri says that the latest IPCC climate change report provides a particular context and reasons for concern about the impact of pesticides on corals: "Corals are already under pressure from rising sea temperatures and pesticides in runoff may be causing additional critical stresses on corals especially during the early life histories such as the larval phase," he says.
*This story was jointly released by AIMS and the Australian Research Council Centre of Excellence for Coral Reef Studies.
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