Dugong populations in the southern Great Barrier Reef Region have undergone a major population decline over the last 10 years. Anthropogenic influences including capture in fishing nets, hunting, loss of seagrass habitat and local water quality degradation caused by coastal and hinterland development all threaten dugong populations. Pesticides including organochlorine compounds have been extensively applied by Queensland's intensive coastal agriculture industry. The persistent nature of many of these types of compounds raises the potential for continued long-term chronic exposure of Great Barrier Reef seagrasses and dugongs. This is important as organochlorines may affect marine mammal reproduction and immune system functioning, and have been implicated in marine mammal population declines elsewhere. Herbicide runoff from agricultural lands has the potential to impact seagrass, the major food consumed by dugongs.
Analysis of sediments and seagrass collected from 15 intertidal and 52 subtidal sites between Torres Strait and Moreton Bay in a region-wide survey of pollutants indicated that concentrations of herbicide (atrazine and diuron) and insecticide (lindane, dieldrin, DDT, and DDE) contamination were typically low. Contaminants were mainly detected in samples collected in subtidal muds along the high rainfall, tropical coast between Townsville and Port Douglas. Of the contaminants detected, diuron and dieldrin occurred at concentrations high enough to present an environmental risk to local biota. Heavy metal concentrations were generally low and are unlikely to present an environmental risk to local biota. Polychlorinated dibenzo-p-dioxins (PCDDs) were also detectable in sediments from 5 Great Barrier Reef sites sampled selectively for dioxins. These results were unexpected, and provide evidence that an unidentified source for higher chlorinated dioxins exists along the Queensland coast.
The impact of diuron at concentrations present along the Queensland coast on photosynthetic activity in three tropical seagrasses was assessed in the laboratorory using a PAM fluorometer. Exposure ofCymodocea serrulata, Halophila ovalis and Zostera capricomi to low (10 and 100 μg Lˉ¹) diuron concentrations resulted in a decline in effective quantum yield within 2 hours of exposure to the herbicide. Effective quantum yield depression was present for extended periods after plants exposed to 10 and 100 μ g Lˉ¹ diuron for 5 days were returned to fresh seawater. These results indicate that exposure to herbicide concentrations present in nearshore Queensland sediments present a potential risk to seagrasses and, as a consequence, an indirect risk to dugongs.
Samples of liver and blubber were salvaged from 31 carcasses of dugong stranded along the Queensland coast between 1996-2000 and analysed for a range of heavy metals and organochlorine compounds (including dioxins) to assess the role of pollutants in local dugong population declines. Concentrations of metals in livers were generally low and within the range typically found in marine mammals, although elevated concentrations of chromium and nickel were detected in liver samples from several animals collected from the southern Queensland coast. Dieldrin, DDT and/or DDE were detected in 72% of blubber samples. Concentrations of organochlorines were low in comparison to concentrations recorded from marine mammal tissues collected elsewhere in the world, and are unlikely to pose a major threat to Great Barrier Reef dugong populations. In contrast, octachlorinated dibenzodioxin was found at concentrations higher than reported for other marine mammals. Accumulation of dioxins in tissues of dugongs is believed to be associated with sediment ingestion during feeding. Dioxins are known teratogens and carcinogens, and the environmental implications of accumulated dioxin concentrations in dugongs are presently unknown.
The most important consequences of coastal pollutant contamination for Great Barrier Reef dugong populations are likely to be through the bioaccumulation of dioxins and indirectly through impacts of the herbicide to their nearshore seagrass food resource. Future research efforts should be directed at further identifying sources, fates and impacts of dioxins and herbicides in the dugong ecosytem. Maintenance of long-term monitoring programs utilising innovative data acquisition techniques should also be regarded as a priority. Collection of these data will enable assessment of change in concentrations of these pollutants overtime. However, improved land management practices, which include an immediate minimisation of vegetation clearance and responsible use of pesticides in Queensland, are essential if water quality in the Great Barrier Reef World Heritage Area is to be maintained and its populations of dugongs protected.