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Relative risks to the Great Barrier Reef from degraded water quality

The greatest water quality risks to the Great Barrier Reef are from nitrogen discharge, associated with crown-of-thorns starfish outbreaks and their destructive effects on coral reefs, and fine sediment discharge which reduces the light available to seagrass ecosystems and inshore coral reefs. Pesticides pose a risk to freshwater and some inshore and coastal habitats.

A combination of qualitative and semi-quantitative assessments was used to estimate the relative risk of water quality constituents to Great Barrier Reef ecosystem health from major sources in the catchments, focusing on agricultural land uses. Risk was defined as the area of coral reefs and seagrass meadows within a range of assessment classes (very low to very high relative risk) for several water quality variables in each natural resource management region.

The variables included:

  • ecologically relevant thresholds for concentrations of total suspended solids and chlorophyll a from daily remote sensing observations
  • the distribution of key pollutants including total suspended solids, dissolved inorganic nitrogen and photosystem II inhibiting herbicides in the marine environment during flood conditions (based on end-of-catchment loads and plume loading estimates)
  • a factor related to water quality influences on crown-of-thorns starfish outbreaks was included for coral reefs.

The main finding was that increased loads of suspended sediments, nutrients (nitrogen and phosphorus) and pesticides all pose a high risk to some parts of the Great Barrier Reef. However, the risk differs between the individual pollutants, the source catchments and the distance from the coast.

Summary of evidence

  1. Overall, nitrogen poses the greatest risk of pollution to coral reefs from catchments between the Daintree and Burdekin Rivers. Runoff from these rivers during extreme and early wet seasons is associated with outbreak cycles of the coral-eating crown-of-thorns starfish on the northern Great Barrier Reef shelf (15 to 17 degrees south) that subsequently generate secondary outbreaks throughout the central Great Barrier Reef. Great Barrier Reef-wide loss of coral cover due to crown-of-thorns starfish is estimated to be 1.4 per cent per year over the past 25 years, and a new outbreak is underway. It is estimated that crown-of-thorns starfish have affected more than 1000 of the approximately 3000 reefs within the Great Barrier Reef over the past 60 years.
  2. Of equal importance is the risk to seagrass from suspended sediments discharged from rivers in excess of natural erosion rates, especially the fine fractions (clays). Whether carried in flood plumes, or resuspended by waves, suspended solids create a turbid water column that reduces the light available to seagrass and corals. High turbidity affects approximately 200 inshore reefs and most seagrass areas. Seagrass loss severely impacts green turtle and dugong populations. On a regional basis, the Burdekin and Fitzroy regions present the greatest risk to the Great Barrier Reef in terms of sediment loads.
  3. At smaller scales, particularly in coastal seagrass habitats and freshwater and estuarine wetlands, pesticides can pose a high risk. Concentrations of a range of pesticides exceed water quality guidelines in many fresh and estuarine water bodies downstream of cropping lands. Based on a risk assessment of the six commonly used photosystem II inhibiting herbicides, the Mackay Whitsunday and Burdekin regions are considered to be at highest risk, followed by the Wet Tropics, Fitzroy and Burnett Mary regions. However, the risk of only a fraction of pesticides has been assessed, with only six of the 34 pesticides currently detected included in the assessment, and therefore the effect of pesticides is most likely to have been underestimated.
  4. The ranking of the relative risk of degraded water quality between the regions in the Great Barrier Reef is (from highest risk to lowest):
    • Wet Tropics
    • Fitzroy
    • Burdekin
    • Mackay Whitsunday
    • Burnett Mary
    • Cape York.
    Priority areas for managing degraded water quality in the Great Barrier Reef are Wet Tropics for nitrogen management; Mackay Whitsunday and the lower Burdekin for photosystem II inhibiting herbicide management; and Burdekin and Fitzroy for suspended sediment management.
  5. From a combined assessment of relative risk of water quality variables in the Great Barrier Reef (using the total area of habitat affected in the areas identified to be of highest relative risk) and end-of-catchment anthropogenic loads of nutrients, sediments and photosystem II inhibiting herbicides, the regional ranking of water quality risk to coral reefs is (from highest risk to lowest):
    • Wet Tropics
    • Fitzroy
    • Mackay Whitsunday
    • Burdekin
    • Cape York
    • Burnett Mary.
    The regional ranking of water quality risk to seagrass is (from highest risk to lowest):
    • Burdekin
    • Wet Tropics
    • Fitzroy
    • Mackay Whitsunday
    • Burnett Mary
    • Cape York.
    Importantly in the Mackay Whitsunday region, 40 per cent of the seagrass area is in the highest relative risk class compared to less than 10 per cent for all other regions. The highly valuable seagrass meadows in Hervey Bay, and the importance to associated dugong and turtle populations in the Burnett Mary region, were not included in the ranking analysis, as they are outside the Great Barrier Reef Marine Park boundaries.
  6. Both dissolved (inorganic and organic) and particulate forms of nutrients discharged into the Great Barrier Reef are important in driving ecological effects. Overall, increased nitrogen inputs are more important than phosphorus inputs. Dissolved inorganic forms of nitrogen and phosphorus are considered to be of greater concern than dissolved organic and particulate forms as they are immediately bioavailable for supporting algal growth. Particulate forms of nitrogen and phosphorus mostly become bioavailable, but over longer time frames. Most dissolved organic nitrogen typically has limited and delayed bioavailability.
  7. Little is known about the types and concentrations of contaminants bound to sediment discharged by rivers into the Great Barrier Reef and the risk that these pose to marine ecosystems.
Last updated:
27 August, 2014
Last reviewed:
13 August, 2013

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