Declining water quality

Declining marine water quality influenced by land-based run-off is recognised as one of the most significant threats to the long-term health and resilience of the Great Barrier Reef.

It is contributing to declines in many of the attributes that make up the outstanding universal value of the world heritage property, particularly those related to coral reefs and seagrass meadows.

Catchment run-off

The Reef receives run-off from 35 major catchments that drain 424,000 square kilometres of coastal Queensland. Most sediment entering the Great Barrier Reef comes from catchments in major pastoral areas such as the Burdekin, Herbert and Fitzroy rivers.

The catchments that deliver water in the Great Barrier Reef can be divided into:

  • coastal catchments that provide a continuous flow of freshwater to the Reef from small catchments supporting intensive farming, for example Tully River and Pioneers River
  • large catchments that drain large inland grazing areas and tend to be seasonal and influenced by flooding, for example Fitzroy River.

Impacts on communities

Changes to the Great Barrier Reef ecosystem have serious economic implications for Reef dependent industries, such as tourism and fishing, and for adjacent communities.

Any problems associated with water quality have the potential to compromise Reef dependent industries. Perceptions about the health of the ecosystem can affect its attractiveness for tourism and recreation.

Examples of Reef-dependent activities are traditional use of marine resources, commercial marine tourism, fishing, recreation and research and educational activities. Reef-dependent activities are likely to be more sensitive to changes in the condition of the Region's values.

Impacts on natural resources

Increased sedimentation and the flow of nutrients and pesticides into the ecosystem affect inshore areas, causing higher algal growth, build-up of pollutants in sediments and marine species, and reduced light and smothered corals.

The coastal zones, especially areas close to river mouths, are most exposed to increased sediments, nutrients and pesticides.

Dissolved inorganic nutrients are quickly removed from seawater by phytoplankton, bacteria and benthic organisms. However, high levels over a long time can create ecological changes across large areas and metabolic changes in most marine plants and animals.

Changes in water quality affect the biodiversity and resilience of Reef systems. Higher concentrations of pollutants, such as suspended sediments, nitrogen and phosphorus, indicated by higher levels of chlorophyll and lower water clarity, leader to more algae and less coral diversity. In these conditions, algae take over and reduce the chances for new hard corals to establish and grow.

Some pollutants have been known to stay in the marine environment for decades and can build up in animals that have high fat contents (e.g. whales and dolphins), are higher on the food web or are long-lived. Pollutants, such as heavy metals, can disrupt reproduction, impact the immune system, and cause neurological disorders and cancers.

Cumulative effect and the timing of exposure to pollutants can magnify the impacts of catchment run-off.

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