Declining marine/coastal 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.

Coral reefs, and the ecosystems they sustain, depend on the quality of the water within and around them.

For the Great Barrier Reef, the main water quality issues are:

• increasing sediment, nutrients and contaminants entering coastal waters in run-off from agricultural, industrial and urban land uses
• rising seawater temperatures and increasing seawater acidity associated with climate change.

The Reef receives run-off from 35 major catchments draining 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.

• Increased sedimentation and nutrients can cause higher algal growth, build-up of pollutants in sediments and marine species, and reduced light and smothered corals.
• Any problems associated with water quality also have the potential to compromise Reef-dependent industries such as tourism, fishing, recreation, research and education. 

Nutrient run-off has also been linked to outbreaks of crown-of-thorns starfish - a significant contributor to the loss of coral cover on the Great Barrier Reef.