Compared to pre-European conditions, the annual discharge of nutrient loads to the Great Barrier Reef has more than doubled with sugarcane farming being the main contributor.
Sugarcane farmers manage 400,000 hectares in the Great Barrier Reef catchment. This is only 1.4 percent of the catchment area but is a high impact land use.
Sugarcane growing areas are the largest contributors of:
- dissolved inorganic nitrogen: 78% of the anthropogenic load
- pesticides: more than 95% of the load.
Nitrogen levels are highest in run-off from catchments where sugarcane is a major crop. The greatest coral reef and seagrass exposure to dissolved inorganic nitrogen is from the Herbert, Haughton, Johnstone, Russell-Mulgrave, Tully, Plane and Murray catchments.
Nitrogen-based fertilisers are needed for sugarcane production but if used inappropriately, they can run off into local waterways and out to the Great Barrier Reef.
Catchment water quality monitoring and modelling estimates that approximately 55 kilotonnes/year of total nitrogen is delivered to the Great Barrier Reef. Modelling suggests approximately 12 kilotonnes/year of that is dissolved inorganic nitrogen (DIN), the form of nitrogen that is used most easily by algae.
Improved management practices, such as applying the correct level of nitrogen for the crops needs at the optimum times for weather, are essential to maximise productivity and minimise the impact on the environment.
What is the problem with nitrogen run-off?
Fertiliser containing nitrogen easily changes in soil and water to a form called dissolved inorganic nitrogen (DIN). DIN is the type of nitrogen that plants and algae can easily absorb to grow. This availability means it is mobile in soil and can easily be lost in run-off or drainage into waterways.
Excess nutrients (particularly nitrogen) can upset the natural balance of Reef ecosystems. There is strong evidence that excess nutrients in the Great Barrier Reef contribute to:
- increased outbreaks of coral-eating crown-of-thorns starfish
- lower coral diversity
- algal blooms
- increased susceptibility to coral bleaching
- some coral diseases.
While most effects occur in the wet season because of greatly increased river discharge, some effects may continue for many years, for example crown-of-thorns starfish outbreaks.
What is the problem with pesticide run-off?
Pesticides are a threat because what they are designed to do on land – kill pests such as weeds and insects – means they also impact plants and animals in rivers and creeks, as well as some coastal and inshore areas. The pesticides commonly used for weed control act by inhibiting photosynthesis, which is why they are so good at controlling weeds, and can affect non-target species such as seagrasses.
Pesticides, including herbicides, insecticides and fungicides, are generally not found in the natural environment and can take months or even years to break down. They are carried in river run-off and have been detected in Great Barrier Reef ecosystems at concentrations high enough to affect organisms.
The effects of ongoing low-level pesticide exposures in inshore environments are unknown but likely to impact coral fertility and reproduction.
Support programs and tools
There are a number of industry and government programs and support tools that can help cane farmers adopt best nutrient and pesticide management practices.
- Last reviewed
- 3 May 2023
- Last updated
- 8 July 2022