Which rivers are polluting the Great Barrier Reef?
Research led by scientists from The Nature Conservancy (TNC) and the University of Queensland has identified the rivers most responsible for polluting the world’s largest coral reef system—The Great Barrier Reef.
The research indicates that the Burdekin, Fitzroy, Tully and Daintree Rivers posed the greatest runoff risk of inorganic nitrogen onto coral reefs.
Biggest polluters to the Great Barrier Reef
It’s disturbing to discover the extent that pollutants can travel from individual rivers. Dr Nick Wolff, Climate Change Scientist at TNC found that during 2011, 275 reefs up to 450 kilometres north of the Burdekin River mouth were impacted.
The research used sophisticated oceanographic models and ‘virtual dyes’ to trace the movement of pollutants in rivers flowing onto the reef. Understanding the flow and direction of pollutants from each river and tracking where these pollutants end up has been a missing piece in the puzzle to protect the reef.
Now that we know where the biggest pollution problems are coming from and flowing to, critical land-based actions can be prioritised to reduce threats to the Great Barrier Reef and protect its future.
How river pollution contributes to coral bleaching
Increased runoff of sediment, nutrients and contaminants from the land has lowered coastal water quality and marine ecosystem health across northern Australia, including the Great Barrier Reef.
Increased nutrients in the water stimulate excessive growth of algae (e.g. seaweed) which can outcompete corals for space1.
Sedimentation runoff reduces the clarity of coastal waters and restricts the growth of light-dependent plants and animals, including corals. Light is an important factor for the growth and survival of coral reefs. Corals are combination of plant and animal parts, and their soft bodies harbour algae that need light to photosynthesize. The animal part of the coral depends on the algae to provide energy. While corals can grow and form reefs over a wide range of water clarity conditions, the nature of the reefs and their continued survival depends on receiving sufficient light. And with sediment particles continually resuspended by waves and currents, it’s difficult for coral in certain areas to get the much-needed light to survive.
Increased nutrients in the water can also promote the breeding success of the destructive Crown-of-Thorns Starfish that is also a problem for the Great Barrier Reef. That’s why reducing nutrients flowing from polluting rivers to the reef is so critical in helping to sustain its health.
Coral bleaching and reef recovery
Scientists have found coral recovery rates across the Great Barrier Reef are declining. During the 18-year (1992 -2010) study period, the ability of coral reefs to bounce back from disturbances, such as bleaching, Crown-of-Thorns Starfish outbreaks or cyclones has declined, on average, by six-fold. It is the first time a decline in recovery rate of this magnitude has been identified in coral reefs2.
It is alarming that this steep decline in recovery was documented before the extreme bleaching events in 2016 and 2017. Recent research demonstrated that this back to back bleaching dealt such a severe blow to adult colonies of some important coral types that the number of young corals has declined by nearly 90%3. The implication is that there are simply not enough surviving adults to replenish the reef and that recovery rates may be even lower.
Coral recovery decline is likely driven by a combination of acute disturbances like coral bleaching and the ongoing effect of chronic pressures like poor water quality. Although it may sound strange to say, it is promising that poor water quality plays a role because it offers an opportunity. Armed with our research showing where river pollution is coming from, management agencies can target land-based interventions (e.g. land use improvements) that will strategically, and cost effectively reduce pollution. While such improvements will improve the resilience of some reefs4, these local interventions need to be coupled with the urgent need to reduce emissions.