The Great Barrier Reef is one of Australia's most important environmental and economic assets. It is estimated to contribute A$56 billion per year and supports about 64,000 full-time jobs.
However, the Reef is facing multiple threats, such as climate change, coral bleaching, ocean acidification, and pollution.
Among these, excess nutrients from land-based sources are considered to be one of the major causes of degradation of the Reef's health and resilience.
What are the sources of nutrients to the Reef?
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Nutrients are essential to support the incredible biodiversity of the Reef. They provide food for the primary producers, such as algae and seagrasses, which in turn sustain the higher trophic levels, such as fish and corals.
However, an excess of nutrients can lead to detrimental issues, such as harmful algal blooms, crown-of-thorns starfish outbreaks, and fish diseases, which have been on the rise in the Reef over the past few decades.
The main sources of nutrients to the Reef are river runoff and submarine groundwater discharge (SGD). River runoff is the water that flows from the land to the sea, carrying sediments, nutrients, and other pollutants.
SGD is any water released to the ocean below the waterline from various sources, including underground aquifers and the seafloor.
Until recently, SGD was largely overlooked as a source of nutrients to the Reef. Most of the efforts to mitigate the impact of nutrients on the Reef were focused on reducing river runoff.
However, a new study by Southern Cross University and other institutions has revealed that SGD exceeds river inputs as a source of nutrients to the Reef.
The study used natural tracers, such as radium isotopes, to track how much nutrient is transported from the land and shelf sediments via invisible groundwater flows.
The researchers collected data from offshore transects, rivers, and coastal bores in an area from south of Rockhampton to north of Cairns.
The study estimated that SGD accounted for approximately one-third of new nitrogen and two-thirds of phosphorus inputs to the Reef.
This means that nearly twice the amount of nitrogen enters the Reef from SGD compared to river waters.
Nitrogen and phosphorus are the two most important nutrients that affect the growth and productivity of algae and seagrasses.
What are the implications for Reef management?
The discovery of invisible nutrient discharge on the Reef raises concerns about the current management approaches aimed at safeguarding the Reef from the effects of excess nutrients.
The study suggests that reducing river runoff alone may not be sufficient to restore the health and resilience of the Reef.
It also highlights the need for a long-term perspective, as nutrients in groundwater could be stored for decades underground before being discharged into coastal waters.
The study calls for a strategic shift in management approaches that consider both river runoff and SGD as sources of nutrients to the Reef.
It also recommends further research to understand the spatial and temporal variability of SGD, its impact on different regions and habitats of the Reef, and its interaction with other stressors, such as climate change.
The study sheds new light on the complex nutrient dynamics within the Reef. It also underscores the importance of using natural tracers to monitor and quantify SGD in coastal ecosystems.
By improving our understanding and ability to manage the sources of nutrients, we can hope to preserve the Reef for generations to come.
