Scientists have identified a growing problem in the Great Barrier Reef that is likely, they say, to be a cause for concern when considering the region’s health.
Rather than the increasing pressure of global warming affecting marine temperatures or the risk of acidification from carbon dioxide absorption by the ocean, a new study has uncovered a hitherto unknown impact on the reef’s health: groundwater.
The study by Southern Cross University, Gothenburg University (in Sweden), the Australian Institute of Marine Science (AIMS) and CSIRO, found unusually high proportions of nutrients in the Reef between Rockhampton and Cairns, originating via groundwater discharge.
When it comes to nutrients in aquatic and marine ecosystems, it is possible to have too much of a good thing.
Rather than simply providing conditions for growth and a functional ecosystem, excessive amounts of elements like nitrogen and phosphorous can lead to negative environmental outcomes, such as algal blooms, eutrophication (excessive algal growth) and promote growth of crown-of-thorns starfish, a deadly predator of juvenile corals. When excess nutrients are released into marine ecosystems, they have a polluting effect.
“It’s something that is a growing problem on the reef, too much nutrient, so it is something that needs to be managed,” says Dr Douglas Tait, a senior researcher at Southern Cross University who led the study published in Environmental Science & Technology.
Tait and his colleagues used radium isotope tracing to monitor nutrient proportions along the Great Barrier Reef. Radium occurs naturally in sediments and makes a good foundation for building a model to track flows of nutrients into the Reef via groundwater discharge.
Using an AIMS research vessel, the study team conducted radium transects up to 120km offshore, then modelled the data alongside measurements of river and groundwater flows.
Their model suggested twice as much nitrogen comes into the reef via groundwater discharge than river flows. This poses a problem because, to date, attempts to manage nutrient runoff has been focussed on river runoff, not groundwater.
Managing nutrients from sources like fertilisers in agriculture has been an ongoing priority for industry for many years, but groundwater systems can store these pollutants for decades. “It’s a sort of hidden problem, behind the scenes,” says Tait.
Ongoing monitoring of historic and present-day flows is something Tait has spent years researching.
“Much like we’re tracking this plume of nutrient-rich groundwater starting from the 1960s, which was about the same time as widespread use of herbicides, pesticides and industrial chemicals in urban environments and we really need to be focussing on where and when those types of contaminants are coming out as well,” Tait says.
“Heavy metals, pharmaceuticals, there’s a whole range of colours that groundwater can carry to the coast.
“What we’re experiencing now is just an indication of what’s coming out, we just really need more information on what might be coming out in the future.”