The Great Barrier Reef is one of the Seven Natural Wonders of the World, and was World Heritage listed in 1981. It is important for food, medicine and the economy, with its value as an asset being estimated to be $56 billion.
But it is also under threat from climate change, severe weather events, over-fishing and poor water quality.
With regards to poor water quality, excess nitrogen is particularly challenging in the Wet Tropics. High rainfall, and the number of short sharp river basins and the close proximity of the reef to the coast mean pollutants are flushed to the reef lagoon quickly.
The problem with nitrogen
Given that nitrogen is invisible and naturally occurring in the environment, it is not necessarily obvious why it is a ‘pollutant’.
However, the reason nitrogen is a problem for the Great Barrier Reef is because it has been washed out into marine waters after being dissolved from inorganic nitrogen sources (fertilisers), which are mainly added to crops on land.
Nitrogen is needed for plant growth and Dissolved Inorganic Nitrogen (DIN) is added to crops to make them grow bigger, faster and better.
Unfortunately, when it flushes off the land into waterways, DIN has the same effect on tiny single-celled plants called phytoplankton. And, single-celled organisms don’t just grow, they multiply and they do it very quickly.
One cell splits into two, two cells become four and four become eight and so on. In a matter of days, this really fast growth turns into billions of phytoplankton, which goes on to create an ‘algal bloom’ covering thousands of square kilometres of ocean.
Meanwhile, as the algae is multiplying and basking in the sunshine, it casts a shadow below the surface of the water, starving coral and seagrass of light.
Finally, when the algal bloom comes to an end, a mass of dead single-celled organisms falls to the ocean floor and starts to decay. The organisms that break them down use a lot of oxygen, which can make a ‘dead zone’ in the ocean.
A system out of balance
Lots of marine creatures – from starfish to whales – eat phytoplankton and algae. But when they’re out of balance and an algal bloom occurs there is suddenly so much food available that other things also start to grow too much, especially the coral-eating Crown of Thorns Starfish (COTS).
COTS occur naturally on the reef and are found in small numbers even though they can produce up to 6 million eggs in a season. The reason they reproduce in such high numbers is because relatively few of them survive to become fully grown starfish.
However, when there is suddenly a huge amount of extra food for the larvae to feast on a much greater proportion of them survive to become adults. It only takes a 2% increase in larvae surviving to adulthood to result in over a million extra starfish for each female laying eggs.
An outbreak of COTS means things are out of balance and it means coral is being eaten faster than new coral can grow.
Reef health is just the same as human health. When we’re in balance we recover much more quickly from illnesses and stress, but if we’re out of balance it’s much harder.
A healthy reef in good clean water can bounce back more quickly from coral bleaching events and severe storms, and this is something we can work on.
Many land-based projects are improving water quality by reducing the amount of nutrients, including nitrogen, and pesticides flushing off into our rivers. The more we can do to manage the health of our waterways, the better able the reef will be to recover quickly from 'stressors' that we can't influence.