Nitrogen & the Great Barrier Reef

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 at $56 billion.

Many scientists believe that the reef is under threat from the effects of climate change, severe weather events, over-fishing and poor water quality.

With regards to water quality, nitrogen has been identified as a potential issue and is particularly challenging in the Wet Tropics. High rainfall, the number of short sharp river basins and the close proximity of the reef to the coast mean pulses of pollutants can be flushed to the reef lagoon quickly.

The problem with nitrogen

Given that nitrogen is invisible and is naturally occurring in the environment, it is not necessarily obvious why it might be considered a ‘pollutant’.

On land, nitrogen is vital for healthy plant growth and it is applied to crops as fertiliser, usually in urea form. It's a complex element and changes form after application, with urea changing to ammonia, nitrate and nitrite. Together, scientists refer to these forms of nitrogen as Dissolved Inorganic Nitrogen (DIN).

Unfortunately, sometimes DIN is flushed from the land during heavy rainfall and makes it way into waterways and eventually the Great Barrier Reef Lagoon. Scientists believe that Dissolved Inorganic Nitrogen presents a problem for the Great Barrier Reef is because it can be a factor in the growth of microscopic, planktonic algae.

These algae can multiply quickly, with one cell splitting to two into two, two cells become four, four become eight and so on. In a matter of days, this really fast growth can 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 create a ‘dead zone’ in the ocean.

A system out of balance

Lots of marine creatures eat phytoplankton and algae. But when an algal bloom occurs things can quickly get out of balance. Many scientists attribute the proliferation of the coral-eating Crown of Thorns Starfish (COTS) to increased presence of marine phytoplankton.

COTS occur naturally on the reef and are found in small numbers even though they can produce up to 60 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, it's believed that 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 ten million extra starfish for each female laying eggs.

An outbreak of COTS might therefore be an indication that things are out of balance and could result in coral being eaten at a rate faster than new coral can grow.

Building resilience

Reef health is a little like 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.

There's good evidence that 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 flushing 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 have less influence over.

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