The good and bad of nutrients

It’s hard to think badly of nutrients. They are, by definition, good for us, for plants and for a variety of other environmental and biological systems that want to grow and prosper.

However, when we’re talking about the health of the Great Barrier Reef, we have to be a little more specific. You can get too much of a good thing.

What are nutrients?

Nutrients are the natural chemical elements and compounds that plants and animals need to grow. There are a number of them, but six are particularly important: carbon, hydrogen, nitrogen, oxygen, phosphorus and sulphur.

Carbon, hydrogen and oxygen are found in large quantities everywhere in nature, while nitrogen, phosphorus and sulphur are a little less abundant.

Coral and algae live in a delicate, mutually-beneficial relationship with each other. Nutrients are the big concerns in reef environments because if their levels are too high they can upset the natural balance of these ecosystems. For example, some plants grab the extra nutrients and grow to dominate in size or quantity more than they normally would.

How do nutrients affect the Reef?

Levels of these nutrients get too high when excess amounts are washed into the water from land. Annual discharge of nutrients into the Great Barrier Reef has increased substantially since European settlement. Potential effects of nutrients in the Great Barrier Reef include:

Increased outbreaks of coral-eating crown-of-thorns starfish.
Reduced coral diversity.
Algal blooms (which reduce light and contribute additional nutrients).
Increased susceptibility to coral bleaching and some coral diseases.

While most effects occur in the wet season, some effects may continue for many years, for example crown-of-thorns starfish outbreaks.

Crown-of-thorns starfish and nutrients

The current scientific consensus suggests that elevated nutrient inputs, particularly nitrogen from land-based runoff, play a role in contributing to crown-of-thorns starfish outbreaks on the Great Barrier Reef, although the relationship is complex and outbreaks are likely influenced by multiple interacting factors.

Impacts on other ecosystems

Excess nutrients can also harm mangroves, wetlands and seagrasses.

Mangroves: Excess nutrients, especially nitrogen, stimulate the growth of shoots relative to roots, which causes physical instability of the mangrove trees and increases their vulnerability to sea level rise.
Wetlands: Excess nutrients can lead to sudden and sometimes irreversible changes such as increased growth, changes in the type and number of species, and excessive growth of algae and weeds.
Seagrasses: Additional nutrients can enhance seagrass growth, they also encourage the growth of plankton and algae, which can reduce the light available to seagrass leaves. This can lead to algae eventually dominating seagrass in a particular ecosystem.

How do nutrients reach the Reef?

Nutrients generally flow to the Reef in run-off in one of two ways:

Particulate nutrients – carried as particles in water these are typically deposited close to river mouths.
Dissolved nutrients – carried in water, these can travel further from the shore and are ready absorbed by marine plants including phytoplankton, algae and bacteria.

Dissolved nutrients, particularly dissolved inorganic nitrogen and phosphorus, can easily travel further along or away from the shore, where they are consumed by phytoplankton (made up of microscopic plants), algae and bacteria. They become part of a natural cycle of growth, decay and transformation, meaning they can stay in the system for quite a while.

Over time, particulate nutrients deposited near the shore can break down and become dissolved. They can be released from sediments for years after they are first deposited.

While most of this impact occurs in the wet season because of greatly increased river discharge, some effects have consequences throughout the year. During the dry season, for example, poor water quality can be made worse by low river flows and an increase in organic matter from dying organisms.

Where do these nutrients come from?

Rainfall and irrigation can wash nutrients, pesticides and sediment into waterways and coastal wetlands. Nutrients and pesticides can also drain through agricultural soils into groundwater and then reach downstream waters.

Monitoring and scientific modelling show that sugarcane is by far the greatest contributor (42%) to the dissolved inorganic nitrogen that is transported to the Great Barrier Reef. This is primarily from applied fertilisers. Urban areas contribute about 7%, which may be important at local scales.

Most particulate nutrients come from grazing areas, although sugarcane dominates contributions in the Wet Tropics and Mackay Whitsunday regions.

Both the Australian and Queensland governments are supporting landholders to make long-term transformational changes on their properties. Farmers are reducing their nutrient run-off through programs such as industry-led Best Management Practices, extension and education services, large-scale social change programs and land restoration initiatives.