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.

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.

Levels of these nutrients get too high when excess amounts are washed into the sea from the land. Annual discharge of nutrients into the Great Barrier Reef has more than doubled since European settlement.

There is strong evidence for several effects of nutrients in the Great Barrier Reef including increased outbreaks of coral-eating crown-of-thorns starfish,  lower coral diversity, algal blooms (that reduce light and add their own 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.

The current scientific consensus is that nitrogen inputs are more likely to cause crown-of-thorns starfish outbreaks than other nutrients.

Sometimes problems are caused by the patterns of the excess growth, such as with mangroves. Nutrients – again especially nitrogen – stimulate the growth of shoots relative to roots, which causes physical instability of the mangrove trees and increases their risk to sea level rise.

Wetlands are also affected by excess nutrients which 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.

In a similar vein, while additional nutrients can enhance seagrass growth, they also favour 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.

So how does this all happen?

Nutrients generally flow to the sea in run-off in one of two ways – as particles carried by the water (particulate nutrients) or actually dissolved in the water (dissolved nutrients). Particulate nutrients are generally more common, but this varies between catchments. However, dissolved inorganic nitrogen and phosphorus are of more immediate concern as they are completely available for uptake by marine plants.

Particulate nutrients are usually deposited close to river mouths, whereas dissolved nutrients 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.

The other twist in the story is that over time excessive nutrients can damage some plants and they lose the ability to retain the nutrients they need. So excess nutrients can actually lead to a plant not getting enough nutrients.

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 (78%) to the dissolved inorganic nitrogen that is transported to the Great Barrier Reef. This is primarily from applied fertilisers. Urban areas contribute about 9%, 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.