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By Virginia Tressider 31 March 2016 5 min read

Coral, seagrass and marine life on the Great Barrier Reef are under threat from many factors, including declining water quality. Image; Kyle Taylor / Flikr

As a largely agricultural state, Queensland is prone to sediment run-off associated with farming. A complication – in this context – is that Queensland also has the spectacular asset of the Great Barrier Reef, as well as a thriving agricultural sector. Coral and crops don’t always go together. Agricultural inputs such as fertiliser, herbicides and pesticides, along with eroded soil, can enter waterways, making their way to the sea. And there, just off shore, is a massive aggregation of coral.

Coral have a symbiotic relationship with single celled algae that live inside each polyp. The algae give off oxygen and other nutrients the coral polyp needs, and take carbon dioxide and other substances in exchange. The algae need sunshine for photosynthesis. When sediment coats coral, even lightly, it blocks the light, preventing the algae from photosynthesising.

With sediments rich in organic material, the problem is exacerbated. If the sediment has relatively high levels of bacteria, they use up the surrounding oxygen when digesting the organic material. Sediment can also prevent coral larvae from attaching.

Within the Great Barrier Reef Lagoon there is growing evidence that sediment export from agricultural land in the coastal catchments have increased turbidity and sedimentation, affecting valuable ecosystems such as seagrass meadows, as well as coral.

Plainly, this is a major problem. Agricultural pollution threatens approximately 25 per cent of the total global reef area, second only to climate change as a risk for coral reefs. The amount of sediment reaching the Great Barrier Reef is estimated to have increased to between five and ten times the volume normal before European settlement of the area in about 1870. Further increases in sediment and nutrient fluxes are projected over the next 50 years. To manage this threat, we need to identify precisely what constitutes the main components of the sediment, to get better at identifying major catchment source areas of this sediment, and better at understanding how it moves through the catchment and disperses in the coastal waters of the reef.

Sediment the key to targeted interventions

But how to work out what came from what part of the catchment, in something that is all mixed together, and may have some variation in the constituent parts according to the weather conditions and the time of year?­ Research scientist Zoe Bainbridge has been working on this, taking ‘sediment fingerprints’ of what is heading out onto the reef. The clay mineral properties of sediment vary according to the parent rock material, time and environmental conditions that influence weathering.

Muddy waters flow through a river
Waters from the Burdekin River in Queensland flow into the Great Barrier Reef. Image: Rob and Stephanie Levy / Flikr

The presence and relative abundance of common clay minerals in a suspended sediment sample can point to the sample’s source area.

After spending several wet seasons between 2005 and 2011 collecting more than 500 river and flood plume water samples from 31 sites across the Burdekin catchment, Zoe analysed them for particle size and clay mineral abundance. From this particle size data Zoe was able to calculate catchment-wide amounts of fine sediment and of clay-only. She then used a clay mineral ratio to distinguish the geological area the sediment had originated in.

The sediment source tracing study identified basaltic, granitic and sedimentary geologies as the main sources of end-of-river and flood plume fine sediments across the Burdekin, and the clay mineral ratio could clearly distinguish between the main catchment source areas - the Upper Burdekin and Bowen River sub-catchments.

Although erosion rates and sediment runoffs to the reef are higher than pre-agricultural levels, there is scope for coexistence of nature and agriculture, with appropriate monitoring and adaptive management. By understanding the source of the sediment and the associated nutrients, pesticides and herbicides, it becomes easier and more effective to target land management interventions that enable us to reconcile agricultural production with environmental protection.

Not all parts of an area erode at the same rate. Even in catchments with the same land use, erosion rates can vary significantly, because of differences in variables such as slope, rainfall, geology, vegetation and soil type. Small hillslopes with patchy ground cover, for example, appear to be an erosion risk. Unless we understand and take into account the catchment’s natural susceptibility, we risk allocating resources for remediation to areas that appear to be producing high sediment yields, but in fact have landscape features that generate large volumes of sediment, even in the absence of agriculture. The measurements may also reflect the climate and rainfall at the time the data was collected, and exclude large episodic runoff events, which are an important influence on soil erosion and delivery.

CSIRO’s Rebecca Bartley has developed the ‘accelerated erosion factor’ (AEF) for the Burdekin Basin catchment. This helps establish what the current erosion rates are and how they compare with the pre-agricultural rate.

The current data comes from measuring sediment fluxes, and the historical data from detecting levels of Berryllium-10 nuclides. All atoms are made up of a nucleus and electrons circling the nucleus. The composition of the nucleus varies, generating different nuclides. The ratio of these nuclides can be used to trace the origin of the atoms. Researchers can then identify the parts of the catchment where the rates of erosion are of particular concern. Obviously, this is of considerable benefit for landholders, but it is also a valuable aid in stemming sediment flow to the reef.

In the reef area catchments, funds for on-ground remediation are currently allocated according to the relative risk of sediment runoff to the marine system. A study of the catchment using the new, more flexible AEF model found that to protect the reef, it would be useful to have multiple catchment targets that consider long-term (>100 year) erosion rates, and accommodate differences in rainfall, geology, soil type and slope.

The study also shows that the resources available for erosion control and catchment remediation can be targeted, with very specific ‘hot-spot’ areas getting priority treatment. If contemporary erosion rates have not increased above long-term (pre-agricultural) erosion rates everywhere, treating the whole catchment is unnecessary, and diverts scarce remediation resources aimed at protecting the reef.

With appropriate design of monitoring, control and remediation programs, the reef will be in good health and better prepared to handle what the future might have in store for it.

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