Setting the sediment record straight

How coral reef research is challenging widely-held assumptions about sedimentation from land-use change

Research has shown that coral reefs in the inner-shelf areas of Australia’s Great Barrier Reef are growing as fast as or faster than clear-water reefs; a result that questions the widely-held assumption that increased sedimentation from land use changes are having ubiquitous negative effects on reef growth.

Professor Chris Perry, Chair in Physical Geography at the University of Exeter, has been studying the Great Barrier Reef for nearly a decade, with a focus on the growth history of nearshore reefs.

“These reefs have developed in naturally very muddy conditions, often in close proximity to rivers, or at least under the influence of rivers that bring a lot of sediment and nutrients onto the shelf. They are also strongly influenced by high levels of wave-induced sediment resuspension from the seafloor. These reefs are really at the heart of the controversial topics of run-off and land use practices,” said Perry.

The Great Barrier Reef (GBR), the world’s largest coral reef ecosystem, is a natural icon and a UNESCO World Heritage Site. There is concern globally about the degradation of coral reefs, including the GBR. One growing concern is the intense agriculture in the major river catchment areas that drain into the GBR. There is evidence that certain land use practices have increased sediments, nutrients and pollutants being carried down river and out onto the nearshore reefs.

Perry and his colleagues are trying to understand more about where and when near- shore reefs developed, how fast they have grown and how they might have altered over the last few thousand years. They are also looking for any changes in the rates of reef growth or in coral species that might correlate with changes in the water quality as a result of human activities. The idea is to look at how these reefs have changed through time in order to put the changes we see today into better context.

Despite other evidence that suggests there’s been an increase in the amount of sediment and nutrients coming onto the shelf, the researchers couldn’t find any strong signal of any obvious negative effect on reef growth within the very nearshore reefs.

“In fact, the overwhelming signal that we saw was one that was more closely associated with the reefs just growing towards the sea surface and the communities changing naturally as a result of more light, higher wave action, and lower rates of net sediment accumulation” said Perry.

Perry’s research has shown that the inner reefs have incredibly fast rates of growth and exhibit resilience despite exposure to high turbidity. These findings are contrary to common understanding.

“A lot of those reefs on the inner part of the GBR are actually OK and doing quite well. This is probably linked to the fact that the coral building these reefs have always grown under high stress conditions and are thus adapted to deal with any recent increased inputs of sediment,” said Perry

Despite the politically charged nature of this subject, Perry’s research is starting to infiltrate the policy arena and his findings have been used in political debates within the Queensland Government.

The impact of Perry’s work is still developing but if it is taken up by organisations such as the Great Barrier Reef Marine Park Authority, it could help put current environmental conditions into context and establish ecologically relevant targets for water quality.