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The Middle Reef, part of Australia’s Great Barrier Reef, is growing more quickly than reefs in other areas with lower levels of sediment stress, a new study has found. Rapid coral reef growth has been identified in environments with large amounts of sediment, conditions previously thought to be detrimental to reef growth.
Middle Reef is located on the inner Great Barrier Reef shelf just 4 km off the mainland coast near Townsville, Australia. Middle Reef grows in water that is always ‘muddy’ unlike most reefs that grow in clear water. The sediment comes from seasonal river flood plumes and the mud churning up from the floor of the sea. Since European settlement, The Queensland coast has changed significantly. The sediment runoff has increased due to the clearing of natural vegetation for agricultural use. It is believed that the poor water quality, due to high levels of sediment, has a detrimental effect on marine biodiversity.
Cores through the structure of Middle Reef were collected by the research team to analyze how it had grown. Radiocarbon dating was used to map out the exact rate of growth of the reef. The results show that the reef started to grow only about 700 years ago but that it has subsequently grown rapidly towards sea level at rates averaging nearly 1 cm per year. These rates are notably greater than those measured on most clear water reefs on the Great Barrier Reef and elsewhere. When the accumulation rates of land-derived sediment within the reef structure were at their peak is when the most rapid growth took place, averaging 1.3 cm a year. They discovered that, while the reef faced high sediment levels after the European settlers arrived in the 1800s, these same conditions were also part of the long-term environmental regime under which the reef grew.
The findings suggest that in some cases reefs can adapt to these conditions and flourish, although there is evidence that other reefs have suffered degradation from high levels of sediment. Middle Reef has probably been aided by the high sedimentation rates causing the rapid rate of vertical growth. The team believe this is because after the coral dies, the accumulating sediment quickly covers the coral skeleton preventing destruction from fish, urchins and other biological eroders thus promoting coral framework preservation and rapid reef growth.
Professor Chris Perry of Geography at the University of Exeter said: “Our research challenges the long-held assumption that high sedimentation rates are necessarily bad news in terms of coral reef growth. It is exciting to discover that Middle Reef has in fact thrived in these unpromising conditions. It is, however, important to remain cautious when considering what this means for other reefs. Middle Reef includes corals adapted to deal with high sedimentation and low light conditions. Other reefs where corals and various other reef organisms are less well adapted may not do so well if sediment inputs increased.”
“Our research calls for a rethink on some of the classic models of reef growth. At a time when these delicate and unique ecosystems are under threat from climate change and ocean acidification, a view endorsed in a recent consensus statement from many of the World’s coral reef scientists, it is more important than ever that we understand how, when and where reefs can grow and thrive.”
Photo Credit: University of Exeter