Barrier Reef sediment up since European arrival
River sediment in Australia's Great Barrier Reef has risen five to 10-fold since Europeans colonised the region, according to the first long-term study of the reef ever compiled.
The research undertaken by a group from the Research School of Earth Sciences at the Australian National University in Canberra, analysed barium levels in 250 year-old coral.
They found that there has been a five to 10-fold increase in the delivery of river sediments to the Great Barrier Reef since the 1860s. The study is published in today's issue of the journal Nature.
"What this study does is provide a baseline that shows you what it was like before any changes to the catchment were made by agricultural practices," said Timothy Wyndham, a doctoral student at the university who worked as a member of the project team. "It allows you to compare the current situation with what it was like then."
The research was based on a site of the inner Great Barrier Reef that is influenced by flood plains from the Burdekin River in the Australian northeastern state of Queensland. On a given flood more than 100 million tonnes of suspended sediment can be deposited.
"We used two indicators," Wyndham told ABC Science Online. "Barium concentration, which tells us the amount of sediment being washed in; and fluorescence in the coral, which tells us when there has been a flood and gives us an approximate measure of how large the flood has been."
As corals build their skeletons from calcium and carbonate, trace amounts of other elements are incorporated. Barium is an element that is bonded to the surface of river sediment, but is set loose when it mixes with salt water.
The technique is new to coral research. "The researchers have developed an effective proxy record of terrestrial sedimentation on the reef that tends from AD 1750 to AD 1985," writes Associate Professor Julia Cole from the University of Arizona, in the United States, in an accompanying article.
By comparing the barium-to-calcium ratio of the coral in a 5.3 metre-long core of Porites coral from Havannah Reef, north of Townsville, the researchers were able to graph peaks of barium uptake - representing the higher levels of sediment landing in the reef.
The year 1870 was the first major flood event that occurred after European settlement of the catchment area. Before 1870, flood waters still influenced the reef, but contained much less barium. As domestic grazing and land clearance intensified after 1870, soil become more vulnerable to erosion during monsoon rains, and the barium content of river waters increased.
"It is something we understood was happening, but didn't know how much about how much sediment was being washed off, and we had a really poor understanding of what it was like before we made any changes to the catchment," Wyndham said.
The researchers conclude in their paper, "Since European settlement, land-use practices - such as clearing and overstocking - have led to major degradation of the semi-arid river catchment."
But researchers acknowledged that the environmental implications of the increase in sediment is still a matter of debate.
"One argument is that the sediment levels are a problem, because an increase of sediments means an increase in the amount of nutrients," said Wyndham. In other corals, this has been linked to a change in the way the coral grows and an increase in the amount of algal growth.
The Great Barrier Reef is a collection of reefs spread over 2000 km along Australia's northeast coast, and like all coral reefs, is based on the growth of hard corals. Soft coral grows on the hard coral and this is what the reef community builds on.
"Hard coral is the type most affected by an algae shift," said Wyndham. "So less hard coral means an increase in soft coral and algae which in turn means a reduction in the area of a real coral reef."