Dirty Story: Farming has increased flow of soil onto reef

Ever since European settlers brought agriculture to Australia, soil has been deposited on the Great Barrier Reef at an accelerated pace, according to new research. The study suggests that the introduction of farming near coastlines can have rapid and continuing consequences offshore.

Eroded sediments that wash down rivers and out to sea can block out sunlight, interfere with corals’ feeding, and introduce pathogens to reef communities (SN: 1/30/99, p. 72: https://www.sciencenews.org/sn_arc99/1_30_99/bob1.htm).

To understand how such processes have affected the Great Barrier Reef, Malcolm McCulloch of Australian National University in Canberra and his colleagues drilled cores of coral about 150 kilometers from the mouth of the Burdekin River in Queensland, Australia. Today, periodic floods spew plumes of sediment onto this portion of the reef.

The cores, at up to 5.3 meters long, record reef growth from about 1750 to 1998, say the researchers. They attempted to gauge sediment deposits over that period by measuring core-layer fluctuations in the ratio of barium to calcium. Growing corals incorporate these metals in proportion to their presence in the surrounding water.

The barium-calcium ratio is normally low in seawater and therefore in coral, but it rises when erosion sweeps barium-rich terrestrial soils into the ocean.

The researchers compared the barium-calcium ratios in the coral with detailed logs of the Burdekin River’s flow since 1921. Each flood during that period temporarily boosted the barium-calcium ratio in proportion to the increase in the river’s flow, the scientists report. The team concludes that the barium-calcium ratio accurately reflects the amount of sedimentation.

Looking back farther into the coral record, McCulloch and his colleagues found that the barium-calcium ratio in the reef rose by about one-third soon after European colonists settled in the river basin in 1862. Moreover, seasonal flooding of the river after farming began caused exaggerated spikes in sediment flow onto the reef.

Within 1 to 2 decades after the arrival of European settlers in northern Queensland, there were already “massive impacts” on the waters of the inner reef, the team says in the Feb. 13 Nature.

The researchers “have cleverly developed a history of sedimentation on [the reef] by quizzing the corals themselves,” says Julia Cole of the University of Arizona in Tucson in a commentary accompanying the new study. They’ve also shown that sedimentation increased dramatically after European settlement, she says.

Research in the western Indian Ocean suggests that reefs there were similarly affected by the advent of European-style agriculture in Kenya in the early 20th century, Cole adds.

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