The floods ransacking the Queensland coast have cost 31 lives and $30 billion worth of property damage. Now, huge volumes of water are pouring into the ocean, threatening the Great Barrier Reef, which extends for thousands of kilometers off the coast. Although it may take years to know the full consequences of the flooding, Australian scientists are already warning of serious damage. For now, the biggest problems are concentrated on the southern part of the reef, where three overflowing rivers—the Burdekin, the Fitzroy, and the Burnett—have released millions of gallons of heavily polluted water into the sea. So far, prevailing winds have confined the majority of the floodwaters to within 65 kilometers of shore. But in time, the damage may grow to affect the entire reef system.
The floodwaters are dangerous for a number of reasons. One problem is the pure volume of freshwater now rushing into the ocean. In the Fitzroy river delta alone, enough flood water has flowed into the reef to fill three Sydney Harbors. Too much freshwater lowers salinity levels and can be deadly for coral. The floodwaters are also cloudy, mixed with dirt and sediment—not to mention debris from cities and fertilizer and pesticide runoff from farms. The cloudy floodwaters block sunlight, impeding photosynthesis and smothering the coral. In one resort area, visibility in the water has already fallen from 30 meters to one meter. In addition, agricultural chemicals and trace metals may hurt coral directly.
Great Barrier Reef in Australia. Photo by: Rhett A. Butler.
The nutrient-rich floodwaters are also likely to encourage the growth of microalgae , which provide food for the larvae of the crown-of-thorns starfish. When mature, the starfish feed on coral. Previous floods have triggered starfish outbreaks that devastated portions of the reef. Because those starfish take time to mature, it will probably be at least three years before we see the full consequences of the flood.
The floods are yet another piece of bad news for the already vulnerable reef, which was damaged by a different set floods that occurred in 2008-2009, and which is already threatened by coastal development, climate change and ocean acidification.