The Great Barrier Reef is the world’s largest reef, extending an extraordinary 1250 miles along Australia’s northeast coast. This amazing formation is one of the great wonders of the world, and the establishment of the Great Barrier Reef Marine Park in 1975 created the largest protected marine area in the world (today it is the second largest). Unfortunately, this diverse ecosystem has been hit with many of the same threats that are destroying reefs around the world, and the reef is suffering. Habitat pollution accounts for much of the decline in the health of the Great Barrier Reef.
Pollution threats to the Great Barrier Reef, and other reefs worldwide, don’t come only from typical sources of water pollution. The main air pollutant is the greenhouse gas carbon dioxide, which has been increasing in the atmosphere. The oceans absorb a large percentage of the excess carbon dioxide in the air, which causes them to become more acidic. In effect, carbon dioxide is a water pollutant as well as an air pollutant.
As the pH of water gets lower, the amount of free carbonate ions in the water decreases, and it becomes more difficult for coral polyps to obtain the calcium carbonate they need to secrete their limestone skeletons.
A study published in the January 2, 2009 issue of the journal Science reports a 14.2% drop in the rate of coral calcification on the Great Barrier Reef between 1990 and 2005.Researchers at the Australian Institute of Marine Science studied 328 colonies of Porite corals on 69 individual reefs. Porite corals grow in layers and, much like tree rings, the layers reveal age and growth rate. Examination of these colonies revealed that calcification began slowing around 1990, and growth is currently at the lowest it has been in 400 years. The likely cause is ocean acidification.
The World Wildlife Fun has estimated that at least 90% of the chemical pollution in the waters of the Great Barrier Reef comes from farm runoff. This runoff is laden with pesticides and fertilizers, and it takes only very low concentrations to harm corals. Corals pass through several stages in their life-cycle, including fertilization, larval development and metamorphosis into the adult polyp. Some stages are more susceptible to pesticides than other stages.
Adults polyps are relatively resistant to pesticides, but pesticides do prevent spawning. Most corals also reproduce asexually, but sexual reproduction is important to regenerating damaged coral, and pesticides prevent this from happening. One particular fungicide used on sugarcane crops, methoxyethylmercury choride (MEMO) is particularly toxic, even to adults. MEMO not only kills baby coral at extremely low concentrations, it causes bleaching in adults.
Fertilizer runoff also introduces high concentrations of nutrients into the waters around the Great Barrier Reef, and these extra nutrients feed the growth of algae and phytoplankton. Algal blooms cause huge mats of algae over the surface of the water, which can block sunlight from reaching the reef, preventing photosynthesis and starving the corals. Algal mats may also grow directly on and suffocate the corals.
In addition, increased populations of phytoplankton may account for the invasion of animal species that damage the coral. The crown-of-thorns starfish, which can decimate entire coral colonies within days, feeds on phytoplankton. When phytoplankton are abundant, the survival rate of crown-of-thorn starfish larvae increases, eventually resulting in a population boom in the adult animals, which then invade the Great Barrier Reef.
In addition to chemicals, agricultural runoff increases the flow of sediments into the Great Barrier Reef. An abundance of sediment and silt creates a muddy, opaque water, which not only blocks sunlight, but coats and chokes the coral.
One study by a University of Queensland scientist suggests that the beginning of large-scale coral deaths coincides with the development of the North Queensland coast. Branching coral especially seem to have been affected by coastal development. Prior to the early 1960s branching coral was abundant, but since then growth of these corals has nearly ceased.
Many farms continue to use outmoded and wasteful methods of farming, applying excessive fertilizer and pesticides. While some farms have updated their methods to limit chemical applications, more needs to be done to ensure the health of the Great Barrier Reef and its continued growth. We cannot allow ourselves to destroy in a few decades what it took the amazing corals hundreds of thousands of years to create.