New insights into coral reef threats

It is impossible to describe the stunning, captivating, and apparent tranquillity of a coral reef — it’s something you have to experience for yourself. Unfortunately, coral reefs are under threat from global warming, pollution, and other human activities. Coral bleaching, a key indicator of reef health, is becoming more frequent and the incidence of coral diseases may also be on the rise. But a new monitoring project hopes to throw more light on these problems. Reefs comprise a complex network of symbiotic interactions involving thousands of species of corals, fish, and algae. Corals themselves are fascinating organisms. They rely on a symbiotic relationship with their algal partners zooxanthellae, which give corals their brown color. When they are stressed they expel the algae, causing them to bleach to bone white or, in the case of corals that produce their own pigments, pink, pale blue, and yellow. Corals can survive short-term bleaching, but prolonged severe events are fatal. Although it is clear that rising global temperatures are partially to blame for bleaching, it appears to involve a combination of causative events that has yet to be determined. It is known that approximately two months before a major bleaching event there is an unusual up-welling of water from the ocean deeps. Normal up­ wellings are associated with the time of year: they generally occur between November and March on the Great Barrier Reef and happen when the boundary between warmer, surface water and cooler, deeper water moves upwards 20 to 40 meters allowing strong currents to sweep slugs of cold water onto the continental shelf. In these events the cold water does not penetrate far across the continental shelf, but unusual up-wellings are more intense and can drive cold, nutrient-rich water all the way to the shore. It is unlikely that the temperature drop alone caused by the cold water is responsible for coral bleaching, as it amounts to only a few degrees and does not go below normal winter temperatures for the reef, so the role of up-welling in bleaching remains unclear. To investigate this, and other aspects of the life of coral reefs, a team of scientists are installing a smart sensor network to monitor the Great Barrier Reef. The sensor network will function in many ways similarly to the internet. Each sensor is network enabled, can be given an IP address, and can be programmed and reprogrammed. They use wireless technology to communicate with each other and to send information back to a central location. Like the internet, if one sensor drops out of the network or a new sensor is added, they can reroute to accommodate the change. Because they use microwaves, smart sensors can send much larger amounts of information much faster than the traditional high-frequency radio systems that have been used to monitor the reef up to now. They can also function over long distances by taking advantage of a phenomenon known as ducting. This involves a region of rapidly changing humidity just above the ocean surface, known as the evaporation duct, that causes refraction of microwaves such that they bend toward the earth and therefore follow the curve of the earth rather than sailing off into the atmosphere. Using this, microwave signals have been reliably sent over distances of 70 km (the greatest distance from the shore to the reef is 100 km), unlike traditional line­ of-site radio transmissions. Although satellite communication can also achieve this, the cost is prohibitive, while the sensor network has been designed to use off-the-shelf technology to keep costs low. The Great Barrier Reef sensor network will consist of hundreds of sensors around Davies Reef and Magnetic, Orpheus, Heron, and Lizard