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Larval Biology and Super-Computers Connect Source and Sink ReefsAdvances in technology and collaboration between mathematicians and marine biologists are providing new insights into ocean currents that connect organisms throughout the Great Barrier Reef.
Inter-disciplinary collaboration between Townsville-based researchers is helping to identify reefs that produce an abundance of coral and fish larvae, and reefs that depend on stocks from upstream sources. Such insights promise to improve the conservation and management of the Great Barrier Reef Marine Park by providing a stronger ecological basis for planning different human uses, such as fishing and tourism, in this multiple use region. Understanding the pattern of connection between groups of reefs is of fundamental importance to reef science and management. Some 'source' reefs are net exporters of fertilized coral and fish eggs, producing enough larvae to maintain their home population as well as supplementing the populations of coral reefs downstream. 'Sink' reefs, in contrast, rely on catching larval emigrants from reefs upstream. Other reefs may even be self-seeding, generating just enough larvae to sustain the local fish or coral populations with very few young larvae available for export. If connecting ocean currents between reefs can be clearly mapped during breeding cycles, and source and sink reefs identified, the potential exists for resource managers to better determine what types of activities are appropriate on each reef. For example, if the breeding adults of a particular fish species on a source reef are heavily fished, then populations of this species on other reefs linked by downsteam currents will also be affected. In contrast, fishing on a sink will only affect local fish populations. Depending on the level of connection, scientists believe one well-protected source reef can provide enough larvae to sustain diversified fish or coral populations on up to ten reefs downstream. The traditional approach to mapping these connections used mathematical models to describe probable water movements between coral reefs. Computer models simulated suspended larvae that traveled passively in currents within and between reefs. The model predictions were then tested against data collected in the field. However, the results of the first generation of models, produced during the 1980s, were disappointing. According to Dr Jamie Oliver, then a researcher at the Australian Institute of Marine Science, "there was no correlation between the observed larval concentration around a coral reef and the concentration predicted by the models." The problem, so it was thought, was that computer models could not accurately describe water movements across a topographically complex reef environment. The limiting factor was the processing speed of computers used to run the simulations. The modellers had to wait for faster, more powerful computers with the ability to model water movement in three dimensions. They waited until 1994 when the mathematical skills of Dr Eric Wolanksi, a research scientist at AIMS, and the computer muscle of IBM, combined in a project named CRAMMM (Coral Reefs and Mangroves: Modeling and Management) to model patterns of water movement in the Barrier Reef's complex topographical environment. However, despite the capacity of the computers to process 250 trillion floating point numbers, once again they were frustrated. While the model could effectively describe the current patterns in and around reefs, it failed to accurately predict the abundance and distribution of fish larvae measured in field tests. A little biology was needed for the breakthrough. The model's simulation overlooked one important factor - it assumed that young fish had no control over their movement in water. But the assumption was untested and it took Dr Jeff Leis, a biologist from the Australian Museum, to get in the water and check it out. His observations revealed that larval fish actually sensed and swam towards nearby reefs. When the swimming abilities and behavior of these larval fish were incorporated into the model, the simulations finally matched observed patterns in the field. The scientists had their breakthrough.
However, while the models now accurately predict the dispersal of young fish around reefs, the dispersal of swimming coral larvae, or planulae, defied the modellers predictive skill. This time, scientists had underestimated the capacity of planulae to influence their own movement. Little research had been undertaken in this field - biology of coral larvae is still a new science - and until recently, researchers lacked easy and reliable ways to propagate them. Previous experimental methods were labor intensive, unreliable and risky. Coral larvae, grown at sea and exposed to the elements could be wiped out in a storm, and a year's work lost in minutes. This was high-risk science and no doubt scared many marine scientists away. But improved techniques now allow coral biologists to breed larvae in the comfort of a laboratory, where progress can be monitored and larvae are safe from the elements. The large number of larvae required for experiments can now be produced and rapid progress is being made towards understanding and documenting their behavior. For example, Dr Craig Mundy of AIMS has found that coral larvae know which way is up, swim into currents, and can sense the intensity and wavelength of light. My own research has shown that some planulae develop much faster, and live much longer, than was previously assumed. These abilities provide coral larvae with a greater potential to influence their fate. Clearly, planulae are not just passive particles that modellers once thought. When data on coral larvae behavior is incorporated into the latest generation of computer models there is a good chance that the patterns of connection can be accurately mapped. Armed with this knowledge, managers will be able to make more informed responses to periodic large-scale natural disturbances such as cyclones, crown-of-thorns starfish and coral bleaching. Source reefs and other areas vulnerable to disturbance, including those isolated by distance or far from interconnecting currents, can be identified and afforded a higher level of protection. If in decline, such reefs may even be targets for rehabilitation work. 'Well-connected' reefs would be ideal locations for future tourist development as they would be less likely to suffer from any longer-term impact. The collaborative approach to understand the implications of reef connectivity is proving immensely rewarding but the message is clear - understanding the level of connection between reefs will remain a mystery unless the biology of the larval organisms is known. We can never hope to understand the complexity of reef ecology without getting wet. By Andrew Baird |
