For several years, loss of live coral habitat due to coral bleaching has become a global concern, prompting researchers and scientists to study the health of coral reefs around the world. One location affected by this phenomenon is the southern Seychelles Islands, in the central-western Indian Ocean, north of Madagascar.
Coral bleaching has been linked to rising ocean temperatures, causing 40% to 50% of the coral in this region to become colorless and vulnerable to a variety of environmental stresses. To better understand the effects of rising water temperature on the coral's ecosystem, researchers are using HOBO® water temperature data loggers as part of a long-term monitoring program.
"Coral reefs are one of the most productive ecosystems on the planet -- equal in many respects to tropical rainforests, and they are a very important component of the ocean's ecosystem,” explains Ray Buckley, a principal investigator with the Aldabra Marine Programme (AMP). “So when you have a coral bleaching impact, it has a major effect on local and regional ecosystems."
"The main focus of our research is Aldabra Atoll because it's a large remote coral reef ecosystem which has received little impact from human habitation. It is one of the last natural laboratories on the planet. What happens there is how nature really responds to an environmental impact."
To monitor water temperature, Buckley and the AMP team have strategically placed HOBO Water Temp Pro data loggers in several locations at Aldabra Atoll, and Assomption, Astove, and St Pierre Islands to the east. Each data logger is deployed using cable ties and stakes, and water temperature measurements were taken every 30 minutes to an hour.
"The data loggers have become encrusted with bioaccumulation of coralline algae after two years on the reef and are still able to collect accurate temperature data," says Buckley.
The data logger provides ±0.2°C accuracy over a wide temperature range and offers a 42,000-measurement storage capacity, making it suitable for long-term deployments.
Data from the loggers can be quickly offloaded directly to a laptop via a USB-based optical interface, which provides high-speed, reliable data offload in wet environments. Its optical design eliminates the need for failure-prone mechanical connectors found in many traditional underwater data logger products.
The data were analyzed using HOBOware® Pro graphing and analysis software, which easily converts the collected data into easy-to-read graphs that reveal spikes and drops in water temperature over a 12-month period.
"Most coral bleaching studies take place in locations where there are major impacts to the corals due to human development," explains Buckley. "While those studies are critical in better understanding coral reef health, they are unable to provide a good baseline for how an ecosystem responds when there is no human impact. Aldabra Atoll is as close to pristine as we have and allows us to see how the coral and fish respond to the rising temperatures without outside interference."
Researchers have found that in coral reef ecosystems impacted by human development, there is usually a shift in the dominant component of the ecosystem from live coral to algae.
"The fish populations then switch to mainly herbivorous species and the algae covering the dead coral make it harder for the coral to recover. This causes a major shift in reef ecosystem," says Buckley.
According to Buckley, for the past 10 years there were no substantial changes to the fish populations at Aldabra Atoll, where there was essentially no human habitat.
"The coral bleaching event did not result in an algae-dominated ecosystem and it wasn't catastrophic for the fish in this area. Fish were able to adapt to the rapid major loss of live coral habitat and development of a new coral habitat. We found that if there were no other variables affecting the ecosystem, fish can respond positively to these habitat changes, even though the matrix of the system changed."